CN112202693B - Anti-interference frequency offset estimation method suitable for OFDM system - Google Patents

Anti-interference frequency offset estimation method suitable for OFDM system Download PDF

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CN112202693B
CN112202693B CN202010914721.9A CN202010914721A CN112202693B CN 112202693 B CN112202693 B CN 112202693B CN 202010914721 A CN202010914721 A CN 202010914721A CN 112202693 B CN112202693 B CN 112202693B
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CN112202693A (en
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解永生
刘建坡
柳军
朱军
周磊磊
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Shanghai Institute of Microsystem and Information Technology of CAS
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H04L25/0224—Channel estimation using sounding signals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H04L25/022—Channel estimation of frequency response
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H04L25/024—Channel estimation channel estimation algorithms
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2602—Signal structure
    • H04L27/261—Details of reference signals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only

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Abstract

The invention relates to an anti-interference frequency offset estimation method suitable for an OFDM system, which comprises the following steps: receiving a pilot frequency symbol, performing fast Fourier transform, and extracting pilot frequency subcarriers in the pilot frequency symbol; estimating the interference noise power and the signal power of a frequency domain, and estimating an interfered pilot frequency subcarrier set according to the interference noise power and the signal power; performing channel estimation to obtain a frequency domain channel response value; and eliminating the frequency domain channel response value affected by the interference, calculating the phase difference of adjacent pilot frequency symbols, and estimating a frequency offset value. The invention can enhance the anti-interference capability.

