Channel estimation method for multi-user uplink low-orbit satellite wireless communication system

Abstract

The invention provides a channel estimation method of a multi-user uplink low earth orbit satellite wireless communication system, which is used for transmitting signals by adopting an OTFS (Over The Over The Fine Switching) modulation technology aiming at a low earth orbit satellite rapid moving scene, and provides a low-complexity uplink channel estimation method combining path number, time delay, Doppler and channel fading. The method comprises the following steps: firstly, designing different pilot symbol grid positions of multiple users in a time delay-Doppler domain; then, performing energy threshold detection on all time delay domains of low-orbit satellite receiving signals to estimate the total number of paths and the time delay value of each path; then, giving a closed expression of channel fading by using a maximum likelihood estimation method, and estimating Doppler frequency shift by using a one-dimensional search method; through simulation verification, the provided uplink joint channel estimation method can effectively realize channel detection, can be further applied to ergodic capacity derivation work, and has relatively good practicability and performance.

Description

Technical Field

[0001] The invention belongs to the field of mobile communications, and in particular relates to a channel estimation method for a multi-user uplink low-orbit satellite wireless communication system. Background Art

[0002] Low-orbit satellite systems provide important support for the development of future communication networks with their unique advantages, but they also need to conduct in-depth research on the impact caused by the Doppler effect. For high-speed communication scenarios, traditional OFDM modulation can no longer be used because it cannot overcome the inter-carrier interference problem caused by the Doppler effect. In order to use OFDM modulation technology in low-orbit satellite communication systems, one way is to consider compensating for the Doppler frequency shift of the main path and ignoring the Doppler effect of different multipaths. Another way is to use the OFDM symbol repetition method to improve the time-frequency domain resolution, thereby improving the performance of uplink channel estimation. Unlike OFDM, OTFS is a new waveform modulation technology proposed for high-speed mobile scenarios. It introduces the delay Doppler (DD) domain in the traditional time-frequency domain dimension. In the DD domain, the multipath time-frequency dual-dispersion channel is converted into a time-invariant delay-Doppler channel. The data in the DD domain is very sparse. Through appropriate sparse channel estimation or data reconstruction algorithms, dynamic channel information and data information can be obtained.

[0003] However, in the research on OTFS modulation of low-orbit satellites, one approach is to consider ideal dual-orthogonal pulses and analyze a series of indicators such as interruption probability and safe interruption probability. However, in the actual environment, the filter is affected by the hardware, and the perfect orthogonality is easily destroyed, so dual-orthogonal pulses are difficult to achieve. Another approach is to study multi-satellite scenarios, where all satellites have single antennas, consider embedded OTFS pilot symbols for channel estimation, and use LMMSE for channel equalization. However, its scenario is only for single-user communication, and does not study the scenario of multi-user communication.

[0004] In summary, the existing research has not yet studied the acquisition of low-orbit satellite OTFS modulation channel information in multi-user scenarios and based on non-biorthogonal waveforms, so the existing research has not given an analysis method. Summary of the invention

[0005] Purpose of the invention: The technical problem to be solved by the present invention is to provide a channel estimation method for a multi-user uplink low-orbit satellite wireless communication system in view of the shortcomings of the prior art. In view of the scenario where the low-orbit satellite moves rapidly, OTFS modulation is used to send signals, and a low-complexity uplink channel estimation method for the joint number of paths, delay, Doppler, and channel fading is proposed. First, multiple users are designed to be located at different pilot symbol grid positions in the delay-Doppler domain; then, the energy threshold detection is performed on the received signal of the low-orbit satellite in all delay domains to estimate the total number of all paths and the delay value of each path; then, the maximum likelihood estimation method is used to give a closed-form expression for channel fading and the Doppler is estimated using a one-dimensional search method. After simulation verification, the uplink joint estimation method provided by the present invention can effectively realize channel detection and can be applied in the derivation of traversal capacity.

[0006] The method of the present invention comprises the following steps:

[0007] Step 1, modeling a multi-user uplink low-orbit satellite communication system;

[0008] Step 2, establishing a pilot transmission model;

[0009] Step 3, estimate the time delay and multipath number;

[0010] Step 4: perform small-scale fading and Doppler estimation.

