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Essay: To increase energy efficiency in a cellular heterogeneous network and improve the network handling capacity

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There are various reasons why diverse mobile network operators and versatile mobile makers are as one comprehensively inquiring about various approaches to optimize their system execution from the energy efficiency and capacity standpoint [1]. In recent years, with the explosive growth of mobile communications in terms of number of connected devices and the demand for new services and ubiquitous connectivity, the energy consumption of wireless access networks is experiencing a significant increase. Soaring cost of energy is not only problem for mobile operators but these are also big challenges for them.

Deployment of increasingly powerful mobile network technologies has taken place within the last decade. Although network efficiency has been growing, the higher access rates have inevitably led to increased energy consumption in base stations (BSs) and network densities have been constantly growing. The mobile communication community has become aware of the large and ever growing energy usage of mobile networks. Yet, the work on the reduction of energy consumption has been mostly carried out by BS manufacturers since high power efficiency provides a competitive advantage.

Mobile networks do not have considerable share in the overall energy consumption of the ICT sector, which itself is responsible for 2% to 10 % of the world energy consumption. ICT, the fifth largest industry in power consumption, emits 2 percent of the world-wide CO2 representing approximately one fourth of the emissions produced by cars. Despite the fact that these numbers look rather small, they are expected to increase by nearly a factor of 3 due to upward trend in energy consumption. This issue motivates governments to take political action in order to prevent global warming. However, reduction in energy consumption of mobile networks is of great importance from economical (cost reduction) and environmental (decreased CO2 emissions) perspective [2].

A typical mobile network consists of three main elements: core network, base stations, and mobile terminals. Base stations contribute 60% to 80% of the whole network energy consumption [9]. Thus the efforts in the reduction of energy consumption focus on the BS equipment, which includes the minimization of BS energy consumption, minimization of BS density (BS density is inversely proportion to cell area) and the use of renewable energy sources.

Coordinated multi-point transmission (CoMP) and heterogeneous networks (HetNet) are two promising concepts for achieving high spectral efficiency and improved coverage in future wireless cellular networks [7]. HetNet combined with CoMP is now a booming research topic, presenting opportunities able to enhance future wireless system bitrates considerably. These two ideas are being widely investigated within the context of LTE-Advanced in various 3GPP release [8].

Coordinate multipoint technique which is introduced in LTE-A has come out as a best solution to decreases the energy consumption and to increase the capacity at cell edge users [10]. Improvement of network performance in term of interference management, network capacity and cell edge spectral efficiency is possible by CoMP techniques. In CoMP technique BSs are coordinated with each other and enable multiple transmissions. Due to this user demands are satisfied by providing best services. Dynamic point selection (DPS), joint transmission (JT), and coordinated scheduling/coordinated beamforming (CS/CB) are the three major downlink CoMP techniques introduced by 3GPP [11], In JT technique, multiple coordinating BSs transmit data simultaneously to a UE and improves the received signal quality. In DPS technique, though UE data is available to multiple coordinating BSs, only one of the BSs is selected for transmission. However, based on the wireless resource availability and channel state information, transmitting BS serving a specific UE can be switched among the coordinating BSs at the subframe level [11]. On the other hand, in CS/CB technique, UE scheduling/beamforming decisions are made through coordination among the cooperating BSs, while data for transmission to UE is only available at and transmitted from one BS. Selection of transmitting BS is chosen in a semi-static manner configured by higher-layer radio resource control signaling [11].

1.2 MOTIVATION

Number of mobile users are increasing day by day so existing cellular network is becoming insufficient to provide high data rates, coverage and capacity. Moreover, Information and communication technology has great impact on worldwide energy consumption. Up to 3% energy is consumed by only ICT infrastructure. So higher energy efficiency is main concern nowadays as it could lead to future “Green Network”.

1.3 PROBLEM STATEMENT

To increase energy efficiency in a cellular heterogeneous network and improve the network handling capacity using,

• Coordinated Multi-Point Technique

1.4 OBJECTIVE

The objective of the work is to study the coordinated multipoint technique. To analyze various parameters like energy efficiency, SINR, throughput, BER and path-loss exponent for coordinated multipoint technique. Also, to compare the coordinated multipoint technique with non-coordinated multipoint technique for various parameters.

