Event-Triggered Synchronization for Mobile Distributed Transmit Beamforming

Distributed transmit beamforming offers $N^{2}$ -fold increase of the received power with $N$ transmitters forming a virtual antenna array, however, it requires the transmitters to be synchronized in-phase without sharing a common wired clock reference. One popular approach to establish such synchronization is based on the receiver periodically providing feedback on phase and possibly frequency offsets of each individual transmitter through a low-rate feedback channel. In this work, we propose a new phase- and frequency-synchronization scheme performing synchronization in an aperiodic manner, when the receiver detects the asynchrony has exceeded allowed limits. We then prototype the proposed scheme in software-defined radio and evaluate its performance with field trials. The proposed scheme significantly reduces the demand for feedback channel bandwidth even when a receiver is mobile, while keeping the phase and frequency synchronization sufficient to achieve the received signal-to-noise ratio (SNR) improvement predicted theoretically.