Joint Design of Digital and Analog Processing for Downlink C-RAN with Large-Scale Antenna Arrays

摘要

In millimeter-wave communication systems with large-scale antenna arrays,conventional digital beamforming may not be cost-effective. A promisingsolution is the implementation of hybrid beamforming techniques, which consistof low-dimensional digital beamforming followed by analog radio frequency (RF)beamforming. This work studies the optimization of hybrid beamforming in thecontext of a cloud radio access network (C-RAN) architecture. In a C-RANsystem, digital baseband signal processing functionalities are migrated fromremote radio heads (RRHs) to a baseband processing unit (BBU) in the "cloud" bymeans of finite-capacity fronthaul links. Specifically, this work tackles theproblem of jointly optimizing digital beamforming and fronthaul quantizationstrategies at the BBU, as well as RF beamforming at the RRHs, with the goal ofmaximizing the weighted downlink sum-rate. Fronthaul capacity and per-RRH powerconstraints are enforced along with constant modulus constraints on the RFbeamforming matrices. An iterative algorithm is proposed that is based onsuccessive convex approximation and on the relaxation of the constant modulusconstraint. The effectiveness of the proposed scheme is validated by numericalsimulation results.