CN108768903B - Multi-user detection method of low-orbit satellite random access system based on CRDSA (China compact disc system discovery and maintenance) protocols - Google Patents
Multi-user detection method of low-orbit satellite random access system based on CRDSA (China compact disc system discovery and maintenance) protocols Download PDFInfo
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Abstract
The invention belongs to the technical field of satellite communication, and discloses a multi-user detection method of a low-orbit satellite random access system based on a CRDSA (reduced random access system) class protocol, wherein a plurality of access users transmit user groups to a satellite according to the CRDSA class protocol; the satellite receiving end orderly processes a plurality of superposed user groups on each time slot in a frame according to a CRDSA protocol; when a satellite receiving terminal processes K superimposed user groups on a certain time slot, a base extension model is adopted to model a channel of each user on the time slot; jointly solving the basic coefficient of each user through a maximum likelihood method; fitting each user channel by using the solved basis coefficients; reconstructing all interference user groups and eliminating interference by utilizing the first K-1 user data which are detected and decoded when other time slots are processed and the fitted first K-1 user channels; detecting the residual signal after decoding interference elimination; iterative channel estimation and interference cancellation are performed using the decoded bits. The invention has high channel estimation precision, accurate interference elimination and low packet error rate.
Description
Technical Field
The invention belongs to the technical field of satellite communication, and particularly relates to a low-orbit satellite random access system multi-user detection method based on a CRDSA (China Mobile station digital subscriber identity System) protocol.
Background
Currently, the current state of the art commonly used in the industry is such that:since the internet of things is proposed, the internet of things is gradually one of the problems of key researches at home and abroad. The internet of things is widely applied to environmental monitoring, disaster prediction and oil and gas exploration. In such a scenario, the internet of things devices are usually distributed in a wide or remote area, the ground network is difficult to provide data acquisition and transmission services for such a wide area or a specific area, and the satellite network can well make up for the defects of the ground network in coverage capability. In the satellite Internet of things, the random access method can avoid overhigh access delay and improve the throughput. In a conventional random access method, such as Aloha and timeslot Aloha, the collision probability of user packets is continuously increased as the number of users increases, and the resulting retransmission will cause a large delay in data transmission of the entire system. In satellite networks, to avoid large access delays due to frequent retransmissions, oneNew random access methods such as CRDSA (contention resolution diversity timeslot Aloha), IRSA (irregular repeat timeslot Aloha) and MuSCA (multislot coding Aloha) are proposed in succession. The basic principle of the CRDSA random access method is that each user sends a plurality of repeated data packets, and a receiver detects the packets which are not collided one by one and further eliminates all copies of the detected packets, thereby achieving the purpose of eliminating interference and improving the detection probability of the packets. In the interference cancellation process, in order to accurately cancel the decoded packet, the receiver needs effective channel estimation to acquire accurate channel state information. Otherwise, the decoded packet cannot be accurately eliminated due to the wrong channel state information, and the residual error is superposed on the un-decoded packet, thereby affecting the performance of system interference elimination and reducing the packet detection probability. At present, some documents have already developed research on multi-user detection under a new random access method in a satellite network. Based on CRDSA random access protocol, a channel estimation algorithm based on EM (maximum expectation) algorithm is provided, and packet detection decoding is realized through interference elimination. Aiming at the deadlock problem existing in the CRDSA random access protocol, the packet position is positioned through correlation, and then the packet is combined and decoded by using the correction parameter. However, the prior art only considers a simple channel model. LEO (low orbit) satellites have the characteristics of short propagation delay and low path loss, and are widely applied to the field of satellite communication with small traffic. Therefore, the LEO satellite has the natural advantages of being combined with the Internet of things with low speed, low time delay and wide coverage. However, since the angular velocity of the LEO satellite in the near-earth flight is very large, the maximum doppler shift due to high-speed movement can reach tens of hundreds of KHz and can cause a fast time-varying problem of the channel, and the quasi-static channel model in the above-mentioned document cannot describe such a channel. The method is used in an LEO satellite scene, so that the packet error rate of the system is improved.
