The combination of differential space-frequency modulation (DSFM) with orthogonal frequency division multiplexing (OFDM) is attractive for transmission over time-and frequency-selective multiple-input multiple-output (MIMO) channels and detection without the need for channel state information (CSI) at the receiver. It is well known that simple differential detection results in a high error floor already for moderate time and frequency selectivity of the channel. More sophisticated multiple-symbol differential detection (MSDD), which jointly processes multiple received symbols, overcomes this limitation, usually at the price of higher detection complexity. In this paper we consider DSFM for MIMO-OFDM transmission and MSDD at the receiver. Inspired by previous work presented in the literature, we devise a novel DSFM scheme, which makes use of spatial and/or spectral (multipath) diversity and is particularly suited for MIMO-OFDM and power-efficient, low-delay MSDD. We further investigate the application of a two-dimensional (2D) observation window to MSDD (2D-MSDD) in order to exploit channel correlations in both time and frequency direction. We develop a representation of the detection problem that is amenable to tree-search decoding, whose application leads to a tremendous reduction in MSDD complexity or "fast" MSDD. An analytical approximation of the symbol-error rate of 2D-MSDD for MIMO-OFDM under spatially correlated fading is derived, which enables quick and accurate performance evaluations. Numerical and simulation results corroborate the efficacy of our approach and show that power efficiency close to that of coherent detection with perfect CSI is feasible in all standard fading scenarios at reasonable decoder complexity.
Index TermsOrthogonal frequency division multiplexing (OFDM), multiple-input multiple-output (MIMO) transmission, differential space-frequency modulation (DSFM), differential space-time modulation (DSTM), multiple-symbol differential detection (MSDD), tree-search decoding, sphere decoding.