CN110113276B - OFDM frequency offset estimation method, system and device based on IEEE802.11 - Google Patents

OFDM frequency offset estimation method, system and device based on IEEE802.11 Download PDF

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CN110113276B
CN110113276B CN201810099899.5A CN201810099899A CN110113276B CN 110113276 B CN110113276 B CN 110113276B CN 201810099899 A CN201810099899 A CN 201810099899A CN 110113276 B CN110113276 B CN 110113276B
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frequency offset
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offset estimation
carrier frequency
time domain
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CN110113276A (en
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朱嘉俊
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Allwinner Technology Co Ltd
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    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00Modulated-carrier systems
    • H04L27/0014Carrier regulation
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00Modulated-carrier systems
    • H04L27/26Systems using multi-frequency codes
    • H04L27/2601Multicarrier modulation systems
    • H04L27/2647Arrangements specific to the receiver only
    • H04L27/2655Synchronisation arrangements
    • H04L27/2656Frame synchronisation, e.g. packet synchronisation, time division duplex [TDD] switching point detection or subframe synchronisation
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00Modulated-carrier systems
    • H04L27/0014Carrier regulation
    • H04L2027/0024Carrier regulation at the receiver end
    • H04L2027/0026Correction of carrier offset

Abstract

The invention discloses an OFDM frequency offset estimation method, a system and a device based on IEEE 802.11. The method comprises the following steps: performing front-back autocorrelation operation on two identical long training sequences in a Preamble of a received signal on a time domain to obtain a first carrier frequency offset estimation value; performing time domain carrier frequency offset compensation on the Signal according to the first carrier frequency offset estimation value; converting the compensated Signal from a time domain Signal into a frequency domain Signal; analyzing the Signal of the frequency domain to obtain a frequency domain reference Signal; performing local correlation operation on the frequency domain reference Signal and the Signal of the frequency domain to obtain a second carrier frequency offset estimation value; and performing time domain carrier frequency offset compensation on Payload according to the first carrier frequency offset estimation value and the second carrier frequency offset estimation value. Combining the time domain carrier frequency offset estimation of the Preamble and the frequency domain carrier frequency offset estimation of the Signal, and finally obtaining more accurate time domain carrier frequency offset estimation compensation in Payload; the method has the advantages of high accuracy, low complexity and strong real-time performance.

Description

OFDM frequency offset estimation method, system and device based on IEEE802.11
Technical Field
The present invention relates to the field of communications technologies, and in particular, to an OFDM frequency offset estimation method, system, and apparatus based on IEEE 802.11.
Background
IEEE802.11 is a communication protocol based on OFDM (orthogonal frequency division multiplexing) technology. This technique improves throughput by transmitting multiple subcarriers simultaneously. Each subcarrier carries different information. Since the subcarriers are orthogonal, there is no mutual interference. OFDM techniques are very sensitive to Carrier Frequency Offset (CFO), however, because the carrier frequency offset not only introduces phase rotation to the subcarrier information, but also destroys orthogonality between subcarriers, allowing subcarriers to interfere with each other (ICI).
Therefore, the OFDM receiver needs to have effective carrier frequency offset estimation and compensation.
For IEEE802.11 protocol, the prior art basically performs carrier frequency offset estimation through a Preamble training sequence (STF or LTF), or performs iterative tracking through some characteristics in the data receiving process. However, these methods have problems of low accuracy, high complexity, and poor real-time performance.
A frequency offset estimation method and device (application number: 201111049869.1) of OFDM communication system remodulates the feedback information obtained by analysis in frequency domain and converts it to time domain, and carries out local correlation operation in time domain. The local correlation in the time domain avoids the noise introduced by ICI interference, resulting in higher accuracy, but introduces greater complexity.
A carrier frequency offset estimation method (application number: 201610147118.6) suitable for MIMO-OFDM system uses autocorrelation of N STFs (short training sequences) on a Preamble to perform rough frequency offset estimation, and then uses autocorrelation of two LTFs (long training sequences) to obtain accurate carrier frequency offset estimation. Finally, the accurate carrier frequency offset estimation value is used for compensating to subsequent Signal and Payload parts, and the demodulation performance of the parts is guaranteed. The autocorrelation of the two LTFs provides a limited accuracy in the frequency offset estimation and, therefore, there is a certain loss in reception performance. Some methods are to perform iterative estimation by using some information of subsequent Payload, and in the iterative process, the performance is gradually improved along with the time, but the method does not have real-time performance, and the front part in the Payload cannot be guaranteed in time.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides an OFDM frequency offset estimation method, system and device based on IEEE802.11, the method, system or device completes local correlation operation in the frequency domain, and the frequency domain local correlation of the Signal part is utilized to improve the accuracy of frequency offset estimation, thereby solving the problems of low accuracy, high complexity and poor real-time performance of the existing carrier frequency offset estimation method.
