Relay selection scheme with adaptive cyclic prefix for cooperative amplify-and-forward relay

In cooperative amplify-and-forward (AF) relay networks, the system performance over multipath channels is impacted by both frequency-selective fading and delay spread. However, most relay selection (RS) schemes choose the best relay node only based on the channel gain while ignoring the delay spread effect on the performance. If orthogonal frequency division multiplexing (OFDM) is used in AF relay networks, the cyclic prefix (CP) length has to be extended to tolerate the accumulated delay spread from source via relay to destination due to the lack of the channel compensation at relay nodes. A long CP, which is the transmission overhead, redeces both the effective data transmission throughput and the overall system transmission efficiency. Therefore, an appropriate RS scheme in cooperation multiple-relay networks should not only enhance the overall transmission reliability but also minimize the relay overhead. To this end, we propose a variable-CP based RS scheme for AF relay networks to maximize the transmission efficiency by dynamically choosing the most suitable relay node. In the proposed scheme, a normalized effective throughput is defined as the selection criterion which depends on both the end-to-end channel gain and the accumulated delay spread. Based on this criterion, the best relay link is selected by achieving the tradeoff between the transmission reliability and overhead. Both the theoretical analysis and simulation results show that when the channel delay spread varies, the proposed scheme can dramatically improve the the effective data transmission throughput compared to the maximum signal-to-noise-ratio schemes with variable/fixed CP.