Three Step Cooperative MIMO Relaying

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Three Step Cooperative MIMO Relaying

Feasibility And Evaluation Study

CHAFIC NASSIF

Masters Degree ProjectStockholm, Sweden 2005


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Abstract

The Cooperative MIMO Relaying (CMIMOR) System is a relatively new topicthat researches the possibility of benefiting from the capacity gains offered by aMultiple Input Multiple Output channel despite the physical limitations of the

mobile phones.This thesis investigates an enhancement to the CMIMOR network that aims

for a reduction of the cost of implementation. The main goal is to study thefeasibility of the proposed Three-Hop CMIMOR Network, and evaluate it withrespect to its original Two-Hop CMIMOR counterpart.

Both systems are presented and a comparison in terms of end to end through-put reveals that the two-hop system performs better. The problem with theThree-Hop system appears to be that the resources are not allocated in an op-timized manner. To improve the performance some modifications are proposed,and the results prove that the proposed modifications produce increased capac-ity. The increase in capacity is especially evident when a proper allocation ofbandwidth or a good relay selection criteria are applied, allowing the Three-Hop

CMIMOR network to perform as well (better for some cases) as the Two-HOPCMIMOR network.

Finally at the end of the study a brief cost analysis reveals that, in addi-tion to the good performance of the proposed system, the cost with respect tothroughput is less than that of the Two-Hop CMIMOR system.

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Acknowledgements

Looking back over the past months that I spent working on my thesis, I realizethat this has been one of the most educational experiences of my life. Not onlydid I gain so much knowledge in the field that I was working on, but I also

learned how to manage my work, communicate my ideas to my colleagues, andabove all I learned how to properly conduct a research.

Along the way I accumulated many memories from the day to day workingenvironment to the sleepless nights spent in the labs. I remember the frustrationof reaching a dead end and the thrill of discovering a new solution. I rememberthe weariness from writing a report and the excitement from stumbling over agreat result. All these memories I cherish, but most importantly I rememberthe people that I have been in contact with. Many of those people to whom Iowe a large debt of gratitude for being there for me throughout the period ofthis thesis.

So I would like to extend my appreciation to them, and I will start off withmy advisor Bogdan Timus whom I thank for all the indispensable advice and

crucial assistance that he has offered me, and for bearing with me when mytime-table got somewhat hectic. I would also like to thank my examiner S. BenSlimane for providing positive feedback and expert opinion.

My sincere gratitude goes to my family (Habib, Reine, and Rami Nassif)back in Lebanon for their continuous moral support and encouragement. Aspecial thank you also goes to Georges and Rita Khoury for providing me witha home away from home. I would also like to thank my friends both here andabroad (especially Nelly Nassar) who have been so warm hearted and supportive,and I apologize for not mentioning all their names but they know who they are.

Finally I would like to say that I am grateful to the Wireless Systems depart-ment for providing me with the opportunity to come and study in the beautifulcity of Stockholm and earn my Masters Degree.

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viii Contents

4 Results 23

4.1 Throughput of the Reference System . . . . . . . . . . . . . . . . 234.2 Throughput of the Proposed System . . . . . . . . . . . . . . . . 274.3 Modifications for the Proposed System . . . . . . . . . . . . . . . 31

4.3.1 Varying Number of Active Relays of FT . . . . . . . . . . 314.3.2 Varying Bandwidth Distribution . . . . . . . . . . . . . . 334.3.3 Alternate Relay Activation Algorithm . . . . . . . . . . . 36

5 Cost Evaluation 41

6 Conclusion 43

7 Suggested Future Work 45

7.1 Propagation Model . . . . . . . . . . . . . . . . . . . . . . . . . . 45

7.2 Multi-Cell Topology . . . . . . . . . . . . . . . . . . . . . . . . . 457.3 Vary the Density of the Relays . . . . . . . . . . . . . . . . . . . 457.4 Relay Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . 467.5 Bandwidth Allocation . . . . . . . . . . . . . . . . . . . . . . . . 467.6 Regenerative Relays . . . . . . . . . . . . . . . . . . . . . . . . . 46

References 47

A First Hop Analysis 49

A.1 Scenario One . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49A.1.1 Capacity for the First Hop . . . . . . . . . . . . . . . . . 49A.1.2 Capacity for the Second & Third Hops . . . . . . . . . . . 50

A.2 Scenario Two . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

A.2.1 Capacity for the First Hop . . . . . . . . . . . . . . . . . 50A.2.2 Capacity for the Second & Third Hops . . . . . . . . . . . 51

A.3 Comparison of the Two Scenarios . . . . . . . . . . . . . . . . . . 51

B Relay Density Derivation 53


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List of Tables

3.1 Table of Simulation Parameters. . . . . . . . . . . . . . . . . . . 21

4.1 Table of densities used within simulation. . . . . . . . . . . . . . 24

5.1 Macro BS costs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415.2 Pico BS costs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

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List of Figures

1.1 Diagram of MIMO wireless transmission system. . . . . . . . . . 31.2 VAA Scheme suggested by Dohler. . . . . . . . . . . . . . . . . . 41.3 VAA groups in a Cell. . . . . . . . . . . . . . . . . . . . . . . . . 41.4 Three-Step Cooperative MIMO Relaying. . . . . . . . . . . . . . 6

2.1 Relay Distribution around Terminal with Direct Path from BS. . 112.2 Example of Channel Assignment - number of active relays is 4. . 122.3 The relays with best gain are activated while the other relays

(faded) are inactive. . . . . . . . . . . . . . . . . . . . . . . . . . 122.4 The Schematic Description of a CMIMOR architecture. . . . . . 132.5 The three-hop CMIMOR scenario. . . . . . . . . . . . . . . . . . 15

3.1 The Terminals are randomly but uniformly generated around theBS at a distance R. . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.1 Normalized throughput of 2-hop system for MT 100m from BS. . 254.2 Normalized throughput of 2-hop system for MT 300m (upperleft), 500m (upper right), 700m (lower left), and 900m (lowerright) away from BS. . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.3 Normalized throughput of the 2-hop system with respect to vary-ing density for MT 100m away from BS and number of relays inthe VAA=5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.4 Normalized throughput of the 2-hop system with respect to vary-ing distance of MT from BS, where number of relays in theVAA=5 and density=157.2 Relays/km2. . . . . . . . . . . . . . . 26

4.5 Normalized throughput of 3-hop system for MT 100m from BS. . 274.6 Normalized throughput of the 3-hop system for MT at 300m (up-

per left), 500m (upper right), 700m (lower left), and 900m (lowerright) away from AP. . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.7 Normalized throughput of the first hop, for distance between theAP and MT equal to 100m (uppermost), 300m (2nd row left),500m (2nd row right), 700m (3rd row left), and 900m (3rd rowright). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.8 Comparison between the 2-hop and 3-hop normalized systemthroughputs for density=157.2 Relays/Km2 where the VAAs arecomposed of 5 relays. . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.9 Comparison between the normalized throughput of the first hopand the combination of second and third hops of the 3-hop systemfor density=157.2 Relays/Km2 and MT at 500m away from AP. 31

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xii List of Figures

4.10 Normalized throughput of 3-hop system, with varying number of

FT relays, where MT is 100m (uppermost), 300m (2nd row left),500m (2nd row right), 700m (3rd row left), and 900m (3rd rowright) from BS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.11 Comparison between the normalized throughput of the first hopand the combination of second and third hops of the 3-hop systemfor T=10, density=509.3 Relays/Km2, and MT at 100m awayfrom BS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4.12 Normalized throughput of the 3-hop system, with improved band-width allocation for distance between the BS and MT equal to100m (uppermost), 300m (2nd row left), 500m (2nd row right),700m (3rd row left), and 900m (3rd row right). . . . . . . . . . . 34

4.13 Comparison between the normalized throughputs of the 2-hopsystem and 3-hop modified (Bandwidth Allocation) system. The

number of relays in FT=5, density=75.34 Relays/Km2, and MTat 500m away from BS. . . . . . . . . . . . . . . . . . . . . . . . 35

4.14 Comparison between the normalized throughputs of the 2-hopsystem and 3-hop modified (Bandwidth Allocation) system. Thenumber of relays in FT=5, density=157.2 Relays/Km2, and MTat 100m away from BS. . . . . . . . . . . . . . . . . . . . . . . . 36

4.15 Normalized throughput of the 3-hop system with Alternate RelayActivation Algorithm for distance between the BS and MT equalto 100m (uppermost), 300m (2nd row left), 500m (2nd row right),700m (3rd row left), and 900m (3rd row right). . . . . . . . . . . 37

