A time-frequency estimation method for low-orbit satellite communication system based on differential pilot

Abstract

The application discloses a low-orbit satellite communication system time-frequency estimation method based on differential pilots, utilizes the good autocorrelation characteristics and the lower cross-correlation characteristics of a ZC sequence, designs a time-domain differential pilot sequence to solve the problems of large frequency offset estimation and timing synchronization in low-orbit satellite communication, and has the advantages of large frequency offset estimation range, no interference between frequency offset estimation and timing estimation, fast initial frequency offset estimation and timing estimation, and subsequent residual frequency offset estimation by using an M-PART algorithm, so as to meet the requirements of a low-orbit satellite mobile communication system on the frequency offset estimation range and precision. The differential design pilot sequence and the time-frequency estimation method based on the pilot sequence have the advantages that the estimation precision of the frequency offset is improved by 2-4 dB compared with the traditional M-PART method, the frequency offset estimation range is much better than that of the traditional method, the design requirements of the low-orbit satellite system are met, and the method has high practical value.

Description

TECHNICAL FIELD

[0001] The application relates to a mobile communication and satellite synchronization method, in particular to a time-frequency estimation method in a low-orbit satellite mobile communication system. BACKGROUND

[0002] With the continuous development of satellite communication technology, low-orbit satellites are widely concerned due to their wide coverage, low transmission delay, small path loss and strong flexibility. The International Standardization Organization 3GPP (3rd Generation Partnership Project) also lists satellite mobile communication as one of the key research directions in the B5G / 6G stage. Low-orbit satellites and 5G technology are integrated to build a sea, land and air integrated communication network, meeting the business needs of users everywhere. However, the high-speed movement of low-orbit satellites will cause a large Doppler frequency offset between the satellite and the mobile communication terminal, greatly increasing the difficulty of time-frequency synchronization of the mobile terminal, which is also a great challenge in the 3GPP 5G NTN (Non-Terrestrial Networks) standardization process.

[0003] Unlike the 4G LTE system, the resource configuration of the 5G NR system will be more flexible, and the frame structure of the downlink will no longer be fixed. In the synchronization channel, the concept of synchronization signal / physical broadcast channel block (SSB) is introduced in the 5G NR system, and each SSB is composed of four parts: primary synchronization sequence, secondary synchronization sequence, physical broadcast channel and demodulation reference signal. SUMMARY

[0004] The application aims to solve the problems of large frequency offset and high delay in low-orbit mobile satellite communication, and proposes a low-orbit satellite communication system time-frequency estimation method based on differential pilot.

[0005] Technical scheme: A low-orbit satellite communication system time-frequency estimation method based on differential pilot, comprising the following steps:

[0006] Step 1: According to the characteristics of large Doppler frequency offset and high transmission delay in the low-orbit satellite communication system, determine the type of differential pilot sequence, differential interval and physical root sequence number parameters;

[0007] Step 2: Perform inverse differential transformation and IFFT transformation on the designed differential pilot to transform the time-domain sequence to the frequency-domain primary synchronization sequence;

[0008] Step 3: Perform down-sampling processing on the received signal in the synchronization process to eliminate the interference of out-of-band signals and reduce the computational complexity;

[0009] Step 4: based on the optimized pilot, utilize the differential cross-correlation algorithm to perform timing estimation and large frequency offset estimation, and perform corresponding frequency offset compensation;

[0010] Step 5: utilize the MPART algorithm to estimate residual frequency offset to meet the design requirements of the low-orbit satellite communication system.

[0011] Further, in the step 1, the differential pilot sequence selects the Zadoff-Chu sequence.

[0012] Further, in the step 1, the differential interval of the differential pilot is designed according to the maximum value of the frequency offset.

[0013] Further, in the step 1, the root sequence number determination method of the three primary synchronization sequences is that the value of the root sequence number is close to half of the sequence length, and the difference value of the root sequence numbers of the three primary synchronization sequences is prime with the sequence length and the root sequence number is prime with the sequence length.

[0014] Further, in the step 4, the differential sequence is constructed by multiplying the down-sampled received sequence and the delayed conjugate sequence, and the timing and large frequency offset are estimated by utilizing the differential cross-correlation algorithm.

