Essay: Broadband/the WiMAX system

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  • Broadband/the WiMAX system
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1.1 Introduction
The experienced growth in the use of digital networks has led to the need for the design of new communication networks with higher capacity. The telecom industry is expected to continue to grow as demand increases for cable and high-speed internet in previously un-serviced locations and as local telephone companies upgrade their lines in response to increasing competitions.
Broadband wireless sits at the confluence of two of the most remarkable growth stories of the telecommunications industry in recent years. Both wireless and broadband have on their own enjoyed rapid mass-market adoption. Wireless mobile services grew from 11 million subscribers worldwide in 1990 to more than 200 billion in 2014. During the same period, the Internet grew from being a curious academic tool to having about a billion users. This staggering growth of the Internet is driving demand for higher-speed Internet-access services, leading to a parallel growth in broadband adoption. In less than a decade, broadband subscription worldwide has grown from virtually zero to over 200 million. Will combining the convenience of wireless with the rich performance of broadband be the next frontier for growth in the industry? Can such a combination be technically and commercially viable? Can wireless deliver broadband applications and services that are of interest to the end users? Many industry observers believe so.
Before we delve into broadband wireless, let us review the state of broadband access today. Digital subscriber line (DSL) technology, which delivers broadband over twisted-pair telephone wires, and cable modem technology, which delivers over coaxial cable TV plant, are the predominant mass-market broadband access technologies today. Both of these technologies typically provide up to a few megabits per second of data to each user, and continuing advances are making several tens of megabits per second possible. Since their initial deployment in the late 1990s, these services have enjoyed considerable growth. Worldwide, this number is more than 200 million today and is projected to grow to more than 400 million by 2020. The availability of a wireless solution for broadband could potentially accelerate this growth.
What are the applications that drive this growth? Broadband users worldwide are finding that it dramatically changes how we share information, conduct business, and seek entertainment.
Broadband access not only provides faster Web surfing and quicker file downloads but also enables several multimedia applications, such as real-time audio and video streaming, multimedia conferencing, and interactive gaming. Broadband connections are also being used for voice telephony using voice-over-Internet Protocol (VoIP) technology. More advanced broadband access systems, such as fiber-to-the-home (FTTH) and very high data rate digital subscriber loop (VDSL), enable such applications as entertainment-quality video, including high-definition TV (HDTV) and video on demand (VoD). As the broadband market continues to grow, several new applications are likely to emerge, and it is difficult to predict which ones will succeed in the future.
So what is broadband wireless? Broadband wireless is about bringing the broadband experience to a wireless context, which offers users certain unique benefits and convenience. There are two fundamentally different types of broadband wireless services. The first type attempts to provide a set of services similar to that of the traditional fixed-line broadband but using wireless as the medium of transmission. This type, called fixed wireless broadband, can be thought of as a competitive alternative to DSL or cable modem. The second type of broadband wireless, called mobile broadband, offers the additional functionality of portability, nomadicity, and mobility. Mobile broadband attempts to bring broadband applications to new user experience scenarios and hence can offer the end user a very different value proposition. WiMAX (worldwide interoperability for microwave access) technology is designed to accommodate both fixed and mobile broadband applications.
In this chapter, we provide a brief overview of broadband wireless. The objective is to present the background and context necessary for understanding WiMAX.
1.2 Evolution of Broadband Wireless
The history of broadband wireless as it relates to WiMAX can be traced back to the desire to find a competitive alternative to traditional wire line-access technologies. Spurred by the deregulation of the telecom industry and the rapid growth of the Internet, several competitive carriers were motivated to find a wireless solution to bypass incumbent service providers. During the past decade or so, a number of wireless access systems have been developed, mostly by start-up companies motivated by the disruptive potential of wireless. These systems varied widely in their performance capabilities, protocols, frequency spectrum used, applications supported, and a host of other parameters. Some systems were commercially deployed only to be decommissioned later. Successful deployments have so far been limited to a few niche applications and markets. Clearly, broadband wireless has until now had a checkered record, in part because of the fragmentation of the industry due to the lack of a common standard. The emergence of WiMAX as an industry standard is expected to change this situation.
Given the wide variety of solutions developed and deployed for broadband wireless in the past, a full historical survey of these is beyond the scope of this section. Instead, we provide a brief review of some of the broader patterns in this development. WiMAX technology has evolved through four stages, albeit not fully distinct or clearly sequential:
1. Narrowband wireless local-loop systems,
2. First-generation line-of-sight (LOS) broadband systems,
3. Second-generation non-line-of-sight (NLOS) broadband systems, and
4. Standards-based broadband wireless systems.
1.2.1 Narrowband Wireless Local-Loop Systems
Naturally, the first application for which a wireless alternative was developed and deployed was voice telephony. These systems, called wireless local-loop (WLL), were quite successful in developing countries such as China, India, Indonesia, Brazil, and Russia, whose high demand for basic telephone services could not be served using existing infrastructure. In fact, WLL systems based on the digital-enhanced cordless telephony (DECT) and code division multiple access (CDMA) standards continue to be deployed in these markets.
In markets in which a robust local-loop infrastructure already existed for voice telephony, WLL systems had to offer additional value to be competitive. Following the commercialization of the Internet in 1993, the demand for Internet-access services began to surge, and many saw providing high-speed Internet-access as a way for wireless systems to differentiate them. For example, in February 1997, AT&T announced that it had developed a wireless access system for the 1,900MHz PCS (personal communications services) band that could deliver two voice lines and a 128kbps data connection to subscribers. This system, developed under the code name ‘Project Angel,’ also had the distinction of being one of the first commercial wireless systems to use adaptive antenna technology. After field trials for a few years and a brief commercial offering, AT&T discontinued the service in December 2001, citing cost run-ups and poor take-rate as reasons.
During the same time, several small start-up companies focused solely on providing Internet-access services using wireless. These wireless Internet service provider (WISP) companies typically deployed systems in the license-exempt 900MHz and 2.4GHz bands. Most of these systems required antennas to be installed at the customer premises, either on rooftops or under the eaves of their buildings. Deployments were limited mostly to select neighborhoods and small towns. These early systems typically offered speeds up to a few hundred kilobits per second. Later evolutions of license-exempt systems were able to provide higher speeds.
1.2.2 First-Generation Broadband Systems
As DSL and cable modems began to be deployed, wireless systems had to evolve to support much higher speeds to be competitive. Systems began to be developed for higher frequencies, such as the 2.5GHz and 3.5GHz bands. Very high speed systems, called local multipoint distribution systems (LMDS), supporting up to several hundreds of megabits per second, were also developed in millimeter wave frequency bands, such as the 24GHz and 39GHz bands. LMDS based services were targeted at business users and in the late 1990s enjoyed rapid but short-lived success. Problems obtaining access to rooftops for installing antennas, coupled with its shorter range capabilities, squashed its growth.
In the late 1990s, one of the more important deployments of wireless broadband happened in the so-called multichannel multipoint distribution services (MMDS) band at 2.5GHz. The MMDS band was historically used to provide wireless cable broadcast video services, especially in rural areas where cable TV services were not available. The advent of satellite TV ruined the wireless cable business, and operators were looking for alternative ways to use this spectrum. A few operators began to offer one-way wireless Internet-access service, using telephone line as the return path. In September 1998, the Federal Communications Commission (FCC) relaxed the rules of the MMDS band in the United States to allow two-way communication services, sparking greater industry interest in the MMDS band. MCI WorldCom and Sprint each paid approximately $1 billion to purchase licenses to use the MMDS spectrum, and several companies started developing high-speed fixed wireless solutions for this band.
The first generation of these fixed broadband wireless solutions was deployed using the same towers that served wireless cable subscribers. These towers were typically several hundred feet tall and enabled LOS coverage to distances up to 35 miles, using high-power transmitters. First-generation MMDS systems required that subscribers install at their premises outdoor antennas high enough and pointed toward the tower for a clear LOS transmission path. Sprint and MCI launched two-way wireless broadband services using first-generation MMDS systems in a few markets in early 2000. The outdoor antenna and LOS requirements proved to be significant impediments. Besides, since a fairly large area was being served by a single tower, the capacity of these systems was fairly limited. Similar first-generation LOS systems were deployed internationally in the 3.5GHz band.
1.2.3 Second-Generation Broadband Systems
Second-generation broadband wireless systems were able to overcome the LOS issue and to provide more capacity. This was done through the use of a cellular architecture and implementation of advanced-signal processing techniques to improve the link and system performance under multipath conditions. Several start-up companies developed advanced proprietary solutions that provided significant performance gains over first-generation systems. Most of these new systems could perform well under non-line-of-sight conditions, with customer-premise antennas typically mounted under the eaves or lower. Many solved the NLOS problem by using such techniques as orthogonal frequency division multiplexing (OFDM), code division multiple access (CDMA), and multiantenna processing. Some systems, such as those developed by SOMA Networks and Navini Networks, demonstrated satisfactory link performance over a few miles to desktop subscriber terminals without the need for an antenna mounted outside. A few megabits per second throughput over cell ranges of a few miles had become possible with second generation fixed wireless broadband systems.
1.2.4 Emergence of Standards-Based Technology
In 1998, the Institute of Electrical and Electronics Engineers (IEEE) formed a group called 802.16 to develop a standard for what was called a wireless metropolitan area network, or wireless MAN. Originally, this group focused on developing solutions in the 10GHz to 66GHz band, with the primary application being delivering high-speed connections to businesses that could not obtain fiber. These systems, like LMDS, were conceived as being able to tap into fiber rings and to distribute that bandwidth through a point-to-multipoint configuration to LOS businesses.
The IEEE 802.16 group produced a standard that was approved in December 2001. This standard, Wireless MAN-SC, specified a physical layer that used single-carrier modulation techniques and a media access control (MAC) layer with a burst time division multiplexing (TDM) structure that supported both frequency division duplexing (FDD) and time division duplexing (TDD).
After completing this standard, the group started work on extending and modifying it to work in both licensed and license-exempt frequencies in the 2GHz to 11GHz range, which would enable NLOS deployments. This amendment, IEEE 802.16a, was completed in 2003, with OFDM schemes added as part of the physical layer for supporting deployment in multipath environments. By this time, OFDM had established itself as a method of choice for dealing with multipath for broadband and was already part of the revised IEEE 802.11 standards. Besides the OFDM physical layers, 802.16a also specified additional MAC-layer options, including support for orthogonal frequency division multiple access (OFDMA).
Further revisions to 802.16a were made and completed in 2004. This revised standard, IEEE 802.16-2004, replaces 802.16, 802.16a, and 802.16c with a single standard, which has also been adopted as the basis for HIPERMAN (high-performance metropolitan area network) by ETSI (European Telecommunications Standards Institute). In 2003, the 802.16 group began work on enhancements to the specifications to allow vehicular mobility applications. That revision, 802.16e, was completed in December 2005 and was published formally as IEEE 802.16e-2005.
It specifies scalable OFDM for the physical layer and makes further modifications to the MAC layer to accommodate high-speed mobility. As it turns out, the IEEE 802.16 specifications are a collection of standards with a very broad scope. In order to accommodate the diverse needs of the industry, the standard incorporated a wide variety of options. In order to develop interoperable solutions using the 802.16 family of standards, the scope of the standard had to be reduced by establishing consensus on what options of the standard to implement and test for interoperability. The IEEE developed the specifications but left to the industry the task of converting them into an interoperable standard that can be certified. The WiMAX Forum was formed to solve this problem and to promote solutions based on the IEEE 802.16 standards. The WiMAX Forum was modeled along the lines of the Wi-Fi Alliance, which has had remarkable success in promoting and providing interoperability testing for products based on the IEEE 802.11 family of standards.
The WiMAX Forum enjoys broad participation from the entire cross-section of the industry, including semiconductor companies, equipment manufacturers, system integrators, and service providers. The forum has begun interoperability testing and announced its first certified product based on IEEE 802.16-2004 for fixed applications in January 2006. Products based on IEEE 802.18e-2005 are expected to be certified in early 2007. Many of the vendors that previously developed proprietary solutions have announced plans to migrate to fixed and/or mobile WiMAX. The arrival of WiMAX-certified products is a significant milestone in the history of broadband wireless.
