A frameless random access method based on rateless code

Abstract

A frameless random access method based on rateless code, relates to the field of communication, and the access method comprises the following steps: step 1, a ground transmitting end generates input symbols; step 2, LDPC generates precoding symbols; step 3, estimates the number of time slots required for access; step 4, calculates the optimal access probability; step 5, modulates the precoding symbols and sends them to the time slots without fixed frame structure according to the number of time slots required for access obtained in step 3 and the optimal access probability obtained in step 4; step 6, a satellite receiving end uses SIC technology to recover the sent precoding symbols in the received aliased signal; step 7, uses LDPC decoding to recover the original input data. The present invention is compatible with data transmission of frameless and fixed frame structure systems, and can adjust the access strategy according to the number of user terminals, thereby reducing the error probability of the random access process in the satellite Internet of Things and improving system performance and resource utilization.

Description

Technical Field

[0001] The present invention relates to the field of communication technology, and in particular to a frameless structure random access method based on a rateless code. Background Art

[0002] Random access technology is mainly divided into two categories: random access for the purpose of establishing a connection and random access for the purpose of short packet transmission. Random access for the purpose of establishing a connection is mainly used in the process of the first communication between a user terminal and a base station in cellular communication. This process is also called a four-step random access process. In the four-step random access process, the random access preamble is usually the focus of research. At present, in the fourth-generation mobile communication (4G) and 5G mobile communication, the preamble adopts a ZadoffChu (ZC) sequence based on a root sequence. The ZC sequence has the advantages of good autocorrelation characteristics, low peak-to-average power ratio, low detection complexity, and large codeword space, and is very suitable for cellular communication. With the deployment of 5G networks and the research on next-generation mobile communication networks, in some new application scenarios, such as NTN (non-terrestrial network), Internet of Things, and Massive Machine-Type Communication (mMTC), the ZC sequence begins to fail to meet the needs of random access. In NTN, since the transmission delay and Doppler shift are higher than those in terrestrial communications, the preamble also bears the task of delay and Doppler estimation, which exceeds the capability of the ZC sequence in 5G. Random access for the purpose of short packet transmission was first used in satellite communications. Satellite links have long transmission delays and large link quality fluctuations. When transmitting short packets, the time to establish a connection is often longer than the data transmission time. Therefore, it is more efficient to use random access to transmit short packets.

[0003] At present, most of these random access technologies are based on slotted ALOHA. ALOHA is the earliest random access model. Each user sends data randomly in the shared communication medium. If a collision occurs, the colliding data will fail to be transmitted. Therefore, the transmission efficiency of the ALOHA system is extremely low. In order to reduce the probability of collision, Robert proposed slotted ALOHA, which requires each user to send data according to the uniformly divided time slots, and the transmission efficiency is improved to about 0.55. In 2007, Casini et al. found that each user sending data once more in a frame can significantly improve the transmission efficiency. This method is named Contention Resolution Diversity Slotted ALOHA (CRDSA). In CRDSA, if the data of two users collides, the receiver will first put aside the current time slot to detect other time slots. If another transmission of one of the users is correctly received, the data on the collision time slot can also be correctly recovered. From the perspective of error correction code, CRDSA is essentially to perform repetition coding (Repetition Coding) of the original data at a code rate of 1 / 2 before sending it to the random access channel. Following this idea, Liva et al. extended the repetition code of code rate 1 / 2 to the repetition code of general code rate and general error correction code in 2010 and 2015 respectively, and proposed Irregular Repetition Slotted ALOHA (IRSA) and Coded Slotted ALOHA (CSA). When coded, ALOHA can achieve a theoretical peak efficiency close to 1 and an actual peak efficiency close to 0.9, which is the highest efficiency under the erasure random access channel at present.

[0004] The idea of ​​rateless code is also applied to random access systems with frameless structures. Stefanovic et al. proposed a frameless slotted ALOHA method to optimize the access method with the goal of maximizing throughput. Professor Zhang Chaoyang's team at Zhejiang University proposed the concept of rateless multiple access (RMA) and analyzed the theoretical performance of the RMA system. When multiple user terminals access the channel to send data to the same satellite receiver, the slotted ALOHA scheme is used to reduce collisions. Some slotted ALOHA methods are suitable for systems with frameless structures. The user terminal divides the data to be sent into multiple data blocks, and each data block is sent in sequence. After the receiving end successfully recovers a data block, the user terminal sends the next data block, but this wastes a lot of time resources and leads to slow data transmission efficiency. The coded slotted ALOHA method is suitable for systems with fixed frame structures. Within a frame, the user terminal uses error correction codes to encode each data block into precoded packets, and randomly assigns the precoded packets to time slots within a frame. After receiving the complete frame, the receiving end performs continuous interference elimination and decoding to recover the data. The above method is not compatible with systems with a frameless structure and a fixed frame structure at the same time, and cannot adjust the access strategy according to the number of user terminals. Summary of the invention

