CN108737029B - Time-frequency interleaving preprocessing combination method for resisting narrow-band interference and impulse noise in OFDM system - Google Patents

Time-frequency interleaving preprocessing combination method for resisting narrow-band interference and impulse noise in OFDM system Download PDF

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CN108737029B
CN108737029B CN201810453605.4A CN201810453605A CN108737029B CN 108737029 B CN108737029 B CN 108737029B CN 201810453605 A CN201810453605 A CN 201810453605A CN 108737029 B CN108737029 B CN 108737029B
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倪阳
赵民建
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Zhejiang University ZJU
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
    • H04L1/0056—Systems characterized by the type of code used
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
    • H04L1/0041—Arrangements at the transmitter end
    • H04L1/0042—Encoding specially adapted to other signal generation operation, e.g. in order to reduce transmit distortions, jitter, or to improve signal shape
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    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
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    • H04L27/26—Systems using multi-frequency codes
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Abstract

The invention provides a time-frequency interleaving preprocessing combination method for resisting narrow-band interference and impulse noise in an OFDM system, which specifically comprises the following steps: in the OFDM system, after the signal is mapped with a corresponding constellation through LDPC coding, time domain interleaver parameters are optimized, and noise is fully whitened; after OFDM modulation at a sending end, carrying out frequency domain interleaving on OFDM subcarriers by using cyclic shift operation of a block interleaver submatrix in a frequency domain; noise which is still higher than a threshold after full interleaving and interference nonlinearity are removed through amplitude limiting method noise preprocessing of a receiving end. The invention fully combines the characteristics of the interleaver and the advantages of nonlinear noise processing, can effectively enhance the resistance of the OFDM system to narrow-band interference and impulse noise, and further improves the transmission performance of the system compared with the traditional block interleaving method.

Description

Time-frequency interleaving preprocessing combination method for resisting narrow-band interference and impulse noise in OFDM system
Technical Field
The invention relates to a method for resisting interference and noise in an OFDM (orthogonal frequency division multiplexing) system, in particular to a time-frequency interleaving preprocessing combination method for resisting narrow-band interference and impulse noise in the OFDM system.
Background
Orthogonal Frequency Division Multiplexing (OFDM) is a high-efficiency multi-carrier modulation technique, and has the characteristics of orthogonal multiplexing among sub-carrier frequencies and long duration, so that the OFDM has the advantages of high spectrum utilization rate, multipath effect resistance and the like, and is widely popularized and applied to various digital communication systems, such as Wireless Local Area Networks (WLANs), PLC power line standard communication IEEE1901.2, digital broadcast audio (DAB) and the like, and is a core representative of 4G communication.
OFDM is evolving through FDM systems. In the conventional FDM system, a certain interval is required in the frequency spectrum to ensure that the system does not generate crosstalk between channels, and the frequency spectrums between sub-carriers cannot overlap. To achieve this, the OFDM system requires that each subcarrier needs to be orthogonal to each other, and that the carriers are synchronized in both time and frequency, and the interval between adjacent subcarriers is the inverse of the effective symbol time interval T.
The OFDM technology divides the data stream of the multi-carrier into a plurality of sub-data streams with lower code rates, and the sub-data streams with the lower code rates are used for modulating corresponding sub-carriers, so that the duration of a transmission signal is prolonged, and intersymbol interference (ISI) is effectively reduced.
The transmission performance of a wideband digital system is affected by many factors, and various noises and interferences are particularly serious. Especially for those interferences and noises that are widely present but have particularity, such as narrowband interference and impulse noise, they have complexity, randomness and sparsity completely different from white gaussian noise.
In the method for resisting such interference and noise, the interleaving and de-interleaving technique is commonly used, and the principle is that continuous bit data is dispersed, and errors in a single frame are effectively dispersed into each frame, so that a string of continuous errors become single or relatively dispersed short errors, and at the moment, the original data of signals can be recovered to the maximum extent by matching with an error correction range accommodated by channel coding, so that the stability and robustness of a communication system are effectively improved. In essence, interleaving and deinterleaving only change the structure of data to disperse burst errors, and do not change the content carried by the data itself, which makes the interleaving and deinterleaving technology widely applied to most communication systems.