1.5 THESIS OUTLINE

This thesis has been organized in six chapters. Chapter 1 has already discussed about introduction, motivation, problem definition and objective. Chapter 2 has the literature survey of various papers related to HetNet, CoMP and Energy Efficiency. Chapter 3 has the basic theories that are related to project domain which are overview of LTE-A, HetNet and CoMP with LTE-A. Chapter 4 presents the system model which is considered for analysis. Chapter 5 shows the implementation results. Finally the research analysis is summarized in the chapter 6 with future work.

2 LITERATURE SURVEY

2.1 REVIEWED PAPERS

Paper-1: Green HetNet Comp: Energy Efficiency Analysis And Optimization

• Authors: Kazi Mohammed Saidul Huq, Shahid Mumtaz, Joanna Bachmatiuk, Jonathan Rodriguez, Xianbin Wang and Rui L. Aguiar

• Published in: IEEE Transactions on Vehicular Technology (Volume: 64, Issue: 10, Oct. 2015)

• Issue:

Main issue that is focused in the paper is backhaul power. Usually, in the literature, the total network power consumption is restricted to the sum of the power consumption of all BSs. In mobile networks, the backhaul contribution to the total power consumption is usually overlooked due to its limited impact compared to that of the radio base stations. But it is affect a lot in total power sum when considered.

• Challenge:

Main challenge of the paper is to maximize the energy efficiency. Second is to mitigate the ICI with respect to coordinate and uncoordinated strategy. Third is to analysis the backhaul power consumption in fiber and microwave link.

• Proposed Method:

To achieve the maximum energy efficiency the novel algorithm is proposed.

• Summary:

Investigates advanced energy-efficient wireless systems in orthogonal frequency division multiple access (OFDMA) downlink networks using coordinated multi-point (CoMP) transmissions between the base stations (BSs) in heterogeneous network (HetNet) which is adopted by 3GPP LTE-Advanced to meet IMT-Advanced targets. The total network power consumption is restricted to the sum of the power consumption of all BSs. In mobile networks, the backhaul contribution to the total power consumption is usually overlooked due to its limited impact compared to that of the radio base stations. For SE and EE analysis of HetNet CoMP, the energy and bandwidth consumption of the backhaul is considered without which the investigation remains incomplete.
The EE is measured as “throughput (bits) per Joule”
, while the power consumption model includes RF transmit, circuit and backhaul power. A novel resource allocation algorithm is proposed–modeled as an optimization problem–which takes into account the total power consumption, including radiated, circuit and backhaul power, and the minimum required data rate to maximize EE. The considered optimization problem is transformed into a convex optimization problem by redefining the constraint using cubic inequality, which results in an efficient iterative resource allocation algorithm. In each iteration, the transformed problem is solved by using dual decomposition with a projected gradient method. Analytical results shed light on future ”green” network planning in advanced OFDMA wireless systems such as envisioned for 5G system. The results presented are obtained for a specific backhaul solution, and may differ for alternative solutions, but the underlining message is that when assessing the benefits of deployment strategies, the backhaul power consumption cannot be simply ignored. Two backhaul strategies: microwave and fiber. The presented results confirm that the power consumption of the backhaul segment is an important part of the total network power consumption. For this reason the backhaul needs to be carefully included in any deployment strategy with the objective of minimizing the total network power consumption. Results also show that from a pure power consumption perspective a complete fiber-based topologies for both inter and intra backhaul is preferable over microwave-based intra backhaul topology.

Paper-2: Spectrum-energy Efficiency Optimization for Downlink LTE-A for Heterogeneous Networks

• Authors: Chan-Ching Hsu and J. Morris Chang

• Published in: IEEE Transactions on Mobile Computing ( Volume: PP, Issue: 99, July 2016)

• Issue:

Main issue that is focused in this paper is System capacity and Power consumption

• Challenge:

Main challenge in this paper is to increase system capacity and reduce power consumption by optimizing spectrum-energy efficiency.

• Proposed Method:

In this paper quasiconvex optimization and heuristic algorithm is use.