1.3 WiMAX versus 3G and Wi-Fi
How does WiMAX compare with the existing and emerging capabilities of 3G and Wi-Fi? The throughput capabilities of WiMAX depend on the channel bandwidth used. Unlike 3G systems, which have a fixed channel bandwidth, WiMAX defines a selectable channel bandwidth from 1.25MHz to 20MHz, which allows for a very flexible deployment. When deployed using the more likely 10MHz TDD (time division duplexing) channel, assuming a 3:1 downlink-to-uplink split and 2 ” 2 MIMO, WiMAX offers 46Mbps peak downlink throughput and 7Mbps uplink. The reliance of Wi-Fi and WiMAX on OFDM modulation, as opposed to CDMA as in 3G, allows them to support very high peak rates. The need for spreading makes very high data rates more difficult in CDMA systems.
More important than peak data rate offered over an individual link is the average throughput and overall system capacity when deployed in a multicellular environment. From a capacity standpoint, the more pertinent measure of system performance is spectral efficiency. The fact that WiMAX specifications accommodated multiple antennas right from the start gives it a boost in spectral efficiency. In 3G systems, on the other hand, multiple-antenna support is being added in the form of revisions. Further, the OFDM physical layer used by WiMAX is more amenable to MIMO implementations than are CDMA systems from the standpoint of the required complexity for comparable gain. OFDM also makes it easier to exploit frequency diversity and multiuser diversity to improve capacity. Therefore, when compared to 3G, WiMAX offers higher peak data rates, greater flexibility, and higher average throughput and system capacity.
Another advantage of WiMAX is its ability to efficiently support more symmetric links’useful for fixed applications, such as T1 replacement’and support for flexible and dynamic adjustment of the downlink-to-uplink data rate ratios. Typically, 3G systems have a fixed asymmetric data rate ratio between downlink and uplink.
What about in terms of supporting advanced IP applications, such as voice, video, and multimedia? How do the technologies compare in terms of prioritizing traffic and controlling quality? The WiMAX media access control layer is built from the ground up to support a variety of traffic mixes, including real-time and non-real-time constant bit rate and variable bit rate traffic, prioritized data, and best-effort data. Such 3G solutions as HSDPA and 1x EV-DO were also designed for a variety of QoS levels.
Perhaps the most important advantage for WiMAX may be the potential for lower cost owing to its lightweight IP architecture. Using an IP architecture simplifies the core network’3G has a complex and separate core network for voice and data’and reduces the capital and operating expenses. IP also puts WiMAX on a performance/price curve that is more in line with general-purpose processors (Moore’s Law), thereby providing greater capital and operational efficiencies. IP also allows for easier integration with third-party application developers and makes convergence with other networks and applications easier.
In terms of supporting roaming and high-speed vehicular mobility, WiMAX capabilities are somewhat unproven when compared to those of 3G. In 3G, mobility was an integral part of the design; WiMAX was designed as a fixed system, with mobility capabilities developed as an add-on feature.
In summary, WiMAX occupies a somewhat middle ground between Wi-Fi and 3G technologies when compared in the key dimensions of data rate, coverage, QoS, mobility, and price.
1.4 Motivation
Nowadays, life does not seem feasible without wireless networks in one or the other form. Wireless is becoming the leader in communication choices among users. In the current era life is converging towards the cable less environment where the last mile connectivity can be easily achievable without the need of physical connections. So the field of wireless communication is continuously emerging one which is the demand for the transfer of data with high speed and with long coverage range. The claim for broadband mobile services continues to grow. Usually, high-speed broadband solutions are based on wired-access technologies such as digital subscriber line (DSL). This type of solution is not easy to deploy in remote rural areas, and furthermore it lacks support for terminal mobility.]
Also the gradual development in the use of wireless networks has led to the requirement for the design of new modern communication networks with higher capacity and lower error rate. The telecommunication industry is also upgrading, with a requirement for a greater range of services, such as video conferences, or applications with multimedia contents. The increased dependence on computer networking and the internetwork has resulted in a larger demand for connections to be allotted any time, any place, leading to a increase in the requirements for greater capacity and ultimate reliable broadband wireless communication systems.
For this issue, new technologies with high throughput with less requirement of bandwidth have been designed. As a matter of fact the requirements on bandwidth and spectrum availability are endless. As a result, the designers working in the domain of wireless communication has to face the lots of difficulties to fulfill the requirement of bandwidth for the efficient and accurate transmission and reception. Moreover the problems of time varying nature of channel such as fading and multipath put the limitation on the performance of high data rate with good quality of service. The demands for greater capacity, high reliability as well as accuracy are the prime requisites for the forth coming generations of the wireless networking systems such as Wi-Fi, WiMAX, etc.
1.5 Objective of Thesis
How to deal with fading and with interference is central aim to the design of wireless communication systems, and by taking the advantage of multi-path fading and improving the system capacity and bit rate of 4G modern wireless system will be the fundamental objective of this research work. The research work includes the performance analysis of following points:
‘ To study and to analysis the performance of forthcoming future generation wireless networking technique i.e. WiMAX as the upcoming 4G standard for meeting the requirements of last mile end to end wireless network with greater system capacity with improved bit error rate.
‘ To analyze the features of antenna diversity techniques in wireless communication for nullifying the limitations due to multipath fading by simulating the system in terms of system throughput and bit error rate under MATLAB based environment.
‘ To simulate the complete WiMAX system by implementing antenna diversity techniques and Alamouti coding in it to fulfill the current demands of the modern wireless networks with the anticipation of improvement in bit error rate thereby increment in system reliability.
1.6 Organization of Thesis
This research work examines the modeling, simulation and comparative analysis of WiMAX system along with the implementation of various antenna diversity techniques in it built with MATLAB. The whole thesis is organized in 6 chapters.
Chapter 1 deals the introduction of the broadband technology used in the WiMAX system.
Chapter 2 deals the all related work by the previous researchers. In this chapter we discuss the paper based on the previous work associated with the WiMAX system and found problem in the WiMAX system.
Chapter 3 deals the fundamentals of the WiMAX system. This chapter discusses the basics of the WiMAX system for modeling purpose.
Chapter 4 deals the technology used in the WiMAX system. In these chapter diversity techniques is conceptually discussed. Also discuss the antenna diversity too.
Chapter 5 is based on the chapter 3 to 4. In this chapter a simulink model is generated with the result to analyzed the WiMAX physical layer for antenna diversity.
Chapter 6 is the conclusion and future work of the thesis.
2.1 Introduction
In this section, a brief review of literature on Performance of Antenna Diversity Techniques, Alamouti Coding Scheme, WiMAX Broadband Wireless Access Technology, Mobile WiMAX Technology, IEEE 802.16 Standards, Efficient Wireless Channels and Orthogonal Frequency Division Multiplexing Technique are reported and discussed.
2.2 Related work
Abdulrahman Yarali, Saifur Rahman [6] describes the overview of the forthcoming most promising wireless system WiMAX-Worldwide Interoperability for Microwave Access. In this research paper the basic WiMAX introduction, comparison with the existing wireless systems, types of IEEE standards as well as layered structure has been included. This paper presented a brief description of some of the major functions of a WiMAX network architecture currently being designed and specified in the network group. This paper is useful to analyze the basic architecture and supporting features of WiMAX for this research work.
Mai Tran, George Zaggoulos, Andrew Nix and Angela Doufexi [7] describes the current demand of wireless communication system is to achieve highest capacity with lowest requirement of bandwidth and improved error rate. The mobile WiMAX is the wonderful invention which is fulfilling the latest demand. This research paper presents the theoretical aspect of the mobile WiMAX system whose remarkable features are scalable OFDM and Advanced antenna techniques such as MIMO. Each and every parameter which are required to build and to model the WiMAX system such as channel coding, sampling frequency, sampling period, symbol duration of OFDM, modulation scheme, etc. have been discussed in this paper which are the useful matters to develop a simulation model of this research work.
Liangshan Ma, Dongyan Jia [8] analyses both the competitive and cooperative relationships between WiMAX, WLAN and 3G from various aspects such as technical standards, current status and future trends, etc. WiMAX and WLAN are two most emerging IEEE standards for providing efficient wireless networking whereas 3G is the most important mobile communication standard for providing highest speed along with maximum accuracy. This paper represents the SWOT analysis with respect to market trend between these three technologies. This paper will be helpful to understand the role of one technique to cope up with other technologies.
Abdulrahman Yarali Bwanga Mbula Ajay Tumula [9] identifies the cost effective, flexible 4th generation standard of IEEE i.e. WiMAX system which is becoming the perfect solution to meet the current demands of the future wireless networks thereby providing the tough competition to the existing 3G standards. This paper includes the modeling of WiMAX layer by considering its physical layer as well as various parameters related to it which is the main utilization in this research work. The two variants of WiMAX system i.e. fixed WiMAX (IEEE 802.16d) and mobile WiMAX (IEEE 802.16e) have been included in the paper which helps to derive the characteristics of WiMAX system. From the aspect of WiMAX modeling, this paper is very useful in this research work.
Sassan Ahmadi, Intel Corporation[10] presents the thorough analysis of IEEE 802.16 architecture which is becoming the most popular 4G standard for the different mobile communication applications. The growing demand for mobile Internet and wireless multimedia applications has motivated the development of broadband wireless access technologies in recent years. Mobile WiMAX has enabled convergence of mobile and fixed broadband networks through a common wide-area radio-access technology and flexible network architecture. The theoretical aspect of WiMAX architecture and parameters of this paper at very minute level helps in this research work to understand the whole WiMAX system.
Hicham Yehia and Hany Kamal [11] discusses the effect of interference in the WiMAX network thereby analyzing the effect of the same on the capacity of the network. Due to imperfections at both the ends of the system i.e. at transmitter and at the receiver, the interference can occur which limits the system performance. At the transmitter due to inefficient filter response, spurious transmission can be encountered and at the receiver, due to insufficient selectivity, the interference will occur. The paper checks the effects of this kind of interference in the WiMAX system which is the main reason behind the degradation in the network capacity.
Raj Jain, Chakchai So-In, And Abdel-Karim Al Tamimi [12] basically deals with the analysis of one to one layer of the WiMAX network which is the very important issue from the view point of service providers and network developers. The paper includes the system level modeling at various levels such as physical layer modeling, MAC layer modeling, interference level modeling, frequency level modeling, etc so as to analyze many system level parameters. The readings and observations generated at the abstract level are the very useful outcomes for this research work.
Nedeljko Cvejic and Seppanen, Tapio [13] illustrate that for the efficient and fruitful wireless communication, the virtual channel i.e. radio channel of propagation should be modeled properly. This paper estimates the efficiency of AWGN channel and Rayleigh channel under different scenarios. As per the nature of application, the type of channel modeling should get selected. In this paper, the concentration is on the modern digital video compression technique i.e. mp3 system where during watermarking, the impairment due to noise in the transmission would be analyzed by modeling the channel as Rayleigh channel which is frequency selective channel not by AWGN channel. This paper justifies the utility and use of channel modeling in this research.
Daniele Lo Iacono, Marco Ronchi, Luigi Della Torre, and Fabio Osnato[14] discuss that in today’s world, the main goal of any system is to achieve highest system capacity with lowest error rate which is not possible with single transmitter and single receiving antenna because it can’t overcome the effects of fading. For this particular reason, the current wireless communication trend is tilting towards the multiple transmitter and multiple receiver antenna systems i.e. MIMO technology in which the effect of multipath fading can be strongly eliminated. This research paper is based on the same fundamental by considering the effect and application of MIMO technique with the implementation of OFDM in wireless communication system. The methodology and results derived from the experiment of MIMO and OFDM are very useful deductions for this research work.
