CN105141562B - Communication system and its synchronous method - Google Patents
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Abstract
The present invention relates to communication system and its synchronous method.Including：Transmitting terminal sends the data frame for including the first training sequence A, the second training sequence C, the 3rd training sequence D, cyclic prefix CP and data symbol, and receiving terminal receives the data frame and performs following steps：S1. use parallel code phase search algorithm to carry out the detection of data frame using the first training sequence A and acquire coarse frequency offset value to correct thick frequency deviation value so as to obtain the data symbol after thick correcting frequency deviation；S2. thin offset estimation is carried out using the second training sequence C, to correct thin frequency deviation value so as to obtain the data symbol after meticulous correcting frequency deviation；And S3. carries out Symbol Timing estimation using the second training sequence C and the 3rd training sequence D.The present invention can be reasonably resistant to the influence of noise, realize larger carrier wave frequency deviation detection range, correct sufficiently large frequency deviation；And hard-wired complexity can be reduced, reduce the consumed resource of algorithm FPGA realizations, obtain sufficiently high synchronization accuracy.
Description
Technical Field
The present invention relates to the field of communications, and more particularly, to a communication system and a synchronization method thereof.
Background
With the development of anti-fading technology and low-power receiving technology of wireless signals, research on SC-FDE under low signal-to-noise ratio conditions will become a hot spot in the future, and particularly, "weight loss" (a wide area internet of things technology standard with low power consumption) has taken the SC-FDE technology as a main transmission scheme of a physical layer. Implementing M2M under low signal-to-noise ratio conditions requires not only a balance and tradeoff between signal-to-noise ratio and data throughput, but also an ability to achieve multipath immunity to the signal over 15 kilometers. For low signal-to-noise ratio M2M (Machine to Machine) digital communication systems, synchronization is a very important issue.
The traditional SC-FDE system synchronization algorithm is used for carrying out algorithm design by correlating the front part and the rear part of a multi-purpose received signal, and the algorithm based on the idea is insensitive to the magnitude of frequency deviation and can solve the problems caused by signal multipath transmission to a certain extent. However, these synchronization algorithms are basically oriented to higher signal-to-noise ratio and have poor synchronization accuracy, and the reason for this is mainly because the method of using the received signal itself to make correlation is very sensitive to noise and has poor synchronization performance under the condition of poor signal-to-noise ratio. When the signal-to-noise ratio of the received signal is low and the carrier frequency offset range is large, the traditional synchronization algorithm is often difficult to complete the synchronization of the signal with higher precision.
Therefore, it is necessary to develop a synchronization technology for a low snr wireless communication receiving system.
Disclosure of Invention
The technical problem to be solved by the present invention is to provide a synchronization method for a communication system and a communication system using the synchronization method, which can achieve accurate synchronization of signals, aiming at the defect of poor synchronization performance of the synchronization algorithm of the conventional SC-FDE system under the condition of low signal-to-noise ratio in the prior art.
The technical scheme adopted by the invention for solving the technical problems is as follows: provide for
A communication system synchronization method, the sending end of the communication system sends the data frame including the first training sequence A, the second training sequence C, the third training sequence D, the cyclic prefix CP and the data symbol, the receiving end receives the data frame and executes the following steps:
s1, detecting a data frame by using a first training sequence A and adopting a parallel code phase search algorithm, and obtaining a coarse frequency offset estimation value to correct the coarse frequency offset value so as to obtain a data symbol subjected to coarse frequency offset correction;
s2, performing fine frequency offset estimation by using a second training sequence C to correct a fine frequency offset value so as to obtain a data symbol subjected to fine frequency offset correction; and
s3, carrying out symbol timing estimation by using a second training sequence C and a third training sequence D;
wherein,
the first training sequence A is formed by repeating T1 times by a ZC sequence with the period length of L1, wherein L1 and T1 are both positive integers, L1 is more than or equal to 4 and less than or equal to 12, T1 is more than or equal to 3000 and less than or equal to 5000, and the value of T1 depends on L1, the maximum frequency offset range and the signal-to-noise ratio;
the second training sequence C is formed by repeating T3 times by a ZC sequence with the period length of L3, wherein L3 and T3 are positive integers, L3 is more than or equal to 128 and less than or equal to 512, and T3 is more than or equal to 12 and less than or equal to 48;
the third training sequence D is formed by repeating T4 times after inverting a ZC sequence with the period length of L3, wherein T4 is a positive integer, and T3/8 is more than or equal to T4 is more than or equal to T3/4.
In the synchronization method of the communication system according to the present invention, the length of the cyclic prefix CP is set such that an advance Ng is introduced to the symbol timing estimation point in step S3, and the magnitude of the advance Ng is at least greater than the maximum delay of the primary path and the first path for a multi-path channel whose primary path is not the first path.