4.16 Comparison between the normalized throughput of the 3-hop un-modified system and 3-hop modified (Alternate Relay Activa-

tion Algorithm) system. The number of relays in FT=5, den-sity=75.34 Relays/Km2, and MT at 500m away from BS. . . . . 38

4.17 Comparison between the normalized throughputs of the 2-hopsystem and 3-hop modified (Alternate Relay Activation Algo-rithm) system. The number of relays in FT=5, density=157.2Relays/Km2 for MT 100m (Upper) and 300m (lower left) awayfrom AP, and density=75.34 Relays/Km2 for MT at 500m (lowerright) away from BS. . . . . . . . . . . . . . . . . . . . . . . . . . 39

A.1 Comparison between the 1st scenario (upper line) and 2nd sce-nario (lower line) for x varying from 1 to 43dB. Note the thicklines represent a bundle of 10 plots each that correspond to valuesof R varying from 1 to 10 relays per VAA. . . . . . . . . . . . . . 52


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List of Notations

Attenuation CoefficientBWhop1 Bandwidth of Hop 1

BWhop23 Bandwidth of Hops 2 and 3BWT Total Bandwidth of the SystemChop1 Capacity of Hop 1

Chop23 Capacity of Hops 2 and 3Co Okumura-Hatta Coefficient

Dsf Correlation Distance of Lognormal Shadow FadingGd Distance Based Attenuation Component

Gff Fast Fading Attenuation ComponentGsf Shadow Fading Attenuation ComponentGtot Overall Power attenuation of the Signal

I Interference Relay Density

No Noisenrand Normal Distributed Random VariableNR Number of Active RelaysNT Number of Transmitters

Pmax Maximum Power of a Unitr Average Distance of the Closest Relay to the MTR Number of Channels between MT and its Relays

sf Standard Deviation of a Lognormal Shadow Fading ComponentT Number of Channels between Transmitter and the RelaysU Uniformly Distributed Variable

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List of Abbreviations

AP Access PointBS Base Station

CMIMOR Cooperative MIMO RelayingFDMA Frequency Division Multiple Access

FT First TierMIMO Multiple Output Multiple Input

MT Mobile TerminalRX Reciever Antenna

SISO Single Input Single OutputST Second Tier

STC Space Time CodesSTTB Space Time Block CodesSTTC Space Time Trellis Codes

TX Transmitter Antenna

VAA Virtual Antenna Array

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Chapter 1

Introduction

Wireless communication dates back to the first human that used physical ges-tures to convey an idea to another human. This concept evolved in parallel withthe evolution of humanity, and its need for more advanced means of interaction.

From smoke signals, beacons, and heliographs (messages by mirrors), to thepresent radio, television, and cellular phones; the evolution process has beendrastic. Numerous technological breakthroughs that were thought to be a lux-ury some time ago have become essential to our everyday life. A very commonphenomenon that endorses the above statement is the massive outbreak of mo-bile phones around the world today. In Europe, for example, nearly everyindividual has his/her own mobile phone.

Originally, voice transmission was the basic idea behind mobile phones, and

then gradually other applications started appearing with the introduction ofdata transfer. The possibilities presented by transfering huge amounts of datawere immense; however, there were certain capacity restrictions dictated princi-pally by limits of physical resources such as the electromagnetic spectrum or theavailable power factors [1], and the cost involved in setting up these channelsand maintaining them.

Recently, the advances in technology and coding techniques have somewhatovercome the physical limitations, through increasing the spectrum efficiency,and allowed for transfer of data at approximately the channel capacity limits,yet there exists a need for even higher data rates to accommodate more demand-ing applications. Towards this end, Multiple Input Multiple Output (MIMO)channels were introduced; the idea was to introduce an additional resource, orextra dimension, namely space that provides diversity. These MIMO channelspromised increased capacity provided that solutions could be discovered to by-pass the mobile terminals spatial capacity limitations.

One possible solution was to shift to higher communication frequencies,which inevitably would result in limiting the transmission range, since the at-tenuation increases with the carrier frequency [1]. Another solution, which hasrecently emerged and is currently under study, is to emulate a MIMO channelthrough the concept of Cooperative MIMO Relaying (CMIMOR).

The CMIMOR scheme basically consist of a base station that transmits,through multiple antennas, to a network of relays, which in turn act as if theywere multiple antennas connected (wirelessly) to one designated receiver. Thedetails of this plan are elaborated in the next section of this report, but the

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2 Chapter 1. Introduction

result, according to [2], is a theoretically higher capacity limit for cellular net-

works than the currently achievable one.The focus of this thesis is to work with the CMIMOR scheme and studythe possibility of implementing it at a lower cost. That is, an adjustment tothe original CMIMOR design is proposed, and the feasibility in terms of thecapacity is evaluated, taking into consideration the radio resources needed fordeployment.

The proposed modification involves replacing the Multiple Antenna Trans-mitting Macro Base Station by a Pico Base Station with a single antennaelement. The Pico Base Station has a shorter range than the Macro Base Sta-tion [3] which would dictate the presence of more relays and more hops for thesignal. Hence, such a modification is proposed to eliminate the huge cost of theMacro Base Station at the expense of a possible loss in system capacity.

The next sections of this chapter provide a background of the relevant con-

cepts and define the objectives. Then chapters two and three discuss the systemmodel and the simulation environment respectively. Chapter four presents theresults of the simulations, and chapter five provides a brief cost analysis. Finally,the conclusions are drawn in chapter six, and the future work are suggested inchapter seven.

1.1 Background

This section covers a general overview about Multiple Input Multiple Output(MIMO) schemes, and Virtual Antenna Arrays (VAA) or Cooperative MIMORelays (CMIMOR).

1.1.1 MIMO

Until recently, researchers have focused mainly on improving coding techniquesand devising methods to eliminate interference. Lately, however, the concept ofMIMO has emerged as a possible solution for offering data rates far in excessof conventional systems [4] through the introduction of the extra dimension ofspace.

As defined in [5], and illustrated in Figure 1.1, a MIMO system consists ofa transmitting end and a receiving end both equipped with multiple antennaelements. The idea behind MIMO is that the signals are sent from the transmit-ter end (TX) through parallel streams over the same frequency band and timeinterval to the receiver end (RX). The receiver then combines the incoming sig-nals via multiple receiver antennas to form the original data stream. Becausethe parallel channels exist over the same frequency and time intervals, high datarates can be achieved without the need for extra bandwidth [4].

This architecture allows the MIMO system to exploit the multipath scat-tering found in the environment to achieve significant gain in link capacity.Consequently, and under the theoretical assumption of uncorrelated fading, ifwe consider the rank of the channel coefficient matrix n = minimum (N, M),where N and M are the number of transmit and receive antennas respectively,then n parallel channels will be created effectively between (TX) and (RX),thus increasing the spectral efficiency n times [6]. This translates into a linear


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1.1. Background 3

Figure 1.1: Diagram of MIMO wireless transmission system.

increase in capacity relative to the increase in the n number of antennas [7].It can also be proven, according to [8], that given a full rank matrix, de-

ploying an unequal number of transmitter and receiver elements is a waste of

resources. If the number of transmitters exceeds the number of receivers thenthe capacity saturates very fast. On the other hand, if the number of receiversis greater than the number of transmitters then the capacity increases logarith-mically. Hence, the only solution is for the elements on both ends to be equalwhich as stated above will result in a linear increase of capacity.

To be accurate, it must be noted here that the assumption of a full rankmatrix is only a simplification. In practice it is not so common to have a fullrank matrix nor can a designer of a system control the matrix such that it isrendered full.

The set of coding schemes that are conventionally implemented at the MIMO(TX) antennas are called Space Time Codes (STC). These coding schemes pro-vide both data rate maximization and diversity maximization. There are twotypes of STC, the Space Time Trellis Codes (STTC) and the Space Time BlockCodes (STTB). The first type of codes provides a diversity benefit equal to thenumber of transmit antennas in addition to a coding gain that depends on thecomplexity - i.e. the number of states in the trellis - without any loss in band-width efficiency. The second type was introduced later on and it provides thesame diversity gain as STTC with minimal coding gain; however, they are muchsimpler to decode so they are more popular [5].

For more detailed information about MIMO systems and their architecturethe following references [5], [6] are recommended. It must also be noted thatin this thesis no specific coding shall be considered, since the calculations arebased on Shannons capacity calculations that no practical coder can provideus with.


