Random access lead code and detection algorithm design based on low earth orbit satellite communication

Abstract

The invention discloses a random access preamble and detection algorithm design based on low earth orbit satellite communication. Specifically, firstly, a novel lead code with a plurality of subsequences is provided, in order to enlarge the number of available lead codes in a wave beam, a ZCM sequence is introduced to form the subsequences, and orthogonal frequency division multiplexing is adopted for modulation and is sent to a channel. Subsequently, a timing advance (TA) detection algorithm is proposed. The first sub-sequence is used for decimal time delay detection, and the other sub-sequences are used for integer time delay detection. In addition, the peak detection ingeniously utilizes the variance of the power delay spectrum, so that the detection performance is improved. Finally, a simulation result shows that the lead code provided by the invention is almost not influenced by frequency offset, does not need pre-compensation, and is suitable for being applied to low-orbit satellite communication with relatively large frequency offset.

Description

Technical Field

[0001] The present invention relates to the field of communication technologies, and more specifically, to a random access preamble and detection algorithm design based on low-earth orbit satellite communication. Background Art

[0002] In recent years, with the exploration of 6G communication, satellite communication, as a very promising communication system, can achieve communication coverage in remote areas and reduce the cost of building ground base stations. Among them, low-earth orbit (LEO) satellite communication has attracted extensive research in the academic and industrial fields due to its relatively low latency and small loss compared with others. However, compared with terrestrial communication systems, LEO satellite communication has a larger latency, and due to the high-speed movement of LEO satellites, the carrier frequency offset is also very large. Therefore, new communication technologies need to be developed. Currently, 3GPP is formulating a number of specifications, and random access technology, as a key technology, needs to be enhanced. In the random access process, first, the user terminal initiates random access, and after receiving it, the base station calculates the timing advance and feeds it back to the UE side. Among them, the preamble is the key for timing advance detection. The maximum Doppler frequency offset supported in the 5G standard is less than the frequency offset of LEO satellite communication, that is, 30 kHz. Therefore, an enhanced preamble format needs to be designed.

[0003] To solve the problems of large latency and large frequency offset in LEO satellite random access, researchers have proposed a preamble format design of multi-segment sequence concatenation. Chougrani proposed a frequency offset estimation algorithm, which can effectively compensate the frequency offset in the transmission link. However, it cannot distinguish the frequency offset caused by the Doppler effect and the frequency offset caused by the crystal oscillator and cannot perform effective compensation. Zhu proposed a frequency offset compensation algorithm for downlink synchronization based on ephemeris, which can compensate both the local oscillator frequency offset and the Doppler frequency offset. However, when the communication frequency band is low or there is a positioning error, accurate compensation cannot be performed. Caus proposed a flexible frequency offset robust preamble design, which determines the subcarrier width used according to the elevation angle to increase the robustness to frequency offset, but it will fail when the frequency offset is too large. Li proposed to use the concatenation of different root sequences to alternately generate extended preambles, which improves the robustness to frequency offset, but requires a high computational complexity to improve the anti-noise performance.

[0004] The existing timing advance estimation methods still have deficiencies. The most important one is that they all need to perform frequency offset estimation or compensation based on satellite ephemeris information and GPS positioning. How to improve the frequency offset robustness of the preamble itself is the key research objective. Summary of the Invention

[0005] In view of the problems of carrier frequency offset and complexity in low-earth orbit satellite communication, a frequency offset-robust preamble and its detection algorithm based on the variance of the power delay profile are proposed. Without the need for frequency offset compensation, the interference of frequency offset on timing advance detection can be completely eliminated, thereby reducing the computational complexity. First, this design presents a preamble structure in the form of cascading multiple sub-sequences. To expand the available preamble set, ZCM sequences are introduced as sub-sequences, which have better frequency offset robustness and can reduce the probability of preamble collision. Finally, a timing advance detection algorithm is proposed, which novelly uses the variance of the power delay profile for peak detection, and can reduce the probability of false detection. To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A design of a random access preamble and a detection algorithm based on low-earth orbit satellite communication, comprising the following steps:

[0007] S1: At the transmitting end, generate a preamble s for timing advance detection;

