Performance Improvement of Dual-Hop OFDM Decode-and-Forward Relay System

Ergodic capacity of dual-hop orthogonal frequency division multiplexing (OFDM) based decode-and-forward (DF) relay system may be significantly enhanced if ordered subcarrier mapping (SCM) is implemented at relay station (R). We proved earlier that this SCM scheme also brings certain bit error rate (BER) performance improvement to assumed system. However, the level of the achieved BER reduction is very small, with no reduction at all for the high signal-to-noise ratios (SNRs) on both hops. Therefore, in this paper we propose a modification of the SCM scheme where one or more subcarriers with the lowest SNRs from both hops are omitted before ordered SCM is performed at the relay station of an OFDM DF relay system. For the scenario with Rayleigh fading statistics on both hops, and no direct communication between the source of information (S) and destination (D), we derived a closed form BER relation of the assumed uncoded BPSK modulated relay system. The analytically obtained BER results, which are verified through simulations, show that the proposed modified SCM scheme brings significant BER performance improvements for all SNR values on both hops. Additionally, in order to provide analytical tool for the assessment of ergodic capacity enhancement achieved through ordered SCM, we derived ergodic capacity expression of the dual-hop OFDM DF relay system with ordered SCM.