On the optimal solution for BER performance improvement in dual-hop OFDM relay systems

In this paper bit error rate (BER) performance of DPSK modulated dual-hop OFDM based relay systems implementing ordered subcarrier mapping (SCM) at relay station (R) is analyzed. We propose a new solution, where in the OFDM amplify-and-forward (AF) relay system with fixed gain (FG) at the R station, one or more subcarriers having the lowest signal-to-noise ratios (SNRs) on both hops are first omitted, and then the SCM is performed. Analytical and simulation results have shown that in this way significant BER performance improvements are attained in the regions of medium and high SNRs on both hops. Then, we compare BER performances of the most common OFDM relay systems with SCM, i.e. the systems applying decode-and-forward (DF), AF variable gain (AF VG) relaying, and AF FG relaying, where the same solution based on omitting the worst subcarriers on both hops is assumed. Through these comparisons, an optimal solution for BER performance improvement in the given channels' conditions, is defined.