Adaptive Distributed Transceiver Synchronization Over a 90 m Microwave Wireless Link

We present an adaptive approach for synchronizing both the phase and frequency of radio frequency transceivers over 90 m wireless links to support distributed antenna array applications. To enable phase-coherent distributed operation between separate wireless nodes, the oscillators in the transceivers must operate at the same frequency, and their phases must be appropriately aligned to support beamsteering. Based on a spectrally sparse waveform, a self-mixing frequency demodulation circuit, and an adaptive control loop, we present a system capable of synchronizing the RF oscillators in separate transceivers over distances of 90 m. The approach is based on a multitone waveform designed for joint internode ranging and frequency transfer. A frequency reference is modulated onto one signal of a two-tone waveform transmitted by the primary node and is then demodulated and used to lock the oscillator of the secondary node. The secondary node retransmits the two-tone signal, which the primary node uses for a high-accuracy range measurement. From this range, the phase of the two transceivers can be aligned to support beamforming. We furthermore implemented an adaptive phase control approach to support high-accuracy phase coordination in changing environmental conditions. We demonstrate continuous high-accuracy links over a 90 m distance in an outdoor environment for durations up to seven days, demonstrating sufficient phase coordination in changing weather conditions to support distributed beamforming at frequencies up to 3 GHz.