In the synchronization method of a communication system according to the present invention, step S1 includes:
s1-1, carrying out frequency mixing operation on the received signal and an orthogonal carrier generated by a digital oscillator (NCO), and then carrying out correlation operation on the output of the frequency mixing and a local training sequence by utilizing an FFT (fast Fourier transform) module and an IFFT (inverse fast Fourier transform) module to obtain a correlation operation result;
s1-2, realizing coherent integration with preset length by using a periodic accumulation mode on the result of the correlation operation, performing modular operation on the I and Q paths of output after the coherent integration, and performing square addition to obtain a final detection judgment value V;
s1-3, comparing the detection judgment value V with a set threshold value Vt to judge whether a data frame is detected; and is
When a data frame is detected, the frame detection module stops working and provides a frequency value of the DDS at the moment as the coarse frequency offset estimation value;
when no data frame is detected, the frequency value of the DDS is updated and the search is re-performed by repeating the steps S1-1 to S1-3 until a data frame is detected.
In the synchronization method of a communication system according to the present invention,
the length N of the FFT (fast Fourier transform) is 1024 points, the length of the cyclic prefix CP is more than 91 points, and the time interval T of adjacent symbols at the transmitting ends0.2 mus; in the first training sequence a, L1 ═ 8 and T1 ═ 3800; in the second training sequence C, L2-256 and T2-24; in the third training sequence D, T3 ═ 4.
In the synchronization method of a communication system according to the present invention,
step S2 includes: performing correlation operation by using the local training sequence and the received second training sequence C, and accumulating the correlation operation result for 8 times to perform fine frequency offset estimation of the carrier;
step S3 includes: carrying out symbol timing estimation on the data symbols by using the correlation operation results of the local training sequence and the received second training sequence C and the third training sequence D; and is
The length of the cyclic prefix CP is 128 points, the signal-to-noise ratio of the communication system is as low as-15 dB, and the maximum normalized carrier frequency offset value is 1.2.
In the synchronization method of the communication system according to the present invention, the step S2 of performing fine frequency offset estimation of the carrier by accumulating 8 times with the result of the correlation operation further includes:
dividing the correlation peak points corresponding to the continuous 16 subsequences in the second training sequence C into a front part and a rear part, and then respectively adding the front 8 values and the rear 8 values to calculate a fine frequency offset estimation value, wherein the formula is as follows:
wherein:
where r (d) represents the data symbols after coarse frequency offset correction, m (d) represents the local training sequence,representing a fine frequency offset estimate.
In the synchronization method of a communication system according to the present invention, step S3 includes: obtaining a peak point by using the output of the following time measurement function, calculating the position of a third training sequence D by using the index of the peak point so as to complete symbol timing estimation,
where r (d) represents the data symbols after coarse frequency offset correction, and m (d) represents the local training sequence.
The other technical scheme adopted by the invention for solving the technical problem is as follows: a communication system is constructed, which includes a synchronization apparatus disposed at a receiving end, a data frame sent by a transmitting end of the communication system includes a first training sequence a, a second training sequence C, a third training sequence D, a cyclic prefix CP and a data symbol, and the synchronization apparatus includes:
the frame detection module is used for detecting a data frame by using a first training sequence A and adopting a parallel code phase search algorithm, and obtaining a coarse frequency offset estimation value to correct the coarse frequency offset value so as to obtain a data symbol subjected to coarse frequency offset correction;
the fine frequency offset correction module is used for performing fine frequency offset estimation by utilizing the second training sequence C so as to correct a fine frequency offset value and obtain a data symbol after fine frequency offset correction; and
a symbol timing module, which utilizes the second training sequence C and the third training sequence D to carry out symbol timing estimation;
wherein,
the first training sequence A is formed by repeating T1 times by a ZC sequence with the period length of L1, wherein L1 and T1 are both positive integers, L1 is more than or equal to 4 and less than or equal to 12, T1 is more than or equal to 3000 and less than or equal to 5000, and the value of T1 depends on L1, the maximum frequency offset range and the signal-to-noise ratio;
the second training sequence C is formed by repeating T3 times by a ZC sequence with the period length of L3, wherein L3 and T3 are positive integers, L3 is more than or equal to 128 and less than or equal to 512, and T3 is more than or equal to 12 and less than or equal to 48
The third training sequence D is formed by repeating T4 times after inverting a ZC sequence with the period length of L3, wherein T4 is a positive integer, and T3/8 is more than or equal to T4 is more than or equal to T3/4; and,
the length of the cyclic prefix CP is set so that the symbol timing module introduces an advance Ng to the symbol timing estimation point, and the advance Ng is at least larger than the maximum time delay of the main path and the first path for the multi-path channel of which the main path is not the first path.
In the communication system of the present invention, the frame detection module includes:
a frequency mixing module for mixing a received signal with a quadrature carrier generated by a digital oscillator (NCO);
a first FFT (fast fourier transform) module for performing fast fourier transform on the mixed output;
a second FFT (fast fourier transform) module for performing fast fourier transform on the local training sequence;
the conjugate module is used for carrying out complex conjugate operation on the output of the second FFT module;
an IFFT (inverse fast Fourier transform) module for performing inverse fast Fourier transform on the mixed output of the fast Fourier transform operation and the complex conjugate operation to obtain an I component and a Q component after the correlation operation;
the I component coherent integration module is used for performing coherent integration with a preset length on the I component after the correlation operation in a periodic accumulation mode;
the Q component coherent integration module is used for performing coherent integration with a preset length on the Q component after the correlation operation in a periodic accumulation mode;
an I component modulus and square module for performing modulus operation and square on the I component after coherent integration to obtain I2；
A Q component modulus and square module for performing modulus operation and square on the Q component after coherent integration to obtain Q2；
An addition module for pair I2And Q2Performing addition operation to obtain a detection judgment value V;
and the comparison module compares the detection judgment value V with a set threshold value Vt to judge whether a data frame is detected.