Onsy Abdel Alim, Hiba S. Abdallah and Azza M. Elaskary [15] discusses two main issues. The first one presents models for simulating OFDM WiMAX system in Simulink including channel estimation and equalization subsystems in MATLAB functions. Next, the effect of channel estimation error on the performance of MIMO VBLAST receivers in uncorrelated Rayleigh flat fading channels is investigated. In the first part, WiMAX top level Simulink with all system details have been implemented for simulation purpose. In the second part, the performance of MIMO VBLAST ZF receivers over uncorrelated Rayleigh flat fading channels in the presence of channel estimation error is investigated. This IEEE transaction is very much useful to derive the simulation model of the WiMAX system along with the reference BER reading with MIMO implementation.
Mohab Shalash, Tallal El Shabrawy and Waleed Diab [16] covers a thorough study of wideband frequency selective channels from the perspective of multi-carrier modulation system. Wideband communications systems suffer from frequency selective channels. Accordingly, 3G/4G systems have endorsed the concept of multi-carrier modulation such as OFDM and MC-CDMA, where the wideband channel is sub-divided into numerous subcarriers. This research paper is useful to analyze the behavior of various wireless channels such as Rayleigh and Rician channel along with the basic parameters of it for modeling purpose. For 3G and 4G systems how the different channels and their parameters are affecting the behavior of the whole system while modeling was the true strength of this paper for this research work point of view.
Muhammad Nadeem Khan, Sabir Ghauri [17] discusses the model building of the WiMAX Physical layer using Simulink in MATLAB. This model is a useful tool for performance evaluation of the WiMAX standards 802.16e under the various parameters like carrier frequency, frequency bands, bandwidth, radio technology etc which have been mentioned. For this research work, this IEEE transaction will be the mile stone. This paper is useful to find the most valuable information regarding the modeling of the WiMAX physical layer with the various aspects of OFDM and MIMO. For WIMAX System modeling, this research paper may come across the very minute detailing of each and every blocks of the WiMAX modeling along with the most precise readings.
Vahid Tarokh, Nambi Seshadr and A. R. Calderbank [18] basically includes the characterization of wide band wireless channel for the future wireless technologies along with the feature of antenna diversity. In this paper the most practical approach of increasing the capacity of the channel has been presented for modern wireless communication systems with the introduction of antenna diversity and space time codes i.e. Alamouti coding. This paper provides the base for this research work regarding the different antenna diversity techniques such as SIMO, MISO, MIMO along with the space time coding.
Tao Jiang and Weidong Xiang [19] describes Multimedia Multicast and Broadcast service (MBS) over wireless links, such as mobile TV and IP radio broadcasting. As one of the most promising enabling technologies, mobile WiMAX can offer scalability in both radio access and network architecture, thus providing important flexibility in terms of network services and deployment options. This paper presents the overview of network architecture of OFDM based WiMAX system. Also the enhanced features such as antenna diversity, multiple modulation schemes, etc which can be a part of WiMAX system for improved network performance are the helpful contents for this research work.
Kamran Etemad [20] discusses the brief of Mobile WiMAX technology with the layered architecture and evolution. Mobile WiMAX combines OFDMA and advanced MIMO schemes along with flexible bandwidth and fast link adaptation, creating a highly efficient air interface that exceeds the capacity of existing and evolving 3G radio access networks. This research paper is useful for the implementation of advanced antenna techniques and OFDMA in the physical layer of WiMAX to improve Quality of Service.
Chengshan Xiao, Jingxian Wu, Sang-Yick Leong and Yahong Rosa Zheng [21] presents a new discrete-time channel model for MIMO systems over space-selective (or spatially correlated), time-selective (or time-varying), and frequency-selective Rayleigh fading channels, which are referred to as triply selective Rayleigh fading channels. MIMO is the current trends in the modern cellular system towards achieving high data rate with low error rate. This paper is helpful in deriving the performance analysis of MIMO system with Rayleigh channel for the research work. Here in this the discrete time channel model has been developed which evaluates the statistical properties of the system.
Zakhia Abichar, Yanlin Peng and J. Morris Chang [22] includes the brief of WiMAX and its layered architecture i.e. physical layer and MAC layer. The theoretical aspects presented in this chapter define the functioning as well features of MAC layer and Physical layer. The MAC layer of the WiMAX technology decides the quality of service and the algorithms related to error control while physical layer is responsible for data transfer with high capacity and low error rate. This paper is purely helpful for the theoretical survey of WiMAX system in the research work.
H. Farhat, G. Grunfelder, A. Carcelen and G. El Zein [23] Illustrate that to improve data rates and to enhance the quality of the system for the future generation wireless systems, the most prominent solution is antenna diversity techniques i.e. MIMO. The referred reference paper gives the design of MIMO channel sounder utilized for the WiMAX technology. The basic illustrations given in this paper related to MIMO system design along with channel characteristic analysis are the useful estimates for this research work.
Onsy Abdel Alim, Nemat Elboghdadly, Mahmoud M. Ashour, Azza M. Elaskary [24] has the objective of building a System level model for a WiMAX Orthogonal Frequency Division Multiplexing based transceiver. OFDM technique theoretically saves the bandwidth about 50%. Modeling irradiation noise as an external effect added to the Additive White Gaussian noise (AWGN). This paper represents the basic simulation model of WiMAX OFDM system which is the most important helpful aspect of the paper in this research work.
Mikko Majanen, Pekka H. J. Perala and Thomas Casey [25] describes that the demand for mobile internet access is continuing its growth at increasing speed. New wireless access technologies compete with each other at the global market and it is still unsure which one will be the winner. One of the most promising ones is WiMAX which is based on IEEE 802.16 air interface standard. This paper includes the WiMAX simulation model along with the handover process. The paper is useful to analyze the three types of handover process i.e. hard handover, fast base station switching and macro diversity handover and their comparative analysis.
Ibrahim A.Z. Qatawneh [26] represents the bit error rate performance comparison of AWGN channel and Rician fading channels by considering their application in multi carrier DE-APSK and single carrier DE-APSK system. Frequency flat Rayleigh fading is a typical channel model found in land mobile radio situations. This model is suitable for modeling urban areas that are characterized by many obstructions where a line of sight path does not exist between the transmitter and receiver. In suburban areas a line of sight path may exist between the transmitter and receiver and this will give rise to Rician fading. The analysis of the channel comparison is the helpful conclusion for this research work.
Alireza Seyedi, Vasanth Gaddam, and Dagnachew Birru [27] represent performance analysis of OFDM UWB system with two antennas at the receiver side. Different antenna selection and combining methods, such as simple antenna selection, antenna selection per sub-carrier, equal gain combining and maximal ratio combining are considered. This paper discusses the simulation model, different antenna selection and combining techniques which are the important conclusions for this research work.
Shigenobu Sasaki Hisakazu Kikuchi Jinkang ZHU [28] discussed the performance of system over the type of frequency non-selective Rayleigh fading scenario. Fading and interferences are the two phenomenons that make the problem domain of modern wireless communication system most challenging and interesting. This paper illustrates the implementation of the diversity techniques for the significant reduction in Bit Error Rate performance over fading channel. The degradation of performance is over come by introducing the selection diversity and time diversity techniques. This research paper is useful to introduce the time diversity technique i.e. Reed-Solomon coding in physical layer modeling of WiMAX system for the remarkable reduction in system BER.
Koon Hoo Teo, Zhifeng Tao, and Jinyun Zhang [29] discussed the IEEE 802.16e Standards for Mobile WiMAX. This paper illustrates the implementation of frequency diversity technique for the high speech mobile service perspective. Also the comparison of WiMAX standards with WLANs and cellular is mentioned. More specifically this paper focused on the exploitation of technology in Mobile WiMAX standards. This paper is useful for the modeling of WiMAX System using IEEE 802.16e standards.
Rick S. Blum, Senior, Jack H. Winters and Nelson R. Sollenberger [30] discussed the benefits of transmitting the information through the multiple antennas over the fading channels. It also describes that the mutual information of a single, isolated, multiple transmit and receive antenna array link is exploited by transmitting the maximum number of independent data streams for a flat fading channel with independent fading coefficients for each path. This paper is useful in this research works as it purely focused on great potential achieved by transmit and receive antenna arrays used in Multiple Input Multiple Output antenna system.
3.1 Introduction
WiMAX, the Worldwide Interoperability for Microwave Access is the highly anticipated technology that aims to provide business and consumer wireless broadband services in form of Metropolitan Area Network (MAN). The technology has a target range of up to 31 miles and a target transmission rate exceeding 100 Mbps and is expected to challenge DSL and T1 lines (both expensive technologies to deploy and maintain) especially in emerging markets.
The mobile WiMAX is the wonderful invention which is fulfilling the latest demand. Through its high coverage and data rate characteristics, it fulfills the idea of complete network architecture thereby providing a flexible and cheap solution for the last-mile. The interoperability is a very critical issue, on which equipment cost and volume of sales will be based. Operators will not be bound to a sole equipment supplier, as the radio base stations will be able to interact with terminals produced by different suppliers. From the point of view of cost and accuracy, the customer’s must get the benefit of supplier’s competition. WiMAX may be seen as the fourth generation (4G) of mobile systems as the convergence of cellular telephony, computing, Internet access, and potentially many multimedia applications become a real fact.
WiMAX’s attributes open the technology to a wide variety of applications. With its large range and high transmission rate, WiMAX can serve as a backbone for 802.11 hotspots for connecting to the Internet. Alternatively, users can connect mobile devices such as laptops and handsets directly to WiMAX base stations without using 802.11 which can be very well observed from Figure 3.1. Developers project this configuration for the WiMAX mobile version, which will provide users broadband connectivity over large coverage areas compared with 802.11 hotspots’ moderate coverage. Mobile devices connected directly to WiMAX base stations likely will achieve a range of 5 to 6 miles, because mobility makes links vulnerable.
Figure 3.1 WiMAX Senario
The technology can also provide fast and cheap broadband access to markets that lack infrastructure (fiber optics or copper wire), such as rural areas and unwired countries. Currently, several companies offer proprietary solutions for wireless broadband access, many of which are expensive because they use chipsets from adjacent technologies, such as 802.11. Manufacturers of these solutions use the physical layer and bypass the medium access control layer by designing a new one. Unlike these proprietary solutions, WiMAX’s standardized approach offers economies of scale to vendors of wireless broadband products, significantly reducing costs and making the technology more accessible. Many companies that were offering proprietary solutions, however, have participated in the WiMAX forum and now offer WiMAX based solutions. WiMAX can be used in disaster recovery scenes where the wired networks have broken down. Similarly, WiMAX can be used as backup links for broken wired links. Additionally, WiMAX will represent a serious competitor to 3G cellular systems as high speed mobile data applications will be achieved with the 802.16e specification.
The main operators have concentrated their interests and efforts on the future applications of this new technology. The WiMAX forum created in April 2002, is a no-profit organization that groups companies promoting the broadband access based on the wireless communication standard, point to multipoint IEEE 802.16 for Metropolitan Area Network. WiMAX forum activities aim to:
‘ support the standardization process of IEEE 802.16 for MAN
‘ select and promote some of the WiMAX profiles defined in the 802.16
‘ certificate the interoperability between WiMAX equipment of different suppliers
‘ make WiMAX a universally accepted technology
3.2 Relationship with other Wireless Technology
Wireless access to data networks is expected to be an area of rapid growth for mobile communication systems. The huge uptake rate of mobile phone technologies, WLANs and the exponential growth that is experiencing the use of the internet have resulted in an increased demand for new methods to obtain high capacity wireless networks. WiMAX is expected to have an explosive growth, as well as the Wi-Fi, but compared with the Wi-Fi, WiMAX provides broadband connections in greater areas, measured in square kilometers, even with links not in line of sight. For these reasons WiMAX is a MAN, highlighting that ‘metropolitan’ is referred to the extension of the areas and not to the density of population. But Wi-Fi and WiMAX are not competing technologies. While WiMAX can provide high capacity internet access to residences and business seats, Wi-Fi allows the extension of such connections inside the corporate sites buildings. Figure-3.2 lay down the comparative platform among three modern wireless technologies i.e. WiMAX, WiFi and 3G cellular telephony.
