協力式多樣技術於WiMAX OFDM(A)系統之運用

國 立 交 通 大 學

電信工程學系碩士班

碩士論文

協力式多樣技術於 WiMAX OFDM(A)系統

之運用

Incorporation of Cooperative Diversity in WiMAX

OFDM(A) Based Systems

研 究 生：廖晨吟 Student:

Chen-Yin

Liao

指導教授：李大嵩 博士 Advisor:

Dr.

Ta-Sung

Lee

協力式多樣技術於 WiMAX OFDM(A)系統之運用

Incorporation of Cooperative Diversity in WiMAX

OFDM(A) Based Systems

研 究 生：廖晨吟 Student: Chen-Yin Liao

指導教授：李大嵩 博士 Advisor:

Dr. Ta-Sung Lee

國立交通大學

電信工程學系碩士班

碩士論文

A Thesis

Submitted to Department of Communication Engineering

College of Electrical and Computer Engineering

National Chiao Tung University

in Partial Fulfillment of the Requirements

for the Degree of

Master of Science

in

Communication Engineering

June 2008

Hsinchu, Taiwan, Republic of China

協力式多樣技術於 WiMAX OFDM(A)系統之運用

學生：廖晨吟

指導教授：李大嵩 博士

國立交通大學電信工程學系碩士班

摘要

正 交 分 頻 多 工 系 統 為 新 一 代 無 線 通 訊 系 統 最 常 使 用 的 技 術 ， 如 IEEE 802.11a/g/n、 IEEE 802.16、IEEE 802.20、數位電視、數位廣播等許多系統均採 用此技術。協力式多樣傳輸為近年來無線通訊系統中被廣泛討論的新技術，其概 念為利用中繼節點幫助傳送以有效提升系統整體效能。在本論文中，吾人首先設 計出兩種協力式多樣傳送的機制來有效執行協力式多樣傳輸，並且推導兩種機制 的錯誤率上界，並以數學模型趨近表示之。吾人在第二部份中，提出在適應性調 變編碼和適應性天線系統的支援下，能讓協力式傳輸更有效改善整體效能的機 制。最後吾人以 IEEE 802.16(e)正交分頻多工存取的訊框結構為範例，提出一連 結適應機制，藉由在基地台進行協力式傳送的評估，並且決定最佳之傳送機制， 可有效使整體系統吞吐量優於傳統的直接傳送方式。吾人藉由電腦模擬驗證提出 之新連結適應法以及上述在低速移動的 IEEE 802.16-2005 環境規格下，可有效改 善位元錯誤率和整體系統吞吐量。

Incorporation of Cooperative Diversity in WiMAX

OFDM(A) Based Systems

Student:

Chen-Yin

Liao

Advisor:

Dr.

Ta-Sung

Lee

Department of Communication Engineering

National Chiao Tung University

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) is a popular technique in modern wireless communications. There are many systems adopting the OFDM technique, such as IEEE 802.11 a/g/n, IEEE 802.16, Digital Video Broadcasting, etc. Cooperative transmission is a new and promising technique in wireless communication systems, and it can improve the end-to-end throughput of the system by introducing relay terminals into networks. We propose two cooperative diversity schemes to efficiently incorporate the cooperative transmission in OFDM(A) based systems, and derive the BER upper bounds of two schemes. In order to make the overall performance of cooperative transmission better, the modified cooperative diversity schemes are proposed to employ cooperative transmission to AAS and AMC zones. At last, we propose a link adaptation method in OFDMA frame structure. The proposed link adaptation method is for base stations to evaluate the performance of each transmission scheme and choose the one which can provide the highest throughput. We evaluate the performance of the proposed system under slow mobility using IEEE 802.16-2005 standard and confirm that it achieves good performance.

Acknowledgement

I would like to express my deepest gratitude to my advisor, Dr. Ta-Sung Lee, for his enthusiastic guidance and great patience. I also wish to thank my friends for their encouragement and help. Finally, I would like to show my sincere thanks to my parents for their inspiration and love.

Contents

Chinese Abstract ...I

English Abstract... II

Acknowledgement ...III

Contents ... IV

List of Figures... VII

List of Tables ... IX

Acronym Glossary ...X

Notations...XII

Chapter 1

Introduction... 1

Chapter 2

Overview of WiMAX System ... 4

2.1 Physical

Layer Description ...4

2.1.1 Randomizer ...6

2.1.2 Forward Error Correction ...7

2.1.3 Interleaver ...8

2.2

Key Features of Scalable OFDMA ...10

2.2.1 Scalable Channel Bandwidth ... 11

2.2.2 Sub-channelization and Permutation ...12

2.3 Transmit

Techniques ...16

2.3.1 Transmit Diversity: Space-Time Coding ...17

2.3.2 Transmit Beamforming: Adaptive Antenna System ...20

2.4 Channel

Model ...22

2.4.1 SUI Channel Model for Fixed Wireless Application ...23

2.5 Summary ...28

Chapter 3

Different Cooperative Diversity Schemes for WiMAX

Systems... 29

3.1 System

Model ...29

3.1.1 Relaying methods...32

3.2 Different

Cooperative

Diversity Schemes ...32

3.2.1 Cooperative Receive Diversity Scheme...33

3.2.2 Cooperative Transmit Diversity Scheme ...38

3.3

Evaluation of Overall Average BER Upper Bound Based on AF

Mode ...43

3.4

Implementing Cooperative Transmission in WiMAX Systems..46

3.5 Computer

Simulations...51

3.6 Summary ...55

Zones ... 56

4.1

AAS for Cooperative Diversity Schemes ...56

4.2

AMC for Cooperative Diversity Schemes ...60

4.3

Proposed Link Adaptation under WiMAX OFDMA AMC Zone64

4.4 Computer

Simulations...67

4.5 Summary ...73

Chapter 5

Conclusion ... 74

List of Figures

Figure 2-1: PRBS generator for randomizer ...6

Figure 2-2: OFDMA randomizer DL initialization vector ...7

Figure 2-3: Convolutional encoder ...7

Figure 2-4: PRBS generator for pilot modulation...9

Figure 2-5: Example of DL preamble for segment 1 ...10

Figure 2-6: Cluster structure ...13

Figure 2-7: Allocated subcarriers into subchannels for PUSC ...14

Figure 2-8: Example of mapping OFDMA slots to subchannels and symbols in DL PUSC...14