[0008] S2: Modulate the preamble, and then transmit it through radio frequency to the wireless channel for transmission. At the receiving end, demodulate the received signal through a demodulator to obtain the demodulated signal s d (n);

[0009] S3: Perform user index detection on the preamble to obtain the preamble sequence s u (n) corresponding to the local index number;

[0010] S4: Perform fractional delay detection. Take out the first sub-sequence y1(n) of the received signal, perform sliding autocorrelation operation, transform the obtained correlation function to the frequency domain for peak detection, and obtain the fractional delay d f ;

[0011] S5: Compensate the received signal with the fractional delay, and then start from the second sub-sequence, take out all the sub-sequences y i (n) of the compensated signal, perform correlation operations on all sub-sequences with the preamble s u (n) corresponding to the local index number to obtain M - 1 power delay profiles;

[0012] S6: Perform integer delay detection. Calculate the variances of all power delay profiles respectively, and set a threshold for decision to obtain the power delay profile where the first peak appears, and calculate the integer delay d i ;

[0013] S7: Calculate the timing advance d from the fractional delay and the integer delay, and output the result.

[0014] Preferably, the design of the preamble in step S1 is specifically as follows:

[0015] The ZCM sequence is used as the preamble sequence that constitutes the preamble, and its expression is:

[0016]

[0017] In the formula, x u (n) is the ZC sequence, and the expression is u is the root sequence index, and N is the length of the preamble sequence. m[n] is the M sequence, and the expression of the m-th order generating polynomial is as follows:

[0018] g[P] = g m P m + g m-1 P m-1 + … + g1P 1 + g0

[0019] The expression of the preamble structure is:

[0020] s = [s0(n), s u (n), …, s u (n)]

[0021] In the formula, s u (n) = x u [(n + vN cs ) mod N] is used as a subsequence of the preamble, and vN cs is the cyclic shift, and s0(n) = 1 1×N .

[0022] Preferably, in step S2, the preamble is modulated and demodulated by orthogonal frequency division multiplexing to obtain the demodulated signal s de at the receiving end, specifically:

[0023] S2.1: Modulate the preamble by orthogonal frequency division multiplexing;

[0024] S2.2: Transmit the modulated signal to the wireless channel through radio frequency for transmission;

[0025] S2.3: Demodulate the received signal by the demodulator at the receiving end.

[0026] Preferably, the expression of the modulated signal in step S2.1 is:

[0027]

[0028] Preferably, when transmitted to the wireless channel in step S2.2, the expression of the received signal is:

[0029]

[0030] In the formula, d is the time delay, and ε = f / ΔfRA is the normalized carrier frequency offset, f is the carrier frequency offset in the channel, and Δf RA is the subcarrier spacing of the transmitted signal. w(n) is additive white Gaussian noise.

[0031] Preferably, in step S2.3, the received signal is demodulated at the receiving end, and the expression of the demodulated signal is:

[0032]

[0033] Preferably, in step S3, user index detection is performed on the preamble, specifically:

[0034] S3.1: Correlate the last subsequence of the demodulated signal with all local root sequences to obtain the power delay profile;

[0035] S3.2: Calculate the variances of all power delay profiles, find the power delay profile with the largest variance, and the corresponding window index is the user index.

[0036] Preferably, the correlation operation in step S3.1 has the following expression:

[0037]

[0038] In the formula, where a and b are constants.

[0039] Preferably, the peak detection in step S3.2 has the following expression:

[0040]

[0041] In the formula, var(·) is the function for calculating variance, and find u corresponding to the power delay profile with the largest variance.

[0042] Preferably, in step S4, fractional delay detection is performed. The first subsequence y1(n) of the demodulated signal is subjected to sliding autocorrelation operation, and the specific expression is:

[0043]

[0044] Subsequently, the correlation function is subjected to Fourier transform to detect the peak index, which is the fractional delay d f 。

[0045] Preferably, in step S5, power delay profile calculation is performed, specifically:

[0046] S5.1: Compensate the demodulated signal using the fractional delay;

[0047] S5.2: For the subsequence y i(n) is correlated with the prefix of the local index number.

