International Journal of Scientific & Engineering Research, Volume 4, Issue 8, August-2013 112

ISSN 2229-5518

Evaluate Comparative of Cooperative Relaying

Protocols in Wireless Communication

Ibrahim Khider Eltahir, Khalid Hamidr Bilal and Samah taha

Abstract— future generations of cellular communications require higher data rates and more reliable transmission link .Cooperative communications have a great significant in modern communications as they improve the information transmission between the source and the destination by using the relays. The relaying schemes are Amplify-and-Forward (AF) in which the relay sends an amplified copy of its receive signal and consequently serves as a repeater. In contrast, for Decode-and-Forward (DF) the relay performs a complete decoding of the receive signal and transmits the re-encoded message. This paper is a complete study of cooperative systems, analyzing its performance and comparing the use of two protocols amplify and forward (AF) and decode and forward(DF) with single and multi relays scenarios by measuring bit error rate using matlab.

Index Terms— wireless network, Relay, AF,DF,simulation

—————————  ——————————

1 INTRODUCTION

ooperative communications is a new communication technique which allows single-antenna mobiles to share
their antennas and to produce virtual multiple-antenna sys- tem. Cooperative communication is built on the broadcast na- ture of wireless communication which suggests that the transmitted signal between source and destination can be overheard at neighbouring nodes.The basic idea of coopera- tive relaying is to introduce intermediate nodes (relays) that forward the received data from the source to the destination [1].Cooperative relaying has large numbers of advantages, it provides spatial diversity since the relay terminals form a dis- tributed antenna array, increasing the range of communication and it is used to provide spatial multiplexing in multiuser communication scenarios [2].the cooperation signalling schemes include amplify and forward ,in this method the partner simply scales its received signal, to satisfy its own power constraints, and retransmit to cooperate. Each user re- ceives a noisy version of the transmitted signal and then am- plifies and retransmits it. Though noise is amplified by coop- eration, the destination can make better decision on the trans- mitted bits by combining the two independently received fad- ed versions of the signals [1].In Decode and Forward strate- gies, the relay tries to detect the received information, re- encodes and retransmits the signal after detection. If detection is unsuccessful the relaying can be harmful when detecting at the destination, therefore the strategy should be implemented such that the relay only retransmits the information when de- tection is successful. DF requires more complex devices than AF methods, but the noise at the receiver is much lower [3]. In detect and forward method the user tries to detect a partner bits and then retransmit the detected bits. In case of com- press and forward , the relay performs a non-linear transfor- mation on the received signal and then retransmits to destina- tion [3]. ————————————————

Ibrahim khider is an assistant professor at SUST ,dept of electronics since

july 2009.his PhD in communication

Email:Ibrahim_khider@hotmail.com,www.sustech.edu.

Khalid hamid is an assistant professor at UST ,head of the dept of com- munication engineering since august 2009. A PhD in electrical engineer- ing.Email: drkhalidbilal@hotmail.com

2 RELATED WORK

Fading affects the performance of wireless network by having an effect on the signal amplitude. To overcome the problem, the joint use of MIMO(Multiple input Multiple output) and cooperative schemes is proposed as an alternative means of alleviating the problem arrived because of robust fading. MIMO which is used to improve the performance of wireless communication in adverse propagation conditions such as fading, multi-path and interference. However, one limitation of MIMO is that installing multiple antennas on mobile station may not be feasible because of limitations in power, cost, and size .To overcome drawbacks of MIMO, distributed wireless nodes (active terminals or fixed relays) can be engaged in a cooperative fashion to extract antenna diversity.
In cooperative transmission, nodes can share their time, fre-
quency, and/or other resources to form a distributed or virtu-
al MIMO [4]. Transmitting independent copies of the signal

generates diversity and can effectively combat the fading, which cooperative diversity generates this diversity [5].
Figure 1: Illustration of cooperative diversity [6]
As shown in the Figure 1, Source information is broadcasted
to relay and destination nodes. Then, the relay node either
amplifies-and-forwards (AF) or decodes-and-encodes the re-
ceived information and forwards to the destination node
[14].cooperative diversity system, each wireless user is as-
sumed to transmit data as well as act as a cooperative agent

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ISSN 2229-5518

for another user [5].
The multi-hop communication in relay networks is a very
promising approach to improve the transmission coverage of
cellular and ad hoc networks. Because of transmit power con-
straints, multi-hop transmission also leads to remarkable cov-
erage extensions by dividing a total end-to-end transmission

into a group of shorter paths. The advantage of multi-hop re- laying has been pointed out, especially for rural areas with low traffic density and sparse population [7].
Figure 2: Example of a multi-hop relay system.