Figure 2-9: Description of a UL PUSC tile...15

Figure 2-10: Allocated subcarriers into subchannels for FUSC...15

Figure 2-11: AMC bin structure ...16

Figure 2-12: Block diagram of STC ...18

Figure 2-13: Illustration of Alamouti scheme ...18

Figure 2-14: Cluster structure for STC PUSC using 2 Antennas...20

Figure 2-15: Illustration of AAS ...21

Figure 2-16: Generalized AAS zone allocation ...22

Figure 2-17: AAS zone structure in OFDMA mode ...22

Figure 2-18: Doppler spectrum of SUI channel models ...26

Figure 3-1: Illustration of Cooperative transmission ...31

Figure 3-2: Coop-SIMO diversity scheme...33

Figure 3-3: Coop-MISO diversity scheme...38

Figure 3-4: Block diagram of the Coop-SIMO OFDM based system ...48

Figure 3-5: Block diagram of weighting algorithm ...49

Figure 3-6: Block diagram of the Coop-SIMO OFDM based system ...51

Figure 3-7: BER performance of simulations and bounds of each cooperative diversity scheme...52

Figure 4-1: Switched beam system ...57

Figure 4-3: Coop-MISO scheme under AAS ...59

Figure 4-4: BER curves for different coding profiles under SUI-3 channel...61

Figure 4-5: Look-up table for stored thr γ ...63 ( )

Figure 4-6: Modified frame structure for proposed link adaptation method ...65

Figure 4-7: Flow chart of proposed link adaptation method...67

Figure 4-8: BER performance of Coop-MISO scheme with and without AAS...69

Figure 4-9: BER performance of Coop-SIMO scheme with and without AAS...70

Figure 4-10: End-to-end throughput under different conditions of relay channel ...71

List of Tables

Table 2-1: Data rate for different modulations and code rates ...5

Table 2-2: Puncturing patterns and orders to realize different code rates ...8

Table 2-3: Useful data payload for a slot ...8

Table 2-4: OFDMA scalability parameters for different bandwidth ...12

Table 3-1: Transmission sequence of Coop-MISO scheme ...39

Acronym Glossary

3GPP third generation partnership project AAS Adaptive Antenna System AF amplify-and-forward

AMC Adaptive Modulation and Coding AOA angle of reception

AOD angle of departure

AWGN additive white Gaussian noise BS base station

CCIR co-channel interference rejection CINR carrier-to-interference-and-noise ratio Coop-MISO cooperative MISO scheme

Coop-SIMO cooperative SIMO scheme CRC cyclic redundancy check DF decode-and-forward

DL downlink

DFT discrete fourier transform FEC forward error correction FFT fast fourier transform FUSC Full Usage of Subchannels

IDFT inverse discrete fourier transform IFFT inverse fast fourier transform ISI inter-symbol interference LOS line-of-sight

MISO multiple-input-single-output MRC maximum ratio combining MS mobile station

NLOS non-line-of-sight

OFDM orthogonal frequency division multiplexing OFDMA orthogonal frequency division multiple access PUSC Partial Usage of Subchannels

QAM quadrature amplitude modulation QOS quality of service

QPSK quadrature phase shift keying SIMO single-input-multiple-output SNR signal-to-noise ratio STC space time coding TDD time division duplexing

UL uplink

Notations

N FFT size

cp

L sample numbers of cyclic prefix

H channel frequency response

h channel time response

g multipath channel tap gain τ multipath delay

x time domain transmit signal

X frequency domain transmit signal

y time domain receive signal

Y frequency domain receive signal

P pilot magnitude

d

N number of subcarriers used for data transmission

g

N number of subcarriers used for guard band

p

N number of subcarriers used for data transmission

s

Chapter 1

Introduction

Wireless communication systems have been in use for quite a long time. Many standards are available based on which user devices communicate, but the present standards fail to provide sufficient data rate, and broadband wireless access is an appealing way to provide flexible and easily-to-deploy solution. In view of this requirement for future mobile wireless communication systems, the 802.16 standard has been proposed by Institute of Electrical and Electronic Engineers (IEEE) [1], [2].

The WiMAX (Worldwide Interoperability for Microwave Access) Forum is committed to providing optimized solutions for fixed, nomadic, portable and mobile broadband wireless access. Two versions of WiMAX address the demand for these different types of access:

• IEEE 802.16-2004: This is based on the 802.16-2004 version of the IEEE 802.16 standard. It uses Orthogonal Frequency Division Multiplexing (OFDM) and supports fixed and nomadic access in Line of Sight (LOS) and Non Line of Sight (NLOS) environments. For LOS environment, the frequency range in 802.16d is from 2GHz to 66GHz and Single Carrier (SC) is mainly adopted as the transmission scheme. For NLOS environment, it focuses on the Broadband Wireless Access (BWA), where the frequency band ranges from 2GHz to 10GHz. In physical layer (PHY), NLOS temps to adopt OFDM and OFDMA techniques.

• IEEE 802.16-2005: Optimized for dynamic mobile radio channels. This version is based on the IEEE 802.16-2005 amendment and provides support for handoffs and roaming. The choice of the subcarrier number becomes more flexible since it provides four options, 128, 512, 1024, and 2048. The frequency band ranges from 2GHz to 6GHz.

Orthogonal Frequency Division Multiplexing (OFDM) is a popular technique in modern wireless communication systems. In an OFDM system, the bandwidth is divided into several orthogonal subchannels for transmission. A cyclic prefix (CP) is inserted before each symbol. Therefore, if the delay spread of the channel is shorter than the length of the cyclic prefix, the intercarrier symbol interference (ISI) can be eliminated due to the cyclic prefix. On the other hand, subcarriers in OFDM are orthogonal to each other over time-invariant channels, so the conventional OFDM system only requires one-tap equalizers [3] to compensate the channel response. This characteristic simplifies the design of the OFDM receiver, and for this reason, the OFDM technique is widely used in wireless communication systems.

Future wireless networks will be highly dynamic with extreme demands on performance, especially in terms of bandwidth and energy. The use of multiple antennas can provide significant improvement in power and spectral efficiency in wireless communications. However, it might be impractical in many cases due to the limited size and power of the terminals. By exploiting the broadcast nature of the wireless networks, a virtual MIMO system can be realized through cooperation among nodes [4-6]. Cooperative diversity has recently emerged as a promising alternative to combat fading in wireless channels. The fundamental idea is that nodes in a wireless network share their information and transmit cooperatively as a virtual antenna array, thus providing diversity without the requirement of additional antennas at each node. In [7], the authors proposed some cooperative strategies including fixed

relaying, selection relaying, and incremental relaying schemes. In [8], user cooperation diversity was proposed for CDMA system in which orthogonal codes are used to mitigate multiple access interference.

In the simplest embodiment of cooperative networking, some nodes may simply relay a message [9]. This type of relaying has been proposed to augment the performance of infrastructure-based networks, such as WiMAX. Since cooperation is a promising architecture for next generation wireless system and OFDM is one of the most popular physical-layer technologies for wireless systems, the combination of the two techniques named OFDM-relay will be a good candidate technology for future wideband wireless communications.