In any case, both WLAN and cellular mobile applications are being widely expanded to offer the demanded wireless access. However, they experience several difficulties for reaching a complete mobile broadband access, bounded by factors such as bandwidth, coverage area, and infrastructure costs.
As shown in following Figure-3.2, Wi-Fi provides a high data rate, but only on a short range of distances and with a slow movement of the user. On the other hand, cellular offers larger ranges and vehicular mobility, but instead, it provides lower data rates, and requires high investments for its deployment.
Figure 3.2 Relationship of WiMAX with other technologies
Figure-3.3 Relationship with other wireless technologies
WiMAX tries to balance this situation which is pictorially depicted in Figure-3.3. WiMAX fills the gap between Wi-Fi and cellular, thus providing vehicular mobility, and high service areas and data rates.
3.3 WiMAX Standards
WiMAX is a technology standardized by IEEE for wireless MANs conforming to parameters which enable interoperability. WiMAX developments have been moving forward at a rapid pace since the initial standardization efforts in IEEE 802.16. In the meantime, the metropolitan area wireless networks development work was progressing under the IEEE 802.16 committee which evolved standards for wireless MANs. The IEEE 802.16 standard was firstly designed to address communications with direct visibility in the frequency band from 10 to 66 GHz. Due to the fact that non-line-of-sight transmissions are difficult when communicating at high frequencies, the amendment 802.16a was specified for working in a lower frequency band, between 2 and 11 GHz.
The IEEE 802.16d specification is a variation of the fixed standard (IEEE 802.16a) with the main advantage of optimizing the power consumption of the mobile devices. Standards for Fixed WiMAX (IEEE 802.16-2004) were announced as final in 2004, followed by Mobile WiMAX (IEEE 802.16e) in 2005. On the other hand, the IEEE 802.16e standard is an amendment to the 802.16-2004 base specification with the aim of targeting the mobile market by adding portability. WiMAX standard-based products are designed to work not only with IEEE 802.16-2004 but also with the IEEE 802.16e specification. While the 802.16-2004 is primarily intended for stationary transmission, the 802.16e is oriented to both stationary and mobile deployments.
The WiMAX forum, an industry body founded in 2001 to promote conformance to standards and interoperability among wireless MAN networks, then brought forth the WiMAX as it is commonly known today. In Europe, the standards for wireless MANs were formalized as HiperMANs. These were also based on IEEE 802.16 standards but did not initially use the same parameters (such as frequency or number of subcarriers).
These were later harmonized with the WiMAX standards. The IEEE 802.16d standards provide for fixed and nomadic access, while the 802.16e standards also provide mobility up to speeds of 120 kilometers per hour. The brief summary of WiMAX standards is given in Table 3.1.
Table 3.1 WiMAX Standards
WMAN Standard Definition Year Frequency Band
802.16 2001 10 to 66 GHZ
802.16(a) 2003 2 to 11 GHz
802.16(b) 2003 5 to 6 GHz
802.16( c ) 2003 10 to 66 GHz
802.16 (d) 2003 2 to 11 GHz
802.16-2004 2004 2 to 11 GHz
802.16 (e) 2005 2.3 to 3.4 GHz
3.4 Technical Overview
The WiMAX standard defines the air interface for the IEEE 802.16-2004 specification working in the frequency band 2 to 11 GHz. This air interface includes the definition of the medium access control (MAC) and the physical (PHY) layers.
3.4.1 Medium Access Control (MAC) layer
Some functions are dedicated to provide service to subscribers that include transmitting data in frames and controlling the access to the shared wireless medium. The medium access control (MAC) layer, which is situated above the physical layer, groups the mentioned functions. The original MAC is enhanced to accommodate multiple physical layer specifications and services, addressing the needs for different environments. It is generally designed to work with point-to-multipoint topology networks, with a base station controlling independent sectors simultaneously. Access and bandwidth allocation algorithms must be able to accommodate hundreds of terminals per channel, with terminals that may be shared by multiple end users. Therefore, the MAC protocol defines how and when a base station (BS) or a subscriber station (SS) may initiate the transmission on the channel. To achieve synchronization during the transmission reception process, total 48 overhead bits summarized in Table-3.2 are added along with data frame as a preamble.
Table 3.2 MAC Layer Header Fields
Type Length (bits) Function
CI 1 CRC Indicator
1=CRC included in the PDU by appending it to the PDU payload after
encryption, if any
0=No CRC is included
CD 16 Connection identifier
EC 1 Encryption Control
1= Payload is encrypted
0=Payload is not encrypted
EKS 2 Encryption Key Sequence
HCS 8 Header Check Sequence For detecting errors
HT 1 Header type
Shall be set to zero
LEN 11 Length. The length in bytes of the MAC PDU including the MAC header
and the CRC if present
Type 6 This field indicates the sub headers and special payload types
The ‘CI’ bit indicates the presence of CRC code for the error checking purpose. ‘CID’ forms the 16 bit data for identifying the connection. ‘EC’ bit justifies whether the data is encrypted or not. ‘HCS’ and ‘HT’ define the characteristic of header field. ‘LEN’ indicates the length of whole MAC PDU. The last field ‘Type’ indicates the sub header.
3.4.2 Physical Layer
The IEEE 802.16-2004 standard defines three different PHYs that can be used in conjunction with the MAC layer to provide a reliable end-to-end link. This PHY layer defines following specifications.
i. Randomizer: Randomization is the first process carried out in the physical layer after the data packet is received from the MAC layer. Randomizer drives on a bit by bit basis. Each burst in transmitter and receiver is randomized. The purpose of the scrambled data is to convert long sequences of 0’s or 1’s in a random sequence to improve the coding performance. The main component of the data randomization is a Pseudo Random Binary Sequence generator which is implemented using Linear Feedback Shift Register.
ii. Time-diversity with Forwarded Error Correction: Diversity in time is provided through Forward Error Correction which is done in transmitter and receiver and consists of concatenation of Reed-Solomon outer code and a rate compatible Convolution inner code. The purpose of using Reed-Solomon code to the data is to add redundancy to the data sequence. This redundancy addition helps in correcting block errors that occur during transmission of the signal. In WiMAX Physical Layer, the Reed-Solomon outer code block is encoded by the inner convolution encoder. Convolution codes are used to correct the random errors in the data transmission.
iii. Block interleaving: Interleaving in its most essential form can be describe as a randomizer but it is quite different from the randomizer in the sense that it does not change the state of the bits but it works on the position of bits. Interleaving is done by spreading the coded symbols in time before the modulation in transmitter reverse process-de-interleaving is carried out at receiver after the demodulation.
iv. M-QAM technique: The interleaver reorders the data and sends the data frame to the M (Modulo)-QAM block. The function of the M-QAM is to map the incoming bits of data from interleaver onto a constellation. In the modulation phase the coded bits are mapped to the IQ constellation and data bursts are transmitted with equal power by using a normalization factor.
v. Frequency-diversity with OFDM technique: Frequency diversity is provided by OFDM technique which allows the transmission of multiple signals using different subcarriers simultaneously. Because the OFDM waveform is composed of multiple narrowband orthogonal carriers, selective fading is localized to a subset of carriers that are relatively easy to equalize.
vi. Space diversity in fading environments: Optional support of both transmits and receives diversity to enhance performance in fading environments through spatial diversity, allowing the system to increase capacity. The transmitter implements space time coding (STC) to provide transmit source independence, reducing the fade margin requirement, and combating interference. The receiver, however, uses Maximum Ratio Combining (MRC) techniques to improve the availability of the system.
3.5 Salient Features of WiMAX
i. OFDM-based physical layer: The WiMAX physical layer is based on OFDM, which is an elegant and effective technique for overcoming multipath distortion.
ii. Very high peak data rates: WiMAX is capable of supporting very high peak data rates. In fact, the peak PHY data rate can be as high as 70Mbps when operating using a 20MHz wide spectrum.
iii. Orthogonal Frequency Division Multiple Access (OFDMA): Mobile WiMAX uses OFDM as a multiple-access technique, whereby different users can be allocated different subsets of the OFDM tones. OFDMA facilitates the exploitation of frequency diversity and multi-user diversity to significantly improve the system capacity.
iv. Adaptive modulation and coding (AMC): WiMAX supports a number of advanced signal-processing techniques to improve overall system capacity. These techniques include adaptive modulation and coding, spatial multiplexing, and multi-user diversity.
v. Link-layer retransmissions: For connections that require enhanced reliability, WiMAX supports Automatic Retransmission Request (ARQ) at the link layer. ARQ enabled connections require each transmitted packet to be acknowledged by the receiver; unacknowledged packets are assumed to be lost and are retransmitted.
vi. Support for advanced antenna techniques: The WiMAX solution has a number of hooks built into the physical-layer design, which allows for the use of multiple-antenna techniques. WiMAX offers very high spectral efficiency, particularly when using higher order MIMO solutions.
vii. Quality-of-Service support: The WiMAX MAC layer has a connection-oriented architecture. WiMAX has a very flexible MAC layer that can accommodate a variety of traffic types, including voice, video, and multimedia, and provide strong QoS.
viii. Robust security: Robust security functions, such as strong encryption and mutual authentication, are built into the WiMAX standard.
ix. IP-based architecture: WiMAX defines a flexible all-IP-based network architecture that allows for the exploitation of all the benefits of IP. The reference network model calls for the use of IP-based protocols to deliver end-to-end functions, such as QoS, security, and mobility management.
Up till now each and every aspect of WiMAX structure, layering approach, functions of individual segments as well as their characteristics have been very well discussed.
3.6 Structure of WiMAX Network System
The WiMax network system mainly comprises of core network and access network. The core network includes the network management system, router, AAA agency or server, user database, and Intern gateway equipment. It mainly provides an IP connection to WiMax users. The access network includes base station (BS), subscriber station (SS) and mobile subscriber station (MS). It mainly provides wireless access to WiMax users. See the following figure.
Figure 3.4 IP-Based WiMax Network Architecture
3.6.1 Core Network
The WiMax core network is mainly responsible for the user authentication, roaming service, network administration and providing interface to other networks. Its network administration system is used to monitor and control all base stations and subscriber stations in the network, and provide the functions of inquiry, condition monitoring, software download, and system parameters configuration. The IP network connected to the WiMax system is generally a traditional switching network or the Internet or other networks. The WiMax system provides the connection interface between the IP network and base stations. However, the WiMax system does not cover these IP networks.
3.6.2 Access Network
The base station provides a connection between the subscriber station and the core network. It generally uses a sector/beam antenna or umbrella antenna, which provides flexible arrangement and configuration of sub-channels, upgrades and expands the network based on the conditions of users. The subscriber station is a kind of base station, which provides the repeater connection between the base station and the equipment of user terminal. It generally uses a beam antenna installed on the roof. The dynamic adaptive modulation mode of the signal is used between base station and subscriber station. MS mainly refers to the mobile WiMax terminal and handheld devices responsible for realizing the wireless access for mobile WiMax users.
3.6.4 Base Station
The base station provides a connection between the subscriber station and the core network. It generally uses a sector/beam antenna or umbrella antenna, which provides flexible arrangement and configuration of sub-channels, upgrades and expands the network based on the conditions of users.
3.6.5 User Terminal Equipment
The WiMAX system defines the connection interface between the user terminal equipment and the base station, and provides the access of terminal equipment. However, the user terminal equipment does not belong to the WiMAX system.
3.7 Working of WiMAX
The WiMAX network uses an approach that is similar to that of cell phones. Coverage for a geographical area is divided into a series of overlapping areas called cells. Each cell provides coverage for users within that immediate vicinity. When subscriber travels from one cell to another, the wireless connection is handed off from one cell to another. A WiMAX system consists of two parts:
i. A Base station, similar in concept to a cell-phone tower – A single WiMAX tower can provide coverage to a very large area — as big as 3,000 square miles (~8,000 square km).is mounted on a tower or tall building to broadcast the wireless signal.
ii. A WiMAX subscriber device, these could be WiMAX enabled notebook, mobile Internet device (MID), or even a WiMAX modem by using the subscriber receivesthe signals.
