Virtual Full-Duplex Distributed Spatial Modulation: A New Protocol for Cooperative Diversity

Spatial modulation, a multi-antenna technology which uses the antenna index as an additional means of conveying information, is an emerging technology for modern wireless communications. In this paper, a new distributed version of spatial modulation is proposed which achieves virtual full-duplex communication (VFD-DSM) in order to increase system throughput. This throughput improvement is achieved by allowing the source to transmit new data while the relays implicitly forward the source's data in every time slot (via the index of the active relay) while explicitly transmitting their own data (via a conventional modulation technique). Motivated by the achievable throughput improvement with VFD-DSM, two maximum a posteriori (MAP) detection methods are proposed for implementation at the destination node: the first, called local MAP, is based on processing the signals received over two or three consecutive time slots, while the second, called global MAP, is based on symbol-error-rate optimal detection over an entire frame of data. For each MAP detection method, an error-aware version of the detector is also proposed which takes into account the demodulation error rate at the relays; this can achieve an extra BER advantage at the cost of additional complexity and an increased channel state information (CSI) requirement. Simulation results demonstrate that the proposed VFD-DSM protocol can provide an improved BER compared to the baseline protocol of successive relaying, while also providing a significant increase in the overall throughput since the relays can forward the source symbols while simultaneously transmitting their own data. The proposed VFD-DSM detectors are shown to provide a range of design choices offering different tradeoffs between BER performance and computational complexity. Finally, the impact of the data frame length in VFD-DSM on the error rate performance and system throughput is investigated, and it is shown how to choose the optimal frame length for a given target system throughput.