Switched Combining Scheme for Relay Cooperative Diversity Systems

Switched Combining Scheme for Relay Cooperative Diversity Systems

1. Kumaresan,

   PG student, Communication systems,

   Dept of ECE,

   Kumaraguru College of Technology

2. T. Nandana,

PG student, Communication systems,

Dept of ECE,

Kumaraguru College of Technology

AbstractWe consider a cooperative diversity system with a source, a relay and a destination. The relay adopts decode and forward protocol. In this work, we propose a switched diversity combining scheme based on the assigned threshold value. Furthermore, We derive the end-to-end symbol error probability of this scheme with M-ary phase shift keying for a slow, flat Rayleigh fading. Numerical results show that the Switched combining scheme performs better than the conventional selection combining.

Index TermsDecode and forward (DF), M-ary phase-shift keying (MPSK), Rayleigh fading, Scaled Selection,, symbol error probability (SEP).

I.INTRODUCTION

Diversity combining technique is one of the effective methods formulated against Fading in wireless Communication. In this cooperative diversity, users in this network will share their antennas or resources to provide diversity. Most commonly using diversity technique is spatial diversity which involves multiple antennas which be used with enough distance separation between them. So that the receiver can receive multiple independently fading signal paths. By focusing on this problem we came across with one alternative which involves the distributed space diversity named as Cooperative Communication. Cooperative diversity error analysis is carried out in [1],[2].In [5], paired error approach based error analysis of a single relay cooperative diversity

source to destination (SD) link.The values of assigned threshold value is obtained by trial and error approach which minimizes the end-to-end symbol error probability(SEP). We derive the end- to-end SEP of this scheme with help of paired error approach for MPSK Signalling. We assume slow, flat Rayleigh fading environment for this paper work.

1. SYSTEM MODEL

   In this paper cooperative communication system with Single relay is considered. The binary information is modulated by M-ary PSK modulator and broadcasted to destination as well as to the relay over frequency non selective flat fading Rayleigh channel.

   Fig.1 Relaying communication

   m s M

   The received signal is demodulated and detected using DF algorithm. Let the information carrying complex signal have energy 2Es to the constellation S, which is refers to symbol and it is given by,

   S = 2 exp 2( 1) , m=1,2,3,..M

   system has been performed and [1]-[3] assume

   source-to-relay link (SR), relay-to- destination(RD),and source to destination (SD) channels to be statistically independent. In this paper, we propose a hybrid diversity combining scheme which is a combination of switched diversity and a scaled selection combining. At the destination, if the instantaneous SNR of Source to destination (SD) is above a certain threshold, then we choose the source to destination (SD) link. Otherwise, we switch over to the Relay to destination (RD) link. If the scaled version of instantaneous SNR of relay to destination (RD) link is higher than the SD link, then we choose Relay to Destination (RD) link else we revert to

   Where, j = 1. In this transmission system the transmission of symbol is done by two time slots. At first time slot, the source transmits the MPSK symbol to the destination as well as the relay. The complex baseband received signal at the destination and at the relay at the first time slot is represented as,

   rsd = hsd.S + nsd rsr = hsr.S + nsr

   Where hsd and hsr are the random complex fading gains of the source to destination (SD) link and

   source to relay (SR) link. Further, the additive white Gaussian noises of the source to destination and source relay links are nsd, nsr respectively. Whereas the fading gains are formed as symmetric Gaussian random variable with zero Mean and Variance sd and sr respectively. The noises are formed as zero mean complex Gaussian random variable with the variance 2No.

   In second time slot, the relay regenerate the symbol

   ^ and transmit to the destination. The destination receives the relay signal as,

   rrd = hrd. ^ + nrd

2. PERFORMANCE ANALYSIS

   We represent the instantaneous SNR as,

   = s 2,

   o

   In this section, we use the mathematical probability model for DF relaying proposed in [4] to derive a closed-form expression for the outage probability of the SC system.

   The conditional error probability for MPSK is given by,

   where hrd and nrd are fading gains and additive white Gaussian noise of the Relay Destination link respectively. We assume that the destination have

   =

   (1)

   1

   0

   exp

   sin 2

   2

   M

   knowledge about the channel state information (CSI) of all links including relay to destination and source to destination, Meanwhile relay has the CSI of SR link.

