CN114401170A - Channel estimation and equalization method of OFDM system - Google Patents
Channel estimation and equalization method of OFDM system Download PDFInfo
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- CN114401170A CN114401170A CN202111631010.1A CN202111631010A CN114401170A CN 114401170 A CN114401170 A CN 114401170A CN 202111631010 A CN202111631010 A CN 202111631010A CN 114401170 A CN114401170 A CN 114401170A
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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
- H04L25/022—Channel estimation of frequency response
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- 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
- H04L27/2655—Synchronisation arrangements
- H04L27/2668—Details of algorithms
- H04L27/2669—Details of algorithms characterised by the domain of operation
- H04L27/2671—Time domain
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- 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
- H04L27/2655—Synchronisation arrangements
- H04L27/2668—Details of algorithms
- H04L27/2669—Details of algorithms characterised by the domain of operation
- H04L27/2672—Frequency domain
Abstract
The invention relates to a channel estimation and equalization method of OFDM system, considering that the received signal on pilot frequency position contains channel information, and the data also contains channel information, the current symbol is equalized by using the channel estimation value of the previous symbol, then the decision is made, the channel of the current symbol is estimated again by using the result of the decision, then the channel estimation values of the previous and the next symbols are combined properly, and the new channel estimation value is obtained for the equalization of the next symbol. By such processing, channel information included in the data is fully utilized, channel estimation errors are reduced, and the error rate after equalization is reduced.
Description
Technical Field
The invention relates to a channel estimation and equalization technology of a receiving end of a communication system, in particular to a channel estimation and equalization method of an OFDM system.
Background
Although some channels of the wired communication system, such as optical fiber communication, etc., are constant parameter channels, the channels are generally multipath, so that intersymbol interference generated between symbols in digital communication needs to be equalized, in single carrier communication, along with the increase of multipath expansion, the number of taps of a required transversal filter is significantly increased, and the complexity of equalization is improved. Orthogonal multi-carrier (OFDM) communication, with the addition of a cyclic prefix of appropriate length, makes equalization simple. Equalization requires an estimate of the channel, whether for single carrier or multi-carrier communication. For an OFDM system, if it is a constant reference channel, it is conventional to use only one OFDM symbol as a pilot, then use the pilot to perform channel estimation, and use the channel estimation value to equalize all subsequent data symbols. This approach does not consider that the data symbols also contain channel information, which reduces the estimation accuracy to some extent, resulting in a higher error rate after equalization. Therefore, if the channel information contained in the data symbols can be fully utilized, the estimation accuracy can be effectively improved, and the error rate after equalization can be reduced.
Disclosure of Invention
In view of the above, an object of the present invention is to provide a channel estimation and equalization method for an OFDM system, which reduces the bit error rate after equalization of the system by fully utilizing channel information in data symbols.
In order to achieve the purpose, the invention adopts the following technical scheme:
a channel estimation and equalization method of OFDM system, under the state of time and frequency synchronization, the output sequence from the preposed coherent demodulation module is processed by the following steps:
s1, defining M to represent the number of subcarriers of the OFDM signal, N to represent the number of OFDM symbols in a group, V1 and V2 to represent vectors formed by the M numbers, and V1/V2 to represent that each number in V1 is divided by each number in the same position in V2;
step S2, the transmitting end groups the transmitted sequence according to a preset rule, and the used pilot frequency is marked as X1= [ X1 ]1,X12,…,X1M]；
Step S3, the receiving end respectively processes the cyclic prefix removal and Fourier transform to all OFDM time domain symbols to obtain all OFDM frequency domain symbols; then, the symbol sequences after the fourier transform are grouped by a group of N OFDM frequency domain symbols, and for each group, the first OFDM frequency domain symbol of the group is taken and recorded as Y1= [ Y1 ]1,Y12,…,Y1M]And using X1 of the same group of transmitting terminals to obtain H1= Y1./X1;
and step S4, for each group, starting from the second OFDM frequency domain symbol, calculating the channel frequency response estimated value of each symbol in turn, and balancing until all the symbols in the group are processed.
