CN112312581A - Aloha enhanced access method for low-orbit constellation system - Google Patents
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Abstract
The invention relates to an Aloha enhanced access method for a low-orbit constellation system, belongs to the field of satellite communication, and is used for solving the problem of quick access when a high-dynamic terminal is communicated with a new generation low-orbit constellation system. Firstly, constructing a system satellite access system model; reserving different numbers of channels for different priority users of related high dynamic terminals in advance; then when the user terminal has access requirement, the specific time for sending the data packet is calculated by using access control, when the data packet is sent, the user terminal divides the channel into time slots by using a local clock, and randomly puts the data packet into the time slots for sending; and finally, by utilizing the characteristic that the user terminal is covered by a plurality of system satellites in the communication process, after each receiving end satellite receives the data packet, the data packet is forwarded to the low-orbit constellation system for interference elimination until all the data packets are successfully decoded or the maximum iteration number is reached. The invention can optimize the throughput rate and the packet loss rate index of the system, improve the resource utilization rate and optimize the first access success rate of the high dynamic terminal.
Description
Technical Field
The invention provides an Aloha enhanced access method for a low-orbit constellation system, and belongs to the field of satellite communication. Mainly relates to a reliable and fast access method of a high dynamic terminal facing a low orbit constellation system.
Background
With the development and maturity of the aerospace technology, the concept of a high-performance Low-Orbit satellite system meeting the requirements of Low Earth Orbit (LEO) information service diversification and real-time development arises. The existing service requires stronger on-track information forwarding and processing capability. In a physical layer, a low-orbit satellite system is close to the earth surface, the link loss and delay of the low-orbit satellite system are relatively smaller than those of a medium-orbit satellite and a high-orbit satellite, the requirements on the volume and the power consumption of the satellite are lower, a global coverage constellation form can be formed through large-scale deployment, and continuous, high-speed and high-quality communication of users in the whole network is realized.
The low orbit satellite system can provide generalized and customized information service for various users, and various users in the traditional strategy can be divided into a high dynamic terminal and a low dynamic terminal according to the speed of the communication terminal. The high dynamic terminal generally refers to a near space vehicle, the speed of the near space vehicle is between 5 and 15 Mach, compared with the LEO ground speed, the near space vehicle is not negligible, and the near space vehicle has the unique advantages of high flying speed, long flying distance, strong maneuvering capability and the like. The low dynamic terminal generally refers to a handheld access communication device and a general vehicle access communication terminal, and the speed of the communication terminal is negligible relative to the LEO to ground speed. In addition, there are a number of mobile devices that have various rates that have not yet reached the mach-zehnder rate standard, but which are not negligible under precise calculations compared to the moving speed of low-orbit satellite beam cells. Therefore, the communication terminal is divided into a high type, a medium type and a low type according to the speed of the communication terminal, the standard is that the speed belongs to a low dynamic terminal from 0km/s to 1000km/s, the speed belongs to a medium dynamic terminal from 1000km/s to 6120km/s, and the speed is higher than 6120km/s (Mach 5) and belongs to a high dynamic terminal. In addition to setting the dynamic performance of the terminal device, in order to further improve the service supporting capability of the low-earth orbit satellite system for the high dynamic terminal, the service type of the high dynamic satellite is further subdivided. The satellite services are specifically classified into voice services, streaming services, interactive services, and background services. Meanwhile, the track prediction can be carried out on the terminal service through the positioning equipment in the terminal equipment, and the newly generated call suppression or the switching requirement is further judged.
The assignment of the related priority policy needs to be based on the characteristic requirements of the related type of service, and the voice service is characterized by small end-to-end time delay and symmetric or almost symmetric uplink and downlink traffic. In order to ensure the delay and delay jitter index of voice services, the services are usually mapped to the highest priority. The most critical QoS indicator for conversational services is transmission delay. Meanwhile, the delay jitter is also an important index affecting the conversational services, and a serious delay jitter can cause that the conversation cannot be normally carried out. Human ears are less sensitive to packet loss and packet error rates, allowing some short speech pauses and picture mosaics. Streaming class traffic is also real-time, but since it is unidirectional, no interaction is required, so the real-time requirements are less stringent than for conversational class traffic. Like the session service, the delay jitter is also an important index affecting the stream QoS, and allows a certain packet loss rate and packet error rate. Since the streaming service does not have the real-time interaction requirement, and a cache is usually arranged locally to keep the service continuous for a certain time, the service is not sensitive to the delay parameter and the conversational service. In addition, the receiving end of the streaming media needs to perform time sorting on the received data, and the maximum delay jitter allowed by the system depends on the sorting capability of the terminal. The latency of the interactive class traffic depends on one's tolerance to latency, and is longer than the conversational class traffic, but may be shorter than the streaming class traffic. This kind of data service has no requirement for delay jitter, but has a high requirement for packet loss rate, and generally needs zero packet loss rate (which can be guaranteed by upper layer application). The time delay requirement of the interactive service is not strict with the conversation class, and the network adopts a weighted fair queue to ensure the priority of the interactive service. Background class services include some automatic background E-mail reception, SMS or some file and database downloads. The service has the characteristics that the user has no special requirement on transmission time, but has high requirement on packet loss rate, and generally needs zero packet loss rate (which can be ensured by upper layer application). When the system is congested, a drop operation is allowed for that type of user. The background service has low requirements on time delay and time delay jitter, and is forwarded in a best effort mode, so that the background service is set as the lowest priority.
