Measurement method of transmitter IQ imbalance in digital coherent communication systems

Abstract

To satisfy the ever-increasing traffic demands in a cost-effective manner, the transmission capacity per wavelength must be increased. The use of high-order modulation and high-baud modulation can increase the capacity per wavelength. However, such signal forms are susceptible to IQ impairments created in the transceiver since symbol distance and symbol duration are extremely short. The IQ impairments can be alleviated by adaptive finite-impulse-response (FIR) filters in the receiver. However, transmitter IQ phase imbalance and transmitter IQ power imbalance cannot be eliminated completely in the receiver-side digital signal processing (DSP). Furthermore, to eliminate transmitter IQ skew, additional filters need to be implemented in the receiver DSP. Therefore, transmitter IQ impairments should be calibrated at the transmitter side to enhance demodulation performance and cost efficiency. In this paper, we propose a novel method for measuring transmitter IQ impairments. In our proposed scheme, transmitter IQ impairments are calculated from the tap coefficients of the adaptive filters adopted in the measurement system. In addition, our proposed scheme can estimate transmitter IQ impairments even when other impairments contaminate the signal. With the estimated value, transmitter-side DSP can easily eliminate the transmitter IQ impairments with digital buffers. Simulations using 64 Gbaud 64-QAM signals show that the estimation error of IQ phase imbalance is less than 1.5 degree, that of IQ power imbalance is less than 0.2 dB, and that of IQ skew is less than 0.05 ps even if the signal is contaminated by other impairments.