A satellite-ground two-step random access enhancement method based on irregular repetitive time slot ALOHA

Abstract

The application provides a satellite-to-ground two-step random access enhancement method based on irregular repeated time slot ALOHA, which comprises the following steps: step one, random access flow method design; step two, preamble design and timing detection; step three, receiver demodulation and interference cancellation process; and step four, simulation verification and performance evaluation. The application designs a satellite-to-ground two-step random access system level method by introducing irregular repeated time slot ALOHA strategy, double preamble sequence (2-rooted PRACH preamble) detection technology, orthogonal pilot channel estimation technology and continuous interference cancellation technology. The simulation results show that the method can improve the throughput of satellite-to-ground random access, significantly reduce the access delay, and improve the access speed and access capacity.

Description

[Technical Field]

[0001] For multi-user scenarios requiring two-step random access between satellite and ground, this invention designs an enhancement method for two-step random access based on irregularly repeating time slots (ALOHA). This method significantly improves the single-access success rate of users by employing interference cancellation technology, even under conditions of high probability of data collisions among multiple users, thereby enhancing the capacity and speed of two-step random access. This invention belongs to the field of communication and signal processing technology. [Background Technology]

[0002] Space-to-ground communication systems are a crucial component in realizing the 6G integrated space-air-ground vision, with Low Earth Orbit (LEO) and Geostationary Orbit (GEO) satellites demonstrating significant potential application value in this field. Among various orbital satellites, LEO satellites stand out due to their low launch costs and short link transmission latency; while GEO satellites, due to their stationary nature relative to the ground, are less affected by the Doppler effect. Space-to-ground communication systems can not only provide supplementary coverage with the same frequency in areas with insufficient terrestrial cellular network coverage, such as urban-rural fringe areas and city edges, thereby improving communication quality, but also provide inter-frequency coverage services in remote mountain villages, polar regions, and other areas where continuous terrestrial network coverage is difficult, ensuring that communication needs in coverage blind spots are met. [1] .

[0003] Random Access (RA), a core step in achieving uplink and downlink time-frequency synchronization and establishing a reliable communication link between User Equipment (UE) and a base station or satellite, typically occurs during the initial power-on of the UE or after a connection interruption with the original base station, as it searches for and reconnects to a new base station. For communication applications involving Low Earth Orbit (LEO) and Geostationary Orbit (GEO) satellites, the satellite-to-ground random access mechanism places even stricter requirements on the success rate of single access attempts, aiming to minimize the time cost introduced by the random access process.

[0004] In traditional random access mechanisms, packet transmission generally follows the Slotted ALOHA (SA) strategy. Within the coverage area of ​​a base station or satellite beam, when a user has a communication need, it immediately initiates an access request to the base station without pre-establishing a link or allocating time-frequency resources. If the receiver fails to demodulate due to packet collisions, the system retransmits according to the Hybrid Automatic Repeat Request (HARQ) mechanism. However, because the SA strategy fails to effectively utilize colliding packets, its access throughput is capped at only 36.8%. [2] .

[0005] To address this issue, the Irregular Repetition Slotted ALOHA (IRSA) strategy was proposed. This strategy allows the transmitter to copy and retransmit data packets multiple times across multiple time slots, while the receiver employs Successive Interference Cancellation (SIC) to utilize colliding data packets, thereby achieving a progressive throughput of up to 80%. Applying this method to random access scenarios in LEO (Low Earth Orbit) or GEO (Geostationary Earth Orbit) communications, which have significant link latency, effectively reduces the number of user data packet retransmissions and significantly lowers access latency.

[0006] Compared to terrestrial random access scenarios, GEO or LEO communication scenarios are often characterized by high altitude, high dynamism, and large coverage areas. Especially in LEO scenarios, random access preamble detection needs to overcome errors caused by significant Doppler frequency shift and accurately estimate large round-trip time delay differences. Furthermore, if IRSA is introduced for random access, more accurate channel estimation and compensation are required to ensure the success rate of demodulation and decoding after interference cancellation. [3][4] However, current random access technologies are not yet fully adapted to the specific needs of satellite-to-ground random access scenarios. Therefore, how to appropriately improve existing technologies to ensure the success rate and robustness of satellite-to-ground random access remains a significant technical challenge.

[0007] References:

[0008] [1] Yan Fuli. Research on 5G+ Satellite Convergence Communication Application [J]. Yangtze Information and Communication, 2021, 34(06):22-24. 3GPP.

[0009] [2] Shi Wenxiao. Communication Network Theory and Application [M]. Peking University Press, 2009.

[0010] [3]Casini E.,De Gaudenzi R.,Herrero ODRContention resolutiondiversity slotted ALOHA

[0011] (CRDSA): An enhanced random access scheme for satellite access packetnetworks[J]. IEEE transactions on wireless communications, 2007, 6(4): 1408-1419.

