Time synchronization method in multi-rate OFDM (Orthogonal Frequency Division Multiplexing) communication system

Abstract

The invention discloses a time synchronization method in a multi-rate OFDM (Orthogonal Frequency Division Multiplexing) communication system. The time synchronization method comprises the following steps: S1, forming a training symbol based on a ZC sequence; s2, quantifying the matching degree of a received signal and a synchronous signal in the multi-rate OFDM communication system; s3, performing sliding window energy normalization on the received signal to generate a dynamic detection threshold value; and S4, when the matching degree of the received signal and the synchronization signal is greater than a dynamic detection threshold, detecting the peak position of the synchronization signal, and outputting a timing estimation position to realize time synchronization. According to the method, a ZC sequence synchronization mechanism is expanded to a dynamic bandwidth scene for the first time, the synchronization problem under a complex channel in unmanned aerial vehicle communication is solved through unique characteristics of the ZC sequence, the technical blank of high-maneuverability unmanned aerial vehicle communication is filled, and efficient and high-precision time synchronization is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of data communication, and particularly relates to a time synchronization method in a multi-rate OFDM communication system. Background Art

[0002] Orthogonal frequency division multiplexing (OFDM) technology has become the core technology of modern wireless communication systems due to its characteristics of anti-multipath fading and high spectral efficiency, and is widely used in fields such as 4G / 5G networks and unmanned aerial vehicle (UAV) communication. In an OFDM system, precise time synchronization is crucial for ensuring the correct alignment of received symbols, mitigating inter-symbol interference (ISI) and inter-carrier interference (ICI).

[0003] Traditional time synchronization algorithms, such as the Schmidl & Cox algorithm, rely on the correlation of training sequences for timing estimation. However, in a multipath channel or under low signal-to-noise ratio conditions, due to the existence of a correlation plateau, this algorithm often leads to timing ambiguity and reduces synchronization accuracy. Especially in UAV communication, high-dynamic channels (such as Doppler frequency shift and fast time-varying channels) further exacerbate this problem. In addition, existing OFDM systems mostly adopt fixed-rate configurations, making it difficult to adapt to the dynamic changes in channel conditions and bandwidth requirements in UAV mission scenarios, resulting in an increase in the bit error rate or waste of spectral resources. Therefore, there is an urgent need for an efficient and robust time synchronization method to meet the performance requirements of multi-rate OFDM systems in a dynamic channel environment. Summary of the Invention

[0004] Aiming at the above deficiencies in the prior art, the time synchronization method in a multi-rate OFDM communication system provided by the present invention solves the problems of timing ambiguity, insufficient robustness, and poor multi-rate adaptability in existing multi-rate OFDM systems when using traditional time synchronization algorithms in multipath channels, low signal-to-noise ratio, and high-dynamic channel environments, and thus it is difficult to meet the high-reliability requirements of UAV communication.

[0005] To achieve the above invention objective, the technical solution adopted by the present invention is as follows: A time synchronization method in a multi-rate OFDM communication system, comprising the following steps:

[0006] S1. Form training symbols based on ZC sequences;

[0007] S2. Use the training symbols as synchronization signals to quantify the matching degree between the received signals and the synchronization signals in the multi-rate OFDM communication system;

[0008] S3. Perform sliding window energy normalization on the received signals to generate a dynamic detection threshold;

[0009] S4. When the matching degree between the received signal and the synchronization signal is greater than the dynamic detection threshold, the peak position of the synchronization signal is detected, and the timing estimation position is output to achieve time synchronization.

[0010] Further, in step S1, a ZC sequence with a length N of 63 is mapped to the pilot subcarriers of the OFDM symbol, and a middle cyclic prefix is added before each ZC sequence to form a serial number symbol.

[0011] Further, the ZC sequence z u (n) is expressed as:

[0012]

[0013] In the formula, u represents the sequence root index, n represents the training sequence length ordinal number, N represents the training sequence length, and is relatively prime to u.

