Adaptive Decode-and-Forward Cooperative Networks with Multiple Relay Nodes

Abstract

We propose an adaptive cooperative scheme with a multi-relay node that achieves high bandwidth efficiency and achieves better SEP performance. In the proposed protocol, if the quality of the direct channel is better than that of the all channels from relays to destination, the source will transmit directly to the destination. Otherwise, the source broadcasts the signal and then a potential relay will be chosen to help the source. A re-transmission will also occur if the potential relay cannot be detected. The spectral efficiency is first derived by calculating the probability of each mode, i.e., direct and cooperation transmission. Subsequently, the SEP performance of M-PSK modulation for the scheme is analyzed by considering each event where the source transmits data to the destination. Finally, the obtained analytical results are verified through computer simulations. Ⅰ. IntroductionCooperative diversity techniques have attracted considerable and increasing attention over the past several years [1]～ [3]. In cooperative communication, relays are assigned to help a source (S) forward its information to a destination (D); hence, a virtual antenna array can be formed, thereby achieving diversity. Various cooperative diversity protocols have been previously proposed and analyzed [1～4]. However, cooperation almost always occurs in these literature schemes. This may waste bandwidth and power allocated for the relaying strategy if the relay fails to decode the source's information or if the direct channel is inadequate for the destination detecting the data. Previous work by others [5] and also by our group [6], which investigated the adaptive cooperative scheme with one R, showed some benefits regarding the data rate and the reliability of transmission when compared with other published alternatives.In this paper, we focus on the adaptive cooperation scheme with multiple relays in which the time and bandwidth can be saved, which has a higher reliability performance than conventional multi-relay schemes. First, by using the instantaneous SNR as the performance measure, D checks which of the direct path or the relay path is better, and then chooses the path having the higher SNR for the transmission. If the direct channel is better than all other relay channels, then the source will directly transmit data to the destination with all power consumption. In contrast, if the direct link is not the best one, then the selection of the DF strategy will occur with the help of the relays that correctly decode the received signal. In this case, the source broadcasts the signal with a lower power in the first time slot. Afterward, the relay having the highest instantaneous SNR to the destination is chosen to retransmit the re-encoded signal. We also apply the re-transmission at S when a potential relay cannot be defined.Through mathematical analysis, we have studied the proposed scheme by investigating the spectral efficiency and the symbol error probability (SEP) performance. We obtain the spectral efficiency by derivin...