In summary, the problems of the prior art are as follows:because the angular velocity of the LEO satellite in the near-earth flight is extremely high, the maximum Doppler frequency shift brought by high-speed movement can reach dozens of hundreds of KHz, and the problem of quick time variation of a channel is caused.
The difficulty and significance for solving the technical problems are as follows:the large doppler shift characteristic of LEO satellites results in fast time-varying channels. Under the scene, the multi-user detection based on the CRDSA random access protocol needs to perform joint channel estimation of multiple users on a time slot. It is difficult to obtain accurate channel estimation accuracy in such fast time-varying channel estimation in the presence of multi-user interference and noise.
And designing high-precision channel estimation of the LEO satellite based on a CRDSA random access protocol so as to accurately eliminate interference and successfully detect users. And a physical layer solution under the LEO satellite for realizing a high-throughput random access protocol and matching the demand of the Internet of things.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a low-orbit satellite random access system multi-user detection method based on a CRDSA type protocol.
The invention is realized in this way, a low orbit satellite random access system multi-user detection method based on CRDSA type protocol, the low orbit satellite random access system multi-user detection method based on CRDSA type protocol includes: a plurality of access users transmit user groups to the satellite according to a CRDSA type protocol; the satellite receiving end orderly processes a plurality of superposed user groups on each time slot in a frame according to a CRDSA protocol; when a satellite receiving terminal processes K superimposed user groups on a certain time slot, a base extension model is adopted to model a channel of each user on the time slot; jointly solving the basic coefficient of each user through a maximum likelihood method; fitting each user channel by using the solved basis coefficients; reconstructing all interference user groups and eliminating interference by utilizing the first K-1 user data which are detected and decoded when other time slots are processed and the fitted first K-1 user channels; detecting the residual signal after decoding interference elimination; iterative channel estimation and interference cancellation are performed using the decoded bits.
Further, the low-orbit satellite random access system multi-user detection method based on the CRDSA type protocol comprises the following steps:
firstly, a plurality of access users transmit user groups to a satellite according to a CRDSA type protocol;
step two, the satellite receiving end processes a plurality of superposed user groups on each time slot in a frame in order according to the CRDSA type protocol;
step three, the satellite receiving end skips the time slot, when the time slot is the last time slot in a frame and the satellite receiving end does not process any time slot in the traversal, the satellite receiving end stops the detection, otherwise, the satellite receiving end jumps to the first time slot in the frame, the satellite receiving end is executed to receive the signal emitted by the user by taking the frame as the unit and cache the signal on all the time slots of the current frame, when the time slot is not the last time slot in the frame, the satellite receiving end continues to detect the next time slot, when the undetected user group does not exist on a certain time slot or a plurality of undetected user groups exist on a certain time slot, the step three is executed, when one undetected user group exists on a certain time slot, the step four is executed;
fourthly, the satellite receiving end carries out channel estimation and interference elimination processing on the time slot with one undetected user group;
step five, the satellite receiving terminal detects and decodes the Kth user data bit by utilizing the fitted Kth user time-varying channel and the residual signal after the interference elimination;
and step six, the satellite receiving terminal reconstructs a Kth user data symbol by using the decoded Kth user data bit through code modulation, and performs iterative channel estimation and interference elimination by combining the front K-1 user data which are detected and decoded and the front guide, the pilot frequency and the back guide of the K users, and after the iteration times are met, the satellite receiving terminal executes the step three.
Further, the first step specifically includes:
(1) all user data bits are respectively coded and modulated to generate corresponding user data symbols, and a group of ZC sequences are respectively randomly selected from a local ZC sequence set and are inserted into the user data symbols as a leader, a pilot frequency and a postleader to form user groups;
(2) all users randomly choose two or more time slots to transmit user packets on a fixed frame.
Further, the local ZC sequence set in (2) is generated by cyclic shifting a ZC root sequence, where the ZC root sequence expression is:
wherein N isZCIs the length of the sequence, mu and NZCCoprime, q is any integer;
and (3) all users in the step (2) randomly select two or more time slots to transmit user packets on the fixed frame, according to different adopted protocols, in a CRDSA protocol, the users randomly select two copies of the two time slot transmission packets, and in an IRSA protocol, the users randomly select a plurality of time slots to transmit a plurality of copies of the packets.
