IQ Imbalance Correction in Wideband Software Defined Radio Transceivers

Abstract

A method for compensation of frequency-selective (FS) in-phase/quadrature (IQ) imbalance of a wideband transceiver is proposed in the paper. It is dedicated for implementation in software defined radio (SDR) cellular base stations. Both transmitter (TX) and receiver (RX) IQ impairments are corrected by complex valued finite impulse response (FIR) filters which are designed based on previously found imbalance correction models. The compensation performance is assessed after the method was implemented in the SDR platform capable of transmitting signals at different central frequencies. At frequencies higher than 3 GHz measured IQ gain and phase error functions exhibit asymmetrical characteristic. In order to reduce the level of asymmetry, adopted IQ gain correction model incorporates odd polynomial elements while the phase correction model includes even polynomial parts. Regardless of utilized central frequency IQ impairments are efficiently compensated. The advantage of the proposed method is low complexity. The method doesn't require specialized hardware for calibration, instead, it uses the RF loopback. At central frequency of 3.5 GHz, transmitter image rejection ratio (IRR) is increased from 20 dBc to 45-50 dBc by applying the proposed method. After receiver imbalance is compensated, the improvement in IRR of more than 25 dBc is achieved.