[0048] Preferably, compensation is performed in step S5.1, and its expression is:

[0049] s′ d (n) = s d (n + d f )

[0050] Preferably, correlation operation is performed in step S5.2, and its expression is:

[0051]

[0052] Preferably, in step S6, the variance of the power delay profile c i (m) is calculated, peak detection is performed, and the integer multiple delay is calculated. Specifically:

[0053] S6.1: Denote the peak of the power delay profile as T. If T < γ, it is determined that this subsequence is not s u (n), and the correlation operation of the next subsequence is performed until there is a correlation peak T > γ. The index corresponding to this subsequence is used as the number of subsequences of s u (n) time shift, where the expression for peak detection is:

[0054]

[0055] In the formula, α is a threshold constant, and its variation range is [0, 0.1, 0.2, …, 1.9, 2]. In the detection of the prefix index number, α is traversed from small to large until the minimum threshold constant that meets the set detection false alarm rate is obtained.

[0056] S6.2: Calculate the integer multiple delay, and its expression is:

[0057] d i = (j - 1)N

[0058] In the formula, j is the index of the power delay profile that meets the peak detection condition, that is, i = j.

[0059] Preferably, in step S7, the timing advance d is calculated, and its expression is:

[0060] d = d i + d f Description of the Drawings

[0061] Figure 1 It is a schematic flowchart of the method of the present invention.

[0062] Figure 2Schematic diagram for comparing the successful detection probabilities of timing advance estimations of different algorithms provided for the embodiments in a large frequency offset channel.

[0063] Figure 3 Schematic diagram for comparing the MSEs of timing advance estimations of different frequency offset channels of the designed algorithm provided for the embodiments. Detailed implementation manners

[0064] The accompanying drawings are only for illustrative purposes and should not be construed as limitations on this patent;

[0065] To better illustrate this embodiment, some components in the accompanying drawings are omitted, enlarged or reduced, and do not represent the dimensions of the actual product;

[0066] For those skilled in the art, it is understandable that some well-known structures and their descriptions in the accompanying drawings may be omitted.

[0067] The technical solutions of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0068] Embodiment 1

[0069] This embodiment provides a design of a random access preamble and a detection algorithm based on low-earth orbit satellite communication, as Figure 1 shown, including the following steps:

[0070] S1: At the transmitting end, generate a preamble s for timing advance detection;

[0071] S2: Modulate the preamble, and then transmit it through radio frequency into the wireless channel for transmission. At the receiving end, demodulate the received signal through a demodulator to obtain the demodulated signal s d (n);

[0072] S3: Perform user index detection on the preamble to obtain the preamble sequence s u (n) corresponding to the local index number;

[0073] S4: Perform fractional delay detection. Take out the first subsequence y1(n) of the received signal, perform sliding autocorrelation operation, and after obtaining the correlation function, transform it to the frequency domain for peak detection to obtain the fractional delay d f ;

[0074] S5: Compensate the received signal with the fractional delay, and then starting from the second subsequence, take out all the subsequences y i (n) of the compensated signal, and perform correlation operations on all the subsequences with the preamble s u (n) of the local index number to obtain M - 1 power delay spectra;

[0075] S6: Perform integer multiple time delay detection, calculate the variances of all power delay profiles respectively, set a threshold for decision-making, obtain the power delay profile where the first peak appears, and calculate the integer multiple time delay d i ;

[0076] S7: Calculate the timing advance d from the fractional multiple time delay and the integer multiple time delay, and output the result.

[0077] Embodiment 2

[0078] Based on Embodiment 1, the following content is further disclosed in this embodiment:

[0079] The design of the preamble in step S1 is specifically as follows:

[0080] Use the ZCM sequence as the preamble sequence that constitutes the preamble. Its expression is:

[0081]

[0082] In the formula, x u (n) is the ZC sequence, and the expression is u is the root sequence index, N is the length of the preamble sequence. m[n] is the M sequence, and the expression of the m-th order generating polynomial is as follows:

[0083] g[P] = g m P m + g m-1 P m-1 + … + g1P 1 + g0

[0084] The expression of the preamble structure is:

[0085] s = [s0(n), s u (n), …, s u (n)]

[0086] In the formula, s u (n) = x u [(n + vN cs ) mod N] is used as a subsequence of the preamble, and vN cs is the cyclic shift, and s0(n) = 1 1×N .