In the communication system according to the present invention,
the fine frequency offset correction module performs correlation operation by using a local training sequence and a received second training sequence C, and accumulates the correlation operation result for 8 times to perform fine frequency offset estimation of a carrier;
and the symbol timing module carries out symbol timing estimation on the data symbols by using the correlation operation results of the local training sequence, the received second training sequence C and the received third training sequence D.
The implementation of the invention has the following beneficial effects: aiming at a communication system with a low signal-to-noise ratio, such as an SC-FDE system, an OFDM system and the like with the low signal-to-noise ratio, the invention adopts a parallel code phase search algorithm and a data frame has a first training sequence A with enough length, thereby effectively resisting the influence of noise, realizing a larger carrier frequency offset detection range, correcting enough frequency offset and completing high-precision synchronization of signals; the problem that the synchronization performance of the traditional synchronization algorithm is poor under the condition of low signal to noise ratio is solved, and better synchronization performance can be obtained under the condition of low signal to noise ratio.
Moreover, due to the length setting of the cyclic prefix CP in the data frame, a lead Ng is introduced to the symbol timing estimation point, the accuracy of the final symbol timing estimation is ensured, and the problem of large demodulation loss caused by the lag of the timing point under the condition that the main path of the wireless fading channel is not the first path is solved.
Because the invention uses the training sequence C and the training sequence D to jointly complete the fine frequency offset estimation and the symbol timing estimation, the complexity of hardware realization can be reduced, and the length of a correlator can be reduced, thereby greatly reducing the resources consumed by the realization of an algorithm FPGA, and being easy to popularize practically.
Drawings
The invention will be further described with reference to the accompanying drawings and examples, in which:
fig. 1 is a schematic diagram of a data frame structure used in the synchronization method of the communication system of the present invention;
FIG. 2 is a flow chart of a communication system synchronization method of the present invention;
FIG. 3 is a flow chart of a parallel code phase search algorithm employed in the synchronization method of the communication system of the present invention;
fig. 4 is a schematic structural diagram of a data frame used in a synchronization method of a communication system according to an embodiment of the present invention;
FIG. 5 is a schematic diagram of the detection of peak variation with carrier frequency offset using a parallel code phase search algorithm in the synchronization method of the communication system of the present invention;
FIG. 6 is a diagram illustrating the correlation and accumulation results of a second training sequence C under AWGN channel conditions;
FIG. 7a is a diagram illustrating the correlation result (real correlation part) of the second training sequence C and the third training sequence D under the noise-free condition;
FIG. 7b is a diagram illustrating the correlation result (correlation imaginary part) of the second training sequence C and the third training sequence D under the noise-free condition;
FIG. 8 is a schematic diagram of the time measurement of the synchronization algorithm of the present invention under AWGN channel conditions;
figure 9a shows a graph of the discrete impulse response of a multi-path channel with the primary path being the first path;
figure 9b shows a graph of the discrete impulse response of a multi-path channel in which the primary path is not the first path.
Detailed Description
The invention relates to a low signal-to-noise ratio SC-FDE (Single Carrier frequency Domain Equalization) system synchronization algorithm in a wide area Internet of things M2M communication system, which is suitable for a low signal-to-noise ratio SC-FDE system with long signal transmission distance and low efficiency requirement and can be popularized to other communication systems, such as an OFDM system.
Fig. 1 is a schematic diagram of a structure of a data frame used in the synchronization method of the communication system of the present invention. As shown in fig. 1, in the present invention, a data frame used for communication between a transmitting end and a receiving end of a communication system includes a training sequence, a cyclic prefix CP, and a payload (i.e., data symbols). The training sequence comprises a first training sequence A, a second training sequence C and a third training sequence D, wherein the first training sequence A is formed by repeating T1 times by a ZC sequence with the period length of L1, the second training sequence C is formed by repeating T2 times by a ZC sequence with the period length of L2, and the third training sequence D is formed by repeating T3 times after inverting by a ZC sequence with the period length of L2. L1 and T1 are positive integers, L1 is more than or equal to 4 and less than or equal to 12, T1 is more than or equal to 3000 and less than or equal to 5000, wherein the value of T1 depends on L1, the maximum frequency offset range and the magnitude of signal-to-noise ratio; l2 and T2 are positive integers, L2 is more than or equal to 128 and less than or equal to 512, and T2 is more than or equal to 12 and less than or equal to 48; t3 is a positive integer, and T2/8 is not less than T3 is not less than T2/4.
In the communication process of the communication system, the sending end sends the data frame with the frame format, and the receiving end receives the data frame and carries out signal synchronization operation. The synchronization method of the communication system is executed by a synchronization device, and the synchronization device comprises a frame detection module, a fine frequency offset correction module and a symbol timing module.