[0005] The present invention provides a frameless structure random access method based on rateless code in a satellite Internet of Things scenario, which is compatible with data transmission of frameless structure and fixed frame structure systems, and can adjust the access strategy according to the number of user terminals, thereby reducing the error probability of the random access process in the satellite Internet of Things and improving system performance and resource utilization.

[0006] The technical solution adopted by the present invention to solve the technical problem is as follows:

[0007] A frameless random access method based on a rateless code of the present invention comprises the following steps:

[0008] Step 1: The ground transmitter generates input symbols;

[0009] Step 2: LDPC generates precoded symbols;

[0010] Step 3: Estimate the number of time slots required for access;

[0011] Step 4: Calculate the optimal access probability;

[0012] Step 5: After the pre-coded symbol is modulated, it is sent to the time slot without a fixed frame structure according to the number of time slots required for access obtained in step 3 and the optimal access probability obtained in step 4;

[0013] Step 6: The satellite receiving end uses the SIC technology to recover the sent precoded symbols from the received aliased signals;

[0014] Step 7: Use LDPC decoding to restore the original input data.

[0015] Furthermore, the specific operation process of step one is as follows:

[0016] Each ground user terminal has an equal amount of data to be transmitted. The gateway first divides the data to be transmitted in the satellite beam cell into K input groups and generates corresponding input symbols.

[0017] Furthermore, the specific operation process of step 2 is as follows:

[0018] An irregular LDPC code is used to perform LDPC code encoding on K input packets to obtain precoded symbols.

[0019] Furthermore, the specific operation process of step three is as follows:

[0020] Based on the asymptotic performance analysis, the number of time slots required for the ground transmitter to achieve reliable access to the satellite receiver is estimated; considering that the number of data sent within the communication range of the satellite beam of the satellite receiver approaches infinity, the number of received time slots approaches infinity. Under this asymptotic assumption, when the position of the ground transmitter obeys a uniform distribution, the transmission delay consists of the waiting delay in the buffer area and the waiting delay in the encoder. The total transmission delay T total The calculation formula is as follows:

[0021] T total =T buffer +T encoder

[0022] Among them, T buffer represents the waiting delay in the buffer, T encoder represents the waiting delay in the encoder;

[0023] Waiting delay T in the buffer buffer Depend on And the Little criterion in queuing theory is calculated as follows:

[0024]

[0025] in It represents the steady-state distribution of state B, B represents the number of packets in the sender's buffer, the change of B is determined by the arrival of packets, Bmax represents the maximum number of packets in the sender's buffer, that is, the maximum value of B is Bmax, which is the buffer capacity. a represents the average arrival rate of packets into the buffer.

[0026] The waiting delay in the encoder is divided into average sending delay and round-trip delay. The average sending delay is related to the average number of packets entering the encoder. The waiting delay T in the encoder is encoderThe expression is:

[0027]

[0028] Where τ represents the time to generate a coded packet and send it to the receiving end, U represents the number of bits of data contained in a coded packet, ο represents the decoding overhead, and K max It represents the maximum number of data packet decoding, and R represents the expectation of the data packet carrying valid information. The calculation formula is as follows:

[0029]

[0030] Where r represents the channel parameter, R(r) represents the effective information contained in U bits of data after being transmitted through this channel, and p(r) represents the probability density function of r.

[0031] Furthermore, the specific operation process of step 4 is as follows:

[0032] Considering the frameless access part, for a satellite IoT consisting of M ground user terminals, there are a total of MK p precoded data participate in frameless access, the number of time slots is N, and the erasure probability of erasing the time slot channel is ε, then the number of input nodes used is MK p , the LT code with the number of coding nodes being (1-ε)N is modeled; for each time slot, M ground user terminals have access probability p access Access is performed, and the degree distribution of the time slot is the access probability p access The binomial distribution of LT code is λ(x)=(p access x+1-p access ) M , where x represents the node access status;

[0033] The access performance is calculated using the performance analysis method of the AND-OR tree: the performance of the frameless random access method is related to the number of ground user terminals M, the number of time slots N, and the access probability p access The number of ground user terminals M is the result of the node number estimation method, and the number of time slots N is set to an initial value. The performance of the frameless random access method of the present invention is only related to the access probability p. access In other words, it is only related to the degree division λ(x); the optimal degree division λ * (x) is obtained through search, and then the optimal access probability p is obtained based on the optimal degree distribution access .

