CN116016067B - Frequency Offset Estimation Method in OFDM System - Google Patents
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Abstract
The invention discloses a frequency offset estimation method in an OFDM system, which comprises the steps of sending a signal x (n) at a moment T 0 by a transmitting end, acquiring a transmitted real-value signal at a moment T 1 by a receiving end to obtain a receiving signal, and obtaining the receiving signal by a processor: And performing fast Fourier transform on the analysis signal by a processor to obtain: Taking W (n) as 0, and establishing a new function Y 1＝Y(1:(N-1))、Y2 =Y (2:N) by the processor to obtain Y New＝X*(n-ε)·X(n-ε);X1＝X(1:(N-1))、X2 = X ((2: n)) is obtainedCreating new functions by a processorObtaining
Description
Technical Field
The invention relates to a frequency offset estimation method in an OFDM system, and belongs to the technical field of carriers.
Background
In a conventional parallel transmission system, the whole signal band is divided into a plurality of parallel non-orthogonal subcarriers, each subcarrier is modulated separately, the whole system forms frequency division multiplexing, and the frequency spectrums between two adjacent subcarriers are not overlapped, so that interference of the subcarriers is eliminated. However, this frequency division multiplexing method reduces the spectrum utilization rate, and in order to improve the spectrum utilization rate, parallel transmission and FDM ideas with spectrum interleaving, that is, orthogonal frequency division multiplexing OFDM, are proposed in the middle 60 s of the 20 th century.
The multi-carrier technology is to divide the transmission bandwidth into a plurality of narrow-band sub-carriers for parallel transmission, so that higher transmission rate can be obtained in a limited bandwidth, in the traditional frequency division multiplexing technology, the sub-carriers are not overlapped with each other and a guard interval is added, the frequency utilization rate is low, in the OFDM technology, the sub-carriers are orthogonal, each sub-carrier has an integer multiple of cycles within one OFDM symbol period from the angle of a time domain, each adjacent sub-carrier is different by one cycle, the other sub-carriers have zero values at the center of each sub-carrier from the angle of a frequency domain, thereby saving precious frequency resources and improving the frequency utilization rate.
In an OFDM system, a digital signal modulated by a transmitting end is converted into an analog signal through D/A, the analog signal is modulated by an RF center frequency and then is transmitted into a wireless channel, the RF analog signal is firstly regulated at a receiving end, the RF analog signal is moved from RF to baseband, the signal is sampled and digitized at a sampling rate F and finally is transmitted to an OFDM demodulator, but if carrier frequency deviation delta F occurs due to unstable crystal oscillator at the receiving end, doppler effect in a mobile environment, phase noise introduced into a nonlinear channel and other reasons, the signal of a corresponding channel of the regulating signal is caused to fall on other channels, error codes occur, orthogonality of subcarriers is even destroyed, and interference among subcarriers occurs.
In the prior art, schmidl, cox and Paul H.moose propose a method for carrying out frequency synchronization by using two OFDM symbols and knowing the starting points of the two repeated OFDM symbols so as to carry out frequency offset estimation (study and analysis of carrier frequency offset in the OFDM of a student AND ANALYSIS of Carrier Frequency Offset (CFO) in OFDM), on one hand, the method at least needs the two repeated OFDM symbols, has large calculated amount and low measuring and calculating speed, and on the other hand, can only realize the estimation of the frequency offset on the bandwidth of +/-0.5 db, and has large error and low precision.
Disclosure of Invention
The invention aims to provide a frequency offset estimation method and device in an OFDM system, which not only solve the problems of channel disorder and mutual interference among subcarriers caused by frequency offset of an OFDM structure signal in the prior art, but also solve the problems of large estimation difference and large correction deviation.
