Stable Beamforming With Low Overhead for C/U-Plane Decoupled HSR Wireless Networks

Abstract

Millimeter wave (mmWave) directional communications have become a vital 5G technique to achieve the goal of multiple gigabit data rate with low latency, which also hold true for high-speed railway (HSR) wireless networks. However, the high mobility in HSR scenarios causes frequent beam realignments and consequently high feedback overheads, degrading the transmission reliability, and efficiency. In mmWave beamforming systems, besides transmitters (TXs) and receivers (RXs) are also equipped with antenna arrays, which equivalently introduces a new degree of freedom for link adaptation. Based on this observation, a stable beamforming scheme with low overhead, consisting of both long-term TX beamforming and short-term RX beamforming, is proposed. In the long-term TX beamforming, under the premise of satisfying the basic receiver sensitivity, TXs are devised to use as wide beams as possible to extend the serving time of each beam. In the short-term RX beamforming, within a TX beam, RXs autonomously adjust RX beams according to the current channel state information instead of frequently feeding it back to TXs, thereby reducing feedback overheads and accelerating link adaptation. Through this scheme, the received signal to noise ratio is kept at a high and stable level. To meet the strict transmission reliability requirements in HSRs, the control/user-plane decoupled architecture is applied to mitigate the inherent blindness of mmWave directional beams. Theoretical and numerical results show that the proposed scheme can significantly reduce feedback overheads and guarantee the desired performance.