[0012] [4]Liva G.Graph-based analysis and optimization of contentionresolution diversity slotted ALOHA[J].IEEE Transactions on Communications,2010,59(2):477-487. [Summary of the Invention]

[0013] The purpose of this invention is to address the random access process in geostationary orbit (GEO) or low Earth orbit (LEO) satellite communication scenarios. This invention designs a two-step random access system-level method for satellite-to-ground communication by introducing an irregular repeating time slot (ALOHA) strategy, a 2-rooted preamble detection technique, orthogonal pilot channel estimation, and continuous interference cancellation. Simulation results show that this method can improve the throughput of satellite-to-ground random access, significantly reduce access latency, and increase access speed and capacity.

[0014] This invention proposes a two-step random access enhancement method for satellite-to-ground communication based on irregularly repeating time slots (ALOHA), comprising the following steps:

[0015] Step 1: Design of Random Access Procedure

[0016] In the existing two-step random access procedure, the user terminal and the base station need to exchange signals at least twice to complete the access: MsgA, which contains the random access preamble and control information, and MsgB, which contains the random access response and contention resolution information. Consider a single-beam coverage area where m users, after demodulating the downlink broadcast signal, choose to initiate random access within the same period. If the random access period is divided into n... s Each time slot contains one MsgA signaling message. The normalized user load of the system is defined as follows:

[0017]

[0018] Assuming the number of users successfully connected in one round is q, the system throughput is defined as:

[0019]

[0020] Among them, each user in this n s Several timeslots are randomly selected and the same MsgA is transmitted. The control information in the MsgA needs to be recorded as the time slot position pointer of these MsgA, and the receiver needs to store these n timeslots. sThe receiver uses all the information from each time slot, including time slot data that caused collisions due to some users selecting the same preamble, to determine which users successfully accessed the network and which users needed to reconnect based on the demodulation results of this data.

[0021] Each time slot's MsgA contains a random access preamble and control information, which are transmitted on the Physical Random Access Channel (PRACH) and the Physical Uplink Shared Channel (PUSCH), respectively. Their time-domain structure is as follows: Figure 2 As shown. Because the MsgA signals from different users cannot be synchronized when they arrive at the satellite receiver, and there is a significant Doppler frequency shift (e.g., ... Figure 3 As shown in the figure, the distance between different users and the receiving base station is different, which causes the starting point of the receiving window to not coincide with the signaling frame header. Therefore, the data packet cannot be demodulated directly and preamble detection is required for time and frequency compensation.

[0022] Step Two: Pre-design and Timing Detection

[0023] To address the uplink synchronization issue of user data, random access utilizes access preambles for timing advance (TA) estimation. In four-step random access, the TA estimate is fed back to each user via the Random Access Response (RAR), followed by timing adjustments to ensure that subsequent PUSCH data arriving at the receiver are essentially synchronized. In two-step random access, the TA value can be used to locate the PUSCH position in the MsgA, thus eliminating the need for feedback and timing adjustments. Figure 4 As shown.

[0024] The Zadoff-Chu (ZC) sequence is widely used as a leader sequence due to its excellent autocorrelation and cross-correlation properties. The ZC sequence is a constant-amplitude complex sequence, and for a length of N... zc ZC sequence x with u as physical root u (n), whose general expression is:

[0025]

[0026] Where n is the sequence symbol index and j is the imaginary part flag, equation (3) is generally called the root sequence, which is known to both the transmitter and the receiver. For a certain round-trip delay of τ i The preamble sequence received by the i-th user in an ideal noiseless channel can be represented as y. pre (n)=x u (n-τ iThe received sequence and the root sequence are cross-correlated to obtain the power delay profile (PDP), which can be expressed as:

[0027]

[0028] in, represents the complex conjugate of the root sequence with u as the physical root, and k is the time index of the delay power spectrum. The user's time delay τ can be detected by detecting the peak position of the PDP spectrum. i This enables uplink synchronization.

[0029] In satellite-to-ground communication scenarios, the high-speed movement of satellites can cause significant Doppler shift effects, severely interfering with the accuracy of the receiver's preamble detection algorithm and significantly impacting synchronization and demodulation performance. To address this challenge, this invention introduces a dual-root preamble sequence method, aiming to achieve robust TA estimation and Doppler shift estimation (details are provided in the subsequent implementation section), thereby significantly improving the system's access success rate.

[0030] Step 3: Receiver demodulation and interference cancellation process

[0031] 1. Receiver time-frequency compensation

[0032] Without loss of generality, considering the transmission of a coded modulated sequence in the frequency domain through a noisy channel, the transmitter and receiver structures are as follows: Figure 5 and Figure 6 As shown. The transmitter uses the Discrete Fourier Transform Extended Orthogonal Frequency Division Multiplexing (OFDM) method to transmit data. That is, the data bits to be transmitted are first processed by serial-to-parallel conversion, and then digitally modulated. Next, the modulated symbols are precoded by Discrete Fourier Transform and mapped to different subcarriers to achieve frequency domain modulation. Finally, these frequency domain signals are converted back to the time domain by inverse Fourier transform and a cyclic prefix is ​​added to complete the data transmission preparation. The receiver is the inverse process of the above steps, which will not be elaborated here. Let x[n] be the time domain baseband transmission signal after OFDM modulation. If N subcarriers are used for transmission and the normalized Doppler frequency shift of the transmission link is ε (see Equation (12)), then the baseband sampling signal r[n] of the receiver can be expressed as:

[0033]

[0034] Where h[n] is the link channel coefficient and z[n] is Gaussian white noise.