However, the conventional interleaving technology has many disadvantages, such as poor flexibility, and for different coded modulation parameters, the interleaver needs to be designed again; the traditional block interleaving of the frequency domain cannot simultaneously avoid errors caused by time domain and frequency domain combination, so that the interleaving performance is not greatly improved, and the like. On the other hand, since the interleaving technique can only reduce the influence of impulse noise in probability and cannot absolutely guarantee the elimination, the interleaving technique has certain limitation when the noise power is too high.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a time-frequency interleaving preprocessing combination method for resisting narrow-band interference and impulse noise in an OFDM system, thereby effectively improving the transmission performance of the system.
In order to achieve the purpose, the specific technical scheme is as follows:
a time-frequency interleaving pretreatment combination method for resisting narrow-band interference and impulse noise in an OFDM system is characterized by comprising the following steps:
the method comprises the following steps: original signal of binary bit stream bkFirstly, at a sending end, LDPC coding is carried out to generate a code word { c }lThe code words are mapped by a constellation diagram to generate complex field symbols { c }tWill then be { c }tInterweaving the symbols by a time domain interweaver with m rows and n columns and adding pilot frequency to obtain complex domain symbols { c }mWherein, the parameters m and n of the time domain interleaver are solved by the following formula xiTThe maximum value of (a) to (b) to obtain an optimal solution;
Figure GDA0002599971230000021
wherein N isIOB(x)The number of independent OFDM blocks in the nth LDPC code word;
LCthe number of LDPC code words in the interleaver;
Lsymthe number of data symbols of LDPC;
m and n are respectively the row number and the column number of the interleaver;
n is the OFDM data subcarrier number and the order of DFT;
ξTthe meaning is NIOB(x)And its theoretical maximum
Figure GDA0002599971230000022
Average ratio of (d);
step two: will complex field symbol cmModulating the symbol into OFDM subcarrier, carrying out frequency domain interleaving treatment on the subcarrier, then carrying out IDFT change, and converting the data symbol into time domain signal xiSending the signal into an antenna for transmission; the frequency domain interleaving method comprises the following steps:
(1) reading the data symbols modulated by the OFDM subcarriers into a frequency domain interleaver in a row unit, and dividing the data symbols into S submatrices under the condition of ensuring that the column number of each submatrix is the same;
(2) within each sub-matrix, cyclically shifting f according to each sub-matrixrThe unit rule is carried out, finally the new matrix after the cyclic shift is read out according to columns and is mapped to the subcarrier to complete the frequency domain interleaving, wherein
fr＝p·r
Wherein, p is a preset coefficient, the value is a prime number greater than or equal to 2, and r is the number of the submatrix;
step three: after a bad channel with coexistence of AWGN noise, impulse noise and narrow-band interference, a signal is received by an antenna, and the signal received by the antenna is yiY is adjusted by the amplitude limiting method of the receiving endiNoise preprocessing is carried out, namely, noise which is still higher than a threshold after full interweaving and interference nonlinearity are removed;
step four: and performing DFT on the signal after the noise is removed, performing corresponding frequency domain de-interleaving, OFDM demodulation, time domain de-interleaving and constellation de-mapping, decoding by LDPC, and outputting the signal.
Further, in the second step, each sub-matrix is circularly shifted by frThe specific process of each unit is as follows:
Figure GDA0002599971230000031
wherein
Figure GDA0002599971230000032
And
Figure GDA0002599971230000033
respectively representing the jth row in the r-th sub-matrix after and before shifting, i.e. c(r)Each row, r-0, 1, K S-1 is displaced down or up by frAnd (4) units.
Further, the OFDM data symbol length is N, which takes 256, 512, 1024, or 2048, the LDPC code length is C, which takes 3072 or 5120, and the code rate is T, which takes 1/2 or 2/3.
Furthermore, the number of the sub-matrixes S is the same as n, and p is 2, 3, 5, 7 or 11.
Further, the threshold T of the clipping non-linear noise preprocessing is performedkThe ratio to the average power of the signal is 1.8-2.2.
The invention has the beneficial effects that:
the combination method of the invention effectively disperses the errors in a single frame to each frame by using a time-frequency combination interweaving and nonlinear noise preprocessing mode, thus leading the serial continuous errors to be changed into single or relatively dispersed short errors, effectively whitening the interference noise, having good resistance performance especially under the influence of strong interference and noise, and further improving the transmission performance, robustness, flexibility and stability of the OFDM system.
