We propose differential distributed quasi-orthogonal space-time-frequency coding (DQSTFC) to cater for broadband cooperative networks operating in scenarios where channel state information (CSI) acquisition is impractical. Our differential DQSTFC scheme combines the concept of differential modulation, space-time-frequency (STF) mapping and quasi-orthogonal coding in cooperative networks. From this, we develop a scheme that exploits the achievable diversity gain in the spatial, temporal and frequency dimensions, and guarantees full code rate in non-coherent broadband cooperative networks. Different from existing STF schemes where full code rate is only achievable for cooperative networks with two relay nodes, and unlike other STF schemes that assume coherent signal recovery, our differential DQSTFC scheme is able to guarantee full code rate for cooperative networks with more than two relay nodes while guaranteeing non-coherent signal recovery. We first introduce our STF mapping scheme, we then show how the quasi-orthogonal codewords are constructed from a special choice of signal sets such that full code-rate signals are obtained. Then we present the full differential encoding and decoding setup for cooperative networks with four relay nodes. Unlike other works where the quasi-static channel assumption is adopted for long symbol periods, we show that performance degradation can be reduced when the quasi-static channel assumption is adopted only across adjacent symbols. Simulation results show that our scheme is robust against time-selective and frequency-selective fading compared to other related schemes.
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