Linearity and Efficiency Improvements in Phased-Array Transmitters with Large Number of Elements and Complex Modulation
Phased array transmit systems use several antennas to combine RF power-amplifiers in free space. If all the RF channels are identical, then the transmit spectrum in the far-field, including both linear and nonlinear components, would be a scaled version of the output spectrum of each channel. However, random variations in the nonlinear components (AM-AM and AM-PM conversion) between the channels improves the nonlinearity of the overall array as the number of elements increases. In this paper, measured results are used to show that the adjacent channel power ratio, which is one metric of linearity, improves with the number of elements at a fixed backoff from the 1 dB compression point. Also, for a 100 Mbaud 64QAM signal and a fixed ACPR of -32 dBc, a 256-element phased-array can be operated at P1dB-2 dB, while an 8-element phased-array would need to be operated at P1dB-4 dB for the same ACPR level. This work has great implications on the overall efficiency of 5G phased arrays since it implies that large phased-arrays can be operated at less back-off than small phased-arrays (or single antennas with high power amplifiers). Thus, in reality and taking the ACPR as the figure of merit, phased-arrays are 2 dB more efficient than what is predicted by standard system simulations which assume the same non-linear response for all the phased-array channels.