Efficient self-calibration of frequency-dependent I/Q-imbalance in direct-conversion OFDM transceivers

For communication systems with direct-conversion architecture, I/Q-imbalance can be a crucial performance limiting effect. Furthermore, as the datarates of the wireless communication systems increase, larger transmission bandwidths (BW) and high-order modulated waveforms are applied. As a result, the systems are sensitive to frequency dependent I/Q-imbalance and the corresponding calibration is more challenging. Fortunately, efficient signal equalization is possible with OFDM, which is a widespread technique for broadband high datarate systems. Moreover, OFDM transceivers may have symmetrical transmitter (Tx) and receiver (Rx) structures, typically in ad-hoc networks like WLAN. In this case, efficient and separate I/Q-imbalance parameter estimation in the Tx and the Rx is possible. Based on the considerations above, a new pilot-based self-calibration scheme is presented in this paper, which requires relatively low hardware overhead and signal processing complexity. Its effectiveness was verified by numerical simulations.