Compensation for In-Phase/Quadrature Phase Mismatch in Coherent Free-Space Optical QPSK Communication Systems

Abstract

The Gram–Schmidt orthogonalization procedure (GSOP) and Löwdin symmetric orthogonalization procedure (SYOP) are the two mainstream algorithms for the compensation of phase mismatch in an imperfect optical 90° hybrid. In this paper, we put forward an algorithm switching orthogonalization procedure (ASOP) according to the quality of in-phase and quadrature signals based on the Q value of the eye diagram with less computation. If the quality of the in-phase and quadrature signals has a significant difference, we use the GSOP and select the signal branch with better quality as the initial reference vector for orthogonalization. If they are of about the same quality, then we use the SYOP. We present computer simulations for a coherent free-space optical (FSO) quadrature phase-shift keying (QPSK) communication system and demonstrate the system improvement that can be achieved using the ASOP. Finally, we also show that the proposed ASOP scheme can contribute to the frequency offset and phase estimation of the FSO system in the environment of atmospheric turbulence.

Design and Performance Analysis of Probabilistically Shaped QAM Signals for Coherent FSO Systems with Gamma-Gamma Turbulence Channels

“…Eventually, the bit streams are obtained by an LPDC decoder and a PS decoder. The coherent detection currents can be expressed as [2]:…”

“…After binary converting, they are encoded together with the remaining binary bits. We utilize the DVB-S2 LDPC code, whose code rate is 2 3 . At the receiver, the LLRs are calculated for bit-metric decoding.…”

“…Free-space optical (FSO) communication has attracted more and more attention due to its large capacity, high security, high data rates and license-free operation [1][2][3]. Nevertheless, the demand for data transmission has shown an explosive growth in the past few years, as a result of the emergence of big data-related technologies.…”