Description

Anti-interference frequency offset estimation method suitable for OFDM system
Technical Field
The invention relates to the technical field of wireless mobile communication, in particular to an anti-interference frequency offset estimation method suitable for an OFDM system.
Background
OFDM distributes a high-speed data stream to a number of subchannels with lower transmission rates for transmission via serial-to-parallel conversion. Each sub-channel occupies a sufficiently narrow bandwidth, and the channel frequency response thereof can be regarded as flat, so that the OFDM system can effectively resist the influence of multipath. Meanwhile, cyclic prefixes are inserted between OFDM symbols, so that inter-symbol interference caused by multipath can be eliminated. However, in the OFDM system, since the frequency spectrums of the subcarriers overlap with each other, orthogonality between the subcarriers is destroyed by frequency spectrum shift of a radio signal or frequency deviation between local oscillators of a transceiver occurring during transmission. Sensitive to frequency offset is one of the main drawbacks of OFDM.
Frequency offset estimation is one of the key techniques of OFDM systems. The commonly used frequency offset estimation method utilizes a frame leader sequence to perform time delay correlation, thereby calculating the phase rotation amount within a period of time and realizing the estimation of the frequency offset. Let the transmitted signal be x (n) and the carrier frequency offset be Δ f. Neglecting the influence of noise and multipath channel, after down-conversion, the baseband receiving signal is
Figure GDA0003141007440000011
In the formula, TsIs the sampling interval. Delay correlation is performed on the received signal, and the corresponding correlation value is obtained as follows:
Figure GDA0003141007440000012
in the formula, L is the cycle period of the training sequence, and the frequency offset estimation result obtained by the above formula is:
Figure GDA0003141007440000013
the above equation holds if the phase shift caused by the frequency shift does not exceed pi, i.e., 2 pi | Δ f, during the time period of point Lmax|LTsIs less than pi, to obtain | Delta fmax|＜1/(2LTs)。
The performance of the above frequency offset estimation algorithm is easily affected by external signal interference. In particular, if the received signal is interfered by the tone signal, the estimated frequency offset value is the frequency of the tone signal, not the actual frequency offset value of the system.
Disclosure of Invention
The invention aims to solve the technical problem of providing an anti-interference frequency offset estimation method suitable for an OFDM system, which can enhance the anti-interference capability.
The technical scheme adopted by the invention for solving the technical problems is as follows: the anti-interference frequency offset estimation method suitable for the OFDM system comprises the following steps:
(1) receiving a pilot frequency symbol, performing fast Fourier transform, and extracting pilot frequency subcarriers in the pilot frequency symbol;
(2) estimating the interference noise power and the signal power of a frequency domain, and estimating an interfered pilot frequency subcarrier set according to the interference noise power and the signal power;
(3) performing channel estimation to obtain a frequency domain channel response value;
(4) and eliminating the frequency domain channel response value affected by the interference, calculating the phase difference of adjacent pilot frequency symbols, and estimating a frequency offset value.
The estimating of the frequency domain interference noise power and the signal power in the step (2) specifically comprises:
(21) conjugate multiplying the extracted pilot frequency sub-carrier and the local pilot frequency to obtain Z (m, k), wherein Z (m, k) is Y (m, k) P*(m, k), Y (m, k) is the kth pilot subcarrier of the mth pilot symbol received, and P (m, k) is the local pilot;
(22) calculating the frequency domain autocorrelation value R of Z (m, k)z(m,k,j)＝Z(m,k)Z*(m,k+j)；
(23) Dividing the pilot frequency sub-carriers into L groups, and calculating the autocorrelation mean value of each group Z (m, k), wherein the autocorrelation mean value of Z (m, k) of the L group
Figure GDA0003141007440000021
QlThe number of pilot frequency sub-carriers in the l group;
(24) by passing
Figure GDA0003141007440000022
Calculating the grouping noise power and the signal power, wherein S (l) is the average signal power of the l group of subcarriers, N (l) is the average interference noise power of the l group of subcarriers, and delta is the frequency domain insertion interval of the pilot subcarriers.
The estimation of the interfered pilot subcarriers in the step (2) is specifically as follows: and if the average interference noise power of the pilot frequency subcarrier group exceeds the noise threshold, the pilot frequency subcarrier group is interfered by external signals, and all the pilot frequency subcarriers in the group are judged to be interfered by the external signals.
When the interfered pilot frequency sub-carrier is estimated in the step (2), if the ratio of the average signal power to the average interference noise power of the pilot frequency sub-carrier group is lower than the signal-to-noise ratio threshold, the pilot frequency sub-carrier group is interfered by signals, and all the pilot frequency sub-carriers in the group are judged to be interfered by external signals.
And (3) performing channel estimation by adopting a least square channel estimation algorithm.
And (4) performing low-pass filtering on the frequency domain channel response value in the step (3).
The low pass filtered parameter is [1/81/41/211/21/41/8 ].
In the step (4) by
Figure GDA0003141007440000031
Calculating the phase difference between adjacent pilot symbols by
Figure GDA0003141007440000032
A frequency offset value is calculated, wherein,
Figure GDA0003141007440000033
frequency domain channel response value, N, for the k pilot subcarrier of the m pilot symbolpIs the number of pilot frequency sub-carriers, I is the set of interfered sub-carriers, L is the number of sampling interval points of adjacent pilot frequency symbols, fsIs the sampling rate.
Advantageous effects
Due to the adoption of the technical scheme, compared with the prior art, the invention has the following advantages and positive effects: the invention can accurately estimate the interference noise power and the signal power, can accurately judge the set of the interfered subcarriers, enhances the anti-interference capability by eliminating the influence of the interfered subcarriers, and has lower calculation complexity.
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FIG. 1 is a diagram illustrating a physical frame format according to an embodiment of the present invention;