The rest of this thesis is organized as follows. In Chapter 2, an overview of WiMAX system is given. The transmit techniques such as Space-Time Coding (STC), Adaptive Antenna System (AAS), Adaptive Modulation and Coding (AMC) are also introduced. In Chapter 3, two cooperative diversity schemes are proposed and the average BER upper bounds are derived. In Chapter 4, we modify cooperative diversity schemes to match up the properties of AAS and AMC, and propose a link adaptation method for the base station to determine the transmission schemes. Several computer simulation results are also given to show the performance improvement of the proposed cooperative diversity schemes in IEEE 802.16-2005 system. In Chapter 5, we conclude this thesis and future work can be the design of the proposed link adaptation method for users with high speeds.

Chapter 2

Overview of WiMAX System

WiMAX is a broadband wireless technology that supports fixed, nomadic, portable and mobile access. To meet the requirements of different types of access, two versions of WiMAX have been defined. The first is based on IEEE 802.16-2004 and is optimized for fixed and nomadic access. The second version is designed to support portability and mobility, and will be based on the IEEE 802.16-2005 amendment to the standard. In this chapter, we will focus on the physical layer of orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) structure in IEEE 802.16-2005. Then, the transmit techniques such as STC and AAS adopted in the system will be introduced. Finally, the channel model for fixed wireless application will be mentioned.

2.1 Physical Layer Description

Worldwide Interoperability of Microwave Access (WiMAX) is a broadband wireless technology that supports fixed, nomadic, portable and mobile access. In other words, WiMAX is a technology based on the IEEE 802.16 specifications to enable the delivery of last mile wireless broadband access as an alternative to cable and DSL. WiMAX will provide wireless broadband connectivity without the requirement for

direct line-of-sight (LOS) with a base station. WiMAX provides metropolitan area network connectivity at speeds of up to 75 Mb/sec, and WiMAX systems can be used to transmit signal as far as 30 miles. However, on the average a WiMAX base-station installation will likely cover between three to five miles [10].

WiMAX covers both LOS and NLOS applications in the 2-66 GHz frequencies. The PHY layer contains several forms of multiplexing and modulation to support different frequency range and application. Data rates determined by exact modulation and encoding schemes are shown in Table 2-1. The IEEE 802.16 standard was originally written to support several physical medium interfaces and it is expected that it will continue to develop and extend to support other PHY specifications. Hence, the modular nature of the standard is helpful in this aspect. For example, the first version of the standard only supported single carrier modulation. Since that time, OFDM has been added [11].

Table 2-1: Data rate for different modulations and code rates

73.2 48.8 24.4 20 Bandwidth (MHz) 26.1 17.5 8.7 7 22.5 15 7.5 6 73.2 48.8 24.4 20 Bandwidth (MHz) 26.1 17.5 8.7 7 22.5 15 7.5 6

Raw bit rate (Mb/s)

QPSK, CC3/4 16QAM, CC3/4 64QAM, CC3/4

In IEEE 802.16-2005, its applications are focused on mobile applications in the 2-6 GHz. Two multi-carrier modulation techniques are supported in 802.16-2005: OFDM with 256 carriers and OFDMA with 128, 512, 1024, or 2048 carriers.

In the following sections, we will introduce the main block diagrams of the transmitter architecture. We will focus on physical layer description on OFDM and OFDMA mode in IEEE 802.16-2005.

2.1.1 Randomizer

The randomization is performed on each burst of data on the DL and UL, which means that for each allocation of a data block, the randomizer shall be used independently. For RS and CC encoded data, padding will be added to the end of the transmission block, up to the amount of data allocated minus one byte, which shall be reserved for the introduction of a 0x00 tail byte by the FEC. The PRBS generator shall be 1 X+ 14 +X15 as shown in Figure 2-1. Each data byte to be transmitted shall enter sequentially into the randomizer. Preambles are not randomized.

LSB MSB

LSB MSB

Figure 2-1: PRBS generator for randomizer

On the downlink, the randomizer shall be re-initialized at the start of each frame. In OFDMA mode, the randomizer shall be re-initialized with the sequence: [LSB]011011100010101[MSB]. At the start of subsequent bursts, the randomizer shall be initialized with the vector shown in Figure 2-2. The frame number used for initialization refers to the frame in which the DL burst is transmitted. The subchannel offset used for initialization refers to the allocated subchannels in which the DL burst is transmitted.

Figure 2-2: OFDMA randomizer DL initialization vector

2.1.2 Forward Error Correction

The encoding is performed by passing the data in block format through a convolutional encoder. A single 0xFF tail byte is appended to the end of each burst after randomization. Each data block is encoded by the binary convolutional encoder, which shall have native rate of 1/2, a constraint length equal to 7, and shall use the generator depicted in Figure 2-3. Puncturing patterns and serialization order that shall be used to realize different code rates are defined in Table 2-2. Table 2-3 gives the data payload sizes and the code rates used for the different modulations.

Table 2-2: Puncturing patterns and orders to realize different code rates Code rates Rate 1/2 2/3 3/4 dfree 10 6 5 X 1 10 101 Y 1 11 110 XY X1Y1 X1Y1Y2 X1Y1Y2X3

Table 2-3 Useful data payload for a slot

2.1.3 Interleaver

After FEC, all encoded data bits shall be interleaved by a block interleaver with a block size corresponding to the number of coded bits over the allocated subchannels per OFDM symbol. The interleaver is defined by two step permutation. The first permutation ensures that adjacent coded bits are mapped onto nonadjacent subcarriers. The second permutation ensures that adjacent coded bits are mapped alternately onto less or more significant bits of the constellation, thus avoiding long runs of lowly reliable bits.

2.1.4 Modulator

The data are entered serially to the constellation mapper after interleaving. In OFDMA mode, Gray-mapped QPSK, 16QAM, and 64QAM shall be supported. The constellation-mapped data shall be subsequently modulated onto all allocated data subcarriers and each subcarrier multiplied by the factor 2 * (1/ 2−w_k) according the subcarrier index, k.

Pilot Modulation

Pilot subcarriers shall be inserted into each data burst in order to constitute the symbol and they shall be modulated according to their carrier location within the symbol. The PRBS generator depicted in Figure 2.4 shall be used to produce a sequence, w . The polynomial for the PRBS generator shall be _k 1 X+ 9+X11. For OFDMA mode, each pilot shall be transmitted with a boosting of 2.5 dB over the average power of each data tone. The pilot subcarriers shall be modulated according to (2-1):

{ }

{ }

The pilot in DL preamble shall be modulated according to (2-2):

{

}

{

}

Preamble Structure

The first symbol of the DL transmission is the preamble and the preamble subcarriers are divided into three carrier-sets. Those subcarriers are modulated using a boosted BPSK modulation with a specific PN code. There are three possible groups consisting of a carrier-set each that may be used by any segment. Each segment uses a preamble composed of a carrier-set out of the three available carrier-sets in the following manner: (In the case of segment 0 under 2048-FFT, the DC carrier will not be modulated at all and the appropriate PN will be discarded; therefore, DC carrier shall be always zero. For the preamble symbol of 2048-FFT, there will be 172 guard band subcarriers on the left side and the right side of the spectrum). For example, Figure 2-8 depicts the preamble of segment 1 for 2048-FFT.