The user pays the service provider for wireless Internet access, just as for a normal Internet connection via a cable network. The service provider provides the end user with the software, a login and a password. Most of the laptop manufacturers today equip high-end models with a built in antenna bundled with the required software for the unit to be WiMAX compatible. They service provider beams the internet signals from the base station. The antenna at the user end catches the signals, providing uninterrupted internet as long as the signal is available. With a laptop equipped with an antenna you could be connected to the Internet wherever the signal is available from the base station. As with mobile station that catch a signal from the nearest tower of the particular service provider, so is it with new generation WiMAX services. One WiMAX base station can send signals over distances of several miles depending on the terrain. The more flat the terrain, more the coverage. If end user moves from one base station area to another, your laptop receiver will hook up to the other base station (of the same service provider) with a stronger signal.
For fixed WiMAX deployments, service providers supply Customer Premises Equipment (CPE) that acts as a wireless ‘modem’ to provide the interface to the WiMAX network for a specific location, such as a home, cafe, or office. WiMAX is also well suited for emerging markets as a cost-effective way to deliver high speed Internet.
Figure 3.5 Fixed WiMAX using CPE
3.8 WiMAX Security:
Designed by the IEEE 802.16 committee, WiMAX was developed after the security failures that plagued early IEEE 802.11 networks. Recognizing the importance of security, the 802.16 working groups designed several mechanisms to protect the service provider from theft of service, and to protect the customer from unauthorized information disclosure.
The standard includes state-of-the-art methods for ensuring user data privacy and preventing unauthorized access, with additional protocol optimization for mobility. A privacy sub layer within the WiMAX MAC handles security. The key aspects of WiMAX security are as follow.
i. Support for privacy: User data is encrypted using cryptographic schemes of proven robustness to provide privacy. Both AES (Advanced Encryption Standard) and 3DES (Triple Data Encryption Standard) are supported. Most system implementations will likely use AES, as it is the new encryption standard approved as compliant with Federal Information Processing Standard (FIPS) and is easier to implement.
ii. Device/user authentication: WiMAX provides a flexible means for authenticating subscriber stations and users to prevent unauthorized use. The authentication framework is based on the Internet Engineering Task Force (IETF) EAP, which supports a variety of credentials, such as username/password, digital certificates, and smart cards. WiMAX terminal devices come with built-in X.509 digital certificates that contain their public key and MAC address. WiMAX operators can use the certificates for device authentication and use a username/password or smart card authentication on top of it for user authentication.
iii. Flexible key-management protocol: The Privacy and Key Management Protocol Version 2 (PKMv2) is used for securely transferring keying material from the base station to the mobile station, periodically reauthorizing and refreshing the keys. PKM is a client-server protocol: The MS acts as the client; the BS, the server. PKM uses X.509 digital certificates and RSA (Rivest-Shamer-Adleman) public-key encryption algorithms to securely perform key exchanges between the BS and the MS.
iv. Protection of control messages: using message digest schemes, such as AES-based CMAC or MD5-based HMAC.11, protects the integrity of over-the-air control messages.
v. Support for fast handover: To support fast handovers, WiMAX allows the MS to use pre-authentication with a particular target BS to facilitate accelerated reentry. A three-way handshake scheme is supported to optimize the re-authentication mechanisms for supporting fast handovers, while simultaneously preventing any man-in-the-middle attacks
3.9 Problem Definition
There are two fundamental phenomenon of wireless communication that makes the problem challenging and interesting. First is the phenomenon of fading: the variations in the signal strength, frequency and time delay i.e. phase as well as time-variation of the channel strengths due to the small-scale effect of multi path fading, as well as larger scale effects such as path loss via distance attenuation, shadowing, refraction or reflections by obstacles [9].
Second, unlike in the wired communication in which each transmitter-receiver pair can often be identified as an isolated point-to-point link, wireless users communicate over the air spectrum and there is significant interference between them in wireless communication. The interference can be between transmitters communicating with single receiver (e.g. uplink of a cellular system), between signals from a single transmitter to multiple receivers (e.g. downlink of a cellular system), or between different transmitter receiver pairs (e.g. interference between users in different cells).
The WiMAX networks form a very important part of the wireless rollout of the next generation networks. They also provide a replacement for major wired extensions of broadcast services; broadcast content feeder networks, and news-gathering networks available today by enriching them with the new broadband features. In this way, the WiMAX may be seen as the last mile solution providing very high data rate along with large coverage area [10].
Figure-1.4 shows the present status of WiMAX system in India. Various operators are now looking for the WiMAX technology as the filling bridge between the existing cellular system and the future demand of highest speed communication with lowest errors. Up till now the WiMAX is configures with the traditional way of single transmitter and receiver antenna but it can’t exactly form the shape of 4G technology.
Figure-1.4 WiMAX in India [7]
Now is the time when the potential of WiMAX to develop an entirely new generation of applications is at its prime. As discussed in the present scenario of WiMAX system, the maximum research work is done in Single Input Single Output WIMAX system physical layer model and maximum data throughput received accordingly. However in present scenario, during the phase of real time voice or image transmission through WiMAX system, the available Bit Error Rate and Signal to Noise Ratio and hence capacity of the systems are serious limitations for real time implementation.
So in 4G transmission system, link reliability and maximum data throughput is the need for transmitting voice as well as image at high speed. Implementation of antenna diversity techniques along with OFDM technique is one of the promising solutions for this. But very few resources are available in which the modeling and critical comparative analysis of WiMAX system with antenna diversity such as Single Input Single Output, Single Input Multiple Output, Multiple Input Single Output and Multiple Input Multiple Output along with Alamouti coding have been done.
Very few results for simulating and modeling of WiMAX system are available for real time data transmission (such as image and speech) to achieve the lower Bit Error Rates, higher Signal to Noise Ratio and higher system capacity.
4.1 Introduction
As per the objective of this research, the main aspect is to study the WiMAX system along with the implementation of antenna diversity techniques coupled with Alamouti scheme in it to improve the capacity of the system with no change in required bandwidth. This chapter contains the detailed analysis of various antenna diversity techniques with Alamouti coding scheme.
Today, hardly any hardware of some complexity is built without first performing extensive computer simulations. Communication and radar systems involving antennas is no exception. However, in almost all cases a communication (or radar) system is simulated with very crude models of the hardware and underlying physics. In contrast, the hardware (e.g. antennas) design is based on detailed electromagnetic simulation, but not taking the system aspects into account. The purpose of this chapter is to describe some steps in bridging the gaps between system and hardware level simulation, based on MATLAB. The goal is to be able to directly see the effect of component design, or architecture, on system level performance measures. The focus in this session is on a wireless communication system with the implementation of antenna diversity techniques, and the performance measures are usually given in the form of bit-error rate (BER) or some other Quality-of-Service (QoS) measure.
This chapter is mainly divided into two sections. In first section, the introduction to diversity phenomenon and its various types has been discussed. The second part is specified on MIMO OFDM concept for WiMAx technology.
4.2 Diversity Techniques
In wireless communication radio waves traveling along different paths arrive at the receiver at different times with random phases and combine constructively or destructively as shown in Figure-4.1.
Some of the signal components travel directly from transmitter to receiver in LOS path while the others will get obstructed by certain objects and then reach to the destination. According to their direction of arrival they form the various angles at the receiver and due to that the amount of received power is going to be altered.
Figure-4.1 Wave propagations mechanism
Figure 4.2 Multipath Components
When two or more multipath components are with the same access delay bin arrive at the same time, the received signal is the vector addition of two multipath signals. Let’s assume that two signals S1 and S2 are arrived at the same time at the receiver and R is the combined signal at receiver.
The net result is a rapid fluctuation in the amplitude of the received signal in a short period of time or distance travelled known as fading. However, the large scale average path loss remains constant. Multipath propagation had previously been considered a problem, but now it is exploited to achieve higher capacity which is the central idea behind the development of diversity phenomenon.
Conventionally the design of wireless systems has been focused on increasing the reliability of the air interface; in this context, fading and interference are viewed as nuisances that are to be countered. Recent focus has shifted more towards increasing the spectral efficiency; associated with this shift is a new point of view that fading can be viewed as an opportunity to be exploited. The one of the objective of this work is to provide a unified treatment of wireless communication from both these points of view.
While dealing with multipath environment, the individual signal path arriving at the receiver faces independent or highly uncorrelated fading. This means that when a particular signal path is in a fade there may be another signal path not in any fade. This phenomenon of independent fading in various paths can be exploited as an advantage to achieve improved performance in wireless communication provided that out of multiple paths, at least one path can be obtained with minimum distortion and maximum signal strength. This phenomenon leads towards the concept of diversity which can dramatically improve the performance over fading channels.
In practice, diversity techniques can be applied in the space, frequency or time domains. Diversity over time can be obtained via coding and interleaving: information is coded and the coded symbols are dispersed over time in different coherence periods so that different parts of the code-words experience independent fades. Analogously, one can also exploit diversity over frequency if the channel is frequency-selective. In a channel with multiple transmit or receive antennas spaced sufficiently far enough, diversity can be obtained over space as well. In a cellular network, macro-diversity can be exploited by the fact that the signal from a mobile can be received at two base-stations. Since diversity is such an important resource, a wireless system typically uses several types of diversity. The following section 4.2.1 illustrates the various types of diversity techniques.
4.2.1 Time Diversity
Time diversity means transmitting identical messages in different time slots as shown in Figure-4.3. This yields two uncorrelated signals at the receiving end. The same information symbol is repeatedly transmitted at different time slots with the hope that they will suffer independent fading and the receiver will combine them properly.
The most suitable example of time diversity is the basic GSM structure where time division multiple access scheme is based on the same principle of time diversity. GSM is a frequency division duplex system and uses two 25 MHz bands, one for the uplink (mobiles to base station) and the other for the downlink (base station to mobiles). The GSM bands are at 890-915 MHz (uplink) and at 935-960 MHz (downlink). The bands are further divided into 200 kHz sub-channels and each sub-channel is shared by 8 users in a time division fashion (time-division multiple access (TDMA)). The data of each user are sent over time slots of length 577 microseconds (”s) and the time slots of the 8 users together form a frame of length 4.615 ms.
Now for the voice processing through GSM, it is coded by a speech encoder into speech frames each of length 20 ms. The bits in each speech frame are encoded by a 1/2 rate convolution coder. The number of coded bits for each speech frame is 456. To achieve time diversity, these coded bits are interleaved across 8 consecutive time slots assigned to that specific user: the 0th, 8th, . . ., 448th bits are put into the first time slot, the 1st, 9th, .. ., 449th bits are put into the second time slot, etc. The process is explained graphically in Figure-4.3.
While highly effective in fast fading environments, time diversity is not as effective in slow fading channels unless a large decoding delay can be tolerated. A coding structure known as interleaving is often used to realize time diversity where the receiver knows the code before any transmission takes place For simplicity, let us consider a flat fading channel. We transmit a codeword of length L symbols and the received signal is given by
where l= 1,2,3′..L 4.2
Figure 4.3 Transmission of code word: An example of time diversity
Assuming ideal interleaving so that consecutive symbols x` are transmitted sufficiently far apart in time, we can assume that are independent. The parameter L is commonly called the number of diversity branches. The additive noises w0, . . . ,wL are random variables.
In practical cases this interleaving process is used to reorder the sequence of coded bits so that burst errors can be avoided to happen. Such kind of interleaving or more specifically block interleaving is used as a system block in many modern wireless systems such as WiMAX. In previous chapter of this thesis, this kind of interleaving has been integrated with the WiMAX physical layer so as to avail the advantage of time diversity.
4.2.2 Frequency Diversity
To discuss about the concept of frequency diversity, consider first the one-shot communication situation when one symbol x[0] is sent at time 0, and no symbols are transmitted after that.
The receiver observes
l=1,2”. 4.3
Let’s assume that the channel response has a finite number of taps L, then the delayed replicas of the signal are providing L branches of diversity in detecting x[0], since the tap gains are assumed to be independent. This diversity is achieved by the ability of resolving the multi-paths at the receiver due to the wideband nature of the channel, and is thus known as frequency diversity.