• Summary:

Heterogeneous networks have been pointed out to be one of the key network architectures that help increase system capacity and reduce power consumption for efficient communications. Optimal policies by employing the techniques of cell size zooming, user migration and sleep mode in the deployment of different base station types. Problem is formulated as a quasiconvex optimization problem and it is transformed into an equivalent form of the MILP problem; the former is solved with a bisection algorithm and the latter is approached by an off-the-shelf software package. Saving energy leads to reduce operating expense that leads to increase revenue.

Paper-3: Energy Efficient Heterogeneous Cellular Networks

• Authors: Yong Sheng Soh, Tony Q. S. Quek, Marios Kountouris and Hyundong Shin

• Published in: IEEE Journal on Selected Areas in Communications ( Volume: 31, Issue: 5, May 2013 )

• Issue:

Main issue that is focused in this paper is network capacity against number of user increased.

• Challenge:

Main challenge of this paper is to meet overwhelming demand of network capacity by increasing energy efficiency.

• Proposed Method:

Active/sleep (on/off) modes in macrocell base stations and Overlaying small cell networks with macrocell networks

• Summary:

With the exponential increase in mobile internet traffic driven by a new generation of wireless devices, future cellular networks face a great challenge to meet this overwhelming demand of network capacity. At the same time, the demand for higher data rates and the ever-increasing number of wireless users led to rapid increases in power consumption and operating cost of cellular networks. One potential solution to address these issues is to overlay small cell networks with macrocell networks as a means to provide higher network capacity and better coverage. Another technique to improve energy efficiency in cellular networks is to introduce active/sleep (on/off) modes in macrocell base stations. Investigation of the design and the associated tradeoffs of energy efficient cellular networks through the deployment of sleeping strategies and small cells. Using a stochastic geometry based model, derived the success probability and energy efficiency in homogeneous macrocell (single-tier) and heterogeneous K-tier wireless networks under different sleeping policies. The gains in terms of energy efficiency depend on the type of sleeping strategy used. Understanding on the deployment of future green heterogeneous networks. Numerical results confirmed the effectiveness of sleeping strategy in homogeneous macrocell networks but the gain in energy efficiency depends on the type of sleeping strategy used. The deployment of small cells generally leads to higher energy efficiency but this gain saturates as the density of small cells increases. The energy efficiency metric investigated here is only dependent on the power consumption and the coverage within the network, and does not take into account the infrastructure cost and backhaul overhead associated with implementing small cell networks.

3 BASIC THEORY

3.1 OVERVIEW OF LTE-ADVANCED

3.1.1 Summary of LTE Features

LTE Advanced is a mobile communication standard and a noteworthy upgrade of the Long Term Evolution (LTE) standard. It was formally submitted as a hopeful 4G framework to ITU-T in late 2009 as meeting the prerequisites of the IMT-Advanced standard, and was institutionalized by the Third Generation Partnership Project (3GPP) in March 2011 as 3GPP release 10. In LTE-Advanced concentrate is on higher data-rates. LTE–Advanced to give higher bitrates in a cost proficient manner and, in the meantime, totally satisfy the necessities set by ITU for IMT Advanced.

• Increased peak data rate, DL 3 Gbps, UL 1.5 Gbps

• Higher spectral efficiency, from a maximum of 16bps/Hz in R8 to 30 bps/Hz in R10

• Increased number of simultaneously active subscribers

• Improved performance at cell edges.

The primary new functionalities presented in LTE-Progressed are Carrier Aggregation (CA), improved utilization of multi-antenna techniques and support for Relay Nodes (RN).

3.1.2 LTE-Advanced Network Architecture

The center system of the LTE-Advanced framework is isolated into many parts. Figure 3.1 shows how every part in the LTE-Advanced system is associated with each other. NodeB in 3G framework was supplanted by developed NodeB (eNB), which is a mix of NodeB and radio network controller (RNC). The eNB speaks with User Equipments (UE’s) and can serve one or a few cells at one time. Home eNB (HeNB) is additionally considered to serve a femtocell that covers a little indoor range. The evolved packet core (EPC) includes the accompanying four segments. The serving gateway (SGW) is in charge of directing and sending parcels amongst UE’s and bundle information arrange (PDN) and charging. What’s more, it fills in as a versatility grapple point for handover. The mobility management entity (MME) oversees UE get to and versatility, and builds up the conveyor way for UE’s. Packet data network gateway (PDN GW) is a gateway to the PDN, and policy and charging rules function (PCRF) oversees arrangement and charging rules.