Figure 2-5: Example of DL preamble for segment 1

2.2 Key Features of Scalable OFDMA

Scalable OFDMA (SOFDMA) is IEEE 802.16-2005 enhanced physical layer and it includes many important features for fixed, nomadic, and mobile networks. Because of these advantages, most of the industry will build their IEEE 802.16-2005 products using SOFDMA technology. However, the IEEE 802.16-2005 standard is not just for mobility. There are also many compelling reasons for using SOFDMA in fixed broadband wireless access (BWA) networks. In this section, we will focus on some key features of SOFDMA for wireless applications [12], [13].

2.2.1 Scalable Channel Bandwidth

Scalability is one of the most important advantages of OFDMA. Spectrum resources for wireless broadband worldwide are still quite different in its allocation. With OFDMA subcarrier structure, it is designed to be able to scale to work in different channelization from 1.25 to 20 MHz to cope with varied worldwide requirements as efforts proceed to achieve spectrum harmonization in the longer term. The scalability is supported by adjusting FFT size according to the different channel bandwidth to fix the subcarrier frequency spacing. By fixing the subcarrier spacing and symbol duration, the basic unit of physical resource is fixed. Therefore, the impact to higher layers is minimal when scaling the bandwidth.

The significant advantage from scalability is the flexibility of deployment. With the little modification to different air interfaces, OFDMA system can be deployed in various frequency bands to flexibly address the requirement for various spectrum allocation and usage model requirements. The OFDMA scalability parameters used in the thesis are listed in Table 2-4. The subcarrier spacing is fixed to 11.16 kHz and the symbol time is fixed to 89.6 sμ . With the flexibility to support wider range bandwidth, OFDMA also enjoys high sector throughput, which allows more efficient multiplexing of data traffic, lower latency and better quality of service (QoS).

Table 2-4: OFDMA scalability parameters for different bandwidth 22.4 us (Tb/4) 89.6 us 11.16 KHz 128 1.43 1.25 256 2.86 2.5 512 5.71 5 1024 11.4 10 Values Parameters CP duration Useful symbol time (Tb) Subcarrier spacing 2048 FFT size 22.8 Sampling frequency (MHz) 20 Bandwidth (MHz) 22.4 us (Tb/4) 89.6 us 11.16 KHz 128 1.43 1.25 256 2.86 2.5 512 5.71 5 1024 11.4 10 Values Parameters CP duration Useful symbol time (Tb) Subcarrier spacing 2048 FFT size 22.8 Sampling frequency (MHz) 20 Bandwidth (MHz)

2.2.2 Sub-channelization and Permutation

Active (data and pilot) subcarriers are grouped into subsets of subcarriers called subchannels. The OFDMA PHY supports sub-channelization in both DL and UL. The minimum frequency-time resource unit of sub-channelization is one slot, which is equal to 48 data tones. There are two major types of subcarriers permutation for subchannelization: diversity and contiguous. The diversity permutation takes subcarriers pseudo-randomly to form a subchannel. The diversity permutations include DL & UL PUSC (Partial Usage of Subchannels), DL FUSC (Full Usage of Subchannels), and additional optional permutations. The contiguous permutation groups a block of adjacent sub-carriers to form a subchannel. The contiguous permutations include DL & UL AMC (Adaptive Modulation and Coding). With DL PUSC, for each pair of OFDM symbols, the usable subcarriers are grouped into

clusters containing 14 adjacent subcarriers per symbol, with pilot and data allocations

in each cluster in the even and odd symbols as shown in Figure 2-6. Other definitions of the PUSC subcarrier allocation are: one subchannel contains two clusters by one OFDMA symbol and one slot is one subchannel by two OFDMA symbols.

Figure 2-6: Cluster structure

Divide these clusters into several Major Groups. The allocation algorithm varies with FFT sizes. For each subchannel, subcarriers are distributed in some clusters that belong to its major group as shown in Figure 2.7. A subchannel contains 2 clusters and is comprised of 48 data subcarriers and 8 pilot subcarriers. Allocating subcarriers to subchannel in each major group is performed separately for each OFDMA symbol by first allocating the pilot carriers within each cluster, and then taking all remaining data carriers within the symbol and using the procedure described in (2-3):

{

}

( , )

[ mod ] _ mod

=

⋅ + +

subchannels k s k subchannels subchannels

subcarrier k s

N n p n N DL PermBase N , (2-3)

where

( , )

subcarrier k s is the subcarrier index k in subchannel s

subchannels

N is the number of subchannels in current partitioned major group ( 13 ) mod

k subccarriers

n = k+ ⋅s N

subccarriers

N is the number of data subcarriers allocated to a subchannel [ ]

s

p j is the series obtained by rotating basic permutation sequence cyclically to

the left s times

The parameters vary with FFT sizes. Figure 2-8 shows an example of mapping OFDMA slots into subchannels and symbols in the DL PUSC.

even symbols odd symbols

subcarriers N_gguard subcarriers Ng-1 guard subcarriers Group 0 Sub-channel 1 Sub-channel 0 Group 0 Group 0 … … … … … … … subcarriers N_gguard subcarriers Ng-1 guard subcarriers Group 0 Sub-channel 1 Sub-channel 0 Group 0 Group 0 … … … … … … …

Figure 2-7: Allocated subcarriers into subchannels for PUSC

Subchannel number

OFDMA symbol index

Spanning two OFDMA symbols

Subchannel number

OFDMA symbol index

Spanning two OFDMA symbols

Figure 2-8: Example of mapping OFDMA slots to subchannels and symbols in DL PUSC

Compared with the cluster structure for DL PUSC, a tile structure is defined for the UL PUSC whose format is shown in Figure 2-9. The slot is comprised of 48 data subcarriers and 24 pilot subcarriers in 3 OFDM symbols.

Figure 2-9: Description of a UL PUSC tile

FUSC achieves full diversity by spreading tones over entire band. The symbol structure is constructed using pilots, data, and zero subcarriers. The symbol is first allocated with the appropriate pilots and with zero subcarriers, and then all the remaining subcarriers are used as data subcarriers. To allocate the data subchannels, the remaining subcarriers are partitioned into groups of contiguous subcarriers. Each subchannel consists of one subcarrier from each of these groups as shown in Figure 2-10. The number of groups is therefore equal to the number of subcarriers per subchannel. The exact partitioning into subchannels is according to the same procedure as (2-3).

subcarriers

Group 0 Group 1 Group 2 Group 3

Sub-channel 1

Sub-channel 0

N_gguard

subcarriers Ng-1 guard_subcarriers

Group N

subcarriers

Group 0 Group 1 Group 2 Group 3

Sub-channel 1

Sub-channel 0

N_gguard

subcarriers Ng-1 guard_subcarriers

Group N

Figure 2-10: Allocated subcarriers into subchannels for FUSC

The contiguous permutation groups a block of adjacent subcarriers to form a subchannel, such as DL AMC and UL AMC. As shown in Figure 2-11, a bin consists

of nine adjacent subcarriers in a symbol, with eight tones for data and one assigned for a pilot. A slot in AMC is defined as a collection of bins of the type (N x M = 6), where N is the number of adjacent bins and M is the number of adjacent symbols. Thus 4 different ways of defining a slot are (6 bins, 1 symbol), (3 bins, 2 symbols), (2 bins, 3 symbols), (1 bin, 6 symbols). AMC permutation enables multi-user diversity by choosing the sub-channel with the best channel frequency response.