[0011] Step 1 includes: the multi-user uplink low-orbit satellite communication system includes a low-orbit LEO satellite and Q ground users, and the LEO satellite is equipped with Uniform linear antenna array ULA;

[0012] On the ground side, each user has a single antenna;

[0013] All Q ground users to LEO satellite uplink communication links use broadband multi-carrier orthogonal time-frequency modulation (OTFS) modulation technology;

[0014] The delay-Doppler (DD) domain wireless channel from the uplink user to the satellite is modeled as:

[0015] (1),

[0016] in represents the DD domain wireless channel of the qth user, , is the intermediate parameter, Indicates The number of multipaths per user, Expressing the small-scale fading channel of the qth user's i-th multipath, the mean is , the variance is The Rice distribution of ; Indicates User The steering vectors of the multipaths, ,in Indicates User The uplink angle of arrival (AoA) of the multipath, represents the number of uniform linear antenna arrays of LEO satellites, represents the imaginary unit, e is a natural constant, represents the Dirac delta function, Respectively represent User The Doppler and delay values ​​of the multipath.

[0017] Step 2 includes: in a multi-user uplink low-orbit satellite communication system, pilot symbols of different users are embedded into the DD domain, and the index of the DD domain grid is It is expressed as: (2), Where k and l represent the discrete Doppler grid index and discrete delay grid index respectively. , , and Represent the subcarrier spacing and symbol spacing respectively, and satisfy .

[0018] Step 2 also includes: , They represent the maximum Doppler grid index and maximum delay grid index of all users within the satellite coverage area respectively;

[0019] Each user sends a single known pilot symbol in the pilot frame, and the pilot symbol of the qth user is set at The discrete Doppler grid index position and discrete delay grid index position in are respectively denoted as .

[0020] Step 2 also includes: due to the characteristics of OTFS, in the discrete delay grid index, the pilot symbol is from To Time dispersion will occur at the location where the first user's pilot position is set. , then the pilot position of the second user only needs to satisfy The grid index of the second user pilot position is , No. The position of the pilot signal of each user on the DD domain grid ; In order to ensure that the pilot signals of all Q users can be deployed in the grid of the DD domain, the following conditions must be met:

[0021] (3),

[0022] Record different users sending DD domain pilot symbols to the satellite ,in represents the pilot symbol of the qth user, The non-zero element positions are , the non-zero element values ​​are recorded as , the rest of the positions are 0; represents a matrix vectorized operation, Represents a complex domain, DD domain In OTFS modulation, after Heisenberg transformation and DD domain window function calculation, it is converted into time-frequency domain and sent out through the user's antenna RF link. The received signal of the u-th antenna of the LEO satellite is It is expressed as: (4), in , , Indicates LEO satellite DD domain received signal matrix of the root antenna; represents the transmission power of the qth user, is the equivalent channel in the DD domain, expressed as ,in is a sparse matrix, , ; for Point DFT matrix, , denote the permutation matrix and diagonal matrix in OTFS modulation, respectively. , , represents the Kronecker product operation, express An all-zero row vector of express The all-zero column vector of express The identity matrix of the user In the The delay and Doppler of the multipath are respectively expressed as and express, ; , Indicates the desired operation; Indicates user The indices of non-zero elements of the pilot symbol vectorization, ; For the The additive white Gaussian noise AWGN of the root receiving antenna has a mean of 0 and a variance of The complex Gaussian distribution of express The identity matrix of .

[0023] Step 3 includes: for the satellite received signal shown in formula (4), a low-complexity delay index estimation algorithm is proposed. The energy detection of the received signals of each user in different pilot areas is performed. First, the formula (4) is transformed into In matrix form, the expression of the received signal is transformed into: (5), in Indicates Root antenna in grid index The received signal at , For vector-to-matrix operations, the intermediate parameters , intermediate parameters , is the intermediate parameter, Indicates Antenna in Grid-indexed AWGN, .