[0035] The preamble sequence arrives at the receiver after a certain time delay τ. Due to the presence of the cyclic prefix (CP), the output sequence after removing the cyclic prefix will be a cyclically shifted version of the original transmitted sequence.

[0036] y[n]=r[(n-τ)modN](6)

[0037] Where N is the sampling sequence length of an OFDM symbol after removing the cyclic prefix, i.e., the number of subcarriers, and τ is the time delay. Before decoding, the time-domain sequence needs to be converted to the frequency domain, so the time-domain signal y[n] needs to be compensated accordingly to make it as close as possible to the ideal received sequence y under no frequency offset and no time delay. cmp [n]:

[0038]

[0039] This ensures a high success rate for frequency domain demodulation and decoding. The preamble detection performed in step two can simultaneously estimate the time delay and the normalized Doppler frequency shift ε, thereby achieving the compensation process.

[0040] 2. Interference cancellation based on irregularly repeating time slots ALOHA

[0041] The receiver needs to receive and store data information from all time slots. During demodulation, the receiver first performs preamble detection on each time slot to determine if a collision has occurred and records the estimated delay value for each time slot. Then, it begins the demodulation and interference cancellation process: for data in non-collision time slots, demodulation and decoding are performed. The decoding result is used to obtain the time slot location of the remaining data replicas, and interference from those replicas in those time slots is eliminated. When traversing to the last time slot, if data in any time slot has not been successfully demodulated, the traversal needs to be repeated. This process is repeated multiple times until all time slots are demodulated, or the remaining undemodulated time slots are all collision time slots, at which point the demodulation process is complete. All users who fail to access the network need to re-initiate access in the next round. Different system throughputs can be achieved by repeating data packets a different number of times for each user; dynamic adjustment during the access process can further improve the overall access speed. See the implementation plan for details.

[0042] 3. Channel estimation for time slots to be eliminated

[0043] To ensure efficient interference cancellation, the receiver needs to perform channel estimation for all time slots occupied by each user MsgA. Since different users may choose the same time slot, the pilots used for channel estimation must be orthogonal, selected from a row or column of the Hadamard matrix. Because the channel experiences time delay and Doppler shift, the receiver needs to perform appropriate time-frequency compensation on the pilot sequence before channel estimation. The specific channel estimation calculation process is detailed in the implementation plan.

[0044] Step 4: Simulation Verification and Performance Evaluation

[0045] Considering both LEO and GEO communication scenarios, under high elevation angles, the transmission link primarily uses direct sunlight components with minimal reflection. Therefore, this invention uses a Ricean channel with a high direct sunlight component. To ensure demodulation and decoding performance, this invention employs Low Density Parity Check (LDPC) codes for encoding and decoding, along with multi-carrier modulation and demodulation techniques. To compare access performance, this invention simultaneously simulates and verifies the improvements in access speed and capacity compared to existing two-step random access methods. The simulation results, including access speed, access capacity, and access success rate, are statistically analyzed. The simulation process is detailed in the implementation steps.

[0046] The advantages and beneficial effects of this invention are as follows:

[0047] This invention addresses the two-step random access scenario between satellite and ground by designing an enhancement method based on irregularly repeating time slot ALOHA. The core of this method lies in repeatedly transmitting MsgA data multiple times at the transmitter on the satellite-ground link, while simultaneously integrating advanced interference cancellation technology and performing efficient demodulation at the receiver, thereby achieving effective utilization of conflicting time slot data. This strategy significantly improves the success rate of single-access for users, thus enhancing the overall capacity and response speed of the two-step random access. Furthermore, this invention cleverly introduces a double-root preamble sequence mechanism, designed to improve the detection success rate of the preamble when facing Doppler shift channels. Simultaneously, combined with time-frequency compensated orthogonal pilots, this invention achieves accurate estimation of multi-time slot channels, ensuring high accuracy in each interference cancellation and demodulation process, thereby solidifying the reliability of access performance.

[0048] In summary, this invention significantly improves the success rate of two-step random access between satellite and ground in both low Earth orbit and geostationary orbit systems, and effectively reduces the frequency of signaling interactions between users and satellites. This innovative achievement has enormous application potential and value in the field of satellite communications. [Attached Image Description]

[0049] Figure 1 This is a flowchart illustrating the overall implementation of the present invention.

[0050] Figure 2 The MsgA time-domain structure is a two-step random access method.

[0051] Figure 3 This is a schematic diagram of uplink frame reception within the beam coverage area.

[0052] Figure 4 This is a schematic diagram of the receiver detection window.

[0053] Figure 5 , Figure 6This is a block diagram of a DFT-s-OFDM transmitter and receiver.

[0054] Figure 7 This is the random access preamble structure used in this invention.

[0055] Figure 8 This is a schematic diagram of the receiver data packet interference cancellation of the present invention.

