Synchronizing Local Oscillator Frequencies in Beamforming Nodes

Collaborative Beamforming (CB) which is a cooperative transmission technique where nodes act as a virtual antenna array can achieve an order of magnitude in improved signal strength, directional signal transmission, energy efficiency and node redundancy when practically deployed. However, signals arrive out of phase at the target receiver due to unsynchronized clock frequencies of local oscillators (LOs). In this article, an iterative procedure synchronizes the common message signal from multiple transmitters to the receiver. LO frequency synchronization and random phase adjustments are carried out by CB nodes at each time slot to increase the signal to noise ratio (SNR). Mean square error (MSE) of the frequency offset of the feedback signal are estimated with the developed LO estimation algorithm. This algorithm is seen to have tracked the modified Cramer-Rao lower bound (MCRLB) more closely for SNR between 7 dB and 20 dB. An improvement of 99.57% and 93.74% is recorded for LO frequency offset MSE for CB node 1 and CB node 2, respectively, when compared to another estimator. When beamforming is enabled, the percentage increase in amplitude between single and combine node with convergence achieved is 66.67%.