Linear chirp z transform based frequency offset estimation algorithm in M-QAM (M-ary Quadrature Amplitude Modulation) coherent optical communication system

Abstract

The invention provides a linear chirp z transform based frequency offset estimation algorithm in an M-QAM (M-ary Quadrature Amplitude Modulation) coherent optical communication system, which comprises the following steps that: a smaller amount of M-QAM signal receiving sample values are selected for four-power; the FFT (Fast Fourier Transform) is conducted to obtain a frequency corresponding to the maximum spectral value to be recorded as a roughly estimated value; the linear CET (Chirp z Transform) of four-power signals is conducted near the roughly estimated value to obtain a frequency corresponding to the maximum spectral value; and the frequency is divided by four to obtain a frequency offset estimated value of M-QAM signals. The linear chirp z transform based frequency offset estimation algorithm does not adopt a gradient descent method, a Newton method and other methods depending on a step size and the number of symbols, but adopts a method that the frequency is finely estimated by CZT with a fixed computation load. Since the CZT can be realized by the FFT with fast algorithm, the complexity of the algorithm can be obviously reduced in the algorithm, and meanwhile, the algorithm is applied to all-order QAM signals.

Description

technical field [0001] The invention relates to the field of communication, more specifically, to a frequency offset estimation scheme in an optical fiber communication system using M-QAM modulation format in coherent optical communication. Background technique [0002] The optical fiber communication system started in the early 1970s, using an intensity-modulated semiconductor laser, and the received light intensity signal is converted into an electrical signal through a photodiode. This approach is called intensity modulation / direct detection (IM / DD), and is widely used in current commercial optical fiber communication systems. However, direct detection / intensity modulation has its inherent shortcomings, such as the reception and sensitivity are limited by noise, the spectral efficiency is not high, and the bandwidth cannot be fully utilized. In earlier studies, a constant-strength differential QPSK modulation format was used, and differential detection was used at the re...

AI Technical Summary

Problems solved by technology

However, the convergence of optimization methods such as gradient descent method or Newton descent method is affected by the number of symbols (or number of samples) involved in the operation, iteration step size, convergence accuracy, etc., which makes the algorithm complexity and estimation of the overall frequency estimation algorithm Difficult to determine accuracy

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