Drawings
FIG. 1 is a flow diagram of a clipped time-frequency interleaved OFDM system;
FIG. 2 is a schematic diagram of the cyclic shift of the frequency domain sub-matrix according to the present invention;
FIG. 3 is a graph of system bit error rate for different threshold to signal average power ratios in the system;
FIG. 4 is a graph comparing the performance of the proposed algorithm of the present invention with conventional block interleaving under impulse noise only;
fig. 5 is a graph comparing the performance of the proposed algorithm with conventional block interleaving under narrowband interference only;
figure 6 is a graph comparing the performance of the algorithm under strong interference noise and weak interference noise for the AWGN channel of the present invention.
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings and preferred embodiments, and the objects and effects of the present invention will become more apparent, and the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
As shown in fig. 1, the flow chart of the OFDM system is expressed as follows:
yi＝hie xi+ei+zi+wi
wherein:
hi＝[hi,0 hi,1...hi,N-1]Trepresenting the multi-path channel impulse response with the maximum multi-path time delay of L;
e is a convolution operation;
ei＝[ei,0 ei,1...ei,N-1]Ta narrowband interference signal corresponding to an ith received time domain OFDM symbol;
zi＝[zi,0 zi,1...zi,N-1]Tis an impulse noise signal corresponding to the ith received time domain OFDM symbol;
wi＝[wi,0 wi,1...wi,N-1]TAWGN noise representing the background;
a time-frequency interleaving preprocessing combination method for resisting narrowband interference and impulse noise in an OFDM system, as shown in fig. 1, the method specifically includes the following steps:
the method comprises the following steps: original signal of binary bit stream bkFirstly, at a sending end, LDPC coding is carried out to generate a code word { c }lThe coded code words are mapped by a QPSK constellation diagram to generate complex domain symbols { c }tWill then be { c }tInterweaving the symbols by a time domain interweaver with m rows and n columns and adding pilot frequency to obtain complex domain symbols { c }m}。
Here, the OFDM data symbol length is N, which takes 256, 512, 1024, or 2048, the LDPC code length is C, which takes 3072 or 5120, and the code rate is T, which takes 1/2 or 2/3. In this embodiment, N is 1024, C is 3072, and T is 2/3.
For one FEC code word, if the number of OFDM data blocks included in the FEC code word increases, the system has better resistance to impulse noise, and as the number of independent OFDM blocks corresponding to each FEC code word increases, the unsuccessful decoding rate caused by burst errors of impulse noise further decreases. As occurs at reference numeral 1 in fig. 1, the following formula is a quantitative evaluation criterion for the theoretical basis:
Figure GDA0002599971230000041
wherein N isIOB(x)The number of independent OFDM blocks in the nth LDPC code word;
LCthe number of LDPC code words in the interleaver;
Lsymthe number of data symbols of LDPC;
m and n are respectively the row number and the column number of the interleaver;
n is the OFDM data subcarrier number and the order of DFT;
ξTthe meaning is NIOB(x)And its theoretical maximum
Figure GDA0002599971230000051
Average ratio of (d);
the optimal solution solved here is as follows:
Figure GDA0002599971230000052
Figure GDA0002599971230000053
wherein the content of the first and second substances,
Figure GDA0002599971230000054
the operator is solved for the least common multiple.
LbIs the number of bits of the LDPC codeword.
p is a prime number which is greater than or equal to 2 and is smaller, such as 2, 3, 5, 7, and 11, because in a general OFDM system and the g.hn9960 power line standard, the values of p can both meet corresponding settings, so that an optimal time domain interleaving scheme exists. In this example, p is 2.
A delay multiplier acceptable to the system;
m is the order of the constellation mapping, 4 in case of QPSK.
Step two: will complex field symbol cmModulating the symbol into OFDM subcarrier, carrying out frequency domain interleaving treatment on the subcarrier, then carrying out IDFT change, and converting the data symbol into time domain signal xiSending the signal into an antenna for transmission;
frequency domain interleaving occurs at reference numeral 2 in the OFDM system depicted in fig. 1. Similar to the above-mentioned optimized time domain interleaving scheme, the problem to be solved by the optimized frequency domain interleaving scheme is to increase the number of different OFDM subcarriers mapped to the data symbols corresponding to the strong error correction codeword. The benefit of uniformly allocating the subcarriers in the codeword is to disperse the error caused by the narrowband interference to each codeword, thereby further improving the performance of the system.
The quantitative evaluation standard of the scheme is as follows:
Figure GDA0002599971230000055
wherein:
NISC(x)the number of different sub-carriers of the data symbol corresponding to each code word.
