ACPR Improvement in Large Phased Arrays With Complex Modulated Waveforms

Abstract

Phased array transmit systems use a number of antennas to combine RF power amplifiers (PAs) 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, it is found that random variations in the nonlinear components (AM–AM and AM–PM) between the channels improve the nonlinearity of the overall array as the number of elements increases. In this article, detailed measurements are done on an 8- to 256-element phased array and show that the adjacent channel power ratio (ACPR), which is one metric of linearity, improves with the number of elements at a fixed backoff from the 1-dB compression point. Also, for a100-Mbaud 64-QAM signal and a fixed ACPR of −32 dBc, a 256-element phased array can be operated at $P_{1\,\text {dB}} - 2$ dB, while an eight-element phased array needs to be operated at $P_{1\,\text {dB}} - 4$ dB to achieve the same ACPR level. These improvements were found to occur at broadside (no scan) for any scan angle and for different modulated waveforms (64-QAM, 16-QAM, QPSK, and so on). This result has great implications on the overall efficiency of 5G phased arrays since it implies that large phased arrays can be operated at less backoff than the small phased arrays (or single antennas with high-PAs). Thus, in practical implementations and taking the ACPR as the figure of merit, large phased arrays are ~2 dB more efficient than what is predicted by standard system simulations that assume the same nonlinear response for all the phased array channels.