Figure 2-11: AMC bin structure

In general, diversity subcarrier permutations perform well in mobile applications while contiguous subcarrier permutations are well suited for fixed, portable, or low mobility environments. These options enable the system designer to trade-off mobility for throughput.

2.3 Transmit Techniques

To increase the converge and reliability of WiMAX systems, the WiMAX standard supports optional multiple-antenna techniques such as Alamouti Space-Time Coding (STC), Adaptive Antenna Systems (AAS) and Multiple-Input Multiple-Output (MIMO) systems.

There are several advantages to using multiple-antenna technology compared with single-antenna technology:

• Array Gain: This is the gain achieved by using multiple antennas so that the signal adds coherently.

• Diversity Gain: This is the gain achieved by utilizing multiple paths so that the probability that any one path is bad does not limit performance. Effectively, diversity gain refers to techniques at the transmitter or receiver to achieve multiple “looks” at the fading channel. These schemes improve performance by increasing the stability of the received signal strength in the presence of wireless signal fading. Diversity may be exploited in the spatial (antenna), temporal (time), or spectral (frequency) dimensions.

• Co-channel Interference Rejection (CCIR): This is the rejection of signals by making use of the different channel response of the interferers.

2.3.1 Transmit Diversity: Space-Time Coding

In order to increase the rate and range of the modem, there are several considerations. Generally, BS can bear more cost and complexity than SS, so multiple-antenna techniques are good option at BS, also called transmit diversity. Among various transmit diversity schemes, STC is the most popular scheme with the feature of open loop (i.e., no feedback signaling is required) as channel information is not required at the transmitter. Therefore we will focus on the scheme of STC with 2 transmit antennas in this section as shown in Figure 2-12.

Sub-channel Modulation Sub-channel Modulation IFFT Input Packing IFFT Input Packing Space-Time Encoder Space-Time Encoder IFFT IFFT IFFT IFFT Filter Filter Filter Filter DAC DAC DAC DAC RF RF RF RF

#

RF ADCADC FilterFilter FFTFFT

#

Sub-channel Demod. Sub-channel Demod.

#

Figure 2-12: Block diagram of STC

The space-time block coding scheme was first discovered by Alamouti for two transmit antennas. Symbols transmitted from those antennas are encoded in both space and time in a simple manner to ensure that transmissions from both the antennas are orthogonal to each other. This would allow the receiver to decode the transmitted information with a slight increment in the computational complexity.

Figure 2-13: Illustration of Alamouti scheme

Figure 2-13 shows the operation of Alamouti scheme. The input symbols to the space-time block encoder are divided into groups of two symbols. At a given symbol period, the encoder takes a block of two modulated symbols s₁ and s₂ in each encoding operation and maps them to the transmit antennas according to a code matrix given by ST Decoder

+

Hs

n

H y

=

H Hs

+

H n

(

)

(

H H

=

h

+

h

I

=

ρ

I

)

y

s

s

s

s

_s

_s

⎡

−

⎤

⎡ ⎤

_⎢

_⎥

⎢ ⎥

_⎢

_⎥

⎢ ⎥

_⎢

_⎥

⎣ ⎦

6

_⎣

_⎦

s

* 1 2 * 2 1 s s s s ⎡ ₋ ⎤ ⎢ ⎥ = ⎢ ⎥ ⎢ ⎥ ⎣ ⎦ s . (2-4)

The encoder outputs are transmitted in two consecutive transmission periods from two transmit antennas. Let h₁ and h₂ be the channel gains from the first and second transmit antennas to the only one receiver antenna. Assume that h₁ and h₂ are scalar and constant over two consecutive symbol periods. The received signals in two consecutive symbol periods, denoted as r₁ and r₂, can be expressed as

1 1 1 2 2 1 * * 2 1 2 2 1 2, r h s h s n r h s h s n = + + = − + + (2-5)

where n₁ and n₂ are AWGN noise modeled as identical independent distributed (i.i.d.) complex Gaussian random variables with zero mean and power spectral density N₀/2 for each dimension. The above equation can be rewritten in a matrix form as

( )

( )

Since the channel matrix H is unitary, i.e. HH H = ρ·I₂, where ρ = h₁2 + h₂2, the ML decoder can perform an MRC operation on the modified signal vector r given by H H = ⋅ = ⋅ + ⋅ = ⋅ + n r H r s H n s n   _  ρ ρ . (2-7)

Therefore, we can obtain the space-time decoded vector s .

For OFDMA mode, STC coding is done on all data subcarriers that belong to an STC coded burst in the two consecutive OFDMA symbols. Pilot subcarriers are not encoded and are transmitted from either antenna 0 or antenna 1. In PUSC, the pilot

allocation to cluster is changed as shown in Figure 2-14. The pilot locations change in period of four symbols to accommodate two antennas transmission with the same estimation capability.

Figure 2-14: Cluster structure for STC PUSC using 2 Antennas

2.3.2 Transmit Beamforming: Adaptive Antenna

System

Future wireless communication systems aim at providing higher data rates with better link quality subject to being interference limited. Smart antenna technology is one of the most promising technologies for increasing both system coverage and capacity as shown in Figure 2-15. AAS, although an optional feature, through the use of more than one antenna elements at BS, can significantly improve range and capacity by adapting the antenna pattern and concentrating its radiation to each individual user. There are several advantages of using beamforming:

• Increase spectral efficiency proportional to the number of antenna elements • Realize an inter-cell frequency reuse of one and an in-cell reuse factor

proportional to the number of antenna elements

• Increase SNR of certain subscribers and steer nulls to others that can enable bursts to be concurrently transmitted to spatially separated users.

Figure 2-15: Illustration of AAS

First, the generalized AAS zone allocation is introduced as shown in Figure 2-16. The frame is divided into two parts: the fist part is allocated to the non-AAS users and the second part (called AAS zone) is allocated to the AAS users. This allows a mixture of non-AAS and AAS users to be supported by the same BS. The BS can dynamically allocate capacity to non-AAS and AAS traffic. The SS without AAS capability will ignore the traffic in the AAS zone.