[0087] In step S2, perform orthogonal frequency division multiplexing modulation and demodulation on the preamble to obtain the demodulated signal s de at the receiving end. Specifically:

[0088] S2.1: Perform orthogonal frequency division multiplexing modulation on the preamble;

[0089] S2.2: Transmit the modulated signal to the wireless channel through radio frequency for transmission;

[0090] S2.3: Demodulate the received signal through a demodulator at the receiving end.

[0091] The expression of the modulated signal in step S2.1 is:

[0092]

[0093] In step S2.2, when transmitted into the wireless channel, the expression of the received signal is:

[0094]

[0095] In the formula, d is the time delay, ε = f / Δf RA is the normalized carrier frequency offset, f is the carrier frequency offset in the channel, and Δf RA is the subcarrier spacing of the transmitted signal. w(n) is the additive white Gaussian noise.

[0096] In step S2.3, demodulate the received signal at the receiving end, and the expression of the demodulated signal is:

[0097]

[0098] In step S3, perform user index detection on the preamble, specifically:

[0099] S3.1: Perform correlation operations between the last subsequence of the demodulated signal and all local root sequences to obtain the power delay profile;

[0100] S3.2: Calculate the variances of all power delay profiles, find the power delay profile with the largest variance, and the corresponding window index is the user index.

[0101] The correlation operation in step S3.1 has the following expression:

[0102]

[0103] In the formula, where a and b are constants.

[0104] The peak detection in step S3.2 has the following expression:

[0105]

[0106] In the formula, var(·) is the function for calculating variance, and find u corresponding to the power delay profile with the largest variance.

[0107] In step S4, perform fractional delay detection, and perform sliding autocorrelation operations on the first subsequence y1(n) of the demodulated signal. The specific expression is:

[0108]

[0109] Subsequently, perform Fourier transform on the relevant function and detect the peak index, which is the fractional delay d f .

[0110] In step S5, calculate the power delay profile, specifically as follows:

[0111] S5.1: Compensate the demodulated signal using the fractional delay;

[0112] S5.2: Perform a correlation operation on the subsequence y i (n) and the preamble of the local index number

[0113] In step S5.1, perform the compensation, and its expression is:

[0114] s′ d (n) = s d (n + d f )

[0115] In step S5.2, perform the correlation operation, and its expression is:

[0116]

[0117] In step S6, calculate the variance of the power delay profile c i (m), perform peak detection, and calculate the integer delay, specifically as follows:

[0118] S6.1: Denote the peak of the power delay profile as T. If T < γ, then it is determined that this subsequence is not s u (n), and perform the correlation operation on the next subsequence until there is a correlation peak T > γ. Take the index corresponding to this subsequence as the number of subsequences of the time shift of s u (n), where the expression for peak detection is:

[0119]

[0120] In the formula, α is a threshold constant, and its variation range is [0, 0.1, 0.2, …, 1.9, 2]. Traverse α from small to large in the detection of the preamble index number until the minimum threshold constant that meets the set detection false alarm rate is obtained.

[0121] S6.2: Calculate the integer delay, and its expression is:

[0122] d i = (j - 1)N

[0123] In the formula, j is the index of the power delay profile that meets the peak detection condition, that is, i = j.

[0124] In step S7, calculate the timing advance d, and its expression is:

[0125] d = d i + d f

[0126] Example 3

[0127] Based on Example 1 and Example 2, the following specific examples are provided in this example:

[0128] Set the simulation parameters as follows: The channel used is an additive white Gaussian noise channel (AWGN), the signal-to-noise ratio (SNR) is set in the range of -20 dB to 0 dB, the carrier frequency offset is 1875 Hz, the transmission round-trip delay difference range is [0, 4.28] ms, the beam radius is 350 km, the number of available preambles within the beam is 64, the subcarrier spacing is 1.25 kHz, the carrier frequency is 1.621 GHz, the minimum communication elevation angle is 10°, the satellite orbital altitude is 780 km, and the preamble duration is 4.8 ms.

