Adaptive channel feedback for coordinated beamforming in heterogeneous networks

In this paper, a channel direction information (CDI) feedback strategy is proposed to assist coordinated multi-point beamforming (CoMP-CB) between a macro base station (MBS) and multiple pico BSs (PBSs) in heterogeneous networks, where the numbers of bits for feedback are adaptive to the average channel gains of multiple users. To maximize the average sum rate achieved by the users with given feedback resource, we optimize the overall number of feedback bits for each user and the codebook sizes for quantizing the per-cell CDIs of each user. To provide feasible feedback scheme for practice use, a low complexity two-step solution is developed from deriving the lower bounds of the per-user average rate. In the first step, we optimize the number of overall feedback bits for each user, which provides a tradeoff between the accuracy of CDI quantization and the reliability of feedback transmission. In the second step, we optimize the bit allocation for quantizing the per-cell CDIs, which accommodates the difference in transmit powers and antennas of the MBS and PBSs and in the channels of macro and pico cells. Two adaptive feedback schemes are developed from two lower bounds. By characterizing the degree of freedom (DoF) achieved by each user, we show that the systems using the proposed adaptive feedback schemes are no longer interference-limited. The maximum per-user DoFs achieved by the proposed schemes depend on the per-antenna feedback resource constraint for each user. Simulation results show significant performance gain of the proposed scheme over NonCoMP and CoMP-CB with fixed codebook size.