In order to achieve the purpose, the technical scheme adopted by the invention is that the frequency offset estimation method in the OFDM system comprises the following steps:
The transmitting end transmits a signal X (n) at the moment T0;
The receiving end is used for acquiring the transmitted real-value signal at the moment T 1 to obtain a receiving signal, and when the receiving signal of the receiving end has frequency offset epsilon, the receiving signal is obtained by a processor: τ=t 1-T0, w (n) is the sampling value of gaussian noise;
And performing fast Fourier transform on the analysis signal by a processor to obtain: Taking W (n) as 0;
A new function is built by the processor, Y 1＝Y(1：(N-1))、Y2 =y (2: n), resulting in Screening constant termY New＝X*(n-ε)·X(n-ε);X1＝X(1：(N-1))、X2 = X ((2: n)) is obtained, yielding
Creating new functions by a processorObtainingAnd output.
The further improved scheme in the technical scheme is as follows:
1. In the above scheme, the receiving end is a microphone or an antenna.
2. In the above scheme, the real-value signal acquired by the receiving end is filtered out by the filter to obtain signals of other frequencies except the real-value signal.
3. In the above scheme, the real-valued signal and the imaginary-valued signal after the real-valued signal is transformed respectively pass through a low-pass filter, and then the real-valued signal and the imaginary-valued signal are downsampled by a sampler to output real-valued data and imaginary-valued data.
4. In the above scheme, a pilot signal is used instead of a real value signal.
5. In the above scheme, the received signal y (λ) is a discrete signal.
6. In the above solution, the processor performs oversampling, where the oversampling multiple is M, m= 2,4,6,8,16Spacing;
Obtained by a processor Taking W (k) as 0;
Epsilon new＝[M·ε]round is obtained by a processor to obtain And output.
7. In the above scheme, synchronous outputAnd
8. In the above scheme, the processor only outputs after the oversampling is adopted
Due to the application of the technical scheme, compared with the prior art, the invention has the following advantages:
1. the frequency offset estimation method in the OFDM system establishes a new function based on the time domain received signal, can realize the rapid measurement and calculation of the frequency offset by only acquiring one OFDM symbol (one frame) through the observation window, can be widely applied to the existing wireless uplink and downlink communication systems, solves the problem of time/frequency asynchronous during communication between terminals, and has high measurement and calculation speed and high acquisition and correction efficiency.
2. The frequency offset estimation method in the OFDM system is different from the measuring and calculating method proposed by Schmidl, cox and Paul H.Moose, can realize frequency offset estimation under the full bandwidth, and has stronger applicability and higher precision.
3. According to the frequency offset estimation method in the OFDM system, the frequency offset estimation precision is improved by utilizing the oversampling through optimizing the sampling method, so that the correction precision is improved conveniently, and the error is reduced.
Detailed Description
The method and the device of the invention are used for solving the frequency offset problem of OFDM with the signal structure and the similar signal structure.
Embodiment 1a method for estimating frequency offset in an OFDM system, the method comprising:
After the transmitting end sends out the signal x (n) at the moment T0, the microphone is used as the receiving end to acquire the transmitted real-value signal at the moment T 1 to obtain an audio signal (receiving signal), and the audio signal is input into the filter, wherein the filtering frequency is set according to the frequency of the audio signal, clutter is filtered, and the subsequent processing is facilitated.
The filtered real-value signal x (n) is subjected to phase transformation by a processor, and then a complex-form analysis signal is obtained: τ=t 1-T0, w (n) is a sampling value of gaussian noise.
The processor is used for further carrying out Fast Fourier Transform (FFT) on the analysis signal, and transforming the analysis signal from a time domain to a frequency domain, so that the frequency offset is convenient to analyze, and the obtained frequency offset is obtained: Taking W (n) as 0, establishing a new function formula Y 1＝Y(1：(N-1))、Y2 =Y (2:N) to obtain Screening constant termY New＝X*(n-ε)·X(n-ε);X1＝X(1：(N-1))、X2 =x (2:n) is obtained, giving
Creating new functions by a processorObtainingAnd output.
Embodiment 2a method for estimating frequency offset in an OFDM system includes:
After the transmitting end sends out the signal x (n) at the moment T 0, the microphone is used as the receiving end to acquire the transmitted real-value signal at the moment T 1 to obtain an audio signal (receiving signal), and the audio signal is input into the filter, wherein the filtering frequency is set according to the frequency of the audio signal, clutter is filtered, and the subsequent processing is facilitated.
