Distributed Transmit Beamforming Based on Frequency Scanning

Distributed beamforming is a form of collaborative communication, in which wireless nodes adjust the phases of their transmitted signals so that the signals are combined coherently at the destination. This enhances the received signal power and extends the communication range. In this paper, we propose a transmit beamforming method that does not require any explicit feedback information from the receiver for the phase adjustment. Instead, sensors use phase-locked loops (PLL) to synchronize their carriers to the received reference signal broadcast by the destination / receiver. Based on the fact that the change of phase over some distance depends not only on the distance but also on the frequency, the destination indirectly adjusts the sensors' phase by changing the frequency of the reference signal. The destination scans all the frequencies within a feasible range to find the optimal frequency at which the received power gets its maximum. The advantage of this method is that it does not require any direct interaction among the sensors and feedback between the sensors and the destination. Every sensor adjusts its own phase independently and simultaneously. Therefore, the proposed method has good scalability and is suitable for the distributed system. We also derive some performance analysis of the method and present simulation results that validate the analysis.