Further, the second step specifically includes:
(1) a satellite receiving terminal receives signals transmitted by a user in a frame unit and caches signals on all time slots of a current frame;
(2) a satellite receiving terminal circularly traverses each time slot in a frame, and a local ZC sequence set is utilized to be respectively related to a received training signal on the time slot so as to detect the number of user groups on the time slot;
(3) and when no undetected user packet exists on a certain time slot or a plurality of undetected user packets exist on a certain time slot, executing a third step, and when one undetected user packet exists on a certain time slot, executing a fourth step.
Further, in the step (2), the local ZC sequence set is used to correlate with the received training signal in the timeslot, the number of user packets in the timeslot is detected, the satellite receiving end sets a corresponding threshold, and when the correlation value between the local ZC sequence and the received training signal in the timeslot exceeds the set threshold, it is determined that the user packet corresponding to the local ZC sequence is transmitted.
Further, the fourth step specifically includes:
(1) the satellite receiving end adopts a base extension model to model time-varying channels of K users on a time slot;
(2) the satellite receiving end constructs a maximum likelihood function according to the relation among the K user leading, pilot frequency, trailing and base coefficients, the base function and the received signal, and obtains the base coefficient estimated value of each user by combining and solving
(3) Satellite receiving end uses base coefficient estimated value of each user
And a basis function muqFitting out time-varying channels of each user;
(4) the satellite receiving end reconstructs all interference user groups by using the data of the first K-1 detected and decoded users and the fitted time-varying channels of the first K-1 users when processing other time slots, and eliminates interference in the received signals.
Further, the satellite receiving end in (1) adopts a base extension model to model time-varying channels of K users in a time slot, and the expression is as follows:
wherein b isqkQ is the Q base coefficient of the k user, Q is the number of base functions, T is the length of channel coefficient, muq＝[μq(0) … μq(T-1)]TRepresenting the qth basis function vector;
constructing a maximum likelihood function in the step (2), and obtaining a base coefficient estimated value of each user by combining and solving
The calculation formula is as follows:
wherein
Represents the training block vector, mu, transmitted by the kth usertqRepresenting the vector of basis functions, U, at the moment of the training blockt＝[μt1…μtQ]，Bk＝[b1k…bQk]T，
Is composed of
The process is repeated for Q times,
representing the Hadamard product, YtRepresenting a received superimposed training block vector;
the satellite receiving end in (3) utilizes the base coefficient estimation value of each user
And a basis function muqFitting a time-varying channel of each user, wherein the expression is as follows:
the satellite receiving end in (4) reconstructs all interference user groups by using the front K-1 user data which are detected and decoded when processing other time slots and the fitted front K-1 user time-varying channels, and eliminates interference in the received signals, and the expression is as follows:
wherein
Representing the residual signal at time n, Y (n) representing the received signal at time n, skAnd (n) represents the transmission packet of the kth user at the time of n.
The invention also aims to provide a satellite communication system applying the low-orbit satellite random access system multi-user detection method based on the CRDSA type protocol.
In summary, the advantages and positive effects of the invention are:adopting BEM (basic extension model) to model a multi-user channel, jointly solving the base coefficients of multiple users through ML (maximum likelihood) algorithm, fitting the time-varying channel of each user through the base coefficients, finally reconstructing all interference user groups and eliminating interference, and decoding the residual signals after the interference is eliminated; the channel estimation and interference cancellation process is repeated using the decoded bits and the training blocks of each user and the detected user data to improve the detection probability of the packet. According to the invention, when the satellite receiving end carries out multi-user detection based on channel estimation and interference elimination on the current system, the base extension model is adopted to model the multi-user time-varying channel, the effectiveness of the base extension model for modeling the time-varying channel is fully utilized, the base extension model can accurately model the unknown time-varying channel coefficient of the multi-user on the whole user group by using a small amount of unknown parameters, compared with the prior art, in an LEO satellite scene, the multi-user time-varying channel estimation method can more accurately estimate the multi-user time-varying channel, thereby carrying out more accurate interference elimination, effectively reducing the system packet error rate and improving the system throughput.
