On direction-of-arrival (DoA) feedback and precoding for D2D assisted massive MIMO

Effective channel state information (CSI) feedback design is difficult for massive multiple-input multiple-output (MIMO) downlink transmissions in 5G frequency division duplexing (FDD) bands, where partial CSI could be directly exchanged between users through device-to-device (D2D) communications. In this work, we propose Direction of Arrival (DoA) adaptive codebook feedback, where the users exchange their CSI knowledge through D2D communications, estimate the precoder, and transmit it back to the base station (BS). That is, we first formulate an optimization problem of the feedback and precoding strategy on a Grassmann manifold. Using that, we are able to determine an optimal bit allocation of the precoder and the throughput of mmWave massive MIMO systems. Using simulations, we show that, compared with existing schemes, our proposed DoA adaptive codebook feedback scheme is able to achieve much higher throughput, and improve robustness and compatibility.