Fig. 2 is a flow chart of a synchronization method of the communication system of the present invention. As shown in fig. 2, the synchronization method of the communication system of the present invention includes a step S1 executed by the frame detection module of the synchronization apparatus in the receiving end, a step S2 executed by the fine frequency offset correction module, and a step S3 executed by the symbol timing module, and each step is as follows:
step S1: the first training sequence A is utilized to perform data frame detection (namely, frame capture) by adopting a parallel code phase search algorithm, so that a coarse frequency offset estimation value is obtained to correct the coarse frequency offset value, and then a data symbol after coarse frequency offset correction is obtained.
Step S2: and performing fine frequency offset estimation by using the second training sequence C to obtain a fine frequency offset estimation value, and correcting the fine frequency offset value to obtain the data symbol after fine frequency offset correction.
Step S3: and carrying out symbol timing estimation by utilizing the second training sequence C and the third training sequence D to carry out timing synchronization.
Steps S1-S3 are described in detail below.
First, the frame detection and the coarse frequency offset estimation in step S1 will be explained.
When the signal-to-noise ratio of the received signal is low and the carrier frequency offset range is large, the traditional synchronization algorithm is often difficult to complete the synchronization of the signal with higher precision. In step S1 of the synchronization method of the present invention, the data frame is detected by using a parallel code phase search algorithm in the GPS receiver for reference. For a GPS receiver, a training sequence which is long enough in a transmitter is generally utilized to complete three-dimensional search of a code sequence, a carrier frequency and a code phase of a satellite signal, and because the training sequence A used for completing frame detection in the communication system is fixed, an algorithm is only required to be designed in a two-dimensional search unit of the carrier frequency and the code phase. The principle of the joint detection method is simple, and theoretically, the influence of noise can be effectively resisted as long as the training sequence is long enough, and a larger frequency deviation detection range can be realized. The length of the training sequence A and the complexity of the algorithm are comprehensively considered, and the parallel code phase search algorithm is adopted in the invention.
In some embodiments of the present invention, a flow of the parallel code phase search algorithm employed in step S1 executed by the frame detection module in the synchronization method of the communication system is shown in fig. 3.
Wherein:
in step S1-1, the received signal is mixed with an orthogonal carrier generated by a digital oscillator (NCO), and then the mixed output and the local training sequence are correlated by an FFT (fast fourier transform) module and an IFFT (inverse fast fourier transform) module to obtain a correlation result;
in step S1-2, coherent integration with a preset length is realized by using a periodic accumulation mode on the correlation operation result obtained in step S1-1, and the I and Q paths after coherent integration are subjected to modular operation and squared addition to obtain a final detection judgment value V;
in step S1-3, the detection determination value V is compared with a set threshold value Vt to determine whether a data frame is detected; and is
When a data frame is detected, the frame detection module stops working and provides a frequency value of the DDS at the moment as the coarse frequency offset estimation value; when no data frame is detected, the frequency value of the DDS is updated and the search is re-performed by repeating the steps S1-1 to S1-3 until a data frame is detected.
Fig. 3 also shows the structure of an exemplary frame detection module, which includes:
a frequency mixing module for mixing a received signal with a quadrature carrier generated by a digital oscillator (NCO);
a first FFT (fast fourier transform) module for performing fast fourier transform on the mixed output;
a second FFT (fast fourier transform) module for performing fast fourier transform on the local training sequence;
the conjugate module is used for carrying out complex conjugate operation on the output of the second FFT module;
an IFFT (inverse fast fourier transform) module for performing inverse fast fourier transform on the output of the mixing of the fast fourier transform operation and the complex conjugate operation to obtain an I component and a Q component after the correlation operation;
the I component coherent integration module is used for performing coherent integration with a preset length on the I component after the correlation operation in a periodic accumulation mode;
the Q component coherent integration module is used for performing coherent integration with a preset length on the Q component after the correlation operation in a periodic accumulation mode;
an I component modulus and square module for performing modulus operation and square on the I component after coherent integration to obtain I2；
A Q component modulus and square module for performing modulus operation and square on the Q component after coherent integration to obtain Q2；
An addition module for pair I2And Q2Performing addition operation to obtain a detection judgment value V;
and the comparison module compares the detection judgment value V with a set threshold value Vt to judge whether a data frame is detected.
For the convenience of understanding of the present invention, the algorithm in the synchronization method of the communication system of the present invention is described below by an embodiment. In this embodiment, several important technical indicators of the communication system are as follows:
(1) the FFT (Fast Fourier Transform) length N is 1024 points, the Cyclic Prefix (CP) length is 128 points, and the symbol interval Ts is equal to 0.2 mu s;
(2) the signal-to-noise ratio is as low as-15 dB, and the maximum normalized carrier frequency deviation value is 1.2;
(3) two multipath channel models are shown in table 1;
(4) the training sequence configuration parameters in the data frame are shown in table 2 and fig. 4, that is, in the first training sequence a, L1 ═ 8 and T1 ═ 3800; in the second training sequence C, L2-256 and T2-24; in the third training sequence D, L2 is 256 and T3 is 4.