FIG. 2 is a flow chart of a method for frequency offset estimation according to an embodiment of the present invention;
fig. 3 is a flow chart of interference noise power and signal power estimation in an embodiment of the present invention.
Detailed Description
The invention will be further illustrated with reference to the following specific examples. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention. Further, it should be understood that various changes or modifications of the present invention may be made by those skilled in the art after reading the teaching of the present invention, and such equivalents may fall within the scope of the present invention as defined in the appended claims.
The embodiment of the invention relates to an anti-interference frequency offset estimation method suitable for an OFDM system, which comprises the following steps: receiving a pilot frequency symbol, performing fast Fourier transform, and extracting pilot frequency subcarriers in the pilot frequency symbol; estimating the interference noise power and the signal power of a frequency domain, and estimating an interfered pilot frequency subcarrier set according to the interference noise power and the signal power; performing channel estimation to obtain a frequency domain channel response value; and eliminating the frequency domain channel response value affected by the interference, calculating the phase difference of adjacent pilot frequency symbols, and estimating a frequency offset value.
The physical layer frame format in a burst transmission OFDM system generally includes a preamble sequence, a physical frame header, and a data payload portion, as shown in fig. 1. The preamble sequence includes a short preamble sequence and a long preamble sequence. The short preamble sequence consists of the same periodic short sequence symbols. The long leader sequence consists of two repeated long sequences, the repetition period is N, and the corresponding frequency domain sequence is P. In this embodiment, the long preamble sequence is used for frequency offset estimation, as shown in fig. 2, the specific process is as follows:
frequency offset estimation process
1) Carrying out N-point fast Fourier transform on the two received long preamble symbols;
2) extracting pilot frequency subcarrier in long preamble symbol, the number of subcarrier is Np(ii) a The pilot frequency sub-carrier is a frequency domain non-zero sub-carrier of a long sequence;
3) estimating the power of frequency domain interference noise and the power of signals; as shown in fig. 3, the method specifically includes the following sub-steps:
A) and carrying out conjugate multiplication on the received frequency domain pilot frequency subcarrier and a local pilot frequency sequence to eliminate modulation data:
Z(m,k)＝Y(m,k)P*(m,k)
in the above equation, Y (m, k) is the received frequency domain long preamble subcarrier, which is located on the kth pilot subcarrier of the mth symbol, P (m, k) is the local long preamble sequence, and m is 1: 2.
B) Computing frequency domain autocorrelation values of Z (m, k)
Rz(m,k,j)＝Z(m,k)Z*(m,k+j)
In the above formula, Rz(m, k, j) represents a correlation value of a kth subcarrier of the mth symbol;
C) computing the autocorrelation mean of Z (m, k)
And dividing the pilot frequency subcarriers into L groups, and calculating the frequency domain autocorrelation mean value of each group of subcarriers. Let the number of subcarriers of the l-th group be QlThe frequency domain autocorrelation statistical mean of the ith group can be obtained by the following formula:
Figure GDA0003141007440000041
D) estimating interference noise power and signal power
Figure GDA0003141007440000042
In the above formula, s (l) is the average signal power of the ith group of subcarriers, n (l) is the average interference noise power of the ith group of subcarriers, and Δ is the frequency domain insertion interval of the pilot subcarriers. After the long preamble is subjected to N-point fast fourier transform as a pilot symbol, the pilot subcarriers are continuously distributed, that is, Δ is 1. Thus, from the above formula can be obtained
Figure GDA0003141007440000051
Using an approximation algorithm, the interference noise power and the signal power can be approximated by:
Figure GDA0003141007440000052
4) estimating a set I of interfered pilot subcarriers:
when estimating the interfered pilot subcarriers, the following concrete criteria are: and if the average interference noise power of the pilot frequency subcarrier group exceeds the noise threshold, the pilot frequency subcarrier group is interfered by external signals, and all the pilot frequency subcarriers in the group are judged to be interfered by the external signals. In order to ensure the accuracy of estimation, the step further determines the average signal-to-noise ratio of the pilot subcarrier group, and if the ratio of the average signal power to the average interference noise power of the pilot subcarrier group is lower than the signal-to-noise ratio threshold, the pilot subcarrier group is interfered by signals, and all the pilot subcarriers in the group are determined to be interfered by external signals, that is, if the average interference noise power of the pilot subcarrier group exceeds the noise threshold and the ratio of the average signal power to the average interference noise power of the pilot subcarrier group is lower than the signal-to-noise ratio threshold, the subcarrier group is interfered by external signals, and all the subcarriers in the group are determined to be interfered by external signals.
5) Channel estimation is performed using a least squares channel estimation algorithm and FIR low pass filtering is performed on the channel response with filter parameters [1/81/41/211/21/41/8 ].
6) Eliminating the frequency domain channel response value affected by interference, calculating the phase difference of adjacent pilot frequency symbols, and estimating a frequency offset value;
wherein, the phase difference is:
Figure GDA0003141007440000053
in the formula (I), the compound is shown in the specification,
Figure GDA0003141007440000054
is a channel estimation value of the kth pilot subcarrier of the 1 st long preamble symbol,
Figure GDA0003141007440000055
channel estimation value of k pilot subcarrier of 2 nd long preamble symbol, NpIs the number of useful sub-carriers of the long preamble, and I is the set of interfered sub-carriers.
The frequency offset estimationThe evaluation value is:
Figure GDA0003141007440000056
wherein N is the repetition period of the long leader sequence, fsIs the sampling rate.
The invention can accurately estimate the interference noise power and the signal power, can accurately judge the set of the interfered subcarriers, enhances the anti-interference capability by eliminating the influence of the interfered subcarriers, and has lower calculation complexity.

