A multi-user low threshold access demodulation device for low-orbit satellite communication system

Abstract

The present invention belongs to the field of low-orbit satellite communications, and discloses a multi-user low-threshold access demodulation device for a low-orbit satellite communication system, which relates to the design technology of a constellation payload gateway of a low-orbit satellite mobile communication system. Aiming at the characteristics of low-orbit satellite communications requiring low user access thresholds, requiring multi-user simultaneous access capabilities to meet system capacity design, and requiring low-orbit constellation system deployment to reduce satellite costs, the present invention proposes a low-threshold flexible access demodulation device based on rapid screening and adaptive adjustment, including: a preprocessing module, a threshold calculation module, a rapid correlation module, a rapid screening module, an adaptive access control module, a low-threshold access module, a time domain demodulation module, and a frequency domain demodulation module. The present invention has the ability to adaptively access and demodulate multiple users under limited processing resources, and can obtain better multi-user access effects, which is particularly beneficial to the development and equipment deployment of my country's independent low-orbit satellite communication system.

Description

Technical Field

[0001] The present invention belongs to the field of low-orbit satellite communications, and relates to a multi-user low-threshold access demodulation device for a low-orbit satellite communication system, which is particularly suitable for the flexible multi-user access and low-threshold high-sensitivity demodulation requirements of the access channel of a low-orbit constellation satellite communication system under satellite-to-ground link conditions. Background Art

[0002] Due to the long transmission distance of low-orbit satellite communication and the limited transmission power of user terminals to ensure mobility, when the signal-to-noise ratio of the satellite payload receiver is low or it is subject to other interference, the user access threshold is required to be low; on the other hand, the satellite mobile communication system requires support for multiple users to access simultaneously to meet the user capacity design of the system. At the same time, since a large number of low-orbit satellites need to be deployed to realize the low-orbit constellation system, the cost of a single satellite must be reduced, and the processing resources of the on-board payload receiver are also strictly limited.

[0003] In order to increase the number of simultaneously supported access users and avoid the pseudo-peak effect caused by low-orbit Doppler frequency deviation, the system design supports more access root sequences and uses different root sequences to distinguish the access requests of each user. At the same time, in order to support the rapid access of multiple users and avoid collisions, it is required that there are enough candidate root sequences, the interval between access opportunities is short enough, and the demodulation time of one access opportunity is limited. On the other hand, the user's access threshold needs to be low enough, that is, the access demodulation capability of the receiver needs to be sensitive enough, and at the same time, it cannot occupy too many processing resources. These restrictions bring challenges to the access demodulation design of the system. Therefore, a multi-user low-threshold access demodulation method and device suitable for low-orbit constellation satellite communication systems is needed, which can meet the system's requirements of multiple users, low access threshold, and low cost at the same time, which is particularly beneficial to the development and equipment deployment of my country's independent low-orbit satellite communication systems. Summary of the invention

[0004] The technical problem to be solved by the present invention is to solve the problems in the background technology and achieve better access demodulation capability and multi-user access capability with lower processing resources. To adapt to the above conditions, the present invention proposes a multi-user low threshold access demodulation device for a low-orbit satellite communication system.

[0005] The technical solution adopted by the present invention is:

[0006] A multi-user low threshold access demodulation device for a low-orbit satellite communication system, comprising the following modules:

[0007] The preprocessing module is used to perform spectrum shifting and down-sampling time domain processing on the received AD sampling signal, and then convert the time domain signal of each time domain repetition unit into a frequency domain signal, and extract the subcarrier signal occupied by the access sequence to obtain the frequency domain characteristic signal of each time domain repetition unit, and output it to the threshold calculation module, the fast screening module and the low threshold access module;

[0008] A threshold calculation module is used to calculate the average power of the frequency domain characteristic signal of each time domain repetitive unit, and multiply it by the threshold coefficient according to a preset table to obtain a threshold for effective signal detection, and output it to the time domain demodulation module;

