Space-time zero-forcing pre-equalization for synchronous dispersive multi-user channels

There are several forms of precoding that can be used to defeat inter-block interference (IBI) in multipulse communication techniques such as OFDM; techniques include cyclic prefixes and (most simply) inter-block time gaps. The dispersion caused by multipath requires some kind of accommodation by using available resources; prefixes use extra time and/or energy. The goal of this paper is to show how extra space (via multiple transmit antenna elements) can also be used. This paper first introduces a synchronous dispersive multi-user multi-input multi-output (MEMO) channel model. Due to dispersion, any single SISO (single-input single-output) block is full column-rank and thus has an empty null space. However, under certain conditions, when enough receive antenna elements are employed, the null space becomes richer; ultimately, the matrix becomes full row-rank. Based on this idea, we identify sufficient conditions for zero-forcing of the inter-block interference. Finally, we propose an approach for the construction of near-optimum signal sets to achieve zero-forcing precoding.