Claims (7)

1. An anti-interference frequency offset estimation method suitable for an OFDM system is characterized by comprising the following steps:
(1) receiving a pilot frequency symbol, performing fast Fourier transform, and extracting pilot frequency subcarriers in the pilot frequency symbol;
(2) estimating the interference noise power and the signal power of a frequency domain, and estimating an interfered pilot frequency subcarrier set according to the interference noise power and the signal power; wherein, the estimating the frequency domain interference noise power and the signal power specifically comprises:
(21) conjugate multiplying the extracted pilot frequency sub-carrier and the local pilot frequency to obtain Z (m, k), wherein Z (m, k) is Y (m, k) P*(m, k), Y (m, k) is the kth pilot subcarrier of the mth pilot symbol received, and P (m, k) is the local pilot;
(22) calculating the frequency domain autocorrelation value R of Z (m, k)z(m,k,j)＝Z(m,k)Z*(m,k+j)；
(23) Dividing the pilot frequency sub-carriers into L groups, and calculating the autocorrelation mean value of each group Z (m, k), wherein the autocorrelation mean value of Z (m, k) of the L group
Figure FDA0003141007430000011
QlThe number of pilot frequency sub-carriers in the l group;
(24) the pilot subcarriers are continuously distributed by
Figure FDA0003141007430000012
Calculating packet noise power and signal power, wherein S (l) isAverage signal power of the sub-carriers of the group I, N (l) is the average interference noise power of the sub-carriers of the group I, and delta is the frequency domain insertion interval of the pilot frequency sub-carriers;
(3) performing channel estimation to obtain a frequency domain channel response value;
(4) and eliminating the frequency domain channel response value affected by the interference, calculating the phase difference of adjacent pilot frequency symbols, and estimating a frequency offset value.
2. The method of claim 1, wherein the step (2) of estimating the interfered pilot subcarriers specifically comprises: and if the average interference noise power of the pilot frequency subcarrier group exceeds the noise threshold, the pilot frequency subcarrier group is interfered by external signals, and all the pilot frequency subcarriers in the group are judged to be interfered by the external signals.
3. The method according to claim 1 or 2, wherein when estimating the interfered pilot subcarriers in step (2), the method further comprises that if the ratio of the average signal power to the average interference noise power of the pilot subcarrier group is lower than the snr threshold, the pilot subcarrier group is interfered by signals, and it is determined that all the pilot subcarriers in the group are interfered by external signals.
4. The method of claim 1, wherein in step (3), a least square channel estimation algorithm is used for channel estimation.
5. The method of claim 1, wherein the step (3) of low-pass filtering the frequency domain channel response value is performed.
6. The method of claim 5, wherein the low pass filtered parameter is [1/81/41/211/21/41/8 ].
7. The method of claim 1, wherein the step (4) is performed by
Figure FDA0003141007430000021
Calculating the phase difference between adjacent pilot symbols by
Figure FDA0003141007430000022
A frequency offset value is calculated, wherein,
Figure FDA0003141007430000023
frequency domain channel response value, N, for the k pilot subcarrier of the m pilot symbolpIs the number of pilot frequency sub-carriers, I is the set of interfered sub-carriers, L is the number of sampling interval points of adjacent pilot frequency symbols, fsIs the sampling rate.
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