Research On Techniques Of Real-Time Coherent Optical Communication With High Sensitivity

High-sensitivity coherent optical communication technology has broad application prospects in space optical communication and unrepeatered optical transmission systems.The m-ary pulse-position combined quadrature phase-shift keying modulation format(mPPMQPSK)has attracted widespread attention in the field of coherent optical communication due to its advantage of high sensitivity.This paper studies the real-time implementation of mPPM-QPSK modulation technology in optical communication systems based on Field Programmable Gate Array(FPGA).The main research content and innovative work of this paper are as follows:1)A survey was conducted on the current research status of highsensitivity real-time digital coherent reception technology and its digital signal processing algorithms both domestically and internationally.The basic principle of high-sensitivity modulation formats was introduced,and the theoretical sensitivity curves of several modulation formats such as QPSK and 4PPM-QPSK were derived.Comparative analysis was conducted,and the results showed that 4PPM-QPSK had a theoretical sensitivity advantage of about 2.5dB compared to QPSK.2)The basic process of digital signal processing algorithm for the receiver of 4PPM-QPSK in optical communication system was introduced,and a 4PPM-QPSK optical communication numerical simulation system was built.The simulation analysis of 4PPM-QPSK modulation in optical communication system was conducted and compared with the theoretical sensitivity curve.3)Complete the hardware programming of the digital signal processing algorithm of 4PPM-QPSK modulation with Verilog language at the receiver in an FPGA-based optical communication system.A real-time hardware-adaptive time slot synchronization scheme was proposed in response to the high resource of convolutional operation in traditional algorithms,with the proportion of resources less than 2.5%in total FPGA resources;A modified carrier frequency recovery algorithm for mPPMQPSK is proposed to solve the problem of low estimation accuracy in that of traditional QPSK modulation.The modified algorithm performs frequency recovery on the signal based on pulse position,and the estimation accuracy is improved by about 0.6dB.The 15Gbit/s 4PPMQPSK offline digital coherent reception experiment based on real-time sampling oscilloscope was carried out,and the performance of Verilogbased hardware programming algorithm and Matlab-based offline algorithm was compared.The results show that the hardware algorithm considering parallelization,fixed-point and time delay has 0.5dB power cost compared with the serial floating-point offline algorithm.4)A real-time optical communication reception experiment of 15Gbit/s 4PPM-QPSK was completed based on ADC and FPGA chips(with model of xcvul3p-flga2577-1-exit).The experimental results show that a record sensitivity of-49dBm is achieved at BER=1 × 10-3,which is 4dB away from the theoretical sensitivity.