Figure 3 1: LTE-Advanced Network Architecture [13]

3.1.3 Summary of LTE Features

The Long Term Evolution venture was started in 2004. The inspiration for LTE incorporated the craving for
a decrease in the cost per bit, the expansion of lower cost administrations with bett
er user encounter, the adaptable utilization of new and existing frequency bands, a disentangled and lower cost connect with open interfaces, and a diminishment in terminal multifaceted nature with a recompense for sensible power consumption.These high level goals led to further expectations for LTE, including reduced latency for packets, and spectral efficiency improvements above Release 6 high speed packet access (HSPA) of three to four times in the downlink and two to three times in the uplink. Adaptable channel transmission capacities—a key component of LTE—are indicated at 1.4, 3, 5, 10, 15, and 20 MHz in both the uplink and the downlink. This permits LTE to be adaptably conveyed where different frameworks exist today, including narrowband frameworks, for example, GSM and a few frameworks in the U.S. in view of 1.25 MHz. Speed is the most presumably element associated with LTE.

Dissimilar to past frameworks, LTE is planned from the earliest starting point to utilize MIMO innovation, which brings about a more incorporated way to deal with this advanced antenna technology innovation than does the expansion of MIMO to legacy framework, for example, HSPA. At long last, as far as mobility, LTE is pointed fundamentally at low mobility applications in the 0 to 15 km/h range, where the most elevated execution will be seen. The framework is equipped for working at higher speeds and will be upheld with superior from 15 to 120 km/h and utilitarian support from 120 to 350 km/h. Support for speeds of 350 to 500 km/h is under consideration.

3.1.4 New Things in LTE-Advanced in Compare with LTE

In the attainability think about for LTE-Advanced, 3GPP confirmed that LTE-Advanced would meet the ITU-R prerequisites for 4G. The consequences of the review are distributed in 3GPP Technical Report (TR) 36.912. Assist, it was resolved that 3GPP Release 8 LTE could meet the majority of the 4G necessities separated from uplink spectral efficiency and the peak information rates. These higher prerequisites are addressed with the addition of the following LTE-Advanced features:

• Wider bandwidths, enabled by carrier aggregation

• Higher efficiency, enabled by enhanced uplink multiple access and enhanced multiple antenna transmission (advanced MIMO techniques)

Other performance enhancements are under consideration for Release 10 and beyond, even though they are not critical to meeting 4G requirements:

• Coordinated multipoint transmission and reception (CoMP)

• Relaying

• Support for heterogeneous networks

• LTE self-optimizing network (SON) enhancements

3.1.5 Features of LTE-A

3.1.5.1 Carrier Aggregation

The most clear approach to increase capacity is to add more bandwidth. Since it is vital to keep in reverse similarity with R8 and R9 mobiles the expansion in transfer speed in LTE-Advanced is given through aggregation of R8/R9 carriers.Carrier aggregation can be utilized for both FDD and TDD.

Each aggregated carrier is alluded to as a component carrier. The component carrier can have a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz and a most extreme of five component carrier can be aggregated. Thus the maximum bandwidth is 100 MHz. The quantity of aggregated carriers can be diverse in DL and UL, however the quantity of UL component carrier is never bigger than the quantity of DL component carrier. The individual segment component carrier can likewise be of various bandwidths capacities, see figure 3.2.

Figure 3 2 : Carrier Aggregation – FDD the R10 UE can be allocated resources DL and UL on up to five Component Carriers (CC). The R8/R9 UEs can be allocated resources on any ONE of the CCs. The CCs can be of different bandwidths. [14]

For practical reasons different carrier aggregation configurations – determined by e.g. mixes of E-UTRA working band and the quantity of component carriers – are presented in steps. In R10 there are two component carriers in the DL and just a single in the UL (thus no carrier aggregation in the UL), in R11 there are two component carriers DL and maybe a couple of component carriers in the UL when carrier aggregation is utilized.

The simplest approach to organize aggregation is to utilize adjacent component carriers inside the same operating frequency band (as characterized for LTE), so called intra-band contiguous. This may not generally be conceivable, because of frequency allocation scenarios. For non-contiguous allocation it could either be intra-band, i.e. the component carriers have a place with the same operating frequency band, yet are isolated by a frequency gap, or it could be inter band, in which case the component carriers have a place with various operating frequency bands, see figure 3.3.

