Limited Feedback Hybrid Precoding for Multi-User Millimeter Wave Systems

29 Jan 2015
The large bandwidths in the mmWave spectrum make mmWave communication desirable for wireless local area networking and also a promising candidate for future cellular systems. Achieving high quality communication links in mmWave systems requires employing large antenna arrays at both the access point or base station (BS) and the mobile stations (MS). For efficient system performance, each BS needs to simultaneously serve a number of MS. Multiplexing different data streams to different users requires some form of precoding be applied to generate the transmitted signal at the BS. In conventional lower frequency systems, this precoding was commonly done in the baseband to have a better control over the entries of the precoding matrix. Unfortunately, the high cost and power consumption of mixed signal components make fully digital baseband precoding unlikely with current semiconductor technologies. Further, the design of the precoding matrices is usually based on complete channel state information, which is difficult to achieve in mmWave systems due to the large number of antennas which would require a huge training overhead, and the small signal-to-noise ratio (SNR) before beamforming. Therefore, new multi-user precoding algorithms that respect the mmWave hardware constraints and require much less complexity need to be developed for mmWave systems.
Leveraging the sparse-nature of mmWave channels and the large MIMO channel characteristics, WNCG graduate student Ahmed Alkhateeb, Delft University Professor Geert Leus, and WNCG Professor Robert W. Heath Jr. developed a low-complexity yet efficient hybrid analog/digital precoding algorithm for downlink multiuser mmWave systems. The proposed algorithm depends on the known (but arbitrary) array geometry and incurs a low training and feedback overhead. Our model assumes that the MS’s employ analog-only combining while the BS performs hybrid analog/digital precoding. The performance of the proposed algorithm was analyzed when the channels are single-path and when the system dimensions are very large. In these cases, the asymptotic optimality of the proposed algorithm, and the gain over beamsteering solutions were illustrated. Their results indicate that interference management in multi-user mmWave systems is required even when the number of antennas is very large. When the feedback channels are limited, the average rate loss due to joint analog/digital codebook quantization was analyzed and numerically simulated. Their results show that the proposed techniques offer higher sum rates compared with analog-only beamforming solutions, and approach the performance of the unconstrained digital beamforming with relatively small codebooks.
This research funded by the National Science Foundation under Grant numbers 1218338 and 1319556 and a gift from Huawei.