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1.1.2 Virtual Antenna Arrays

The benefits of the MIMO scheme and STC codes are realizable only in thecase when we have an antenna array at both the transmitter and the receiver;however, the number of antenna elements on a terminal and the fading indepen-dence between them is limited by the space constraint. To solve this problemDohler suggested a scheme called Virtual Antenna Arrays (VAA [8]), whichwill be described in this section.

Figure 1.2: VAA Scheme suggested by Dohler.

Figure 1.3: VAA groups in a Cell.

The existing cellular systems are designed so that a Base Station (BS) com-municates with each Mobile Terminal (MT) individually; hence, the BS hastotal control over a cell [1]. In the VAA concept, Dohler suggests that MTsform a mutually communicating entity that emulates a real MIMO system [8].

To better understand this scheme let us consider the downlink case as anexample. A base station array (refer to Figure 1.2 and Figure 1.3) consistingof several antenna elements transmits a space time encoded data stream to theassociated mobile terminals which can form several independent VAA groups.


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1.2. Problem Definition 5

The entire data stream is received by every MT in the group. Each individual

MT extracts its own information and relays further information to the othersurrounding MTs. The target MT then receives more of its own informationfrom these surrounding MTs and finally it processes the entire data stream ithas received. The wired links within a traditional receiving antenna array arethus replaced by wireless links [8].

Of course this concept is easier theorized than physically applied. There aremany issues that remain to be resolved, most obvious of which is the assumptionthat MTs are aware of each other and manage to establish intelligent synchro-nization and data scheduling algorithms amongst one another. Moreover, thereare concerns such as distance estimation, power control, encoding, and datarelaying. Dohler does not provide definitive solutions for these concerns, ratherhe suggests possible scenarios. For example he proposes the use of Bluetoothtechnology for inter-MT communications while assuming that each MT is able

to process the signal it has received and regenerate it (regenerative relaying)[2].

Although, as discussed above, the VAA model is in need of many refine-ments that are outside the scope of my study, it still holds a lot of potentialfor providing higher capacity gains than the conventional systems and is worthlooking into.

1.2 Problem Definition

The problem definition and the objectives of the thesis work are stated explic-itly in this section. The Motivation subsection explains the logic of the thesistopic while the second subsection relays the objectives.

1.2.1 Motivation

Technology has always suffered from trade-offs between cost and efficiency. Tolower the cost one has to sacrifice some efficiency, as long as it is within certainacceptable limits. This also applies to communication systems; hence, if thedifference between the capacities of two systems is small while the cost reduc-tion is significant then the use of the system with the lower capacity may beeconomically sustainable.Taking the above into consideration, the thesis being proposed here was initially

formulated according to the following logic:

1. A multi-antenna Macro BS provides good performance but is expensive.

2. A wireless relay (Pico BS) costs much less than a Macro BS and can actas an access point (transmitter-receiver).

3. A wireless relay doesnt possess multiple antennas; hence, it cannot benefitfrom the advantages offered by the MIMO concept, unless CooperativeMIMO relaying is employed.

4. Thus, instead of deploying a new Macro BS, an existing wireless relaycould be converted to a Pico BS (using CMIMOR). Then the question


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would be if it is possible to achieve approximately the same performance

as is attainable by the Macro BS?

Stated in one sentence, the motivation of this thesis is to: Explore the pos-sibility of implementing a three step MIMO communication system and evaluateits capacity gains with respect to Dohlers (Two-Hop distributed MIMO Com-munication) reference system.

The way to do that is by suggesting a scheme that could reduce the cost ofBSs needed to cover a certain region by replacing some Macro BSs with PicoBSs and utilizing the concept of cooperative MIMO relaying (VAA).

To better understand this notion, consider a BS that needs to send a signalto a MT that is out of range. The BS would then send the signal to a wirelessarray of relay antennas that are perhaps located on lamp posts (in range). These

relay antennas would then send the message to their neighbouring antennas andso on in a virtual MIMO channels fashion until the target MT is reached (referto Figure 1.4).

Figure 1.4: Three-Step Cooperative MIMO Relaying.

For this study, a relay antenna is transformed into an access point (AP)which will represent a Pico BS. We shall assume that the signal is transmittedfrom this AP and the analysis starts from the first hop until the target MT.


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1.2. Problem Definition 7

1.2.2 Objectives

The objectives of this thesis can now be stated explicitly in the following points:

Evaluate the 3-hop proposed system with respect to the 2-hop referencesystem.

Explore the optimality of the initial 3-hop system settings.

A brief comparison of the approximate expenses involved in the imple-mention of both the 2-hop and 3-hop systems.


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Chapter 2

System Model

The approach that will be adopted throughout this thesis will focus mainlyon obtaining the maximum end-to-end throughput of the system as stated inthe previous chapter. Therefore the assumptions and parameters of the systemmodel shall be discussed herein. The system units are presented first. Thenthe reference and proposed systems are described consecutively along with thecapacity calculations.

2.1 System Units

There are four system units and they are: the Macro BS, the AP, the MT, and

the Relays. In this section the characteristics of each unit shall be presentedand discussed.

2.1.1 The Macro BS

The Macro BS unit exists only in the 2-hop reference system and is consideredto be placed at the center of the system with the rest of the units distributedall around it. It is a structure possessing multiple antenna elements that areused (in this study) for transmitting a signal to the MT and the Relays. Theantennas are assumed to be omni-antennas (0dB gain, to simplify the analy-sis), which might not be the optimum assumption since the use of directionalantennas might yield better results. The macro base station is assumed to be

mounted on a high location, such as a mast, and therefore has high infrastruc-ture costs, especially in terms of deployment [3]. The power of the Macro BS isdivided equally over the number of antenna elements and these elements forma MIMO channel together with both the relays and the MT.

2.1.2 The Acess Point

The AP unit exists only in the 3-hop proposed system. It has only one antennaelement (0dB gain) that is used for transmitting a signal to the first virtualantenna array composed of regenerative relays. The antenna element of the APsends the signal to the regenerative relays via independent orthogonal channels.

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10 Chapter 2. System Model

This means that the AP is responsible for dividing the signal, distributing it

over the orthogonal channels, and defining which specific part of the signal issent by which relay in the first tier of relays. The AP is the unit that substitutesthe Macro BS (of the 2-hop system) but it utilizes much less power (equal to thepower consumed by the individual relays). It is expected to lead to a decreasein the deployment costs, due to the fact that it is not located on a high mast,which is the reason for the substitution. It must be noted; however, that thedeployment of the AP affects the type of propagation of each connection, butthis aspect is beyond the scope of the thesis.

2.1.3 The Mobile Terminal

The MT unit represents the final destination of the transmitted signal. Thisunit exists in both the reference system and the proposed one. It possesses oneantenna element (0dB gain) for receiving the signal from the Macro BS (or APin the 3-hop case) and the surrounding relays. The MT is placed at varyingdistances from the BS such that the transmitted signal is influenced by differentpower attenuation and noise factors. During the interval of analysis the MT isassumed to be stationary, and the study is carried out according to snapshotsof the stationary terminal at different positions.

2.1.4 The Relays

The relays are assumed to have the same physical properties as the AP, that isthey are not mounted on a high mast, they consume the same amount of lowpower, and they have one omni-directional antenna element. The relays imple-mented in this thesis can be divided into two types according to functionality:Regenerative and Non-Regenerative (Transparent). The difference between thetwo types of relays is that the regenerative relays receive a signal, process it (de-code it and then re-encode it) before re-transmitting it, while the transparentrelays simply amplify the received signal before re-transmitting it. In general,regenerative relaying outperforms the non-regenerative in terms of end-to-endthroughput as proved in [9] for the SISO multi-hop case over flat Rayleigh fad-ing channels. This improved p erformance, however, is achieved at the expenseof implementing more complex systems [10], and much costlier ones.

The transparent relays are found around the MT in both the 2-hop and 3-

hop systems (to stay consistent with reference [11]). The regenerative relays,on the other hand, are only utilized around the AP in the 3-hop case.

The relays are uniformly distributed to form VAAs according to a predefinedactivation criteria. The relay density is denoted by and is measured as thenumber of relays available in one squared kilo meter. To give the reader a moreperceptive idea of what a certain distribution of the relays means, I have relatedthe density to the average distance of the closest relay to the terminal denoted asr. Thus a certain r corresponds to a certain through the following formula(refer to Appendix B for the derivation based on [12]):

r = 0.5

1/ (2.1)


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2.2. The Reference System Model 11

The power of the relays is assumed to be constant and equal for both types.

