Orthogonal Space-Time Block Codes with Feedback

Abstract

In this paper we consider how Orthogonal Space-Time Block Codes (OSTBC) can be used in the presence of feedback from the receiver to the transmitter. First, we survey how some of the feedback techniques for AWGN channels with fading can be applied to OSTBC. Then we consider a simple scheme with diagonal weighting. The optimal diagonal weighting matrix, which minimizes the probability of error, is derived. The optimal weights depend on the channel and, hence, a feedback becomes necessary. However, the required feedback can be accomplished using log2 (nt) bits, where nt is the number of transmit antennas. Simulations show that relatively significant gains can be achieved with the diagonal weighting scheme. In a practical system it is quite possible that the bits that are fed back to the transmitter are in error. In that case we show that there will be a loss of diversity. To overcome this loss, we develop weighting schemes that are error tolerant and always perform better than the unweighted OSTBC (even in the presence of feedback errors).