Frame synchronization method and frame synchronization detection module for inter-cabin OFDM (Orthogonal Frequency Division Multiplexing) wireless communication system

Abstract

The invention discloses an inter-cabin OFDM (Orthogonal Frequency Division Multiplexing) wireless communication system frame synchronization method and a frame synchronization detection module. Frame synchronization is the first step of baseband processing of a receiving end demodulator, namely, frame header detection is carried out on received wireless signals, and the initial position of a frame is determined by utilizing the physical characteristics of a lead code so as to detect the arrival of an effective data packet. A preamble is a known short training sequence for receiving and transmitting, frame synchronization detection is carried out by utilizing good correlation characteristics and repeatability characteristics of the preamble, local correlation and self-correlation processing is respectively carried out on an input signal at a receiving end, and a peak value is generated if a threshold value is greater than the threshold value for multiple times continuously in a threshold value comparison mode. And when the interval between the peak values is the length of a group of short training sequences, determining that the frame start is found, and completing the accurate frame synchronization process. Frame synchronization accuracy directly affects OFDM symbol positioning performance, and synchronization performance of the OFDM symbol positioning performance is the key to influence normal work of a demodulator.

Description

Technical Field

[0001] The invention discloses a frame synchronization method and a frame synchronization detection module for an inter-cabin OFDM wireless communication system, and belongs to the field of wireless communication signal processing. Background Art

[0002] Slip rings are often used for inter-cabin data transmission. However, slip rings have the following problems during use: (1) their service life is easily affected by rotational wear and deformation; (2) they have the inherent characteristic of instantaneous open circuit, which is unreliable. For high-speed (e.g., above 20Mbps) data transmission requirements, data communication interruptions, frame loss, and packet loss will be very obvious due to their instantaneous open circuit characteristics.

[0003] In view of the above problems that may exist in slip ring communication and the demand for high-speed and reliable data transmission, OFDM multi-carrier modulation and demodulation processing technology is combined on the basis of traditional coding and modulation processing. OFDM signal transmission and reception are realized through antennas to form a wireless transmission link, which effectively solves the rate bottleneck and communication interruption problems of traditional slip ring wired links and improves the system's anti-multipath performance and reliability.

[0004] The inter-cabin receiver receives OFDM signals through the antenna, first filters and extracts the input signal, recovers the baseband sample data, and sends the baseband data to the demodulator for frame synchronization to determine the frame start. Here, according to the characteristics of the data transmission frame format, the continuous symbol sequence correlation characteristics in the known physical frame header are fully utilized for accurate detection, and then OFDM demodulation and decoding are performed and the results are output. Therefore, it can be seen that frame synchronization detection is the key to subsequent OFDM demodulation and decoding. Summary of the invention

[0005] The technical problem solved by the present invention is: to overcome the shortcomings of the prior art, to provide a frame synchronization method and a frame synchronization detection module for an inter-cabin OFDM wireless communication system, to solve the problem of physical frame header frame synchronization of an inter-cabin OFDM receiver in a low signal-to-noise ratio and interference environment, to accurately locate the frame start position, and to reduce missynchronization and OFDM symbol synchronization performance loss.

[0006] The technical solution of the present invention is: a frame synchronization method for an inter-cabin OFDM wireless communication system, the method comprising the following steps:

[0007] S1, storing the received input signal in a buffer and performing complex conjugate correlation with the short training sequence repeated in the preamble, accumulating and summing the complex conjugate correlation results within the observation window period to obtain the correlation value within the observation window period;

[0008] S2, summing up the energy of the input signal during the observation window period to obtain the energy value during the observation window period;

[0009] S3, calculating the ratio of the local correlation value modulo the fourth power within the observation window period to the energy value squared within the observation window period, and recording it as the discrimination value;

[0010] S4, comparing the discrimination value with a preset threshold value, if the discrimination value is greater than the preset threshold value, it is considered that the discrimination value has generated a peak, and the position of the input signal corresponding to the peak value in the buffer is recorded;

[0011] S5. Repeat steps S1 to S4 until M peaks appear. Compare whether the position difference of the input signals corresponding to adjacent peaks in the M peaks in the buffer is a set of short training sequence lengths. If so, it is considered that the frame start is found and accurate frame synchronization is completed, and M is greater than or equal to 1.