In order to achieve the purpose, the technical scheme of the invention is as follows:
an OFDM frequency offset estimation method based on IEEE802.11 comprises the following steps: performing front-back autocorrelation operation on two identical long training sequences in a Preamble of a received signal on a time domain to obtain a first carrier frequency offset estimation value; carrying out time domain carrier frequency offset compensation on the Signal of the received Signal according to the first carrier frequency offset estimation value; converting the compensated Signal from a time domain Signal into a frequency domain Signal; analyzing the Signal of the frequency domain, and modulating the analyzed information again to obtain a frequency domain reference Signal; performing local correlation operation on the frequency domain reference Signal and the Signal of the frequency domain to obtain a second carrier frequency offset estimation value; and performing time domain carrier frequency offset compensation on Payload of the received signal according to the first carrier frequency offset estimation value and the second carrier frequency offset estimation value.
Further, the formula for performing the front-back autocorrelation operation on two identical long training sequences in the Preamble of the received signal in the time domain is as follows:
Figure BDA0001566019590000021
where N is the length of the long training sequence, yLTF1And yLTF2Respectively, a first long training sequence and a second long training sequence in the Preamble, (.)*Represents a conjugation;
the first carrier frequency offset estimation value
Figure BDA0001566019590000022
Comprises the following steps:
Figure BDA0001566019590000023
wherein f issIs the sample rate of the received signal.
Further, the formula for performing time domain carrier frequency offset compensation on the Signal of the received Signal according to the first carrier frequency offset estimation value is as follows:
Figure BDA0001566019590000024
wherein the content of the first and second substances,
Figure BDA0001566019590000025
in order to compensate the time domain Signal after the compensation,
Figure BDA0001566019590000026
for compensated white Gaussian noise, foIs the actual carrier frequency offset value.
Further, the formula for converting the compensated Signal from the time domain Signal to the frequency domain Signal is as follows:
Figure BDA0001566019590000027
wherein the content of the first and second substances,
Figure BDA0001566019590000028
for the converted frequency domain Signal, L is the distance between OFDM symbols; f. ofΔThe carrier frequency deviation value after compensation.
Further, the formula for performing local correlation operation on the frequency domain reference Signal and the frequency domain Signal is as follows:
Figure BDA0001566019590000029
wherein the content of the first and second substances,
Figure BDA0001566019590000031
in order to convert the frequency domain Signal,
Figure BDA0001566019590000032
is the frequency domain reference signal;
the second carrier frequency offset estimation value is:
Figure BDA0001566019590000033
an OFDM frequency offset estimation system based on IEEE802.11 comprises a first frequency offset estimation unit, a Signal frequency offset compensation unit, a Fourier transform unit, a frequency domain reference Signal acquisition unit, a second frequency offset estimation unit and a Payload frequency offset compensation unit.
The first frequency offset estimation unit is used for performing front-back autocorrelation operation on two identical long training sequences in a Preamble of a received signal in a time domain to obtain a first carrier frequency offset estimation value.
And the Signal frequency offset compensation unit is used for performing time domain carrier frequency offset compensation on the Signal of the received Signal according to the first carrier frequency offset estimation value.
And the Fourier transform unit is used for converting the compensated Signal from a time domain Signal into a frequency domain Signal.
The frequency domain reference Signal acquisition unit is used for analyzing the Signal of the frequency domain and remodulating the analyzed information to obtain the frequency domain reference Signal.
And the second frequency offset estimation unit is used for carrying out local correlation operation on the frequency domain reference Signal and the frequency domain Signal to obtain a second carrier frequency offset estimation value.
And the Payload frequency offset compensation unit is used for performing time domain carrier frequency offset compensation on Payload of the received signal according to the first carrier frequency offset estimation value and the second carrier frequency offset estimation value.
Further, the formula of performing a pre-autocorrelation and post-autocorrelation operation on two identical long training sequences in the Preamble of the received signal by the first frequency offset estimation unit on the time domain is as follows:
Figure BDA0001566019590000034
where N is the length of the long training sequence, yLTF1And yLTF2Respectively, a first long training sequence and a second long training sequence in the Preamble, (.)*Representing conjugation.