[0014] Further, the length of the cyclic prefix is N / 4.

[0015] Further, in step S2, the matching degree between the received signal and the synchronization signal is:

[0016] |C ss (d)| 2 =Re{C ss (d)} 2 +Im{C ss (d)} 2

[0017] In the formula, C ss (d) represents the sliding cross-correlation between the received signal and the synchronization signal, Re{·} represents taking the real part, Im{·} represents taking the imaginary part, where, y(n) represents the received signal, s * (n - d) represents the conjugate of the training symbol based on the ZC sequence, n represents the sample point position, and d represents the time delay variable.

[0018] Further, in step S3, the dynamic detection threshold γ ss (d) is:

[0019] γ ss (d)=max(k·P ss (d), γ min )

[0020] In the formula, P ss (d) represents the energy of the received signal within a 128-sample sliding window, γ min represents the lower limit threshold, k represents the normalization coefficient, k = 0.6025595, γ min =5.9605×10-8 , y(n) represents the received signal, n represents the sample point position, and d represents the time delay variable.

[0021] Furthermore, in the step S4, the timing estimation position is:

[0022]

[0023] In the formula, timeingOffset represents the peak position of the synchronization signal.

[0024] The beneficial effects of the present invention are:

[0025] The method of the present invention first extends the ZC sequence synchronization mechanism to the dynamic bandwidth scenario, solves the synchronization problem in complex channels in UAV communication through the unique characteristics of the ZC sequence, fills the technical gap in high-mobility UAV communication, and realizes efficient and high-precision time synchronization. The specific advantages are as follows:

[0026] (1) Significantly improves the time synchronization accuracy and reduces the false detection probability:

[0027] This method utilizes the sharp autocorrelation peak characteristic of the ZC sequence (Zadoff-Chu sequence) to overcome the correlation platform effect of the traditional Schmidl & Cox algorithm in multipath channels or low signal-to-noise ratio environments. Experiments show that in an additive white Gaussian noise channel with a signal-to-noise ratio greater than 0 dB, the synchronization false detection probability of this method is lower than 10-3, which is nearly an order of magnitude lower than 10-2 of the traditional method. This enables the method of the present invention to more accurately determine the starting position of the OFDM symbol in the high-dynamic channel scenario of UAV communication (such as the existence of multipath effects and Doppler frequency shifts), avoid inter-symbol interference, and thus ensure the stability of subsequent demodulation.

[0028] (2) Enhances the synchronization robustness in low signal-to-noise ratio environments

[0029] The synchronization method based on the ZC sequence of the present invention can still maintain high synchronization performance in low signal-to-noise ratio (SNR < 0 dB) environments. Compared with the traditional method with a high synchronization failure rate (> 20%) at low signal-to-noise ratio, the synchronization success rate of this method can still reach more than 90% when SNR = -5 dB. This is particularly important for the common high-noise interference scenarios in UAV communication (such as urban environments or bad weather), ensuring the reliability of the communication link, reducing the frequency of data retransmission, and improving the communication efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a flowchart of the time synchronization method in the multi-rate OFDM communication system provided by the present invention.

[0031] Figure 2 Schematic diagram of training symbols provided by the present invention Detailed implementation manners

[0032] The following describes the detailed implementation manners of the present invention to facilitate those skilled in the art to understand the present invention. However, it should be clear that the present invention is not limited to the scope of the detailed implementation manners. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions created using the concept of the present invention are within the scope of protection.

[0033] An embodiment of the present invention provides a time synchronization method in a multi-rate OFDM communication system, including the following steps:

[0034] S1. Form training symbols based on ZC sequences;

[0035] S2. Use the training symbols as synchronization signals to quantify the matching degree between the received signals and the synchronization signals in the multi-rate OFDM communication system;

[0036] S3. Perform sliding window energy normalization on the received signals to generate a dynamic detection threshold;

[0037] S4. When the matching degree between the received signals and the synchronization signals is greater than the dynamic detection threshold, detect the peak position of the synchronization signals and output the timing estimation position to achieve time synchronization.