[0056] Figure 9 This is the channel estimation method adopted in this invention.

[0057] Figure 10 The program of this invention includes a simulation module.

[0058] Figure 11 This is a comparison of the initial access throughput curves of the present invention and the baseline scheme based on the SA strategy as a function of user load.

[0059] Figure 12 This is a comparison of the initial access success rate curves of the present invention and the baseline scheme based on the SA strategy as a function of user load.

[0060] Figure 13 This is a comparison of the average number of signaling interactions as a function of user load between the present invention and a baseline scheme based on the SA strategy.

[0061] Figure 14 This is a comparison of the curves showing the total number of rounds required for all user access to change with user load between the present invention and a baseline scheme based on the SA strategy.

[0062] Figure 15 This is a comparison of the access speed curves of the present invention and the baseline scheme based on the SA strategy as a function of user load.

Detailed Implementation Methods

[0063] To more clearly illustrate the purpose, technical solution, and advantages of this invention, the following will be described in conjunction with the appendix. Figure 1-15 The present invention will be further described in detail below. The described embodiments are some, but not all, of the embodiments of the present invention. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of the present invention.

[0064] This invention proposes a two-step random access enhancement method for satellite-to-ground communication based on irregularly repeating time slots (ALOHA), comprising the following steps:

[0065] Step 1: Design of Random Access Procedure

[0066] Consider a low-Earth orbit satellite single-beam coverage area. Assume that m users in this area demodulate the downlink broadcast signal and initiate random access within the same period. Define the time domain length of MsgA as a time slot length. The time period from when all users initiate random access to when reception is completed is divided into n. s For each time slot, the normalized user load of the system is defined as:

[0067]

[0068] Assuming the number of users successfully connected in the first round is q, the system throughput is defined as:

[0069]

[0070] Figure 1 The diagram illustrates an implementation of the present invention. Based on a two-step random access framework, after obtaining cell timing information and preamble format information, the user generates a usable preamble code pool. First, the user randomly selects a preamble root sequence from the preamble code pool and adds a cyclic prefix, guard interval, etc., according to current system parameters to construct the access preamble. Simultaneously, the user generates a data packet of a certain code length, constituting MsgA signaling. The preamble code (on the PRACH channel) and data packet (on the PUSCH channel) of the same user are frequency-division multiplexed, and the selection of the user's time slot is independent of the preamble code selection. In the time domain, the preamble code and data packet are time-division multiplexed, with a certain interval between them to ensure that adjacent time slots do not interfere with each other. Figure 2 As shown.

[0071] Next, the user randomly selects several time slots to send the exact same MsgA, where the MsgA data portion contains pointers to the selected time slots. Each user repeats MsgA a different number of times; the different repetition patterns can be represented by a polynomial:

[0072]

[0073] The exponent β of each term in the polynomial represents the number of repetitions, and the coefficient λ β This represents the proportion of users who repeat the same number of times out of the total number of users, satisfying the following condition:

[0074]

[0075] Different repetition patterns will result in different system throughput performance. The number of repetitions can be dynamically adjusted according to different user loads to ensure that access efficiency remains at its best.

[0076] The satellite receives and stores the MsgA information of all accessing users and uses interference cancellation technology for demodulation and decoding. For successfully accessing users, the satellite receiver returns MsgB to the user terminal on the corresponding Physical Downlink Shared Channel (PDSCH) resource, which contains information about the successfully accessing user and their TA estimate.

[0077] Finally, the user terminal receives MsgB, detects and determines whether reconnection is required. If the connection is successful, the timing advance information TA can be used to adjust the timing of subsequent data transmission.

[0078] Step 2: Preamble Design and Uplink Synchronization

[0079] The satellite-to-ground communication link exhibits a significant Doppler frequency shift. The normalized Doppler frequency shift ε is defined as the absolute Doppler frequency shift f. d The ratio to the subcarrier spacing Δf:

[0080]

[0081] The fractional part of ε will lead to multiple peaks, causing false detections; the integer part will cause a shift in the peak value of the delay power spectrum, causing false detections. Therefore, preamble design is required for high-speed moving scenarios. For preambles transmitted by multiple carriers, the impact of Doppler frequency shift is also related to the absolute Doppler frequency shift and the subcarrier spacing.

[0082] This invention employs a dual preamble sequence as the access preamble. These two preamble sequences have different physical roots, and both ZC sequences are root sequences without cyclic shifting. In actual transmission, these two preamble sequences can be transmitted at different times but at the same frequency (i.e., Mode A), or at the same time and at the same frequency (i.e., Mode B). Compared to traditional preambles, each sequence in the enhanced preamble uses two root sequences, such as... Figure 7 As shown, assume the two root sequences are:

[0083]

[0084] Where, N zc Let u1 and u2 represent the length of the preamble sequence, and u1 and u2 represent the two physical roots selected by the user. After passing through the Doppler shift channel, these two preamble sequences are used at the receiver to calculate the PDP spectrum using the cyclic correlation of the root sequences corresponding to the two preambles, resulting in two peaks, p1 and p2. Analysis shows that the positions of these two peaks are related to the normalized Doppler shift ε, the physical roots u1 and u2, and the actual normalized delay τ, satisfying the following:

