Analyzing the Effects of PA Variations on the Performance of Phased Array Digital Predistortion

Abstract

This paper shows how digital predistortion of a phased array can benefit from the parametric variations over parallel power amplifiers (PAs). Different antenna configurations are simulated by varying the PA input drive levels by the Monte-Carlo method. The error vector magnitude (EVM) at the steering angle and total radiated adjacent channel power ratio (TRACPR) are used as performance metrics. The simulation results indicate that array predistortion can benefit from the variations between the PAs to improve the EVM significantly. However, at the same time, the TRACPR performance is reduced. This gives a new trade-off to balance between in-band and outoff-band distortion in the fifth generation beamforming systems.

Average Linearization of Phased Array Transmitters Under Random Amplitude and Phase Variations

“…This is because, in the presence of amplitude variations, some of the PAs of the array are driven harder into compression, while others are operated in relatively linear region. Over-the-air nonlinearities of the hard driven PAs are compensated for by the low driven PAs thus improving linearity performance [7]. Figs.…”

“…The aim is to design the DPD that can tolerate the variations without the need of updating the DPD coefficients. The large arrays are able to average out some of the variations [7]. Thus the average behavior of a group of multiple parallel nonlinear elements fed with various amplitude and phase weights is expected to be more predictable compared to a single branch behavior.…”

Digital Predistortion Concepts for Linearization of mmW Phased Array Transmitters

“…That is why 3GPP/NR has specified that EVM and ACPR of mmW 5G transmitter have to be characterized using OTA measurements [4]. 3GPP/NR specifies adjacent channel power (ACP) as total radiated power (TRP) integrated or approximated over the space [4], [5], [6]. This means that ACP direction does not matter for the specifications and hence only the total amount of radiated ACP compared to the total amount of TRP channel power counts.…”

“…in [7]- [14]. Traditional DPD requires Cartesian feedback receiver and several alternative feedback approaches have been presented all the way from switchable feedback [7], [8], [ to combined channel-emulating feedback [6], [12] and even over-the-air (OTA) feedback [13], [15]. Some studies e.g.…”

“…[6], [8], [11], [12] present DPD methods for linearizing the array in the intended beamforming direction, but this does not necessary guarantee the total radiated adjacent channel power ratio (TRACPR) performance. On the other hand, EVM describing the modulation accuracy is specified in the intended beamforming direction and it benefits from the directive linearization schemes as shown in [6]. Hence, there is a trade-off between the total radiated ACPR and directive EVM if both are desired to be improved by the DPD.…”