A timing and frequency joint synchronization method for a delay-doppler domain communication system

Abstract

The application discloses a timing and frequency combined synchronization method of a delay Doppler domain communication system, uses a dual-chirp signal as a preamble sequence, obtains time-frequency offset information according to the peak position offset of the FrFT of the chirp signal, and realizes synchronization of the delay Doppler domain communication system. The method comprises the following steps: taking the dual-chirp signal as the preamble sequence, first performing FrFT transformation on the dual-chirp signal, and recording the peak position; padding the preamble sequence with zeros to the same dimension as the data sequence and transforming the preamble sequence to the DD domain; superimposing the preamble sequence and the data sequence in the DD domain, taking the superimposed data as the transmitted DD domain data, transforming the transmitted DD domain data to the time domain, and then transmitting the transmitted DD domain data; performing FrFT transformation on the received signal according to a sliding window, and recording the sliding window position where energy is concentrated; solving the timing offset and the frequency offset according to the difference between the received peak position and the transmitted peak position, and completing the synchronization process. The preamble sequence designed in the application realizes high-precision synchronization in a high-carrier-frequency high-speed mobile scene without occupying additional resources.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of wireless communication, and relates to a communication synchronization method, in particular to a synchronization method based on timing and frequency joint synchronization of fractional Fourier transform of chirp signal in a delay-Doppler domain communication system. BACKGROUND

[0002] The sixth generation wireless network will support a wide range of mobile terminals and ubiquitous connectivity, from autonomous vehicles to drones, low earth orbit satellites and high-speed trains. One of the key challenges faced by these services is to provide reliable communication in high-speed mobile environments. In addition, the congestion of the spectrum below 6 GHz brings a fundamental bottleneck for capacity improvement and system sustainability, prompting mobile providers to use higher frequency bands, such as millimeter wave bands. However, the Doppler spread caused by the relative motion between the transmitter, receiver and scatterer poses a huge challenge to wireless communication in high carrier frequency high-speed mobile scenarios. Orthogonal time-frequency space modulation modulates information in the delay-Doppler (DD) domain instead of the time-frequency domain of classical orthogonal frequency division multiplexing (OFDM), has superior delay and Doppler performance, and has the potential for full diversity, and has great application prospects in high carrier frequency high-speed mobile scenarios.

[0003] In a wireless communication system, the starting time of the transmitted signal, the transmission delay caused by the multipath channel and noise, are unknown to the receiving end. Therefore, in order to accurately obtain the transmitted information at the receiving end, timing synchronization is first needed to ensure that the received data symbols can be correctly demodulated. At the same time, frequency synchronization is needed to estimate and compensate for the crystal oscillator offset between the transmitter and receiver, and the Doppler spread caused by the relative motion between the transmitter, receiver and scatterer. In the design of any actual communication system, synchronization is a challenging problem, and the delay-Doppler domain communication system is no exception. Due to the backward compatibility of oFDM for delay-Doppler domain communication, many synchronization methods used for OFDM can be applied in principle to delay-Doppler domain communication systems. However, given that delay-Doppler domain communication is mainly used in high carrier high mobility scenarios, while most existing synchronization techniques are designed for low mobility and low carrier scenarios, delay-Doppler domain communication faces new challenges in synchronization. Therefore, in order to fully exploit the potential of delay-Doppler domain communication, the synchronization problem of delay-Doppler domain communication systems in high carrier frequency high-speed mobile scenarios must be solved. SUMMARY

[0004] In order to solve the problems of the prior art, the present application provides a joint timing and frequency synchronization method based on Fraction Fourier Transform (FrFT) of chirp signal for delay Doppler domain communication system.

[0005] The baseband processing of the sending end comprises the following steps:

[0006] (1) taking the dual-chirp signal of M length as the synchronization preamble sequence, firstly performing FrFT transform on the signal, recording the optimal rotation angle a and the position P of the peak value tx1 and P tx2 ;

[0007] (2) adding M(N-1) 0s at the end of the dual-chirp signal of M length, and constructing a M*N matrix, sequentially performing Wigner transform and Symplectic Finite Fourier Transform (SFFT) on the matrix, transforming to the DD domain, and recording as P;

[0008] (3) after the bit information to be transmitted of each delay Doppler domain communication system is modulated by QAM, constructing a M*N matrix, and recording as X;

[0009] (4) superimposing the matrix P on the matrix X to form a DD domain superimposed signal, then performing Inverse Symplectic Finite Fourier Transform (ISFFT) and Heisenberg transform, obtaining the time domain signal and sending after adding CP;

[0010] The synchronization processing of the receiving end comprises the following steps:

[0011] (5) performing FrFT transform of rotation a on the received signal according to the sliding window, when the energy aggregation peak appears, determining the sliding window as the window where the dual-chirp signal is located, and recording the starting position of the sliding window as b;

[0012] (6) recording the peak position of the FrFT transform of the signal in the receiving window as P rxl and P rx2 , calculating the difference between the peak position of the sending dual-chirp signal and the received signal, and recording as ΔN1 and ΔN2;

[0013] (7) calculating the timing offset d and the normalized frequency offset ζ from ΔN1 and ΔN2, obtaining the starting position of the delay Doppler domain communication system frame as b+d, and completing the synchronization process of the received signal.

