Beam-Level Frequency-Domain Digital Predistortion for OFDM Massive MIMO Transmitters

In this article, a novel digital predistortion (DPD) solution for fully digital multiple-input–multiple-output (MIMO) transmitters (TXs) is proposed. Opposed to classical DPD solutions that operate at TX chain or antenna level, the proposed DPD operates at the stream or beam level, and hence, its complexity is proportional to the number of spatially multiplexed streams or users rather than to the number of antennas. In addition, the proposed beam-level DPD operates in the frequency domain (FD), which makes it possible to provide flexible frequency-dependent linearization of the transmit waveforms. This feature is very well suited to the linearity requirements applicable at the 5G new radio (NR) frequency-range 2 (FR2), where the inband quality requirements commonly limit the feasible TX power and can also vary significantly within the channel bandwidth depending on the utilized data modulation and coding schemes of the different frequency-multiplexed users. Altogether, the proposed solution enables a large reduction in the computational complexity of the overall DPD system, and its performance is demonstrated and verified through both experimental and simulation-based results.