A simple communication scheme can be built on the above idea by sending an information symbol every L symbol times. The maximal diversity gain of L can be achieved, but the problem with this scheme is that it is very wasteful of degrees of freedom: only one symbol can be transmitted every delay spread. This scheme can actually be thought of as analogous to the repetition codes used for both time and spatial diversity, where one information symbol is repeated L times. In this setting, once one tries to transmit symbols more frequently, Inter Symbol Interference occurs: the delayed replicas of previous symbols interfere with the current symbol.
The problem is then how to deal with the ISI while at the same time exploiting the inherent frequency diversity in the channel. Broadly speaking, there are three common approaches
1. Single-carrier systems with equalization
By using linear and non-linear processing at the receiver, ISI can be mitigated to some extent. Optimal ML detection of the transmitted symbols can be implemented using the Viterbi algorithm. However, the complexity of the Viterbi algorithm grows exponentially with the number of taps, and it is typically used only when the number of significant taps is small. Alternatively, linear equalizers attempt to detect the current symbol while linearly suppressing the interference from the other symbols, and they have lower complexity.
2. Direct sequence spread spectrum
In this method, information symbols are modulated by a pseudo noise sequence and transmitted over a bandwidth W much larger than the data rate. Because the symbol rate is very low, ISI is small, simplifying the receiver structure significantly. Although this leads to an inefficient utilization of the total degrees of freedom in the system from the perspective of one user, this scheme allows multiple users to share the total degrees of freedom, with users appearing as pseudo noise to each other.
3. Multi-carrier systems
Here, transmit pre-coding is performed to convert the ISI channel into a set of non-interfering, orthogonal sub-carriers, each experiencing narrowband flat fading. Diversity can be obtained by coding across the symbols in different sub-carriers. This method is also called Discrete Multi-Tone (DMT) or Orthogonal Frequency Division Multiplexing (OFDM) that has been explained in detail at the end of this chapter. Frequency-hop spread spectrum can be viewed as a special case where one carrier is used at a time. For example, GSM is a single-carrier system, IS-95 CDMA and IEEE 802.11b (a wireless LAN standard) are based on direct sequence spread spectrum, and IEEE 802.11a is a multi-carrier system. An important conceptual point is that, while frequency diversity is something intrinsic in a wideband channel, the presence of ISI is not, as it depends on the modulation technique used. For example, under OFDM, there is no ISI, but subcarriers that are separated apart by more than the coherence bandwidth fade more or less independently and hence frequency diversity is still present.
4.2.3 Antenna Diversity Techniques
To exploit time diversity, interleaving and coding over several coherence time periods is necessary. When there is a strict delay constraint and/or the coherence time is large, this may not be possible. In this case other forms of diversity i.e. Antenna diversity or Space diversity have to be obtained. Figure 4.4 (a), (b) and (c) show various types of antenna diversity techniques.
Two kinds of space diversity can be obtained to improve the capacity of the system. Tx-Diversity in which multiple transmit antennae are used for the signal transmission which in term results in Multiple Input Single Output diversity (MISO) (n x 1 system). While Rx-Diversity in which multiple receive antennae are used for the signal reception which in term results in Single Input Multiple Output (SIMO) (1 x n system). Channels with Multiple Transmit and Multiple Receive antennas so called Multi Input Multi Output (MIMO) (n x n) channels provide even more potential.
Figure-4.4 (a) SISO (b) MISO (c) MIMO diversity techniques
Antenna diversity, or spatial diversity, can be obtained by placing multiple antennas at the transmitter and/or the receiver. If the antennas are placed sufficiently far apart, the channel gains between different antenna pairs fade more or less independently, and independent signal paths are created. The required antenna separation depends on the local scattering environment as well as on the carrier frequency. For a mobile which is near the ground with many scatters around, the channel de-correlates over shorter spatial distances, and typical antenna separation of half to one carrier wavelength is sufficient. For base stations on high towers, larger antenna separation of several to 10’s of wavelengths may be required.
The forthcoming section elaborates the various antenna diversity techniques from the detail analysis point of view so that it would be much easier to implement the same for real time scenario.
4.3 MIMO
The rapid development of wireless communication sets forth stricter requirements for the system capacity and frequency spectrum efficiency. There have been various attempts to meet these requirements, such as the expanding band width of system, optimizing modulation mode, or adopting a complex CDMA system. However, the application of these methods is restrictive. Obviously, neither the expansion of band width nor the increase of modulation order is limitless, and the channels of CDMA system are not orthogonal to each other perfectly. The MIMO (Multiple Input Multiple Output) system was born at the right moment. By using Space Time Coding (STC) technology, it realizes space division multiplexing using multi-element array, which greatly improves the frequency spectrum efficiency within the limited bandwidth. For this reason, MIMO becomes one of the necessary key technologies for WiMAX, LTE, 802.11n and nearly all ‘popular’ wireless communication systems in the future.
MIMO means using multiple transmitting and receiving antennae at the transmitting and receiving terminals respectively. The signals are transmitted and received by multiple antennae at the transmitting and receiving terminals, and accordingly the quality of service is improved for each user. Compared with the traditional single-element system, MIMO technology greatly improves the unitization rate of frequency spectrum, which enables the system to transmit data by higher speed under limited bandwidth. The block diagram of MIMO system with N transmitting antennae and M receiving antennae is shown in Figure 4.5.
Figure 4.5 MIMO system
WiMAX802.16e defines three options of MIMO. They are Space-time Transmit Diversity (STTD), Spatial Multiplexing (SM) and adaptive switching. It also defines three coding matrix: Matrix A, Matrix B and Matrix C.
In the WiMAX802.16e system, MIMO and OFDMA are combined to improve the network coverage and redouble the WiMAX system capacity. Accordingly, the costs of network construction and maintenance are reduced greatly, which promotes the development of mobile WiMAX.
4.3.1 Advantage of MIMO
MIMO advantages can be achieved without any expansion in required bandwidth or increases in transmit power.
(i) Array gain: Array gain results an increase in average receive SNR and hence enhance the coverage area and range of network.
(ii) Diversity gain: MIMO opens a new dimension space to offer an advantage of diversity. With multiple numbers of independent copies transmitted, there are fewer chances to loss the information.
(iii) Multiplexing gain: MIMO system significantly increases the channel capacity which immediately translates to higher data rate through spatial multiplexing.
(iv) Interference reduction: Interference is minimized in MIMO system by exploiting the spatial dimension to increase the distance between users.
4.4 OFDM
The Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier digital modulation technology. The research on the technology is traceable to the middle of 1960s. The concept of OFDM has remained for years. However, it was recognized as a good approach for high-speed bi-directional wireless data communication until the development of media industry recently. The technology is adopted by the European Digital Television Standard (DVB-T) and Digital Audio Broadcasting (DAB) standard, and it is the core technology of WLAN (ETSI HiperLAN/2 and IEEE802.11a) and broadband wireless access (IEEE 802.16). Along with the development of DSP CMOS chips, some mature technologies adopted by Fourier Transform/Inverse Transform and high-speed Modem, such as 64/128/256QAM, Trellis Coding, soft decision, channel adaptive, inserting guard time, reducing equilibrium calculation, are gradually introduced into the field of mobile communication. People devote more and more energy to the application of OFDM in mobile communication. It is predicted that the mainstream technology for the 4th generation of mobile commutation will be the OFDM.
OFDM is a high-speed transmission technology in wireless environment. Most frequency response curves of the wireless channel are not flat. The main idea of OFDM is to divide the bonded channel into many orthogonal sub-channels in the frequency range, and use a sub-carrier on each sub-channel for modulation, in which the sub-carriers are transmitted in parallel. In this way, in spite of the non-flat channel with different frequency options, every sub-channel is flat relatively, and narrow-band transmission is completed on these sub-channels, with the band width of signal less than the corresponding band width of channel. Accordingly, the interference between signal waves will be eliminated. Because the carriers on these sub-channels in OFDM are orthogonal to each other, they have overlapped frequency spectrum, which reduces the interference between sub-carriers and improves the utilization rate of frequency spectrum. The key technologies of OFDM are guard space (cyclic prefix/postfix), simultaneous techniques, training sequence/pilot frequency and channel estimation, control of PAPR (Peak to Average Power Ratio), channel coding and interleaving and equalization technique.
FDM/FDMA(Frequency-Division Multiplexing/ Frequency-Division Multiple Access) is actually a traditional technique. It is the plainest approach to realize the broadband transmission to divide a broader frequency band into several narrower sub-bands (subcarriers) and transmit them in parallel. However, a large space has to be reserved between neighboring sub-carriers to avoid the interference, which reduces frequency spectrum efficiency. Therefore, the TDM/TDMA (Time-Division Multiplexing/ Time-Division Multiple Access) and CDM/CDMA of greater frequency spectrum efficiency turn to be the core transmission technology for wireless communication. In recent years, FDM has revolutionary changed the development of digital modulation technique FFT (Fast Frontier Transform). FFT allows the arrangement of overlapped sub-carriers in FDM, and maintains the orthogonality of sub-carriers at the same time to avoid interference between sub-carriers. As shown in the figure, the arrangement of partly overlapped sub-carriers greatly improves the frequency spectrum efficiency because more sub-carriers are contained in the same bandwidth.
4.4.1 Advantages of OFDM
OFDM tends to replace CDMA to be the new generation core technology of wireless communication, mainly because of its advantages:
(i) Higher frequency spectrum efficiency: As sub-carriers partly overlap with FFT processing, they approach the Nyquist limit theoretically. The OFDM-based OFDMA (Orthogonal Frequency Division Multiple Access) realizes the orthogonality of various users in a residential area, which effectively avoids the interference between users. As a result, the OFDM system realizes a very large capacity.
(ii) Good expandability of band width: As the signal bandwidth in the OFDM system depends on the quantity of sub-carriers used, the OFDM system has good expandability of band width. It easily realizes the band widths as small as hundreds of KHz and as large as hundreds of MHz. In particular, along with the broadband service of mobile communication (increased from ‘ 5MHz to maximum 20MHz above), the efficient support of the OFDM system to broader band width turns to be its ‘decisive advantage’ to single-carrier techniques such as CDMA.
(iii) Anti-multipath-fading: As OFDM transforms the broadband transmission into narrowband transmission on many sub-carriers, the channel on each sub-carrier can be considered as a horizontal fading channel, which greatly reduces the complexity of the receiver equalizer. On the contrary, the complexity of the multi-path equalization of the single-carrier signal sharply increases with the increase of bandwidth, for which broader bandwidth is hard to support (such as 20MHz above).
(iv) Flexibly allocation of frequency spectrum resource: By selecting suitable sub-carriers for transmission, the OFDM system realizes dynamic allocation of frequency range resource, and fully utilizes the frequency diversity and multiuser diversity to achieve optimum system performance.
(v) Simple realization of MIMO: As the channel of each OFDM sub-carrier can be considered as a horizontal fading channel, the additional complexity introduced by the MIMO system is controlled in lower level which presents a linear increase with the quantity of antenna. On the contrary, the complexity of the single-carrier MIMO system is in direct proportion to the power of product by multiplying the quantity of antennae and the quantity of multi-paths. It adversely affects the application of the MIMO technique.
4.5 Combinations of OFDM and MIMO
In the WiMAX802.16e system, MIMO and OFDMA are combined to improve the network coverage and redouble the WiMAX system capacity. Accordingly, the costs of network construction and maintenance are reduced greatly, which promotes the development of mobile WiMAX.
MIMO is applicable for all wireless communication technologies. In the WiMAX802.16e system, the perfect combination of MIMO and OFDMA embodies the technical advantages of MIMO better.