TABLE 1 multipath channel model
Table 2 training sequence parameter configuration
The detection of data frame is realized by adopting parallel code phase search algorithm, and the frequency search step f needs to be determinedbinAnd coherent integration time TcohTwo parameters. The I and Q output signals after coherent integration can be respectively expressed as:
I(n)＝aR(τ)sinc(feTcoh)cosφe+nI(1)
Q(n)＝aR(τ)sinc(feTcoh)sinφe+nQ(2)
where a denotes the amplitude of the input signal, τ is the phase difference between the received training sequence and the local training sequence, feRepresenting the difference, phi, between the carrier frequency of the received signal and the local carrier frequencyeFor the phase difference between the two, TcohRepresenting the time of coherent integration, R (t) representing the autocorrelation function of the ZC sequence with a maximum value of 1, nIAnd nQRepresenting the noise of the I and Q paths, respectively. Irrespective of the influence of noise, when the training sequence a is detected (τ is 0), the detection determination value at this time can be expressed as:
V＝I2+Q2＝a2|sinc(feTcoh)|2(3)
as can be seen from the above equation, the carrier frequency deviation feWill introduce | sinc (f) to the final time metriceTcoh)|2Loss of multiples, which increases the miss rate of the signal and reduces the sensitivity of signal acquisition. In order to reduce the probability of false negative rate events, it is generally required that the difference between the carrier frequency of the received signal and the local carrier frequency is controlled within 3dB, becauseThe absolute value of the frequency error should not exceed theoretically0.443/T of the solutioncohI.e. frequency search step fbin<0.886/Tcoh. In practical application, the search bandwidth value of the frequency is required to be smaller to further reduce the missing rate, so the final design generally satisfies the following conditions:
here, the factor 2/3 is such that there is some overlap between two adjacent 3dB bandwidths, and the above equation also indicates the frequency search step fbinAnd coherent integration time TcohIn inverse proportion to each other. Considering the range of the fine frequency offset estimation in the subsequent step S2, after the algorithm requires the coarse frequency offset value to be corrected, the maximum normalized carrier frequency offset value of the signal is limited to be within 0.25, i.e. fbin<0.25/NTsAfter conversion, the length of the coherent integration training sequence corresponding to a single frequency band is at least (T)coh/Ts)>The 8N/3 ≈ 2730 points, i.e., the subsequence with period length L1 of 8 needs to be repeated at least 341 times. Since the maximum normalized carrier frequency offset value of the system is 1.2, theoretically, only the central frequency f of the signal is neededeSince the frequency points are only required to be 11 frequency bands, in this embodiment, the total repeated times T1 of the algorithm design is 3800 times, and is larger than 341 × 11 times to 3751 times. In other embodiments of the present invention, the period length L1 and the total repetition number T1 can be flexibly designed according to practical requirements, mainly by designing parameters according to the maximum frequency offset range and the signal-to-noise ratio.
Fig. 5 is a schematic diagram of a detection peak value varying with carrier frequency offset by using a parallel code phase search algorithm in the synchronization method of the communication system of the present invention, which is a simulation result of the parallel code phase search algorithm detecting a peak value varying with carrier frequency offset when a simulation signal-to-Noise ratio is-15 dB under an AWGN (Additive White Gaussian Noise) channel condition. It can be seen from the simulation diagram that when the correct frequency band is searched, the peak value of the frame detection is much larger than the detection result of the adjacent frequency band, so that the data frame can be ensured to be correctly detected as long as the capture threshold can be reasonably set, and the requirement of the coarse frequency offset estimation precision is also met.
Next, the fine frequency offset estimation of step S2 will be explained.
In step S2, a correlation operation is performed by using the local training sequence and the received second training sequence C, and the correlation operation result is accumulated for 8 times to perform fine frequency offset estimation of the carrier.
Since the subsequence cycle length L2 of the training sequence C is only 256, a significant correlation peak cannot be obtained when the signal-to-noise ratio is low, using only one correlation result. In order to obtain a good detection effect, the algorithm adopted by the synchronization method resists the influence of noise by accumulating the correlation result of the training sequence C for 8 times. Fig. 6 shows simulation results of 8 times of correlation between the local training sequence and the received training sequence C when the SNR is-15 dB under AWGN channel conditions. As can be seen from the simulation diagram, the training sequence C can be accurately detected only by reasonably setting the detection threshold, and the starting position of a certain subsequence of the training sequence C can be determined at the same time. However, the starting position of the training sequence C cannot be determined at this time, because the number of sub-sequences consumed by detection varies with the simulated signal-to-noise ratio, but it can be known that only 8 sub-sequences are consumed at most when the training sequence C is detected.
The fine frequency offset estimation of the synchronization algorithm is realized by directly calculating the phase difference corresponding to two related peak points, but because the length of the training sequence directly determines the range and the precision of the fine carrier frequency offset estimation, in order to obtain good fine frequency offset estimation precision, the fine frequency offset estimation can be completed by using the result of the next 16 subsequences related operation after the training sequence C is detected. The specific calculation flow is as follows: dividing the correlation peak points corresponding to the 16 consecutive subsequences into two parts, i.e. front and back, and then adding the front 8 values and the back 8 values respectively, so as to obtain two correlation values equivalent to those in structure 1, and can be expressed as:
wherein:
where r (d) represents the data symbols after coarse frequency offset correction and m (d) represents the local training sequence. By using the above calculation method, a sufficient fine carrier frequency offset estimation accuracy can be theoretically obtained.