The first carrier frequency offset estimation value
Figure BDA0001566019590000035
Comprises the following steps:
Figure BDA0001566019590000036
wherein f issIs the sample rate of the received signal.
Further, the formula of the Signal frequency offset compensation unit performing time domain carrier frequency offset compensation on the Signal of the received Signal according to the first carrier frequency offset estimation value is as follows:
Figure BDA0001566019590000037
wherein the content of the first and second substances,
Figure BDA0001566019590000041
in order to compensate the time domain Signal after the compensation,
Figure BDA0001566019590000042
for compensated white Gaussian noise, foIs the actual carrier frequency offset value.
Further, the formula for the fourier transform unit to convert the compensated Signal from the time domain Signal to the frequency domain Signal is as follows:
Figure BDA0001566019590000043
wherein the content of the first and second substances,
Figure BDA0001566019590000044
for the converted frequency domain Signal, L is the distance between OFDM symbols; f. ofΔThe carrier frequency deviation value after compensation.
The frequency domain reference Signal obtaining unit performs local correlation operation on the frequency domain reference Signal and the frequency domain Signal according to the following formula:
Figure BDA0001566019590000045
wherein the content of the first and second substances,
Figure BDA0001566019590000046
is the frequency domain reference signal.
The second carrier frequency offset estimation value is:
Figure BDA0001566019590000047
an IEEE 802.11-based OFDM frequency offset estimation apparatus, comprising a memory, a processor and a computer program stored on the memory and executable on the processor; the computer program, when executed by the processor, implements the steps of any of the methods described above.
The invention has the beneficial effects that:
the invention utilizes the autocorrelation operation of two LTFs of the Preamble part to obtain a more accurate carrier frequency offset estimation value, and compensates the carrier frequency offset estimation value to the subsequent Signal part in the time domain; converting the time-domain compensated Signal to a frequency domain for demodulation and analyzing information; the obtained information can be assumed as accurate information and is remodulated and fed back, and local correlation estimation is carried out in a frequency domain to obtain a more accurate carrier frequency offset value; by combining the time domain carrier frequency offset estimation in the Preamble and the frequency domain carrier frequency offset estimation in the Signal, more accurate time domain carrier frequency offset estimation compensation can be obtained in time in the Payload part finally; the method has the advantages of high accuracy, low complexity and strong real-time performance.
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FIG. 1 shows a physical layer Legacy frame format of the IEEE802.11a/g protocol.
Fig. 2 shows the physical layer HT-GF frame format of the ieee802.11n protocol.
Fig. 3 shows the physical layer HT-MM frame format of the ieee802.11n protocol.
Fig. 4 is a flowchart illustrating an OFDM frequency offset estimation method based on IEEE802.11 according to the present invention.
Fig. 5 is a diagram of a simulation result of carrier frequency offset estimation according to the present invention.
Detailed Description
The invention is further illustrated below with reference to the figures and examples.
IEEE802.11 is a communication protocol for WLANs under which both the transmitting and receiving parties can effectively communicate. Since the protocol content to which the present invention relates is only the physical layer communication frame format, its physical layer frame format is first briefly stated.
As shown in fig. 1 to 3, 3 physical layer frame formats of ieee802.11a/g/n are listed, respectively, and these frame formats are based on the OFDM modulation technique. Basically, the frame format is divided into three major parts: preamble, Signal, Payload. Ieee802.11a, ieee802.11g, and ieee802.11n are 3 standard protocols that have been further developed based on IEEE 802.11.
(1) Preamble: this part does not carry any information and the content is well-defined by the protocol and used for frame synchronization, i.e. frame detection, reception timing, power adjustment, Carrier Frequency Offset (CFO) estimation, etc.
(2) Signal: carrying characteristic information of the frame such as bandwidth, length, modulation format, etc. The characteristic information has high robustness and noise immunity and is a prerequisite for correctly receiving information. Therefore, the information is generally difficult to interfere with to make errors.
(3) Payload: carrying the transmission content.
Example 1:
as shown in fig. 4, an OFDM frequency offset estimation method based on IEEE802.11 includes the following steps:
s1: performing front-back autocorrelation operation on two identical Long Training Sequences (LTFs) in a Preamble of a received signal on a time domain to obtain a first carrier frequency offset estimation value
Figure BDA0001566019590000051
S2: according to the first carrier frequency deviation estimated value
Figure BDA0001566019590000052
And performing time domain carrier frequency offset compensation on the Signal of the received Signal.
