Optimization of Training and Feedback for Beamforming Over a MIMO Channel

Abstract
We examine the capacity of beamforming over a block Rayleigh fading multi-input/multi-output (MIMO) channel with finite training for channel estimation and limited feedback. A fixed-length packet is assumed, which is spanned by T training symbols, B feedback bits, and the data symbols. The training symbols are used to obtain a minimum mean squared error (MMSE) estimate of the channel matrix. Given this estimate, the receiver selects a transmit beamforming vector from a codebook containing 2B i.i.d. random vectors, and relays the corresponding B-bit index back to the transmitter. We derive bounds on the large system capacity, i.e., as the number of transmit antennas Nt rarr infin and receive antennas Nr rarr infin with fixed ratio Nt/Nr. The bounds are used to show that the optimal T, which maximizes the capacity, increases as Nt/ log Nt, whereas the optimal B increases as Nt/log2 Nt.

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