Modeling and performance analysis of differential cqi feedback in ofdm cellular systems

Abstract

Reduced feedback schemes play a critical role in contemporary orthogonal frequency division multiplexing systems such as Long Term Evolution (LTE). They ensure that the channel state information required for downlink rate adaptation and scheduling is available at the base station (BS) without overwhelming the uplink with feedback overhead. We present a novel model and analysis of the single-user and multi-user throughput of the widely used differential feedback scheme of LTE, which is called eNodeB-configured subband feedback. In it, a user feeds back a 2-bit differential channel quality indicator (CQI) for each subband relative to a 4-bit wideband CQI, which indicates the rate that the user can decode if the BS were to transmit over the entire system bandwidth. Our analysis applies to many multi-antenna modes. In addition to bringing out several insights, it shows that differential feedback can reduce the feedback overhead significantly while incurring only a marginal reduction in throughput.