[0014] In the step (1), the dual-chirp signal is:

[0015]

[0016] wherein m=0, 1, …, M, f0 is an initial frequency of the chirp signal, is a frequency modulation of the chirp signal, and T is a pulse width of the chirp signal.

[0017] In the step (1), the FrFT transform of the signal x(t) is defined as:

[0018]

[0019] wherein K α (t, u) is a kernel function, and is defined as:

[0020]

[0021] wherein, k is an integer.

[0022] In the step (2), M is a delay axis length of one frame of the delay-Doppler domain communication system, and N is a Doppler axis length.

[0023] In the step (4), the matrix P and the matrix X are superimposed together, and only the FrFT of the matrix P has the energy concentration characteristic, so that the chirp preamble sequence can be recognized at the receiving end.

[0024] In the step (6), the calculation method of ΔN1 and ΔN2 is as follows:

[0025] ΔN1=P rx1 -P tx1

[0026] ΔN2=P rx2 -P tx2

[0027] In the step (7), the timing offset and the frequency offset estimation depend on the time shift and the frequency shift characteristics of the FrFT transform:

[0028] If F α (u)=F α [f(t)](u), then

[0029]

[0030]

[0031] The time shift τ makes the two peak values move in the same direction by τcosα, and the frequency shift ε makes the two peak values move in the opposite direction by εsinα.

[0032] ΔN1 = d cos α - ε T sin α

[0033] ΔN2 = d cos α + ε T sin α

[0034] In step (7), the way of calculating timing offset d and normalized frequency offset ζ from ΔN1 and ΔN2 is:

[0035]

[0036]

[0037] Where Δf is the subcarrier spacing.

[0038] The advantages of the present application are embodied in:

[0039] (1) The present application superimposes the preamble sequence on the DD domain data sequence, and the preamble sequence does not need to occupy additional resource grid, thereby ensuring the spectral efficiency of the transmitted signal.

[0040] (2) The present application pads the preamble sequence and converts it to the DD domain, and the average power of the converted signal is 1 / N of the original chirp signal, which is much smaller than the useful signal power. Therefore, the receiving end does not need to perform preamble sequence elimination, and can directly perform demodulation, thereby reducing the implementation complexity.

[0041] (3) The chirp signal is used for synchronization in the high carrier frequency high-speed mobile scenario, which can resist the influence of large frequency offset on the initial timing, and achieve superior synchronization performance. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 The present application adopts a delay Doppler domain communication system block diagram;

[0043] Figure 2 The present application designs a superposition manner of the preamble sequence and the user data;

[0044] Figure 3 The accuracy performance comparison of several timing synchronization methods. DETAILED DESCRIPTION

[0045] The method described in the present application is described in detail in combination with the drawings and embodiments.

[0046] This invention proposes a joint timing and frequency synchronization method based on the FrFT of chirp signals. Using a dual-chirp signal as a preamble sequence, the time-frequency offset information is obtained based on the peak position offset of the chirp signal's FrFT, thus achieving synchronization of the delayed Doppler domain communication system. Taking a delayed Doppler domain communication system with M=256 and N=16 as an example, employing 16-QAM modulation, the system block diagram is as follows... Figure 1 As shown, the specific implementation method is as follows:

[0047] (1) A dual-chirp signal of length 256 is used as a synchronization preamble sequence. The parameters of the chirp signal are f0 = 1.4MHz and μ = 1.44e10. First, FrFT transform is performed on the signal, and the optimal rotation angle α and the peak position P are recorded. tx1 and P tx2 ;

[0048] (2) Add 256×15=3840 zeros to the end of the dual-chirp signal of length 256 and construct a matrix of 256×16. Perform Wigner transform and SFFT on the matrix in sequence to transform it to the DD domain, denoted as P.

[0049] (3) After the bit information to be transmitted in each delayed Doppler domain communication system is modulated by 16-QAM, it is constructed into a 256×16 matrix, denoted as X;

[0050] (4) Superimpose matrix P onto matrix X to form a superimposed signal in the DD domain, such as Figure 2 As shown; then ISFFT and Heisenberg transform are performed to obtain the time-domain signal, which is then sent after adding CP;

[0051] The receiver synchronization process includes the following steps:

[0052] (5) Perform FrFT transformation on the received signal by rotating α using a sliding window. When an energy concentration peak appears, the sliding window is determined as the window where the dual-chirp signal is located. The starting position of the sliding window is recorded as b.