[0085]

[0086] Where [ε] represents rounding the normalized Doppler frequency shift to the nearest integer. The modular multiplicative inverses of u1 and u2 are those that satisfy:

[0087]

[0088] The above can estimate the normalized Doppler frequency shift. and delay They are respectively:

[0089]

[0090] Equation (16) shows that the Doppler frequency shift can be roughly estimated by rounding and the normalized time delay can be estimated more accurately by using two delay power spectra. To maximize the accuracy of the normalized time delay estimation using the above method, u1 and u2 should be selected in pairs and satisfy the following relationship:

[0091]

[0092] The physical roots available for use by mode A satisfy:

[0093]

[0094] The physical roots available for use by mode B satisfy:

[0095]

[0096] Furthermore, when designing the frame structure, the relationship between user delay difference within a single beam coverage area and preamble length and preamble subcarrier spacing must be considered. Assume the maximum user delay difference is RTD. max With a subcarrier spacing of Δf, the position of the PDP spectral peak needs to be limited to a preamble length of N. zc Within the range to ensure detection, the three should satisfy the following relationship:

[0097]

[0098] Wherein, the length of the leader sequence is N. zc =839, with a preamble subcarrier spacing of Δf = 1.25kHz, it can be used as a preamble for LEO systems with a beam radius R = 45km and for satellite-to-ground random access systems with a beam radius R = 60km. In addition, it is necessary to consider the RTD... max The cyclic prefix (CP) and guard interval (GT) of appropriate length are designed. In this invention, the time domain lengths of both CP and GT are equal to the maximum round-trip time difference (RTD). max .

[0099] Step 3: Receiver demodulation and interference cancellation process

[0100] 1. Time-frequency compensation of the receiver

[0101] according to Figure 2 The receiver MsgA structure shown requires the receiver to detect the preamble for each time slot and record the delay of the detected user. For colliding time slots, if the preambles in the time slots are different, the delay of the colliding user can be estimated separately based on the properties of the ZC sequence. Therefore, if two or more preambles are detected in a single time slot, it indicates that a collision has occurred in that time slot. In this invention, after completing the preamble detection of all time slots, the receiver begins the traversal demodulation process. Demodulation requires skipping the colliding time slots and demodulating the time slots with only one MsgA receiver. The sequence y[n] after removing the cyclic prefix is ​​represented as:

[0102] y[n]=r[(n-τ)modN](21)

[0103] Where τ represents the time delay, N represents the length of the time-domain sampling sequence of one OFDM symbol after removing the cyclic prefix, i.e., the number of subcarriers, and r[n] represents the receiver baseband sampling signal, which is related to the transmitter baseband signal x[n] as follows:

[0104]

[0105] Where h[n] is the link channel coefficient, and z[n] is Gaussian white noise. Substituting equation (22) into equation (21) yields:

[0106]

[0107] Using the timing advance detected by the leader To estimate the time delay τ, use By estimating the actual normalized Doppler frequency shift ε, time-frequency compensation can be achieved, and the compensation result... It can be represented as:

[0108]

[0109] The compensation result can be used for channel estimation, and then for demodulation and decoding. Channel estimation methods are detailed in Section 3 of this book.

[0110] 2. Interference cancellation based on irregularly repeating time slots ALOHA

[0111] Consider as Figure 8In the simple access scenario shown, user 1 and user 2 copied their data frames twice during the current transmission, selecting time slots 2 and 4 and time slots 2 and 3 respectively; user 3 copied their data frames three times, selecting time slots 1, 2, and 4. Therefore, only one data frame was transmitted in time slots 1 and 3, and no collision occurred, while three and two data frames were stacked in time slots 2 and 4 respectively. According to the definition of equation (10), this access frequency can be expressed as Λ(x) = 0.67x 2 +0.33x 3 This indicates that 2 / 3 of the users' data was repeated twice, and 1 / 3 of the users' data was repeated three times.

[0112] The first time slot contains only user 3's data frames. After demodulation and decoding of this time slot, the time slot pointer information contained in the decoded data can simultaneously eliminate interference from user 3 in time slots 1, 2, and 4. The third time slot contains only user 2's data frames. After demodulation and decoding of this time slot, the interference from user 2 in time slots 2 and 3 can be simultaneously eliminated. Since the data interference from users 2 and 3 has been eliminated, user 1's data in time slots 2 and 4 is "easy to demodulate," and therefore can also be demodulated and decoded. At this point, decoding for all users has been completed.

[0113] The above analysis shows that the user's data frames are randomly distributed across various time slots after being copied multiple times. As long as one of these data replicas can be demodulated and decoded, all replicas can be eliminated. Unlike the time slot ALOHA protocol, the improved ALOHA protocol utilizes conflicting time slots and makes time slots that were originally undemodulated "easy to demodulate" through continuous interference cancellation technology, reducing the probability of retransmission and further improving throughput.