Finally, the symbol timing estimation in step S3 will be described.
In step S3, symbol timing estimation is performed on the data symbols using the results of correlation operations between the local training sequence and the received second training sequence C and third training sequence D. When four detected sequences are subsequences of the training sequence C and 4 detected sequences are subsequences of the training sequence D, a large correlation peak value can appear due to the opposite characteristics of the four detected sequences and the 4 detected sequences, so that the position of the training sequence D can be found, and corresponding symbol timing estimation is completed.
Because the second training sequence C and the third training sequence D are in an opposite relationship, and after the fine frequency offset correction, the residual frequency offset of the data symbol is already small, and only a random phase is superimposed, at this time, the simulation results shown in fig. 7a and 7b can be obtained by performing correlation operation on the training sequences C and D and the local training sequence again, where fig. 7a shows a correlation live state, and fig. 7b shows a correlation imaginary part. It should be noted that, in order to clearly show the comparison between the two training sequence correlation calculation results, the simulation is obtained without noise, and actually, the correlation peak of the training sequence is basically submerged by the noise around SNR < -2 dB. The time metric function of the algorithm design is as follows:
figure 8 shows the time metric simulation results of the algorithm under AWGN channel conditions with a simulated signal-to-noise ratio of-15 dB. It can be seen from the simulation diagram that at this time, an obvious peak point can be obtained from the output of the time measurement function of the algorithm, and the position of the training sequence D can be accurately calculated by using the index of the peak point, thereby completing the symbol timing estimation.
That is, in the communication system synchronization method of the present invention, only the sequence C can be detected, but the start position thereof cannot be determined. The symbol timing estimation is to perform correlation operation with the local training sequence according to the training sequences C and D, obtain a very obvious peak point by using the time measurement function output of algorithm design, and accurately calculate the position of the training sequence D by using the index of the peak point, thereby completing the symbol timing estimation.
Finally, considering the problem of the delay of the symbol timing point caused when the main path of the received signal is not the first path, the position of the timing point needs to be corrected. As mentioned above, step S3 is to perform symbol timing estimation on the data symbols using the local training sequence and the result of the correlation operation of the received training sequences C and D. However, some communication systems, such as SC-FDE systems, are inherently multipath resistant systems, and various synchronization algorithms must be designed to ensure that synchronization with higher accuracy can still be achieved under multipath channel conditions. The algorithm for realizing symbol timing estimation by using the correlation operation result of the local training sequence and the received training sequence is mainly used for solving the problem of symbol timing point lag caused when the main path of a received signal is not the first path of the received signal, and the large demodulation loss is caused by the timing point lag. Fig. 9a and 9b show the discrete impulse responses of two typical multipath channels, where the multipath channel in fig. 9a shows a multipath channel where the primary path is the first path and fig. 9b shows a multipath channel where the primary path is not the first path. In fact, the two channel models given in table 1 represent the two multipath channels with different characteristics.
The main path of most multipath channels in wireless communicationIs its first path, the delay between the two is typically small for multipath channels where the primary path is not the first path. Assume that the symbol timing point index determined by the main path is NuThen, it is only necessary to advance it by NgThe point ensures that the symbol timing estimate point does not lag the first path. Assuming that the position of the symbol timing point after correction is Nv, it can be expressed as:
Nv＝Nu-Ng(8)
in the formula NgMust be at least larger than the maximum delay of the main path and the first path, so as to ensure that the position of the corrected symbol timing point leads the arrival time of the first path. Meanwhile, in order to ensure that the symbol timing estimation point does not exceed the ISI-free region, the length of the CP also meets certain requirements. Since the accurate symbol timing estimation points for both SC-FDE and OFDM systems are a range, the final symbol timing estimate can be guaranteed to be accurate by increasing the length of the cyclic prefix appropriately.
Therefore, in the present invention, in the design of the data frame structure, the length of the cyclic prefix CP is set so that an advance Ng is introduced to the symbol timing estimation point in step S3, and the magnitude of the advance Ng is at least greater than the maximum delay of the primary path and the first path for the multipath channel whose primary path is not the first path.
For example, referring to two multipath channel models in table 1, the maximum signal delay of the second multipath channel with respect to the first path is 2.4 μ s, and combining the first channel model, it can be understood that the time of the main path lagging the first path after the wireless signal is transmitted through the actual channel is between 0 and 2.4 μ s. Since the time interval between adjacent symbols at the transmitting end is 0.2 mus, considering the limit, in practice, only N is satisfiedg≧ 12 ensures that the symbol timing point does not lag the first path. Meanwhile, it can be seen from the table that the delay time of the first path and the last path of the signal is 15.8 μ s, that is, corresponding to 79 symbol points, and the loss caused by the estimation of the symbol timing is comprehensively considered, as long as the length of the CP is greater than 91 points, but in this embodiment, the length of the CP set in the system is 1And point 28, obviously meets the design requirement.