[0034] The beneficial effects of the present invention are:

[0035] In view of the problem of poor transmission efficiency of satellite Internet of Things networks and the highly dynamic and centralized characteristics of satellite Internet of Things networks, the present invention proposes a random access method with a frameless structure based on a rateless code. The present invention first adopts an access method of LDPC precoding and frameless access cascade, and uses SIC and LDPC decoding for data recovery, thereby realizing efficient and flexible distributed frameless access. The error probability of the access method is reduced by estimating the number of time slots required for access, calculating the optimal access probability, and designing an access scheme. The random access method with a frameless structure based on a rateless code of the present invention can meet the requirements of reliable access under the required number of time slots, and can significantly reduce the bit error rate and block error rate compared to the existing access methods. Therefore, the present invention well meets the requirements of future satellite Internet of Things networks for the flexibility and reliability of random access systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a random access scene diagram for satellite IoT.

[0037] Figure 2 This is the random access message structure diagram of the satellite IoT ground transmitter.

[0038] Figure 3 The present invention is a schematic diagram of a frameless random access method based on a rateless code.

[0039] Figure 4 The present invention is an overall flow chart of a frameless structure random access method based on a rateless code. DETAILED DESCRIPTION

[0040] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0041] The present invention discloses a frameless structure random access method based on a rateless code, which mainly consists of two parts: LDPC encoding and decoding and frameless structure access. The present invention mainly studies the influence of the frameless structure access part on the access performance, without considering the LDPC coding parameters. In the performance analysis method of frameless structure access, the existing methods mostly adopt ideal assumptions. Under this assumption, single time slot data can be decoded without error and signal interference can be perfectly eliminated. This assumption is based on the basis of the performance analysis framework represented by the graph. Subsequent studies will also continue to adopt ideal assumptions to study access methods when data decoding and interference elimination are correct. Therefore, the performance analysis of the access method in the present invention also adopts ideal assumptions.

[0042] The present invention is a frameless random access method based on rateless code. Considering the centralized network structure, the beam cell of the low-orbit satellite is within a certain area. There is a satellite beam cell scenario of a gateway station such as Figure 1 As shown in the figure, the gateway is responsible for collecting data from ground user terminals (UE) and grouping them. Ground user terminals (UE) divide the interactive information according to the synchronized time slots. The frame length is not limited but there is only one available channel. The information sent in a time slot is divided into two parts: the preamble and the data. Figure 2 As shown in the figure, t is time, ① is preamble, ② is data part, ③ is time slot, and ④ is protection interval. Each terrestrial user terminal (UE) has a unique preamble and it is assumed that the preamble can be successfully detected. The data part may be affected by noise or interference, so SIC technology (serial interference cancellation) is needed for detection.

[0043] See also Figures 1 to 4 To illustrate, a frameless random access method based on a rateless code of the present invention has a specific operation process as follows:

[0044] Step 1: The ground transmitter generates input symbols;

[0045] Considering the centralized network structure, the beam cell of the low-orbit satellite is within a certain area, and there is a satellite beam cell scenario with a gateway station as follows: Figure 1 As shown. Figure 3 As shown, each ground user terminal (UE) has an equal amount of data to be transmitted. The gateway first divides the data to be transmitted in the satellite beam cell into K input groups and generates corresponding input symbols.

[0046] Step 2: LDPC generates precoded symbols;

[0047] An irregular LDPC code is used to perform LDPC code encoding on K input packets to obtain precoded symbols.

[0048] Step 3: Estimate the number of time slots required for access;

[0049] The number of time slots required for the ground transmitter to achieve reliable access to the satellite receiver is estimated based on asymptotic performance analysis.