Further, the preset rule is specifically as follows: n OFDM frequency domain symbols are used as a group, one OFDM frequency domain symbol is M in length, pilot frequency is inserted into the position of the first OFDM frequency domain symbol of each group, and data is not transmitted.
Further, the step S4 is specifically to calculate a channel frequency response estimated value H2 of the current OFDM symbol, combine the channel frequency response estimated value H2 with a channel frequency response estimated value H1 of the previous symbol according to a preset ratio, record the result as H2_ new, equalize the current symbol with H2_ new, then replace the original H1 with H2_ new, then use the next OFDM symbol as the current OFDM symbol, and loop until all OFDM symbols in the packet complete channel estimation and equalization.
Further, the loop process is performed until all OFDM symbols in the packet complete channel estimation and equalization, specifically:
step S41: let i = 2;
step S42: taking the ith OFDM frequency domain symbol of the group as a current symbol, marking as Y2, and obtaining X2= Y2./H1;
step S43: judging X2 according to the mapping rule of a constellation diagram used by digital modulation, and recording the obtained complex sequence as X2_ new;
step S44: calculating H2= Y2./X2_ new;
step S45: calculating H2_ new = a H1+ b H2, a and b each taking a real number between 0 and 1;
step S46: calculating X2_ equ = Y2./H2_ new, and judging X2_ equ according to a mapping rule of a constellation diagram used by digital modulation to obtain a binary sequence, wherein the sequence is used as an equalization result of the ith symbol;
step S47: and judging whether i is equal to N, if not, enabling H1= H2_ new and i = i +1, returning to S41 to continue calculation, and otherwise, finishing channel estimation and equalization.
Compared with the prior art, the invention has the following beneficial effects:
the invention combines the channel estimation value of the current symbol and the channel estimation value of the previous symbol to properly integrate the current symbol and the previous symbol, and effectively reduces the bit error rate after the system is balanced by fully utilizing the channel information in the data symbol.
Drawings
FIG. 1 is a schematic of the process of the present invention;
fig. 2 is a bit error rate curve of embodiment 1 of the present invention.
Detailed Description
The invention is further explained below with reference to the drawings and the embodiments.
Referring to fig. 1, the present invention provides a channel estimation and equalization method for an OFDM system, which performs signal processing on an output sequence from a pre-coherent demodulation module in a time and frequency synchronization state according to the following steps:
s1, defining M to represent the number of subcarriers of the OFDM signal, N to represent the number of OFDM symbols in a group, V1 and V2 to represent vectors formed by the M numbers, and V1/V2 to represent that each number in V1 is divided by each number in the same position in V2;
step S2, the transmitting end takes N OFDM frequency domain symbols as a group, one OFDM frequency domain symbol has a length of M, pilot is inserted into the first OFDM frequency domain symbol position of each group, and data is not transmitted, the transmitted sequences are grouped, and the used pilot is marked as X1= [ X1 ]1,X12,…,X1M]；
Step S3, the receiving end respectively processes the cyclic prefix removal and Fourier transform to all OFDM time domain symbols to obtain all OFDM frequency domain symbols; then, the symbol sequences after fourier transform are grouped according to the rule of "a group of N OFDM frequency domain symbols", and for each group, the first OFDM frequency domain symbol of the group is taken and is marked as Y1= [ Y1 ]1,Y12,…,Y1M]And using X1 of the same group of transmitting terminals to obtain H1= Y1./X1;
step S4, for each group, starting from the second OFDM frequency domain symbol, calculating the channel frequency response estimated value H2 of the current OFDM symbol, and combining the channel frequency response estimated value H1 of the previous symbol according to a certain proportion, the result is recorded as H2_ new, H2_ new is used for equalizing the current symbol, H2_ new is used for replacing the original H1, then the next OFDM symbol is used as the current OFDM symbol, and the process is circulated until all the OFDM symbols in the group complete channel estimation and equalization;
preferably, step S4 specifically includes:
step S41: let i = 2;
step S42: taking the ith OFDM frequency domain symbol of the group as a current symbol, marking as Y2, and obtaining X2= Y2./H1;
step S43: judging X2 according to the mapping rule of a constellation diagram used by digital modulation, and recording the obtained complex sequence as X2_ new;
step S44: calculating H2= Y2./X2_ new;
step S45: calculating H2_ new = a H1+ b H2, a and b each taking a real number between 0 and 1;
step S46: calculating X2_ equ = Y2./H2_ new, and judging X2_ equ according to a mapping rule of a constellation diagram used by digital modulation to obtain a binary sequence, wherein the sequence is used as an equalization result of the ith symbol;
step S47: and judging whether i is equal to N, if not, enabling H1= H2_ new and i = i +1, returning to S41 to continue calculation, and otherwise, finishing channel estimation and equalization.