The patent aims at providing an Aloha enhanced access method facing a low-orbit constellation system, which fully considers the service type and the terminal dynamic property, for a high dynamic terminal to solve all the problems of quick access and frequent switching of the high dynamic terminal.
The low-orbit satellite system has stronger on-orbit information processing and service capability, can replace the existing ground command facilities to carry out information decision, and utilizes the high-speed laser link to transmit data information, thereby optimizing the data information transmission link and reducing the propagation delay. When the high dynamic terminal executes a task, the high dynamic terminal is accessed to a low earth orbit satellite by using a high-speed laser link to realize information binding, and the difficulty of information transmission between the high dynamic terminal and a command system is optimized.
And secondly, a relative motion prediction strategy can be adopted, whether the service flow has the requirement of subsequent switching is judged during access, so that a dynamic channel reservation strategy based on priority is adopted for switching in a subsequent cell, the call blocking rate is further reduced, and the service quality is improved. Prediction strategies can be divided into two categories: when the terminal has positioning systems such as GPS and the like, the access in the service cell is combined with the current access timeAnd calculating the retention time T of the terminal in the source access cell. The residence time and the maximum residence time T of the cellmaxIn comparison, D is the diameter of a single serving cell of a low-orbit satellite, and V is the movement speed of a high-dynamic terminal.
The purpose of access in the satellite network is to establish a logical communication link between the user terminal and the satellite network, which is one of the important technologies for ensuring the quality of satellite wireless communication. Existing satellite network multiple access techniques can be divided into two categories: conflict-free access technologies and contention-based access technologies, wherein conflict-free access technologies can be further classified into fixed allocation access technologies and on-demand allocation access technologies.
Common fixed allocation Access technologies are mainly Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), and Space Division Multiple Access (SDMA). The fixed allocation access technology has higher channel utilization rate and is suitable for the scenes of fixed user terminals and constant service. However, when the traffic is in a bursty state and has a high duty ratio, the channel utilization rate is low, which easily causes waste of channel resources. The low earth orbit satellite constellation researched by the patent is mainly oriented to a high dynamic terminal object, and has the advantages of high moving speed, high task burstiness, large service type change range, uncertain service types and different required time delay and accuracy, so that the fixed allocation strategy is obviously not suitable for the scene.
Demand Assignment Multiple Access (DAMA) technology combines the ideas of random contention and fixed allocation. The main idea is that a user terminal side requests channel resources in a random competition mode, and a receiving end side distributes satellite channels to the user terminal according to the received request. However, when the access request of the ue is frequent, the process of requesting resource allocation reduces efficiency and increases unnecessary propagation delay. In a low-earth-orbit satellite communication system, the service requirement of the control signaling class requires extremely short time delay, so the strategy of allocation according to demand cannot adapt to the service diversity of a high-dynamic system.
Contention-based Access technologies have been developed on the basis of Random Access (RA) as a research base. The random access technology can be classified into a contention type and a non-contention type access technology. Aloha and S-Aloha are the "cornerstones" of contention-based random access techniques. Existing contention-based random access techniques can be classified into the following two categories according to the standard of whether synchronization of time slots is required:
(1) slot synchronization is required: DSA (scalable Slotted Aloha) is based on S-Aloha, and actively transmits a data packet twice in the same channel at the side of a user terminal, so as to improve the probability of successfully decoding the data packet at the side of a receiving end. With the development of channel coding technology, researchers have proposed a Contention Resolution Diversity Slot Aloha (CRDSA) based on DSA as a research basis. The CRDSA actively sends the same data packet twice on one side of a user terminal, detects packets which are not collided one by one on one side of a receiving end, and further eliminates all copies of the detected packets, thereby achieving the purpose of eliminating interference and improving the detection probability of the packets. The theoretical peak throughput rate of the CRDSA can reach 0.55bit/symbol, which is higher than 0.36bit/symbol of S-Aloha and 0.184bit/symbol of Aloha.
(2) No time slot synchronization is required: researchers have introduced the contention Resolution idea and iterative interference cancellation into Aloha as a research basis, and proposed Contention Resolution Aloha (CRA). In order to optimize throughput rate and packet loss rate index, the CRA actively sends the same data packet twice in a channel at a user terminal side, eliminates partial interference of the data packet in the channel by using Forward Error Correction (FEC) technology, and improves the probability of successfully receiving the data packet by using iterative interference elimination at a receiving end side. The CRA reduces the requirement of time slot synchronization, and has no limit to the time and the size of data packet transmission. Enhanced contention Resolution Aloha (enhanced Content Resolution Aloha, ECRA) is based on CRA, and a merging technique is introduced at the receiving end side to optimize the throughput and packet loss index of the system. ECRA initiatively sends the same data packet twice at one side of a user terminal, and a merging technology is introduced at one side of a receiving end, so that the possible situation of 'ring effect' that the data packets are mutually interfered in pairs is eliminated, and the probability of successfully decoding the data packets is improved. ECRA uses a virtual frame structure to transmit packets, reducing the probability of partial collisions of packets in the channel. Meanwhile, the virtual frame structure divides the channel frame into time slots by using a local clock, eliminates the requirement of time slot synchronization of the whole network, reduces signaling interaction required by time slot synchronization, saves propagation delay and obtains better indexes of peak throughput rate and packet loss rate.
