A time synchronization method, system and storage medium based on constant envelope zero autocorrelation sequence in OFDM communication

Abstract

The application belongs to the field of wireless communication, and particularly discloses a time synchronization method based on a constant envelope zero autocorrelation sequence in OFDM communication, a system and a storage medium, which comprises the following steps: at the sending end, a synchronization preamble is inserted at the front end of each OFDM frame, the synchronization preamble is constructed by using a constant envelope zero autocorrelation sequence with superior correlation characteristics, a sequence weighting is introduced in the preamble, and a symmetric sequence is additionally added to offset the weighting effect; and then, by fully utilizing the structural characteristics of the synchronization preamble, a fast Fourier transform starting point estimation algorithm is designed, so that the timing metric curve only presents one sharp main peak without obvious side lobes. The application utilizes the constructed preamble and the corresponding starting point estimation algorithm to design a time synchronization method, which can further improve the time synchronization success rate under the condition that the low signal-to-noise ratio and the weighting sequence at the receiving end do not need to be strictly aligned with the sending end, and simulation experiments prove that the method can realize higher synchronization accuracy.

Description

Technical Field

[0001] This invention belongs to the field of wireless communication, and more specifically, relates to a time synchronization method, system and storage medium based on constant envelope zero autocorrelation sequence in OFDM communication. Background Technology

[0002] Orthogonal Frequency Division Multiplexing (OFDM) has been widely used in digital communication systems due to its excellent resistance to multipath fading and high spectral efficiency. In OFDM systems, accurate time synchronization is crucial, directly affecting subsequent signal demodulation and detection. Inaccurate synchronization (i.e., a deviation between the estimated Fast Fourier Transform (FFT) start point and the actual start point) will lead to severe inter-carrier interference (ICI) and inter-symbol interference (ISI). However, existing synchronization methods exhibit significantly degraded performance in low signal-to-noise ratio (SNR) environments or when the weighted sequence at the receiver cannot be strictly aligned with that at the transmitter.

[0003] Against this backdrop, the development of time synchronization technology has significant theoretical and practical value, prompting numerous domestic and international scholars to conduct research on it. Currently, research findings on time synchronization mainly fall into two categories: one is non-data-aided synchronization methods, which do not rely on additional information and directly utilize the structural characteristics and inherent properties of OFDM signals to achieve synchronization; the other is data-aided synchronization methods, which typically involve pre-inserting a synchronization preamble with a specific structure at the transmitting end, and the receiving end using the characteristics of this preamble to calculate the corresponding timing metric to achieve time synchronization.

[0004] Synchronization from received OFDM signals often faces the following challenges: ① In low signal-to-noise ratio (SNR) environments, the aforementioned methods are prone to generating multiple noise-induced interference peaks during synchronization. Due to the randomness of noise, the positions of these interference peaks are difficult to predict, resulting in inaccurate identification of the FFT start point. ② When data-assisted methods improve synchronization performance by weighting the preamble, additional implementation overhead is introduced, requiring the receiver and transmitter to strictly align the weighted sequences, thus increasing the system's implementation complexity.

[0005] Therefore, a robust time synchronization method for OFDM systems is urgently needed. Summary of the Invention

[0006] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention provides a time synchronization method, system and storage medium based on constant envelope zero autocorrelation sequence in OFDM communication, the purpose of which is to improve the robustness of OFDM system time synchronization.

[0007] To achieve the above objectives, according to a first aspect of the present invention, a time synchronization method based on a constant envelope zero autocorrelation sequence in OFDM communication is proposed, comprising the following steps:

[0008] At the OFDM system transmitter, a synchronization preamble is inserted at the beginning of each OFDM frame;

[0009] The synchronization preamble consists of four equal-length parts, specifically represented as: [A[n]-B * [n]-B[n]·v[n]-A[n]·s[n]], where A[n] represents a constant envelope zero autocorrelation sequence of length N / 4, B[n] is symmetric to A[n], and * denotes the conjugate operation; v[n] represents a complex exponentially weighted sequence, s[n] is symmetric to v[n]; N represents the number of subcarriers in each OFDM symbol, and n represents the sequence index;

[0010] At the OFDM system receiver, the start position of the synchronization preamble is determined based on the received signal. This allows us to obtain the starting position of the Fast Fourier Transform and complete time synchronization.

