CN104836769B - A kind of joint timing leading based on conjugated structure and frequency synchronization method - Google Patents
A kind of joint timing leading based on conjugated structure and frequency synchronization method Download PDFInfo
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Abstract
It is as follows the step of this method the invention provides a kind of leading based on conjugated structure joint timing and frequency synchronization method：1st, reception signal r (n) and leading symbol c (n) coherent signal r are calculated0(n,d)；2nd, differential correlation signal p (m, d) is calculated；3rd, differential correlation signal p (m, d) is weighted to add up and obtains weighting correlation function P (d)；4th, weighting correlation function P (d) is normalized using the energy of reception signal in sliding window, obtains normalizing timing metric value M (d)；5th, maximum is asked for normalization timing metric M (d), obtains timing slip estimator6th, fractional part Nonlinear Transformation in Frequency Offset Estimation amount is calculated7th, the Nonlinear Transformation in Frequency Offset Estimation amount of fractional part is utilizedThe reception signal after being compensated is compensated to reception signal8th, the reception signal after being compensated using decimal frequency bias calculates the offset estimation amount of integer part9th, Nonlinear Transformation in Frequency Offset Estimation amountThis method only with one there is the leading symbol of conjugated structure to carry out union of symbol timing and carrier frequency synchronization, timing synchronization not to be influenceed by frequency departure.
Description
Technical Field
The invention belongs to the technical field of digital wireless communication transmission, and particularly relates to a combined timing and frequency synchronization method based on a conjugate structure preamble.
Background
Orthogonal Frequency Division Multiplexing (OFDM) can effectively combat the multipath effect of the channel, has high spectrum utilization rate, and can provide high-speed data transmission under a wireless fading channel. The OFDM system is applied to the fields of Digital Video Broadcasting (DVB) and a new generation of terrestrial mobile communications, and becomes an international and industrial standard. In the field of satellite communications, OFDM is suitable for high-speed data transmission over broadband satellite communication channels, for example, the european satellite standard DVB-SH has devised a hybrid terrestrial and satellite communication system employing OFDM or TDM techniques.
Assuming that the high speed serial data stream information rate is R b The symbol interval being T b ＝1/R b . The OFDM sending end firstly carries out serial-to-parallel conversion on high-speed serial data streams to obtain relatively low-speed parallel sub-data streams. The information rate of the parallel sub-data stream obtained by serial-to-parallel conversion (S/P) is reduced to 1/N (namely R) of the information rate of the input serial data stream b /N), the symbol interval is extended by N times (i.e. NT) b ). The parallel data streams are then modulated onto N mutually orthogonal subcarriers by an inverse fourier transform (IFFT). Finally, inserting a Cyclic Prefix (CP) with the length larger than the maximum multipath time delay of the channel into the frequency domain after IFFT conversion, and performing parallel-to-serial conversion (P/S) to obtain a baseband OFDM symbol of a transmitting end, which is expressed as a baseband OFDM symbol of the transmitting end
Wherein X (k) is data on the k subcarrier, N is the size of IFFT, and N is g Is the cyclic prefix length.
After experiencing a multipath fading channel, the OFDM signal usually has time delay and frequency offset introduced by a wireless channel, and a receiving end baseband OFDM symbol is represented as
Wherein epsilon is unknown symbol timing deviation, v is normalized carrier frequency deviation, w (n) is complex Gaussian white noise, h (m) is channel impulse response, and L is channel memory length.