Each relay has one antenna element (0dB gain) that is used for receiving signalsand transmitting them alternatively. The transparent relays form orthogonalSISO channels with the MT while the regenerative relays form orthogonal SISOchannels with the AP. In the 3-hop case the regenerative relays and the trans-parent relays form the MIMO channel.

2.2 The Reference System Model

The reference system is Dohlers Two-Hop distributed MIMO CommunicationSystem [13], which was portrayed earlier in Figure 1.2. It is assumed that thereis one transmitting Macro BS with a fixed number of antennas. The signal is

sent from this BS towards a tier of relays (with a varying number of relays),that in turn amplify the signal and resend it to the destination terminal. Tomodel this process, certain assumptions and parameters are presented in thissection in detail, keeping in mind that the description also applies for the secondand third hops of the proposed system - the difference is that the BS antennaelements are replaced by individual relays.

2.2.1 General Architecture of the 2-hop System

The signal is initially transmitted by the BS antenna elements towards the relaysand the terminal. The terminal receives a direct transmission of the signal fromthe BS and an amplified one from the relays around it. Figure 2.1 shows thetopology of the system with one BS and one user terminal surrounded by apre-specified density of relays per km2.

Figure 2.1: Relay Distribution around Terminal with Direct Path from BS.

The relays around the MT are activated according to the criteria discussed inthe next section. After the required number of relays is chosen, each active relayreceives a signal from all the BS antenna elements, amplifies it and resends itover an independent orthogonal channel to the terminal as shown in Figure 2.2.


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Figure 2.2: Example of Channel Assignment - number of active relays is 4.

2.2.2 Relay Activation Criteria

The decision of which relays are to be chosen (activated) is done based on thechannel conditions. For this system it is assumed that the relays with the bestpath gain to the terminal are chosen. Of course this method takes for grantedthat the relays are able to communicate with each other and the terminal, tocalculate the individual path gains and decide upon the best of them.

Figure 2.3: The relays with best gain are activated while the other relays (faded)are inactive.

As can be observed in Figure 2.3, the relays that are chosen dont need tobe the closest to the terminal since both Shadow and Rayleigh fading are takeninto consideration. Moreover, it must be noted that the activated relays maynot have the best path gain from the BS (since the activation criteria is the bestpath gain from the terminal). As for the number of active relays and theirdensity, these variables will be regarded as simulation parameters.


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2.2. The Reference System Model 13

2.2.3 Radio Resource Allocation

The total available bandwidth of the system is set at a fixed value denoted byBWT as assumed in the previous work done in [14] and [11]. This bandwidth isdivided equally into 1+ NR channels, where NR represents the number of activerelays. IN other words, only 1/(1 + NR) of the available bandwidth is usedby the MIMO channel in an amplify-and-forward (non-regenerative) CMIMORarchitecture.

2.2.4 Capacity Calculations

The analysis presented herein is a summary of document [15] which calculatesthe capacity expression for a non-regenerative CMIMOR connection. This anal-ysis applies to both the 2-hop reference system and the combined capacity of the

second and third hops of the the proposed 3-hop system. It is assumed that thedownlink connection is established between a BS with T antenna elements(forthe three hop case the T antenna elements are the T relays of the first VAA),through a number of R relays, to a target terminal with one antenna element,as shown in the figure 2.4 (which is extracted from [15]).

Figure 2.4: The Schematic Description of a CMIMOR architecture.

The following capacity calculations are based on two assumptions. Thefirst assumption is that the channels are Gaussian, i.e. the received signals arenormally distributed. The second assumption is that the channel state matricesare assumed to be known both by the sender and the receiver. The signal modelfor a two hop CMIMOR connection is given by the equations 2.2 and 2.3 andexemplified in Figure 2.4, where r and y are the signals received by the R relaysand the target MT respectively. x is the signal initially sent by the BS, while n


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and m represent the noise vectors. Finally H and K are the channel coefficient

matrices of the MIMO channel and the R orthogonal channels respectively.

r = Hx + n (2.2)

y = AKr + m (2.3)

A is the amplification factor that is multiplied by the signal before the relaysresend it to the MT. The expression for A is depicted below (derivation is foundin [15]):

|Aii|2

= Psend,i

2nn,i +

PtT

Tt=1

|Hi,t|2

1(2.4)

The capacity calculations of the system are based on the mutual information;thus, the expression for capacity is as follows:

Chop23 = max {I(y, x)} where, (2.5)

I(y, x) = h (y) h (y|x) (2.6)

Computation of the values of h(y) and h(y|x) lead us to the following ex-pressions:

h (y) =1

2ln

(2)R e det(Cyy)

(2.7)

h (y|x) =

1

2 ln

(2)

R

e det(Cww)

(2.8)Cww = AKCnnK

HAH + Cmm

Hence, after substituting the values above into the equation of I(y, x) andsimplifying, we obtain an expression for Chop23 of the following form:

Chop23 =1

2

di=1

ln(1 + B,i) (2.9)

The capacity is calculated in terms of (Bits/sec/Hz). B,i is the set ofeigenvalues of matrix B which is calculated to be as follows:

B =Pt

T

[AiiKii]RR |H|2 [AiiKii]

TRR C

1ww (2.10)

Finally, equation ( 2.9 ) is multiplied by the Bandwidth of the channels tomake it comparable with the calculations of the first hop capacity for the 3-hopsystem; hence, the unit of measurment becomes (Bits/sec).

2.3 The Three-Hop Model

The Three-Hop Model is the system that is proposed by this thesis and evaluatedwith respect to the reference 2-Hop Model. It can be viewed as an enhancementto the reference system which means that the description of the reference systemapplies to the second and third hops of this system.


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2.3. The Three-Hop Model 15

2.3.1 General Architecture of the 3-hop System

The Macro BS and its antenna elements of the reference system are replacedby the AP and the First Tier (FT) of regenerative relays. The signal is initiallytransmitted by the AP towards the FT of relays through independent orthogonalchannels. The reason for choosing to have these links as orthogonal channelsand not let the AP simply broadcast is explained in the Appendix A. Theregenerative relays receive the signal process it and then send it over the MIMOchannel to both the Second Tier (ST) of relays and the MT. The terminalreceives a direct transmission of the signal from the FT relays and an amplifiedone from the ST relays around it. Figure 2.5 shows the topology of the systemwith one AP, the FT and ST of relays, and one MT surrounded by a pre-specifieddensity of relays per km2.

Figure 2.5: The three-hop CMIMOR scenario.

2.3.2 Relay Activation CriteriaThe decision of which relays are to be chosen (activated) in the FT and STis done as in the case for the 2-hop system, i.e. depending on the channelconditions. For this system the ST relays with the best path gain to the terminalare chosen, and similarly the relays of the FT with the best path gain to the APare activated. Of course this method is not the optimum and it takes for grantedthat the relays are able to communicate with each other and the terminal (orAP), to calculate the individual path gains and decide upon the best of them.

The activated relays are not necessarily the closest to the terminal since bothShadow and Rayleigh fading are taken into consideration. As for the numberof active relays and their density, these variables will be regarded as simulation


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parameters to be discussed in the next chapter.

2.3.3 Radio Resource Allocation

The total available bandwidth of the system is set at a fixed value denoted byBWT. However, unlike the 2-hop case this bandwidth is divided equally into1 + NR + NT channels, where NR represents the number of active relays in theST, NT represents the number of active relays in the FT, and the remainingchannel represents the MIMO channel.

2.3.4 Capacity Calculations

Knowing that the three-hop system utilizes regenerative relays in the FT, wecan split our capacity analysis into two parts:

The capacity of the first hop.

The capacity of the second and third hops combined.

Thus, we could calculate the capacity of the two separate parts and thencompute the total capacity of the proposed system to be the minimum betweenthe two capacities.

Moreover, the proposed three-hop system resembles the reference systemexcept for the addition of one extra hop in the beginning. This means that

the capacity calculations for the reference system are the same as the capacitycalculations for the combined second and third hops of the proposed system.Keeping the above in mind this section of the report calculates only the first

hop capacity, since the capacity for the second and third hops is summarizes insection 2.2.4.