S3: and converting the compensated Signal from a time domain Signal into a frequency domain Signal.
S4: analyzing the Signal of the frequency domain, and re-modulating the analyzed information to obtain a frequency domain reference Signal
Figure BDA0001566019590000053
S5: reference signal of frequency domain
Figure BDA0001566019590000054
Carrying out local correlation operation with the Signal of the frequency domain to obtain a second carrier frequency offset estimation value
Figure BDA0001566019590000055
S6: according to the first carrier frequency deviation estimated value
Figure BDA0001566019590000056
And a second carrier frequency offset estimate
Figure BDA0001566019590000057
And carrying out time domain carrier frequency offset compensation on Payload of the received signal.
The invention utilizes the autocorrelation operation of two LTFs of the Preamble part to obtain a more accurate carrier frequency offset estimation value, and compensates the carrier frequency offset estimation value to the subsequent Signal part in the time domain; converting the time-domain compensated Signal to a frequency domain for demodulation and analyzing information; the obtained information can be assumed as accurate information and the feedback is remodulated, and local correlation estimation is carried out in a frequency domain to obtain a more accurate carrier frequency offset value. And finally, more accurate time domain carrier frequency offset estimation compensation can be obtained in time in the Payload part by combining the time domain carrier frequency offset estimation in the Preamble and the frequency domain carrier frequency offset estimation in the Signal. The method has the advantages of high accuracy, low complexity and strong real-time performance.
Specifically, in step S1, the formula for performing the pre-post autocorrelation operation on the time domain is:
Figure BDA0001566019590000061
where N is the length of the LTF, yLTF1And yLTF2First and second LTF, (. DEG) within the Preamble, respectively*Is conjugated.
The received signal y (n) has, in addition to noise, a carrier frequency offset, expressed as:
Figure BDA0001566019590000062
wherein x (n) is a useful signal; z (n) is white Gaussian noise,. sigma2Is the power;
Figure BDA0001566019590000063
causing phase rotation for carrier frequency offset estimation; f. ofoIs the carrier frequency offset value; f. ofsIs the sample rate of the received signal.
Therefore, the pre-post autocorrelation estimation of the Preamble can be expressed as:
Figure BDA0001566019590000064
wherein the content of the first and second substances,
Figure BDA0001566019590000065
is white noise after autocorrelation and follows Gaussian distribution, and the power is 2 NxXLTF1|2σ2;xLTF1=xLTF2
By obtaining the angle θ (angle) (g) for g, the angle can be obtained
Figure BDA0001566019590000066
Thus, a first carrier frequency offset estimation value is obtained
Figure BDA0001566019590000067
First carrier frequency offset estimation value
Figure BDA0001566019590000068
Is the actual carrier frequency offset foSince it is impossible to accurately obtain the actual carrier frequency offset value foSo there is also a residual value f of carrier frequency offsetΔ
The accuracy of the carrier frequency offset estimation can be generally expressed by the signal-to-noise ratio after autocorrelation, and the expression is as follows:
Figure BDA0001566019590000069
specifically, in step S2, the formula for time-domain carrier frequency offset compensation on Signal is as follows:
Figure BDA0001566019590000071
wherein the content of the first and second substances,
Figure BDA0001566019590000072
the compensated time domain Signal part is obtained; z is a radical ofsig(n) and
Figure BDA0001566019590000073
for compensating Gaussian white noise before and after, the power is sigma2;fΔThe compensated carrier frequency offset residual value is obtained.
Obtaining a first carrier frequency offset estimation value by utilizing LTF time domain autocorrelation operation
Figure BDA0001566019590000074
When the Signal of the received Signal is compensated in the time domain, the noise introduced by ICI (inter-carrier interference) can be basically ignored when the Signal is converted from the time domain to the frequency domain, and the demodulation performance of the Signal is improved.
Specifically, in step S3, Fast Fourier Transform (FFT) is performed on the compensated Signal, and the Signal is converted from the time domain to the frequency domain, which is a necessary step for demodulating OFDM. The formula is as follows:
Figure BDA0001566019590000075
wherein the content of the first and second substances,
Figure BDA0001566019590000076
is an FFT frequency point sequence;
Figure BDA0001566019590000077
represented as a time-domain compensated frequency domain Signal; zsig(k) Expressed as frequency domain white Gaussian noise, with a power of
Figure BDA0001566019590000078
Further, in the present invention,
Figure BDA0001566019590000079
can be expressed as:
Figure BDA00015660195900000710
wherein the content of the first and second substances,
Figure BDA00015660195900000711
is Gaussian white noise (including frequency domain noise and ICI-induced noise) and has power of
Figure BDA00015660195900000712
L is the distance from the previous OFDM symbol (L is more than or equal to N).