[0038] In step S1 of the embodiment of the present invention, a ZC sequence with a length N of 63 is mapped to the pilot subcarriers of the OFDM symbols, and a middle cyclic prefix is added before each ZC sequence to form serial number symbols; the formed training symbols are as Figure 2 shown, and the training sequence therein is the ZC sequence.

[0039] In this embodiment, the sequence length N is selected to be 63 to ensure constant amplitude and zero autocorrelation characteristics.

[0040] The ZC sequence z u (n) in this embodiment has the following expression:

[0041]

[0042] In the formula, u represents the sequence root index, n represents the training sequence length ordinal number, and N represents the training sequence length, which is relatively prime to u.

[0043] In this embodiment, the length of the cyclic prefix CP is set to N / 4. The cyclic prefix is a sequence segment copied from the end part to the head of the training symbols and has periodicity.

[0044] In this embodiment, focusing on time synchronization rather than signal multiplexing, a single ZC sequence is independently used as the preamble symbol of the training symbol. By optimizing the cyclic prefix length (fixed at 1 / 4 of the FFT point number N), the anti-multipath ability is improved. The ZC sequence has ideal autocorrelation characteristics, that is, the main peak is sharp (the autocorrelation value is N, where N is the sequence length), and the sidelobes are zero (zero correlation zone), which can significantly improve the synchronization accuracy in a multipath channel and a low signal-to-noise ratio environment.

[0045] In this implementation, by adjusting the ZC sequence parameters (such as the index u being relatively prime to the FFT point number N) to adapt to different modulation methods and coding rates, the dependence on a fixed-rate system in the prior art is solved, and the dynamic configuration of a multi-rate OFDM system is supported.

[0046] In step S2 of the embodiment of the present invention, the sliding cross-correlation between the received signal y(n) and the synchronization signal is calculated:

[0047]

[0048] Then, by calculating the squared magnitude of the cross-correlation value, the matching degree of the signal is quantified, which is expressed as:

[0049] |C ss (d)| 2 =Re{C ss (d)} 2 +Im{C ss (d)} 2

[0050] In the formula, C ss (d) represents the sliding cross-correlation between the received signal and the synchronization signal, Re{·} represents taking the real part, Im{·} represents taking the imaginary part, where y(n) represents the received signal, s * (n - d) represents the conjugate of the training symbol based on the ZC sequence, n represents the sample point position, and d represents the time delay variable.

[0051] In the embodiment of the present invention, in order to enhance the adaptability of the method under complex channel conditions, a sliding window energy normalization mechanism and a dynamic detection threshold strategy are introduced, effectively alleviating the misjudgment risk caused by the fluctuation of the received signal power. This decision method based on energy ratio normalization and cross-correlation amplitude enables the method of the present invention to adaptively adjust the detection threshold under different signal-to-noise ratio conditions, improve the synchronization success rate, and reduce the probability calculation of false detection and missed detection.

[0052] Specifically, in step S3 of the embodiment of the present invention, the energy P ss (d) within the 128-sample sliding window of the received signal is calculated as:

[0053]

[0054] To ensure that the threshold is not less than a certain lower limit, values below the lower limit are adjusted during the calculation of the threshold to generate a dynamic detection threshold γ ss (d) is:

[0055] γ ss (d) = max(k·P ss (d), γ min )

[0056] In the formula, P ss (d) represents the energy of the received signal within a 128 - sample sliding window, γ min represents the lower - limit threshold, k represents the normalization coefficient, k = 0.6025595, γ min = 5.9605×10 -8 , y(n) represents the received signal, n represents the sample - point position, and d represents the time - delay variable.

[0057] When the matching degree of the signal is greater than the dynamic detection threshold, that is:

[0058] |C ss (d)| 2 > γ ss (d)

[0059] It is considered that the peak position of the synchronization signal is detected Output the timing - estimation position as:

[0060]

[0061] In the formula, timeingOffset represents the peak position of the synchronization signal.