In some embodiments of the invention, the length of the CP and the length of the FFT are both calculated to an integer power of 2. Obtaining N point data after calling fft (x, N), wherein the N point data is actually the sampling frequency from 0Hz to 5120Hz, the frequency difference between adjacent data points is 1/Ts (5120/N Hz), and N is 5120 Ts (1024); in table 1, the delay time of the first path and the last path is 15.8us, the maximum signal delay of the main path relative to the first path is 2.4 μ s, the time interval of adjacent symbols at the transmitting end is 0.2 μ s, and the length of the CP is greater than 15.8/0.2+2.4/0.2, which is 91. So long as the integer powers of 2 greater than 91 all satisfy the above requirements.
The communication system synchronization method provided by the invention jointly completes fine frequency offset estimation and symbol timing estimation by utilizing the training sequence C and the training sequence D, can reduce the complexity of hardware realization and reduce the length of a correlator, thereby greatly reducing the resources consumed by realizing the algorithm FPGA, simultaneously obtaining sufficiently high synchronization precision and being easy to popularize practically.
By comparing the advantages and disadvantages of the synchronization algorithm of the conventional SC-FDE system and the limitation under the condition of low signal to noise ratio, the synchronization algorithm of the communication system under the condition of low signal to noise ratio is provided by the invention, is suitable for the SC-FDE system with low signal to noise ratio, which has long signal transmission distance and low efficiency requirement, and can be popularized to other communication systems, such as an OFDM system. Because most of the synchronization algorithms of the SC-FDE system are designed based on the received training sequence to perform correlation operation, the synchronization precision is poor under the condition of low signal-to-noise ratio. The invention provides a method for carrying out correlation operation by utilizing a received training sequence and a local training sequence, thereby reducing the influence of noise on the signal synchronization precision and solving the problem of symbol timing point lag caused when the main path of a received signal is not the first path. The communication system synchronization technology is provided on the basis of using the existing algorithm ideas such as the synchronization algorithm of OFDM (Orthogonal Frequency Division Multiplexing) and the signal search algorithm of a GPS receiver, and is suitable for M2M communication under the condition of low signal-to-noise ratio.
The above embodiments are merely illustrative of the technical ideas and features of the present invention, and are intended to enable those skilled in the art to understand the contents of the present invention and implement the present invention, and not to limit the scope of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be covered by the claims of the present invention.
Claims (10)
1. A synchronization method of communication system is characterized in that a sending end sends a data frame containing a first training sequence A, a second training sequence C, a third training sequence D, a cyclic prefix CP and a data symbol, and a receiving end receives the data frame and executes the following steps:
s1, detecting a data frame by using a first training sequence A and adopting a parallel code phase search algorithm, and obtaining a coarse frequency offset estimation value to correct the coarse frequency offset value so as to obtain a data symbol subjected to coarse frequency offset correction;
s2, performing fine frequency offset estimation by using a second training sequence C to correct a fine frequency offset value so as to obtain a data symbol subjected to fine frequency offset correction; and
s3, carrying out symbol timing estimation by using a second training sequence C and a third training sequence D;
wherein,
the first training sequence A is formed by repeating T1 times by a ZC sequence with the period length of L1, wherein L1 and T1 are both positive integers, L1 is more than or equal to 4 and less than or equal to 12, T1 is more than or equal to 3000 and less than or equal to 5000, and the value of T1 depends on L1, the maximum frequency offset range and the signal-to-noise ratio;
the second training sequence C is formed by repeating T2 times by a ZC sequence with the period length of L2, wherein L2 and T2 are positive integers, L2 is more than or equal to 128 and less than or equal to 512, and T2 is more than or equal to 12 and less than or equal to 48;
the third training sequence D is formed by repeating T3 times after inverting a ZC sequence with the period length of L2, wherein T3 is a positive integer, and T2/8 is more than or equal to T3 is more than or equal to T2/4.
2. The method according to claim 1, wherein the length of the cyclic prefix CP is set such that an advance Ng is introduced to the symbol timing estimation point in step S3, and the magnitude of the advance Ng is at least larger than the maximum delay of the primary path and the first path for a multi-path channel in which the primary path is not the first path.
3. The communication system synchronization method according to claim 2, wherein step S1 comprises:
s1-1, carrying out frequency mixing operation on the received signal and an orthogonal carrier generated by a digital oscillator (NCO), and then carrying out correlation operation on the output of the frequency mixing and a local training sequence by utilizing an FFT (fast Fourier transform) module and an IFFT (inverse fast Fourier transform) module to obtain a correlation operation result;
s1-2, realizing coherent integration with preset length by using a periodic accumulation mode on the result of the correlation operation, performing modular operation on the I and Q paths of output after the coherent integration, and performing square addition to obtain a final detection judgment value V;
s1-3, comparing the detection judgment value V with a set threshold value Vt to judge whether a data frame is detected; and is
When a data frame is detected, the frame detection module stops working and provides a frequency value of the DDS at the moment as the coarse frequency offset estimation value;
when no data frame is detected, the frequency value of the DDS is updated and the search is re-performed by repeating the steps S1-1 to S1-3 until a data frame is detected.