[0009] A fast screening module is used to select a frequency domain signal of a single time domain repetition unit and output it to a fast correlation module, and output all root sequence index values ​​to the fast correlation module in sequence, and then transform the frequency domain correlation results of all root sequences output by the fast correlation module into the time domain, find the peak value with the largest energy of each frequency domain correlation result as the correlation result of the corresponding root sequence and the received signal, and arrange them in order from large to small and output them to the low threshold access module;

[0010] The fast correlation module is used to perform a correlation operation on the frequency domain characteristic signal of a single time domain repetition unit with a root sequence frequency domain signal generated by a root sequence index value each time, obtain a frequency domain correlation result, and output it to the fast screening module; it is also used to perform a correlation operation on the frequency domain signals of all time domain repetition units output by the low threshold access module with the root sequence frequency domain signal generated by each root sequence index value in sequence, obtain a frequency domain correlation result, and output it to the low threshold access module;

[0011] An adaptive access control module, used to calculate the maximum number of simultaneously accessing users supported by one access opportunity according to different access preamble sequence formats, and output the result to the low threshold access module;

[0012] The low threshold access module is used to obtain the maximum number of simultaneous access users N obtained by the adaptive access control module. max , find the top N permutations of the output of the quick filter module max root sequences, output each root sequence index value and the frequency domain signals of all time domain repetition units to the fast correlation module, transform the frequency domain correlation results output by the fast correlation module to the time domain, and average all the time domain correlation results to obtain the time domain correlation results of the corresponding root sequence and the received signal, find the peak value and peak position with the maximum energy, and output them to the time domain demodulation module and the frequency domain demodulation module;

[0013] The time domain demodulation module is used to demodulate the N obtained by the low threshold access module. maxThe energy peak of each root sequence is compared with the threshold value obtained by the threshold calculation module. For each energy peak value higher than the threshold, it is determined that there is a user access request sent with the corresponding root sequence, and the corresponding root sequence index value is output to the frequency domain demodulation module. The time offset value of the corresponding root sequence is converted using the peak position, and then the uplink timing offset of the user is obtained to complete the time demodulation of the access channel;

[0014] The frequency domain demodulation module is used to calculate the frequency deviation estimate of the received signal for each root sequence with access request obtained by the time domain demodulation module according to the time domain correlation results and energy peak positions of each time domain repetition unit, as the user's frequency deviation feedback result, to complete the frequency demodulation of the access channel.

[0015] Furthermore, the fast correlation module includes the following components:

[0016] The local sequence generation parameter preset table module is used to store and search for local sequence generation parameters according to the index value i of different root sequences, including sequence number U, reverse sequence index V, local root sequence initial value and the length of the local root sequence, output the sequence number U, the reverse sequence index V and the length of the local root sequence to the CORDIC index value calculation module, and convert the local sequence initial value Output to the fast correlation frequency domain processing module;

[0017] A CORDIC index value calculation module is used to generate output parameters of the parameter preset table module according to the local sequence, calculate the index value of the fast CORDIC operation preset table, and output it to the fast CORDIC operation preset table module;

[0018] A fast CORDIC operation preset table module is used to store all possible local root sequence frequency domain phase rotation results according to all possible local root sequence lengths, and output the phase rotation result corresponding to the current index value to the fast correlation frequency domain processing module according to the index value obtained by the CORDIC index value calculation module;

[0019] The fast correlation frequency domain processing module is used to multiply the phase rotation result with the local root sequence generation parameter to obtain the root sequence frequency domain signal, and then calculate the conjugate multiplication of the frequency domain characteristic signal of the time domain repetition unit and the root sequence frequency domain signal to obtain the frequency domain correlation result.