Compared with an access technology requiring time slot synchronization, the access technology without time slot synchronization does not need whole network clock synchronization, and a user terminal sends a data packet without the limitation of time and size, so that the method is more suitable for a low-earth-orbit satellite system. The ECRA in the access technology without time slot synchronization obtains better peak throughput rate and packet loss rate indexes, but has the following problems:
(1) under the condition that the ECRA is low in channel load, the packet loss rate index is far lower than 10-2The access quality can be well ensured. However, since the user terminal sends the same data packet in the same channel for multiple times, under a high load condition, both throughput and packet loss index of the ECRA become very poor, and a receiving end side can hardly successfully decode a complete data packet, which cannot guarantee the access of the user terminal and the success rate of the first access of the high dynamic terminal.
(2) In the ECRA, the user terminal sends the same data packet in the same channel for multiple times, which causes the waste of channel resources and can further improve the channel utilization rate.
In summary, the characteristics of high dynamics and strong burstiness of the executed service of the high dynamic terminal provide more strict requirements for the access method of the low earth orbit constellation satellite system. The access method not only needs to optimize the throughput rate and the packet loss rate index, but also needs to ensure the success rate of the first access of the high dynamic terminal, so that the high dynamic terminal can be quickly accessed into the low orbit satellite system with high reliability.
In addition, a motion trajectory prediction technique may be employed for the terminal to predict the motion state of the highly dynamic terminal.
Disclosure of Invention
In order to solve the above problems, the present invention provides an Aloha enhanced access method for a low-orbit constellation system.
The access method provided by the patent has the following assumed conditions:
(1) the priority of each user accessing the satellite network is consistent with the priority of the transmission service, the user can only transmit the service with one priority at the same time, and the priority of the user is changed along with the type of the transmitted service.
(2) Once a user transmitting low-priority service detects that a user having high-priority service in a channel transmits high-priority service, the low-priority service enters a waiting queue, and the low-priority service cannot be transmitted again until the user transmitting the low-priority service detects an idle spectrum.
(3) When users access the satellite channel, the transmitted services do not interfere with each other, and each priority user can only use a single frequency band to transmit data.
The method comprises the following specific steps:
step one, building a satellite access system model, and classifying various users according to the speed of a communication terminal and dynamic terminals with the same speed type according to the service type;
step two, ensuring the transmission of the high-priority service by adopting a mode of reserving a proper number of channels for the high-priority users, wherein the number of the reserved channels is m (m is less than n), namely the reserved channels are { c1, c2, …, cm };
step three, calculating the corresponding channel maximum load threshold value G according to the access success rate standard of the classified user serviceth；
Step four, the receiving end satellite calculates the real-time channel load G according to the data packet received by the previous time slice, and broadcasts the real-time channel load G to the user terminal with the access requirement;
step five, when the relative priority service arrives and the reserved channel is occupied, making a channel data packet sending strategy based on the priority, and establishing a priority judging function PRO (K, Q, T) ═ alpha0K+α1Q+α2T), dividing the obtained product into n grades according to a preset standard. And considers the call requests of user terminal service access and switching. Determining the value of a data packet sending load threshold and the time slice length of the data packet waiting for sending by using a priority judgment function;
step six, when the user terminal relative reserved channel is occupied and the enhanced contention resolution Aloha access technology is needed, the real-time channel load G of the satellite broadcast and the maximum load threshold value G calculated by the user terminal are usedthComparing, if the real-time load is smaller than the threshold value, turning to the seventh step, and if the real-time load is larger than the load threshold, turning to the fifth step;
step seven, on the premise of dividing various users into two types of priorities of a high dynamic terminal and a low dynamic terminal, generating a corresponding waiting time slice according to the corresponding priority of the user terminal, inserting the data packet to be transmitted into a waiting queue, waiting for transmission, and if the waiting time slice is exhausted and the waiting queue has no data packet to be transmitted with higher priority, turning to step four;
step eight, the user terminal selects a data packet to be sent, codes the data packet, divides a channel into time slots by using a local clock, randomly puts the coded data packet into the time slots, and inserts the position of the time slot into the head of the data packet to send the data packet;
step nine, after receiving the data packet, the LEO covering the user terminal decodes the data packet;
step ten, the satellite processing system sets a sliding time window TwinAnd carrying out data packet integrity detection on the received data packets in the sliding time window, and carrying out interference elimination until all the data packets are successfully decoded in the time window or the maximum iteration number is reached.
Further, in the first step, firstly, according to the speed of the communication terminal, the users are divided into three types of high dynamic terminals, medium dynamic terminals and low dynamic terminals; if the terminal is dynamic, the terminal is not further subdivided, if the terminal is dynamic, the service type of the terminal is continuously judged, and the judgment criterion of the service type is to set a service priority function: the PRO (K, Q,T)＝(α0K+α1Q+α2T)。
wherein, K is an identifier indicating which service class belongs to the voice service, the streaming service, the interactive service and the background service. A Q of 1 indicates a new call type when the previous call is a handover call Q of 0. T represents the distinction of the terminal user belonging to the civil user and the military user, when T is 1, the current calling user is the military user, and when T is 0, the current calling user is the common civil user. Alpha is alpha0α1α2Is the correlation coefficient.
