Digital Predistortion of Phased-Array Transmitter With Shared Feedback and Far-Field Calibration
Digital predistortion (DPD) of a phased array requires that multiple transmit paths must be measured by a feedback (FB) receiver (Rx). In this article, we propose an FB concept for DPD in a time-division-duplex (TDD) phased arrays. We use a single FB line to collect the waveform samples from the parallel transmit paths to the FB Rx. The TDD switches are used to enable and disable individual transmit paths. The FB is calibrated by comparing the FB outputs from individual power amplifiers (PAs) to over-the-air (OTA) measurement reference performed with a frequency-modulated continuous-wave (FMCW) signal. The individual PA measurements are postequalized before the DPD training to model the far-field signal. Three alternative strategies are considered for training the DPD through the calibrated FB line and compared with the OTA DPD. The performance is verified by OTA measurements of a 28-GHz phased-array transmitter and with fifth-generation new radio waveform in terms of total radiated adjacent channel power ratio (TRACPR), cumulative absolute ACP (CACP), and main lobe error vector magnitude (EVM). The best EVM and ACPR performance is achieved by the strategy where the individual PA responses are treated independently. The methods were comparable to the OTA DPD performance, achieving all <-37-dB TRACPR, -29-dBm/MHz CACP, and ≤7 % EVM.