Specifically, in step S4, for the frequency domain
Figure BDA00015660195900000713
And demodulating and analyzing the information to obtain bit information of a Signal part of the sending end, and feeding back the information. Demodulation may select hard decisions, or channel decoding. The feedback is more accurate after the channel decoding is adopted, because the channel decoding has an error correction function and needs a certain time delay; while the hard decision feedback is directly passed through
Figure BDA0001566019590000081
The most likely bit information is found without considering delay. The two feedback performances are slightly different through experiments.
Then, the feedback information is remodulated by the same processing as the sending end, and the frequency domain reference signal is obtained
Figure BDA0001566019590000082
Frequency domain reference signal
Figure BDA0001566019590000083
Is estimated according to the received signal, not necessarily the signal X actually modulated by the receiving endsig(k) Are completely consistent.
Specifically, in step S5, the frequency domain reference signal is transmitted
Figure BDA0001566019590000084
Received signal in frequency domain
Figure BDA0001566019590000085
Carrying out local correlation operation, wherein the formula is as follows:
Figure BDA0001566019590000086
assuming an estimated frequency domain reference signal
Figure BDA0001566019590000087
Is completely correct, then
Figure BDA0001566019590000088
The local correlation result can be expressed as:
Figure BDA0001566019590000089
finally, the angle is calculated by the angle of G
Figure BDA00015660195900000810
Thus, a second carrier frequency offset estimation value is obtained
Figure BDA00015660195900000811
The modulation format of Signal is BPSK with the highest robustness, and the channel coding is 1/2 code rate with the highest robustness, so the demodulation performance of Signal is much higher than that of payload, and errors are difficult to make. Therefore, Signal information can basically be assumed to be correct, and it is a very reliable way to use its feedback as carrier frequency offset estimation. Feedback modulating the information analyzed by Signal, modulating the obtained
Figure BDA00015660195900000812
Received signal in frequency domain
Figure BDA00015660195900000813
The local correlation is performed, and the performance gain improvement is higher than the performance gain of the self-correlation before and after the time domain.
After local correlation, assuming N ═ L, the signal-to-noise ratio is expressed as:
Figure BDA00015660195900000814
in contrast, it can be seen that the signal-to-noise ratio of the local correlation is a 3dB higher gain than the autocorrelation.
Specifically, in step S6, the final time domain carrier frequency offset compensation value for Payload is
Figure BDA00015660195900000815
As shown in fig. 5, which is a simulation result diagram of carrier frequency offset estimation according to the present invention, the frequency offset estimation error is CDF, and the channel environment is AWGN (SNR ═ 1.5 dB). The solid line is the result of the LTF autocorrelation estimation before and after use alone, with a 10% error greater than 11 KHz. The dotted line is the result obtained by adding Signal frequency domain local correlation estimation, and the error of 10% of the frequency offset estimation of the invention is more than 4 KHz. Obviously, the invention has excellent performance.
Example 2:
an OFDM frequency offset estimation system based on IEEE802.11 comprises a first frequency offset estimation unit, a Signal frequency offset compensation unit, a Fourier transform unit, a frequency domain reference Signal acquisition unit, a second frequency offset estimation unit and a Payload frequency offset compensation unit.
A first frequency offset estimation unit, configured to perform a pre-and-post autocorrelation operation on two identical long training sequences in a Preamble of a received signal in a time domain to obtain a first carrier frequency offset estimation value
Figure BDA0001566019590000091
A Signal frequency offset compensation unit for estimating the frequency offset according to the first carrier frequency offset
Figure BDA0001566019590000092
And performing time domain carrier frequency offset compensation on the Signal of the received Signal.
And the Fourier transform unit is used for converting the compensated Signal from a time domain Signal into a frequency domain Signal.
A frequency domain reference Signal obtaining unit, configured to analyze the Signal in the frequency domain, and remodulate the analyzed information to obtain a frequency domain reference Signal
Figure BDA0001566019590000093
A second frequency offset estimation unit for estimating the frequency domain reference signal
Figure BDA0001566019590000094
Carrying out local correlation operation with the Signal of the frequency domain to obtain a second carrier frequency offset estimation value
Figure BDA0001566019590000095
A Payload frequency offset compensation unit for estimating the first carrier frequency offset
Figure BDA0001566019590000096
And a second carrier frequency offset estimate
Figure BDA0001566019590000097
And carrying out time domain carrier frequency offset compensation on Payload of the received signal.