[0114] 3. Channel estimation for interference cancellation time slots

[0115] During interference cancellation, due to the different channel environments in different time slots, after successful demodulation and decoding in a certain time slot, to ensure the accuracy of interference cancellation and the success rate of subsequent demodulation and decoding, it is necessary to obtain the channel coefficients of the time slot to be cancelled. Assume the channel coefficient of the j-th time slot for the i-th user is h. ij [n] = h ij Its MsgA data sequence x i [n] contains nested pilot sequences x pi [d], the pilot spacing is Δn, which means that:

[0116] x pi [d] = x i [(d-1)Δn+1] (25)

[0117] The pilot sequence is a row or column of the Hadamard matrix, and different pilots are orthogonal to each other. This satisfies...

[0118]

[0119] Where, N p The length is the pilot length. Pilot sequences are associated with preambles; different preambles correspond to different pilot sequences. Therefore, if the preambles in the same time slot are different, channel estimation can be performed by mapping the corresponding pilot sequences through the preamble, regardless of whether it's a collision or non-collision time slot. Within a time slot, the Doppler shift and delay of each user can be considered to be only related to their spatial location and not to time. Therefore, time-frequency compensation of the pilots can be performed using the Doppler shift and delay estimated from the preamble. Figure 9 For example, assuming preamble detection and time-frequency compensation are performed, the receiver output pilot sequence can be represented by equation (24). Based on the properties of the Hadamard matrix, the receiver utilizes the local pilot x... pi [n] is calculated if the time delay is correctly estimated, i.e. Then the estimated channel coefficient It can be represented as:

[0120]

[0121] in, This can be obtained from equation (24), which shows that the channel estimation error mainly comes from the Doppler frequency shift estimation bias. And Gaussian white noise z[n] in the channel. In the absence of Gaussian noise (z[n] = 0) and with completely accurate Doppler shift estimation. In this case, the channel estimation is a perfect estimation.

[0122] Step 4: Simulation Verification and Performance Evaluation

[0123] For single-beam low Earth orbit (LEO) and geostationary orbit (GEO) systems, this invention designs and constructs, based on the relevant parameters listed in Table 1, as follows: Figure 10 The simulation module shown.

[0124] Table 1

[0125]

[0126] In the simulation process of this invention, a certain number of users are first randomly generated within the beam coverage area. Then, these users are simulated to perform a series of operations, including preamble selection, data encoding and modulation, time slot selection, and data transmission. At the receiver, this invention simulates preamble detection, channel estimation, data demodulation and decoding, and interference cancellation. Simultaneously, the system collects real-time access data for subsequent analysis. To comprehensively verify the performance of this invention in random access, the simulation results are compared in detail with existing two-step random access baseline schemes to intuitively evaluate the performance advantages of this invention in key indicators such as access success rate and access latency.

[0127] Figures 11-15 The performance of this invention was compared with existing random access methods from different perspectives. Considering a signal-to-noise ratio of 6.5 dB, a Rice factor K = 10 dB, and a code rate of R... c In scenarios where the low-density parity-check code (LDPC) with a value of 0.5 is transmitted in a multi-carrier configuration, the preamble uses a double-root preamble sequence that is robust to Doppler shift. Figure 11 The initial access throughput shown represents the number of users who can access the network when all users simultaneously initiate access under the current load. It represents the access capacity of the two schemes. In LEO and GEO scenarios, the peak throughput proposed in this invention is approximately 70% and 75% higher than the current baseline scheme, respectively. The initial access success rate s is defined as the ratio of the number of users successfully accessing the network for the first time q to the total number of users currently accessing the network m, i.e., s = q / m = T / G. The resulting curve is shown in the figure. Figure 12 As shown, with increasing user load, the probability of MsgA data collisions increases, thus reducing the initial access success rate of existing baseline solutions. However, thanks to interference cancellation demodulation technology, this invention can still guarantee a success rate of over 90% when the normalized user load is below 0.6.

[0128] Simulation results show that in LEO and GEO scenarios with a signal-to-noise ratio of 6.5 dB and Rice factor K = 10, the throughput of the present invention can be increased by up to about 70% and 75%, respectively, and the access latency can be reduced by up to about 56% and 60%, respectively, which significantly improves the performance of random access.

[0129] Regarding access speed, the average number of signaling interactions directly reflects the average access latency per user, such as... Figure 13 As shown, existing solutions generally exhibit linearly increasing latency per user with normalized user load, meaning that average access latency per user is entirely limited by user load. This invention, however, guarantees that when the normalized user load is below 0.6, the average access latency per user is approximately the time required for two signaling interactions, thereby improving access speed. Simultaneously... Figure 14The displayed number of rounds required for all users to complete access visually demonstrates the overall access speed, and records the total number of rounds C when the last user in the beam completes access. Figure 15 This reflects the average number of users (m) who join per round after all users have completed their tasks. c That is, m c =m / C, directly reflecting the access speed. In LEO and GEO scenarios, the access latency proposed in this invention can reduce the latency by up to approximately 56% and 60% respectively compared to existing baseline solutions.