Hence, from Shanons channel limit [16], and as explained in ( [17], p.585), wecan obtain the following formula for the capacity (Bits/sec) of each individualchannel of the first hop:

Ct = W1 log2[1 +P Gt

TNoW1] (2.11)

W1

=WT

T + R + 1(2.12)

where,

Ct = the capacity of the channel between the AP and Relay t.W1 = the bandwidth of the channels of the first hop.WT = the total bandwidth of the system.T = the number of channels in the first hop.R = the number of channels in the third hop.P = the maximum power transmitted by a relay = cst.Gt = Gt,dist(d) Gt,shadow Gt,Rayleigh


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2.4. Modified 3-hop System 17

Thus the expression of the total capacity that could be provided by the

first hop would be limited by the minimum capacity of the multiple channels asfollows:

Chop1 = T min[Ct], t = 1 T (2.13)

Now that we have the expression for the first hop capacity, the end to endthroughput of the 3-hop system can be expressed as:

Ctotal = minimum{Chop1, Chops23} (2.14)

where, Chops23 = BWhop2,3d

i=1 ln(1 + B,i) (Bits/sec).

2.4 Modified 3-hop System

In this section three modifications to the already discussed 3-hop system arepresented. The aim of introducing these modifications is to investigate the pos-sibility of increasing the total throughput of the system. The first modificationconsists of increasing the number of active relays around the AP. The secondmodification handles the optimization of the bandwidth distribution over thechannels. The third, and last modification, implements a new algorithm foractivating relays around the AP.

2.4.1 Varying Number of Active Relays of FT

One of the advantages of the 3-hop system over the reference system is thatthere are no physical limitations on the number of relays used in the FT. Inthe 3-hop system discussed in the previous section we chose this number to beequal with the Macro BS antenna elements of the 2-hop reference system. Herewe make use of this advantage by varying the number of relays in the FT aswell as the number of relays in the second VAA. The possible advantage offeredby increasing the number of relays is to increase the efficiency of the MIMOchannel provided that there are enough resources.

2.4.2 Varying Bandwidth Distribution

The unmodified 3-hop system distributes the bandwidth equally among all thechannels. This, as mentioned earlier, is not the optimum resource allocationtechnique. The reason is that the first hop of the system will have a muchgreater capacity than the second and third hops combined due to the fact thatthe FT relays are closer to the AP than to the rest of the units of the system.Given that the system is bounded by the lowest throughput it is importantthat the resources be allocated in a way that would render the capacity of thefirst hop and the combined capacity of the second and third hops equal (tosome extent). Thus, this modification considers assigning most of the band-width to the second and third hops such that the first hop will have a band-width BW1 and the second and third hops will have a bandwidth BW2,3, where


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BW1+BW2,3 = BWT. Then the bandwidth of each channel in the first hop will

become BWhop1 = BW1/NT and the bandwidth of each channel in the secondand third hops will become BWhop2,3 = BW2,3/(NR + 1).

2.4.3 Alternate Relay Activation Algorithm

The capacity of the second and third hops acts as the bottle-neck of the unmod-ified 3-hop system since the relays of the FT are usually quite far away from therelays of the ST and the MT. One way of attempting to rectify this situationis through presenting an alternate method for choosing which relays in the FTshould be activated. The activation criteria for the relays of the second VAAis kept as it is so that the system remains comparable with the 2-hop referencecase.

The modified activation criteria consists of activating those relays (within acertain predefined area around the AP) that have the best path gain with theterminal rather than with the AP. The logic behind this modification is thatthe line of sight paths between the FT relays and the mobile terminal consti-tute a very significant impact on the capacity of the second and third hops asproved in [14] (for the 2-hop system). Another incentive is that if the relays arechosen to have a better path gain with the MT, then chances are that they willalso be closer to the ST relays which will improve the capacity of the MIMOchannel. Therefore, if the path gain for the direct path is optimized then thecapacity should increase at the expense of an acceptable decrease in the firsthop capacity.


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Chapter 3

Simulation Environment

The programming environment used to virtually emulate the system model (de-scribed in the previous section of this report) was Matlab. Of course, a de-scription of the environment is necessary for comprehending how the study iscarried out. Therefore, this section is dedicated to explaining the simulationenvironment and its parameters.

3.1 System Layout

In every simulation environment, the real life objects (such as relays) are placedin a certain setting to enable the study to focus on the required result while

keeping the system as genuine as possible. The system layout herein is no ex-ception, and in the following is a description of the chosen settings. Keeping inmind that the main interest is to calculate the capacity of only one link fromthe BS (or AP) to the terminal, where the BS and the AP are chosen to beplaced at the origin of the system.

The reference system and the proposed one have similar topologies with afew differences. Throughout this discussion the discrepancies are p ointed outand explained clearly.

3.1.1 Placing the Terminal

The program uniformly generates hundreds of terminals along the circumferenceof multiple circles with predefined radiuses. The reason for this sort of genera-tion is to allow each generated terminal to be subjected to a different path gaindue to distance, Shadow fading, and Rayleigh fading variations. The differentpath gains make it possible for us to study the average throughput to terminalsat a certain distance, which renders the results more realistic. Figure 3.1 showsthe positions of terminals generated along a circle of radius=R.

3.1.2 Placing the Relays

It has been explained earlier that there are two types of relays: regenerativerelays (forming the First VAA), and non-regenerative relays (forming the Sec-

19


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20 Chapter 3. Simulation Environment

Figure 3.1: The Terminals are randomly but uniformly generated around theBS at a distance R.

ond VAA). The regenerative relays are randomly but uniformly distributed ina circular area (with a certain predefined density) around the AP. Similarly thenon-regenerative relays are randomly but uniformly distributed around the userMTs (also with a certain predefined density).For the reference system only the non-regenerative relays exist; hence, there isonly one VAA whose distribution is identical to that of the relays of the secondVAA of the proposed three hop system.

3.2 Path Gain Computation

The propagation model that will be adopted is based on three methods of powerattenuation: distance based attenuation (Gd), a slow fading component (Gsf),and a fast fading component (Gff). We will not consider the antenna elementsgain since as noted in chapter two, it is assumed to be 0dB gain. The dis-tance attenuation model is established according to the Okumura-Hata pathloss model [18]:

Loss = 69.55 + 26.16 log(f) 13.82 log(hBase) a(hMobile) (3.1)+(44.9 6.55 log (hBase)) log(d)

Where,

a(hMobile) = (1.1 log(f) 0.7) hMobile (1.56 log (f) 0.8) (3.2)

This is not the most suitable model for the system at hand; however, it is oneof the most commonly used models, and it is the model that was implementedin the previous work that was done in this area [11], [14].

The slow fading component models the shadow fading effect. Shadow fadingis due to the existence of major terrain obstacles such as hills, large buildings,


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3.3. Interference and Noise Models 21

etc. . . obstructing the line of sight path of the signal being sent [19], and intro-

ducing a lognormal fading variable with standard deviation sf and a spatialcorrelation for all extracted variables within a distance of Dsf.As for the fast fading component, it is there to represent the change of the

reflectors around the relays which would result in different path losses for thesignal arriving at the receiver [19]. This type of fading is modeled by means ofa Rayleigh fading function such that the fast fading received by a unit from oneantenna element is utterly uncorrelated with the fast fading received from theother antenna elements. Fast fading is the only factor in the propagation modelthat changes between the same MT and BS, since both the shadow fading andpath loss are assumed fixed (due to the fact that the analysis of the system isbased on snapshots).

Thus, we can model the overall power attenuation of the system as the sumof the above three attenuation models (in dB):

Gtot = Gd + Gsf + Gff (3.3)

3.3 Interference and Noise Models

The noise model that was adopted consisted of a fixed thermal noise No =200dB. The interference component, on the other hand, was modeled asa normally distributed random function with mean 127.5dB and standarddeviation of 2nrand = 2.5. The range of the interference component was takenfrom the study done by [11] where multiple cell topology was considered.

3.4 Simulation ParametersThe values of the parameters1 that were used in the simulation are listed inTable 3.1 below. It is assumed that the parameters are fixed unless otherwiseindicated in the Results section.

Table 3.1: Table of Simulation Parameters.

Parameter Significance

= 3.5 Attenuation CoefficientCo = 38.8 Okumura-Hatta Coefficient

T = 5 Number of BS AntennasBWT = 5M Hz Total available bandwidth

No = 200dB Fixed Noise levelI = nrand 127.5dB Interference

2nrand = 2.5 Normal distributed variable with mean zeroPBS = 20W Power of the BSPrelay = 1w Power of indivisual relays

sf = 1 Standard deviation of lognormal shadow fadingDsf = 100 Correlation distance of lognormal shadow fading

Omni Antennas 0dB Amplification

1The value of the Bandwidth is assumed 5MHz only for simulation purposes. In reality

BWT should be smaller since this study is assumed valid for Narrow-Band channels.