   Now we indicate the instantaneous SNRs of the SD, SR, and the RD links, respectively as

   1. SEP for Source-Destination link

      The probability of error in SD link is derived in the form under the condition > or > is given by,

      s

      s

      1

      1 = 1

      sin 2

      2 M ×

      sd =

      2 ,sr =

      2 ,

      0

      o

      o

      1

      rd = s 2

      o

      The corresponding average SNRs are given by,

      Simplify error probability equation by applying integral over x,

      =

      = ES ,

      NO

      1 ( 1) 1

      1 =

      E

      0

      sin 2 1 ×

      2M +

      = = S ,

      NO sin 2 1

      =

      = ES

      NO

      M +

      .

      2

      Where, th is known as assigned average SNR threshold value and E [.] is known as expectation operator. The destination detects received signal and finally detected symbol is obtained from the

      Probability of error in source to destination link > is obtained by applying integral over x and further substitution we get total symbol error probability of S-D link is given as,

      proposed decision rule is,

      max Sh SD

      =

      ,

      ,,

      arg

      = arg

      max

      SD

      SR

      Sh SR

      ,,

      +

      ,,,

      max Sh RD

      arg

      RD

   2. SEP for Source-Relay-Destination Link

   partial to the following conditions,

   SD < th ,

   SD < th and SD < RD ,

   SD < th and SD > RD .

   The probability of error in R-D ( Relay- Destination) link, under the condition of < is given by ,

   =

   (, ) 1

   ×

   2 = 4 1 1 ×

   0

   0

   0 sin 2 4+ 1 sin 2 4+ 1 + 1

   1 1

   2 2

   1 sin 2 4 + 1 + 1

   2

   Lets define 1 , , 3

   1 = 1 2 , 0 1 () ×

3. NUMERICAL RESULTS

   1

   1

   0

   2 = ()

   1

   0

   3 = , () , ()

   , () is probability of error in S-R link when Sk is transmitted in S-R link and it is detected as Sl at the relay .( ) is probability of error in R-D link when Sl is transmitted from relay to destination and it is detected as Sk at the destination node.

   By solving above equations we get,

   1

   Fig. 2. BER Comparison for Non cooperative, Scaled Selection combining and Threshold (switched) combining.

   In this section, we discuss about the error analysis

   0

   1 = 1 1

   sin 2 1

   1 ×

   0

   for DF relaying protocol and Symbol error probability (SEP) measure is carried out . Plots are

   M +

   2

   zd

   done for received signal SEP with varying

   average SNR the decision rule.Fig.2 shows the

   Now applying integral over y in as, By Substituting we get,

   1 = 1 2 , 0 × 1 ×

   results for Non cooperation, conventional Scaled selection combining and Threshold switching selection combining.

   1

   1

   sin 2

   0

   sin

   2 1

   M

   +

   M +

   2

   2

   1

   By applying integral over x and by further substitution, we get total symbol error probability of R-D link is given as

   =

   ,

   =,

   , × ( )

   Where, P1 and P2 is given as,

   1 = 3 1 1 ×

   0 sin 2 4+ 1 sin 2 3+ 1 + 1

   2 2

   Fig 3. shows that the result of BPSK,QPSK and

   1 sin 2 3 + 1 + 1

   8-PSK performance for threshold switched

   2

   selection scheme.

   The plots shows that the BER performance of proposed scheme is improved compared to the conventional selection combining.Fig.3 shows the results of Symbol error probability for various instantaneous SNR values of MPSK.In this evaluated result we use switched threshold decision rule for performance analysis.

4. CONCLUSION

   In this paper, we proposed the threshold switching technique which incorporates the effect of source to relay link to reduce the probability of error. The end to end symbol error probability is derived for M-PSK signalling with DF relaying in flat Rayleigh fading. We have proved that the BER of proposed scheme is reduced than conventional non cooperation and scaled selection combining schemes.

   ACKNOWLEDGEMENT

   The authors thank the management and principal of Kumaraguru College of Technology, Coimbatore for providing excellent computing facilities and encouragement.

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A.Kumaresan received his degree BE degree in Electronics and Communication Engineering from Paavai Engineering college affiliated to Anna university in 2011.He is currently pursuing M.E(Communication Systems) in Department of ECE at Kumaraguru College of technology.

B.T.Nandana received her degree B.Tech degree in Electronics and Communication Engineering from MEA Engineering College affiliated to Calicut university in 2010.She is currently pursuing M.E(Communication systems) in Department of ECE at Kumaraguru College of technology.