[0053] (6) Record the peak position of the signal FrFT transform in the receiving window as P. rx1 and P rx2 Calculate the difference between the peak positions of the transmitted dual-chirp signal and the received signal, denoted as ΔN1 and ΔN2;

[0054] ΔN1=P rx1 -P tx1

[0055] ΔN2=P rx2 -P tx2

[0056] At the same time, the time shift and frequency domain characteristics of the FrFT transform are obtained as follows:

[0057] ΔN1 = d cos α - εT sin α

[0058] ΔN2 = d cos α + εT sin α

[0059] (7) Calculate the timing offset d and the normalized frequency offset ε from ΔN1 and ΔN2.

[0060]

[0061]

[0062] The starting position of a frame of the delay-Doppler domain communication system is b+d, and the normalized frequency offset is ε, and the signal synchronization process is completed.

[0063] Simulation results:

[0064] The performance of the application will be analyzed below in combination with simulation.

[0065] The simulation uses Figure 1 The delay-Doppler domain communication system shown in the figure, M = 256, N = 16, the CP length is 17, and the system carrier frequency is 28 GHz. The parameters of the dual-chirp signal are f0 = 1.4 MHz and μ = 1.44e10. The subcarrier spacing is 15 kHz, the channel sampling rate is 3.84 MHz, and the modulation mode is 16-QAM.

[0066] The channel model is a 3GPP EVA channel. There are 9 multipath transmission paths, and the path delays are 0us, 0.03us, 0.15us, 0.31us, 0.37us, 0.71us, 1.09us, 1.73us, and 2.51us, respectively. The path gains are 0dB, -1.5dB, -1.4dB, -3.6dB, -0.6dB, -9.1dB, -7dB, -12dB, and -16.9dB, respectively. The relative motion speed between the transceiver is 500km / h. At the same time, the signal is introduced into the influence of additive white Gaussian noise, and the range of E b / N0 is -1dB~20dB.

[0067] The proposed FrFT method based on chirp signals is compared with the S&C method, the Minn method, and the Park method of the OFDM system. Several methods are simulated under different E b / N0, and the simulation results are shown in the figure Figure 3 . It can be seen that the performance of the proposed method is far superior to that of the other three methods. The simulation results prove the superiority of the proposed FrFT method based on chirp signals in the high-carrier high-speed mobile scenario.

[0068] The above is only further description of the present application, and is not used to limit the implementation of the patent. Any equivalent implementation of the present application shall be included in the scope of the claims of the patent.

Claims

1. A method of timing and frequency co-synchronization of a delay-Doppler domain communication system, characterized in that, The dual-chirp signal is used as a preamble sequence, time-frequency offset information is obtained according to the peak position offset of the FrFT of the chirp signal, and synchronization of a delay Doppler domain communication system is realized, including the following steps: (1) The dual-chirp signal of M length is taken as the synchronization preamble sequence, M is the delay axis length of a frame of the delay-Doppler domain communication system, the signal is first subjected to FrFT transformation, and the optimal rotation angle α and the position P of the peak value are recorded tx1 and P tx2 ; (2) M(N-1) 0s are added at the end of the M-length dual-chirp signal, and a M*N matrix is constructed, wherein N is the length of the Doppler axis; the matrix is subjected to Wigner transformation and SFFT in sequence, is transformed into the DD domain, and is recorded as P; (3) after bit information to be transmitted by each delay Doppler domain communication system is subjected to QAM modulation, a M*N matrix is constructed, and is recorded as X; (4) the matrix P is superimposed on the matrix X to form a DD domain superimposed signal, then ISFFT and Heisenberg transformation are performed, a time domain signal is obtained, a CP is added, and then the signal is transmitted; (5) the received signal is subjected to FrFT transformation of rotation alpha according to a sliding window, when an energy aggregation peak appears, the sliding window is determined as a window in which the dual-chirp signal is located, and the starting position of the sliding window is recorded as b; (6) record the peak position of the FrFT transform of the signal in the receiving window as P rx1 and P rx2 , calculate the difference between the peak position of the sending dual-chirp signal and the receiving signal, denoted as ΔN1 and ΔN2; (7) timing offset d and normalized frequency offset epsilon are calculated from delta N1 and delta N2, the starting position of a frame of the delay Doppler domain communication system is b+d, and the synchronization process of the received signal is completed.

2. The method of timing and frequency co-synchronization of a delay-Doppler domain communication system of claim 1, wherein, In the step (1), the dual-chirp signal is: wherein m = 0, 1,..., M, f0is the initial frequency of the chirp signal, is the frequency modulation of the chirp signal, and T is the pulse width of the chirp signal.

3. The method of Claim 1, wherein the delay-Doppler domain communication system is a time and frequency joint synchronization method, characterized in that, In the step (4), the matrix P and the matrix X are superimposed together, only the FrFT of the matrix P has energy aggregation characteristics, and therefore the chirp preamble sequence can be identified at the receiving end.

4. The method of Claim 1, wherein the delay-Doppler domain communication system is a time and frequency joint synchronization method, characterized in that, In the step (7), the way of calculating timing offset d and normalized frequency offset epsilon from delta N1 and delta N2 is: Wherein, delta f is a subcarrier spacing, and alpha is an optimal rotation angle of the FrFT transformation.

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