Drawings
Fig. 1 is a flowchart of a multi-user detection method for a low-orbit satellite random access system based on a CRDSA type protocol according to an embodiment of the present invention.
Fig. 2 is a schematic diagram of CRDSA random access applicable according to an embodiment of the present invention.
Fig. 3 is a user grouping structure diagram provided in the embodiment of the present invention.
FIG. 4 is a diagram of a multi-user system model according to an embodiment of the present invention.
Fig. 5 is a comparison graph of the packet error rate performance simulation provided by the embodiment of the present invention and the prior art.
Fig. 6 is a simulation comparison diagram of mean square error performance of channel estimation provided by the embodiment of the present invention and the prior art.
Fig. 7 is a graph comparing throughput performance simulation provided by an embodiment of the present invention and prior art.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The invention discloses a multi-user detection algorithm for channel estimation and interference elimination of a low-orbit satellite random access system, which can be used for the random access system of the satellite Internet of things and aims to solve the technical problem of high system packet error rate in an LEO satellite scene in the prior art.
As shown in fig. 1, the method for detecting multiple users in a low-orbit satellite random access system based on CRDSA type protocols according to an embodiment of the present invention includes the following steps:
s101: a plurality of access users transmit user groups to the satellite according to a CRDSA type protocol; the satellite receiving end orderly processes a plurality of superposed user groups on each time slot in a frame according to a CRDSA protocol;
s102: when a satellite receiving end processes K superposed user groups on a certain time slot, a base extension model is adopted to model a channel of each user on the time slot; jointly solving the basic coefficient of each user through a maximum likelihood method;
s103: fitting each user channel by using the solved basis coefficients; reconstructing all interference user groups and eliminating interference by utilizing the first K-1 user data which are detected and decoded when other time slots are processed and the fitted first K-1 user channels;
s104: detecting the residual signal after decoding interference elimination; iterative channel estimation and interference cancellation are performed using the decoded bits.
The application of the principles of the present invention will now be described in further detail with reference to the accompanying drawings.
As shown in fig. 2, user 1, user 2, and user 3 transmit two copies of the same packet on two different time slots in a frame, respectively. Only some copy 1b of the user 1 packet is on slot 3, assuming the receiver first detects packet 1b and decodes it successfully. Then, the packet 1a on the time slot 1 is reconstructed by using the decoding result of the packet 1b and interference cancellation is carried out, so that the packet 2a on the time slot 1 becomes a collision-free packet, and the receiver detects the packet 2a again and successfully decodes the packet. Considering LEO satellite channels with several tens of KHz doppler shifts, the channel parameters of each user on different time slots are significantly different. Therefore, the receiver needs to accurately estimate the channel parameters of each user in each time slot. First the receiver needs to accurately estimate the channel parameters of user 1 on slot 3 and then successfully detect and decode packet 1 b. To reconstruct the packet 1a on the slot 1 accurately, the first step is to decode the packet 1b successfully, and the channel parameters of the user 1 on the slot 1 need to be estimated accurately. Assuming that the receiver has accurately reconstructed packet 1a on slot 1 and performed interference cancellation, the receiver then needs to detect and decode packet 2a using accurate channel parameters, so that the user 1 and user 2 packets are successfully decoded. It can be seen that in order to successfully decode a packet, the receiver must increase the accuracy of the channel estimate as much as possible. Otherwise, the erroneous channel estimation may cause a large residual interference to be introduced when reconstructing the interference packet, thereby deteriorating the packet detection performance. In order to fully demonstrate the present invention, in combination with the existing literature, the present embodiment employs a CRDSA random access protocol, with reference to a satellite channel model in ITU-t m.1225, with a rice factor of 7dB and a doppler shift of 35 KHz.
As shown in fig. 3, the user grouping structure adopts a single carrier, the preamble and the postamble are respectively inserted in the front and the back of the grouping structure, and the pilot frequency is uniformly inserted in the grouping structure data. In this embodiment, the front, pilot, and back pilots adopt ZC sequences with lengths of 40, 12, and 12, respectively, and the number of pilots is 7.
