Joint simple blind IQ imbalance compensation and adaptive equalization for 16-QAM optical communications

We present a novel simple blind adaptive compensation method for in-phase/quadrature (IQ) imbalance in m-ary quadrature amplitude modulation (m-QAM) coherent optical fiber communication systems. IQ-imbalance compensation is integrated into butterfly-structured finite impulse response (FIR) filters, resulting in a significant computational effort reduction in comparison to conventional methods. A reduction in hardware complexity by a factor of about 3 is achieved by the proposed joint method. The proposed structure is experimentally validated with a 40-Gbit/s 16-QAM signal. A 7-dB power penalty reduction is experimentally achieved at a bit error rate (BER) of 10⁻³ in the presence of a 10° phase imbalance, confirming the effectiveness of the proposed algorithm. The equalization capability remains even in the presence of group velocity dispersion along the link, which is numerically confirmed with optical fiber transmission up to 1200 km and 20° phase imbalance.