Performance Evaluation of Asynchronous Distributed Space Time Block Coded

System for Cooperative Communication

Varsha Vimal

Regd. No. 901306001

With the rapid growth of multimedia services, future generations of

wireless communications require

• higher data rates and

• a more reliable transmission link

• satisfactory quality of service.

Multiple-input multiple-output (MIMO) antenna systems have been

considered as an efficient approach to address these demands by

offering significant multiplexing and diversity gains over single antenna

systems without increasing requirements on radio resources such as

bandwidth and power

• The size of the mobile devices and the requirement on the distance

between antennas (to make the channel-fading between antenna pairs

uncorrelated) may limit the multiple antennas that can be deployed.

• Also the propagation environment may not support MIMO because,

for example, there is not enough scattering.

• In the later case, even if the user has multiple antennas installed,

MIMO is not achieved because the paths between several antenna

elements are highly correlated.

• Also in cases when large –scale shadow fading contaminates the

wireless links, all the channels tend to fade together, rather than

independently, hence eroding the achievable diversity gain.

• To overcome these drawbacks, the concept of cooperative

communications has recently been proposed and gained large interest

in the research community.

WIRELESS COOPERATION

• The key idea in user-cooperation is that of resource-sharing among multiple nodes in a network.

• Due to the broadcast nature of wireless medium, as the data is transmitted to its destination in multiple hops, many nodes in the vicinity can hear these transmissions.

• In a cooperative network, two or more nodes share their information and transmit jointly as a virtual antenna array.

• Willingness to share power and computation with neighboring

nodes can lead to savings of overall network resources.

• This enables them to obtain higher data rates and diversity

than they could have individually.

COOPERATIVE DIVERSITY

• A three-terminal network is a fundamental unit in user cooperation.

Diversity obtained through multi-hop transmissions is referred to as

the cooperative diversity.

• cooperative diversity encompasses wireless nodes relaying the

signals of other nodes on to their respective destinations

• In cooperative communications, the system depends on the relay

channel to generate the independent paths between the source and

destination. The relay channel can be thought of as an auxiliary

channel to the direct channel between the source and destination.

• The relay channel is a three-terminal network, in which Terminal 1

(source) aims to transmit to Terminal 3 (destination) with the help of

Terminal 2 (relay)

Relay channel, two hop channel and direct link

Cooperative Communication Protocols

The ways in which the relays process the received signal and forwards

them to the destination are known as cooperative communication

protocols.

Amplify and Forward

• Each user overhears the noisy version of signal transmitted by its

partner and then retransmits after amplification to equalize the effect

of the channel fade between the source and the relay.

• The relay does that by simply scaling the received signal by a factor

that is inversely proportional to the received power.

• It is also known as scale and forward relaying.

Decode and Forward

• In this kind of cooperative communication the partner tries to decode

the source transmission The relay then retransmits the decoded signal

after possibly compressing or adding redundancy.

• The decode-and-forward protocol is close to optimal when the source-

relay channel is excellent, which practically happens when the source

and relay are physically near to each other.

Compress and Forward

• The key idea of Compress and Forward is that the relay quantizes and

compresses the received signal using source coding and transmits the

compressed version to the destination. Then, the destination combines

the received message from the source and its quantized/compressed

version from the relay.

• In Estimate and Forward mode, the relay forwards an analog estimate

of its received signals.

Coded Cooperation

• Coded cooperation is a method that integrates cooperation into

channel coding. Here different portions of each user’s (partners) code

word are sent via independent fading path similar to the other

cooperative schemes. Then each user tries to transmit incremental

redundancy (for eg. The parity bits) to its partner. For unsuccessful

decoding of partners second code partition, user transmits its own

second partition.

Selective Relaying

• Is a scheme where relays are selected to retransmit the source message

only if the relay path satisfies certain threshold criteria.

Incremental Relaying

• In fixed protocols category, although if the destination correctly detect

the transmitted symbols in phase 1, the channel resources divided

between the source and relay node which waste the bandwidth that

reduce the overall data rate in the system. This issue can be overcome, if

there is feedback channel from the destination to the relay nodes. This

called incremental relaying

COOPERATIVE COMMUNICATION

System structure for two-phase cooperative Communications

• A conventional cooperative communication system consists of a

source node S, n relay nodes, N1,...,NR, and a destination node D.

Each node deploys only one antenna. A two-phase communication

protocol is adopted. In the first phase, the information data is

broadcasted from the source node S to the relay nodes, N1,...,NR. In

the second phase, all the relay nodes forward the received

information data to the destination node D in a cooperative manner.

Asynchronicity in Cooperative diversity

• Synchronization means that all relays are assumed to have the

identical timing, carrier frequency and propagation delay.

• Perfect synchronization is almost impossible to be achieved because

that the relay nodes will be in random locations and their

transmissions will be affected by random different conditions.

• However, because relays are at different locations (i.e., different

propagation delays) and they have their own local oscillators with no

common timing reference; it is an asynchronous technique in nature.