In an OFDM receiver, symbol timing and carrier frequency synchronization is required due to delay and frequency offset of the received signal. A typical preamble-based symbol timing and carrier frequency synchronization method is the Schmidl algorithm. The algorithm utilizes two sections of preambles with the same structure, and the timing measurement of the preambles is poor in timing estimation performance due to the fact that a platform exists, so that the carrier frequency offset estimation performance is influenced. In order to improve the timing estimation accuracy of the Schmidl algorithm and expand the range of carrier frequency offset estimation, the Ren algorithm adopts a pseudo-random sequence to weight the same leading symbols of the front section and the rear section. Suppose the pseudorandom sequence is s n The weighted preamble symbol is denoted as x n '＝s n x n . Ren algorithm timing metric is expressed as
Selecting d corresponding to the maximum value of the timing metric M (d) as the timing metric estimatorThe Ren algorithm divides the carrier frequency offset v = α +2 β into a fractional part frequency offset α and an integer part frequency offset β. By usingIs calculated as a fractional part of the frequency offset estimate, i.e.According toAfter compensating fractional part frequency deviation of received signal, the result is compared with known leader sequence x n ' conjugate multiplication to obtain data sequenceAccording toAnd calculating an objective function I (beta), and solving the beta corresponding to the maximum value of the objective function, namely the integer part frequency offset estimator. The objective function I (beta) is expressed as
Based on the above calculation, the carrier frequency offset estimator is represented asThe frequency offset estimation range is +/-N/2. The Ren algorithm adopts the pseudo-random sequence to weight the preamble symbols, thereby improving the performance of symbol timing and carrier frequency synchronization. However, performing pseudo-random sequence weighting on the preamble symbols degrades the power spectrum of the OFDM signal, generates a more severe out-of-band leakage, and affects the adjacent band signals. Therefore, the method aims at the problem that the performance of the traditional symbol timing and carrier frequency synchronization algorithm in the OFDM receiver under the multipath fading channel is poor.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a combined timing and frequency synchronization method based on a conjugate structure preamble, which only needs to adopt a preamble symbol with a conjugate structure to carry out combined symbol timing and carrier frequency synchronization, wherein the symbol timing synchronization is not influenced by frequency deviation, the frequency offset estimation range can reach +/-N/2, and the symbol timing and carrier frequency synchronization performance of the method is superior to that of the traditional algorithm under a multipath fading channel.
The above object of the present invention is achieved by the following scheme:
a combined timing and frequency synchronization method based on conjugate structure preamble comprises the following steps:
(1) And carrying out conjugate multiplication on a received signal r (N) and a preamble symbol c (N) of the OFDM receiver in a sliding window with the length of N to obtain a correlation signal r 0 (n,d)：
r 0 (n,d)＝r(n+d)c * (n)；
Wherein N =0, 1, …, N-1; d =0, 1, …, M s ×N s ；N s ＝N+N g N is the set number of samples of one OFDM symbol, M s For a set number of OFDM symbols per frame of data, N g The number of the cyclic prefixes of the OFDM symbols; and has a conjugate structure at the preamble symbol c (n), that is:
[c(0) c(1) … c(N/2-1)]＝[c * (N/2) c * (N/2+1) … c * (N-1)]；
(2) The sliding correlation signal r obtained according to the step (1) 0 (n, d) calculating to obtain a differential correlation signal p (m, d):
wherein M =1, 2, …, M 0 ，M 0 Is a positive integer and M 0 ≤N-1；
(3) Carrying out weighted accumulation on the differential correlation signals P (m, d) obtained by calculation in the step (2) to obtain a weighted correlation function P (d);
(4) Normalizing the weighted correlation function P (d) obtained by calculation in the step (3) by using the energy of the received signal in the sliding window to obtain a normalized timing measurement value M (d);
(5) Calculating the maximum value of the normalized timing metric M (d); differential correlation timing offset estimatorEqual to the value of d corresponding to said maximum, i.e.
(6) Calculating the decimal part carrier frequency offset estimation quantity
Wherein, angle () represents a phase taking operation;
(7) And (4) utilizing the decimal part carrier frequency offset estimator calculated in the step (6)Performing compensation operation on the received signal to obtain a received signal after fractional frequency offset compensation
(8) Receiving signal compensated by decimal frequency deviationObtaining an objective function gamma (q), then obtaining a maximum value of the objective function gamma (q), and taking a parameter q corresponding to the maximum value as a frequency offset estimator of an integer partThe specific implementation process is as follows:
(8a) Calculating an objective function Γ (q):
(8b) Then, the maximum value of the target function gamma (q) is obtained, and the parameter q corresponding to the maximum value is used as the frequency deviation of an integer partNamely, it is
(9) Estimating the carrier frequency offset of the fractional partAnd an integer part of the frequency offset estimatorSumming to obtain carrier frequency offset estimatorNamely, it is
In the above method for joint timing and frequency synchronization based on conjugate structure preamble, in step (3), the weighted correlation function P (d) is calculated as follows:
in the above method for joint timing and frequency synchronization based on conjugate structure preamble, in step (3), the weighted correlation function P (d) is calculated as follows:
in the above method for joint timing and frequency synchronization based on conjugate structure preamble, in step (4), the calculation formula of the normalized timing metric value M (d) is as follows:
compared with the prior art, the invention has the following advantages:
1) The invention only needs a simple conjugate structure preamble without scrambling the preamble symbol, thereby avoiding worsening the power spectrum characteristic of the OFDM signal;
2) The invention adopts the intermediate variable of timing estimation in the frequency offset estimation, and the joint synchronization method simplifies the frequency offset estimation process.
