Achieving near Beamforming Performance with Quantized CSI

Beamforming needs to have the channel state information (CSI) at the transmitter. The CSI is measured at the receiver and is fed back to the transmitter in the quantized form. Quantized CSI affects the BER performance. Casting the problem of quantization as a vector quantization (VQ) problem, we try to quantize the channel information in such a way as to minimize the loss in the performance due to quantization. We examine a reduced parameter feedback of the channel information for beamforming (maximal ratio transmission or MRT) and show that the performance of MRT is better than that of phasealone feedback (equal gain transmission or EGT) systems for identical number of bits used. We also derive a new clustering algorithm which aims at maximizing the beamforming gain and propose a new cost function that minimizes the performance loss in using the quantized beamforming vector for full channel information feedback.