Unquantized and Uncoded Channel State Information Feedback on Wireless Channels

In this paper we consider a system where a mobile terminal obtains the downlink channel state information (CSI) and feeds it back to the base station using an uplink feedback channel. If the downlink channel is an independent Rayleigh fading channel, then the CSI may be viewed as an output of a complex independent identically distributed Gaussian source. Further, if the uplink feedback channel is AWGN, it can be shown that that unquantized and uncoded (UQ-UC) CSI transmission (that incurs zero delay) is optimal in that it achieves the same minimum mean squared error (MMSE) distortion as a scheme that optimally (in the Shannon sense) quantizes and encodes the CSI while incurring infinite delay. Since the UQ-UC transmission is suboptimal on correlated wireless channels, we propose a simple linear CSI feedback receiver that can be used in conjunction with the UQ-UC transmission while still retaining the attractive zero-delay feature. We provide bounds on the performance of the UQ-UC scheme and also explore the performance in multiple antenna multiuser wireless systems.