Secondly, in order to maintain the universality of the user terminal accessing the system and simplify the access process of accessing the user terminal, the following assumptions are made for the model:
(1) the user terminal and the uplink channel of the satellite adopt a shared channel model, and the user terminal shares the satellite channel resource to perform random competitive access;
(2) when a user terminal is in communication, the user terminal is covered by two LEOs, communication channels of the satellites are all positioned on the same frequency channel, and when a data packet is sent from one side of the user terminal, the two covered satellites can receive the data packet;
(3) the power of the user terminal for sending the data packets is the same, and the power imbalance effect is simplified.
Further, in the third step, since the larger the channel load is, the higher the probability of collision of the data packet is, the lower the probability of successfully decoding the data packet at the receiving end side is, and the lower the access success rate of the user terminal is. Maximum load threshold value G corresponding to each priority terminalthCorresponding to the lowest access success rate required by each user terminal. As can be seen from the following formula, the access success rate of the ue is related to the channel load, so the ue can calculate the corresponding maximum channel load threshold G according to the access success rate standardthWhen there is an access request, GthIn contrast to the real-time load G of the channel broadcast.
T＝G(1-(1-P)n) (1)
PLR＝1-T/G (2)
Psuccess＝1-PLR (3)
In the formula, T and PLR are system throughput rate and packet loss rate indexes, P represents the probability of successfully obtaining data after iterative interference elimination, n represents the number of the user terminal covered by the satellite, and P represents the number of the user terminal covered by the satellitesuccessIndicating the access success rate of the i-th class user terminal.
Further, in the fourth step, the satellite side calculates the real-time load G according to the data packets received in the previous time slice, where the unit of the time slice is TframeAnd indicates the time length of the dummy frame transmitted by the user terminal.
Further, a specific method for generating the waiting time slice in the step five is as follows:
1) in the case of low dynamic terminals:
X＝min(2*X，CWmax)，G≥Glt (4)
X＝X/2，G＜Glt (5)
in the formula, X represents the waiting time, CWmaxRepresents the maximum value of the waiting window, GlrRepresenting the maximum load threshold of the low dynamic terminal, and G representing the real-time load of the channel.
(1) Setting an initial waiting time t, an upper limit max of the waiting time of the data packet and a maximum value CW of the waiting windowmax. Where t is set to a channel frame length;
(2) in the interval from 0 to the current waiting window value, randomly selecting one number Rand and multiplying the number Rand by the initial waiting time t, wherein the obtained result is the waiting time needed by the data packet transmission of the user terminal: t ═ Rand × T;
(3) when the waiting time of the data packet is over, if G is less than GltWhen the data packet is transmitted normally, the size of the waiting window is reduced by half; if G is greater than or equal to GltThen, doubling the waiting window, randomly selecting a number in the waiting window, and multiplying the number by the initial waiting time to obtain the waiting time;
(4) and when the total waiting time of the data packet is greater than or equal to max, reporting the failure of data packet transmission.
2) In the case of a high dynamic terminal:
and establishing a maximum load threshold matrix, wherein the matrix stores the maximum access loads corresponding to different services.
X＝min(X+1，CWmax)，G≥Gt (6)
X＝1，G＜Gt (7)
In the formula, X represents the waiting time, CWmaxRepresents the maximum value of the waiting window, GtAnd representing the maximum load threshold of the high dynamic terminal, wherein the threshold load value is obtained from the matrix, and G represents the real-time load of the channel.
(1) Setting an initial waiting time t, an upper limit max of the waiting time of the data packet and a maximum value CW of the waiting windowmax. Where t is set to a channel frame length;
(2) in the interval from 0 to the current waiting window value, randomly selecting one number Rand and multiplying the number Rand by the initial waiting time t, wherein the obtained result is the waiting time needed by the data packet transmission of the user terminal: t ═ Rand × T;
(3) when the waiting time of the data packet is over, if G is less than GtWhen the data packet is transmitted normally, the size of a waiting window is set to be 1; if G is greater than or equal to GtThen, adding 1 to the waiting window, randomly selecting a number in the window, multiplying the number by the initial waiting time, and waiting;
(4) and when the total waiting time of the data packet is greater than or equal to max, reporting the failure of data packet transmission.
Further, the specific steps of the seventh and eighth steps for the user to send the data packet are as follows:
when a data packet is to be sent, the user terminal divides a channel into a plurality of time slots by using a local clock, and randomly places the data packet to be sent in the time slots. Wherein the time slots form a virtual frame structure, each virtual frame consisting of NslotEach time slot is formed by TslotThe duration of the whole virtual frame structure is Tframe＝Nslot*Tslot. And assumes that the duration of one slot is equal to the duration of one packet.
Further, the iterative interference cancellation in the step ten specifically includes the following steps:
(1) the system first sets a sliding time window value TwinAnd the maximum number of iterations Niter. The sliding window value should be larger than the maximum propagation delay of the data packet, and at least one T should be covered in the time windowframe。TwinHas a starting value of TstartThe end value is Tend＝Tstart+Twin；
(2) After receiving LEO retransmission data packet message, the low earth orbit satellite system transmits the data packet message in a time window TwinDecoding and data packet integrity detection are carried out, the same data packet of the data packet on the rest virtual frames is eliminated by utilizing the position information of the detected head part of the complete data packet, and the interference elimination step is repeated until the T is successfully decodedwinAll data packets or the number of repetitions in the packet reach the maximum number of iterations NiterUntil the end;
(3) after the data packet in the window is processed, the window is slid by delta T, then iterative interference elimination is carried out again, and the starting and stopping position of the sliding window is [ T ]start+ΔT，Tend+ΔT]；
(4) And stopping sliding the window until no new data packet exists, and finishing the iterative interference elimination.
