Performance enhancement of OFDM non-regenerative Variable Gain relay systems

In this paper the level of bit error rate (BER) performance enhancement achieved by subcarrier mapping (SCM) at the relay (R) station in dual-hop OFDM non-regenerative Variable Gain (VG) relay system, is examined. Closed form BER expressions are derived for the assumed M-QAM (M-ary Quadrature Amplitude Modulation) modulated system, in the case of Rayleigh fading channels on both hops, when two SCM schemes denoted as Best-to-Best SCM (BTB SCM) and Best-to-Worst SCM (BTW SCM) are implemented. The obtained results show that, for all the signal-to-noise ratio values of interest, the analyzed relay system with BTB SCM achieves better BER performance, or at least the BER performance equal to the case when BTW SCM scheme is implemented. Having in mind that BTB SCM scheme also maximizes the system capacity, it is shown that it can be considered as the optimal SCM scheme for the OFDM non-regenerative VG relay system.