The invention utilizes the autocorrelation operation of two LTFs of the Preamble to obtain a more accurate carrier frequency offset estimation value, and compensates the carrier frequency offset estimation value to a subsequent Signal part in a time domain; converting the time-domain compensated Signal to a frequency domain for demodulation and analyzing information; the obtained information can be assumed as accurate information and the feedback is remodulated, and local correlation estimation is carried out in a frequency domain to obtain a more accurate carrier frequency offset value. And finally, more accurate time domain carrier frequency offset estimation compensation can be obtained in time in the Payload part by combining the time domain carrier frequency offset estimation in the Preamble and the frequency domain carrier frequency offset estimation in the Signal. The method has the advantages of high accuracy, low complexity and strong real-time performance.
Specifically, the formula of the first frequency offset estimation unit performing the pre-and post-autocorrelation operation on the time domain is as follows:
Figure BDA0001566019590000098
where N is the length of the LTF, yLTF1And yLTF2First and second LTF, (. DEG) within the Preamble, respectively*Is conjugated.
The received signal y (n) has, in addition to noise, a carrier frequency offset, expressed as:
Figure BDA0001566019590000101
wherein x (n) is a useful signal; z (n) is white Gaussian noise,. sigma2Is the power;
Figure BDA0001566019590000102
causing phase rotation for carrier frequency offset estimation; f. ofoIs the carrier frequency offset value; f. ofsIs the sample rate of the received signal.
Therefore, the pre-post autocorrelation estimation of the Preamble can be expressed as:
Figure BDA0001566019590000103
wherein the content of the first and second substances,
Figure BDA0001566019590000104
is white noise after autocorrelation and follows Gaussian distribution, and the power is 2 NxXLTF1|2σ2;xLTF1=xLTF2
By obtaining the angle θ (angle) (g) for g, the angle can be obtained
Figure BDA0001566019590000105
Thus, a first carrier frequency offset estimation value is obtained
Figure BDA0001566019590000106
First carrier frequency offset estimation value
Figure BDA0001566019590000107
Is the actual carrier frequency offset foSince it is impossible to accurately obtain the actual carrier frequency offset value foSo there is also a residual value f of carrier frequency offsetΔ
The accuracy of the carrier frequency offset estimation can be generally expressed by the signal-to-noise ratio after autocorrelation, and the expression is as follows:
Figure BDA0001566019590000108
specifically, the formula of the Signal frequency offset compensation unit performing time domain carrier frequency offset compensation on the Signal is as follows:
Figure BDA0001566019590000109
wherein the content of the first and second substances,
Figure BDA00015660195900001010
the compensated time domain Signal part is obtained; z is a radical ofsig(n) and
Figure BDA00015660195900001011
for compensating Gaussian white noise before and after, the power is sigma2;fΔThe compensated carrier frequency offset residual value is obtained.
Obtaining a first carrier frequency offset estimation value by utilizing LTF time domain autocorrelation operation
Figure BDA00015660195900001012
When the Signal of the received Signal is compensated in the time domain, the noise introduced by ICI (inter-carrier interference) can be basically ignored when the Signal is converted from the time domain to the frequency domain, and the demodulation performance of the Signal is improved.
Specifically, the fourier transform unit performs Fast Fourier Transform (FFT) on the compensated Signal, and converts the Signal from the time domain to the frequency domain, which is a necessary step for demodulating OFDM. The formula is as follows:
Figure BDA0001566019590000111
wherein the content of the first and second substances,
Figure BDA0001566019590000112
is an FFT frequency point sequence;
Figure BDA0001566019590000113
represented as a time-domain compensated frequency domain Signal; zsig(k) Expressed as frequency domain white Gaussian noise, with a power of
Figure BDA0001566019590000114
Further, in the present invention,
Figure BDA0001566019590000115
can be expressed as:
Figure BDA0001566019590000116
wherein the content of the first and second substances,
Figure BDA0001566019590000117
is Gaussian white noise (including frequency domain noise and ICI-induced noise) and has power of
Figure BDA0001566019590000118
L is the distance from the previous OFDM symbol (L is more than or equal to N).
Specifically, the frequency domain reference signal acquisition unit is for the frequency domain
Figure BDA0001566019590000119
And demodulating and analyzing the information to obtain bit information of a Signal part of the sending end, and feeding back the information. Demodulation may select hard decisions, or channel decoding. The feedback is more accurate after the channel decoding is adopted, because the channel decoding has an error correction function and needs a certain time delay; while the hard decision feedback is directly passed through
Figure BDA00015660195900001110
The most likely bit information is found without considering delay. The two feedback performances are slightly different through experiments.