Figure 3 3 : Carrier Aggregation – Intra- and inter-band alternatives [14]

At the point when carrier aggregation is utilized there is various serving cells, one for every component carrier. The coverage of the serving cells may contrast – due to e.g. component carrier frequencies. The RRC association is taken care of by one cell, the Primary serving cell, served by the Primary component carrier (DL and UL PCC). The other component carriers are altogether alluded to Secondary serving cells.

In the inter-band CA illustration appeared in figure 3.4, carrier aggregation on each of the three component carriers is workable for the black UE, the white UE is not inside the coverage region of the red component carrier.

Figure 3 4 : Carrier Aggregation; Serving Cells Each Component Carrier corresponds to a serving cell. The different serving cell a may have different coverage [14]

3.1.5.2 Relay Nodes

In LTE-Advanced, the possibility for efficient heterogeneous network planning – i.e. a mix of large and small cells – is increased by introduction of Relay Nodes (RNs). The Relay Nodes are low power base stations that will provide enhanced coverage and capacity at cell edges, and hot-spot areas and it can also be used to connect to remote areas without fiber connection. The Relay Node is connected to the Donor eNB (DeNB) via a radio interface, Un, which is a modification of the E-UTRAN air interface Uu. Hence in the Donor cell the radio resources are shared between UEs served directly by the DeNB and the Relay Nodes. When the Uu and Un use different frequencies the Relay Node is referred to as a Type 1a RN, for Type 1 RN Uu and Un utilize the same frequencies, see figure 3.5. In the latter case there is a high risk for self-interference in the Relay Node, when receiving on Uu and transmitting on Un at the same time (or vice versa). This can be avoided through time sharing between Uu and Un, or having different locations of the transmitter and receiver. The RN will to a large extent support the same functionalities as the eNB – however the DeNB will be responsible for MME selection.

Figure 3 5 : The Relay Node (RN) is connected to the DeNB via the radio interface Un. UEs at the edge of the donor cell are connected to the RN via Uu, while UEs closer to the DeNB are directly connected to the DeNB via the Uu interface. The frequencies used on Un and Uu can be different, outband, or the same, inband. In the inband case there is a risk for self interference in the RN [14]

3.1.5.3 Coordinated Multi Point operation (CoMP) – R11

LTE-Advanced continues to evolve. New CA configurations are added (additions of new bands for CA are not bound to specific releases) and there are new features introduced in coming releases of the 3GPP specifications, such as Coordinated Multi Point (CoMP) introduced in R11.

The main reason to introduce CoMP is to improve network performance at cell edges. In CoMP a number of TX (transmit) points provide coordinated transmission in the DL, and a number
of RX (receive) points provide coordinated reception in the
UL. A TX/RX-point constitutes of a set of co-located TX/RX antennas providing coverage in the same sector. The set of TX/RX-points used in CoMP can either be at different locations, or co-sited but providing coverage in different sectors, they can also belong to the same or different eNBs. CoMP can be done in a number of ways, and the coordination can be done for both homogenous networks as well as heterogeneous networks. In figure 3.6 two simplified examples for DL CoMP is shown. In both these cases DL data is available for transmission from two TX-points. When two, or more, TX-points, transmit on the same frequency in the same subframe it is called Joint Transmission. When data is available for transmission at two or more TX-points but only scheduled from one TX-point in each subframe it is called Dynamic Point Selection. For UL CoMP there is for example Joint Reception, a number of RX-points receive the UL data from one UE, and the received data is combined to improve the quality. When the TX/RX-points are controlled by different eNBs extra delay might be added, since the eNBs must communicate, for example in order to make scheduling decisions. When CoMP is used additional radio resources for signaling is required e.g. to provide UE scheduling information for the different DL/UL resources.

Figure 3 6 : DL CoMP a) Joint Transmission; two TX-points transmit to one UE in the same radio resource, b) Dynamic Point Selection; two TX points are ready to transmit, but only one will be scheduled in each subframe [14]

3.2 OVERVIEW OF HETEROGENEOUS NETWORK

Long-Term Evolution (LTE) allows operators to use new and wider spectrum and complements 3G networks with higher data rates, lower latency and a flat, IP-based architecture. To further improve the broadband user experience in a ubiquitous and cost-effective manner, 3GPP has been working on various aspects of LTE advanced including higher order MIMO-multiple antennas, carrier aggregation-multiple component carriers, and heterogeneous networks. Since improvements in spectral efficiency per link are approaching theoretical limits with 3G and LTE, the next generation of technology is about improving spectral efficiency per unit area.