The problem described above can then be understood as finding the equation such that ξ is found in the case of a time-domain interleaving determinationFAnd (4) maximizing. Through optimization, the following results are obtained:
Figure GDA0002599971230000056
the frequency domain interleaving method is as follows (as shown in fig. 2):
(1) reading the data symbols modulated by the OFDM subcarriers into a frequency domain interleaver in a row unit, and dividing the data symbols into S submatrices under the condition of ensuring that the column number of each submatrix is the same; the number S of the sub-matrixes can be the same as n;
(2) within each sub-matrix, cyclically shifting f according to each sub-matrixrThe rule of each unit is carried out, and the specific thought of the rule is as follows:
Figure GDA0002599971230000061
wherein
Figure GDA0002599971230000062
And
Figure GDA0002599971230000063
respectively representing the r-th sub-matrix after and before shiftingRow j, intra-row cyclic operation is about to c(r)Each row, r-0, 1, K S-1 is displaced down or up by frAnd finally, reading out the new matrix after the cyclic shift according to columns, and mapping the new matrix to a subcarrier to complete frequency domain interleaving. In view of simplifying the amount of computation of the system, f may be maderP is a preset coefficient, the value of p is a prime number which is greater than or equal to 2, and r is the number of the submatrix;
in the proposed cyclic shift scheme, it can be found that each individual sub-matrix has its own cyclic unit, and before mapping to OFDM data, it corresponds to different frCyclic shift of unit, therefore, it can be deduced that the symbol pattern in the LDPC code word also has S mutually non-repeating patterns, so that xi in the above formulaFAre present.
Step three: after a bad channel with coexistence of AWGN noise, impulse noise and narrow-band interference, a signal is received by an antenna, and the signal received by the antenna is yiY is adjusted by the amplitude limiting method of the receiving endiNoise preprocessing is carried out, namely, noise which is still higher than a threshold after full interweaving and interference nonlinearity are removed;
noise preprocessing takes place at the point of fig. 1, labeled 3, by: in an OFDM system, peak reduction is performed when a point exceeding a threshold is detected.
Figure GDA0002599971230000064
k(t)＝∑Akq(t-tk)
Figure GDA0002599971230000065
Wherein:
Figure GDA0002599971230000066
is the signal after amplitude limiting processing, and S (t) is the signal before processing;
Akas weighting factor, by a threshold TkAnd the original signal S (t)k) Processing and generating;
q (t) is a reference function, a sinc function is typically used under general conditions.
k (t) is AkAnd by a displacement tkThe product of q (t) for time domain sample points.
Because the threshold value selection of the amplitude limiting method is very critical, the optimal threshold value needs to be obtained through pre-calculation, when the threshold value lower than the optimal threshold value is selected, the false alarm probability of the system is improved, and when the threshold value higher than the optimal threshold value is selected, the false alarm probability of the system is improved.
FIG. 3 shows that the optimum threshold value of the simulation system is determined by finding the BER performance of the system under a certain SNR, wherein the abscissa A is the set value TkAnd the ratio of the average power of the signal to the average power of the signal, the ordinate of the mean power of the system is the BER of the system, and the ratio of the average power of the signal to the average power of the impulse noise is-10 db and-15 db respectively. The threshold T of the clipping method can be seenkThe ratio A to the average power of the signal is 1.8-2.2, and the performance effect is best.
Step four: and performing DFT on the signal after the noise is removed, performing corresponding frequency domain de-interleaving, OFDM demodulation, time domain de-interleaving and constellation de-mapping, decoding by LDPC, and outputting the signal.
Fig. 4 is a comparison between the performance of the amplitude-limiting time-frequency interleaving combination method and the performance of the conventional block interleaving in the presence of impulse noise, which shows that the amplitude-limiting time-frequency interleaving combination scheme provided by the present invention obtains a gain of 0.2-0.3 db.
Fig. 5 is a comparison between the performance of the clipping time-frequency interleaving combination method and the performance of the conventional block interleaving under the condition of only having the narrowband interference, and it can be seen that the clipping time-frequency interleaving combination scheme provided by the present invention obtains a gain of about 0.6 db. The addition of frequency domain interleaving, as compared to conventional block interleaving, enhances the ability of the system to combat narrowband interference.