When the data packets interfere with each other pairwise and the ring effect appears, the invention utilizes the merging technology in ECRA to eliminate the ring effect caused by the mutual interference of the data packets pairwise. If the merging effect can eliminate the ring effect, the data packet is successfully decoded, and the number of current iteration interference elimination rounds does not reach the preset NiterThen the interference cancellation in the current time window will continue.
In conclusion, the positive effects and advantages of the invention are as follows:
(1) by utilizing the characteristic that the user terminal can be covered by a plurality of satellites in the communication process, iterative interference elimination at one side of a satellite receiving end is improved, and the throughput rate and the packet loss rate of the system are optimized. Through simulation analysis, the peak throughput rate of the proposed method can reach 1.1bit/symbol and is superior to 0.7bit/symbol of ECRA under the condition of not considering power imbalance.
(2) And priority access control is introduced, and when the high dynamic terminal has access requirements, the success rate of the first access of the high dynamic terminal is optimized. The simulation analysis shows that the first access success rate of the high dynamic terminal can be maintained above 99%, and the problem that the access success rate is reduced under the condition that the data packet arrival rate of the ECRA is too high is solved.
Drawings
Fig. 1 is a schematic diagram of a low earth constellation satellite system architecture according to an embodiment of the present invention;
fig. 2 is a flow chart of an enhanced contention resolution Aloha-based access method according to the present invention;
fig. 3 is a schematic diagram of a low-orbit constellation access system model provided in an embodiment of the present invention;
fig. 4 is a schematic diagram of throughput rate index of an access method provided in an embodiment of the present invention;
fig. 5 is a schematic diagram of a packet loss rate indicator of an access method according to an embodiment of the present invention;
fig. 6 is a schematic diagram comparing throughput indexes provided by the embodiment of the present invention with those of the existing access technology;
fig. 7 is a schematic diagram illustrating a comparison between packet loss indicators in the present invention and those in the prior art;
fig. 8 is a schematic diagram illustrating a comparison between a high dynamic terminal access success rate index provided by an embodiment of the present invention and a high dynamic terminal access success rate index in the prior art.
Detailed Description
The invention is described in further detail below with reference to the accompanying drawings.
The invention mainly aims at the scene of communication between a high dynamic terminal and a low orbit constellation system in various user terminals, and when an access requirement exists on one side of the user terminal, the type of the accessed service is firstly analyzed, then the service is judged to be a new call or a switching call, and a positioning device is utilized to predict whether the service needs to be switched at the later moment. Calculating the specific time for sending the data packet by using the priority function of access control; when the data packet is sent, the user terminal divides the channel into time slots by using a local clock, and randomly puts the data packet into the time slots for sending. On one side of a satellite receiving end, serial interference elimination is carried out through a plurality of satellites by utilizing the characteristic that a user terminal is covered by a plurality of satellites in the communication process, and the covering satellites can receive data packets sent by the user terminal, and decoding is carried out by utilizing a set sliding time window and adopting an improved serial iterative interference elimination (MSIC) technology. If the ring effect is generated, a selective combination technology is introduced to eliminate the ring effect. The method is better than ECRA in the aspects of throughput rate and packet loss rate index, and optimizes the first access success rate of the high dynamic terminal.
The model of the low-earth constellation satellite system architecture is shown in fig. 1, and the present invention mainly relates to the access portion of the low-earth constellation system.
As shown in fig. 2, the specific steps are as follows:
step one, establishing a satellite access system model, and classifying various users according to the speed of a communication terminal;
and dividing users into high dynamic terminals and low dynamic terminals according to the communication rate of the terminals. The method comprises the steps that users with the terminal speed being greater than Mach 5 are divided into high-dynamic high-priority terminals, and users with the terminal speed being less than Mach 5 are divided into low-dynamic common-priority terminals. And further subdividing the service types of the high-dynamic satellites. The satellite services are specifically classified into voice services, streaming services, interactive services, and background services. Meanwhile, the track prediction can be carried out on the terminal service through the positioning equipment in the terminal equipment, and then the condition that the terminal service belongs to a newly generated call or has a switching requirement is judged.
As shown in fig. 3, a user terminal is typically covered by two or more LEO satellites. The user terminal is accessed to the low earth orbit constellation satellite system through the LEO, and the long-distance communication is realized. To maintain the versatility of accessing the user terminal to the system and simplify the access procedure for accessing the user terminal, the following assumptions are made for the model:
(1) the user terminal and the uplink channel of the satellite adopt a shared channel, an uplink communication system based on multi-beam, frequency sharing and multi-carrier is adopted, and the user terminal shares satellite channel resources and carries out random competitive access; the downlink adopts a downlink communication system based on 'beam hopping + time slicing + single carrier'.
(2) When a user terminal is in communication, the user terminal is covered by two LEOs, communication channels of the satellites are all positioned on the same frequency channel, and when a data packet is sent from one side of the user terminal, the two covered satellites can receive the data packet;
(3) the power of the user terminal for sending the data packets is the same, and the power imbalance effect is simplified.