4. The communication system synchronization method of claim 3,
the length N of the FFT (fast Fourier transform) is 1024 points, the length of the cyclic prefix CP is more than 91 points, and the time interval T of adjacent symbols at the transmitting ends0.2 mus; in the first training sequence a, L1 ═ 8 and T1 ═ 3800; in the second training sequence C, L2-256 and T2-24; in the third training sequence D, T3 ═ 4.
5. The communication system synchronization method of claim 4,
step S2 includes: performing correlation operation by using the local training sequence and the received second training sequence C, and accumulating the correlation operation result for 8 times to perform fine frequency offset estimation of the carrier;
step S3 includes: carrying out symbol timing estimation on the data symbols by using the correlation operation results of the local training sequence and the received second training sequence C and the third training sequence D; and is
The length of the cyclic prefix CP is 128 points, the signal-to-noise ratio of the communication system is as low as-15 dB, and the maximum normalized carrier frequency offset value is 1.2.
6. The method for synchronizing communication systems according to claim 5, wherein the step S2 of performing fine frequency offset estimation of carriers by accumulating 8 times with the result of the correlation operation further comprises:
dividing the correlation peak points corresponding to the continuous 16 subsequences in the second training sequence C into a front part and a rear part, and then respectively adding the front 8 values and the rear 8 values to calculate a fine frequency offset estimation value, wherein the formula is as follows:
wherein:
where r (d) represents the data symbols after coarse frequency offset correction, m (d) represents the local training sequence,representing a fine frequency offset estimate.
7. The communication system synchronization method of claim 5,
step S3 includes: obtaining a peak point by using the output of the following time measurement function, calculating the position of a third training sequence D by using the index of the peak point so as to complete symbol timing estimation,
where r (d) represents the data symbols after coarse frequency offset correction, and m (d) represents the local training sequence.
8. A communication system, which includes a synchronization apparatus disposed at a receiving end, wherein a data frame sent by a transmitting end of the communication system includes a first training sequence a, a second training sequence C, a third training sequence D, a cyclic prefix CP and a data symbol, and the synchronization apparatus includes:
the frame detection module is used for detecting a data frame by using a first training sequence A and adopting a parallel code phase search algorithm, and obtaining a coarse frequency offset estimation value to correct the coarse frequency offset value so as to obtain a data symbol subjected to coarse frequency offset correction;
the fine frequency offset correction module is used for performing fine frequency offset estimation by utilizing the second training sequence C so as to correct a fine frequency offset value and obtain a data symbol after fine frequency offset correction; and
a symbol timing module, which utilizes the second training sequence C and the third training sequence D to carry out symbol timing estimation;
wherein,
the first training sequence A is formed by repeating T1 times by a ZC sequence with the period length of L1, wherein L1 and T1 are both positive integers, L1 is more than or equal to 4 and less than or equal to 12, T1 is more than or equal to 3000 and less than or equal to 5000, and the value of T1 depends on L1, the maximum frequency offset range and the signal-to-noise ratio;
the second training sequence C is formed by repeating T2 times by a ZC sequence with the period length of L2, wherein L2 and T2 are positive integers, L2 is more than or equal to 128 and less than or equal to 512, and T2 is more than or equal to 12 and less than or equal to 48;
the third training sequence D is formed by repeating T3 times after inverting a ZC sequence with the period length of L2, wherein T3 is a positive integer, and T2/8 is more than or equal to T3 is more than or equal to T2/4; and,
the length of the cyclic prefix CP is set so that the symbol timing module introduces an advance Ng to the symbol timing estimation point, and the advance Ng is at least larger than the maximum time delay of the main path and the first path for the multi-path channel of which the main path is not the first path.
9. The communication system of claim 8, wherein the frame detection module comprises:
a frequency mixing module for mixing a received signal with a quadrature carrier generated by a digital oscillator (NCO);
a first FFT (fast fourier transform) module for performing fast fourier transform on the mixed output;
a second FFT (fast fourier transform) module for performing fast fourier transform on the local training sequence;
the conjugate module is used for carrying out complex conjugate operation on the output of the second FFT module;
an IFFT (inverse fast Fourier transform) module for performing inverse fast Fourier transform on the mixed output of the fast Fourier transform operation and the complex conjugate operation to obtain an I component and a Q component after the correlation operation;
the I component coherent integration module is used for performing coherent integration with a preset length on the I component after the correlation operation in a periodic accumulation mode;
the Q component coherent integration module is used for performing coherent integration with a preset length on the Q component after the correlation operation in a periodic accumulation mode;
an I component modulus and square module for performing modulus operation and square on the I component after coherent integration to obtain I2；
A Q component modulus and square module for performing modulus operation and square on the Q component after coherent integration to obtain Q2；
An addition module for pair I2And Q2Performing addition operation to obtain a detection judgment value V;
and the comparison module compares the detection judgment value V with a set threshold value Vt to judge whether a data frame is detected.
10. The communication system of claim 9,
the fine frequency offset correction module performs correlation operation by using a local training sequence and a received second training sequence C, and accumulates the correlation operation result for 8 times to perform fine frequency offset estimation of a carrier;
and the symbol timing module carries out symbol timing estimation on the data symbols by using the correlation operation results of the local training sequence, the received second training sequence C and the received third training sequence D.
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