[0129] Compare the correct detection performance of the timing advance of the new method proposed in the present invention (hereinafter referred to as NZP) with the existing timing advance estimation algorithms DCP and FAP in an AWGN channel with frequency offset, specifically as follows: As Figure 2 shown, when there is a large normalized carrier frequency offset (NCFO), the correct detection probability of the timing advance of all preambles increases with the increase of SNR. Among them, FAP shows strong anti-noise performance, while due to the multi-window sliding detection used in DCP detection, adjacent window leakage is likely to occur, resulting in errors. Therefore, it is observed that DCP cannot resist excessive frequency offset and requires a higher SNR to correctly detect the delay, while NZP designed in this paper is less affected by large frequency offset and can better complete the timing advance detection.

[0130] The above examples show that the scheme proposed in the present invention can achieve timing advance detection in a high-mobility channel. The preamble structure of different subsequence differential cascades and the detection algorithm based on the power delay spectrum variance used have advantages under large frequency offset, and the proposed scheme has better detection performance.

[0131] On this basis, while keeping the simulation parameters unchanged, in order to conduct a more in-depth analysis, we use the mean square error (MSE) between the detected delay and the accurate delay as the evaluation index. As Figure 3 shown, the MSE curves of the preambles under different frequency offset conditions have extremely small differences, indicating their robustness to small frequency offset changes. In addition, although FAP can accurately detect the delay under low signal-to-noise ratio conditions, the mean square error of the detected delay is ultimately the smallest for NZP proposed in this paper.

[0132] The above examples show that when the channel frequency offset is small, the method described in the present invention can achieve significantly better timing advance estimation accuracy and calculate a smaller delay error.

[0133] The same or similar reference numerals correspond to the same or similar components;

[0134] The descriptions of the positional relationships in the drawings are for illustrative purposes only and should not be construed as limitations of this patent;

[0135] Obviously, the above examples of the present invention are merely examples for clearly illustrating the present invention and are not intended to limit the embodiments of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all embodiments here. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention shall be included in the protection scope of the claims of the present invention.

Claims

1. Design of a random access preamble and detection algorithm based on low Earth orbit satellite communication, characterized in that, It includes the following steps: S1: At the sending end, generate a preamble s for timing advance detection; S2: Modulate the preamble, and then transmit it through radio frequency into the wireless channel for transmission. At the receiving end, demodulate the received signal through a demodulator to obtain the demodulated signal s d (n); S3: Perform user index detection on the preamble to obtain the preamble sequence s corresponding to the local index number u (n); S4: Perform fractional delay detection. Extract the first subsequence y1(n) of the received signal, perform a sliding autocorrelation operation, transform the resulting correlation function to the frequency domain for peak detection, and obtain the fractional delay d f ; S5: Compensate the received signal for the fractional delay, and then starting from the second subsequence, extract the subsequences y i (n) of all the compensated signals, and perform correlation operations on all the subsequences and the preamble s u (n) of the local index number to obtain M-1 power delay profiles; S6: Perform integer - multiple delay detection, calculate the variances of all power - delay profiles respectively, set a threshold for decision - making, obtain the power - delay profile where the first peak appears, and calculate the integer - multiple delay d i ; S7: Calculate the timing advance d from the fractional delay and the integer delay, and output the result.

2. The random access preamble and detection algorithm design based on low-earth orbit satellite communication according to claim 1, wherein The design of the preamble in step S1 is specifically as follows: Use the ZCM sequence as the preamble sequence that constitutes the preamble, and its expression is: where x u (n) is a ZC sequence, and its expression is u is the root sequence index, N is the length of the preamble sequence. m[n] is an M sequence, and the expression of the m-th order generating polynomial is as follows: g[P] = g m P m + g m-1 P m-1 + … + g1P 1 + g0 The expression of the preamble structure is: s = [s0(n), s u (n), …, s u (n)] where s u (n) = x u [(n + vN cs ) mod N] as a subsequence of the preamble, vN cs is a cyclic shift, s0(n) = 1 1×N .