Wireless cellular systems have evolved to the point where an isolated system with just one base station achieves near optimal performance, as determined by information theoretic capacity limits. Future gains of wireless networks will be obtained more from advanced network topology, which will bring the network closer to the mobile users. The concept of LTE Advanced-based Heterogeneous Networks is about improving spectral efficiency per unit area. Using a mix of macro, pico, femto and relay base stations, heterogeneous networks enable flexible and low-cost deployments and provide a uniform broadband experience to users anywhere in the network. Heterogeneous networks, utilizing a diverse set of base stations, can be deployed to improve spectral efficiency per unit area. Heterogeneous cellular networks for a very compelling approach for cellular networks to provide the coverage and capacity needed to move forwards.

Heterogeneous networks consists of regular placement of macro base stations that typically transmit at high power level (5W – 40W), overlaid with several pico base stations, femto base stations and relay base stations, which transmit at substantially lower power levels (100mW – 2W) and are typically deployed in a relatively unplanned manner.

The low-power base stations can be deployed to eliminate coverage holes in the macro-only system and improve capacity in hot spots. While the placement of macro base stations in a cellular network is generally based on careful network planning, the placement of pico/relay base stations may be more or less ad hoc, based on just a rough knowledge of coverage issues and traffic density (e.g. hot spots) in the network. Due to their lower transmit power and smaller physical size, pico/femto/relay base stations can offer flexible site acquisitions. Relay base stations offer additional flexibility in backhaul where wireline backhaul is unavailable or not economical.

In a homogeneous network, each mobile terminal is served by the base stations with the strongest signal strength, while the unwanted signals received from other base stations are usually treated as interference. In a heterogeneous network, such principles can lead to significantly suboptimal performance. In such systems, smarter resource coordination among base stations, better server selection strategies and more advanced techniques for efficient interference management can provide substantial gains in throughput and user experience as compared to a conventional approach of deploying cellular network infrastructure.

A Heterogeneous Network (HetNet) is a mix of high-power macro-eNBs responsible for umbrella coverage mainly for outdoor users, and low-power micro/Pico/Femto/relay BSs that are deployed for incremental capacity growth and coverage enhancement.

Macrocells: A Macrocell provides the largest area of coverage within a mobile network. Its antennas can be mounted on ground-based masts, rooftops or other structures and must be high enough to avoid obstruction. Macrocells provide radio coverage over varying distances, depending on the frequency used, the number of calls and the physical terrain. Typically they have a power output in tens of watt. Macrocells are conventional base stations with power about 20W, that use dedicated backhaul, are open to public access and range is about 1 km to 20 km.

Microcells: Microcells provide additional coverage and capacity in areas where there are high numbers of users, for Example, urban and suburban areas. Microcells cover around10% of the area of a Macrocell. The antennas for microcells are mounted at street level, are smaller than Macrocell antennas and can often be disguised as building features so that they are less visually intrusive. Microcells have lower output powers than marocells, usually a few watts. Microcells are base stations with power between 1 to 5W, that use dedicated backhaul, are open to public access and range is about 500 m to 2 km.

Picocells: Picocells provide more localized coverage. These are generally found inside buildings where coverage is poor or where there is a dense population of users such as in airport terminals, train stations and shopping centers. Picocells are low power base stations with power ranges from 50 mW to 1W, that use dedicated backhaul connections, open to public access and range is about 200 m or less.

Femtocells: Femtocell base stations allow mobile phone users to make calls inside their homes via their Internet broadband connection. Femtocells provide small area coverage solutions operating at low transmit powers. Femtocells are consumer deployable base stations that utilize consumer’s broadband connection as backhaul, may have restricted association and power is less than 100 mW.

