Outage performance analysis and optimization of OFDM-based relaying protocols

This paper is dedicated to studying the outage performance of half-duplex relaying protocols that convey data over slow-fading multipath channels using OFDM transmission. It also addresses the optimization of these protocols i.e., time and power allocation, with respect to this outage performance. In the context of communications in slow-fading scenarios, the outage probability P_o is the relevant information-theoretic performance metric. However, it is generally hard to derive the expression of P_o for all possible values of the Signal to Noise Ration (SNR) ρ, even for flat-fading channels. This is why we focus on the high SNR regime. It is known that when a single relay operates in this regime over flat-fading channels, ρ²P_o usually converges to a constant that has been referred to in the literature as the “outage gain”. In this paper, we extend this result to the case of frequency-selective channels having L independent channel taps in their discrete-time base-band equivalent model. We show that for these channels ρ^2LP_o converges to a non-zero outage gain ξ that provides crucial information about the behavior of the outage probability. We compute ξ for three different relaying schemes: the non-orthogonal Decode-and-Forward (DF), the Decode-or-Quantize-and-Forward (DoQF) and the Compress-and-Forward (CF). We next show that the DoQF outperforms in terms of outage gain the other two protocols over frequency-selective channels as well as in flat-fading scenarios. Finally, we numerically compute the optimal time and power allocation that minimizes the derived outage gain of the considered protocols.