[0015] Further, in the step 4, the received sequence is compensated for the corresponding frequency offset by utilizing the estimated value of the large frequency offset.

[0016] Beneficial effects: the application discloses a low-orbit satellite communication system time-frequency estimation method based on differential pilot, utilizes the good autocorrelation characteristics and low cross-correlation characteristics of the ZC sequence, designs the time-domain differential pilot sequence to solve the problems of large frequency offset estimation and timing synchronization in the low-orbit satellite communication. The autocorrelation estimation method of the differential sequence has the advantages of large frequency offset estimation range and no interference between the frequency offset estimation and the timing estimation, and the initial frequency offset estimation and timing estimation are quickly obtained, and then the traditional M-PART algorithm is utilized to estimate the residual frequency offset, so that the requirements of the low-orbit satellite mobile communication system on the frequency offset estimation range and precision are met. The differential design pilot sequence and the time-frequency estimation method based on the pilot sequence have the advantages that the estimation precision of the frequency offset is improved by 2-4dB compared with the traditional M-PART method, the frequency offset estimation range is much better than that of the traditional method, the design requirements of the low-orbit satellite system are met, and the application has high practical value. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 It is a flow chart of the method of the application;

[0018] Figure 2 It is a low-orbit satellite mobile communication system terminal information transmission scene schematic diagram adopted by the embodiment of the application;

[0019] Figure 3 Performance curve of the embodiment of the application in the case of a frequency offset of 120 KHz;

[0020] Figure 4 Performance curve of the embodiment of the application in the case of a frequency offset of 120 KHz;

[0021] Figure 5 Performance curve of the embodiment of the application in the case of a frequency offset of 816 KHz. DETAILED DESCRIPTION

[0022] The application will be further explained in conjunction with the accompanying drawings.

[0023] As Figure 1 shown is a flow chart of a time-frequency estimation method for a low-orbit satellite communication system based on differential pilots according to the application. In the embodiment of the application, a low-orbit satellite mobile communication system adopts a 5G NR air interface system to communicate with ground terminals through point beams. Figure 2 is a schematic diagram of a low-orbit satellite mobile communication system terminal information transmission scenario. The low-orbit mobile communication satellite flies in orbit at a high speed along a predetermined orbit, causing a serious Doppler shift. In addition, there is also a transmission delay in the communication process between the low-orbit satellite and the ground. Without loss of generality, the satellite-ground transmission channel fading model is modeled as an AWGN channel, and the satellite-side transmission signal x(n) is represented as:

[0024]

[0025] In the formula, d represents the timing deviation, and ε represents the normalized frequency offset. The downlink of the 5G protocol adopts an OFDM modulation mode, so here N i represents the number of IFFT points in OFDM modulation, v(n) represents a Gaussian white noise, and n represents a sampling time point.

[0026] For the low-orbit satellite mobile communication system terminal information transmission scenario, the primary synchronization sequence in the 5G protocol is optimized. The time-frequency estimation method designed by the application combines the differential method with the M-PART method, and has certain requirements for the pilot sequence before and after the differential, and the specific criteria are as follows:

[0027] (1) Good autocorrelation characteristics: the time-domain pilot after the differential has good autocorrelation characteristics, which is conducive to improving the accuracy of initial access and large frequency offset estimation.

[0028] (2) Lower cross-correlation characteristics: different sequences need to have lower cross-correlation characteristics, which are used to transmit cell ID and other information.

[0029] (3) Constant modulus characteristic: when residual frequency offset estimation is performed by using the M-PART algorithm, the original pilot sequence is required to have a constant modulus characteristic.

[0030] (4) Range requirement of frequency offset estimation:

[0031] When the frequency offset estimation is performed by using the differential time-domain pilot sequence, the interval m of the differential pilot directly affects the range and precision of the large frequency offset estimation. The larger the m value is, the smaller the frequency offset estimation range is, but the higher the frequency offset estimation precision is.

