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Distributed beamforming between separate wireless nodes in a distributed antenna array requires significant coordination of the relative electrical states of the systems to achieve and maintain a phase-coherent state. A principal factor impacting distributed phase coherence is the relative stability of the local oscillators on each node. Ensuring a coherent state requires the distribution of a reference frequency such that all nodes are operating on the same basis frequency. To support distributed beamforming, the reference frequency must furthermore be distributed wirelessly, typically using a phase-locked loop (PLL) on the secondary nodes. In large arrays, the wireless link used for frequency distribution will have limited capacity, necessitating intermittent updates during which the oscillators are locked, and between which their frequencies will drift. The stability of the oscillator therefore plays an important role in the overall performance relative to the update time. In this paper, we discuss the sources of phase noise generated by the reference oscillator and the PLL, and analyze the impacts of phase noise and update interval on distributed beamforming performance. We provide a framework for analyzing distributed beamforming performance from oscillator and PLL parameters in general, and we analyze beamforming performance for two specific cases using nominal high-stability and low-stability voltage-controlled oscillators, with a parametric comparison between their impact on beamforming performance.

Impact of VCO and PLL Phase Noise on Distributed Beamforming Arrays With Periodic Synchronization