• The lack of synchronization may result in inter symbol interference

(ISI) and dispersive channels .Also the lack of a common timing

reference can affect the structure of the code matrix and result in a

rank deficient space-time code.

Techniques to mitigate the effects of asynchronicity

There are three classes of techniques to mitigate the

effects of

asynchronicity:

• Equalizers

• Frequency domain

• Time domain approaches

Time Domain Approaches

Time reversal space time block codes(TR-STBC)

Every symbol of an STBC codeword is replaced by a

block of B symbols whereas the conjugate operation

refines the corresponding block to be transmitted in a

time reversed order such that the embedded

orthogonality can be explored.

 

Distributed Threaded algebraic space time

code (TAST)

These codes choose a suitable algebraic number

for each layer so that different layers are laid in

different algebraic space.

Linear asynchronous distributed space-

time block codes (DSTBC)

These codes employ a linear dispersive structure

using sufficient guard intervals for combating the

imperfect synchronization.

Linear Asynchronous Distributed Space-Time Block Codes

• The term Distributed-space time block codes (D-STBC)system is

referred to a wireless communication system which implements the

space time block codes over cooperative network. Such distributed

space–time codes can be based on a linear dispersion code(LD).

Linear Dispersion Codes

• The relay transmits a linear combination of the T1 symbols in s and

their complex conjugate (i)

where is the column vector containing the complex conjugates of s

and the complex T2 × T1 matrices and are called dispersion

matrices. These matrices define the space-time code. The

individual symbols , T1 are each drawn from a complex

constellation of M symbols. The family of space-time block codes

that can be represented by (i) are called linear dispersion (LD)

codes .

• This means that the relays are not required to decode. Only simple

signal processing is done at the relays. This has two main benefits.

First, the operations at the relays are considerably simplified, and

second, we can avoid imposing bottlenecks on the rate by requiring

some relays to decode.

• For communications in wireless relay networks, a two-step protocol is

used, where in the first step, the transmitter sends information and in

the other, the relays encode their received signals into a “distributed”

LD code, and then transmit the coded signals to the receive node.

Literature Survey

• Nosratinia et al.[2003] describe wireless cooperative communication, a

technique that allows single-antenna mobiles to share their antennas and

thus enjoy some of the benefits of multiple-antenna systems.

• J. N. Laneman et al.[2003]In this paper several strategies employed by

the cooperating radios are outlined, including fixed relaying schemes

such as amplify-and-forward and decode-and-forward, selection relaying

schemes that adapt based upon channel measurements between the

cooperating terminals, and incremental relaying schemes that adapt

based upon limited feedback from the destination terminal.

Sendonaris et al.[2002] The first part presents an analytical study

that demonstrates that how an increase in capacity can be traded for

an increase in cell coverage. The second part of the paper investigates

the cooperation concept further and considers practical issues related

to its implementation. The authors also illustrate the benefits of

cooperation and addresses practical issues within a CDMA

framework.

T. Hunter et al.[ ] In coded cooperation a user transmits additional

parity symbols for its partner according to some overall coding

scheme instead of repeating the symbols initially transmitted by

the partner. Bit and block error rate analysis has been performed

for coded cooperation and examples with specific coding

schemes show significant improvement over non cooperation

transmission.

S Wei et al. propose two delay diversity protocols to achieve the

cooperative diversity gain in an ad hoc wireless network without

requiring synchronization of the relayed symbols at the destination

and a novel joint DFE-MMSE equalizer was derived. Based on the

proposed outage probability criterion, simulation results demonstrate

the performance improvements of the protocols over the single hop

scheme, as well as performance comparable to protocols requiring

strict symbol synchronization.

Y. Jing and B. Hassibi [] The idea of space-time coding is devised for

multiple-antenna systems is applied a wireless relay network with

Rayleigh fading channels. A two-stage protocol is used where in one stage

the transmitter sends information and in the other, the relays encode their

received signals into a “distributed” linear dispersion (LD) code, and then

transmit the coded signals to the receive node. It is further shown that the

optimal power allocation is for the transmitter to expend half the power

and for the relays to collectively expend the other half. At low and high

SNR, the coding gain is the same as that of a multiple-antenna system

with R antennas. However, at intermediate SNR, it can be quite different,

which has implications for the design of distributed space-time codes.

• Nan Wu et al.[29] propose a family of CLDCs for cooperative

networks and demonstrated its ability to achieve full spatial diversity,

as well as its vulnerability under the situation of asynchronous

reception The linear dispersion structure is employed to accommodate

the dynamic topology of cooperative networks, as well as to achieve

higher throughput than conventional space–time codes based on

orthogonal designs. A novel time-domain delay-tolerant ACLDC

scheme is also proposed and being demonstrated that the desirable

cooperative diversity can be maintained, even if severe propagation

delay differences exist by introducing guard intervals and block

encoding/decoding techniques, the interference signals caused by

asynchronous reception can be exploited rather than discarded.