Step two, calculating a corresponding channel maximum load threshold value G according to the access success rate standard of the users after the priority classificationth；
Step three, the receiving end satellite calculates the real-time channel load G according to the data packet received by the previous time slice, and broadcasts the real-time channel load G to the user terminal with the access requirement;
and step four, when the user terminal has an access requirement, analyzing the priority of the service needing to be accessed, judging whether an idle reserved channel exists, if so, directly accessing, and carrying out backup recording. If the idle channel is full, the real-time channel load G of the satellite broadcast and the maximum load threshold value G calculated by the user terminal are usedthComparing, if the real-time load is smaller than the threshold value, turning to the step six, and if the real-time load is larger than the load threshold, turning to the step five;
generating a corresponding waiting time slice according to the corresponding priority of the user terminal on the premise of successfully distinguishing various services, inserting the data packet to be transmitted into a waiting queue, and waiting for transmission, wherein if the waiting time slice is exhausted and no data packet to be transmitted with higher priority exists in the waiting queue, the fourth step is carried out;
step six, the user terminal selects the data packet to be sent, codes the data packet, divides the channel into time slots by using a local clock after coding, and randomly places the coded data packet into the time slots for sending;
seventhly, decoding the data packet after the low orbit satellite covering the user terminal receives the data packet;
step eight, setting a sliding time window T for the low-orbit satellitewinReceiving data packet in sliding time windowAnd carrying out data packet integrity detection and interference elimination until all data packets are successfully decoded in a time window or the maximum iteration number is reached.
The concrete steps in the step eight are as follows:
(1) the system first sets a sliding time window value TwinAnd the maximum number of iterations Niter. The sliding window value should be larger than the maximum propagation delay of the data packet, and at least one T should be covered in the time windowframe。TwinHas a starting value of TstartThe end value is Tend＝Tstart+Twin；
(2) After receiving the message of the forwarding data packet, in the time window TwinDecoding and data packet integrity detection are carried out, the same data packet of the data packet on the rest virtual frames is eliminated by utilizing the position information of the detected head part of the complete data packet, and the interference elimination step is repeated until the T is successfully decodedwinAll data packets or the number of repetitions in the packet reach the maximum number of iterations NiterUntil the end;
(3) after the data packet in the window is processed, the window is slid by delta T, then iterative interference elimination is carried out again, and the starting and stopping position of the sliding window is [ T ]start+ΔT，Tend+ΔT]；
(4) And stopping sliding the window until no new data packet exists, and finishing the iterative interference elimination.
When the data packets interfere with each other pairwise and the ring effect appears, the invention utilizes the merging technology in ECRA to eliminate the ring effect caused by the mutual interference of the data packets pairwise. If the merging effect can eliminate the ring effect, the data packet is successfully decoded, and the number of current iteration interference elimination rounds does not reach the preset NiterThen the interference cancellation in the current time window will continue.
The effects of the present invention will be described in detail below in conjunction with simulations.
The simulation conditions were as follows:
low orbit constellation system LEO constellation refers to iridium network topological structure, and each user terminal is assumedBoth ends are simultaneously covered by two LEOs in the low-orbit constellation system. The user terminal service information source obeys poisson distribution with parameter lambda. The length of the data packet is 100 bits, the length of the virtual data frame is 10000 bits, and each frame time slot can be used for placing one data packet. The data packet is transmitted after QPSK modulation, and the channel coding rate R is 1/2. The satellite channel simulates an additive white gaussian noise channel. The user terminal types are classified into a high dynamic terminal, a medium dynamic terminal, and a low dynamic terminal. Maximum iteration number N of iterative interference cancellation at receiving end sideiterIs 14.
And (3) simulation result analysis:
(1) simulation and comparison of throughput rate and packet loss rate performance
First, we simulate the method without access control. And counting the throughput rate and packet loss rate curves of the access method in the normalized load interval of 0-2bit/symbol, as shown in fig. 4 and 5.
As can be seen from fig. 4, when the normalized load is in the interval of [0-1.1bit/symbol ], the throughput of the whole system increases approximately linearly, which can indicate that under this condition, the data packet sent by the user terminal side can be completely decoded after reaching the receiving end side of the low-orbit constellation system. After the normalized load exceeds 1.0bit/symbol, the probability of collision of the data packets in the channel is increased with the further increase of the load, and the throughput rate of the system is reduced after slowly reaching the peak value. The proposed access method will reach peak throughput at a normalized load of 1.2bit/symbol, at which time the throughput may approach 1.1 bit/symbol.
As can be seen from fig. 5, when the normalized load is less than 0.85bit/symbol, the packet loss rate of the system is less than 10-2When normalized load is less than 1.2bit/symbol, the packet loss rate of the system is less than 10-1. The normalized load of radix Angelicae sinensis is in the range of 0-0.85bit/symbol]In time, the packet loss rate of the system rises slowly. When the normalized load exceeds 1.0bit/symbol, the packet loss rate of the system is increased. The simulation result proves that the probability of the collision of the data packets sent by the user terminal is higher as the channel load is increased.
Next, we compare the access method proposed by the present invention with CRDSA, Irregular Repeated Slotted Aloha (IRSA), CRA, and ECRA in simulation. And counting the throughput rate and packet loss rate curves of each access method in the interval with the normalized load of 0-1.4bit/symbol, as shown in fig. 6 and 7.
As can be seen from the throughput comparison curve in fig. 6, when the normalized load is low, the throughput index of the access method in the graph shows a linear increasing trend along with the normalized load, and when the normalized load of the system continues to increase to a certain extent, the throughput of the access method in the graph shows a decreasing trend. In the access method provided by the invention, when the load reaches 1.2bit/symbol, the throughput rate of the system has a nonlinear descending trend, which is superior to CRDSA, IRSA, CRA and ECRA adopting a selective combining technology, and the peak throughput rate of the system is highest. Therefore, the effectiveness of the access method in improving the system access throughput rate can be proved.