[0050] Specifically, the asymptotic assumption is: considering that the number of data sent within the communication range of the satellite beam of the satellite receiving end approaches infinity, the number of received time slots approaches infinity. Under this assumption, when the position of the ground transmitter obeys a uniform distribution, the transmission delay consists of the waiting delay in the buffer area and the waiting delay in the encoder, and the total transmission delay T total The calculation formula is as follows:

[0051] T total =T buffer +Tencoder

[0052] Among them, T buffer represents the waiting delay in the buffer, T encoder Represents the waiting delay in the encoder.

[0053] Waiting delay T in the buffer buffer Depend on And the Little criterion in queuing theory is calculated as follows:

[0054]

[0055] in It represents the steady-state distribution of state B, B represents the number of packets in the sender's buffer, the change of B is determined by the arrival of packets, Bmax represents the maximum number of packets in the sender's buffer, that is, the maximum value of B is Bmax, which is the buffer capacity. a represents the average arrival rate of packets into the buffer.

[0056] The waiting delay in the encoder is divided into average transmission delay and round-trip time (RTT). The average transmission delay is related to the average number of packets entering the encoder, which can also be calculated through the steady-state distribution of state B. Therefore, the waiting delay T in the encoder is encoder The expression is:

[0057]

[0058] Where τ represents the time to generate a coded packet and send it to the receiving end, U represents the number of bits of data contained in a coded packet, ο represents the decoding overhead, and K max Indicates the maximum number of data packets decoded. It indicates the expectation of data packets carrying valid information. The calculation formula is as follows:

[0059]

[0060] Where r represents the channel parameter, R(r) represents the effective information contained in U bits of data after being transmitted through this channel, and p(r) represents the probability density function (PDF) of r.

[0061] Through the above steps, the number of time slots can be dynamically estimated according to specific communication requirements and the changing number of nodes.

[0062] Step 4: Calculate the optimal access probability;

[0063] The access probability is crucial to the performance of the frameless random access method of the present invention. Under the premise that the ground transmitting end randomly sends precoded bits, the performance of the frameless random access method is determined by the degree distribution of the precoded bits and time slots. The output of density evolution can be used as a metric for optimization, and the degree distribution of the time slot can be decomposed to obtain a near-optimal access probability.

[0064] In order to reduce the amount of calculation in the optimization process, the present invention uses the erasure time slot channel equivalent to the Gaussian time slot channel for optimization. Specifically, considering the frameless structure access part, for the satellite Internet of Things composed of M ground user terminals (UE), there are a total of MK p precoded data participate in frameless access, the number of time slots is N, and the erasure probability of erasing the time slot channel is ε, then the number of input nodes that can be used in the present invention is MK p , the LT code with the number of coding nodes being (1-ε)N is modeled. For each time slot, M ground user terminals (UEs) access with probability p access Access is performed, and the degree distribution of the time slot is the access probability p access The binomial distribution of LT code is λ(x)=(p access x+1-p access ) M , where x represents the node access status.

[0065] The access performance can be calculated by using the performance analysis method of the AND / OR tree. Specifically, the performance of the frameless random access method of the present invention is related to the number M of ground user terminals (UE), the number N of time slots, and the access probability p access The number of ground user terminals (UEs) M is the result of the node number estimation method, and the number of time slots N is set to an initial value. The performance of the frameless random access method of the present invention is only related to the access probability p. access , or in other words, it is only related to the degree division λ(x). The optimal degree division λ * (x) can be obtained through search, and then the optimal access probability p can be obtained based on the optimal degree distribution access .

[0066] Step 5: Transmit precoded symbols;

[0067] The precoded symbols are modulated and sent to the time slot without a fixed frame structure according to the number of time slots required for access obtained in step three and the optimal access probability obtained in step four.

[0068] Step 6: SIC process;

[0069] During the data recovery process, the satellite receiver uses SIC technology (serial interference cancellation) to recover the transmitted precoded symbols from the received aliased signals.

[0070] Step 7: LDPC decoding;

[0071] LDPC decoding is further used to restore the original input data.

[0072] Due to the large-scale unlicensed access scenario of satellite Internet of Things uplink, the satellite link transmission delay is long, and the link quality fluctuates greatly. When transmitting short packets, the time to establish a connection is often longer than the data transmission time. Therefore, it is more efficient to transmit short packets in a random access manner. However, there is a certain probability that short packets will be lost during data transmission, resulting in data transmission failure. The present invention is mainly based on the unlicensed access scenario of satellite Internet of Things uplink, and completes data transmission within a limited access time. According to the distance to the satellite and the access node and the number of ground user terminals (UE) of the access node, the time to complete access to the access target is estimated. According to the number of time slots set, the data volume of the ground user terminal (UE) and the number of ground user terminals (UE) of the access target access node, the optimal access probability is determined, and data is sent according to the optimal access probability. The optimal access probability is determined according to the number of ground user terminals (UE) accessed, the access strategy is flexibly adjusted, resource utilization and communication performance are improved, and it can be applied to communication systems without frames and with frame structures at the same time.

