IQ Imbalance Compensation and Digital Predistortion for Millimeter-Wave Transmitters Using Reduced Sampling Rate Observations

Abstract

This paper expounds on the mitigation of the hardware nonidealities exhibited by millimeter-wave (mm-wave) transmitters (TXs) generating ultra-wideband signals. In particular, it tackles the IQ imbalance and nonlinear distortions attributed to the IQ modulator and power amplifier (PA) stages. For that, an offline frequency-domain-based IQ imbalance detection and compensation method using an interleaved multitone test signal with an interleaved I and Q frequency grid is first proposed. Then, the multitone signal settings were modified to enable the reduction of the required sampling rate of the analog-to-digital converter in the TX observation receiver (TOR) needed for IQ imbalance detection. Subsequently, a rearranged pruned Volterra series digital predistortion (DPD) scheme was exploited to separately set the nonlinearity order and linear and nonlinear memory depths for reducing the number of DPD coefficients needed for mm-wave PA linearization. The application of the proposed IQ imbalance detection and compensation method to a commercial IQ mm-wave modulator is stimulated by an interleaved multitone test signal spanning a modulation bandwidth of 4 GHz allowed for the reduction in the normalized mean squared errors (NMSEs) between the input and output signals from −14.0 to −37.7 dB at a complex baseband sampling rate of 8 GS/s and −37.1 and −36.5 dB with the sub-Nyquist complex baseband sampling rates of 1 and 500 MS/s, respectively. With the full and reduced complex baseband sampling rates of the TOR, the subsequent application of the rearranged DPD scheme was used to linearize a PA under test with an output 1 dB compression point of 23 dBm operating at 30 GHz. The PA stimulated by carrier aggregated orthogonal frequency division multiplexing signals with the bandwidths of 320 and 800 MHz is demonstrated the NMSEs of −36.5 and −32.4 dB and the adjacent channel power ratios of 50 and 45 dBc, using 77 and 66 DPD coefficients, respectively.