As can be seen from the packet loss ratio comparison curve in fig. 7, when the normalized load is low, the packet loss ratio of the access method in the graph is always less than 10-2When normalized load continues to increase after reaching a certain degree, the packet loss rate of the access method in the graph will continue to increase. If the packet loss rate is controlled below 0.1, the normalized load of the access method provided by the invention needs to be controlled to be 1.2bit/symbol, which is superior to 0.5bit/symbol of CRDSA and 0.6bit/symbol of ECRA adopting selective combining technology.
(2) High dynamic terminal first access success rate simulation and comparison
The simulation statistics is carried out on the first access success rate index of the high-dynamic terminal under the condition that the access arrival rate of the user terminal is continuously increased under the access method, the CRDSA, the IRSA, the CRA and the ECRA. As shown in fig. 8.
As can be seen from the access success rate comparison curve in fig. 8, as the access arrival rate of the ue increases, the access method added for access control can maintain the first access success rate of the high dynamic ue at more than 99%, and the first access success rate indexes of the remaining access methods decrease gradually as the access arrival rate of the ue increases. The priority access control provided by the invention can enable some low dynamic terminals to suspend accessing the low orbit satellite constellation system under the condition of higher load, reduce the collision probability of data packets in a channel and optimize the success rate of the first access of the high dynamic terminals. Therefore, the access method can be proved to optimize the first success rate of the access of the high dynamic terminal, and further ensure the high reliable and quick access of the high dynamic terminal.
Although the preferred embodiments of the present invention have been described in detail, the present invention is not limited to the specific embodiments, and modifications and equivalents within the scope of the claims may be made by those skilled in the art and are included in the scope of the present invention.
Claims (8)
1. An Aloha enhanced access method for a low-orbit constellation system is characterized by comprising the following specific steps:
establishing a satellite access system model, dividing various users into high, medium and low dynamic terminals according to the speed of a communication terminal, dividing the users into voice services, stream services, interactive services and background services according to the types of supported services, and predicting the track of the terminal services by positioning equipment in terminal equipment so as to judge whether the terminal services belong to newly generated calls or have switching requirements;
step two, ensuring the transmission of the high-priority service by adopting a mode of reserving a proper number of channels for the high-priority users, wherein the number of the reserved channels is m (m is less than n), namely the reserved channels are { c1, c2, …, cm };
step three, calculating the corresponding channel maximum load threshold value G according to the access success rate standard of the classified user serviceth；
Step four, the receiving end satellite calculates the real-time channel load G according to the data packet received by the previous time slice, and broadcasts the real-time channel load G to the user terminal with the access requirement;
and step five, when the reserved channel is occupied after the related priority service arrives, establishing a channel data packet sending strategy based on the priority, establishing a priority judgment function PRO (K), and dividing the priority judgment function PRO (K) into n levels according to a preset standard. And considers the call requests of user terminal service access and switching. Determining the value of a data packet sending load threshold and the time slice length of the data packet waiting for sending by using a priority judgment function;
step six, when the user terminal relative reserved channel is occupied and the enhanced contention resolution Aloha access technology is needed, the real-time channel load G of the satellite broadcast and the maximum load threshold value G calculated by the user terminal are usedthComparing, if the real-time load is smaller than the threshold value, turning to the seventh step, and if the real-time load is larger than the load threshold, turning to the fifth step;
step seven, on the premise of dividing various users into two types of priorities of a high dynamic terminal and a low dynamic terminal, generating a corresponding waiting time slice according to the corresponding priority of the user terminal, inserting the data packet to be transmitted into a waiting queue, waiting for transmission, and if the waiting time slice is exhausted and the waiting queue has no data packet to be transmitted with higher priority, turning to step four;
step eight, the user terminal selects a data packet to be sent, codes the data packet, divides a channel into time slots by using a local clock, randomly puts the coded data packet into the time slots, and inserts the position of the time slot into the head of the data packet to send the data packet;
step nine, after receiving the data packet, the LEO covering the user terminal decodes the data packet;
step ten, the satellite processing system sets a sliding time window TwinAnd carrying out data packet integrity detection on the received data packets in the sliding time window, and carrying out interference elimination until all the data packets are successfully decoded in the time window or the maximum iteration number is reached.
2. The Aloha enhanced access method for the low-orbit constellation system according to claim 1, wherein the first step specifically is: dividing users into three types of high, medium and low dynamic terminals according to the speed of the communication terminal, and continuously subdividing the high dynamic terminal according to the different types of the service flow; in order to keep the versatility of accessing the user terminal to the system and simplify the access process of accessing the user terminal, the following assumptions are made:
(1) the user terminal and the uplink channel of the satellite adopt a shared channel, an uplink communication system based on multi-beam, frequency sharing and multi-carrier is adopted, and the user terminal shares satellite channel resources and carries out random competitive access; the downlink adopts a downlink communication system based on 'beam hopping + time slicing + single carrier';
(2) when a user terminal is in communication, the user terminal is covered by two LEOs, communication channels of the satellites are all positioned on the same frequency channel, and when a data packet is sent from one side of the user terminal, the two covered satellites can receive the data packet;
(3) the power of the user terminal for sending the data packets is the same, and the power imbalance effect is simplified.