3) The invention adopts a differential correlation structure with adjustable time domain parameters, has pulse-shaped timing measurement and solves the problem of fuzzy timing measurement of the traditional timing algorithm;
4) Under the multipath fading channel with low signal-to-noise ratio, the MSE performance of timing estimation is improved by 8dB compared with the traditional algorithm.
Drawings
FIG. 1 is a functional block diagram of a correlation averaging and windowing based timing estimation method of the present invention;
FIG. 2 is a schematic time domain waveform of a normalized timing metric value M (d) calculated according to the present invention;
fig. 3 is a comparison of MSE performance of timing estimators obtained by simulation in an embodiment of the present invention.
Detailed Description
The invention is described in further detail below with reference to the following figures and specific examples:
in the method, an OFDM system only adopts a preamble with a simple conjugate structure, a receiving end obtains a timing estimator by carrying out parameter-adjustable differential correlation and weighted summation on the preamble, and respectively calculates fractional part frequency offset and integer part frequency offset according to the timing estimator, and the fractional part frequency offset and the integer part frequency offset are added to obtain a total frequency offset estimator which is used for compensating the frequency offset of a received signal in frequency synchronization. The symbol timing synchronization is not affected by the frequency deviation, and the frequency deviation estimation range can reach +/-N/2. Under the multipath fading channel, the symbol timing and carrier frequency synchronization performance of the method is superior to that of the traditional algorithm.
As shown in the schematic block diagram of the method in fig. 1, the joint timing and frequency synchronization method based on the preamble with the conjugate structure of the present invention includes the following steps:
(1) And carrying out conjugate multiplication on a received signal r (N) and a preamble symbol c (N) of the OFDM receiver in a sliding window with the length of N to obtain a correlation signal r 0 (n,d)：
r 0 (n,d)＝r(n+d)c * (n)；
Wherein N =0, 1, …, N-1; d =0, 1, …, M s ×N s ；N s ＝N+N g N is the number of samples per OFDM symbol, M s For the number of OFDM symbols in each frame of data, N g The number of cyclic prefixes of OFDM symbols; and has a conjugate structure at the preamble symbol c (n), that is:
[c(0) c(1) … c(N/2-1)]＝[c * (N/2) c * (N/2+1) … c * (N-1)]；
in the above sliding correlation calculation, when the sliding window position is equal to the starting position of the preamble symbol in the received signal, the conjugate multiplication of the received signal and the known preamble can obtain a data sequence having the same structure in two previous segments and two subsequent segments, and the data sequence can be used for carrier frequency synchronization.