Example 1
The number of OFDM subcarriers M =1024, the cyclic prefix length L =64, the number of OFDM symbols N =1000 per group, and 20 packets in total. The channel is superimposed with white gaussian noise. Using the constellation of QPSK, the mapping relationship is as follows: 00-1 + i, 01- (1 + i, 11) -1-i, 10-1-i. The channel impulse response used is represented in the form of a row vector:
[0.1148 - 0.1886i，0 ，0，0.8147 + 0.0166i，0，0，0，0，-0.3245 + 0.1297i，0，0，0.0404 + 0.1520i，0，-0.3737 + 0.0260i]
the following explains the processing of the 1 st received symbol packet. Other received packets, except for different transmitted and received data, are processed and used in the same formula as the 1 st received symbol packet.
Step S1: v1 and V2 each represent a vector consisting of M numbers, and V1./V2 each represent the number of V1 divided by the number of V2 at the same position;
step S2: the transmitting end groups the transmitted sequence according to a rule that "N =1000 OFDM frequency domain symbols are a group, one OFDM frequency domain symbol has a length of M =1024, a pilot is inserted into the first OFDM frequency domain symbol position of each group, and no data is transmitted", where the pilot used is denoted as X1= [ X1 ]1,X12,…,X1M]=[1+i,1+i,...,1+i]；
Step S3: the receiving end respectively carries out the treatment of removing the cyclic prefix and Fourier transform on all the OFDM time domain symbols to obtain all the OFDM frequency domain symbols; then, the fourier-transformed symbol sequence is also grouped according to the rule of "a group of N =1000 OFDM frequency-domain symbols", and for each group, the first OFDM frequency-domain symbol of the group is taken and is denoted as Y1= [ Y1 ]1,Y12,…,Y1M]And using X1 of the same group of transmitting terminals to obtain H1= Y1./X1;
step S4: calculating a channel frequency response estimated value H2 of a current OFDM symbol, combining the channel frequency response estimated value H1 of a previous symbol with the channel frequency response estimated value H1 of the previous symbol according to a certain proportion, recording the result as H2_ new, balancing the current symbol by using H2_ new, then replacing the original H1 by using H2_ new, then using the next OFDM symbol as the current OFDM symbol, and circulating the process until all the OFDM symbols in a group complete channel estimation and balance;
the method is characterized in that the step S4 is implemented as follows:
step S41: let i = 2;
step S42: taking the ith OFDM frequency domain symbol of the group as a current symbol, marking as Y2, and obtaining X2= Y2./H1;
step S43: according to the mapping rule of a constellation diagram used by digital modulation, judging X2, judging a complex number in X2 to be 1+ i if the phase angle is greater than or equal to 0 and less than 90 degrees, judging the complex number to be-1 + i if the phase angle is greater than or equal to 90 degrees and less than 180 degrees, judging the complex number to be-1-i if the phase angle is greater than or equal to 180 degrees and less than 270 degrees, judging the complex number to be 1-i if the phase angle is greater than or equal to 270 degrees and less than 360 degrees, and marking a new sequence as X2_ new after all the complex numbers in X2 are judged;
step S44: calculating H2= Y2./X2_ new;
step S45: calculate H2_ new = a × H1+ b × H2, here, take a =1, b = 0.1;
step S46: calculating X2_ equ = Y2./H2_ new, judging X2_ equ according to a mapping rule of a constellation diagram used by digital modulation, judging a certain complex number in the X2_ equ to be 00 if the phase angle is greater than or equal to 0 and less than 90 degrees, judging the complex number to be 01 if the phase angle is greater than or equal to 90 degrees and less than 180 degrees, judging the complex number to be 11 if the phase angle is greater than or equal to 180 degrees and less than 270 degrees, judging the complex number to be 10 if the phase angle is greater than or equal to 270 degrees and less than 360 degrees, and obtaining a binary sequence which is an equalization result of the current symbol after all the complex numbers in the X2_ equ are judged to be finished;
step S47: and judging whether i is equal to 1000, if not, letting H1= H2_ new and i = i +1, and returning to S41 to continue calculation, otherwise, completing channel estimation and equalization.