Figure 2-17 shows the AAS zone structure in OFDMA mode. AAS_DLFP in an AAS zone is preceded by an AAS DL preamble of one symbol duration. All other data bursts within an AAS zone have a preamble whose duration is specified in AAS_DL_IE. AAS_DLFP provides a robust transmission of required BS parameters to enable SS access allocation. Each AAS_DLFP requires not carry the same information. Different beams may be used within the AAS diversity map zone. For

OFDMA mode, REP-RSP MAC message shall be sent by SS in response to a REP-REQ message from the BS to report estimation of the mean DL CINR (carrier-to-interference-and-noise ratio).

Regular DL Bursts Regular UL Bursts

DL

UL

Regular DL Bursts

Regular UL Bursts FDD

TDD

Regular DL Bursts Regular UL Bursts

DL

UL

Regular DL Bursts

Regular UL Bursts FDD

TDD

Figure 2-16: Generalized AAS zone allocation

…

AAS_DLFP

AAS Diversity Map Zone

AAS DL preamble Frequency Time SS #1 SS #2 SS #3 SS #4 SS #5 Non AAS portion

…

AAS_DLFP

AAS Diversity Map Zone

AAS DL preamble Frequency Time SS #1 SS #2 SS #3 SS #4 SS #5 Non AAS portion

Figure 2-17: AAS zone structure in OFDMA mode

2.4 Channel Model

Wireless propagation channels have been studied for many years, and a large number of channel models are already available. The signal that has propagated

through a wireless channel consists of multiple echoes of the originally transmitted signals; this phenomenon is known as multipath propagation. The different multipath components are characterized by different attenuations and delays. The correct modeling of the parameters describing the multipath components is the key point of channel modeling.

In first generation systems, a super-cell architecture is used where the base station and subscriber station are in LOS condition and the system uses a single cell with no co-channel interference. For second generation systems, a scalable multi-cell architecture with NLOS conditions becomes necessary. In WiMAX system, the wireless channel is characterized by:

¾ Path loss (including shadowing) ¾ Multipath delay spread

¾ Fading characteristics ¾ Doppler spread

Because we consider mobile users with relatively low speed, the main channel model in our study is Stanford University Interim (SUI) channel models [14].

2.4.1 SUI Channel Model for Fixed Wireless

Application

SUI channel models were proposed in [14] to model a statistic environment in IEEE 802.16-2004. There are many possible combinations of parameters to obtain different channel descriptions. A set of six typical channels were selected for the three terrain types that are typical of the continental US. The channel parameters are related to terrain type, delay spread, and antenna directionality and each channel model has three taps with distinct K-factor and average power. Table 2-5 shows an example of

time domain attribute of the SUI-3 channel, which is chosen to evaluate the performance.

Table 2-5: Parameters of SUI-3 channel models

Multipath Delay Profile

Due to the scattering environment, the channel has a multipath delay profile. It is characterized by τ_rms (RMS delay spread of the entire delay profile) which is defined as

(

)

_∑

_∑

τ is the delay of the jth delay component of the profile and P is given by _j

j

P = (power in the jth delay component) / (total power in all components)

RMS delay spread

A delay spread model was based on a large body of published reports. It was found that the RMS delay spread follows lognormal distribution and that the median of this distribution grows as some power of distance. The model was developed for

rural, suburban, urban, and mountainous environments. The model is of the following form

1

rms T d yε

τ = (2-9)

where τ_rms is the RMS delay spread, d is the distance in km, T₁ is the median value of τ_rms at d = 1 km, ε is an exponent that lies between 0.5 ~ 1.0, and y is a lognormal variant. Depending on the terrain, distance, antenna directivity and other factors, the RMS delay spread values can span from very small values (tens of nanoseconds) to large values (microseconds).

Fading distribution, K-factor

The narrow band received signal fading can be characterized by a Ricean fading. The key parameter of this distribution is the K-factor, defined as the ratio of the “fixed” component power and the “scatter” component power. The narrow band K-factor distribution was found to be lognormal, with the median as a simple function of season, antenna height, antenna beamwidth and distance. The model for the K-factor (in linear scale) is as follows:

s h b o

K =F F F K d uγ (2-10)

where

s

F is a season factor; F =1.0 in summer; 2.5 in winter _s

h

F is the received antenna height factor

b

F is the beamwidth factor

o

K and γ are regression coefficients

u is a lognormal variable which has 0 dB mean and a standard deviation of 8 dB. Using this model, one can observe that the K-factor decreases as the distance increases and as antenna beamwidth increases.

The random components of the coefficients generated in the previous paragraph have a white spectrum since they are independent of each other. The SUI channel model defines a specific power spectral density (PSD) function for these scatter component channel coefficients called “rounded” PSD which is given as

2 4 0 0 1 1.72 0.785 ( ) 0 f f S f = ⎨⎧⎪ −⎪⎪ + ⎪⎪⎪⎩ 0 0 1 1 f f ≤ > (2-11) where ₀ m f f f

= . In fixed wireless channels the shape of the spectrum is therefore different than the classical Jake’s spectrum for mobile channels. Figure 2-18 shows that its shape of Doppler spectrum is convex.

Figure 2-18: Doppler spectrum of SUI channel models

Antenna correlation

The SUI channel models define an antenna correlation, which has to be considered if multiple transmit or receive elements, i.e. multiple channels, are being simulated. Antenna correlation is commonly defined as the envelope correlation coefficient between signals transmitted at two antenna elements. The received baseband signals are modeled as two complex random processes X(t) and Y(t) with an

{ }

(

)

(

)

{

}

{

}

{

}

Note that this is not equal to the correlation of the envelopes of two signals, a measure that is also used frequently in cases where no complex data is available.

Antenna gain reduction factor

The use of directional antennas requires to be considered carefully. The gain due to the directivity can be reduced because of the scattering. The effective gain is less than the actual gain. This factor should be considered in the link budget of a specific receiver antenna configuration.

Denote ΔG_BW as the Gain Reduction Factor. This parameter is a random quantity which dB value is Gaussian distributed with a mean μ_grf and a standard deviation σ_grf given by 2 )( ) (0.53 0.1 ) ln( / 360) (0.5 0.04 ln( / 360) grf I I μ = − + β + + β (2-13) (0.93 0.02 ) ln( / 360) grf I σ = − + β , (2-14) where

β is the beamwidth in degrees

I = 1 for winter and I = -1 for summer.

In the link budget computation, if G is the gain of the antenna (dB), the effective gain of the antenna equals G− ΔG_BW. For example, if a 20-degree antenna is used, the mean value of ΔG_BW would be closed to 7 dB.

2.5 Summary

Specification of IEEE 802.16-2005 system has been introduced in this chapter. We also introduce some key transmit techniques and their operations. By using these transmit techniques, the capacity and range of the system can be improved significantly. Finally, the SUI-3 channel model for slow mobility is introduced.

Chapter 3

Different Cooperative Diversity

Schemes for WiMAX Systems

This section describes our analysis based on the following system model in which single antenna terminals are considered and relay stations (RS) are used for relaying in an OFDM wireless network.