[0020] Furthermore, the adaptive access control module includes the following components:

[0021] A preamble format preset table module is used to store and search for preamble length, access channel subcarrier spacing, cyclic prefix length, preamble sequence length and guard interval length under different access preamble sequence format index values, and output them to the access time calculation module;

[0022] The access time calculation module is used to calculate the maximum time length of an access opportunity in the current format, that is, the maximum time T for a single access processing, based on the found preamble length, access channel subcarrier spacing, cyclic prefix length, preamble sequence length, and guard interval length. max , output to the access user number calculation module;

[0023] The fast screening time calculation module is used to calculate the total number of candidate root sequences N. root , the number of parallel resources P of the fast correlation module and the single correlation operation time T corr , calculate the fast screening time T1, and output it to the access user number calculation module, that is, fast screening time T1 = (the total number of candidate root sequences N root × Single correlation operation time T corr )÷Number of parallel resources P;

[0024] The low threshold processing time calculation module is used to calculate the maximum number of simultaneous access users N max , access sequence repetition times R t , the number of parallel resources P of the fast correlation module and the single correlation operation time T corr , calculate the low threshold related time T2, that is: low threshold access processing time T2 = (maximum number of simultaneous access users N max ×Access sequence repetition times R t × Single correlation operation time T corr )÷Number of parallel resources P;

[0025] The access user number calculation module is used to calculate the maximum time T for a single access process. max , fast screening time T1 and low threshold related time T2, calculate the maximum number of simultaneous access users N supported within a single access processing time max , meet the conditions: the maximum processing time for a single access is T max ≥ Rapid screening time T1 + low threshold access processing time T2.

[0026] Furthermore, the frequency domain demodulation module includes the following components:

[0027] The energy peak calculation module is used to obtain the correlation result at the peak position of each time domain repetitive unit according to the correlation peak value and position of the root sequence obtained by the low threshold access module for the root sequence currently having an access request, record it as the energy peak symbol of the corresponding time domain repetitive unit, and output it to the phase difference algorithm module and the transform domain algorithm module;

[0028] An algorithm selection module is used to determine the number of time domain repetition units, and select a frequency offset estimation method according to a repetition number threshold value and output it to a phase difference algorithm module and a transform domain algorithm module; wherein, when the number of time domain repetition units is less than the threshold value, the phase difference estimation method is used, otherwise, the transform domain estimation method is used;

[0029] The phase difference algorithm module is used to use the phase difference estimation method when the number of time domain repetition units is less than the threshold value: conjugate multiply the energy peak symbols of two adjacent time domain repetition units, take the phase value of the conjugate multiplication result and average it, and convert it to obtain the frequency estimation value;

[0030] The transform domain algorithm module is used to use the transform domain estimation method when the number of time domain repetitive units is greater than or equal to the threshold: the energy peak symbols of all time domain repetitive units are padded with zeros and then transformed to the frequency domain, the energy of each sample point in the frequency domain is calculated, and the position of the maximum energy is found, and the frequency estimation value is converted.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] The present invention has the ability to adaptively demodulate multi-user access under limited processing resources, and can obtain better multi-user access effects. In particular, under conditions of low signal-to-noise ratio between the satellite and the ground, through initial rapid screening and secondary low-threshold access demodulation, accurate multi-user access requests and demodulation information such as timing frequency deviation can be obtained while saving processing resources. At the same time, for different access formats, the maximum number of users supported within a single access opportunity can be adaptively adjusted to obtain the maximum multi-user access capability while ensuring low-threshold access performance. In addition, relevant results are obtained by simply and quickly searching a preset table, which is suitable for practical applications. The device of the present invention has a simple structure and is very suitable for resource-constrained application environments such as onboard payloads of low-orbit constellation communication systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is the overall structure diagram of the device of the present invention.

[0034] Figure 2 It is a schematic diagram of the structure of the fast correlation module of the present invention.

[0035] Figure 3 It is a schematic diagram of the structure of the adaptive access control module of the present invention. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further explained below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0037] In an OFDM system, a subframe consists of 8 time slots, each of which contains 12 OFDM symbols. The length of the random access preamble sequence is L LRA =251, the subcarrier spacing in the frequency domain is 30kHz, and the repetition unit in the time domain is R t =13, the leading code sequence in this format, record its logical index i logic =12, the number of all possible root sequences is N root =64.