• Pierluigi Salvo Rossi [44]The performance of Distributed Linear

Block Codes(DLBC) has been presented in terms of bit error

rate(BER) and outage probability(OP). A packet-based

communication scheme using DLBC has been described and its

performance shown via numerical simulations. The effects of the

number of pilots, decoding errors, and channel estimation errors have

been studied, with a focus on a static set of users transmitting to a

mobile or static BS. The two scenarios emphasize the cases in which

the cooperative channels improve or not with the channel to the BS: in

the first case the advantage of the cooperative system saturates, as

shown by the presence of an error floor in the performance, while in

the second case the cooperative system is always effective.

• P.A. Anghel and M. Kaveh [46] propose a Distributed Space-Time Coding

(DSTC) system based on the Alamouti codes. The symbol error rate of

systems with one and two non-regenerative relays using bounds and high

signal-to-noise ratio (SNR) approximations is characterized. The asymptotic

(high SNR) symbol error probability formulas are used to optimize the power

allocation in the DSTC system. Furthermore, using the asymptotic symbol

error probability formulas it is shown that the DSTC system has at least 1.5

times the diversity achieved by point-to-point transmissions with the same

bandwidth. Simulations show not only that the DSTC outperforms the

amplify-and-forward cooperative system with orthogonal transmissions, but

also convolutional encoded one-hop transmissions with the same information

rate as the DSTC system. Numerical results show that the DSTC system with

two relays performs very close to the optimum cooperative system.

Walid Qaja et al.[ ] consider amplify-and-forward (AF) type cooperative

wireless relay networks employing single bit closed-loop extended

orthogonal space-time block coding (CL EO-STBC) over two selected

cooperating relay nodes. Selection is performed from a set of NR

available relay nodes each equipped with two antennas and outer

convolutive coding is used to improve performance. A near-optimum

detection scheme is used at the destination node for overcoming the

effects of imperfect synchronization among selected relay nodes. End-to-

end simulation results show that the employed detection scheme can

effectively eliminate the interference components induced by

asynchronism with low detection complexity. Furthermore, the one-bit

feedback scheme and relay selection technique can enhance the overall

system performance and outperform previous feedback method.

S. Ding and R. Li [49] have proposed the combination of Low Density

Parity Check -Space Time block Codes(LDPC-STBC) coder supporting

Set Partitioning in Hierarchical Trees (SPIHT) compression image

transmission in the asynchronous cooperative communication. It is found

that the increase of the compression rate doesn’t necessarily increase the

PSNR value in Rayleigh fading channels. The simulation results showed

that the LDPC-STBC with the guard intervals is better than the traditional

coding method in BER performance. The coder proposed can overcome

the ISI effectively. The encoding scheme is used to simulate the image

transmission, and the PSNR values of images using the different encoding

scheme through the fading channel in the asynchronous structure are

compared. The proposed scheme is effective in increasing the PSNR

values of the reconstructed images.

Feng-Kui Gong et al.[ ] propose a distributed orthogonal space-

time block code (STBC) by making use of the Alamouti coding

scheme and jointly processing the signals from the two antennas at

the relay node. Such a code turns out to make the equivalent

channel at each source node be a product of the two Alamouti

channels and thus, is called distributed concatenated Alamouti

STBC. In addition, the asymptotic formula of exact symbol error

probability (SEP) for a quadrature amplitude modulation (QAM)

constellation with the maximum likelihood (ML) detector is

derived. This result shows that the full diversity gain function is

achieved and proportional to ln SNR/.  

Feng-Kui Gong et al.[51]consider a half-duplex amplify-and-forward

two way relaying network consisting of two sources with each having

a single antenna and N relays with each having two antennas. For such

a system with a general distributed linear dispersion code, a tight

lower bound of pair wise error probability (PEP) of the maximum

likelihood (ML) detector is derived, showing that diversity gain

function cannot decay faster than / , where SNR is signal to noise

ratio. Particularly for N = , a new distributed concatenated space-time

block code (STBC) is proposed and its asymptotic PEP formula is

attained, showing that the code presented in this paper achieves the

maximum diversity gain, i.e., meeting the lower bound of the diversity

gain function, as well as the maximum coding gain.

Gaps in Study

• Not much study has been done in the area of linear dispersive

distributed STBC in asynchronous cooperative communication

system.

• The asynchronous Distributed MIMO system is not explored much in

the light of channel coding for eg. LDPC

• The channel coded linear dispersive distributed STBC has not been

extensively studied in different fading environments.

Objectives

• To evaluate the performance of channel coded distributed space time

block codes in asynchronous cooperative MIMO system

• To evaluate the performance of the proposed concatenated DSTBC

scheme in various fading channels.

• To evaluate the performance of concatenated DSTBC in time reversal

(TR) and Linear dispersive (LD) structures respectively.

Research Methodology

1.Research and latest literature related to Distributed Space time

Block codes in asynchronous cooperative communication system

shall be explored and simulated using MATLAB software.

2.The research literature related to channel codes (for eg. LDPC) and

its implementation in asynchronous cooperative communication

shall be explored. Different parameters shall be studied and

implemented using MATLAB software.

3.Further bit error rate(BER) and outage analysis shall be done using

the MATLAB software.

References