I. Introduction
The advantages of using multiple antennas at both the transmit and receive ends of a wireless communications link have recently been noted [1], [2]. A number of space-time codes (STCs) have been proposed that exploit the potential for increased throughput and diversity that such systems offer. For most algorithms, these gains can only be realized when that the multiple-input multiple-output (MIMO) channel separating the transmitter and receiver has been identified. While training data can be used to estimate the channel, this approach consumes precious bandwidth and reduces throughput. One approach to overcoming this difficulty is the use of differential STCs [3], [4], although such techniques incur a 3-dB penalty in SNR.