3. The random access preamble and detection algorithm design based on low-earth orbit satellite communication according to claim 2, wherein In step S2, orthogonal frequency division multiplexing modulation and demodulation are performed on the preamble to obtain the demodulated signal s at the receiving end de , specifically: S2.1: Modulate the preamble by orthogonal frequency division multiplexing; S2.2: Transmit the modulated signal to the wireless channel through radio frequency for transmission; S2.3: Demodulate the received signal at the receiving end through a demodulator.

4. The random access preamble and detection algorithm design based on low-earth orbit satellite communication according to claim 3, characterized in that, The expression of the modulated signal in step S2.1 is: 。 5. The random access preamble and detection algorithm design based on low-earth orbit satellite communication according to claim 4, characterized in that, When transmitted to the wireless channel in step S2.2, the expression of the received signal is: Wherein, d is the time delay, ε = f / Δf RA is the normalized carrier frequency offset, f is the carrier frequency offset in the channel, and Δf RA is the subcarrier spacing of the transmitted signal. w(n) is additive white Gaussian noise.

6. The random access preamble and detection algorithm design based on low Earth orbit satellite communication according to claim 5, characterized in that, When demodulating the received signal at the receiving end in step S2.3, the expression of the demodulated signal is: 。 7. The random access preamble and detection algorithm design based on low-earth orbit satellite communication according to claim 6, characterized in that In step S3, perform user index detection on the preamble, specifically: S3.1: Perform a correlation operation on the last subsequence of the demodulated signal and all local root sequences to obtain the power delay profile; S3.2: Calculate the variance of all power delay profiles, find the power delay profile with the largest variance, and the corresponding window index is the user index.

8. The random access preamble and detection algorithm design based on low-earth orbit satellite communication according to claim 7, characterized in that, The expression for performing the correlation operation in step S3.1 is: In the formula, where a and b are constants.

9. The random access preamble and detection algorithm design based on low-earth orbit satellite communication according to claim 8, characterized in that, The expression for performing peak detection in step S3.2 is: In the formula, var(·) is the function for calculating variance, and find u corresponding to the power delay profile with the largest variance.

10. The random access preamble and detection algorithm design based on low-earth orbit satellite communication according to claim 9, characterized in that, In step S4, perform fractional delay detection, and perform a sliding autocorrelation operation on the first subsequence y1(n) of the demodulated signal. The specific expression is: Subsequently, the relevant function is Fourier-transformed to detect the peak index, which is the fractional delay d f .

11. The random access preamble and detection algorithm design based on low-earth orbit satellite communication according to claim 10, characterized in that, In step S5, perform power delay profile calculation, specifically: S5.1: Compensate the demodulated signal using the fractional delay; S5.2: Correlate the subsequence y i (n) with the leading code of the local index number.

12. The random access preamble and detection algorithm design based on low-earth orbit satellite communication according to claim 11, characterized in that, The expression for performing the compensation in step S5.1 is: s ′ d (n) = s d (n + d f )。 13. The random access preamble and detection algorithm design based on low-earth orbit satellite communication according to claim 12, characterized in that, The expression for performing the correlation operation in step S5.2 is: 。 14. The random access preamble and detection algorithm design based on low-earth orbit satellite communication according to claim 13, characterized in that, In step S6, for the power delay profile c i (m), calculate the variance, perform peak detection, and calculate the integer multiple delay, specifically as follows: S6.1: Denote the peak of the power delay profile as T. If T < γ, then it is determined that this subsequence is not s u (n), and perform the correlation operation for the next subsequence until there is a correlation peak T > γ. Take the index corresponding to this subsequence as s u (n) is the number of time-shifted subsequences, where the expression for peak detection is: In the formula, α is a threshold constant, and its range of variation is [0, 0.1, 0.2, …, 1.9, 2]. In the detection of the preamble index number, traverse α from small to large until the smallest threshold constant that meets the set detection false alarm rate is obtained. S6.2: Calculate the integer delay, and its expression is: d i = (j - 1)N In the formula, j is the index of the power delay profile that meets the peak detection condition, that is, i = j.

15. The random access preamble and detection algorithm design based on low-earth orbit satellite communication according to claim 14, characterized in that, The expression for calculating the timing advance d in step S7 is: d = d i + d f 。

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