As shown in fig. 4, the multi-user detection algorithm in the low-orbit satellite random access system based on the CRDSA protocol provided by the embodiment of the present invention includes the following steps:
step 1) a plurality of access users transmit user packets to a satellite according to a CRDSA protocol, and the implementation steps are as follows:
step 1a) all user data bits are respectively coded and modulated to generate corresponding user data symbols, and a group of ZC sequences are respectively randomly selected from a local ZC sequence set and inserted into the user data symbols as a leader, a pilot frequency and a postleader to form user groups;
in this embodiment, 1/2 Turbo coding and QPSK modulation are adopted, and the user data symbol length is 456.
in this embodiment, a frame includes 100 slots.
Step 2) the satellite receiving end processes a plurality of superposed user groups on each time slot in a frame according to the CRDSA protocol in order, and the realization steps are as follows:
step 2b) the satellite receiving end circularly traverses each time slot in a frame, and the local ZC sequence set is respectively related to the received training signals on the time slots to detect the user grouping number on the time slots;
step 2c) when there is no undetected user packet in a certain time slot or there are multiple undetected user packets in a certain time slot, executing step (3), when there is one undetected user packet in a certain time slot, executing step (4);
step 3) the satellite receiving end skips the time slot, when the time slot is the last time slot in a frame and the satellite receiving end does not process any time slot in the traversal, the satellite receiving end stops detecting, otherwise, the satellite receiving end skips to the first time slot in a frame, the step (2b) is executed, when the time slot is not the last time slot in a frame, the satellite receiving end continues to detect the next time slot, and the step (2b) is executed;
step 4), the satellite receiving end carries out channel estimation and interference elimination processing on the time slot with an undetected user group, and the realization steps are as follows:
step 4a) the satellite receiving end adopts a base extension model to model time-varying channels of K users on a time slot, and the expression is as follows:
in this embodiment, the base extension model is a DPS-BEM (discrete ellipsoid base extension model), Q takes a value of 11, and T takes a value of 592;
step 4b) the satellite receiving end constructs a maximum likelihood function according to the relation among the K user leading, pilot frequency, back leading and base coefficients, the base function and the received signal, and obtains the base coefficient estimated value of each user by combining and solving
The calculation formula is as follows:
step 4c) the satellite receiving terminal utilizes the base coefficient estimated value of each user
And a basis function muqFitting a time-varying channel of each user, wherein the expression is as follows:
step 4d) the satellite receiving end reconstructs all interference user groups by utilizing the first K-1 user data which are detected and decoded when processing other time slots and the fitted first K-1 user time-varying channels, and eliminates interference in the received signals, wherein the expression is as follows:
step 5) the satellite receiving terminal detects and decodes the Kth user data bit by utilizing the fitted Kth user time-varying channel and the residual signal after the interference elimination;
step 6) the satellite receiving end reconstructs a Kth user data symbol by using the decoded Kth user data bit through code modulation, and performs iterative channel estimation and interference elimination by combining the front K-1 user data which are detected and decoded and the front guide, the pilot frequency and the back guide of K users, and executes the step 3 after certain iterative times are met;
in this embodiment, the number of iterations is two.
The application effect of the present invention will be described in detail with reference to the simulation.
1. Simulation conditions are as follows:
the system suitable for the invention is a satellite Internet of things random access system, based on a CRDSA random access protocol, a satellite channel model in ITU-R M.1225 is adopted in simulation, the Rice factor is 7dB, and a user adopts a Turbo code with the code rate of 1/2 and QPSK modulation. The packet preamble, the pilot and the postamble are respectively 40, 12 and 12 in length, 7 pilot blocks are uniformly inserted into data containing 456 symbols, and the base extension model adopts DPS-BEM. The simulation software environment is Intel (R) core (TM) i7-4790CPU @3.60GHz, MATLAB R2015b under Windows 764bit operating system.