Fig. 6 is a comparison of the performance of the amplitude-limiting time-frequency interleaving combination method, the conventional block interleaving and the simple time-frequency interleaving scheme under the condition that the system has strong and weak narrowband interference and impulse noise, and it can be seen that the amplitude-limiting time-frequency interleaving combination scheme provided by the present invention has a gain of about 1db under the condition that strong narrowband interference and impulse noise exist, which indicates that the receiving-end noise preprocessing method can make up the defect that the interference and noise power possessed by the interleaving cannot be effectively coped with when the interference and noise power are too large.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and although the invention has been described in detail with reference to the foregoing examples, it will be apparent to those skilled in the art that various changes in the form and details of the embodiments may be made and equivalents may be substituted for elements thereof. All modifications, equivalents and the like which come within the spirit and principle of the invention are intended to be included within the scope of the invention.

Claims (5)

1. A time-frequency interleaving pretreatment combination method for resisting narrow-band interference and impulse noise in an OFDM system is characterized by comprising the following steps:
the method comprises the following steps: original signal of binary bit stream bkFirstly, at a sending end, LDPC coding is carried out to generate a code word { c }lThe code words are mapped by a constellation diagram to generate complex field symbols { c }tWill then be { c }tInterweaving the symbols by a time domain interweaver with m rows and n columns and adding pilot frequency to obtain complex domain symbols { c }mWherein, the parameters m and n of the time domain interleaver are solved by the following formula xiTObtaining an optimal solution for the maximum value of the parameters;
Figure FDA0002599971220000011
wherein N isIOB(x)The number of independent OFDM blocks in the nth LDPC code word;
LCthe number of LDPC code words in the interleaver;
Lsymthe number of data symbols of LDPC;
m and n are respectively the row number and the column number of the interleaver;
n is the OFDM data subcarrier number and the order of DFT;
ξTthe meaning is NIOB(x)And its theoretical maximum
Figure FDA0002599971220000012
Average ratio of (d);
step two: will complex field symbol cmModulating the symbol into OFDM subcarrier, carrying out frequency domain interleaving treatment on the subcarrier, then carrying out IDFT change, and converting the data symbol into time domain signal xiSending the signal into an antenna for transmission; the frequency domain interleaving method comprises the following steps:
(1) reading the data symbols modulated by the OFDM subcarriers into a frequency domain interleaver in a row unit, and dividing the data symbols into S submatrices under the condition of ensuring that the column number of each submatrix is the same;
(2) within each sub-matrix, cyclically shifting f according to each sub-matrixrThe unit rule is carried out, finally the new matrix after the cyclic shift is read out according to columns and is mapped to the subcarrier to complete the frequency domain interleaving, wherein
fr＝p·r
Wherein, p is a preset coefficient, the value is a prime number greater than or equal to 2, and r is the number of the submatrix;
step three: after a bad channel with coexistence of AWGN noise, impulse noise and narrow-band interference, a signal is received by an antenna, and the signal received by the antenna is yiY is adjusted by the amplitude limiting method of the receiving endiNoise preprocessing is carried out, namely, noise which is still higher than a threshold after full interweaving and interference nonlinearity are removed;
step four: and performing DFT on the signal after the noise is removed, performing corresponding frequency domain de-interleaving, OFDM demodulation, time domain de-interleaving and constellation de-mapping, decoding by LDPC, and outputting the signal.
2. The joint time-frequency interleaving preprocessing method for resisting narrow-band interference and impulse noise in OFDM system as claimed in claim 1, wherein each sub-matrix cyclic shift f in said step tworThe specific process of each unit is as followsThe following:
Figure FDA0002599971220000021
wherein
Figure FDA0002599971220000022
And
Figure FDA0002599971220000023
respectively representing the jth row in the r-th sub-matrix after and before shifting, i.e. c(r)Each row, r-0, 1, K S-1 is displaced down or up by frAnd (4) units.
3. The time-frequency interleaving preprocessing combination method for resisting narrow-band interference and impulse noise in the OFDM system according to claim 1, wherein the OFDM data symbol length is N, and takes 256, 512, 1024, or 2048, the LDPC code length is C, and takes 3072 or 5120, and the code rate is T, and takes 1/2 or 2/3.
4. The time-frequency interleaving preprocessing combination method for resisting narrow-band interference and impulse noise in the OFDM system as claimed in claim 1, wherein the number S and n of the sub-matrices are the same, and p is 2, 3, 5, 7 or 11.
5. The joint time-frequency interleaving pre-processing method against narrowband interference and impulse noise in an OFDM system as claimed in claim 1, wherein the threshold T of clipping non-linear noise pre-processingkThe ratio to the average power of the signal is 1.8-2.2.
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