[0038] like Figure 1 As shown, a multi-user low threshold access demodulation device for a low-orbit satellite communication system includes the following modules:

[0039] The preprocessing module A is used to perform spectrum shifting and downsampling time domain processing on the received AD sampling signal, and then convert the time domain signal of each time domain repetition unit into a frequency domain signal, and extract the subcarrier signal occupied by the access sequence to obtain the frequency domain characteristic signal Y(k,t) of each time domain repetition unit, t=1,2,…,R t ,k=1,2,…,L LRA , output to the threshold calculation module B, the fast screening module D and the low threshold access module F;

[0040] The threshold calculation module B is used to calculate the average power of the frequency domain characteristic signal of each time domain repetitive unit, and multiply it by the threshold coefficient according to the preset table to obtain the threshold for effective signal detection, and output it to the time domain demodulation module G; the average power and effective signal detection threshold T r Respectively expressed as:

[0041]

[0042]

[0043] in, Indicates the threshold coefficient in the preset table.

[0044] The fast screening module D is used to select a frequency domain signal of a single time domain repetition unit and output it to the fast correlation module C, and output all the root sequence index values ​​to the fast correlation module C in sequence, and then transform the frequency domain correlation results of all the root sequences output by the fast correlation module C into the time domain, and find the peak value with the largest energy of each frequency domain correlation result as the correlation result rx of the corresponding root sequence and the received signal. corr (i),i=1,2,…,N root , and are arranged in order from large to small and output to the low threshold access module F, recorded as

[0045] The fast correlation module C is used to perform a correlation operation on the frequency domain characteristic signal of a single time domain repetition unit with a root sequence frequency domain signal generated by a root sequence index value each time, obtain a frequency domain correlation result, and output it to the fast screening module D; it is also used to perform a correlation operation on the frequency domain signals of all time domain repetition units output by the low threshold access module F with the root sequence frequency domain signal generated by each root sequence index value in sequence, obtain a frequency domain correlation result, and output it to the low threshold access module F;

[0046] An adaptive access control module E, used to calculate the maximum number of simultaneously accessing users supported by one access opportunity according to different access preamble sequence formats, and output the result to the low threshold access module F;

[0047] The low threshold access module F is used to obtain the maximum number of simultaneous access users N obtained by the adaptive access control module E. max , find the first N permutations of the output of the quick screening module D max root sequences, output each root sequence index value and the frequency domain signals of all time domain repetition units to the fast correlation module C, transform the frequency domain correlation results output by the fast correlation module C to the time domain, and average all the time domain correlation results to obtain the time domain correlation results of the corresponding root sequence and the received signal, find the peak value and peak position with the maximum energy, and output them to the time domain demodulation module G and the frequency domain demodulation module H;

[0048] The time domain demodulation module G is used to demodulate the N obtained by the low threshold access module F. max The energy peak value of the root sequence is compared with the threshold value obtained by the threshold calculation module B. For each energy peak value higher than the threshold value, it is determined that there is a user access request sent with the corresponding root sequence, and the corresponding root sequence index value is output to the frequency domain demodulation module H. The time offset value of the corresponding root sequence is converted using the peak position, and then the uplink timing offset of the user is obtained to complete the time demodulation of the access channel;

[0049] The frequency domain demodulation module H is used to calculate the frequency deviation estimate of the received signal for each root sequence with access request obtained by the time domain demodulation module G according to the time domain correlation results and energy peak positions of each time domain repetition unit, as the user's frequency deviation feedback result, to complete the frequency demodulation of the access channel.

[0050] like Figure 2 As shown, the fast correlation module C includes the following components:

[0051] The local sequence generation parameter preset table module C1 stores and searches for local sequence generation parameters according to the index values ​​i of different root sequences, including sequence number U, reverse sequence index V, local sequence initial value X u 0and the length of the local root sequence, and output the sequence number U, the reverse sequence index V and the length of the local root sequence to the CORDIC index value calculation module C3, and the local sequence initial value X u 0 Output to the fast correlation frequency domain processing module C4;

[0052] The CORDIC index value calculation module C3 calculates the index value x of the fast CORDIC operation preset table according to the output parameters of the local sequence generation parameter preset table module C1. k , output to the fast CORDIC transport preset table module C2:

[0053] x k =mod(x tmp (k),2L LRA ),

[0054] Where k = 0, 1, ..., L LRA -1, xtmp(k)=U×V×k×(V×k+1), mod(a,b) represents the remainder when a is divided by b.