[0024] Step 3 also includes: designing a multipath and delay index estimation algorithm, defining the detection area of ​​the qth user as , then the LEO satellite is in the region The energy detection within is expressed as:

[0025] (6),

[0026] in Indicates Users in discrete delay grid index The energy detected at the ,when When , the discrete delay grid index is determined There are multipaths on represents the estimated delay, then ;when There is no multipath when , estimating the number of users The total number of multipaths.

[0027] Step 4 includes: analyzing the sparsity of formula (4), using a received signal preprocessing method to Bring in In, due to The pilot symbols of each user do not interfere with each other, and the column vector Simplified to ,in Represents modulo operation;

[0028] The closed form expression of is:

[0029] (7),

[0030] The intermediate parameters , intermediate parameters , Remaining operation;

[0031] At this time, the expression of formula (4) is transformed into:

[0032] (8),

[0033] in represents the simplified received signal, , represents the simplified additive white Gaussian noise AWGN, ;

[0034] For Doppler estimation and small-scale fading estimation , using the maximum likelihood estimation MLE method, we can get the small-scale fading estimate for:

[0035] (9),

[0036] Finally, the formula (9) obtained Substituting into equation (8), the Doppler estimation The maximum likelihood estimate MLE is expressed as:

[0037] (10),

[0038] In the Doppler domain Top pair Perform a one-dimensional search for Find the minimum to determine the Doppler estimate ,in Indicates the maximum Doppler grid index of all users within the satellite coverage area.

[0039] The present invention also provides an electronic device, comprising a processor and a memory, wherein the memory stores program code, and when the program code is executed by the processor, the processor executes the steps of the described method.

[0040] The present invention also provides a storage medium storing a computer program or instruction. When the computer program or instruction is run on a computer, the steps of the method described are executed.

[0041] The present invention has the following beneficial effects: The present invention conducts an in-depth study on multi-user uplink communication in a broadband LEO satellite system. First, a multi-user to satellite channel model is established; considering the actual scenario of satellite and user movement, the present invention uses OTFS modulation, and combines fractional Doppler and rectangular pulses to design a pilot symbol frame, analyzes the sparsity of the uplink pilot reception signal, and proposes a low-complexity joint multipath, delay, Doppler, channel estimation algorithm; the present invention verifies the effectiveness of channel estimation by comparing with other channel estimation algorithms using the mean square error (NMSE) indicator. In addition, the present invention also provides a closed-form expression of the transmission rate based on the statistical CSI condition in the data transmission frame, taking into account the small-scale information estimation error, and verifies the reliability of the theoretical results through simulation. Studies have shown that this method can achieve efficient channel estimation and communication performance analysis in a multi-user uplink broadband LEO satellite communication system, providing an important reference for subsequent research and practical applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments, and the above and / or other advantages of the present invention will become more clear.

[0043] Figure 1 Schematic diagram of a method for estimating and analyzing LEO satellite channels for multi-user uplink in an embodiment of the present invention.

[0044] Figure 2 This is a model diagram of a multi-user uplink LEO satellite communication system in an embodiment of the present invention.

[0045] Figure 3 Schematic diagram of a multi-user uplink OTFS pilot frame in an embodiment of the present invention.

[0046] Figure 4 The graph is a comparison of different estimation algorithms for the channel estimation method used in the embodiment of the present invention.

[0047] Figure 5 It is a curve diagram of simulation and theory of transmission rate for signal detection using a channel estimation method in an embodiment of the present invention. DETAILED DESCRIPTION

[0048] like Figure 1 As shown, an embodiment of the present invention provides a channel estimation method for a multi-user uplink low-orbit satellite wireless communication system, specifically comprising:

[0049] Step 1: Modeling a multi-user uplink massive MIMO low-orbit satellite network;

[0050] Multi-user uplink massive MIMO low-orbit satellite network Figure 2 As shown, the system includes a low-orbit satellite and terrestrial users, represents the user set. The LEO satellite orbit height is , equipped with In addition, at the ground end, each user has a single antenna. All multi-user uplink communication links to LEO satellites use broadband multi-carrier OTFS modulation technology. Considering the dynamic scenarios of the actual high-speed movement of LEO satellites and the movement of ground users, the DD domain wireless channel from the uplink user to the satellite is modeled as: (1) in represents the delay-Doppler domain, Expressing the small-scale fading channel, its obedience parameter is Rice distribution. Under the ULA model ,in Indicates user The uplink angle of arrival (AoA) of the ith path to the satellite, and .

