CAI Rong-wen (蔡融文), SUN En-chang (孫恩昌), ZHANG Yan-hua (張延華)

(College of Electronic Information and Control Engineering, Beijing University of Technology, Beijing 100124, China)

An efficient multi-relay selection scheme in cooperative communication system

CAI Rong-wen (蔡融文), SUN En-chang (孫恩昌), ZHANG Yan-hua (張延華)

(College of Electronic Information and Control Engineering, Beijing University of Technology, Beijing 100124, China)

Cooperative relaying has played an important role in rapid evolution of wireless communications. The cooperative performance strongly depends on the selected relays. In this paper, we concentrate on relay selection in amplify-and-forward (AF) cooperative communication system, and an optimal multi-relay selection scheme is put forward to minimize the average symbol error rate(SER) of the system. Firstly, for the minimum average SER, on the basis of the statistic channel information, we define a parameter named equivalent channel gain which describes the channel status of two phases in the cooperative process. Then, under the constraint of equal power allocation, an optimal relay selection scheme is proposed in ascending order of equivalent channel gain (ECG). The scheme implies that the suitable number of relay nodes should be selected under the different signal-to-noise ratio (SNR) ranges to minimize the average SER. Computer simulation results show that the average SER rate of the proposed scheme is lower than these of the other schemes.

cooperative relaying; amplify-and-forward (AF); symbol error rate(SER); equal power allocation; multi-relay selection

CLD number： TN925 Document code： A

0 Introduction

Cooperative communication, which generates independent paths between the source and the destination by introducing a relay channel, has attracted the attention of both academia and industry during the past decade as a benefit of its spatial diversity gain achieved by sharing the resources of cooperative user terminals[1-3]. The basic idea of cooperative communication can date from the 1970s[4,5], when a three-terminal communication model was first introduced. The most popular cooperative protocols are decode-and-forward(DF) protocol and amplify-and forward(AF) protocol. In terms of AF protocol, the relay scales the received signal and transmits an amplified signal to the destination. Main advantage of AF strategy is its simplicity. On the other hand, performance is limited since the noise at the relay is also amplified and forwarded to the destination at the same time.

Relay node selection is one main research direction because it can significantly improve the performance of the cooperative communication system[6-8]. Bletsas A, et al. proposed a relay node selection scheme based on instantaneous channel state information(CSI) between all the nodes[6]. On the condition of instantaneous CSI, he investigated the outage probability of relay selection in both AF and DF cooperative networks[7]. Considering power allocation between the source and the selected relay, ZHAO Yi, et al. put forward an optimal single relay selection scheme in AF cooperative system[8].

In this paper, on the condition of the statistic channel state information, an optimal multi-relay selection scheme is proposed to minimize the average symbol error rate (SER) in AF cooperative network. For the sake of simplicity, we only consider that all nodes are allocated the equal power. Because the proposed selection scheme is based on statistic channel state information, we can change the selected relay nodes periodically. When a relay node is not selected in a period, it can keep silent to save energy.

The remainder of this paper is organized as follows. In section 1, system model is given, and the average SER of AF cooperative communication system employing binary phase shift keying (BPSK) is described briefly. In section 2, we construct the proposed scheme. Finally, we discuss the simulation results in section 3 and give a conclusion in section 4.

1 System model

Fig.1[9]shows the system model of AF cooperative communication network. With the help of N relay nodes which are denoted by r1,r2,…,rN, the source node s communicates with the destination node d. We assume that M relay nodes will be selected from the total N relay nodes in a relay node selection scheme and the selected relay nodes set is BM.

Fig.1 System model of AF cooperative communication network

In phase 1, the source node sends its information to both the destination and the selected relay nodes set BM. The received signals ys,dand ys,riat the destination and the i-th relay can be written as[10]where x is the transmitted information symbol and P0denotes the transmitted source power; hs,dand hs,riare the channel coefficients from the source to the destination and the source to the i-th relay, respectively; ηs,dand ηs,riare additive noise and modeled as zero-mean, complex Gaussian random variables with variance N0.

In phase 2, each selected relay node amplifies its received signal from the source and retransmits it to the destination. The received signal at the destination in phase 2 due to the i-th relay transmission is given as

At high SNR, the SER of the AF cooperative protocol with N relay nodes employing M-PSK signals can be approximated as[10]

where b=sin2(π/M),

Then we derive the average SER of the AF cooperative system employing BPSK and it can be given as

Because

whether X is odd or even, 2(X+1) is even, then the expression q(N) can be simplified as

2 Proposed relay node selection algorithm

In this section, we will discuss how to select the best relay nodes set to minimize the average SER of the system.

For the sake of simplicity, we assume P0=Pri=P, then we can get the average SER by selecting M relay nodes as

It is obvious that the average SER of Boptis minimal in all potential relay nodes sets.

First of all, for the minimum average SER of the system, a parameter named equivalent channel gain (ECG) is defined and the ECG of a relay node riis

Fig.2 Flow chart of global-search algorithm

Assume that

then the number of the best relay nodes set is

and the best relay nodes set is

Eq.(14) can be proved as follows: First of all, we assume that the average SER is minimized by selecting M best relay nodes and then it can be obtained as

According to the above inequalities and Eq.(11), we have

where f(i) is an increasing function with i and viis also an increasing sequence with i. Then we can get

To further simplify the above inequalities, we have

So we can get the result as shown in Eq.(14). The chart of design flow of the proposed algorithm is shown in Fig.3.

In order to evaluate the performance of the proposed algorithm, we will give the other two algorithms. One scheme is the traditional algorithm[8], in which all relay nodes amplify and forward(AAF) the received signal in phase 2, namely, M=N. The other scheme is pre-selecting signal relay node method which minimizes the SER of the system to amplify and forward(SAF) the signal. In this scheme, only the pre-selected relay nodes will be activated.

Fig.3 Flow chart of the proposed algorithm

3 Simulation results and discussion

In this part, we provide simulation results to characterize the attainable SER performance of the proposed scheme.

Fig.4 Comparison of SER of proposed scheme and those of different selected nodes for N=4

Fig.5 Performance comparison of SER in SAF, AAF and proposed scheme

Fig.5 shows that the SER of proposed scheme is lower than that of AAF in low-medium SNR and lower than that of SAF in medium-high SNR. Therefore the SER of proposed scheme is always lower than the other two schemes. If SNR is very low, the number of proposed scheme will be small and the performance will be close to SAF; if SNR is very high, the number of proposed scheme is large and the performance is close to AAF.

4 Conclusion

In this paper, we focus on selecting different relay nodes to optimize the average SER performance in AF cooperative communication system. On the condition of the statistic channel state information, ECG is defined to describe the channel status of two phases in the cooperative process. Then, a multi-relay selection scheme is proposed to minimize the average SER of system. From the proposed scheme, different relay nodes should be selected with different SNR range. There is a certain relationship between the number of relay nodes and the average SER under the different SNRs. It is not that the more relay nodes participate in, the better performance it will be. Simulation results show that the average SER of the proposed scheme is lower than the other schemes.

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date： 2013-08-07

National Natural Science Foundation of China (No.61072088, No.61101113 and No.61201198); Beijing Natural Science Foundation of China (No.4132019, No.4132015 and No.4132007); Doctorate Subject Foundation of the Ministry of Education (No.20111103120017)

CAI Rong-wen (crwwg@163.com)

1674-8042(2013)04-0365-05

10.3969/j.issn.1674-8042.2013.04.014