Control Channel Design for Many-Antenna MU-MIMO

Many-antenna MU-MIMO faces a critical, previously unaddressed challenge: it lacks a practical control channel. At the heart of this challenge is that the potential range of MU-MIMO beamforming systems scales with up to the square of the number of base-station antennas once they have channel state information (CSI), whereas the range of traditional control channel operations remains constant since they take place before or during CSI acquisition. This range gap between no-CSI and CSI modes presents a critical challenge to the efficiency and feasibility of many-antenna base stations, as their operational range is limited to the no-CSI mode. We present a novel control channel design for many-antenna MU-MIMO, Faros, that allows the number of base-station antennas to scale up to 100s in practice. Faros leverages a combination of open-loop beamforming and coding gains to bridge the range gap between the CSI and no-CSI modes. Not only does Faros provide an elegant and efficient control channel for many-antenna MU-MIMO, but on a more fundamental level it exposes flexible, fine-grained, control over space, time, and code resources, which enables previously impossible optimizations. We implement our design on the Argos many-antenna base station and evaluate its performance in bridging the range gap, synchronization, and paging. With 108 antennas, Faros can provide over 40 dB of gain, which enables it to function reliably at over 250 meters outdoors with less than 100 μW of transmit power per antenna, 10 mW total, at 2.4 GHz.