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Chapter 4

Results

This chapter is divided into four main sections that encompass a summary ofthe simulation results and the verification of these results. First, the studydone on the reference system is presented, followed by the study done on theproposed system. The performance measure in both systems is the bit-rate ofthe throughput (in bits/sec); however, it is normalized by 106 bits to stress onthe fact that the focus of this work is on the behavior of the graphs and not onthe absolute values of the results. After the two systems are considered, somemodifications that were made to the proposed system (to render it more efficient)are revealed. Finally, a brief insight into the approximate cost of implementingthe proposed 3-hop system is presented.

4.1 Throughput of the Reference System

The 2-hop system was chosen to be the reference system since it has been studiedthoroughly by [11], [14], [8]. In addition, the proposed 3-hop system has beenconstructed as a modification to the 2-hop case which makes the latter the mostlogical reference system.

In this section the total throughput of the 2-hop system is studied in terms oftwo variables. The first variable is the number of relays in the VAA distributedaround the terminal, while the second variable is the distribution density of theVAA relays. It is important to note that the focus of this section (and thisthesis - as mentioned in Chapter 1) is not on optimizing the performance of the2-hop case, but on presenting it as a valid reference system. This means that ifthe 2-hop case is optimized through the use of multi-cell resource managementas was done in [11], then the 3-hop case will also perform better in accordancewith it.

Now we need to set our parameters and restrict the variables to a certainrange. The first parameter to fix is the number of BS antenna elements whichwas set to 5. The choice of that specific number of antenna elements was basedon the physical restrictions imposed on an antenna in order to have all thesignals emitted experience the same shadow and distance fading with a non-correlated fast fading component.

As explained in the System Model chapter, the bandwidth is divided by thenumber of relays in the VAA plus one (the direct path). The number of relays

23


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24 Chapter 4. Results

is assumed to vary from zero to 10 relays. This range is logical since using more

relays will consume too many resources to justify their existence.The density on the other hand was somewhat tricky to place within a certainrange; thus, as explained in subsection 2.1.2, the density was defined in terms ofthe average distance of the closest relay to the terminal (equation: 2.1). Thenin order to be consistent with the study done in [11], the range ofr was chosento be:

20m r 65m (4.1)

Which is roughly equivalent to a density range within the following bound-aries:

56.59(Relays/km2) 509.3(Relays/km2) (4.2)

Six specific densities have been chosen within the given range. Table 4.1shows the chosen densities and their corresponding r.

Table 4.1: Table of densities used within simulation.

(Relays/km2) r (m)509.3 22.15259.8 31.02157.2 39.88105.2 48.7575.34 57.60

56.59 66.46

With the relevant variables discussed, the results of simulating the 2-hopreference system are presented below for five different cases. Figure 4.1 repre-sents the first case where the throughput of the system is measured for the sixdifferent densities (discussed above) when the MT is 100m away from the BS.The horizontal axis is the number of relays in the VAA, while the vertical axisis the throughput in bits/sec. Figure 4.2 contains plots of the throughput forthe six densities when the MT is 300, 500, 700, and 900 meters away from theBS respectively.

As can be observed in Figure 4.1, when the Terminal is relatively close tothe BS (100m away), then the total throughput of the system is proportionalto the density of the relays. This means that the more we increase the densityof the relays, then the better our throughput will be. Figure 4.3 shows how thethroughput varies when the density of relays is increased.

However, this does not apply for the other cases where the terminal is placedat a further distance from the BS. We observe from the other plots for distanceslarger than 100m that the different densities converge together in a bundle. Thisphenomenon can be explained by the fact that when the MT is far from the BSthen the distance dependent path loss from the BS to the relays becomes themost prominent factor. Thus, the relays (which are activated according to thebest gain to the terminal criteria) have approximately the same gain to the BS(with small variations due to shadow and fast fading) no matter how dense theyare.


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4.1. Throughput of the Reference System 25

2 4 6 8 102

4

6

8

10

12

14

16

18

Number of Relays in the VAA

Throughput(Bits/sec)

DENSITY = 509.3 Relays/(square km)






Figure 4.1: Normalized throughput of 2-hop system for MT 100m from BS.

2 4 6 8 101

2

3

4

5

6

7

8

9






DENSITY = 105.2 Relays/(square km)DENSITY = 75.34 Relays/(square km)


2 4 6 8 101

1.5

2

2.5

3

3.5

4

4.5

5






DENSITY = 105.2 Relays/(square km)DENSITY = 75.34 Relays/(square km)


2 4 6 8 100.5

1

1.5

2

2.5

3









2 4 6 8 100.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4









Figure 4.2: Normalized throughput of 2-hop system for MT 300m (upper left),500m (upper right), 700m (lower left), and 900m (lower right) away from BS.


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100 200 300 400 5005.5

6

6.5

7

7.5

Density (Relays/Km)


Figure 4.3: Normalized throughput of the 2-hop system with respect to varyingdensity for MT 100m away from BS and number of relays in the VAA=5.

Another significant trend that can be observed is that the total throughputdecreases logarithmically with the increase in the number of relays. This be-havior appears in the plots regardless of the distance of MT from the BS, and itis attributed to the fact that the resources needed to support more relays over-weigh the benefits in performance. The solution to this problem is presentedin [11] through the use of tight resource allocation among the cells. In this studyit is not possible to implement resource management techniques that are typical

to multi-user systems since only one cell is considered.

100 200 300 400 500 600 700 800 9000

1

2

3

4

5

6

7

Distance Between MT and BS (m)


Figure 4.4: Normalized throughput of the 2-hop system with respect to vary-ing distance of MT from BS, where number of relays in the VAA=5 and den-sity=157.2 Relays/km2.

Finally it is important to note that as the distance of the terminal from theBS increases the total throughput decreases. This behaviour is emphasized inFigure 4.4 which plots the throughput in terms of distance between MT andBS. This is expected, of course, because the path loss increases with distance.


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4.2. Throughput of the Proposed System 27

4.2 Throughput of the Proposed System

The 3-hop system is the system being evaluated and in order for the comparisonwith the reference system to be meaningful, they have to be studied under thesame circumstances.

Again we are interested in the total throughput (Bits/sec) of the system interms of the number and density of relays. In this case the number of relays inthe FT is fixed to 5 so as to be compatible with the 5 BS antenna elements ofthe reference system, while the number of relays in the second VAA is variedbetween 1 and 10.

As for the density of relays, both tiers are assumed to have the same density;

hence, when we simulate for different density levels, this applies to both VAAs.The total bandwidth remains fixed (as in the case of the 2-hop), but now we

divide it by the sum of: the number of relays in the FT, the number of relaysin the second VAA, and one (the MIMO channel). The rest of the parametersare set to the same values as in the 2-hop case, and the results of this systemssimulation are depicted in Figures 4.5, and 4.6.

1 2 3 4 5 6 7 8 9 102

2.5

3

3.5

4

4.5

5

5.5

6

6.5

Number of Relays in the Second Tier VAA


509.3 Relays/km

259.8 Relays/km

157.2 Relays/km

105.2 Relays/km

75.34 Relays/km

56.59 Relays/km

Figure 4.5: Normalized throughput of 3-hop system for MT 100m from BS.

If we were to study the plots of the 3-hop case we can note some remarks todiscuss and explain the behavior of the system:

The first detail to note about the 3-hop system plots is that the magnitudeof the total capacity for each case is much less than the magnitude of


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1 2 3 4 5 6 7 8 9 101.2

1.4

1.6

1.8

2

2.2

2.4

2.6



509.3 Relays/km

259.8 Relays/km157.2 Relays/km

105.2 Relays/km

75.34 Relays/km

56.59 Relays/km

1 2 3 4 5 6 7 8 9 100.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5



509.3 Relays/km

259.8 Relays/km157.2 Relays/km

105.2 Relays/km

75.34 Relays/km

56.59 Relays/km

1 2 3 4 5 6 7 8 9 100.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75



509.3 Relays/km

259.8 Relays/km

157.2 Relays/km

105.2 Relays/km

75.34 Relays/km

56.59 Relays/km

1 2 3 4 5 6 7 8 9 100.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55



509.3 Relays/km

259.8 Relays/km

157.2 Relays/km

105.2 Relays/km

75.34 Relays/km

56.59 Relays/km

Figure 4.6: Normalized throughput of the 3-hop system for MT at 300m (upperleft), 500m (upper right), 700m (lower left), and 900m (lower right) away fromAP.

the corresponding 2-hop case. To show this more explicitly, Figure 4.8compares the throughput of the two systems for the same relay density(157.2 Relays/Km2) with respect to the distance between the MT andthe AP (or BS). It is obvious that the 2-hop system outperforms the 3-hop system and this is due to the fact that in the latter system moreresources are required to establish a connection. Since we are using thesame bandwidth but dividing it into more channels, then it is only naturalthat we have worse performance.