[0011] As a further preferred option, A[n] is represented as:

[0012]

[0013] B[n] is represented as:

[0014] B[n] = A[N / 4-n-1]

[0015] v[n] is represented as:

[0016] v[n]=exp(jα n ),α n ∈[0,2π]

[0017] s[n] is represented as:

[0018] s[n] = v[N / 4 - n - 1]

[0019] Where j represents the imaginary unit, α n Indicates phase.

[0020] As a further preferred method, the start position of the synchronization preamble is determined based on the received signal. include:

[0021] The correlation P(d) and the corresponding normalization factor R(d) of different parts of the received signal are calculated based on the received signal.

[0022] The timing metric M(d) is calculated using P(d) and R(d);

[0023] Determine the starting position of the synchronization preamble based on M(d).

[0024] As a further preferred method, the correlation P(d) of different parts of the received signal is calculated using the following formula:

[0025]

[0026] Where r(·) represents the received signal, r(d) represents the received signal segment starting from the d-th sampling point, and the length of each signal segment is not less than N; N represents the number of subcarriers in each OFDM symbol, k represents the summation range, and * represents the conjugate operation.

[0027] As a further preferred method, the normalization factor R(d) is calculated using the received signal, and the formula is as follows:

[0028]

[0029] As a further preferred method, the timing metric M(d) is calculated using P(d) and R(d), and the formula is as follows:

[0030]

[0031] As a further preferred option, the starting position of the synchronization preamble is determined based on M(d). The calculation formula is:

[0032]

[0033] As a further preferred option, the formula for calculating the starting position of the Fast Fourier Transform is:

[0034]

[0035] in, N represents the starting position of the Fast Fourier Transform of the q-th OFDM symbol. cp This represents the length of the cyclic prefix.

[0036] According to a second aspect of the present invention, a time synchronization system based on a constant envelope zero autocorrelation sequence in OFDM communication is provided, comprising a processor, the processor being configured to execute the aforementioned time synchronization method based on a constant envelope zero autocorrelation sequence in OFDM communication.

[0037] According to a third aspect of the present invention, a computer-readable storage medium is provided having a computer program stored thereon, which, when executed by a processor, implements the time synchronization method based on a constant envelope zero autocorrelation sequence in OFDM communication described above.

[0038] In summary, compared with the prior art, the above-described technical solutions conceived by this invention mainly possess the following technical advantages:

[0039] This invention constructs a synchronization preamble consisting of four equal-length parts based on the Constant Amplitude Zero Autocorrelation (CAZAC) sequence, which exhibits superior correlation characteristics. Additional sequence weighting is introduced into the preamble to enhance the sharpness of the timing metric at the correct point. A symmetric sequence is also added to counteract the weighting effect, avoiding the need for the receiver to align with the transmitter's weighted sequence. Furthermore, by fully utilizing the structural characteristics of the four parts of the synchronization preamble and the phase characteristics introduced by the sequence weighting, the starting point of the Fast Fourier Transform (FFT) is estimated, resulting in a timing metric curve exhibiting only a sharp main peak without significant sidelobes. Therefore, even when the received OFDM signal is severely affected by noise and the receiver's weighted sequence is not strictly aligned with the transmitter's, this invention can still accurately estimate the FFT starting point, improving the time synchronization success rate and reducing implementation complexity. This allows for stable operation in various complex environments, demonstrating good robustness. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of a time synchronization method based on a constant envelope zero autocorrelation sequence in OFDM communication according to an embodiment of the present invention;

[0041] Figure 2 , Figure 3 The figures show the test results of timing measurements in an ideal environment within an OFDM system, using the method of this invention and existing data-assisted methods.