[0055] The fast CORDIC operation preset table module C2 stores all possible local root sequence frequency domain phase rotation results exp according to all possible local root sequence lengths. lut :

[0056]

[0057] Among them, l idx =1:1:2L LRA Indicates all index values ​​of the preset table. Further, according to the index value x obtained by the CORDIC index value calculation module k , output the phase rotation result exp corresponding to the current index value lut (x k ) to the fast correlation frequency domain processing module C4;

[0058] The fast correlation frequency domain processing module C4 combines the phase rotation result with the local root sequence generation parameter Multiply to get the frequency domain signal of the local root sequence Then calculate the frequency domain characteristic signal of the time domain repetitive unit and the conjugate multiplication of the root sequence frequency domain signal to obtain the frequency domain correlation result in Represents X u conjugation of;

[0059] like Figure 3 As shown, the adaptive access control module E includes the following components:

[0060] The preamble format preset table module E1 is used to store and search the preamble length L under different access preamble sequence format index values LRA , access channel subcarrier spacing Δf RA , cyclic prefix length Preamble sequence length N u and guard interval length The information is output to the access time calculation module E2; according to the current access format, the preamble length L LRA =251, subcarrier spacing Δf RA =30kHz, cyclic prefix length Preamble sequence length N u =851968T c , and the guard interval length The time scale unit is T c =1 / (Δf max ×N f ), Δf max =480kHz, N f =4096.

[0061] The access time calculation module E2 calculates the maximum time length of an access opportunity in the current format, that is, the maximum time T for a single access processing, based on the found preamble length, access channel subcarrier spacing, cyclic prefix length, preamble sequence length, and guard interval length. max , which is output to the access user number calculation module E5 and can be expressed as:

[0062]

[0063] The fast screening time calculation module E3 is based on the number of all candidate root sequences N root =64, the number of parallel resources of the fast correlation module P = 2 and the single correlation operation time T corr =8224T c , calculate the operation processing time T1 of the fast screening module, and output it to the access user number calculation module E5, which can be expressed as:

[0064]

[0065] The low threshold processing time calculation module E4 calculates the maximum number of simultaneously connected users N. max , access sequence repetition times R t =13, the number of parallel resources of the fast correlation module P = 2 and the single correlation operation time T corr =8224T c , calculate the lower threshold correlation time T2, which can be expressed as:

[0066]

[0067] The access user number calculation module E5 is based on the maximum single access processing time T max , fast screening time T1 and low threshold related time T2, calculate the maximum number of simultaneous access users N supported within a single access processing time max , that is, the maximum number of access processing root sequences, so that it meets the condition: T max ≥T1+T2, as the output of the adaptive access control module.

[0068]

[0069] The frequency domain demodulation module H includes the following components:

[0070] The energy peak calculation module, for the root sequence currently having an access request, takes out the correlation result at the peak position of each time domain repetitive unit according to the correlation peak value and position of the root sequence obtained by the low threshold access module, records it as the energy peak symbol of the time domain repetitive unit, and outputs it to the phase difference algorithm and transform domain algorithm module;

[0071] An algorithm selection module is used to determine the number of time domain repetition units, and select a frequency offset estimation method according to a repetition number threshold value to output to a phase difference algorithm and a transform domain algorithm module, wherein when the number of time domain repetition units is less than the threshold value, the phase difference estimation method is used, otherwise, the transform domain estimation method is used;

[0072] Phase difference algorithm module, used to use the phase difference estimation method when the number of time domain repetition units is less than the threshold: for the root sequence with the current access request, according to the time domain correlation result obtained by the low threshold access module Where 1≤m≤L LRA , 1≤t≤R t The index indicating the number of repeating units, rx corr (:,t) represents the fast correlation result obtained by the tth time domain repetition, and the energy peak position of each time domain repetition unit can be further obtained. At the same time, the energy peak signs of the two adjacent time domain repetition units at this position are conjugate multiplied, that is:

[0073] z(n)=rx corr (P max ,1:R t -1)×[rx corr (P max ,2:R t )] * ,

[0074] Where 1≤n≤R t-1. Further, the phase value of the conjugate multiplication result is taken and averaged to obtain the frequency estimate:

[0075]

[0076] Here, angle(z(n)) represents the phase value of z(n).