[0051] Step 2, establishing a pilot transmission model; In such Figure 3 In the satellite system, the pilot symbols of different users are embedded in the DD domain, and the index of the DD domain grid is expressed as: (2) in , Denote the discrete Doppler grid index and discrete delay index respectively. The multipath delay and Doppler of user q are respectively and Indicates that , , the subcarrier spacing and symbol spacing are and , and satisfy In addition, , They respectively represent the maximum Doppler grid index and maximum delay grid index of all users within the coverage of the satellite.

[0052] Each user sends a single known pilot symbol in the pilot frame. Let us assume that The pilot symbol position of each user is In the delay domain, the pilot signal will only extend Based on this phenomenon, the pilot position of the first user is set to , then the pilot position of the second user only needs to satisfy The guard interval of , and so on, The position of the pilot signal of each user on the DD domain grid is In order to ensure that the pilot signals of all Q users can be deployed in the grid of the DD domain, the following conditions must be met: (3) Record different users sending DD domain pilot symbols to the satellite ,in represents the pilot symbol of the qth user, the DD domain In OTFS modulation, the signal is converted into the time-frequency domain through the Heisenberg transformation and the DD domain window function, and then sent out through the user's antenna RF link. Here, it can be concluded that the received signal of the u-th antenna of the LEO satellite is expressed as: (4) in , Indicates LEO satellite DD domain received signal matrix of the root antenna. represents the user's transmit power, The equivalent channel in the DD domain can be expressed as ,in In addition, according to the modulation method of OTFS, is a sparse matrix, and its dimension can be expressed as .

[0053] For the size The pilot signal matrix , define the index position: (5) in For pilot symbols, Figure 3 The grid position satisfies . For the The AWGN of the receiving antenna. is given, while the small-scale fading, delay, Doppler, and multipath number in the channel information are unknown. The purpose of channel estimation is to design a corresponding estimation algorithm for the user. Channel parameters to the satellite channel Make an estimate.

[0054] Step 3, estimate the time delay and multipath number;

[0055] For the satellite received signal shown in formula (4), a low-complexity delay index estimation algorithm is proposed. Its main method is to Each user performs energy detection on the received signals in different pilot areas. First, transform formula (4) into In matrix form, the expression of the received signal is transformed into: (6) in ,and Due to the characteristics of modulo operation in the delay domain, the detection area is Based on the above principles, a multipath and delay index estimation algorithm is designed to define the The detection area of ​​a user is , then the LEO satellite is in the region The energy detection within is expressed as: (7) By setting the detection threshold ,when When , it is considered that there are multipaths on the delay grid, and In contrast, when There is no multipath when The value of is explained in the simulation part. Finally, by accumulating all The total number of paths for user q can be estimated based on the number of paths.

[0056] Step 4, perform small-scale fading and Doppler estimation; Firstly, the sparsity of formula (4) is analyzed and a received signal preprocessing method is proposed. Figure 3 It can be seen that Only in , and 0 elsewhere, so Bring in In, due to The pilot symbols of each user do not interfere with each other, and the column vector The expression is: (8) because ,and represents the identity matrix, and combined with the properties of the Kronecker product, we get the column vector in formula (8) Only when the horizontal axis is There is a non-zero element at In addition, for the matrix , analyze the The values ​​of the row elements. First, In the Travel time , which is There is a non-zero element at . and Matrix multiplication, according to the permutation matrix The property of right multiplication is equivalent to Shift the elements to the left , so the non-zero elements in the matrix Middle The position of the row can be recorded as , combined with , it can be deduced that The values ​​of the elements of the row are .

[0057] Finally, and Substituting into formula (8), we can use the sparse characteristics to obtain The closed form expression of is:

[0058] (9)

[0059] in At this time, the expression of formula (4) is transformed into: (10) in Representation Matrix No. Column, and , .