[0042] Figure 4 , Figure 5 The figures show the test results of the ratio of sidelobes to peak values ​​as a function of signal-to-noise ratio in an OFDM system with 64 subcarriers, using the method of this invention and an existing data-assisted method.

[0043] Figure 6 , Figure 7 The figures show the test results of the synchronization success rate as a function of signal-to-noise ratio in an OFDM system with 64 subcarriers, using the method of the present invention and the existing data-assisted method.

[0044] Figure 8 , Figure 9The figures show the test results of the method of the present invention and the existing data-assisted method in an OFDM system when the number of subcarriers is 64 and the weighted sequences of the receiver and transmitter are not strictly aligned, respectively, as the signal-to-noise ratio changes.

[0045] Figure 10 , Figure 11 The figures show the test results of the synchronization success rate of the present invention and the existing data-assisted method in an OFDM system when the signal-to-noise ratio is 0dB, as a function of the number of subcarriers. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0047] For OFDM systems, at the receiver, multiple interference peaks may occur in the timing measurement of the received signal due to channel noise. These interference peaks are often highly misleading and can easily lead to misjudgments during timing determination, thus becoming a major technical bottleneck for achieving high-precision synchronization under low signal-to-noise ratio conditions. Furthermore, when the weighted sequence at the receiver is not strictly aligned with that at the transmitter, it will further interfere with the accuracy of the timing measurement and increase the probability of synchronization errors. Therefore, this invention, based on the OFDM system, provides a time synchronization method in OFDM communication based on a constant envelope zero autocorrelation sequence, such as... Figure 1 As shown, it includes the following steps:

[0048] S1. At the transmitting end, a synchronization preamble is inserted at the beginning of each OFDM frame.

[0049] Specifically, to increase the difference in timing measurements between each sampling point, a synchronization preamble consisting of four distinct parts was designed. This preamble was initially constructed as follows:

[0050] [A[n]-B * [n]-B[n]-A[n]]

[0051] Where A[n] is a CAZAC sequence of length N / 4 with root index 1, and its specific expression is:

[0052]

[0053] B[n] and A[n] are symmetrical, represented as

[0054] B[n] = A[N / 4-n-1]

[0055] Considering that additional sequence weighting can further improve the sharpness of the timing metric at the correct point, the third part of the preamble is weighted with a complex exponential sequence v[n], denoted as...

[0056] v[n]=exp(jα n ),α n ∈[0,2π]

[0057] To avoid the need for the receiver to align the weighted sequence v[n], the fourth part uses a sequence s[n] that is symmetrical to v[n] for weighting, so that the third and fourth parts of the preamble can cancel out the influence of the weighted sequences. s[n] is represented as

[0058] s[n] = v[N / 4 - n - 1]

[0059] Where N represents the number of subcarriers in each OFDM symbol, n represents the sequence index, * represents the conjugate operation, j represents the imaginary unit, and α n Indicates phase.

[0060] S2. Calculate the correlation P(d) and the corresponding normalization factor R(d) using the received signal r(d).

[0061] Specifically, to achieve efficient calculation of the correlation part of the timing metric, the structural and phase characteristics among the four different parts of the synchronization preamble are fully utilized, while also incorporating the differences between the first and second halves of the preamble after sequence weighting. Specifically, the formula for P(d) is:

[0062]

[0063] Using half the energy of the complete symbol as the normalization factor, R(d) is expressed as:

[0064]

[0065] Where r(d) represents the received signal segment starting from the d-th sampling point, and the length of each signal segment is not less than N, and k represents the summation range.

[0066] S3. After obtaining the received signal correlation P(d) and normalization factor R(d), the timing metric M(d) is expressed as:

[0067]

[0068] S4. Determine the starting position of the preamble based on M(d). Then, the starting position of the FFT can be obtained. Synchronization complete.

