Soft-Decode-and-Forward for Asynchronous Wireless Networks with Doubly-Selective Fading

A new soft-decode-and-forward (SDF) relay strategy for asynchronous wireless networks is presented in this paper. Unlike most previous works that assume simplified or idealized channel conditions, the new SDF relay strategy is designed to operate in practical asynchronous wireless networks with doubly-selective (both time-selective and frequency selective) fading. To combat the impairments caused by doubly-selective fading, as well as to preserve the correlation between signals delivered by the source and the relay, the relay nodes perform soft decoding, and forward reliability information to destination. A block decision feedback equalizer (BDFE) is employed at relay nodes to extract the soft information from the distorted signals. Distributed orthogonal frequency division multiplexing (OFDM) is adopted to cope with node asynchronism and fading time dispersion. Distributed OFDM systems in the literature were usually developed based on the assumption of perfect decoding at relay nodes, and such unrealistic assumption is not required by the new SDF relay strategy. Simulation results show that the new SDF method can flexibly adapt to the dynamic signal qualities at relay nodes, and it always outperforms the decode-and-forward relay strategy with hard decoding at relay.