Two-Way Protocol for Downlink Channel State Information Reporting in FDD Networks

The user equipment (UE) of cellular systems operating in frequency division duplexing (FDD) mode feeds back (report) the channel state information (CSI) to the new generation base node (gNB). We exploit the correlation between the uplink and downlink channels, although they are at different frequencies, to obtain a two-way protocol to report the downlink CSI. First, the gNB estimates the uplink channel and sends a feedforward message to the UE to select the codebook used by the UE to quantize the downlink CSI; then, the UE transmits the index of the quantized code vector in feedback to the gNB. This protocol is modeled as the cascade of two rate-distortion source coding schemes, for the feedforward and feedback signals, where the feedback scheme also has mixed-side information. We design both encoders and compare their performance with existing schemes, including an approach using natural networks.