Hybrid beamforming in RoF fronthauling for millimeter-wave radio

In this paper we consider a centralized radio access network (C-RAN) architecture with a fully analog fronthaul link between remote antenna units (Raus) and baseband units (BBUs) based on the radio over fiber (RoF) paradigm. Beamforming to separate uplink signals received from a pool of users is splitted between Rau and BBU. Optical beamforming is performed at the Rau site with the aim of reducing the number of fronthaul channels and the corresponding analog/digital converters at the BBU, as mandatory for millimeter wave (mmWave) radio communication. Digital baseband processing completes the beamforming at the BBU and compensates for optical beamforming imperfections. In this paper the minimum mean square error (MMSE) beamforming is considered, paired with all-analog fronthauling for the uplink. Spatial sparsity of the mmWave channel is leveraged for fronthaul compression through phase-only approximation of the dominant eigenvectors of the channel. Validation is on the radio-link capabilities employing multiple antennas to meet the demand for massive MIMO technology.