Blind skew compensation and digital combining with widely linear equalizer for multi-aperture coherent FSO communication

Abstract

Coherent digital combining technology using multiple small apertures has a lot of advantages over doing so with a single large aperture, including the effective mitigation of deep fading under strong turbulence, ease of scalability, and potential higher collected optical power. However, the in-phase/quadrature (I/Q) imbalance and I/Q skew induced by manufacturing imperfections of the coherent receiver front end, and the time mismatch caused by the unequal length of multi-aperture branches will induce a high OSNR penalty and reduce the digital combining efficiency, especially when the system scales to a larger number of apertures, such as massive aperture system. In this work, a complex-valued multiple-input multiple-output (MIMO) 4N×2 widely linear (WL) equalizer is designed to combine multi-aperture signals. Using WL complex analysis, a general analytical model is derived and it is indicated that multi-aperture channel equalization and combining operations can be achieved simultaneously using a MIMO equalizer as long as appropriate tap coefficients are selected. Moreover, the feasibility of the proposed WL equalizer is verified by a 10-Gbps PM-QPSK modulation and a 20-Gbps PM-16QAM modulation four-aperture offline simulated turbulence experiment. The four-aperture combining efficiency of PM-QPSK exceeds 96% even at a single-aperture extremely low OSNR of −6 dB, and 80% for PM-16QAM at a single-aperture OSNR of 0 dB.