FUNDAMENTAL OF WiMAX SYSTEM MODELING
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 .
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 .
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 
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.
...(download the rest of the essay above)