Claims

1. A two-step random access enhancement method for satellite-to-ground communication based on irregularly repeating time slots (ALOHA), characterized in that, Includes the following steps: Step 1: Design of Random Access Procedure In the two-step random access procedure, MsgA contains the random access preamble and control information, and MsgB contains the random access response and contention resolution information. Consider a single-beam coverage area where m users, after demodulating the downlink broadcast signal, choose to initiate random access within the same period. Each time slot’s MsgA contains a random access preamble and control information, which are transmitted on the Physical Random Access Channel (PRACH) and the Physical Uplink Shared Channel (PUSCH), respectively. Preamble detection is performed on data packets for time-frequency compensation; Step Two: Pre-design and Timing Detection Random access uses the access preamble to estimate the timing advance (TA); the estimated TA value is fed back to each user through the random access response (RAR), and then timing adjustments are made to synchronize the PUSCH data arriving at the receiver subsequently; the TA value is used to locate the PUSCH position in MsgA, omitting the feedback and timing adjustment steps. Step 3: Receiver demodulation and interference cancellation process 3.1 Receiver time-frequency compensation; 3.2 Interference cancellation based on irregularly repeating time slots ALOHA; 3.3 Channel estimation for time slots to be eliminated; Step 4: Simulation Verification and Performance Evaluation At high elevation angles, the transmission link is dominated by the direct component, the channel is set to a Ricean channel with a large direct component, low-density parity-check code (LDPC) is used as the encoding and decoding method, and multi-carrier modulation and demodulation are employed. In step three, The receiver's time-frequency compensation is as follows: Considering the transmission of the coded modulation sequence in the frequency domain through a noisy channel, the transmitter uses Discrete Fourier Transform Extended Orthogonal Frequency Division Multiplexing (DFT) to transmit data. First, the data bits to be transmitted are processed through serial-to-parallel conversion, followed by digital modulation. Then, the modulated symbols undergo Discrete Fourier Transform precoding and are mapped onto different subcarriers to achieve frequency domain modulation. Finally, these frequency domain signals are converted back to the time domain through Inverse Fourier Transform and a cyclic prefix is ​​added, thus completing the data transmission preparation. The receiver performs the reverse process of the above steps. Interference cancellation based on irregularly repeating time slots ALOHA is specifically as follows: The receiver needs to receive and store data information from all time slots. During demodulation, the receiver first performs preamble detection on each time slot to determine whether a collision has occurred and records the estimated delay value for each time slot. Then, it begins to traverse demodulation and interference cancellation: for data in non-collision time slots, demodulation and decoding are performed, and the time slot positions of the remaining data copies are obtained through the decoding results, and interference of the copies in these time slots is eliminated. When traversing to the last time slot, if there is still data in the time slot that has not been successfully demodulated, the traversal needs to be repeated. The above process is repeated multiple times until all time slots have been demodulated, or the remaining undemodulated time slots are all collision time slots, at which point the demodulation process is completed. All users who fail to access the system need to re-initiate access in the next round; if each user repeats the data packet a different number of times, they will obtain different system throughput. The channel estimation for the time slots to be eliminated is as follows: The receiver needs to perform channel estimation for all time slots where each user MsgA is located; the orthogonal pilots are selected from a row or column of the Hadamard matrix, and the receiver performs channel estimation only after performing corresponding time-frequency compensation on the pilot sequence.

2. The satellite-to-ground two-step random access enhancement method based on irregularly repeating time slot ALOHA according to claim 1, characterized in that: In step one, if the random access period is divided into n... s Each time slot contains one MsgA signaling message. The normalized user load of the system is defined as follows: Let q be the number of users successfully connected in one round, and define the system throughput as: Among them, each user in this n s Several timeslots are randomly selected and the same MsgA is transmitted. The control information in the MsgA needs to be recorded as the time slot position pointer of these MsgA, and the receiver needs to store these n timeslots. s The receiver uses all the information from each time slot, including time slot data that caused collisions due to some users selecting the same preamble, to determine which users successfully accessed the network and which users needed to re-access based on the demodulation results of this data.

3. The satellite-to-ground two-step random access enhancement method based on irregularly repeating time slot ALOHA according to claim 1, characterized in that: In step two, the Zadoff-Chu (ZC) sequence is a constant-amplitude complex sequence, for a length of N zc Let u be the ZC sequence of the physical root x u (n), whose expression is: Where n is the sequence symbol index and j is the imaginary part flag, equation (3) is called the root sequence, which is known to both the transmitter and the receiver; for a certain round-trip delay of τ i The i-th user receives a preamble sequence y in an ideal noiseless channel. pre (n)=x u (n-τ i The received sequence and the root sequence are cross-correlated to obtain the delayed power spectrum (PDP), which is represented as follows: in, The complex conjugate of the root sequence with u as the physical root, and k is the time index of the delay power spectrum; the user delay τ is detected by detecting the peak position of the PDP spectrum. i This enables uplink synchronization.