The second remark deals with the shape of the plots. It is obvious thatthe plots in the 2-hop case decrease logarithmically, while the 3-hop plotstend to rise to a maximum before starting to decrease as a function of thenumber of relays in the second VAA. The explanation for this phenomenoncan be found in knowing two facts about the 3-hop system. The first factis that the total capacity (as discussed before) is the minimum betweenthe first hop capacity and the combination of the second and third hopcapacities. The plots of the first hop capacity (refer to Figure 4.7) showus that it is much higher than the second and third hops combined whichmeans that the total capacity of the system is limited by and identicalto the combined capacity of the second and third hops. The second factis that Chop23 is approximately the same as the capacity for the 2-hopreference system with a few discrepancies. The most major of these dis-crepancies is the bandwidth; hence, if we were to make the assumptionthat the other discrepancies are insignificant compared to the bandwidth


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4.2. Throughput of the Proposed System 29

1 2 3 4 5 6 7 8 9 100

5

10

15

20

25

30


Through

put(Bits/sec)

509.3 Relays/ km

259.8Relays/ km

157.2 Relays/ km

105.2 Relays/ km

75.34 Relays/ km

56.59 Relays/ km

1 2 3 4 5 6 7 8 9 100

5

10

15

20

25

30



509.3 Relays/ km

259.8 Relays/ km

157.2 Relays/ km

105.2 Relays/ km

75.34 Relays/ km

56.59 Relays/ km

1 2 3 4 5 6 7 8 9 108

10

12

14

16

18

20

22

24

26

28


Throughput(Bits/s

ec)

509.3 Relays/ km259.8 Relays/ km

157.2 Relays/ km105.2 Relays/ km75.34 Relays/ km56.59 Relays/ km

1 2 3 4 5 6 7 8 9 108

10

12

14

16

18

20



509.3Relays/ km259.8 Relays/ km157.2 Relays/ km105.2 Relays/ km75.34 Relays/ km56.59 Relays/ km

1 2 3 4 5 6 7 8 9 106

8

10

12

14

16

18

20



509.3 Relays/ km259.8 Relays/ km157.2 Relays/ km105.2 Relays/ km

75.34 Relays/ km56.59 Relays/ km

Figure 4.7: Normalized throughput of the first hop, for distance between theAP and MT equal to 100m (uppermost), 300m (2nd row left), 500m (2nd rowright), 700m (3rd row left), and 900m (3rd row right).


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100 200 300 400 500 600 700 800 9000

1

2

3

4

5

6

7

Distance between MT and BS (m)


2hop3hop

Figure 4.8: Comparison between the 2-hop and 3-hop normalized systemthroughputs for density=157.2 Relays/Km2 where the VAAs are composedof 5 relays.

then we can mathematically prove that the capacity of the second andthird hops combined decreases logarithmically as in the 2-hop case. Itonly appears to rise since it is divided by a different number of channels.

Another thing to note is that the capacity of the first hop is much higherthan the capacity of the combined second and third hops. To see this factmore clearly I have plotted (as an example) both the first hop capacityand the combined second and third hops capacity in Figure 4.9 for thecase when the MT is 500m away from the AP and the relay density is157.2 Relays/Km2. The explanation for this is that the relays of theFT are relatively close to the AP; thus, the path loss is not too great

and the available bandwidth is more than enough to provide for a goodthroughput. This fact is used in the next section in order to improve theperformance of the whole system.

The final remark deals with the different densities and their behavior. Atfirst glance it appears that the density plots contain no logical patternand in every plot it appears as though a different density is providingthe optimum performance. This is not true because these density plotsrepresent the average of many simulations that are varying over largeintervals. Therefore, these values are not statistically relevent.


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4.3. Modifications for the Proposed System 31

1 2 3 4 5 6 7 8 9 100.1

1

10

100

Number of relays in Second VAA


First Hop

Second & Third Hops

Figure 4.9: Comparison between the normalized throughput of the first hop andthe combination of second and third hops of the 3-hop system for density=157.2Relays/Km2 and MT at 500m away from AP.

4.3 Modifications for the Proposed SystemFrom the above results it is clear that the 3-hop system is performing muchworse than the 2-hop reference system. The graphs show that the proposedsystem needs to be modified to render comparable throughput to the 2-hopcase if it is to be considered as a possible cheaper alternative. Towards this end,three modifications on the originally proposed 3-hop system are presented anddiscussed in this section.

4.3.1 Varying Number of Active Relays of FT

The results depicted in Figure 4.10 portray ten plots per graph that correspondto the capacity of the system for a different number of relays in the FT. Eachgraph considers the terminal at a different distance from the AP, with a prede-fined density of the relays. The density is chosen to be the one that yielded thebest throughput (refer to the plots for the respective distances) in the previoussection.

The remarks that can be inferred from these plots are the following:

From the graphs we can deduce that considering 5 relays in the FT isnot the optimum choice; however, choosing another number does not offermuch larger total capacity.

It appears that choosing to activate 10 relays yields the best results whenthe distance of the terminal from the AP is short. This can be explained


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1 2 3 4 5 6 7 8 9 100

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5



T=1

T=2

T=3

T=4

T=5

T=6

T=7

T=8

T=9

T=10

1 2 3 4 5 6 7 8 9 100

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2



T=1

T=2

T=3

T=4

T=5

T=6T=7

T=8

T=9

T=10

1 2 3 4 5 6 7 8 9 100

0.2

0.4

0.6

0.8

1.0

1.2

1.4



T=1

T=2

T=3

T=4

T=5

T=6

T=7

T=8

T=9

T=10

1 2 3 4 5 6 7 8 9 100

0.2

0.4

0.6

0.8

1.0

1.2

1.4



T=1

T=2T=3

T=4T=5

T=6T=7

T=8T=9

T=10

1 2 3 4 5 6 7 8 9 100

0.05

0.1

0.15

0.2

0.25

0.3

0.35



T=1

T=2

T=3

T=4

T=5

T=6

T=7

T=8

T=9

T=10

Figure 4.10: Normalized throughput of 3-hop system, with varying number ofFT relays, where MT is 100m (uppermost), 300m (2nd row left), 500m (2ndrow right), 700m (3rd row left), and 900m (3rd row right) from BS.

by the fact that the existence of more relays increases the probability thatthese relays have better gain towards the relays of the second VAA. On theother hand, this observation does not apply to the further distance cases,


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and thus we can deduce that the the optimum number of relays is not

absolute and depends on many factors such as the range of the terminal.

1 2 3 4 5 6 7 8 9 101

10

100



Second &Third Hops

First Hop

Figure 4.11: Comparison between the normalized throughput of the first hopand the combination of second and third hops of the 3-hop system for T=10,density=509.3 Relays/Km2, and MT at 100m away from BS.

The total capacity of the system is still limited by the combined capacityof the second and third hops, while the first hop capacity is much larger.Figure 4.11 shows both the first hop capacity and the combined secondand third hops capacity when the MT is at 100m from the BS.

4.3.2 Varying Bandwidth DistributionDistributing the bandwidth equally among all the channels has been establishedas a sub-optimum solution to say the least. For this reason another way of dis-tributing the bandwidth is presented here.

The new distribution methodology consists of splitting the available band-width into two unequal parts, one for the first hop and one for the second andthird hops. This split is justifiable since the first hop is separated from the restof the system by a tier of regenerative relays.

The bandwidth is not split into two equal parts since it is evident that thefirst hop does not require as much resources as the rest of the system. For thisreason the following distribution was chosen empirically such that the majority


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1 2 3 4 5 6 7 8 9 104

5

6

7

8

9

10

11

12



System Capacity

Capacity of 2nd and 3rd hops

Capacity of 1st hop

1 2 3 4 5 6 7 8 9 101.5

2

2.5

3

3.5

4

4.5

5



System Capacity


Capacity of 1st hop

1 2 3 4 5 6 7 8 9 100.5

1

1.5

2

2.5

3

3.5

4



System Capacity


Capacity of 1st hop

1 2 3 4 5 6 7 8 9 100.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2


Thro

ughput(Bits/sec)

System Capacity


Capacity of 1st hop

1 2 3 4 5 6 7 8 9 100.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3


Thro

ughput(Bits/sec)

System Capacity


Capacity of 1st hop

Figure 4.12: Normalized throughput of the 3-hop system, with improved band-width allocation for distance between the BS and MT equal to 100m (upper-most), 300m (2nd row left), 500m (2nd row right), 700m (3rd row left), and900m (3rd row right).