The invention adopts Packet Error Rate (PER) to evaluate the performance of the multi-user detection method of the system, the K-th user grouping of the decoding is detected in each simulation, if the simulation times is N, and the times of successfully detecting the K-th user grouping of the decoding is M, the packet error rate is
The invention adopts the Mean Square Error (MSE) of channel estimation to evaluate the performance of the system channel estimation method, the assumed simulation times are N, and the actual value and the estimated value of the user channel coefficient at N moments in the ith simulation are h respectivelyi(n) and
then the channel estimate mean square error is
2. And (3) simulation result analysis:
as shown in fig. 5, the abscissa represents the signal-to-noise ratio and the ordinate represents the packet error rate. It can be seen from the figure that under the same conditions, the algorithm of the present invention is obviously superior to the algorithm of the prior art. In addition, both the algorithm in the invention and the algorithm in the prior art, the iterative algorithm using the decoded data as the pilot frequency is greatly improved compared with the non-iterative algorithm only using the training block. As the number of users increases, the performance of the algorithm is slightly reduced, but the performance of the algorithm in the prior art is obviously reduced, and the algorithm can tolerate more multiple users. As the signal-to-noise ratio increases, the performance of the algorithm in the invention is improved more rapidly. The algorithm of the invention can better work in a multi-user scene with high signal-to-noise ratio.
As shown in fig. 6, the abscissa represents the signal-to-noise ratio and the ordinate represents the mean square error of the channel estimate. The cramer-perot boundary for initial channel estimation using the training block is labeled in fig. 6. It can be seen from the figure that the number of users does not affect the channel estimation mean square error performance of the two non-iterative algorithms, and the algorithm of the invention has more obvious advantages compared with the algorithm in the prior art.
As shown in fig. 7, the abscissa represents normalized load and the ordinate represents throughput. Throughput is defined as the ratio of the number of successfully decoded users to the number of time slots, and normalized load is defined as the ratio of the number of users to the number of time slots. The signal to noise ratio in fig. 7 is 10dB, and it can be seen that under the same conditions, the algorithm of the present invention is superior to the algorithm of the prior art. Due to the limitation of the theoretical throughput of the CRDSA protocol, the performance difference between the algorithm in the invention and the algorithm in the prior art is more obvious under the non-iterative condition.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.
Claims (6)
1. A multi-user detection method for a low-orbit satellite random access system based on a CRDSA type protocol is characterized in that the multi-user detection method for the low-orbit satellite random access system based on the CRDSA type protocol comprises the following steps:
firstly, a plurality of access users transmit user groups to a satellite according to a CRDSA type protocol;
step two, the satellite receiving end processes a plurality of superposed user groups on each time slot in a frame in order according to the CRDSA type protocol;
step three, when the satellite receiving end processes K superimposed user groups on a certain time slot, a base extension model is adopted to model channels of all users on the time slot;
step four, jointly solving the basis coefficients of all users through a maximum likelihood method; fitting each user channel by using the solved basis coefficients; reconstructing all interference user groups and eliminating interference by utilizing the first K-1 user data which are detected and decoded when other time slots are processed and the fitted first K-1 user channels;
step five, detecting the residual signal after the decoding interference is eliminated;
step six, iterative channel estimation and interference elimination are carried out by utilizing decoding bits;
in the third step, the satellite receiving end skips the time slot, when the time slot is the last time slot in a frame and the satellite receiving end does not process any time slot in the traversal, the satellite receiving end stops detection, otherwise, the satellite receiving end jumps to the first time slot in the frame, the satellite receiving end is executed to receive the signal transmitted by the user by taking the frame as a unit and cache the signals on all the time slots of the current frame, when the time slot is not the last time slot in the frame, the satellite receiving end continues to detect the next time slot, when no undetected user packet exists on a certain time slot or a plurality of undetected user packets exist on a certain time slot, the third step is executed, and when an undetected user packet exists on a certain time slot, the fourth step is executed;
the satellite receiving end carries out channel estimation and interference elimination processing on the time slot with an undetected user group; the method specifically comprises the following steps:
(1) the satellite receiving end adopts a base extension model to model time-varying channels of K users on a time slot;