Then, the feedback information is remodulated by the same processing as the sending end, and the frequency domain reference signal is obtained
Figure BDA00015660195900001111
Frequency domain reference signal
Figure BDA00015660195900001112
Is estimated according to the received signal, not necessarily the signal X actually modulated by the receiving endsig(k) Are completely consistent.
Specifically, the second frequency offset estimation unit is used for estimating a frequency domain reference signal
Figure BDA00015660195900001113
Received signal in frequency domain
Figure BDA00015660195900001114
Carrying out local correlation operation, wherein the formula is as follows:
Figure BDA00015660195900001115
assuming an estimated frequency domain reference signal
Figure BDA00015660195900001116
Is completely correct, then
Figure BDA00015660195900001117
The local correlation result can be expressed as:
Figure BDA0001566019590000121
finally, the angle is calculated by the angle of G
Figure BDA0001566019590000122
Thus, a second carrier frequency offset estimation value is obtained
Figure BDA0001566019590000123
The modulation format of Signal is BPSK with the highest robustness, and the channel coding is 1/2 code rate with the highest robustness, so the demodulation performance of Signal is much higher than that of payload, and errors are difficult to make. Therefore, Signal information can basically be assumed to be correct, and it is a very reliable way to use its feedback as carrier frequency offset estimation. Feedback modulating the information analyzed by Signal, modulating the obtained
Figure BDA0001566019590000124
Received signal in frequency domain
Figure BDA0001566019590000125
The local correlation is performed, and the performance gain improvement is higher than the performance gain of the self-correlation before and after the time domain.
After local correlation, assuming N ═ L, the signal-to-noise ratio is expressed as:
Figure BDA0001566019590000126
in contrast, it can be seen that the signal-to-noise ratio of the local correlation is a 3dB higher gain than the autocorrelation.
Specifically, the final time domain carrier frequency offset compensation value of Payload by the Payload frequency offset compensation unit is
Figure BDA0001566019590000127
Example 3:
the embodiment provides an OFDM frequency offset estimation apparatus based on IEEE802.11, including: a memory, a processor and a computer program stored on the memory and executable on the processor, the computer program, when executed by the processor, implementing the steps S1-S6 of the method of embodiment 1.
What has been described above is only a preferred embodiment of the present invention, and the present invention is not limited to the above examples. It is to be understood that other modifications and variations directly derivable or suggested by those skilled in the art without departing from the basic concept of the present invention are to be considered as included within the scope of the present invention.

Claims (10)

1. An OFDM frequency offset estimation method based on IEEE802.11 is characterized in that:
the method comprises the following steps:
performing front-back autocorrelation operation on two identical long training sequences in a Preamble of a received signal on a time domain to obtain a first carrier frequency offset estimation value;
carrying out time domain carrier frequency offset compensation on the Signal of the received Signal according to the first carrier frequency offset estimation value;
converting the compensated Signal from a time domain Signal into a frequency domain Signal;
analyzing the Signal of the frequency domain, and modulating the analyzed information again to obtain a frequency domain reference Signal;
performing local correlation operation on the frequency domain reference Signal and the Signal of the frequency domain to obtain a second carrier frequency offset estimation value;
and performing time domain carrier frequency offset compensation on Payload of the received signal according to the first carrier frequency offset estimation value and the second carrier frequency offset estimation value.
2. The method of estimating OFDM frequency offset based on IEEE802.11, according to claim 1, wherein:
the formula for performing the front and back autocorrelation operation on two identical long training sequences in the Preamble of the received signal on the time domain is as follows:
Figure FDA0001566019580000011
where N is the length of the long training sequence, yLTF1And yLTF2Respectively, a first long training sequence and a second long training sequence in the Preamble, (.)*Represents a conjugation;
the first carrier frequency offset estimation value is:
Figure FDA0001566019580000012
wherein f issIs the sample rate of the received signal.
3. The method of estimating OFDM frequency offset based on IEEE802.11, according to claim 2, wherein:
the formula for performing time domain carrier frequency offset compensation on the Signal of the received Signal according to the first carrier frequency offset estimation value is as follows:
Figure FDA0001566019580000013
wherein the content of the first and second substances,
Figure FDA0001566019580000014
in order to compensate the time domain Signal after the compensation,
Figure FDA0001566019580000015
for compensated white Gaussian noise, foIs the actual carrier frequency offset value.
