Method for accurately estimating frame synchronization and frequency offset in orthogonal frequency division multiplexing (OFDM) system

Abstract

The invention discloses a method for accurately estimating frame synchronization and frequency offset in an orthogonal frequency division multiplexing (OFDM) system and relates to the field of OFDM. The method comprises the following steps of: fetching the OFDM data of a frame, performing average removal on each character, performing complex exponential modulation on each character, adding a pseudo random binary sequence (PRBS) frame header to the OFDM data and adding a complex exponential sequence to the PRBS frame header, and transmitting the OFDM data to a channel for transmitting; performing a fast Fourier transform (FFT) operation on the data in a receiving window, comparing the modulus square maximum of an FFT operation result with a setting threshold, obtaining a primary frequency offset estimator according to the position offset of the modulus square maximum, and compensating the primary frequency offset estimator; performing a cross correlation operation on the data in a new window and a local known PRBS, comparing the modulus square maximum of a cross correlation operation result with the setting threshold, obtaining the position of the frame header, dividing characters, and calculating the average of the characters; and performing the FFT operation on an average sequence, obtaining a secondary frequency offset estimator according to the position offset of the modulus square maximum of an FFT operation result, and compensating the secondary frequency offset estimator. According to the method, the accuracy positioning of the frame synchronization and the frequency offset compensation is realized by two-time frequency offset estimations and related operations, and the performance of the whole OFDM system is improved.

Description

technical field [0001] The present invention relates to the field of OFDM (Orthogonal Frequency Division Multiplexing, Orthogonal Frequency Division Multiplexing), in particular to a method for frame synchronization and accurate frequency offset estimation in an OFDM system. Background technique [0002] In the OFDM system, frame synchronization is the starting point of all algorithms. First, the starting position of each frame data must be accurately determined in order to carry out various subsequent algorithms smoothly. Generally, PRBS (Pseudo-Random Binary Sequence, pseudo-random binary sequence) is used as the frame header, because PRBS has good autocorrelation characteristics: its autocorrelation function R(τ) has a large modulus when τ=0 peak; while τ≠0, its modulus is almost zero. Therefore, the position of the frame header can be accurately judged by setting the threshold. Take the length as L (L is generally 2 l , l is an integer) PRBS as an example, when it is ...

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Problems solved by technology

[0010] The square of its modulo is also obtained as 4L 2 , but because it is the result of cross-correlation, when the first L data and the last L data are slightly misaligned, there will be a larger modulus output, so the output cross-correlation modulo curve is a slowly rising and slowly falling curve , and in the case of poor signal-to-noise ratio, there will be a large peak platform, which makes it difficult to determine the exact position of frame synchronization

At the same time, for estimating the frequency difference, the result of autocorrelation e -j2πΔfLΔt However, for actual processing circuits such as FPGA (Field Programmable Gate Array, Field Programmable Gate Array), it is a very complicated operation process to obtain one L of the complex number, and the limitation of the number of sampling bits leads to its estimation The accuracy will not be very high, and at the same time, the noise is not eliminated in the estimation process, which will inevitably affect the accuracy of its estimation, and eventually the estimated frequency offset cannot be accurately compensated

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