[0077] The transform domain algorithm module is used to use the transform domain estimation method when the number of time domain repetition units is greater than the threshold: for all energy peak symbols of time domain repetition units, the energy peak symbols are converted from the repetition number R to the repetition number R. t Zero padding to N fft Point, that is:

[0078]

[0079] Then transform the expanded sequence to the transform domain (frequency domain), calculate the energy of each sample point in the transform domain, and record the location of the maximum energy Convert to get the frequency estimate:

[0080]

Claims

1. A multi-user low threshold access demodulation device for a low-orbit satellite communication system, characterized in that: Includes the following modules: The preprocessing module is used to perform spectrum shifting and down-sampling time domain processing on the received AD sampling signal, and then convert the time domain signal of each time domain repetition unit into a frequency domain signal, and extract the subcarrier signal occupied by the access sequence to obtain the frequency domain characteristic signal of each time domain repetition unit, and output it to the threshold calculation module, the fast screening module and the low threshold access module; A threshold calculation module is used to calculate the average power of the frequency domain characteristic signal of each time domain repetitive unit, and multiply it by the threshold coefficient according to a preset table to obtain a threshold for effective signal detection, and output it to the time domain demodulation module; A fast screening module is used to select a frequency domain signal of a single time domain repetition unit and output it to a fast correlation module, and output all root sequence index values ​​to the fast correlation module in sequence, and then transform the frequency domain correlation results of all root sequences output by the fast correlation module into the time domain, find the peak value with the largest energy of each frequency domain correlation result as the correlation result of the corresponding root sequence and the received signal, and arrange them in order from large to small and output them to the low threshold access module; The fast correlation module is used to perform a correlation operation on the frequency domain characteristic signal of a single time domain repetition unit with a root sequence frequency domain signal generated by a root sequence index value each time, obtain a frequency domain correlation result, and output it to the fast screening module; it is also used to perform a correlation operation on the frequency domain signals of all time domain repetition units output by the low threshold access module with the root sequence frequency domain signal generated by each root sequence index value in sequence, obtain a frequency domain correlation result, and output it to the low threshold access module; An adaptive access control module, used to calculate the maximum number of simultaneously accessing users supported by one access opportunity according to different access preamble sequence formats, and output the result to the low threshold access module; The low threshold access module is used to obtain the maximum number of simultaneous access users N obtained by the adaptive access control module. max , find the top N permutations of the output of the quick filter module max root sequences, output each root sequence index value and the frequency domain signals of all time domain repetition units to the fast correlation module, transform the frequency domain correlation results output by the fast correlation module to the time domain, and average all the time domain correlation results to obtain the time domain correlation results of the corresponding root sequence and the received signal, find the peak value and peak position with the maximum energy, and output them to the time domain demodulation module and the frequency domain demodulation module; The time domain demodulation module is used to demodulate the N obtained by the low threshold access module. max The energy peak of each root sequence is compared with the threshold value obtained by the threshold calculation module. For each energy peak value higher than the threshold, it is determined that there is a user access request sent with the corresponding root sequence, and the corresponding root sequence index value is output to the frequency domain demodulation module. The time offset value of the corresponding root sequence is converted using the peak position, and then the uplink timing offset of the user is obtained to complete the time demodulation of the access channel; The frequency domain demodulation module is used to calculate the frequency deviation estimate of the received signal for each root sequence with access request obtained by the time domain demodulation module according to the time domain correlation results and energy peak positions of each time domain repetition unit, as the user's frequency deviation feedback result, to complete the frequency demodulation of the access channel.

