Performance of Analog Feedback in Closed-Loop Transmit Diversity Systems

This paper is concerned with the efficient transmission of channel state information (CSI) in the uplink, and data in the downlink, of a closed-loop transmit diversity system. Two ideas were explored: 1) the employment of analog CSI feedback whereby estimates of the downlink complex gains were fed back periodically to the base-station at a rate of only 1 analog symbol per estimate per frame, and 2) the use of position dependent modulation (PDM) to counter the signal-to-noise ratio variation across a downlink data frame. Simulation results indicated that as far as the bit-error-rate (BER) is concerned, analog feedback is superior to digital feedback because it does not experience unrecoverable quantization errors in the feedback channel estimates. The simulation also indicated that at a given target BER, PDM can attain a substantially higher throughput than fixed modulation.