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

Xueliang Li,Shijie Gao,Tianwen Geng,Ruotong Tian,Yucong Gu

doi:10.3390/app11062543

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.

Highlights

Optical coherent detection, in conjunction with electrical digital signal processing (DSP), is considered an essential technology for next-generation optical communication systems [1,2]
The performance of free-space coherent optical communication systems is seriously impaired by imperfections of the systems, such as phase noise induced by the transmitter laser, local oscillator, and wavefront distortion, as well as an imbalance between the in-phase (I) and quadrature (Q) branches in the front end of the coherent optical receiver
We mainly study the effects of phase mismatch, which means that the phase difference of IQ signals may be deviated from 90◦ because of the imperfect optical hybrid

Summary

Introduction

In conjunction with electrical digital signal processing (DSP), is considered an essential technology for next-generation optical communication systems [1,2]. Compared to traditional direct detection (DD) systems, coherent detection presents the following advantages [3,4,5] It has a higher spectrum efficiency and more flexibility to advanced modulation formats such as M-array phase-shift keying (MPSK) and quadrature amplitude modulation (MQAM), followed by conventional direct detection. The performance of free-space coherent optical communication systems is seriously impaired by imperfections of the systems, such as phase noise induced by the transmitter laser, local oscillator, and wavefront distortion, as well as an imbalance between the in-phase (I) and quadrature (Q) branches in the front end of the coherent optical receiver. A key component to provide the phase diversity of an optical optical coherent receiver.

Methods

Results

Conclusion