[0012] Preferably, the observation window period is N consecutive preamble code periods, 1≤M≤LN, and L is the number of short training sequence groups that are continuously repeated in the preamble code.

[0013] Preferably, the frame starting position is:

[0014] The Mth peak corresponds to a cache position of -(M-1+N)×a set of short training sequence lengths.

[0015] Preferably, the preset threshold is 0.45-0.55.

[0016] Preferably, the energy of the received input signal is calculated by calculating the modulus of the input signal and squaring it.

[0017] Another technical solution of the present invention is: a frame synchronization detection module of an inter-cabin OFDM wireless communication system, the detection module comprising a local correlation branch, an autocorrelation branch, and a decision module;

[0018] The local correlation branch performs complex conjugate correlation summation on the input signal and the short training sequence stored in the buffer and in the preamble within the observation window period, and then calculates the local correlation value within the window period modulo the fourth power to obtain the correlation value within the observation window period;

[0019] Autocorrelation branch, the energy value of the input signal within the window period;

[0020] The judgment module records the ratio of the correlation value to the energy value within the observation window as the judgment value, and compares the judgment value with the preset threshold. If the judgment value is greater than the preset threshold, it is considered that the judgment value has generated a peak, and the position of the input signal corresponding to the peak in the buffer is recorded. The difference in the position of the input signal corresponding to the adjacent peaks in the M peaks in the buffer is compared to see whether it is a group of short training sequence lengths. If so, it is considered that the frame start is found and accurate frame synchronization is completed. M is greater than or equal to 1, 1≤M≤LN, and L is the number of short training sequence groups repeated continuously in the preamble code.

[0021] Preferably, the observation window period is N consecutive preamble code periods, 1≤M≤LN, and L is the number of short training sequence groups that are continuously repeated in the preamble code.

[0022] Preferably, the preset threshold is 0.45-0.55.

[0023] Preferably, the energy of the received input signal is calculated by calculating the modulus of the input signal and squaring it.

[0024] Preferably, the frame starting position is:

[0025] The Mth peak corresponds to a cache position of -(M-1+N)×a set of short training sequence lengths.

[0026] The advantages of the present invention compared with the prior art are:

[0027] The present invention aims at the situation that multiple continuous correlation peak points will appear in the frame synchronization processing process, the peak points are all greater than the threshold value, and there is a group of short training sequence sample point intervals between the peak points. By judging whether the number of sample points between the correlation peak points that are continuously greater than the threshold value meets the above characteristics, the virtual synchronization judgment can be strengthened, the frame synchronization accuracy can be improved, and the synchronous demodulation performance of the receiver can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a block diagram of a physical frame structure of wireless communication according to an embodiment of the present invention;

[0029] Figure 2 This is a block diagram of comparing thresholds of local correlation and autocorrelation branches according to an embodiment of the present invention;

[0030] Figure 3 A frame synchronization implementation block diagram of an embodiment of the present invention;

[0031] Figure 4 This is a frame synchronization simulation effect block diagram of an embodiment of the present invention;

[0032] Figure 5 This is a feature block diagram of multiple continuous peak points in an embodiment of the present invention. DETAILED DESCRIPTION

[0033] The present invention will be further described below in conjunction with the accompanying drawings.