3.1 System Model

In broadband communications, OFDM is an effective technique to capture multipath energy, mitigate the intersymbol interference, and offer high spectral efficiency. Therefore, OFDM is used in many communications system including IEEE-802.16 system. Multiple-input-multiple-output (MIMO) signal processing techniques for communication over point-to-point links using multiple collocated antennas at the transmitter and the receiver have improved tremendously in reliability and throughput. These techniques can be employed to improve the weaker link problem. However due to size, cost and hardware in practical issue constraints the use of MIMO techniques, which may not always be feasible and practical especially in small devices. Therefore, the idea of that making single antenna network nodes cooperatively transmit and receive by forming virtual antenna arrays has been proposed recently [5]. This method is broadly named as cooperative communication. In other words, the idea is that users in the network share their information and transmit cooperatively as a virtual antenna array, hence providing diversity without

the requirement of additional antennas at user terminal. Cooperative diversity has recently emerged as a promising alternative to combat fading in wireless channels [6]. Since OFDM is one of the most popular physical-layer technologies for wireless systems and cooperation is a promising architecture for next generation wireless system, the combination of the two techniques named OFDM-relay will be a good candidate technology for future wideband wireless communications. To improve the performance of ODFM based systems, the fundamental concept of cooperative diversity can be employed. However, special cooperation protocols and modulation and coding are needed to efficiently exploit the available multiple carriers. In [15], an oversampling technique is employed to provide efficient resource utilization due to OFDM properties. In [16], an application a space-time cooperation in OFDM systems was proposed.

Here our system model is based on fixed relaying protocols, in which the relays always repeat the source information. Moreover, our studies rely on an assumption of fixed network topology and fixed source-relay pairs. We take IEEE-802.16 system as a reference and consider mobile users with relatively low speed. For these slow mobile users, the channel is almost unchanged for the duration of a frame which consists of a certain number of OFDM symbols. Since users with slow mobility are considered, we use instantaneous SNR as channel state information. Further considerations should be taken for high-mobility users, such as impact of imperfect channel state information, etc.

We consider a time-division transmission in the source-to-relay (S-R), source-to destination (S-D), and relay-to-destination (R-D) links and Time Division Duplexing (TDD) is assumed. Figure 3-1 shows the cooperation system model, whereγ_SD,γ_SR,γ_RD are the signal-to noise ratio (SNR) of the S-D link, S-R link, and the R-D link respectively, and σ represents the noise variance. The relay which 2

is intermediate nodes and present in close vicinity to either the source or destination form the basis for cooperative communication. The cooperative protocol divides the transmission into tow phase. In the first phase, the source transmits its packets to the destination and the packets are also received at the relay due to the broadcast nature. The received signals at destination y n and relay _D[ ] y n_R[ ] are

[ ] [ ] [ ] D s SD D y n = E h x n +n n , (3-1) and [ ] [ ] [ ] R s SR R y n = E h x n +n n , (3-2)

where E is the transmit energy at the source, _s h_SD and h_SR are the channel coefficients of the S-D link and the S-R link, and n n and _D[ ] n n_R[ ] are the noise received at the destination and the relay, respectively. After receiving the source’s packet, the relay forwards the signals [ ]x n to the destination at the R-D link, whereγ_RD represents the SNR of the R-D link. The signals received from the relay at destination y n + is _D[ 1]

[ 1] [ ] [ 1]

D R RD D

y n+ = E h x n +n n+ , (3-3)

where E_R is the transmit energy at the relay, and h_RD is the channel coefficients of the R-D link. After receiving the signals during two phases, the destination combines two signals from the source and the relay to make decision. Next we study the basic relaying models based on the cooperative communication system studied.

Figure 3-1: Illustration of Cooperative transmission

2 2 SD,i SD,i h γ = σ 2 2 SR,i SR,i h γ = σ 2 2 RD,i RD,i h γ = σ S D R

3.1.1 Relaying methods

Two basic relaying methods are commonly used for cooperation [17]. Decode and forward:

In decode and forward method, the relay first decodes signals received from the source, re-encodes the signals, and then retransmits them. The receiver at the destination uses information retransmitted from the relay and the source to make decisions. It should be noted that it is possible for the relay to decode symbols in error resulting in error propagation. Perfect regeneration at the relays may require retransmission of symbols or use of forward error correction (FEC) depending on the quality of the channel between the source and the relay. This may not be suitable for a delay limited networks.

Amplify and forward:

In this method each cooperating node receives the signals transmitted by the source node but do not decode them. These signals in noisy form are amplified to compensate for the attenuation suffered between the S-R link and retransmitted. The destination requires knowledge of the channel state between the S-R link to correctly decode the symbols sent from the source. This requires transmission of pilots over the relays resulting in overhead in terms of additional bandwidth. Additionally sampling, amplifying, and retransmitting analog values is a nontrivial task for real-time implementation.

3.2 Different Cooperative Diversity Schemes

This section describes various cooperative schemes: cooperative-multiple input single output (MISO) and cooperative-single input multiple output (SIMO) are considered to achieve diversity. Trying various cooperative diversity schemes for

OFDM and OFDMA TDD based networks (802.16e) in low mobility scenarios to maximize system end-to-end throughput. The average transmit energy of the source and relay terminals per sub-carrier is fixed and represented by E and _s E_R , respectively.

3.2.1 Cooperative Receive Diversity Scheme

In the first time slot of cooperative receive diversity scheme, the source transmits the signals while both the relay and the destination receive and both the relay and the destination bugger this information. In the second time slot, the relay forwards the signals to destination and the BS remains silent. For each one of the AF or DF based forwarding, the destination combines the signals received form the source and the destination via Maximum Ratio Combining (MRC) [18]. After MRC, the destination achieves cooperative receive diversity. We refer it to Cooperative-single input multiple output scheme (Coop-SIMO), as shown in Figure 3-2. The derivations are done for a given sub-carrier. In the following sub-sections, the equivalent end-to-end instantaneous SNR is derived for both AF and DF based relaying.

Figure 3-2: Coop-SIMO diversity scheme

z AF Based Coop-SIMO Diversity Scheme

from the source. The transmitted signal by the relay using AF at sub-carrier i is given by [ 1] R [ ] AF R R E S n y n β + = , (3-4)

where β = N_o

(

)

{

}

[ ]

R

y n is the signal which received from the S-R link.

