Crossover digital predistorter for the compensation of crosstalk and nonlinearity in MIMO transmitters

This paper proposes a novel crossover digital predistorter (CO-DPD) model to compensate for crosstalk and nonlinearity in multiple-input multiple-output (MIMO) radio systems. Crosstalk can take place before or after the power amplifiers, designated herein as nonlinear and linear crosstalk, respectively. This paper demonstrates that, contrary to linear crosstalk, nonlinear crosstalk significantly affects the performance of the digital predistortion algorithm; and, it cannot be embedded and compensated for by the conventional channel matrix inversion algorithm at the receiver side of MIMO links. Based on a parametric study of system-level simulations and measurements, it was found that for a -20-dB nonlinear crosstalk level, the use of a memory multi-branch polynomial predistorter, along with the channel matrix inversion algorithm, bounds the adjacent channel power ratio (ACPR) for a wideband code division multiple access (WCDMA) signal to -46 dbc and the error vector magnitude (EVM) for a world interoperability for microwave access (WIMAX) signal to -43 dB in MIMO links. The proposed CO-DPD was investigated and analyzed for the MIMO transmitter with N = 2, where N is the number of RF front-ends. Its performance was evaluated through measurements, the experimental results obtained show that, in the presence of -20-dB nonlinear crosstalk, the proposed CO-DPD improve the ACPR of the WCDMA signal by 13 dB to -56.81 dBc from those obtained using a conventional digital predistorter. The same improvement was observed in the EVM measurement of the WIMAX signal, where the EVM decreases from -21.22 dB for the conventional DPD to -49.71 dB for the proposed CO-DPD. Index Terms--Antenna crosstalk, digital predistortion (DPD), multiple input multiple output (MIMO), power amplifier (PA), RF crosstalk, wideband code division multiple access (WCDMA), world interoperability for microwave access (WIMAX).