Figure 3 7 : Heterogeneous network [15]

3.2.1 Several aspects of HetNets

• Use of multiple radio access technologies: Although, HetNet technology is being talked about in conjunction with LTE, one of the key concepts of heterogeneous networks, HetNets is that they will be able to use a variety of radio access networks including LTE, HSPA and also Wi-Fi and CDMA2000.

• Operation of different cell sizes and approaches: In order to maintain the flexible operation of the network different cell sizes and approaches are being used. The different cell sizes and types are able to fulfil different applications and provide a different type of serve better.

• Backhaul: With operators needing to carry an ever increasing amount of data, they will need to be creative about ways of transferring this back to the network. With everything from the ADSL links typically used by femtocells to the more tradition
al methods used by macro-cells, the whol
e system must operate in a seamless heterogeneous manner.

3.2.2 LTE HetNet Features

• Carrier aggregation: With spectrum allocated for 4G networks, operators often find they have a variety of small bands that they have to piece together to provide the required overall bandwidth needed for 4G LTE. Making these bands work seamlessly is a key element of the LTE heterogeneous network operation.

• Coordinated multipoint: In order to provide the proper coverage at the cell edges, signal from two or more base stations may be needed. Again, providing the same level of service regardless of network technology and areas within the cell can prove to be challenging. Adopting a heterogeneous network approach can assist in providing the same service quality regardless of the position within the cell, and the possibly differing cell and backhaul technologies used for the different base stations.

Heterogeneous network’s ability to manage and control interference in networks will allow for substantial gains in the capacity and performance of wireless systems. Maximizing bits per seconds per hertz per unit area by controlling inter-base station fairness in the context of macro/pico networks enables a more uniform user experience throughout the cell.

Heterogeneous networks allow for a flexible deployment strategy with the use of different power base stations including femtos, picos, relays and macros to provide coverage and capacity where it is needed the most. These techniques provide the most pragmatic, scalable and cost-effective means to significantly enhance the capacity of today’s mobile wireless networks by inserting smaller, cheaper, self-configurable base-stations and relays in an unplanned, incremental manner into the existing macro cellular networks.

3.3 COMP WITH LTE

4G LTE Advanced CoMP, coordinated multipoint is used to send and receive data to and from a UE from several points to ensure the optimum performance is achieved even at cell edges.

LTE CoMP or Coordinated Multipoint is a facility that is being developed for LTE Advanced – many of the facilities are still under development and may change as the standards define the different elements of CoMP more specifically.

LTE Coordinated Multipoint is essentially a range of different techniques that enable the dynamic coordination of transmission and reception over a variety of different base stations. The aim is to improve overall quality for the user as well as improving the utilization of the network.

Essentially, LTE Advanced CoMP turns the inter-cell interference, ICI, into useful signal, especially at the cell borders where performance may be degraded. Over the years the importance of inter-cell interference, ICI has been recognized, and various techniques used from the days of GSM to mitigate its effects. Here interference averaging techniques such as frequency hopping were utilized. However as technology has advanced, much tighter and more effective methods of combating and utilizing the interference have gained support.

3.3.1 LTE CoMP and 3GPP

The concepts for Coordinated Multipoint, CoMP, have been the focus of many studies by 3GPP for LTE-Advanced as well as the IEEE for their WiMAX, 802.16 standards. For 3GPP there are studies that have focused on the techniques involved, but no conclusion has been reached regarding the full implementation of the scheme.

However basic concepts have been established and these are described below.

CoMP has not been included in Rel.10 of the 3GPP standards, but as work is on-going, CoMP is likely to reach a greater level of consensus. When this occurs it will be included in future releases of the standards.

Despite the fact that Rel.10 does not provide any specific support for CoMP, some schemes can be implemented in LTE Rel.10 networks in a proprietary manner. This may enable a simpler upgrade when standardization is finally agreed.

3.3.2 LTE CoMP – The Advantages

Although LTE Advanced CoMP, Coordinated Multipoint is a complex set of techniques, it brings many advantages to the user as well as the network operator.

• Makes better utilization of network: By providing connections to several base stations at once, using CoMP, data can be passed through least loaded base stations for better resource utilization.

• Provides enhanced reception performance: Using several cell sites for each connection means that overall reception will be improved and the number of dropped calls should be reduced.

• Multiple site reception increases received power: The joint reception from multiple base stations or sites using LTE Coordinated Multipoint techniques enables the overall received power at the handset to be increased.