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of the bandwidth is assigned to the second and third hops:

BWhop1 =

BWT5T

, BWhop2,3 =

4BWT

5(R + 1)

(4.3)

where, T = Number of channels from AP to Active Relays in the FT,and R + 1 = Number of channels from Active Relays in the second VAA to

terminal plus the MIMO channel.The capacity is also a function of the distance between the AP and the ter-

minal; thus, the optimum division of the total bandwidth varies for the differentdistances.That is when the MT is at a distance of 100m from the AP the opti-

mum bandwidth division could be BWhop1 =BWT5T

and BWhop2,3 =

4BWT5(R+1)

;

however, when the distance is 500m (for example) then the optimum bandwidth

division could be BWhop1 = BWT7T and BWhop2,3 = 6BWT7(R+1). The divisionchosen above in equation 4.3 is only intended to provide an insight into thebandwidth optimization possibilities. The subject of optimizing the bandwidthfor all the different cases requires a complete thorough study by itself and thetime limitations set on this thesis allow only for a glimpse that proves howimportant resource optimization is for improving the performance.

2 4 6 8 100.5

1

1.5

2

2.5

3

3.5

Number of relays of VAA (second VAA for the 3hop system)


2hop

3hop

Figure 4.13: Comparison between the normalized throughputs of the 2-hopsystem and 3-hop modified (Bandwidth Allocation) system. The number ofrelays in FT=5, density=75.34 Relays/Km2, and MT at 500m away from BS.

The depicted graphs in Figure 4.12 represent the case where the bandwidthis divided according to equation 4.3. By observing these graphs closely, thefollowing remarks can be inferred:

The total capacity of the system shows a notable increase and approachesthe capacity provided by the 2-hop system (refer to Figure 4.13). In fact


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2 4 6 8 104

6

8

10

12

14

Number of relays in VAA (second VAA for the 3hop system)


2hop3hop

Figure 4.14: Comparison between the normalized throughputs of the 2-hopsystem and 3-hop modified (Bandwidth Allocation) system. The number ofrelays in FT=5, density=157.2 Relays/Km2, and MT at 100m away from BS.

for some cases, such as for example the case of 7 active relays when the

terminal is 100m away from the AP (refer to Figure 4.14), we notice thatthe capacity of the 3-hop system outperforms the 2-hop case by a slightmargin.

When the Terminal is within a radius of 100 and 300 meters from the APthen the combined capacity for the second and third hops is higher thanthe first hop capacity especially when the number of relays in the secondtier is low.

All the graphs show that the capacity of the first hop decreases to a certainextent while the bottle-neck capacity of the rest of the system increasesthus raising the total capacity of the system.

The shape of the curve for the second and third hops capacity resembels

that of the 2-hop system since now we divide the bandwidth by only R + 1and not R + 1 + T.

From these results we can deduce that if the relays were to know (by someexternal means) the approximate distance of the target terminal then thebandwidth allocation can be optimized in such a way as to allow the 3-hopsystem to perform as well as the 2-hop case (perhaps better even).

4.3.3 Alternate Relay Activation Algorithm

The simulations so far have proved that the capacity of the second and thirdhops is indeed acting as the bottle-neck of the system (except for the case when


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1 2 3 4 5 6 7 8 9 100

5

10

15

20

25

Throu

ghput(Bits/sec)


System Capacity

1st Hop Capacity

1 2 3 4 5 6 7 8 9 104

6

8

10

12

14

16

Throughput(Bits

/sec)


System Capacity

1st Hop Capacity

1 2 3 4 5 6 7 8 9 103

4

5

6

7

8

3

4

5

6

7

Throughput(Bits

/sec)


System Capacity

1st Hop Capacity

1 2 3 4 5 6 7 8 9 102

2.5

3

3.5

4

4.5

5

5.5

6

T

hroughput(Bits/sec)


System Capacity

1st Hop Capacity

1 2 3 4 5 6 7 8 9 101.2

1.4

1.6

1.8

2

2.2

2.4

T

hroughput(Bits/sec)


System Capacity

1st Hop Capacity

Figure 4.15: Normalized throughput of the 3-hop system with Alternate RelayActivation Algorithm for distance between the BS and MT equal to 100m (up-permost), 300m (2nd row left), 500m (2nd row right), 700m (3rd row left), and900m (3rd row right).


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optimized bandwidth allocation is applied). To rectify this situation the alter-

nate activation criteria for choosing which relays in the FT should be activatedis utilized. The activation criteria for the relays of the second VAA is kept as itis so that the system remains comparable with the 2-hop reference case.

The modified activation criteria chooses those relays (within a certain pre-defined area around the AP) that have the best path gain with the terminalrather than with the AP. For the cases when the MT is at 100 and 300 metersfrom the AP the predefined area is taken to be of radius 450 meters with density157.2 Relays/Km2. Alternatively for the cases when the MT is at 500 and 700meters from the AP the predefined area is taken to be of radius 650 meters withdensity 75.34 Relays/Km2. As for the case when the MT is 900 meters awayfrom the AP then the predefined area is taken to be of radius 750 meters withdensity 56.59 Relays/Km2.

2 4 6 8 100

1

2

3

4

5

Number of relays in second VAA

Th

roughput(Bits/sec)

Unmodified 3hop

Modified 3hop

Figure 4.16: Comparison between the normalized throughput of the 3-hop un-modified system and 3-hop modified (Alternate Relay Activation Algorithm)system. The number of relays in FT=5, density=75.34 Relays/Km2, and MTat 500m away from BS.

The simulation results for the terminal at different distances from the APwith the same densities used in section 4.3.2. are displayed in the graphs ofFigure 4.15. The following comments can be made:

The increase in capacity over the unmodified 3-hop case is clear and posi-tively drastic. It can be observed clearly in Figure 4.16 for the case of theMT being 500 meters away from the AP.

The first hop capacity is still higher than the second and third hops capac-ity but the difference is decreased. This is natural since the active relaysof the first tier no longer have the best path gain with the AP.


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2 4 6 8 102

4

6

8

10

12

14

Number of relays in VAA (second VAA for 3hop system)


2hop3hop

2 4 6 8 101

2

3

4

5

6

Number of relays in VAA (second VAA for 3hop system)


2hop

3hop

2 4 6 8 101

2

3

4

5

6

Number of relays in VAA (second VAA for the 3hop system)


2hop

3hop

Figure 4.17: Comparison between the normalized throughputs of the 2-hopsystem and 3-hop modified (Alternate Relay Activation Algorithm) system. Thenumber of relays in FT=5, density=157.2 Relays/Km2 for MT 100m (Upper)and 300m (lower left) away from AP, and density=75.34 Relays/Km2 for MTat 500m (lower right) away from BS.

Compared to the 2-hop reference system, we observe that for the casewhen the terminal is 100 meters away from the AP, the performance isapproximately the same (refer to Figure 4.17). Furthermore, for the casewhen the distance is 300 meters and above, the modified 3-hop systemoutperforms the reference system by a clear and notable margin.

The results prove that the direct path gains between the FT active re-lays and the terminal constitute an extremely significant parameter in theanalysis and the choice of which relays to activate can radically influencethe performance of the whole system.


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Chapter 5

Cost Evaluation

The cost evaluation presented in this chapter is a simple analysis based on thecost estimation of different types of BS [3] in a typical Wideband Code DivisionMultiple Access (WCDMA) system.

According to [3] the cost involved in setting up a BS can be split into twomajor categories. The first category is the Initial Cost and this includes the priceof the equipment, the cost of building the site, and the site installation costs.The second category is called the Annual Cost and it encompasses the annualoperations and management (O&M) dues, the site lease, and the transmissioncost.

The cost involved in setting up a Macro BS is depicted in Table 5.1 and thevalues are in European Euro (e ). Similarly the cost involved in setting up a

Pico BS is depicted in Table 5.2 and the values are also in European Euro (e

).

Table 5.1: Macro BS costs.

Initial Cost Annual Cost

Equipment = 50K Annual O&M = 3KBuilding Site = 70K Site Lease = 10K

Site Installation = 30K Transmission = 5KTotal = 150K Total = 18K

Table 5.2: Pico BS costs.

Initial Cost Annual Cost

Equipment = 5K Annual O&M = 1KBuilding

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Three Step Cooperative MIMO Relaying