On the Viability of Using a Subset of Transmitter- Observation Receivers for Training a Common DPD in Fully Digital MIMO Transmitters

This letter lays the foundation for reducing the required number of transmitter-observation receivers (TORs) for training digital predistortion (DPD) in fully digital massive multiple-input, multiple-output (MIMO) transmitters. Specifically, it investigates the viability of applying the same, common set of DPD coefficients to linearize all RF chains in fully digital massive MIMO transmitters. First, it is shown that if all RF chains are operated at the same output power, the common set of DPD coefficients can be found by simply averaging the coefficients obtained by training each RF chain on its own. This suggests that only a few chains may be needed for training provided the chains are a representative sample. Experimental results are then conducted where one and three chains are used for training. It is found that when training for one chain, significant variations in normalized mean square error (NMSE) and adjacent channel power ratio (ACPR) of up to 9 dB across the chains are realized. For training with three chains, the common set of DPD coefficients can reduce the variation to 1–2 dB. Finally, after over-the-air (OTA) combining, excellent linearization performance is found for three chains.