Modeling and Analysis of Differential CQI Feedback in 4G/5G OFDM Cellular Systems

Reduced feedback schemes are crucial in achieving the high data rates expected of orthogonal frequency-division multiplexing-based 4G and 5G cellular systems. They ensure that the feedback overhead required for acquiring the channel-state information to enable downlink scheduling and adaptive modulation and coding at the base station (BS) does not overwhelm the uplink. We present a novel modeling and analysis of the single-user and multi-user throughputs of the differential feedback scheme that is used in both 4G and 5G standards. In this feedback scheme, a user feeds back a 4-bit wideband channel quality indicator (CQI), which indicates the rate that the user can decode if the BS was to transmit to it over the entire system bandwidth, and a 2-bit differential CQI for each subband relative to it. Our analysis incorporates co-channel interference, different single-stream multi-antenna modes, and different schedulers, which cover a wide range of the tradeoff between the throughput and user fairness. It brings out several insights such as the increase in the throughput as the correlation between subbands increases and how differential feedback reduces the overhead significantly while only marginally reducing the throughput.