Downlink Optimization in Cloud Radio Access Networks with Hybrid RF/FSO Fronthaul

Abstract

This paper studies the downlink of a cloud radio access network (C-RAN) that incorporates a baseband central processor (CP), multiple remote radio units (RUs), and a network of wireless fronthaul links that connect the RUs to the CP. The fronthaul network utilizes dedicated point-to-point free-space optical (FSO) links along with a broadcast radio frequency (RF) channel. The spectrum of the RF channel is also utilized for downlink transmission from the RUs to the mobile users. That is, the available RF spectrum is time-shared (in a half-duplex manner) among the fronthaul and downlink. The data symbols intended for different users are linearly precoded at the CP in the form of quantized in-phase and quadrature (IQ) samples. These samples are compressed then delivered via the fronthaul network to the corresponding RUs. The RUs, in turn, perform decompression and IQ modulation, before broadcasting their RF signals to the users. We focus on the joint design of the linear precoders, quantizers, and capacity of the RF fronthaul links, along with the time allocation of the RF spectrum, in order to maximize the weighted sum-rate of the users, subject to power constraints and capacity limitations of the hybrid fronthaul network. The resulting problem is nonconvex and difficult to handle. Therefore, we propose a computationally-tractable algorithm that utilizes line search and alternating convex optimization in order to obtain a high-quality suboptimal solution. We provide numerical examples to demonstrate the performance of the proposed algorithm under different weather conditions. We also show the performance gain of hybrid RF/FSO fronthaul, as compared to FSO-only fronthaul, during unfavorable weather conditions.