• Interference reduction: By using specialized combining techniques it is possible to utilize the interference constructively rather than destructively, thereby reducing interference levels.

3.3.3 LTE CoMP – The Basics

Coordinated multipoint transmission and reception actually refers to a wide range of techniques that enable dynamic coordination or transmission and reception with multiple geographically separated eNBs. Its aim is to enhance the overall system performance, utilize the resources more effectively and improve the end user service quality

One of the key parameters for LTE as a whole, and in particular 4G LTE Advanced is the high data rates that are achievable. These data rates are relatively easy to maintain close to the base station, but as distances increase they become more difficult to maintain.

Obviously the cell edges are the most challenging. Not only is the signal lower in strength because of the distance from the base station (eNB), but also interference levels from neighboring eNBs are likely to be higher as the UE will be closer to them.

4G LTE CoMP, Coordinated Multipoint requires close coordination between a numbers of geographically separated eNBs. They dynamically coordinate to provide joint scheduling and transmissions as well as proving joint processing of the received signals. In this way a UE at the edge of a cell is able to be served by two or more eNBs to improve signals reception / transmission and increase throughput particularly under cell edge conditions.

Figure 3 8 : Concept of LTE Advanced CoMP – Coordinated Multipoint [6]

The fundamental principle of CoMP is to coordinate multiple BSs or antennas located within a certain geographical area. In CoMP, multiple point coordination with each other can make in such a way that the transmission signals from/to other points do not incur serious interference or even can be exploited as a meaningful signal. There are several techniques available for CoMP. Most of the CoMP approaches require some scheduling information regarding the users at the different BSs that must be shared among them. This sharing is very important in low-latency links, such as microwave links, where information need to be exchanged between coordinated nodes within the order of milliseconds. By coordinating and combining signals from multiple antennas, CoMP makes it possible for UEs to enjoy consistent performance and quality when they access and share videos, photos and other high band width services whether they are close to the center of an LTE cell or at its outer edges. CoMP is considered by 3GPP as a tool to improve coverage, cell-edge throughput, and/or system efficiency. Thus, CoMP is a prominent technique in LTE wireless broadband networks as well as other 4G networks to ensure consistent service quality.

There are mainly two types of CoMP architectures, which are centralized and decentralized.

3.3.3.1 Centralized Architecture

In centralized approach, a central entity gathers channel information from a
ll the UEs, present in the area covered by coordinating BSs. Thi
s entity performs user scheduling and signal processing operations such as precoding. This operation can be possible when user data are available at all collaborating nodes and tight time synchronization is required among BSs to access them. Figure 3.9 describes the centralized framework for coordination among different BSs, UEs first estimate the channel related corresponding to the cooperating BSs. The information is feedback to a single cell, which acts as the serving cell of the UE when coordination is being applied. Once the information are gathered, each BS forwards it to the central entity, which eventually decides the scheduling with transmission parameters, and forwards these new information to the BSs. The main challenges of this architecture are related to the associated communication links between central entity and BSs. They must support very-low latency data transmissions. In addition to this, a communication protocols need be designed to support these information exchange.

Figure 3 9 : Centralized CoMP Architecture [17]

3.3.3.2 Distributed Architecture

Apart from the centralized architecture, distributed architecture is another solution to perform coordination that relieves the requirements of a centralized approach. It is based on the assumption that schedulers in all BSs are identical and channel information regarding the whole coordinating set can be available to all cooperating nodes.

Thus, inter- BS communication links are no longer necessary to perform cooperation. Figure 3.10 describes the distributed frame work for coordination among different BSs. With these, this architecture has the great advantage of minimizing the infrastructure and signaling protocol cost associated with these links and the central processing unit, so conventional systems need not undergo major changes.

The UE estimates the channel from all the coordinated BSs in the same way as in the centralized approach. The estimates are then sent back to all cooperating BSs and the scheduling is independently performed in each of them. Since the schedulers are identically designed, the same input parameters produce same output decisions and therefore, same UEs are selected in the entire BS cluster. Further, the drawback of this scheme is the handling of errors on the different feedback links.

The same UE reports its channel conditions to all the BSs in the set but the wireless links to the different nodes might be different and can affect the system performance. The above CoMP architectures use different approaches to enhance network performances.

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