[0069] Specifically, the start position of the preamble Estimated as

[0070]

[0071] FFT start position of the q-th OFDM symbol Determined as

[0072]

[0073] Where, N cp This represents the length of the cyclic prefix.

[0074] The following specific embodiments further illustrate the effects of using the present invention:

[0075] (1) Timed measurement results

[0076] Figure 2 and Figure 3 This paper compares the timing measurement performance of the method of this invention with existing data-assisted synchronization methods under ideal conditions. It can be seen that the timing measurement curve of the Schmidl method exhibits a clear plateau effect at its highest peak; the Minn method shows two peaks, with a slow rise and fall on both sides of the peak, rather than a sudden transition from low to high values; the Park method's timing measurement curve shows multiple sidelobes with large amplitudes; the Meng method's timing measurement curve not only has a clear secondary peak with an amplitude close to the main peak, but also has many sidelobes with large amplitudes overall; the Xie method's timing measurement curve, although having a clear main peak, still has several sidelobes with large amplitudes; the Fang method's timing measurement curve is quite similar to the method of this invention, both showing a clear and prominent main peak, with fewer sidelobes and lower amplitudes. Overall, the method of this invention produces a pulse-shaped timing measurement, which enables it to achieve more robust and accurate time synchronization than the other six methods.

[0077] Specifically, the Schmidl method described in this embodiment is derived from "Robust frequency and timing synchronization for OFDM", the Minn method from "On timing offset estimation for OFDM systems", the Park method from "A novel timing estimation method for OFDM systems", the Fang method from "A novel synchronization algorithm based on CAZAC sequence for OFDM systems", the Xie method from "Improved Algorithm for Timing Synchronization of OFDM Systems Based on ZC Sequences", and the Meng method from "Research on timing synchronization algorithm for OFDM systems in Rician channel".

[0078] (2) Results of the ratio of sidelobes to peaks under different signal-to-noise ratio environments

[0079] Figure 4 and Figure 5 The performance of the proposed method versus existing data-assisted synchronization methods in timing metrics was compared with that of existing methods with a subcarrier configuration of 64, focusing on the ratio of sidelobes to peak values. As shown in the figure, the proposed method exhibits a lower sidelobe-to-peak ratio than the other six methods, highlighting its more pronounced peak sharpness in timing metrics.

[0080] (3) Synchronization success rate results under different signal-to-noise ratio environments

[0081] Figure 6 and Figure 7 The performance of the proposed method versus existing data-assisted methods in terms of synchronization success rate is demonstrated with a subcarrier configuration of 64. Within a signal-to-noise ratio range of -5 dB to 5 dB, the proposed method consistently outperforms existing methods. Specifically, within this range, the proposed method achieves a maximum improvement of 12.59% in synchronization success rate compared to the best-performing Fang method.

[0082] (4) Test results when there are differences in the weighted sequences at the receiving end

[0083] It is worth noting that the synchronization performance of the Fang method is highly dependent on the strict alignment of the weighted sequences at the receiving and sending ends. For example... Figure 8 and Figure 9 As shown, when the weighted sequence at the receiving end differs from that at the transmitting end by 50%, the ratio of sidelobes to the peak in the timing metric curve of the Fang method increases significantly, and the synchronization success rate decreases significantly. In contrast, the method of this invention does not rely on strict alignment of the weighted sequences at the receiving and transmitting ends, and still maintains excellent performance.

[0084] (5) Synchronization success rate results under different subcarrier configurations

[0085] Figure 10 and Figure 11 The figures demonstrate the performance of the proposed method versus existing data-assisted methods in terms of synchronization success rate under a signal-to-noise ratio (SNR) of 0 dB, with subcarrier numbers of 64, 128, 256, 512, and 1024. As can be seen from the figures, the proposed method performs comparably to existing methods at high subcarrier numbers, while exhibiting superior performance at low subcarrier numbers.