[0032] (5) Precision requirement of frequency offset estimation:

[0033] In the design scheme of the application, the M-PART method is used to estimate the residual frequency offset, and the precision requirement of the frequency offset estimation is relatively high. The smaller the divided M is, the higher the estimation precision is. Therefore, the M value is taken as 2, and M represents the number of segments divided by the segment correlation. At this time, the estimation range of the residual frequency offset by using the M-PART method is [-Δf c , Δf c ], and Δf c is the subcarrier bandwidth. This requires that the error of the large frequency offset estimation by using the differential algorithm cannot exceed one subcarrier bandwidth. The low-orbit satellite is required to work at a signal-to-noise ratio of -6dB, that is, under this signal-to-noise ratio condition, the error of the large frequency offset estimation cannot exceed one subcarrier bandwidth.

[0034] According to the above design criteria of the pilot sequence, the sequence is selected and the related parameters are designed.

[0035] Sequence selection: the ZC (Zadoff-Chu) sequence is one of the CAZAC (Const Amplitude Zero Auto-Corelation) sequences, has good autocorrelation and cross-correlation characteristics, and is defined as follows:

[0036]

[0037] In the formula, μ is a physical root sequence number, and Z μ (n) represents the ZC sequence.

[0038] The ZC sequence has good autocorrelation, low cross-correlation, good frequency offset resistance, and low peak-to-average ratio, and meets the many requirements of the differential pilot of the main synchronization signal in the low-orbit satellite communication, so the ZC sequence is selected as the differential pilot of the system.

[0039] Selection of the differential sequence length: in combination with the application scene, a 30GHz carrier frequency is selected, and at this time, the maximum value of the low-orbit satellite frequency offset is f dmax= 780 KHz, and the estimation range of frequency offset measured by the differential cross-correlation algorithm is [-NΔf c / 2m, NΔf c / 2m] According to the 5G protocol, the subcarrier bandwidth Δf c The 120 KHz is selected, the sequence length N is 128, and according to NΔf c / 2m≥f dmax The value range of the differential interval m is m≤9.8462. When the differential algorithm is used to estimate the frequency offset, the greater the value of m, the higher the accuracy of the frequency offset estimation, so that the value of m is as large as possible on the basis of ensuring the estimation range of the frequency offset, and m=9 is selected, that is, the differential is performed every 9 points, and the length of the differential pilot is 119 at this time, and 119 is a prime number, so that the selection of the physical root sequence number of the ZC pilot sequence is facilitated.

[0040] The selection of the physical root sequence number is used in the 5G protocol, and the primary synchronization signal is used for detecting the identification number of three cell groups Therefore, three physical root sequence numbers μ1, μ2 and μ3 are selected, so that the cross-correlation of the three ZC sequences is low. According to the research of 3GPP RAN1, the closer the physical root sequence number μ is to half of the sequence length, the smaller the sensitivity of the frequency offset. According to the characteristics of the ZC sequence, when (μ1-μ2) is prime with the sequence length N, and μ1 and μ2 are prime with N, the cross-correlation of the two sequences is good. Based on this, μ1=57, μ2=59 and μ3=61 are selected, which correspond to Three cases, and the specific expressions are as follows:

[0041]

[0042] Generation of the primary synchronization sequence: the initial value of the sequence is a pseudo-random gold sequence with a length of 9, which is represented by a bipolar code of positive and negative 1, and the modulus is always 1, and the specific definition is as follows:

[0043] d(n) = 1-2x(m), wherein

[0044] In the formula, d(n) is a time domain synchronization signal.

[0045] The differential sequence obtained every 9 points is a ZC sequence, that is:

[0046]

[0047] In the formula, the superscript * represents taking a conjugate.

[0048] Then:

[0049]

[0050] The frequency domain synchronization sequence corresponding to the generated time domain synchronization signal is:

[0051]

[0052] In the formula, k is a frequency domain sampling point, and P(k) is a frequency domain synchronization sequence.

[0053] The optimized pilot after the above design has a constant modulus characteristic, and has good autocorrelation characteristics after difference. The time-frequency estimation algorithm of the low-orbit satellite system will be described according to the above characteristics:

[0054] Step one: down-sampling

[0055] In the synchronization process of the low-orbit satellite communication system, the received data is large, and needs to be down-sampled. In order to eliminate the interference of out-of-band signals, filtering is first performed, and the filtered received signal is defined as r LPF (n). Then, down-sampling is performed, and the number of OFDM modulation points N1 is 4096, and the number of primary synchronization sequences is 128, so the down-sampling multiple can be between 1 and 32. Considering the calculation complexity and the best phase angle and other factors, the down-sampling multiple c is selected to be 4. Let the down-sampled signal be Then

[0056] Step two: using a differential cross-correlation algorithm to determine time and large frequency offset

[0057] The down-sampled received signal is multiplied by its delayed conjugate signal to construct a differential sequence, and the timing deviation d of the delay is m×(4096 / 128) / c, wherein c is the down-sampling multiple, m=9, c=4, and d=72. Then, the differential sequence Z(n) of the received signal is:

[0058]

[0059] The sliding correlation of the differential pilot is performed, and the correlation value C(k) is obtained.