The MIMO system has the capacity of anti-multipath fading, but it cannot do anything about the selective fading of frequency. Other communication systems generally adopt equalization technique to solve this problem in the MIMO system. The OFDMA of WiMAX conquers the selective fading of frequency successfully. The next generation of mobile communication requires technologies with higher frequency spectrum utilization rate, but the ability of OFDMA to improve the frequency spectrum utilization rate is limited after all. Combined with the MIMO, the frequency spectrum efficiency is further improved without increasing the bandwidth of system. The MIMO + OFDMA technology not only offers higher data transmission speed, but also achieves strong reliability and stability of system by diversity. Moreover, for lower code rate and additional guard interval, OFDMA has powerful capacity of anti-multipath interference. The multipath delay being less than guard interval releases the system from the inter-symbol interference. In this way, single-frequency network uses the broadband OFDMA system to eliminate the shade influence relying on MIMO technology. It realizes the seamless coverage of network truly.
4.5.1 Network Coverage Ability
Because of higher frequency range, the transmission loss of WiMAX802.16e is much higher than other mobile communication systems. Expand the network coverage is a challenge to WiMAX. The application of MIMO technology in the WiMAX system greatly improves the network coverage. In the diversity mode, MIMO increases the coverage radius of residential areas by diversity gain. In the multiplexing mode, it increases the coverage radius by diversity gain obtained from increase of speed at the edges of residential area. In the adaptive switching mode, the edges of residential area work in diversity mode, and the coverage gain are identical to that of diversity mode.
4.5.2 System Capacity
The WiMAX802.16e system provides very high data throughput and mobility, which keeps the users on-line at any time. The users can experience the true broadband service even if they are moving. In the multiplexing mode, the MIMO technology multiplies the system throughput and frequency spectrum efficiency, and also multiplies the peak speed of a single user. In the diversity mode, the system throughput and the frequency spectrum efficiency are improved by increasing the proportion of High Order Modulation (HOM).
In the adaptive switching mode, the center of residential area works in multiplexing mode, and the edges work in diversity mode. As a result, the improvement of the system throughput and frequency spectrum efficiency falls in between the two modes. The application of the OFDMA and MIMO technologies enables the WiMAX802.16e system to improve the frequency spectrum efficiency to the largest extent and provide high speed and broader band width necessary for high quality mobile video and televisions service.
4.5.3 Cost
In high-density urban areas and CBD areas, there are many high-end users who have higher requirements on the system throughout and peak rate. The capacity may be restrictive sometimes. By adopting the MIMO Matrix B technique, the WiMAX system capacity is improved by 55% for downstream and 33% for upstream. In case of restrictive capacity, the quantity of base stations is decreased by 25% approximately. Compared with other multi-antenna technologies (such as AAS, adaptive antenna system, also called advanced antenna system), MIMO has obvious advantages in the capacity gain for high density urban areas, which effectively reduces the costs of network construction or capacity expansion in intensive call areas.
In case of restrictive coverage, the MIMO technology increases the coverage radius by 50% or higher, and increases the coverage area of single station by nearly 100%, which saves 40% to 60% base stations under certain coverage. The introduction of MIMO technology in suburban areas and villages realizes the maximum coverage by minimum base stations, which greatly reduces the cost of network construction.
Additionally, in respect of installation and maintenance, the AAS antenna is at least 4-beam antenna requiring large installation space. Compared with the traditional antenna, the AAS antenna requires much more feeder lines and wider chamfer of transmission tower, which greatly increases the work quantities. The AAS antenna is heavy, requiring higher installation carrying capacity. Additionally, the AAS antenna is too large to resist the wind. With higher requirements on the wind resistance, it is not suitable for areas of heavy wind or frequent typhoon. MIMO needs fewer antennae than AAS. A piece of common ”45”dual-polarized antenna is sufficient to support 2”2MIMO. With lower requirements on the installation space and carrying capacity, it is easy to arrange and maintain, which effectively saves the costs of antenna installation and maintenance.
4.6 Advantage of MIMO-OFDM
The main advantage of the combination of MIMO-OFDM system in WiMax system is listed bellow
1. Less interference
2. High Diversity Gain
3. High data capacity
4. Increase Power efficiency
5. High Bandwidth gain.
5.1 Simulation
Figure 5.1 Simulink Model of MIMO based WiMAX system
In this thesis we design a physical layer of the MIMO based WiMAX system with ALAMOUTI scheme. Figure 5.1 shows the Simulink model of the thesis work. The simulation of the physical layer is in MATLAB environment. We use communication block set for this purpose. For designing of the system we need a small code for generation of data and communication purpose.
The whole simulation is made in three major parts. They are:
1. Transmitter
2. Channel
3. Receiver
5.1.1 Transmitter
The main aim of the transmitter is to transmit data. For transmission it uses some process. Firstly the logical operation between data and PN sequence is occurs. Then encode the data by using Reed Solomon encoder, convolutional encoder and puncturing process. This are used for noise less transmission of data. Then the data is passing through the modulator. Here we have used QPSK (Quadrature Phase Shift Keying Modulation) modulator. It is very efficient modulator for digital transmission. Then signal is converted to the OFDM symbol. Then pass through the channel.
The WiMAX transmitter has some major components. They are:
‘ Source
‘ Randomizer
‘ Encoder
‘ Modulation
‘ OFDM symbol
‘ Add cyclic prefix
‘ Transmitter antenna Source
In source block we have use a just use an input data from workspace block. It just read data from the workspace. After running the code, a set of integer is generating. Then the source read the data from the workspace. Randomizer
The randomizer is used to operate logical operation between PN sequence and the main data. It is used to identify easily for the receiver. Here we have use a PN sequence generator to generate a PN sequence. Then we have use a logical block which operates logical operation between data and PN sequence. Here we can see the eye diagram of randomizer. Encoder
Figure 5.2 shows the simulink model of the signal coder block. In this block we code the input signal. Firstly we randomized the input data then add a tail bit for each data which is code with Reed Solomon Coding technique.
Figure 5.2 Simulink of Coding of the signal
Here we see that the 88 bit input data is converted into 96 bit by adding of the tail bit the after coding it is converted into 192 bit.
After randomizing the signal we have passed our signal through the encoding process. Encoding is used for distortion less transmission of signal. It is very important part for transmission of signal. Here we can see the how the encoding process is occur by block diagram.
Figure 5.3 Block Diagram of the Encoding Process Reed-Solomon Encoder
The Reed-Solomon encoding is mainly used to recover the main signal if it is distorted. The properties of Reed-Solomon codes make them suitable to applications, where errors occur in bursts. Reed-Solomon error correction is a coding scheme which works by first constructing a polynomial from the data symbols to be transmitted, and then sending an over sampled version of the polynomial instead of the original symbols themselves. A Reed-Solomon code is specified as RS (n, k, t) with l-bit symbols. This means that the encoder takes k data symbols of l bits each and adds 2t parity symbols to construct an n-symbol codeword. Thus, n, k and t can be defined as:
‘ n: number of bytes after encoding,
‘ k: number of data bytes before encoding,
‘ t: number of data bytes that can be corrected.
The error correction ability of any RS code is determined by (n ‘ k), the measure of redundancy in the block. If the location of the erroneous symbols is not known in advance, then a Reed-Solomon code can correct up to t symbols, where t can be expressed as t = (n ‘ k)/2.Here we have used n=255, k=239 and t=8 because this values are standard.
The Reed-Solomon Encoder is mainly work with integer values. So here we have first convert the bit into integer format using bit to integer converter then reorder the vector using zero padding then operate RS encoding process Then we have use a selector to select the data from our vector then convert it to the bit format again using integer to bit converter block. Convolutional Encoder:
The purpose of a convolutional encoder is to take a single or multi-bit input and generate a matrix of encoded outputs. One reason why this is important is that in digital modulation communications systems (such as wireless communication systems, etc.) noise and other external factors can alter bit sequences. By adding additional bits we make bit error checking more successful and allow for more accurate transfers. By transmitting a greater number of bits than the original signal we introduce a certain redundancy that can be used to determine the original signal in the presence of an error.
After the RS encoding process, the data bits are further encoded by a binary convolutional encoder. It converts the single or multi bit into matrix form. It is use to discard noise from the main signal. It is another process of error correction. It deals with bit data. It is work using poly2trellis function.
The generator polynomials used to derive its two output code bits, denoted X and Y, are specified in the following expressions:
G1 = 171oct for X G2 = 133oct for Y
A convolutional encoder accepts messages of length k0 bits and generates code words of n0 bits. Generally, it is made up of a shift register of L segments, where L denotes the constraint length. The binary convolutional encoder that implements the described code is shown in figure. A connection line from the shift register feeding into the adder means a “one” in the octal representation of the polynomials, and no connection is represented by a “zero”.
We have used here a convolutional encoder block for convolutional encoding.
Figure 5.4 Convolutional Encoder of Binary Rate 1/2
Encoding consist two types of process for distortion less transmission of data. They are:
‘ Puncturing process
‘ Interleaver
Here is the description of those processes. Puncturing Process:
Puncturing process is mainly used to convert a long bit stream data into short bit stream. It compress the long bit data. If we pass any balloon through a narrow pipe we have puncture it first then pass through then pump that again. Same process is applied here. It deals with binary bit stream.
Puncturing is the process of systematically deleting bits from the output stream of a low-rate encoder in order to reduce the amount of data to be transmitted, thus forming a high-rate code. The bits are deleted according to a perforation matrix, where a “zero” means a discarded bit. The process of puncturing is used to create the variable coding rates needed to provide various error protection levels to the users of the system. There are different types of puncturing process like as 1/2 rate, 2/3 rate,3/4 rate, 5/6 rate. Here is the scenario of puncturing vector.
Table 5.1 Various rate of Puncturing Process
Rate Puncturing Vector
1/2 Rate [1]
2/3 Rate [1 1 1 0]
3/4 Rate [1 1 0 1 1 0]
5/6 Rate [1 1 0 1 1 0 0 1 1 0]
Puncturing process and Convolutional encoding is directly support by SIMULINK in MATLAB. Interleaver:
Interleaver is mainly used to correct burst error. After puncturing process the data is passed through the interleaver. The main purpose to use it to minimizing burst error. Interleaving is normally implemented by using a two-dimensional array buffer, such that the data enters the buffer in rows, which specify the number of interleaving levels, and then, it is read out in columns. The result is that a burst of errors in the channel after interleaving becomes in few scarcely spaced single symbol errors, which are more easily correctable.
WiMAX uses an interleaver that combines data using 12 interleaving levels. The effect of this process can be understood as a spreading of the bits of the different symbols, which are combined to get new symbols, with the same size but with rearranged bits.
The interleaver of the simulator has been implemented in two steps. First, data passes through a matrix interleaver which performs block interleaving by filling a matrix with the input symbols row by row, and then sending this matrix content column by column. Modulation Technique
There are several modulation technique can be implemented in WiMAX communication. Here we use QAM modulator baseband for modulation purpose of the signal. For modulation of the signal first we convert input signal into integer form by Bit to integer converter block then it is modulate through general QAM modulator baseband. Figure 5.5 shows the complete simulink model of the signal mapping block.
Figure 5.5 Simulink Model of QAM Modulation OFDM Symbol
WiMAX OFDM symbol have 256 sub carriers. There are 3 types of sub carriers are used here. They are data, training, pilot and dc sub carrier. 200 of the total 256 sub carriers are used for data and pilot sub carriers, eight of which are pilots permanently spaced throughout the OFDM spectrum. The remaining 192 active carriers take up the data sub carriers.
Now we have to convert our data into OFDM symbol. For this reason we have to take the data as row format then add pilot signal and training signal with our data then use a vertical concatenation to make an OFDM symbol consists of data, pilot signal and training sequence and guard band.