The following conditions are also guaranteed during access:
(1) the priority of each user accessing the satellite network is consistent with the priority of the transmission service, the user can only transmit the service with one priority at the same time, and the priority of the user is changed along with the type of the transmitted service;
(2) once a user transmitting low-priority service detects that a user transmitting high-priority service in a channel transmits high-priority service, the low-priority service enters a waiting queue, and the low-priority service can not be transmitted again until the user transmitting the low-priority service detects that an idle frequency spectrum is detected;
(3) when users access the satellite channel, the transmitted services do not interfere with each other, and each priority user can only use a single frequency band to transmit data.
3. The Aloha enhanced access method for the low-orbit constellation system according to claim 1, wherein the second step specifically comprises: and ensuring the transmission of the high-priority service by reserving a proper number of channels for the high-priority users, wherein the number of the reserved channels is m (m is less than n), namely the reserved channels are { c1, c2, …, cm }. The strategy employs a dynamic channel reservation strategy that varies according to different priority levels. When the terminal service flow arrives, the reserved channel is accessed preferentially, the reserved channel numbers with different priorities are stored through a matrix, and the matrix element value which can reach the global optimal solution can be obtained by utilizing convex optimization. The contention access scheme using the Aloha enhanced access method is considered when the reserved channel is already fully occupied. Simulation results and relevant theoretical analysis can prove that the channel utilization rate can be improved and the channel blocking rate can be reduced by adopting a properly combined channel reservation strategy.
4. The Aloha enhanced access method for the low-orbit constellation system according to claim 1, wherein the third step specifically is: as can be seen from the following formula, the access success rate of the ue is related to the channel load, so the ue can calculate the corresponding maximum channel load G according to the user's motion state, the supported service type, and whether the subsequent handover is requiredthAnd comparing with the real-time load G of the satellite broadcast; wherein, P represents the probability of successfully obtaining data after iterative interference elimination, n represents the number of the user terminal covered by the satellite, T and PLR are system throughput rate and packet loss rate indexes, PsuccessIndicating the access success rate of the user terminal.
T＝G(1-(1-P)n)
PLR＝1-T/G
Psuccess＝1-PLR 。
5. The Aloha enhanced access method for the low-orbit constellation system according to claim 1, wherein the fourth step specifically is: the satellite side calculates the real-time load according to the data packets received by the last time slice, wherein the unit of the time slice is TframeAnd indicates the time length of the dummy frame transmitted by the user terminal.
6. The Aloha enhanced access method for the low-orbit constellation system according to claim 1, wherein the fifth step specifically is: the system divides the users into high dynamic terminal and low dynamic terminal according to the speed of the communication terminal, and then the high dynamic terminal is accessedThe lines calculate the value PRO (K, Q, T) of the corresponding priority function (alpha) according to the subdivision of the corresponding priority0K+α1Q+α2T), access control is performed, and as shown in the following formula, a slot is generated according to the corresponding priority to perform backoff waiting, where X represents the waiting time and CWmaxRepresents the maximum value of the waiting window, GlrRepresents the maximum load threshold, G, of a low dynamic terminaltRepresenting the maximum load threshold of the high dynamic terminal, which is determined by a priority function, and G represents the real-time load of the channel.
Under low dynamic terminal conditions:
X＝min(2*X，CWmax)，G≥Glt
X＝X/2，G＜Glth
under high dynamic terminal conditions:
X＝min(X+1，CWmax)，G≥Gt
X＝1.G＜Ghth。
7. the Aloha enhanced access method for the low-orbit constellation system according to claim 1, wherein the seventh step specifically is: the user terminal divides the channel into time slots by using a local clock, and randomly puts the data packet to be sent into the time slot to send the data packet under the assumption that the length of the data packet is consistent with the length of the time slot. Wherein the time slots constitute virtual frames, each virtual frame consisting of NslotEach time slot is formed by TslotThe duration of the virtual frame is Tframe＝Nslot*Tslot。
8. The Aloha enhanced access method for the low-orbit constellation system according to claim 1, wherein the eight and nine steps are specifically: combining the satellite coverage user model in the step one, assuming that each user terminal is covered by two LEOs when communicating with the low-orbit satellite constellation system, and when the user terminal sends a data packet, the two coverage satellites can detect the data packet; after receiving and detecting the virtual frame message sent by the user terminal, the two LEOs forward the data packet message to the Tianji information Port. The following interference elimination implementation steps are as follows:
(1) the system first sets a sliding time window value TwinAnd the maximum number of iterations NiterThe sliding window value should be larger than the maximum propagation delay of the data packet, and the time window should cover at least one Tframe，TwinHas a starting value of TstartThe end value is Tend＝Tstart+Twin；
(2) After the low earth orbit satellite constellation system receives the forwarding data packet message, the time window T is carried outwinDecoding and data packet integrity detection are carried out, the same data packet of the data packet on the rest virtual frames is eliminated by utilizing the position information of the detected head part of the complete data packet, and the interference elimination step is repeated until the T is successfully decodedwinAll data packets or the number of repetitions in the packet reach the maximum number of iterations NiterUntil the end;
(3) after the data packet in the window is processed, the current window is slid by delta T, then the interference elimination is carried out again, and the starting and stopping position of the sliding window is [ T ]start+ΔT，Tend+ΔT]；
(4) And stopping sliding the window until no new data packet exists, and finishing interference elimination.
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