[0073] The present invention discloses a frameless structure random access method based on a rateless code, which reduces the error probability of a random access process in a satellite Internet of Things.

[0074] 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 scope of protection of the present invention.

Claims

1. A frameless random access method based on rateless code, It is characterized in that The following steps are involved: Step 1: The ground transmitter generates input symbols; Step 2: LDPC generates precoded symbols; Step 3: Estimate the number of time slots required for access; Based on the asymptotic performance analysis, the number of time slots required for the ground transmitter to achieve reliable access to the satellite receiver is estimated; considering that the number of data sent within the communication range of the satellite beam of the satellite receiver approaches infinity, the number of received time slots approaches infinity. Under this asymptotic assumption, when the position of the ground transmitter obeys a uniform distribution, the transmission delay consists of the waiting delay in the buffer area and the waiting delay in the encoder. The total transmission delay T total The calculation formula is as follows: T total =T buffer +T encoder Among them, T buffer represents the waiting delay in the buffer, T encoder represents the waiting delay in the encoder; Waiting delay T in the buffer buffer Depend on And the Little criterion in queuing theory is calculated as follows: in represents the steady-state distribution of state B, B represents the number of packets in the sender's buffer, the change of B is determined by the arrival of packets, Bmax represents the maximum number of packets in the sender's buffer, that is, the maximum value of B is Bmax, i.e., the buffer capacity, and a represents the average arrival rate of packets into the buffer; The waiting delay in the encoder is divided into average sending delay and round-trip delay. The average sending delay is related to the average number of packets entering the encoder. The waiting delay T in the encoder is encoder The expression is: Where τ represents the time to generate a coded packet and send it to the receiving end, U represents the number of bits of data contained in a coded packet, ο represents the decoding overhead, and K max Indicates the maximum number of data packets decoded. It indicates the expectation of data packets carrying valid information. The calculation formula is as follows: Where r represents the channel parameter, R(r) represents the effective information contained in U bits of data after transmission through this channel, and p(r) represents the probability density function of r; Step 4: Calculate the optimal access probability; Step 5: After the pre-coded symbol is modulated, it is sent to the time slot without a fixed frame structure according to the number of time slots required for access obtained in step 3 and the optimal access probability obtained in step 4; Step 6: The satellite receiving end uses the SIC technology to recover the sent precoded symbols from the received aliased signals; Step 7: Use LDPC decoding to restore the original input data.

2. A frameless random access method based on rateless code according to claim 1, It is characterized in that The specific operation process of step one is as follows: Each ground user terminal has an equal amount of data to be transmitted. The gateway first divides the data to be transmitted in the satellite beam cell into K input groups and generates corresponding input symbols.

3. The frameless random access method based on rateless code according to claim 2, It is characterized in that The specific operation process of step 2 is as follows: An irregular LDPC code is used to perform LDPC code encoding on K input packets to obtain precoded symbols.

4. The method for random access with a frameless structure based on a rateless code according to claim 1, It is characterized in that The specific operation process of step 4 is as follows: Considering the frameless access part, for a satellite IoT consisting of M ground user terminals, there are a total of MK p precoded data participate in frameless access, the number of time slots is N, and the erasure probability of erasing the time slot channel is ε, then the number of input nodes used is MK p , the LT code with the number of coding nodes being (1-ε)N is modeled; for each time slot, M ground user terminals have access probability p access Access is performed, and the degree distribution of the time slot is the access probability p access The binomial distribution of LT code is λ(x)=(p access x+1-p access ) M , where x represents the node access status; The access performance is calculated using the performance analysis method of the AND-OR tree: the performance of the frameless random access method is related to the number of ground user terminals M, the number of time slots N, and the access probability p access The number of ground user terminals M is the result of the node number estimation method, and the number of time slots N is set to an initial value. The performance of the frameless random access method of the present invention is only related to the access probability p. access In other words, it is only related to the degree division λ(x); the optimal degree division λ * (x) is obtained through search, and then the optimal access probability p is obtained based on the optimal degree distribution access .

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