[0060] After the dimension is reduced, for different users The i-th path of is independent of i according to formula (9). The maximum likelihood estimation method is used for formula (10), where and It is known in the pilot transmission frame that the Doppler and small-scale fading to be estimated are , it can be found is a Gaussian vector with mean , the variance is The received signals of the ULA antenna are combined to better estimate the parameters Detection is performed. The likelihood function of the received signal is expressed as: (11) So maximize To estimate It is expressed as: (12) Expand to get: (13) For the minimization problem shown in formula (13), let it be 0 to obtain the minimum solution, then The closed form expression of is: (14) Finally, the above formula Substitute into formula (12), about Doppler estimation The maximum likelihood estimate of is expressed as: (15) It can be observed that formula (15) is a one-dimensional optimization problem with a single variable. By performing a one-dimensional search on its peak value, the Doppler estimate of the i-th path from the q-th user to the satellite can be obtained as .

[0061] Step 5, analyzing the transmission rate; In satellite uplink communication, the number of paths, delay index and Doppler index In addition, by increasing the transmission power of the pilot frame, the Therefore, in the signal transmission frame, set The information is completely known and the signal will be received Substituting into formula (14), the estimate of small-scale fading is obtained as: (16) in , represents an equivalent Gaussian white noise channel. In the signal transmission frame, according to the form of formula (4), the transmission rate of the uplink qth user is: (17) in They represent the desired signal, inter-symbol interference signal, inter-user interference signal, and noise respectively.

[0062] Simulation parameter setting: Now consider that the carrier frequency of communication is 10GHz and the subcarrier spacing is is 300KHz, OTFS modulation are (64, 8) respectively, the symbol uses QPSK, and the number of satellite antennas is , the satellite altitude is 800km, and its operating speed is about , after delay compensation and Doppler compensation, the delay and Doppler range is and The channel estimation result is as follows: Figure 4 As shown in Figure 1, the low-complexity optimization algorithm used is compared with the simulation curve under the perfect delay Doppler condition, which reflects the effectiveness of the algorithm. In addition, the curve of the system transmission rate can be obtained from Figure 5 It is reflected in the figure that, under different numbers of antennas, the theoretical value and the simulation value can coincide, so as to verify the transmission performance in the data transmission frame.

[0063] The present invention provides a channel estimation method for a multi-user uplink low-orbit satellite wireless communication system. There are many methods and ways to implement the technical solution. The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the protection scope of the present invention. All components not specified in this embodiment can be implemented by existing technologies.

Claims

1. A channel estimation method for a multi-user uplink low-orbit satellite wireless communication system, characterized in that: The following steps are involved: Step 1, modeling a multi-user uplink low-orbit satellite communication system; Step 2, establishing a pilot transmission model; Step 3, estimate the time delay and multipath number; Step 4: perform small-scale fading and Doppler estimation.

2. The method according to claim 1, characterized in that Step 1 includes: a multi-user uplink low-orbit satellite communication system including a low-orbit LEO satellite and Ground users, LEO satellites are equipped with Uniform linear antenna array ULA; On the ground side, each user has a single antenna; All Q ground users to LEO satellite uplink communication links use broadband multi-carrier orthogonal time-frequency air-conditioning modulation technology; The delay-Doppler domain wireless channel from the uplink user to the satellite is modeled as: （1）， in Representative DD domain wireless channel for each user, , is the intermediate parameter, Indicates The number of multipaths per user, Expression User The multipath small-scale fading channel has a mean value of , the variance is The Rice distribution of ; Indicates User The steering vectors of the multipaths, ,in Indicates User The uplink arrival angle of the multipath, represents the number of uniform linear antenna arrays of LEO satellites, represents the imaginary unit, e is a natural constant, represents the Dirac delta function, Respectively represent User The Doppler and delay values ​​of the multipath.

3. The method according to claim 2, characterized in that Step 2 includes: in a multi-user uplink low-orbit satellite communication system, pilot symbols of different users are embedded into the DD domain, and the index of the DD domain grid is It is expressed as: （2）， Where k and l represent the discrete Doppler grid index and discrete delay grid index respectively. , , and Represent the subcarrier spacing and symbol spacing respectively, and satisfy .