[0062] In this embodiment, a dynamic - threshold detection mechanism is introduced. By setting the lower - limit threshold (such as γ_min = 5.96×10 -8 ) and the normalization coefficient k = 0.6025595, the detection sensitivity is adaptively adjusted under low signal - to - noise ratio (SNR < 0dB), and the false - detection probability is reduced to 10 -3 or less.

[0063] In this embodiment, through efficient sliding - window correlation calculation, the synchronization process is simplified by using the characteristics of the ZC sequence. The correlation calculation only requires one sliding - window operation, and the computational complexity is O(N). The peak detection quickly determines the symbol start position through threshold comparison.

[0064] The method of the present invention aims at the high dynamic characteristics of UAV communication (such as fast time-varying channels, Doppler frequency shift), optimizes the ZC sequence length (N = 63) and the sliding window design, supports low-complexity real-time processing (computational complexity is O(N)), and has the characteristics of high synchronization accuracy, low SNR robustness, and multi-rate.

[0065] In the present invention, specific embodiments are used to elaborate on the principles and implementation manners of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manners and application scopes. In summary, the content of this specification should not be construed as a limitation on the present invention.

[0066] Those of ordinary skill in the art will realize that the embodiments described herein are for helping readers understand the principles of the present invention, and it should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those of ordinary skill in the art can make various other specific deformations and combinations that do not depart from the essence of the present invention according to these technical revelations disclosed by the present invention, and these deformations and combinations are still within the protection scope of the present invention.

Claims

1. A time synchronization method in a multi-rate OFDM communication system, characterized in that Including the following steps: S1. Form training symbols based on ZC sequences; S2. Use the training symbols as synchronization signals to quantify the matching degree between the received signals and the synchronization signals in a multi-rate OFDM communication system; S3. Perform sliding window energy normalization on the received signals to generate a dynamic detection threshold; S4. When the matching degree between the received signals and the synchronization signals is greater than the dynamic detection threshold, detect the peak position of the synchronization signals and output the timing estimation position to achieve time synchronization.

2. The time synchronization method in the multi-rate OFDM communication system according to claim 1, characterized in that In step S1, a ZC sequence with a length N of 63 is mapped to the pilot subcarriers of the OFDM symbols, and a middle cyclic prefix is added before each ZC sequence to form serial symbols.

3. The time synchronization method in the multi-rate OFDM communication system according to claim 2, wherein The ZC sequence z u (n) is expressed as: In the formula, u represents the sequence root index, n represents the training sequence length ordinal number, and N represents the training sequence length, which is relatively prime to u.

4. The time synchronization method in the multi-rate OFDM communication system according to claim 2, characterized in that, The length of the cyclic prefix is N / 4.

5. The time synchronization method in the multi-rate OFDM communication system according to claim 1, characterized in that, In step S2, the matching degree between the received signals and the synchronization signals is: |C ss (d)| 2 =Re{C ss (d)} 2 +Im{C ss (d)} 2 where C ss (d) represents the sliding cross-correlation between the received signal and the synchronization signal, Re{·} represents taking the real part, Im{·} represents taking the imaginary part, where y(n) represents the received signal, s * (n - d) represents the conjugate of the training symbol based on the ZC sequence, n represents the sample point position, and d represents the time delay variable.

6. The time synchronization method in a multi-rate OFDM communication system according to claim 1, characterized in that In the step S3, the dynamic detection threshold γ ss (d) is: γ ss (d) = max(k·P ss (d), γ min ) where P ss (d) represents the energy of the received signal within a 128-sample sliding window, γ min represents the lower threshold, k represents the normalization coefficient, k = 0.6025595, γ min = 5.9605×10 -8 , y(n) represents the received signal, n represents the sample point position, and d represents the time delay variable.

7. The time synchronization method in the multi-rate OFDM communication system according to claim 1, characterized in that In the step S4, the position is estimated at regular intervals which is In the formula, timeingOffset represents the peak position of the synchronization signals.

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