2. A low-orbit satellite communication system multi-user low threshold access demodulation device according to claim 1, characterized in that: The quick correlation module consists of the following components: The local sequence generation parameter preset table module is used to store and search for local sequence generation parameters according to the index value i of different root sequences, including sequence number U, reverse sequence index V, local root sequence initial value X u 0 and the length of the local root sequence, output the sequence number U, the reverse sequence index V and the length of the local root sequence to the CORDIC index value calculation module, and convert the local sequence initial value X u 0 Output to the fast correlation frequency domain processing module; A CORDIC index value calculation module is used to generate output parameters of the parameter preset table module according to the local sequence, calculate the index value of the fast CORDIC operation preset table, and output it to the fast CORDIC operation preset table module; A fast CORDIC operation preset table module is used to store all possible local root sequence frequency domain phase rotation results according to all possible local root sequence lengths, and output the phase rotation result corresponding to the current index value to the fast correlation frequency domain processing module according to the index value obtained by the CORDIC index value calculation module; The fast correlation frequency domain processing module is used to multiply the phase rotation result with the local root sequence generation parameter to obtain the root sequence frequency domain signal, and then calculate the conjugate multiplication of the frequency domain characteristic signal of the time domain repetition unit and the root sequence frequency domain signal to obtain the frequency domain correlation result.

3. The low-orbit satellite communication system multi-user low threshold access demodulation device according to claim 1, characterized in that: The adaptive access control module consists of the following components: A preamble format preset table module is used to store and search for preamble length, access channel subcarrier spacing, cyclic prefix length, preamble sequence length and guard interval length under different access preamble sequence format index values, and output them to the access time calculation module; The access time calculation module is used to calculate the maximum time length of an access opportunity in the current format, that is, the maximum time T for a single access processing, based on the found preamble length, access channel subcarrier spacing, cyclic prefix length, preamble sequence length, and guard interval length. max , output to the access user number calculation module; The fast screening time calculation module is used to calculate the total number of candidate root sequences N. root , the number of parallel resources P of the fast correlation module and the single correlation operation time T corr , calculate the fast screening time T1, and output it to the access user number calculation module, that is, fast screening time T1 = (the total number of candidate root sequences N root × Single correlation operation time T corr )÷Number of parallel resources P; The low threshold processing time calculation module is used to calculate the maximum number of simultaneous access users N. max , access sequence repetition times R t , the number of parallel resources P of the fast correlation module and the single correlation operation time T corr , calculate the low threshold related time T2, that is: low threshold access processing time T2 = (maximum number of simultaneous access users N max ×Access sequence repetition times R t × Single correlation operation time T corr )÷Number of parallel resources P; The access user number calculation module is used to calculate the maximum time T for a single access process. max , fast screening time T1 and low threshold related time T2, calculate the maximum number of simultaneous access users N supported within a single access processing time max , meet the conditions: the maximum processing time for a single access is T max ≥ Rapid screening time T1 + low threshold access processing time T2.

4. The low-orbit satellite communication system multi-user low threshold access demodulation device according to claim 1, characterized in that: Frequency domain demodulation module It includes the following components: The energy peak calculation module is used to obtain the correlation result at the peak position of each time domain repetitive unit according to the correlation peak value and position of the root sequence obtained by the low threshold access module for the root sequence currently having an access request, record it as the energy peak symbol of the corresponding time domain repetitive unit, and output it to the phase difference algorithm module and the transform domain algorithm module; An algorithm selection module is used to determine the number of time domain repetition units, and select a frequency offset estimation method according to a repetition number threshold value and output it to a phase difference algorithm module and a transform domain algorithm module; wherein, when the number of time domain repetition units is less than the threshold value, the phase difference estimation method is used, otherwise, the transform domain estimation method is used; The phase difference algorithm module is used to use the phase difference estimation method when the number of time domain repetition units is less than the threshold value: conjugate multiply the energy peak symbols of two adjacent time domain repetition units, take the phase value of the conjugate multiplication result and average it, and convert it to obtain the frequency estimation value; The transform domain algorithm module is used to use the transform domain estimation method when the number of time domain repetitive units is greater than or equal to the threshold: the energy peak symbols of all time domain repetitive units are padded with zeros and then transformed to the frequency domain, the energy of each sample point in the frequency domain is calculated, and the position of the maximum energy is found, and the frequency estimation value is converted.

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