The received baseband signal at the destination at sub-carrier i over two successive OFDM symbols can be written as

[ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] H 1 ₁ 1 1 SD S D D AF SIMO AF _R D _{RD R D} SD S D S R R SR RD RD R D D S SD R R S RD R D SR RD AF SIMO h E x n n n y n Y _E y n _{h y n n n} h E x n n n E E E h h x n h n n n n n n E h x n _E E E _{h n n n n} h h x n β β β β β ⎡ ₊ ⎤ ⎡ ⎤ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ =_{⎢ ⎥} = _{⎢ ⎥} + ⎢ ⎥ ⎢ + + ⎥ ⎣ ⎦ _{⎢ ⎥} ⎣ ⎦ ⎡ ₊ ⎤ ⎢ ⎥ ⎢ ⎥ = ⎢ ⎥ ⎢ + + + ⎥ ⎢ ⎥ ⎣ ⎦ ⎡ ⎤ _{⎡ ⎤} ⎢ ⎥ _{⎢ ⎥} ⎢ ⎥ _{⎢ ⎥} =_{⎢ ⎥} +_{⎢ ⎥} + + ⎢ ⎥ _{⎢ ⎥} ⎢ ⎥ ⎢_⎣ ⎥_⎦ ⎣ ⎦ = +nAF_SIMO, (3-5)

where HAF_SIMO and nAF_SIMO are given by

H SD S AF SIMO _{S R} SR RD h E E E h h β ⎡ ⎤ ⎢ ⎥ ⎢ ⎥ = ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎣ ⎦ , (3-6) [ ] [ ] [ ] n 1 D AF SIMO _R RD R D n n E h n n n n β ⎡ ⎤ ⎢ ⎥ ⎢ ⎥ = ⎢ _{+ +} ⎥ ⎢ ⎥ ⎢ ⎥ ⎣ ⎦ , (3-7)

and y n is the signal received from the source at the time slot 1 and _D[ ] y_DAF[n + 1]

Assuming that the destination has perfect channel state information (CSI), which are the channel coefficients h_SD, h_SR, and h_RD. We combine the received signals from the source and the relay by MRC:

(

)

(

)

(

)

H H

AF AF AF AF SIMO SIMO SIMO SIMO

z Y

x n

=

= + (3-8)

The instantaneous SNR per symbol achieved after MRC in the AF mode can be derived form (3-8) as follows:

(

)

After combining the signals at the sub-carrier i, the destination observes a post processing SNR as given by (3-9), which can be referred as that of an effective end-to-end link. In an end-to-end flat fading link with instantaneous SNR γ , the end-to-end throughput can be given as

( ) ( )

(

)

thr γ =R γ −PER γ , (3-10)

where the term R γ in b/s/Hz represents the nominal rate of the specific ( ) modulation and coding and the term PER γ represents the packet error rate with ( ) the specific modulation and coding based on SNR γ [19]. With AF based relaying at the sub-carrier i, the end-to-end throughput based on cooperative receive diversity scheme is finally given by

(

)

where the factor of 0.5 accounts for the fact that two times of time slots is needed for cooperative transmission compared to direct transmission.

z DF Based Coop-SIMO Diversity Scheme

When the relay uses the DF mode at a given sub-carrier, it detects and decodes the signal received from the S-R link at the given sub-carrier. Then, the relay encodes and then forwards it to the destination. Let x n[ ] _{represent the decoded decision with}

unit energy. The decision of the relay severely affects the performance of the DF mode. If the relay decodes x n[ ] incorrectly, it causes the error propagation.

In the DF mode, the relay terminal decodes and forwards the signal received from the source. In the first time slot, the relay demodulates the OFDM symbols received from the source. After the FFT operation, the relay decodes the signal at each sub-carrier and performs cyclic redundancy check (CRC) to check the packets that are correctly received. The relay only encodes and forwards the packets that are correctly received. At the end of the first time slot, it informs the destination about the decoding status of each packet. The transmitted signal by the relay using DF at sub-carrier i is given by

[ 1] [ ]

DF

R R

s n + = E x n . (3-12)

The received baseband signal at the destination at sub-carrier i over two successive OFDM symbols can be written as

[ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] H n , 1 ₁ 1 D _{SD S D} DF SIMO DF D RD R D D S SD D R RD DF DF SIMO SIMO y n _{h E x n n n} Y y n _{h E x n n n} n n E h x n n n E h x n ⎡ ⎤ ⎡ ⎤ _⎢ + _⎥ ⎢ ⎥ =_{⎢ ⎥ = ⎢ ⎥} + ⎢ + + ⎥ ⎢ ⎥ ⎣ ⎦ ⎣ ⎦ ⎡ ⎤ _{⎡ ⎤} ⎢ ⎥ _{⎢ ⎥} =_{⎢ ⎥} +_{⎢ ⎥} + ⎢ ⎥ _{⎢⎣ ⎥⎦} ⎣ ⎦ = +  (3-13) where HDF_SIMO is given by

HDF_SIMO S SD R RD E h E h ⎡ ⎤ ⎢ ⎥ = ⎢ ⎥ ⎢ ⎥ ⎣ ⎦ , (3-14)

and nDF_SIMO is given by [ ] [ ] n 1 D DF SIMO D n n n n ⎡ ⎤ ⎢ ⎥ = ⎢ ₊ ⎥ ⎢ ⎥ ⎣ ⎦ . (3-15)

Assuming that the destination has perfect channel state information (CSI), which the channel coefficients are h_SD , h_SR , and h_RD. We combine the received signals from the source and the relay by MRC:

(

)

DF DF DF SIMO SIMO SIMO

z = . (3-16)

Since the relay only forwards when it can decode the packets correctly, the post processing instantaneous SNR achieved at the destination can be derived from (3-16):

HDF_SIMO 2 DF F SIMO D SD RD o N γ = = γ +γ (3-17)

The available data rate of multi-hop is determined by the link with minimal capacity. Therefore, the end-to-end throughput with DF based relaying is the minimum of the end-to-end throughout of the S-R link and the effective R-D and S-D link, respectively. On the other hand, if γ_SIMODF >γ_SR, the end-to-end throughput is determined by γ_SR ; if γ_SIMODF <γ_SR, the end-to-end throughput is determined by

DF SIMO

γ . Therefore, With DF based relaying at the sub-carrier i, the end-to-end throughput based on cooperative receive diversity scheme is finally given by

(

)

(

)

(

)

(

)

where P_c( )γ represents the probability of correct reception of a packet based on SNR γ and the factor of 0.5 accounts for the fact that two times of time slots is needed for cooperative transmission compared to direct transmission.

3.2.2 Cooperative Transmit Diversity Scheme

In order to achieve cooperative transmit diversity, in the first time slot, only the relay listens to the broadcast of the source. In the second time slot, both the source and the relay transmit simultaneously by using the same radio resource. This way, an equivalent MISO channel is observed at the destination by cooperative transmission. Therefore, the cooperative transmit diversity scheme can be refereed to as Cooperative-multiple input single output scheme (Coop-MISO), as shown in Figure 3-3.

Figure 3-3: Coop-MISO diversity scheme

To achieve diversity, cooperative space time coding is used by the source and the relay [20] and we employ Alamouti spce time coding [21]. When Coop-MISO scheme is employed, the time slot is divided into four sub time slots. During the first tow time slots, the relay listens to the source only; During the second two time slots, both the relay and the source transmit simultaneously employing Alamouti space-time coding. The time division transmission structure to achieve cooperative transmit diversity with Alamouti space-time coding is presented in Table 1. In the table, S represents the