BER performance improvement for OFDM DF infrastructure relay links

OFDM based infrastructure relay station (R), implementing decode-and-forward (DF) relaying, has been included in IMT-Advanced standards, in dual-hop scenarios. Ergodic capacity of such a relay system may be significantly enhanced if ordered subcarrier mapping (SCM) is implemented at R, and, at the same time, bit error rate (BER) performance is improved in this way. However, the level of the achieved BER improvement is very small, and in the region of high signal-to-noise ratios (SNRs) on hops, there is no improvement at all. Thus, in this paper we propose a solution where in the OFDM DF relay system with ordered SCM, one or more subcarriers with the lowest SNRs from both hops are omitted. The analytically obtained BER results, for the assumed uncoded BPSK modulated system, in Rayleigh fading scenario on both hops, are completely verified through simulations. They show that significant BER performance improvement is attained for all the whole range of SNR values on both hops. Moreover, we made comparisons of the proposed solution with other OFDM DF relay systems, and with the case of OFDM direct transmission, thus showing that the achieved level of BER performance improvement justifies additional signal processing introduced at R.