[0034] The present invention provides a frame synchronization method and a frame synchronization detection module for an inter-cabin wireless communication system. In an inter-cabin wireless communication system, due to the complex closed and semi-closed application environment inside and outside the satellite cabin and the influence of multipath interference, OFDM modulation and demodulation technology is mostly used in the system. In order to accurately locate the OFDM symbol for subsequent OFDM demodulation, the receiving end first needs to complete the precise frame synchronization process. Frame synchronization is the first step of the demodulator baseband processing at the receiving end, that is, the frame header detection is performed on the received wireless signal, and the starting position of the frame is determined by the physical characteristics of the preamble code to detect the arrival of a valid data packet. The preamble code is composed of a short training sequence known to the transceiver, and the transceiver is known. The receiving end can use its good correlation characteristics and repeatability characteristics to perform frame synchronization detection. The input signal is processed locally and autocorrelated respectively at the receiving end, and by comparing the threshold value, the threshold value is greater than the threshold for multiple consecutive times, a peak value is generated, and the interval between the peak values ​​is a group of short training sequence lengths, it is considered that the frame start is found, and the precise frame synchronization process is completed.

[0035] like Figure 1 As shown, in a specific embodiment of the present invention, the physical frame structure of inter-cabin wireless transmission consists of a preamble, signaling, and valid data fields. In order to better complete frame synchronization, the preamble selects a training sequence with good correlation performance, including a short training sequence and a long training sequence, wherein the short training sequence L-STF consists of ten groups of completely identical code elements: STS1 to STS10. The data field consists of an integer OFDM symbol consisting of payload data and padding data.

[0036] The present invention proposes a frame synchronization method for an inter-cabin OFDM wireless communication system, the method comprising the following steps:

[0037] S1, storing the received input signal in a buffer and performing complex conjugate correlation with the short training sequence repeated in the preamble, accumulating and summing the complex conjugate correlation results within the observation window period to obtain the correlation value within the observation window period;

[0038] S2. Accumulate and sum the energy of the input signal during the observation window to obtain the energy value during the observation window; the energy of the received input signal is calculated by calculating the modulus of the input signal and squaring it.

[0039] S3, calculating the ratio of the fourth power of the local correlation value within the observation window period to the square of the energy value within the observation window period, and recording it as the discrimination value;

[0040] S4, comparing the discrimination value with a preset threshold value, if the discrimination value is greater than the preset threshold value, it is considered that the discrimination value has generated a peak, and the position of the input signal corresponding to the peak value in the buffer is recorded;

[0041] S5. Repeat steps S1 to S4 until M peaks appear. Compare whether the position difference of the input signals corresponding to adjacent peaks in the M peaks in the buffer is a set of short training sequence lengths. If so, it is considered that the frame start is found and accurate frame synchronization is completed, and M is greater than or equal to 1.

[0042] The observation window period is N consecutive preamble code periods, 1≤M≤LN, and L is the number of short training sequence groups that are continuously repeated in the preamble code.

[0043] The frame starting position is:

[0044] The Mth peak corresponds to a cache position of -(M-1+N)×a set of short training sequence lengths.

[0045] like Figure 2 As shown, in a specific embodiment of the present invention, a method of comparing thresholds using local correlation and autocorrelation branches is adopted. The local correlation branch performs complex conjugate correlation summation on the input signal and the local known data within the window period, and then the local correlation value within the window period is calculated modulo the fourth power; the autocorrelation branch calculates the modulus value of the input signal and squares it to obtain the energy sum of the input signal within the window period, and then the energy value within the window period is squared; finally, continuous sliding is performed to determine the fixedness of the data interval between peaks to start with precise frame synchronization.

[0046] Based on the above method, the present invention also proposes a frame synchronization detection module for an inter-cabin OFDM wireless communication system, the module comprising a local correlation branch, an autocorrelation branch, and a decision module;

[0047] The local correlation branch performs complex conjugate correlation summation on the input signal and the short training sequence stored in the buffer and in the preamble within the observation window period, and then calculates the local correlation value within the window period modulo the fourth power to obtain the correlation value within the observation window period;

[0048] The autocorrelation branch calculates the energy value of the input signal within the window period; the energy of the received input signal is obtained by calculating the input signal modulus and squaring it.

