Spatial modulation over multiple-input / multiple-output (MEMO) communication channels has been a rich area of research for terrestrial wireless communication and, more recently, underwater acoustic communication. By appropriately including spatial as well as temporal degrees of freedom in the signal design, it has been shown that one can potentially achieve significant capacity gains, reduce power requirements, and increase spectral efficiency. The benefits are analogous to having additional bandwidth and, as such, are well suited for bandwidth-limited conditions, as opposed to power-limited conditions. Spatial modulation uses separate, resolvable propagation paths between a transmitter and receiver array as parallel communication channels and, as such, requires both availability of arrays at both ends of the link and a propagation channel with some level of multipath. The combination of rich, complex reverberation and the inherent bandwidth limitations of the underwater acoustic telemetry channel make it well suited for application of spatial modulation techniques. This discussion will focus on issues relating to design of the adaptive receiver algorithm for spatially modulated signals. A principal trade-off that has been studied extensively for cellular systems is one between sequential versus joint detection and estimation approaches as exemplified by the well-known D-BLAST versus V-BLAST approaches to space-time coding proposed by Lucent Technologies. While joint detection may improve performance, the added algorithm complexity may impose untenable practical constraints on processor speed and latency.
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