(2) toiletThe satellite receiving end constructs a maximum likelihood function according to the relation among the K user leading, pilot frequency, trailing and base coefficients, the base function and the received signal, and obtains the base coefficient estimated value of each user by combining and solving
(3) Satellite receiving end uses base coefficient estimated value of each user
And a basis function muqFitting out time-varying channels of each user;
(4) the satellite receiving end reconstructs all interference user groups by utilizing the front K-1 user data which are detected and decoded when other time slots are processed and the fitted front K-1 user time-varying channels, and eliminates interference in the received signals;
the satellite receiving end in the step (1) adopts a base extension model to model time-varying channels of K users in a time slot, and the expression is as follows:
wherein b isqkQ is the Q base coefficient of the k user, Q is the number of base functions, T is the length of channel coefficient, muq＝[μq(0) … μq(T-1)]TRepresenting the qth basis function vector;
constructing a maximum likelihood function in the step (2), and obtaining a base coefficient estimated value of each user by combining and solving
The calculation formula is as follows:
wherein
Represents the training block vector, mu, transmitted by the kth usertqRepresenting the vector of basis functions, U, at the moment of the training blockt＝[μt1 … μtQ]，Bk＝[b1k … bQk]T，
Is composed of
The process is repeated for Q times,
representing the Hadamard product, YtRepresenting a received superimposed training block vector;
the satellite receiving end in (3) utilizes the base coefficient estimation value of each user
And a basis function muqFitting a time-varying channel of each user, wherein the expression is as follows:
the satellite receiving end in (4) reconstructs all interference user groups by using the front K-1 user data which are detected and decoded when processing other time slots and the fitted front K-1 user time-varying channels, and eliminates interference in the received signals, and the expression is as follows:
wherein
Representing the residual signal at time n, Y (n) representing the received signal at time n, sk(n) represents the transmission packet of the kth user at time instant n;
step five, the satellite receiving end detects and decodes the Kth user data bit by utilizing the fitted Kth user time-varying channel and the residual signal after the interference elimination;
and sixthly, the satellite receiving terminal reconstructs a Kth user data symbol by using the decoded Kth user data bit through code modulation, and performs iterative channel estimation and interference elimination by combining the front K-1 user data which are detected and decoded and the front guide, the pilot frequency and the back guide of the K users, and after the iteration times are met, the satellite receiving terminal executes the step three.
2. The CRDSA-based low-orbit satellite random access system multi-user detection method of claim 1, wherein the first step specifically comprises:
(1) all user data bits are respectively coded and modulated to generate corresponding user data symbols, and a group of ZC sequences are respectively randomly selected from a local ZC sequence set and are inserted into the user data symbols as a leader, a pilot frequency and a postleader to form user groups;
(2) all users randomly choose two or more time slots to transmit user packets on a fixed frame.
3. The method for detecting multiple users of a low-orbit satellite random access system based on a CRDSA protocol as claimed in claim 2, wherein the local ZC sequence set is generated by cyclic shift of a ZC root sequence, the ZC root sequence expression is as follows:
wherein N isZCIs the length of the sequence, mu and NZCCoprime, q is any integer;
the user randomly selects two or more time slots to transmit user groups on the fixed frame, and according to different protocols, the user randomly selects two copies of two time slot transmission groups in the CRDSA protocol, and randomly selects a plurality of time slots to transmit a plurality of copies of the groups in the IRSA protocol.
4. The CRDSA-based low-orbit satellite random access system multi-user detection method of claim 2, wherein the second step specifically comprises:
(1) a satellite receiving terminal receives signals transmitted by a user in a frame unit and caches signals on all time slots of a current frame;
(2) a satellite receiving terminal circularly traverses each time slot in a frame, and a local ZC sequence set is utilized to be respectively related to a received training signal on the time slot so as to detect the number of user groups on the time slot;
(3) and when no undetected user packet exists on a certain time slot or a plurality of undetected user packets exist on a certain time slot, executing a third step, and when one undetected user packet exists on a certain time slot, executing a fourth step.
5. The method as claimed in claim 4, wherein the user packets corresponding to the local ZC sequences are considered to be transmitted when the correlation value between the local ZC sequences and the received training signals in the time slots exceeds the threshold.
6. A satellite communication system applying the CRDSA-based low-orbit satellite random access system multi-user detection method of any one of claims 1 to 5.
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