(2) The sliding correlation signal r obtained according to the step (1) 0 (n, d) calculating to obtain a differential correlation signal p (m, d):
wherein m =1, 2, …、M 0 ，M 0 Is a positive integer and M 0 ≤N-1；
(3) When the differential correlation signal p (m, d) is calculated in the step (2), differential correlation is performed on the received signals at different differential intervals m, and the number of accumulation terms for calculating p (m, d) is N-m, that is, the number of accumulation terms is different when the differential intervals m are different, that is, the differential correlation data obtained at different differential intervals m have different influences on the timing metric. The weighted correlation function P (d) is obtained by performing weighted accumulation on the differential correlation signals P (m, d) for the time. The specific weighted accumulation formula is as follows:
wherein, when M 0 When the value is small, for example, when the FFT size N =64, 128, 256, 512, 1024, 2048, 4096 of the OFDM system, M is selected 0 Less than or equal to 10, 9, 8, 6, 4, 3 and 2, then M 0 The summation terms of the differential correlation results P (m, d) have small differences, and in order to simplify the calculation process, the weighted correlation function P (d) can be calculated in an average weighted manner, that is, the weighted correlation function P (d) is calculated
(4) Normalizing the weighted correlation function P (d) calculated in the step (3) by using the energy of the received signal in the sliding window to obtain a normalized timing metric value M (d):
(5) Calculating the maximum value of the normalized timing metric M (d); as shown in fig. 2. Selecting d corresponding to the maximum value of M (d) as a timing offset estimatorNamely that
(6) Calculating the decimal part carrier frequency offset estimation quantity
Wherein, angle () represents a phase taking operation;
(7) And (4) utilizing the decimal part carrier frequency offset estimator calculated in the step (6)Performing compensation operation on the received signal to obtain a received signal after fractional frequency offset compensation
(8) Receiving signal compensated by decimal frequency deviationObtaining an objective function gamma (q), then obtaining a maximum value of the objective function gamma (q), and taking a parameter q corresponding to the maximum value as a frequency offset estimator of an integer partThe specific implementation process is as follows:
(8a) Calculating an objective function Γ (q):
(8b) Then, the maximum value of the target function gamma (q) is obtained, and the parameter q corresponding to the maximum value is used as the frequency deviation of an integer partNamely, it is
(9) Estimating the carrier frequency deviation of the fractional partAnd an integer part of the frequency offset estimatorSumming to obtain carrier frequency offset estimatorNamely, it is
The carrier frequency offset estimator is obtained by calculationPerforming frequency offset compensation on the received signal r (n) and estimating the amount according to the timing offsetAnd determining the position of the FFT window of the OFDM receiving end, namely completing the joint timing and frequency synchronization of the OFDM receiving signal.
Example (b):
in this example, the present invention is describedThe invention carries out simulation analysis based on the conjugate structure preamble joint timing and frequency synchronization method. The simulation conditions were set as follows: the OFDM system adopts block-shaped preamble symbols, the number of system subcarriers is N =256, and the length of cyclic prefix is N g And the signal bandwidth is 3MHz, and the subcarrier spacing is 15kHz. The simulation channel adopts an 11-path Rayleigh fading channel, and each path of delay component is [00.4630.9261.3891.8522.3152.7783.2413.7044.1674.4630 ]]μ s, the channel has an exponential power delay characteristic, i.e. for the path gain A i Comprises the following steps:where i represents the ith multipath.
Under the simulation conditions, the method, the Ren algorithm and the Schmidl algorithm are respectively adopted in the OFDM system for symbol timing synchronization, the simulation is carried out for 10000 times, and the MSE performance of the timing estimator is counted as shown in figure 3. When the signal-to-noise ratio is greater than 5dB (SNR)>, 5 dB), the timing estimation performance of the method and the Ren algorithm is superior to that of the Schmidl algorithm. When signal-to-noise ratio (SNR)<, 10 dB), the MSE performance of the method is superior to that of Ren algorithm, and it can be seen that when the adjustable parameter M is smaller 0 Where =3,7, the MSE performance of the timing estimator may be improved by 5dB and 8dB, respectively. When signal to noise ratio (SNR)>, 10 dB), the MSE performance of the two algorithms is similar. It is worth noting that the method of the present invention is along with the adjustable parameter M under low signal-to-noise ratio 0 Increasing, the MSE performance of the timing estimator is significantly improved. When M is 0 &And 7, the MSE performance of the timing estimator tends to be stable, but the computational complexity of the method is continuously increased. Thus, in a practical OFDM system, the parameter M can be adjusted 0 The choice of (c) requires a trade-off between algorithm performance and implementation complexity.
The above description is only one embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention.
Those skilled in the art will appreciate that the invention may be practiced without these specific details.