According to the above steps, as shown in fig. 2, the obtained equalized bit error rate has about 1dB of gain under low signal-to-noise ratio compared with the conventional LS-based channel estimation and equalization method using only pilot frequency, and the gain is gradually increased under the condition of high signal-to-noise ratio, and the bit error rate is the same as the bit error rate of equalization using a completely accurate channel estimation value from 21 dB.
The above description is only a preferred embodiment of the present invention, and all equivalent changes and modifications made in accordance with the claims of the present invention should be covered by the present invention.
Claims (4)
1. A channel estimation and equalization method of OFDM system is characterized in that under the state of time and frequency synchronization, the output sequence from the preposed coherent demodulation module is processed by the following steps:
s1, defining M to represent the number of subcarriers of the OFDM signal, N to represent the number of OFDM symbols in a group, V1 and V2 to represent vectors formed by the M numbers, and V1/V2 to represent that each number in V1 is divided by each number in the same position in V2;
step S2, the transmitting end groups the transmitted sequence according to a preset rule, and the used pilot frequency is marked as X1= [ X1 ]1,X12,…,X1M]；
Step S3, the receiving end respectively processes the cyclic prefix removal and Fourier transform to all OFDM time domain symbols to obtain all OFDM frequency domain symbols; then, the symbol sequences after the fourier transform are grouped by a group of N OFDM frequency domain symbols, and for each group, the first OFDM frequency domain symbol of the group is taken and recorded as Y1= [ Y1 ]1,Y12,…,Y1M]And using X1 of the same group of transmitting terminals to obtain H1= Y1./X1;
and step S4, for each group, starting from the second OFDM frequency domain symbol, calculating the channel frequency response estimated value of each symbol in turn, and carrying out equalization until all the symbols in the group are processed.
2. The method of claim 1, wherein the predetermined rule is specifically: n OFDM frequency domain symbols are used as a group, one OFDM frequency domain symbol is M in length, pilot frequency is inserted into the position of the first OFDM frequency domain symbol of each group, and data is not transmitted.
3. The method for channel estimation and equalization in an OFDM system as claimed in claim 1, wherein said step S4 comprises calculating an estimated value H2 of the channel frequency response of the current OFDM symbol, combining the estimated value H2 with an estimated value H1 of the channel frequency response of the previous symbol according to a predetermined ratio, recording the result as H2_ new, equalizing the current symbol with H2_ new, replacing the original H1 with H2_ new, and then taking the next OFDM symbol as the current OFDM symbol, and repeating the process until all the OFDM symbols in the packet complete channel estimation and equalization.
4. The method according to claim 3, wherein the loop process is performed until all OFDM symbols in the packet complete channel estimation and equalization, specifically:
step S41: let i = 2;
step S42: taking the ith OFDM frequency domain symbol of the group as a current symbol, marking as Y2, and obtaining X2= Y2./H1;
step S43: judging X2 according to the mapping rule of a constellation diagram used by digital modulation, and recording the obtained complex sequence as X2_ new;
step S44: calculating H2= Y2./X2_ new;
step S45: calculating H2_ new = a H1+ b H2, a and b each taking a real number between 0 and 1;
step S46: calculating X2_ equ = Y2./H2_ new, and judging X2_ equ according to a mapping rule of a constellation diagram used by digital modulation to obtain a binary sequence, wherein the sequence is used as an equalization result of the ith symbol;
step S47: and judging whether i is equal to N, if not, enabling H1= H2_ new and i = i +1, returning to S41 to continue calculation, and otherwise, finishing channel estimation and equalization.
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