[0060]

[0061] In the formula, l=119, which is the number of differential pilots.

[0062] When C(k) takes the maximum value, the estimated timing position and the intra-group cell number are obtained, that is:

[0063]

[0064] When the correlation value is:

[0065]

[0066] Because The estimated range of is [-π, π], so the normalized frequency offset The estimated range of The estimated range of frequency offset is Δf c is the subcarrier bandwidth.

[0067] According to formula 11, the estimated value of large frequency offset is is:

[0068]

[0069] Where, N = 128, m = 9.

[0070] Step three: large frequency offset compensation

[0071] Then the received signal is compensated for frequency offset, and the compensated received signal is denoted as Its expression is as follows:

[0072]

[0073] In the formula, the number of points of IFFT N i = 4096.

[0074] Step four: M-PART algorithm to measure residual frequency offset

[0075] Find the timing position by differential cross-correlation algorithm According to the M-PART correlation principle, the M-PART correlation is performed on the timing point of the time domain primary synchronization sequence d(n) and the frequency offset compensated signal , and the value of the residual frequency offset is measured.

[0076]

[0077] Where, L = N / M, N = 128, M = 2.

[0078] The residual frequency offset is estimated by formula 15:

[0079]

[0080] In summary, the position of the primary synchronization signal The intra-group cell number And the total frequency offset estimation value is:

[0081]

[0082]

[0083] The embodiment selects the subcarrier bandwidth 120KHz in the 5G protocol, selects the IFFT point number as 4096, selects the frequency offset as the maximum value 816KHz under the millimeter wave frequency 30GHz, selects the delay as 16 points, selects the symbol number as 14, selects the CP length of the first symbol as 544, selects the CP length of the remaining symbols as 288, selects the signal-to-noise ratio as -6-10dB, and simulates under the system parameters. Here, the RMSE (Root Mean Squared Error) is used to measure the accuracy of the frequency offset and timing, and the calculation formula is: Wherein, is the estimated value, epsilon is the theoretical value, N ε is the simulation times.

[0084] It can be seen from Figure 3 and Figure 4 that when the frequency offset is 120KHz, with the increase of the signal-to-noise ratio, the estimation error of the frequency offset and the timing is continuously reduced. For the frequency offset, under the same RMSE condition, the proposed optimized pilot time-frequency estimation method reduces the required signal-to-noise ratio by 2-4dB than the traditional M-PART method; for the timing, under high signal-to-noise ratio, the timing effect is obviously better than the traditional M-PART method. The frequency offset in the actual system is often greater than one subcarrier bandwidth, at this time the traditional M-PART method will not be able to estimate. Therefore, the optimized pilot time-frequency estimation method is obviously better than the M-PART method in terms of estimation accuracy and estimation range.

[0085] Figure 5 The RMSE curves of the normalized frequency offset estimation value after the pilot optimization and without pilot optimization are given when the frequency offset is 816KHz. It can be seen from Figure 5 that under the condition of the maximum frequency offset that may exist in the system, the error of the optimized pilot frequency offset estimation method designed is obviously smaller than that without pilot optimization, because the correlation of the optimized differential pilot has been obviously improved, which further improves the accuracy of the frequency offset estimation. At -6dB, the estimation error value of the normalized frequency offset is 0.0218, which meets the requirements of the frequency offset estimation in the low-orbit satellite system.

[0086] The above only describes the preferred embodiments of the present application, and it should be noted that for ordinary skilled persons in the art, without departing from the principles of the present application, some improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present application.