The filtered real value signal) x (n) is subjected to phase transformation by a processor, the real value signal and the imaginary value signal respectively pass through a low-pass filter, then the real part and the imaginary part data are output through downsampling of a sampler, and the complex-form analytic signals are obtained through combination: τ=t 1-T0, w (n) is a sampling value of gaussian noise.
The processor is used for further carrying out Fast Fourier Transform (FFT) on the analysis signal, and transforming the analysis signal from a time domain to a frequency domain, so that the frequency offset is convenient to analyze, and the obtained frequency offset is obtained: Taking W (n) as 0, establishing a new function formula Y 1＝Y(1：(N-1))、Y2 =Y (2:N) to obtain Screening constant termY New＝X*(n-s)·X(n-ε);X1＝X(1：(N-1))、X2 = x ((2: n)) is obtained, yielding
Creating new functions by a processorObtaining
Oversampling is performed by the processor by an oversampling multiple of M, m= 2,4,6,8,16At intervals, let k=m×n, further perform Fast Fourier Transform (FFT) on the resolved signal by the processor, transforming from time domain to frequency domain, facilitating resolving frequency offset, resulting in: W (k) is taken to be 0.
By the processor, a new function is established based on Y (k) by the same steps as before non-oversampling to obtain an estimate of epsilon, thereby obtaining epsilon new＝[M·ε]round
Here, assuming that m=4, ε=1.31, the result is obtained by normal samplingWhile The mean square error is 0.0036<0.0961, and it can be seen that the over-sampling method can improve the accuracy of the frequency offset estimation, and the larger M is, the more accurate the estimation is;
thus, take And outputting the frequency offset value.
Wherein, the pilot signal can be used to replace the real value signal, and the measurement is performed to obtain the correction data.
By adopting the scheme, based on the time domain received signals, a new function is established, and the frequency offset can be rapidly calculated by only acquiring one OFDM symbol (one frame) through the observation window, so that the method and the device can be widely applied to the existing wireless uplink and downlink communication systems, solve the problem of time/frequency asynchronous communication between terminals, have high calculation speed and high acquisition and correction efficiency.
In addition, different from the measuring and calculating methods proposed by Schmidl, cox and Paul H.Moose, the frequency offset estimation under the full bandwidth can be realized, and the applicability and the accuracy are higher.
In addition, by optimizing the sampling method, the over-sampling is utilized to improve the estimation precision of the frequency offset, thereby being convenient for improving the correction precision and reducing the error.
The above embodiments are provided to illustrate the technical concept and features of the present invention and are intended to enable those skilled in the art to understand the content of the present invention and implement the same, and are not intended to limit the scope of the present invention. All equivalent changes or modifications made in accordance with the spirit of the present invention should be construed to be included in the scope of the present invention.
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| CN102984114A (en) * | 2012-12-27 | 2013-03-20 | 上海创远仪器技术股份有限公司 | Signal timing and frequency offset compensation control method applied to orthogonal frequency division multiplexing system |
| CN103095613A (en) * | 2013-01-21 | 2013-05-08 | 西安电子科技大学 | Integer frequency doubling offset and channel parameter joint estimation algorithm with ultra-low complexity in single carrier frequency domain equalization (SC-FDE) system |
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| WO2017049303A1 (en) * | 2015-09-17 | 2017-03-23 | Cohere Technologies, Inc. | Compatible use of orthogonal time frequency space modulation within an lte communication system |
| CN111683034B (en) * | 2020-06-09 | 2022-04-01 | 中国电子科技集团公司第五十四研究所 | OFDM-based large Doppler wireless communication time-frequency synchronization method |
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| CN102984114A (en) * | 2012-12-27 | 2013-03-20 | 上海创远仪器技术股份有限公司 | Signal timing and frequency offset compensation control method applied to orthogonal frequency division multiplexing system |
| CN103095613A (en) * | 2013-01-21 | 2013-05-08 | 西安电子科技大学 | Integer frequency doubling offset and channel parameter joint estimation algorithm with ultra-low complexity in single carrier frequency domain equalization (SC-FDE) system |
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