3 COOPERATIOVE MODEL

Cooperative relaying techniques can be realized in systems with either single relay or multiple relays per user [8].The basic model for cooperative system is a three terminal system model with on source, one relay and one destination.
Figure 3: Cooperative system model with single relay [9]
In multi relay model, requires multiple relays in the system,

relays form a virtual antenna array and exploit some of the benefits of MIMO systems.
Figure 4: Cooperative system model with multiple relays [9]

4 SIMULATION DESCRIPTION

In this paper we model and simulated two Cooperative Communication protocols Amplify and Forward and decode and Forward with Single and Multi-Relay. Wireless system was modelled using the parameters in table.1 and table.2.
Table1: Simulation Parameter of AF-DF Single and multi- Relay

Protocol Used/Relay Mode	Fixed Amplify &Forward & Decode &Forward
Number of Bits	10000
Modulation Scheme	BPSK
Number of Relays	1,2
SNR Vector	0 to 20 dB
Combining Technique	Maximal Ratio Combiner

Table 2: Parameters for system configuration

5 SIMULATION RESULTS

figure 5 shows that the BER decrease with increase of SNR, BER at the beginning is high and SNR is low because of am- plifying noise of signal.
In figure 6 with Multi relay cooperation we have much better performance than that of single node cooperation & Source Only AF relaying scheme. In this case at .0001 BER improve- ment is achieved with the help of two Relay operating in AF mode. However, with noncooperation only .0061 BER is achieved.

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0 the actual BER of Direct and Direct, AF

-1

Direct

0 the actual BER of Direct, and DF

Direct

10 -1

-2

10 -2

-3

10 -3

-4

-5

0 2 4 6 8 10 12 14

SNR(dB)

-5

0 2 4 6 8 10 12 14

Figure 5: SNR versus BER at AF-Single Relay

SNR(dB)

0 the actual BER of Direct and Direct,Multi relay AF

Figure 7: SNR versus BER at DF-Single Relay

Direct

Multi relay AF

-1

0 the actual BER of Direct, and Multi relay DF

Direct

Multi relay DF

-1

-2

-3

-4

-5

0 2 4 6 8 10 12 14

SNR(dB)

-5

0 2 4 6 8 10 12 14

Figure 6: SNR versus BER at AF-Multi Relay
In figure 7,one relay is use with DF Scheme although signifi-
cant gain is not achieved as compare to AF but fixed DF relay-
ing has the advantage over AF relaying in reducing the effects
of additive noise at the relay.
In figure 8 for example, at 6 dB .00005 BER is achieved
.However, with non-cooperation almost .003 BER is achieved.
Though Multi relay somehow improves the results but still
significant gain is not achieved as compare to AF because DF
protocol has a responsibility of reducing error propagation.

SNR(dB)

Figure 8: SNR versus BER at DF-Multi Relay
In these figures 9 and 10 the BS transmits to the RS in the first
phase. While in the second phase the RS transmits to the MS. It
is assumed that the RS is placed in such a way that it can use

the maximum rate of 64-QAM 5/6 AMC.
Figure 9: Time versus Throughput without relay

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4.5

3.5

2.5

1.5

0.5

No relay

Simple relay

Tx diversity AF Tx diversity DF Rx Diversity AF

Rx diversity DF

0 10 20 30 40 50 60

Figure 10:Time versus Throughput with Simple relay

In these figures 11 and 12 show where relaying actually im- proves performance for two scenarios: RS(relay station) transmit power 20.3 dB and RS transmit power of 17.3 dB.The relay transmit power should also be considered while choos- ing relays and the diversity scheme.

Figure 11:Time versus RS power of 20.3dB
Figure 12:Time versus RS power of 17.3dB
Transmit diversity seems to perform better than receive diver-
sity. However receive diversity does not need the BS to trans-
mit during the second phase.
Figure 13: Throughput versus Time at diversity schemes

6 CONCLUSION

In this paper, the performance of wireless communication has been gained tremendous attention by using cooperative com- munication among wireless nodes.
The advantages of AF relaying scheme of simple implementa- tion and low computation load for the relay nodes. Drawback of the AF protocol is that it amplifies the noise in the signal leading to some performance degradation. Benefit of the DF relaying scheme is not having any amplified noise in the transmitted signal to the destination .Drawbacks of the DF relaying scheme are error propagation at the relay due to the possibility of incorrect decoding of the coded signals and high computation load on the relay nodes.

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