Claims

1. A method for time-frequency estimation in a differential pilot based low earth orbit satellite communication system, characterized in that, Comprising the following steps: Step 1: According to the characteristics of large Doppler frequency offset and high transmission delay in low-orbit satellite communication system, the type of differential pilot sequence, differential interval and physical root sequence number parameters are determined; Selection of sequence: ZC sequence is selected as the differential pilot of the system, and the specific definition of ZC sequence is as follows: In the formula, μ is a physical root sequence number, Z μ (n) represents a ZC sequence, and n represents a sampling time point. Selection of the length of the differential sequence: combined with the application scenario, select 30GHz carrier frequency, at this time the maximum value of the low-orbit satellite frequency offset f dmax = 780KHz, while the estimation range of the frequency offset measured by the differential cross-correlation algorithm is [-NΔf c / 2m, NΔf c / 2m], according to the 5G protocol, the subcarrier bandwidth△f c is selected as 120KHz, the sequence length N is taken as 128, then according to NΔf c / 2m≥f dmax , the value range of the differential interval m is obtained as m≤9.8462; When using the differential algorithm for frequency offset estimation, the larger the value of m, the higher the accuracy of frequency offset estimation, so here m=9, that is, differential is made every 9 points, and the length of the differential pilot at this time is 119; The selection of the physical root sequence number in the 5G protocol, the primary synchronization signal is used to detect the identification number in the three cell groups The root sequence number determination method of the three primary synchronization sequences is that the value of the root sequence number is close to half of the sequence length, and the difference value of the root sequence numbers of the three primary synchronization sequences is prime with the sequence length and the root sequence number is prime with the sequence length; Therefore, 3 physical root sequence numbers μ1, μ2, μ3 are selected, so that the cross-correlation of the three ZC sequences is low, μ1=57, μ2=59, μ3=61, which respectively correspond Three cases, the specific expression is as follows: Generation of primary synchronization sequence: the initial value of the sequence is a pseudo-random gold sequence with a length of 9, which is represented by bipolar code of positive and negative 1, and the modulus is always 1, and the specific definition is as follows: In the formula, d(n) is a time domain synchronization signal; x(m) is a satellite side transmission signal; The differential sequence obtained every 9 points is a ZC sequence, that is: In the formula, the superscript * represents taking the conjugate; Then: The frequency domain synchronization sequence corresponding to the generated time domain synchronization signal is: In the formula, k is a frequency domain sampling point, and P(k) is a frequency domain synchronization sequence; Step 2: Perform inverse differential transformation and IFFT transformation on the designed differential pilot to transform the time domain sequence to the frequency domain primary synchronization sequence; Step 3: Perform down-sampling processing on the received signal in the synchronization process to eliminate the interference of out-of-band signals and reduce the calculation complexity; Step 4: Based on the optimized pilot, use the differential cross-correlation algorithm for timing estimation and large frequency offset estimation, and perform corresponding frequency offset compensation; The down-sampled received signal The differential sequence is constructed by multiplying the delayed conjugate signal, the timing deviation d of the delay is m x (4096 / 128) / c, wherein c is the down-sampling multiple, m=9, c=4, d=72, and the differential sequence Z(n) of the received signal is: correlate it with the designed differential pilot correlate it with the designed differential pilot In the formula, l=119, which is the number of differential pilots; When C(k) takes a maximum value, the estimated timing position is obtained and a cell global identity That is: When Correlation value is: In the formula, ε represents the normalized frequency offset, and v(n) represents Gaussian white noise; Because the estimated range of is [-π, π], the normalized frequency offset is estimated to be in the range Δf c is the subcarrier bandwidth; According to (formula 11), an estimated value of a large frequency offset is obtained is: Wherein, N=128, m=9; Then frequency offset compensation is performed on the received signal, and the compensated received signal is denoted as The expression is as follows: In the formula, the number of points N of IFFT i = 4096; Step 5: Use the MPART algorithm to estimate the residual frequency offset to meet the design requirements of the low-orbit satellite communication system; Finding timing position by differential cross-correlation algorithm According to M-PART correlation principle, using time-domain primary synchronization sequence d(n) and frequency offset compensated signal At timing point , M-PART correlation is made, and the value of residual frequency offset is measured ; Wherein, L=N / M, N=128, M=2; residual frequency offset From (15) we obtain: Position of primary synchronization signal Intra-group cell number And total frequency offset estimate Is:

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