Figure 5.4 Simulink model of OFDM modulator
The major parts of the OFDM symbol are:
‘ Training sub carrier
‘ Pilot sub carrier
‘ Guard band
Here we have generated the Training and pilot sub carriers using constant block here we have used complex function to generate complex training and pilot signal. We have use complex function because of after modulation our data transform into complex form. The main purpose of training to generate guard band we have create a complex null vector. The main purpose to use guard band to prevent inter symbol interference (ISI). Then we have used a Matrix concatenation as an assembler. It is used to create an OFDM symbol and put the Sub carrier into that sequentially. IFFT Block
The IFFT is used to produce a time domain signal, as the symbols obtained after modulation can be considered the amplitudes of a certain range of sinusoids. This means that each of the discrete samples before applying the IFFT algorithm corresponds to an individual sub carrier. Besides ensuring the orthogonality of the OFDM sub carriers, the IFFT represents also a rapid way for modulating these sub carriers in parallel, and thus, the use of multiple modulators and demodulators, which spend a lot of time and resources to perform this operation, is avoided. Add Cyclic Prefix
The robustness of any OFDM transmission against multipath delay spread is achieved by having a long symbol period with the purpose of minimizing the inter-symbol interference. This ISI is a great drawback of digital communication system. To avoid this problem we have used cyclic prefix in our WiMAX communication. Here we have used the guard band cyclically in the long period of symbol to avoid ISI. This guard interval, that is actually a copy of the last portion of the data symbol, is known as the cyclic prefix (CP). Transmitter Antenna
Here we have used two antennas for transmitting data antenna to the receiver. Here the signal is same as the signal after adding cyclic prefix.
5.1.2 Channel
Here we have passed our transmitted signal through a channel. Here we have mainly used additive white Gaussian noise channel. In communications, the additive white Gaussian noise (AWGN) channel model is one in which the only impairment is the linear addition of wideband or white noise with a constant spectral density (expressed as watts per hertz of bandwidth) and a Gaussian distribution of amplitude. The model does not account for the phenomena of fading, frequency selectivity, interference, nonlinearity or dispersion. However, it produces simple, tractable mathematical models which are useful for gaining insight into the underlying behavior of a system before these other phenomena are considered.
Wideband Gaussian noise comes from many natural sources, such as the thermal vibrations of atoms in antennas (referred to as thermal noise or Johnson-Nyquist noise), shot noise, black body radiation from the earth and other warm objects, and from celestial sources such as the sun.
The AWGN channel is a good model for many satellite and deep space communication links. It is not a good model for most terrestrial links because of multipath, terrain blocking, interference, etc. However for terrestrial path modeling, AWGN is commonly used to simulate background noise of the channel under study, in addition to multipath, terrain blocking, interference, ground clutter and self interference that modern radio systems encounter in terrestrial operation.
This is the commonly used channel in communication
5.1.3 Receiver
Receiver is grabbing the transmitted signal which is come through the channel. The receiver is mainly reverse of transmitter. It consists of some important part. They are –
‘ Receiver antenna
‘ Remove cyclic prefix
‘ OFDM data
‘ Demodulator
‘ Decoding
‘ Derandomizer
These parts are used for processing the transmitted signal and then recover themain signal from that. Now we will see how they work. Receiver Antenna:
Receiver antenna is used to receive the transmitted data from the channel. Here we have used a ‘From’ block which grab data from the transmitter. Hence it is an ideal case so the received signal is same as transmitted signal. Remove Cyclic Prefix:
Remove cyclic prefix is the same as Add cyclic prefix. Here we have used a selector to add cyclic prefix with the data. Here we have use vector as input and define one based index to initialize the vector from one. It works same as Add cyclic prefix. Here is the eye diagram of the received signal after removing cyclic prefix. Fast Fourier Transform:
The IFFT algorithm represents a rapid way for modulating a group of sub carriers in parallel. Either the FFT or the IFFT are a linear pair of processes, therefore the FFT is necessary to convert the signal again to the frequency domain. It’s needed because the decoding processes are work on frequency domain based signal. It is same as IFFT. The details of FFT are discussed in IFFT part. Demodulator:
Demodulator is the same as modulator. Here we have used QPSK modulation technique. So in receiver part to demodulate the data we have used QPSK demodulator to demodulate the data. It is the same as modulator but it’s the reverse process of modulation. Here is the eye diagram of the signal after demodulation. Decoding:
After demodulating the signal we have to decode the signal. Then we have to decode the signal to recover the main signal from the demodulated signal. We have to pass through some step. They are ‘
‘ Deinterleaver
‘ Inserting zero
‘ Viterbi Decoder
‘ Reed Solomon decoder
Figure 5.5 Block Diagram of Decoding Process Deinterleaver:
Deinterleaver is the reverse process of interleaver. After the demodulation the signal is passes through the deinterleaver. It consists of two blocks, a general block deinterleaver and a matrix deinterleaver. These blocks work similarly as the ones used in the interleaver. The general block deinterleaver rearranges the elements of its input according to an index vector. The matrix deinterleaver performs block deinterleaving by filling a matrix with the input symbols column by column, and then, sending its contents to the output row by row. The parameters used in both blocks are the same as those ones used in the interleaving process. This matrix deinterleaving process is done by the code. Inserting Zero:
The block named “Insert Zeros” deals with the task of reversing the process performed by the “Puncture” block. The receiver does not know the value of the deleted bits but it can know their position from the puncturing vectors. Thus, zeros are used to fill the corresponding hollow of the stream in order to get the same code rate as before performing the puncturing process.
The inserted zeros can also be seen as erasures from the channel. They have no influence on the metric calculation of the succeeding Viterbi decoder described in the following section. Viterbi Decoder:
The Viterbi Algorithm (named after Andrew Viterbi) is a dynamic algorithm that uses certain path metrics to compute the ‘most likely’ path of a transmitted sequence. From this ‘most likely’ path, certain bit errors can be corrected to decode the original bit sequence after it has been sent down a communicative line. An important feature of the Viterbi algorithm is that ties are arbitrarily solved (can be picked randomly) and still yield an original sequence. What the Viterbi algorithm can do is correctly replicate your input string at the output even in the presence of one or more errors. Obviously, with more errors introduced the likelihood of a successful decryption does go down. But the algorithm has proved to be effective.
The main purpose of Viterbi decoder is to decode the convolutional encoded data. The Viterbi algorithm reduces the computational load by taking advantage of the special structure of the trellis code. Another advantage is its complexity, which is not a function of the number of symbols that compose the codeword sequence. The Viterbi algorithm performs approximate maximum likelihood decoding. It involves calculating a measure of similarity or distance between the received signal at time ti, and all the trellis paths entering each state at the same time.
The algorithm works by removing those trellis paths from consideration that could not possibly be candidates for the maximum likelihood choice. When two paths enter the same state, the one that has the best metric is chosen as th “surviving” path. The selection of the different “surviving” paths is performed for all the states. The decoder continues in this way to advance deeper into the trellis making decisions by eliminating the least likely paths. The early rejection of unlikely paths is the fact that reduces the complexity. The goal of selecting the optimum path can be expressed equivalently as choosing the codeword with the maximum likelihood metric, or as choosing the codeword with the minimum distance metric.
The Viterbi decoder block has 3 parameters to do the decoding. They are decision type, operation mode and trace back depth.
‘ As the decision process has been implemented in the demapper, the last kind of decision type, that is the “unquantized”, is the one used in our simulator. It accepts real numbers as inputs for the decoder block. The positive numbers are interpreted as a logical zero, and the negative ones, as a logical one.
‘ The operation mode parameter controls which method the block uses for transitioning between successive frames. The “truncated” mode, in which each frame is treated independently and the trace back depth parameter starts at the state with the best metric and ends in the all-zeros state, is the operation mode used in the simulator.
‘ Here we have use the trace back depth as 8 because every data is consist of 8 bit. Reed-Solomon Decoder:
The last part of the decoding process is the Reed-Solomon decoding. It performs the necessary operations to decode the signal, and get, at the end, the original message sent by the source. Thus, the RS decoder takes code words of length n, and, after decoding the signal, it returns messages of length k, being n = 255 and k = 239, the same as the ones described in the RS encoder.
The Reed-Solomon decoder is works with integer. So after Viterbi decoder we have to convert the data into integer form then operate the Reed-Solomon decoding process. The Reed-Solomon decoding process is same as Reed-Solomon encoding process. This was discussed before. After that we have use selector to select the rows then we have reorder the vector and then convert the data into bit format. Here is the eye diagram of the received signal after decoding and reorder the vector. Derandomizer:
Derandomizer is the same as randomizer. The decoding data is same as the data after randomizer. We have use the same PN sequence generator as randomizer which produces a PN sequence. Now we operate a logical operation same as randomizer (XOR). Then we get the main transmitted data which was transfer by the transmitter.
Figure 5.6 Simulink Model of Alamouti Reciever
5.2 Results Analysis
In this section we discuss the results generated by the simulation for different delay spreading. Figure 5.7 to 5.11 shows the outputs of the WiMAX simulation run in MATLAB.
Figure 5.7 WiMAX Transmitting signal
Figure 5.8 WiMAX Receiving Signal
Figure 5.9 MIMO output signal
Figure 5.7 to 5.9 shows the output of the transmitter, receiver and MIMO system with 1/4 cyclic prefix for OFDM. Here the transmitted signals bandwidth is 10MHz. the signal intensity is 100 dB and received signal is 95dB. Figure 5.10 is eye diagram of the transmitted data and figure 5.11 shows the received data. The vertical opening of the eye indicate the inter symbol interference is minimum and smaller opening shows the interference. Here by the figure 5.10 shows the interference is maximum in transmitter side. At receiver side it is minimum, which is shown in figure 5.11. It also shows the zero error.
Figure 5.10 Eye Diagram of Transmitting Signal
Figure 5.11 Eye Diagram of Receiving Signal
6.1 Conclusion
WiMAX technology brought revolution in both fixed and mobile wireless communication. In present communication world, wireless communication does not mean only data and voice transmission. It also supports high data rate transmission which supports various types of service (voice, data, multimedia).
Since, WiMAX supports high data rate transmission. So it can fulfill the demand of the present end users. Wi-Fi system is widely being used in the first world countries. WiMAX embedded devices support the Wi-Fi standards. So the people who are using Wi-Fi can easily switch to WiMAX technology. Moreover in the developing countries where high data rate wireless communication infra structure is not strong enough. WiMAX can be a good solution for these countries which is more secured, reliable and cheap. For these reasons the user of this technology is increasing day by day. As WiMAX is the latest technology and better solution in the wireless communication world, we have chosen this technology for our thesis. Our objective was to model complete WiMAX system on MATLAB platform exactly according to IEEE 802.16 standards.
This research is devoted to the modeling and performance analysis of the physical layer of the most emerging wireless technique ‘ WiMAX along with the effect of various parameters such as presence of coding, modulation order, and value of cyclic prefix etc. Another important aspect studied through the research work is the performance analysis of quality based algorithms of antenna diversity techniques and its implementation in OFDM based WiMAX system along with Alamouti coding for achieving the drastic improvement in its performance regarding error rates as compared to conventional antenna systems (SISO). Through this the BER dependency over channel SNR can be almost vanished.
6.2 Future Scope
In this research work, the main focus was on achieving the lowest BER i.e. achieving highest quality system performance. It has been fulfilled by improving the diversity gain of the system by the implementation of antenna diversity algorithms with Alamouti coding in WiMAX system. The higher capacity can be obtained by increasing more no. of antennas and through spatial multiplexing, transmitting different signals through multiple antennas which lead to the designing and implementation of BLAST structure in WiMAX system.
We have some future plan. We will implement the following thing in future. They are’
‘ Implemented MIMO transmission of Simulation: In future we will implement multiple inputs and multiple outputs with our mainsimulation of WiMAX security system. That will be more practical.
‘ Introduce sensor & control unit to the hardware: We will introduce sensor for more security purpose and the whole system will be controlled by a control unit which can be a logical device or CPU.
‘ Introduce Interfacing to the hardware: To make our hardware part more practical we can introduce hardware interfacing to our hardware part. Here the reference password is set from the bread board and the user enters the input password from pc. Then the door security system check whether the password matches or not.
‘ Simulate network architecture of WiMAX with other networks like Wi-Fi, 3G mobile and RF etc:
‘ Furthermore BLAST technique can be analyzed with OFDM environment using different size of IFFT and FFT, different type of prefix added to symbol, using modulation technique GMSK etc.
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