Submitted to IEEE Communications Magazine in October 2003.
Feedback in a communications system can enable the transmitter to exploit channel conditions and avoid interference. In the case of a multiple-input multiple-output (MIMO) channel, feedback can be used to specify a precoding matrix at the transmitter, which activates the strongest channel modes. In situations where the feedback is severely limited, important issues are how to quantize the information needed at the transmitter and how much improvement in associated performance can be obtained as a function of the amount of feedback available. We give an overview of some recent work in this area. Methods are presented for constructing a set of possible precoding matrices, from which a particular choice can be relayed to the transmitter. Performance results show that even a few bits of feedback can provide performance close to that of full transmit channel knowledge. We also compare the performance of the constructions for systems with a small number of antennas with asymptotic performance results derived for a system with a large number of antennas. The asymptotic analysis can be used to predict achievable performance gains as a function of system parameters.
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