Hybrid Precoding Based on Monopulse Ratio for Millimeter Wave Systems With Limited Feedback

Multiuser hybrid precoding in 5G NR mmWave communication system faces significant challenges such as establishing accurate directional radio links and maintaining links for mobile stations (MSs) moving in outdoor environments. A conventional solution relies on finite codebook-based beam sweeping for initial first-stage beam acquisition and subsequent second-stage beam tracking by sweeping adjacent beam pairs. However, such a conventional solution has inevitable residual AoA/AoD errors even after the best beam pair is established in the first stage and incurs nonnegligible overheads to sound adjacent beam pairs for maintaining the best beam pair in the second stage. To overcome these problems, a novel codebook-based two-stage solution that combines a novel beam tracking protocol with a low sounding overhead, a unique receiver structure employing a beam scheduler and a beam tester, and a fine accuracy residual AoA/AoD error estimation algorithm based on the monopulse ratio concept is proposed. The solution is unique because of the receiver structure for the residual AoA/AoD error estimation that exploits the cyclic prefix in OFDM systems, inspired by the monopulse ratio in radar systems. Moreover, it can be applied to MSs with a single RF chain. This solution, using the proposed receiver structure and algorithm, can establish a more accurate directional beam pair right after the initial beam sweeping in the first stage. For beam tracking in the second stage, it estimates the residual AoA/AoD errors of the current best beam pair rather than sweeping adjacent beam pairs, thereby reducing beam tracking overheads. Numerical evaluation and computer simulations show that the proposed solution offers more accurate beam acquisition (i.e., average array gain improvement of several dB) and costs considerably reduced beam sounding overheads compared to the conventional solution. Lastly, a ray-tracing tool is used to demonstrate that our solution is effective in practical channel parameters for outdoor environments.