[0049] The judgment module records the ratio of the correlation value to the energy value within the observation window as the judgment value, and compares the judgment value with the preset threshold. If the judgment value is greater than the preset threshold, it is considered that the judgment value has generated a peak, and the position of the input signal corresponding to the peak in the buffer is recorded. The difference in the position of the input signal corresponding to the adjacent peaks in the M peaks in the buffer is compared to see whether it is a group of short training sequence lengths. If so, it is considered that the frame start is found and accurate frame synchronization is completed. M is greater than or equal to 1, 1≤M≤LN, and L is the number of short training sequence groups repeated continuously in the preamble code.

[0050] The observation window period is N consecutive preamble code periods, 1≤M≤LN, and L is the number of short training sequence groups that are continuously repeated in the preamble code.

[0051] The preset threshold is 0.45-0.55.

[0052] The frame starting position is:

[0053] The Mth peak corresponds to a cache position of -(M-1+N)×a set of short training sequence lengths.

[0054] like Figure 3 As shown, in a specific embodiment of the present invention, the frame synchronization detection is mainly composed of: cache, local correlation and autocorrelation two branches and a decision module, which mainly involves cache, correlation, cumulative sum, modulo fourth power and energy square operations. It is known that a group of short training sequence STS codewords has 16 sample data, so a sharp correlation peak will appear after 16 sample data. If a group of known codewords is used to slide correlate with 10 groups of repeated STS, 10 sharp correlation peaks will appear continuously, and the interval between the peaks is 16 sample data. Window C is the complex conjugate correlation between the received input signal and the local short training sequence data. The length of the participating samples is four groups of 4*STS code elements, with a length of 4*16=64. The sum is accumulated and then modulo the fourth power; window P is the energy obtained by the autocorrelation of the received signal, the square of the accumulated sum is taken, and the length of the participating samples is the same as window C; the ratio of the two is used and compared with the threshold, and the threshold is 0.5; instead of using a single comparison value greater than the threshold as the criterion, continue to slide, and find that the threshold is greater than 0.5 for more than three consecutive times, record the peak position each time, and judge whether the three sequence number differences are all D, D is a group of STS sequence sample length, which is 16 here, and it can be considered that the frame synchronization detection is completed; back off from the buffer area, the starting position of the entire frame can be determined and output.

[0055] The frame start position = the position in the cache corresponding to the Mth peak - (M-1+N)×D, where M=3, N=4, and D=16 in this example.

[0056] like Figure 4 As shown in the figure, with 4 local STS groups as the correlation window and after frame synchronization processing with the received data, the decision threshold will have 7 groups of very high sharp peaks in succession at the positions of 10 groups of consecutive short training sequences, and the comparison thresholds on both sides will be gradually reduced. In other data segments, the relevant comparison thresholds are very low and there are no peaks. At this time, the threshold decision threshold is set to 0.5.

[0057] like Figure 5 As shown, when the value is greater than the threshold, the 7 correlation peaks all meet the characteristic of a difference of 16 sample data points (the length of a group of STS data). This continuous feature can be used to strengthen the missynchronization judgment and determine the accuracy of frame synchronization.

[0058] In summary, the method of the present invention adopts a processing method in which the thresholds of the local correlation and autocorrelation branches are compared and are greater than the threshold value; the local correlation branch performs complex conjugate correlation summation on the input signal and the local known data within the window period, and then calculates the modulus of the local correlation value within the window period to the fourth power; the autocorrelation branch calculates the modulus value of the input signal and squares it, obtains the energy sum of the input signal within the window period, and then squares the energy value within the window period; this method can produce multiple correlation peak points continuously, and the peak points differ by a group of STS sample point intervals. By judging the number of difference samples between the correlation peaks that are continuously greater than the threshold for multiple times, the virtual synchronization judgment can be strengthened, the frame synchronization accuracy can be determined, and the synchronous demodulation performance of the receiving end can be met under low signal-to-noise ratio conditions.