Claims (4)
1. A combined timing and frequency synchronization method based on conjugate structure preamble is characterized by comprising the following steps:
(1) And carrying out conjugate multiplication on a received signal r (N) of the OFDM receiver and a preamble symbol c (N) in a sliding window with the length of N to obtain a correlation signal r 0 (n,d)：
r 0 (n,d)＝r(n+d)c * (n)；
Wherein N =0, 1, …, N-1; d =0, 1, …, M s ×N s ；N s ＝N+N g N is the set number of samples of one OFDM symbol, M s For a set number of OFDM symbols per frame of data, N g The number of cyclic prefixes of OFDM symbols; and has a conjugate structure at the preamble symbol c (n), that is:
[c(0) c(1) … c(N/2-1)]＝[c * (N/2) c * (N/2+1) … c * (N-1)]；
(2) The correlation signal r obtained according to the step (1) 0 (n, d) calculating to obtain a differential correlation signal p (m, d):
wherein M =1, 2, …, M 0 ，M 0 Is a positive integer and M 0 ≤N-1；
(3) Performing weighted accumulation on the differential correlation signals P (m, d) obtained by calculation in the step (2) to obtain a weighted correlation function P (d);
(4) Normalizing the weighted correlation function P (d) calculated in the step (3) by using the energy of the received signal in the sliding window to obtain a normalized timing metric value M (d);
(5) Calculating the maximum value of the normalized timing metric M (d); differential correlation timing offset estimatorEqual to the value of d corresponding to said maximum, i.e.
(6) Calculating the decimal part carrier frequency offset estimation quantity
Wherein, angle () represents a phase taking operation;
(7) And (4) utilizing the decimal part carrier frequency offset estimator calculated in the step (6)Performing compensation operation on the received signal to obtain a received signal after fractional frequency offset compensation
(8) Receiving signal compensated by decimal frequency deviationObtaining an objective function gamma (q), then obtaining a maximum value of the objective function gamma (q), and taking a parameter q corresponding to the maximum value as a frequency offset estimator of an integer partThe specific implementation process is as follows:
(8a) Calculating an objective function Γ (q):
(8b) Then, the maximum value of the target function gamma (q) is obtained, and the parameter q corresponding to the maximum value is used as the frequency deviation of an integer partNamely, it is
(9) Estimating the carrier frequency offset of the fractional partAnd an integer part of the frequency offset estimatorSumming to obtain carrier frequency offset estimatorNamely, it is
2. The method of claim 1, wherein the method for joint timing and frequency synchronization based on the preamble with the conjugated structure comprises: in step (3), the weighted correlation function P (d) is calculated as follows:
3. the method of claim 1, wherein the method for joint timing and frequency synchronization based on the preamble with the conjugated structure comprises: in step (3), the weighted correlation function P (d) is calculated as follows:
4. the method of claim 1, wherein the method for joint timing and frequency synchronization based on the preamble with the conjugated structure comprises: in step (4), the calculation formula of the normalized timing metric value M (d) is:
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| CN110061944B (en) * | 2018-01-18 | 2021-11-26 | 瑞昱半导体股份有限公司 | Symbol timing determination apparatus and method |
| CN108512789B (en) * | 2018-03-16 | 2021-05-04 | 中国电子科技集团公司第七研究所 | Synchronization detection method, synchronization detection device, communication-capable device and storage medium |
| CN110213190A (en) * | 2019-05-27 | 2019-09-06 | 浙江万胜智能科技股份有限公司 | A kind of OFDM symbol timing offset estimation method |
| CN111917675B (en) * | 2020-06-22 | 2021-09-03 | 西安交通大学 | Frame detection method suitable for frequency offset robustness in broadband non-line-of-sight communication |
| CN111740814B (en) * | 2020-06-30 | 2023-03-31 | 西安烽火电子科技有限责任公司 | Low-complexity folding product synchronization algorithm suitable for short-wave communication |
| CN113037671B (en) * | 2021-03-02 | 2023-12-05 | 哈尔滨工业大学 | Low-complexity efficient SOQPSK symbol timing and phase joint synchronization algorithm |
| CN113904904B (en) * | 2021-11-12 | 2023-09-15 | 北京信息科技大学 | Integer frequency offset estimation method, system, medium and equipment based on OFDM |
| CN116319206B (en) * | 2022-09-08 | 2023-11-21 | 深圳市国电科技通信有限公司 | Signal compensation method, processor, system and storage medium |
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