4. The method according to claim 3, characterized in that Step 2 also includes: , They represent the maximum Doppler grid index and maximum delay grid index of all users within the satellite coverage area respectively; Each user sends a single known pilot symbol in the pilot frame, setting the The pilot symbols of the users are The discrete Doppler grid index position and discrete delay grid index position in are respectively denoted as .

5. The method according to claim 4, characterized in that Step 2 also includes: in the discrete delay grid index, the pilot symbol is from To Time dispersion will occur at the location where the first user's pilot position is set. , then the pilot position of the second user only needs to satisfy The grid index of the second user pilot position is , No. The position of the pilot signal of each user on the DD domain grid ; In order to ensure that the pilot signals of all Q users can be deployed in the grid of the DD domain, the following conditions must be met: （3）， Record different users sending DD domain pilot symbols to the satellite ,in represents the pilot symbol of the qth user, The non-zero element positions are , the non-zero element values ​​are recorded as , the rest of the positions are 0; represents a matrix vectorized operation, Represents a complex domain, DD domain In OTFS modulation, after Heisenberg transformation and DD domain window function calculation, it is converted into time-frequency domain and sent out through the user's antenna RF link. The received signal of the u-th antenna of the LEO satellite is It is expressed as: （4）， in , , Indicates LEO satellite DD domain received signal matrix of the root antenna; represents the transmission power of the qth user, is the equivalent channel in the DD domain, expressed as ,in is a sparse matrix, , ; for Point DFT matrix, , denote the permutation matrix and diagonal matrix in OTFS modulation, respectively. , , represents the Kronecker product operation, express An all-zero row vector of express The all-zero column vector of express The identity matrix of the user In the The delay and Doppler of the multipath are respectively expressed as and express, ; , Indicates the desired operation; Indicates user The indices of non-zero elements of the pilot symbol vectorization, ; For the The additive white Gaussian noise AWGN of the root receiving antenna has a mean of 0 and a variance of The complex Gaussian distribution of express The identity matrix of .

6. The method according to claim 5, characterized in that Step 3 includes: for the satellite received signal shown in formula (4), a low-complexity delay index estimation algorithm is proposed. The energy detection of the received signals of each user in different pilot areas is performed. First, the formula (4) is transformed into In matrix form, the expression of the received signal is transformed into: （5）， in Indicates Root antenna in grid index The received signal at , For vector-to-matrix operations, the intermediate parameters , intermediate parameters , is the intermediate parameter, Indicates Antenna in Grid-indexed AWGN, .

7. The method according to claim 6, characterized in that Step 3 also includes: designing a multipath and delay index estimation algorithm, defining the detection area of ​​the qth user as , then the LEO satellite is in the region The energy detection within is expressed as: （6）， in Indicates Users in discrete delay grid index The energy detected at the ,when When , the discrete delay grid index is determined There are multipaths on represents the estimated delay, then ;when There is no multipath when , estimating the number of users The total number of multipaths.

8. The method according to claim 7, characterized in that Step 4 includes: analyzing the sparsity of formula (4), using a received signal preprocessing method to Bring in In, due to The pilot symbols of each user do not interfere with each other, and the column vector Simplified to ,in Represents modulo operation; The closed form expression of is: （7）， The intermediate parameters , intermediate parameters , Remaining operation; At this time, the expression of formula (4) is transformed into: （8）， in represents the simplified received signal, , represents the simplified additive white Gaussian noise AWGN, ; For Doppler estimation and small-scale fading estimation , using the maximum likelihood estimation MLE method, we can get the small-scale fading estimate for: （9）， Finally, the formula (9) obtained Substituting into equation (8), the Doppler estimation The maximum likelihood estimate MLE is expressed as: （10）， In the Doppler domain Top pair Perform a one-dimensional search for Find the minimum to determine the Doppler estimate ,in Indicates the maximum Doppler grid index of all users within the satellite coverage area.

9. An electronic device, characterized in that: The method comprises a processor and a memory, wherein the memory stores program codes, and when the program codes are executed by the processor, the processor executes the steps of the method according to any one of claims 1 to 8.

10. A storage medium, characterized in that: A computer program or instruction is stored, and when the computer program or instruction is run on a computer, the steps of the method according to any one of claims 1 to 8 are executed.

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