Adaptive Optical Processing for Wideband Hybrid Beamforming

Multiuser centralized radio access networks (C-RANs) need to adapt to time-varying radio environments, and adaptation is advantageously split between remote antenna units (RAUs) and centralized baseband units (BBUs) to minimize the required fronthaul throughput. To this end, hybrid beamforming to spatially multiplex signals to/from a pool of users is split between RAU and BBU, and optical analog signal processing can be performed at the RAU. The goal here is to reduce the number of fronthaul channels and the corresponding number of analog/digital converters at the BBU in wideband (>100 MHz) millimeter wave (mmWave) radio communication. An algorithm for hybrid beamforming design is developed, and shown to approach the performance of ideal digital beamforming with a number of fronthaul channels comparable to the number of simultaneously served users rather than with the number of array elements. To cope with the radio-access of multiple users, optical analog processing is made adaptive by tuning delay lines built with optical ring resonators (ORRs). The tunability of analog beamforming by means of thermo-optic phase shifters is designed for time-varying configuration compliant to the 5G new radio specifications and the performance degradation of beamforming transients due to the tunability of optical components is assessed.