An efficient feedback method for MIMO systems with slowly time-varying channels

The capacity of a multiple-input multiple-output (MIMO) channel can be improved if the transmitter has a knowledge of the channel. In this paper, we propose an efficient and practical feedback method based on parameterization and quantization of channel parameters. The spatial information of the channel at the transmitter, which is represented as a matrix with orthonormal columns, has a geometrical structure. In parameterization, the geometrical structure is exploited to extract a set of parameters that has a one-to-one mapping to the original matrix. In slowly time-varying channels, the parameters are also found to he smoothly changing in time. We employ adaptive delta modulation to quantize and feed back each parameter. The results show that the proposed feedback scheme has a channel tracking feature and achieves a capacity very close to the perfect feedback case with a reasonable feedback rate.