Claims

1. A frame synchronization method for an inter-cabin OFDM wireless communication system, characterized in that The steps include: S1, storing the received input signal in a buffer and performing complex conjugate correlation with the short training sequence repeated in the preamble, accumulating and summing the complex conjugate correlation results within the observation window period to obtain the correlation value within the observation window period; S2, summing up the energy of the input signal during the observation window period to obtain the energy value during the observation window period; S3, calculating the ratio of the local correlation value modulo the fourth power within the observation window period to the energy value squared within the observation window period, and recording it as the discrimination value; S4, comparing the discrimination value with a preset threshold value, if the discrimination value is greater than the preset threshold value, it is considered that the discrimination value has generated a peak, and the position of the input signal corresponding to the peak value in the buffer is recorded; S5. Repeat steps S1 to S4 until M peaks appear. Compare whether the position difference of the input signals corresponding to adjacent peaks in the M peaks in the buffer is a set of short training sequence lengths. If so, it is considered that the frame start is found and accurate frame synchronization is completed, and M is greater than or equal to 1.

2. The inter-cabin OFDM wireless communication system frame synchronization method according to claim 1, characterized in that: The observation window period is N consecutive preamble code periods, 1≤M≤LN, and L is the number of short training sequence groups that are continuously repeated in the preamble code.

3. The inter-cabin OFDM wireless communication system frame synchronization method according to claim 1, characterized in that: The frame starting position is: The Mth peak corresponds to a cache position of -(M-1+N)×a set of short training sequence lengths.

4. The inter-cabin OFDM wireless communication system frame synchronization method according to claim 1, characterized in that: The preset threshold is 0.45-0.

55.

5. The inter-cabin OFDM wireless communication system frame synchronization method according to claim 1, characterized in that: The energy of the received input signal is calculated by calculating the modulus of the input signal and squaring it.

6. A frame synchronization detection module for an inter-cabin OFDM wireless communication system, characterized in that It includes local correlation branch, autocorrelation branch and decision module; The local correlation branch performs complex conjugate correlation summation on the input signal and the short training sequence stored in the buffer and in the preamble within the observation window period, and then calculates the local correlation value within the window period modulo the fourth power to obtain the correlation value within the observation window period; Autocorrelation branch, the energy value of the input signal within the window period; The judgment module records the ratio of the correlation value to the energy value within the observation window as the judgment value, and compares the judgment value with the preset threshold. If the judgment value is greater than the preset threshold, it is considered that the judgment value has generated a peak, and the position of the input signal corresponding to the peak in the buffer is recorded. The difference in the position of the input signal corresponding to the adjacent peaks in the M peaks in the buffer is compared to see whether it is a group of short training sequence lengths. If so, it is considered that the frame start is found and accurate frame synchronization is completed. M is greater than or equal to 1, 1≤M≤LN, and L is the number of short training sequence groups repeated continuously in the preamble code.

7. The inter-cabin OFDM wireless communication system frame synchronization detection module according to claim 1, characterized in that: The observation window period is N consecutive preamble code periods, 1≤M≤LN, and L is the number of short training sequence groups that are continuously repeated in the preamble code.

8. The inter-cabin OFDM wireless communication system frame synchronization detection module according to claim 1, characterized in that: The preset threshold is 0.45-0.

55.

9. The inter-cabin OFDM wireless communication system frame synchronization detection module according to claim 1, characterized in that: The energy of the received input signal is calculated by calculating the modulus of the input signal and squaring it.

10. The inter-cabin OFDM wireless communication system frame synchronization detection module according to claim 1, characterized in that: The frame starting position is: The Mth peak corresponds to a cache position of -(M-1+N)×a set of short training sequence lengths.

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