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An orthogonal decomposition of a general wideband space-time frequency selective channel is derived assuming antenna arrays at both the transmitter and receiver. Knowledge of channel state information is assumed at both the transmitter and receiver. The decomposition provides a framework for efficiently managing the degrees of freedom in the space-time channel to optimize any combination of bit-error rate and throughput in single-user or multiuser applications. The decomposition is used to derive efficient signaling schemes and receiver structures for a variety of scenarios. For a fixed throughput system, we investigate a power allocation scheme that minimizes the effective bit-error rate. In addition, a strategy to maximize the throughput under a worst-case bit-error rate constraint is proposed. For multiuser applications, we propose a signaling scheme that achieves orthogonality among users by exploiting the temporal channel modes which are common to all users. The effect of imperfect channel state information at the transmitter is also investigated.