4. The method of estimating OFDM frequency offset based on IEEE802.11, according to claim 1, wherein:
the formula for converting the compensated Signal from the time domain Signal to the frequency domain Signal is as follows:
Figure FDA0001566019580000021
wherein the content of the first and second substances,
Figure FDA0001566019580000022
for the converted frequency domain Signal, L is the distance between OFDM symbols; f. ofΔFor compensated loadingAnd (5) wave frequency deviation value.
5. The method of estimating OFDM frequency offset based on IEEE802.11, according to claim 1, wherein:
the formula for performing local correlation operation on the frequency domain reference Signal and the frequency domain Signal is as follows:
Figure FDA0001566019580000023
wherein the content of the first and second substances,
Figure FDA0001566019580000024
in order to convert the frequency domain Signal,
Figure FDA0001566019580000025
is the frequency domain reference signal;
the second carrier frequency offset estimation value is:
Figure FDA0001566019580000026
6. an OFDM frequency offset estimation system based on IEEE802.11 is characterized in that:
the device comprises a first frequency offset estimation unit, a Signal frequency offset compensation unit, a Fourier transform unit, a frequency domain reference Signal acquisition unit, a second frequency offset estimation unit and a Payload frequency offset compensation unit;
the first frequency offset estimation unit is used for carrying out front-back autocorrelation operation on two identical long training sequences in a Preamble of a received signal on a time domain to obtain a first carrier frequency offset estimation value;
the Signal frequency offset compensation unit is used for performing time domain carrier frequency offset compensation on the Signal of the received Signal according to the first carrier frequency offset estimation value;
the Fourier transformation unit is used for converting the compensated Signal from a time domain Signal into a frequency domain Signal;
the frequency domain reference Signal acquisition unit is used for analyzing the Signal of the frequency domain and remodulating the analyzed information to obtain a frequency domain reference Signal;
the second frequency offset estimation unit is used for performing local correlation operation on the frequency domain reference Signal and the Signal of the frequency domain to obtain a second carrier frequency offset estimation value;
and the Payload frequency offset compensation unit is used for performing time domain carrier frequency offset compensation on Payload of the received signal according to the first carrier frequency offset estimation value and the second carrier frequency offset estimation value.
7. The IEEE802.11 based OFDM frequency offset estimation system of claim 6, wherein:
the formula of the first frequency offset estimation unit performing the front and back autocorrelation operation on two identical long training sequences in the Preamble of the received signal in the time domain is as follows:
Figure FDA0001566019580000027
where N is the length of the long training sequence, yLTF1And yLTF2Respectively, a first long training sequence and a second long training sequence in the Preamble, (.)*Represents a conjugation;
the first carrier frequency offset estimation value
Figure FDA0001566019580000031
Comprises the following steps:
Figure FDA0001566019580000032
wherein f issIs the sample rate of the received signal.
8. The IEEE802.11 based OFDM frequency offset estimation system of claim 7, wherein:
the formula of the Signal frequency offset compensation unit for performing time domain carrier frequency offset compensation on the Signal of the received Signal according to the first carrier frequency offset estimation value is as follows:
Figure FDA0001566019580000033
wherein the content of the first and second substances,
Figure FDA0001566019580000034
in order to compensate the time domain Signal after the compensation,
Figure FDA0001566019580000035
for compensated white Gaussian noise, foIs the actual carrier frequency offset value.
9. The IEEE802.11 based OFDM frequency offset estimation system of claim 6, wherein:
the formula for converting the compensated Signal from the time domain Signal to the frequency domain Signal by the Fourier transform unit is as follows:
Figure FDA0001566019580000036
wherein the content of the first and second substances,
Figure FDA0001566019580000037
for the converted frequency domain Signal, L is the distance between OFDM symbols; f. ofΔThe carrier frequency deviation value after compensation is obtained;
the frequency domain reference Signal obtaining unit performs local correlation operation on the frequency domain reference Signal and the frequency domain Signal according to the following formula:
Figure FDA0001566019580000038
wherein the content of the first and second substances,
Figure FDA0001566019580000039
is that it isA frequency domain reference signal;
the second carrier frequency offset estimation value is:
Figure FDA00015660195800000310
10. an OFDM frequency offset estimation device based on IEEE802.11 is characterized in that:
comprising a memory, a processor, and a computer program stored on the memory and executable on the processor; the computer program, when executed by the processor, implementing the steps of the method of any one of claims 1 to 5.
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