4. The satellite-to-ground two-step random access enhancement method based on irregularly repeating time slot ALOHA according to claim 3, characterized in that: In step two, a method using two root leader sequences is introduced to achieve robust TA estimation and Doppler shift estimation; specifically: Define the normalized Doppler frequency shift ε as the absolute Doppler frequency shift f. d The ratio to the subcarrier spacing Δf: A double preamble sequence is used as the access preamble. These two preamble sequences have different physical roots, and both ZC sequences are root sequences without undergoing cyclic shifting. Let the two root sequences be: Where, N zc Let u1 and u2 represent the length of the preamble sequence, and u1 and u2 represent the two physical roots selected by the user. After passing through the Doppler shift channel, these two preamble sequences are used at the receiver to calculate the PDP spectrum using the cyclic correlation of the root sequences corresponding to the two preambles, resulting in two peaks, p1 and p2. The positions of these two peaks are related to the normalized Doppler shift ε, the physical roots u1 and u2, and the actual normalized delay τ, satisfying the following: Where [ε] represents rounding the normalized Doppler frequency shift to the nearest integer. The modular multiplicative inverses of u1 and u2 are those that satisfy: u z ·u z -1 ≡1(modN zc ),z=1.2 (15) Estimate the normalized Doppler frequency shift and delay They are respectively: The selection of u1 and u2 should be paired and satisfy the following relationship:

5. The satellite-to-ground two-step random access enhancement method based on irregularly repeating time slot ALOHA according to claim 4, characterized in that: In step two, when designing the frame structure, it is also necessary to consider the relationship between the user delay difference within the coverage area of ​​a single beam and the preamble length and preamble subcarrier spacing; let the maximum user delay difference be RTD. max With a subcarrier spacing of Δf, the position of the PDP spectral peak needs to be limited to a preamble length of N. zc Within the range to ensure detection, the three should satisfy the following relationship: Wherein, the length of the leader sequence is N. zc =839, with a preamble subcarrier spacing of Δf = 1.25kHz, used as the preamble for LEO systems with a beam radius of R = 45km and satellite-to-ground random access with a radius of R = 60km.

6. The satellite-to-ground two-step random access enhancement method based on irregularly repeating time slot ALOHA according to claim 1, characterized in that: In step three, let x[n] be the time-domain baseband transmission signal modulated by OFDM. If N subcarriers are used for transmission and the normalized Doppler frequency shift of the transmission link is ε, then the baseband sampling signal r[n] of the receiver is expressed as: Where h[n] is the link channel coefficient and z[n] is Gaussian white noise; The preamble sequence arrives at the receiver after a certain time delay τ. Due to the presence of the cyclic prefix CP, the output sequence after removing the cyclic prefix will be a cyclically shifted version of the original transmitted sequence. y[n]=r[(n-τ)modN] (6) Where N is the sampling sequence length of an OFDM symbol after removing the cyclic prefix, i.e., the number of subcarriers, and τ is the time delay; before decoding, the time-domain sequence needs to be converted to the frequency domain, so the time-domain signal y[n] needs to be compensated accordingly to be as close as possible to the ideal received sequence y under no frequency offset and no time delay. cmp [n]: To ensure the success rate of frequency domain demodulation and decoding; the preamble detection simultaneously estimates the time delay and normalized Doppler frequency shift ε, thereby realizing the compensation process.

7. The satellite-to-ground two-step random access enhancement method based on irregularly repeating time slot ALOHA according to claim 6, characterized in that: Using the timing advance detected by the leader To estimate the time delay τ, use To estimate the actual normalized Doppler frequency shift ε, the purpose of time-frequency compensation is achieved, and the compensation result is... Represented as: The compensation result is used for channel estimation, which in turn enables demodulation and decoding.

8. The satellite-to-ground two-step random access enhancement method based on irregularly repeating time slot ALOHA according to claim 7, characterized in that: Let h be the channel coefficient of the j-th time slot for the i-th user. ij [n] = h ij Its MsgA data sequence x i [n] contains nested pilot sequences x pi [d], the pilot spacing is Δn, which means that: x pi [d]=x i [(d-1)Δn+1] (25) The pilot sequence is a row or column of the Hadamard matrix, and different pilots are orthogonal to each other; satisfying... Where, N p The pilot length is denoted by . The pilot sequence is associated with the preamble, and different preambles correspond to different pilot sequences. Therefore, if the preambles in the same time slot are different, channel estimation can be performed by mapping the corresponding pilot sequence to the preamble, regardless of whether it is a collision time slot or a non-collision time slot.

9. The satellite-to-ground two-step random access enhancement method based on irregularly repeating time slot ALOHA according to claim 8, characterized in that: Within a time slot, it is assumed that the Doppler frequency shift and time delay of each user are only related to their spatial location and not to time. Therefore, the Doppler frequency shift and time delay estimated by the preamble are used to perform time-frequency compensation on the pilot. According to the properties of the Hadamard matrix, the receiver uses the local pilot x pi [n] is calculated if the time delay is correctly estimated, i.e. Then the estimated channel coefficient Represented as: The channel estimation error comes from the Doppler frequency shift estimation bias. The Gaussian white noise z[n] of the channel; in the absence of Gaussian noise, z[n] = 0 and the Doppler frequency shift estimation is completely accurate. In this case, the channel estimation is a perfect estimation.

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