[0086] In summary, this invention utilizes a constructed preamble and a corresponding start point estimation algorithm to design a time synchronization method. Even with low signal-to-noise ratios and when the weighted sequence at the receiver does not need to be strictly aligned with that at the transmitter, it can still further improve the time synchronization success rate and synchronization accuracy, demonstrating superior synchronization performance compared to existing methods.

[0087] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A time synchronization method based on a constant envelope zero autocorrelation sequence in OFDM communication, characterized in that, Includes the following steps: At the OFDM system transmitter, a synchronization preamble is inserted at the beginning of each OFDM frame; The synchronization preamble consists of four equal-length parts, specifically represented as: [A[n] -B * [n] -B[n]·v[n] -A[n]·s[n]], where A[n] represents a constant envelope zero autocorrelation sequence of length N / 4, B[n] is symmetric to A[n], and * denotes the conjugate operation; v[n] represents a complex exponentially weighted sequence, s[n] is symmetric to v[n]; N represents the number of subcarriers in each OFDM symbol, and n represents the sequence index; At the OFDM system receiver, the start position of the synchronization preamble is determined based on the received signal. This allows us to obtain the starting position of the Fast Fourier Transform and complete time synchronization.

2. The time synchronization method based on a constant envelope zero autocorrelation sequence in OFDM communication as described in claim 1, characterized in that, A[n] is represented as: B[n] is represented as: B[n] = A[N / 4-n-1] v[n] is represented as: v[n]=exp(jα n ),a n ∈[0,2π] s[n] is represented as: s[n] = v[N / 4 - n - 1] Where j represents the imaginary unit, α n Indicates phase.

3. The time synchronization method based on a constant envelope zero autocorrelation sequence in OFDM communication as described in claim 1 or 2, characterized in that, Determine the start position of the synchronization preamble based on the received signal. Specifically, it includes: The correlation P(d) and the corresponding normalization factor R(d) of different parts of the received signal are calculated based on the received signal. The timing metric M(d) is calculated using P(d) and R(d); Determine the starting position of the synchronization preamble based on M(d).

4. The time synchronization method based on a constant envelope zero autocorrelation sequence in OFDM communication as described in claim 3, characterized in that, The correlation P(d) of different parts of the received signal is calculated using the following formula: Where r(·) represents the received signal, r(d) represents the received signal segment starting from the d-th sampling point, and the length of each signal segment is not less than N; N represents the number of subcarriers in each OFDM symbol, k represents the summation range, and * represents the conjugate operation.

5. The time synchronization method based on a constant envelope zero autocorrelation sequence in OFDM communication as described in claim 4, characterized in that, The normalization factor R(d) is calculated using the received signal, and the formula is as follows:

6. The time synchronization method based on a constant envelope zero autocorrelation sequence in OFDM communication as described in claim 5, characterized in that, The timing metric M(d) is calculated using P(d) and R(d), and the formula is as follows:

7. The time synchronization method based on a constant envelope zero autocorrelation sequence in OFDM communication as described in claim 6, characterized in that, Determine the starting position of the synchronization preamble based on M(d). The calculation formula is:

8. The time synchronization method based on a constant envelope zero autocorrelation sequence in OFDM communication as described in claim 7, characterized in that, The formula for calculating the starting position of the Fast Fourier Transform is: in, N represents the starting position of the Fast Fourier Transform of the q-th OFDM symbol. cp This represents the length of the cyclic prefix.

9. A time synchronization system based on a constant envelope zero autocorrelation sequence in OFDM communication, characterized in that, Includes a processor for executing a time synchronization method based on a constant envelope zero autocorrelation sequence in OFDM communication as described in any one of claims 1-8.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements the time synchronization method based on constant envelope zero autocorrelation sequence in OFDM communication as described in any one of claims 1-8.

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