Aloha network transmission method for optimizing information age

Abstract

The invention discloses an Aloha network transmission method for optimizing information age, and the method comprises the following steps: S1, modeling a queuing system into a dual-server queue system for a time slot-based V2X Aloha network limited by limited retry; s2, representing the average AoI of the dual-server queue system; s3, on the basis of Markov modeling of the queue head state, representing the average service time and the steady-state probability of successfully transmitting the data packet; and S4, selecting the node transmission probability and retransmission times of the minimum AoI. According to the method, the randomly generated data packets are utilized, the buffer area is large, any number of data packets can be accommodated, and the average AoI performance is improved by adopting limited retry times.

Description

Technical Field

[0001] The present invention relates to the technical field of wireless communication networks, and in particular to an Aloha network transmission method for optimizing information age. Background Art

[0002] With the rapid popularization of vehicles, vehicle-to-everything (V2X) communication has received extensive research attention. In V2X communication, vehicles are usually equipped with sensors to communicate with everything around them. Cellular systems (such as 5G systems) are considered to be a simple and scalable solution to facilitate V2X communication, and the so-called cellular-V2X communication is widely used in various fields such as intelligent transportation systems. For intelligent transportation systems, traffic safety information is time-critical and fresh information should be updated in a timely manner. In order to measure the freshness of updated information, the age of information (AoI) indicator is widely adopted, where the AoI of a data packet is defined as the time after the most recently received data packet was generated. In order to ensure that the vehicle's updates are fresh enough, the average AoI of the data packet should be small.

[0003] Some prior art approaches improve the average AoI performance by implementing arbitrary generation strategies, limited retry limit strategies with single buffer nodes, and unlimited retry limit strategies with single buffer nodes. In these strategies, the waiting process of each node buffer is assumed to be instantaneous, so the waiting time is zero. However, in the considered Aloha network, this process cannot be ignored. In the Aloha network with retry limit, the behavior of the head of queue (HOL) packets is different from the behavior of packets in arbitrary generation strategy and threshold-based strategy. Therefore, a special Markov chain is required to accurately model the behavior of HOL packets. When the retry limit and buffer size are infinite, the average AoI can be obtained by considering the queue processing and deriving the probability mass function from the probability generating function (PGF) of the service time. However, in the Aloha network with limited retry limit and unlimited buffer, the same method to calculate the PGF of the service time becomes complicated. The reason for this complexity is that the queue processing in the system is intricate and packet loss occurs, which is more complicated than the situation in the Aloha network. Therefore, obtaining the average AoI for an Aloha network with bounded retry limits and unlimited buffers is a significant challenge. Summary of the invention

[0004] In order to solve the problem that the prior art cannot effectively obtain the average AoI of an Aloha network with limited retry limits and unlimited buffers, the present invention proposes an Aloha network transmission method for optimizing information age to solve the above problem.

[0005] The present application discloses an Aloha network transmission method for optimizing information age, comprising the following steps:

[0006] S1. For the slotted V2X Aloha network with limited retry constraints, the queuing system is modeled as a dual-server queue system;

[0007] S2, characterizes the average AoI of the dual-server queue system modeled in S1;

[0008] S3, Markov modeling based on queue head state, characterizes the average service time and steady-state probability of successful packet transmission;

[0009] S4. Based on the obtained characterization information, the node transmission probability and the number of retransmissions that minimize the AoI are selected.

[0010] Preferably, the S1 comprises the following steps:

[0011] In a slotted V2X Aloha network, n vehicles send information to one receiver, assuming that each vehicle node has a Bernoulli arrival process with parameter λ, where λ is the input rate of each vehicle node, defined as the long-term average number of packets arriving at each vehicle node buffer per time slot;

[0012] In each time slot, as long as the buffer is not empty, each vehicle node will send its HOL data packet with probability q∈(0,1], and all HOL data packets are provided in the order of arrival, that is, first come first served;

[0013] At the receiver, the classic collision model is adopted, that is, the HOL packet can be successfully received if and only if there is no concurrent transmission from other nodes;

[0014] Each vehicle node adopts a retransmission limit M, that is, the HOL data packet will be discarded only when the number of unsuccessful transmissions reaches M. When M is infinite, the HOL data packet always stays in the vehicle node queue until the transmission is successful. When M is a finite value, the HOL data packet of each vehicle node is discarded with a certain probability;

[0015] The queue of each vehicle node is regarded as a two-server queue system, where the probability of each HOL packet being dropped in each time slot is p d , otherwise the probability of successful transmission is 1-p d .

[0016] Preferably, S2 comprises the following steps:

[0017] For a two-server queue system, if The average AoI is characterized as:

[0018]

[0019] Among them, λs =λ(1-p d ) is the input rate of successfully sent HOL packets, is the average service time of a HOL data packet, is the average service time of a successfully sent HOL packet, function Service time D T The probability generating function of The second derivative of at z=1.

[0020] Preferably, the Markov modeling based on the queue head state comprises the following steps:

[0021] A discrete-time Markov chain is built for each vehicle node to simulate the behavior of each HOL packet:

[0022] A new HOL packet is initially in state T. If the transmission is successful, the HOL packet will stay in state T for one time slot. If the transmission is paused, the HOL packet will enter state R0. If the transmission fails, the HOL packet will enter state R1.

[0023] For the state R i (0≤i≤M-2) HOL packets, if the transmission is suspended, the HOL packets will be in state R i If the transmission is successful, the HOL packet will go to state T; if the transmission fails, the HOL packet will go to state R i+1 ;

[0024] For the state R M-1 As long as the transmission probability reaches q, the HOL packet will move to the initial state T because the number of transmission attempts has reached the retry limit M.

[0025] Preferably, characterizing the average service time and the steady-state probability of successfully transmitting a data packet comprises the following steps:

[0026] Service time D T The probability generating function of is:

[0027]

[0028] Probability Generating Function The second-order derivative of is:

[0029]

[0030] Among them, z is the input of the probability generation function, p is the probability of successfully transmitting a HOL data packet, and q is the node transmission probability;

[0031] The average service time is characterized by:

[0032]

[0033] The average service time of successfully transmitted HOL packets is characterized by:

[0034]

[0035] The steady-state probability of successfully transmitting a data packet is characterized as:

[0036]

[0037] Substituting equations (2) to (6) into equation (1), we can get the exact average AoI.

[0038] Preferably, S4 comprises the following steps:

[0039] According to the monotonicity of AoI with respect to the transmission probability q, the node transmission probability of AoI is set to:

[0040]

[0041] Among them, p S is the solution representing the intermediate value of the steady-state probability p of successfully transmitting a data packet;

[0042] Minimizing the number of retransmissions is to make the maximum retry limit M that has a solution to equation (6). The specific value of the retry limit M is found through the bisection method.

[0043] Beneficial effects of the present invention:

[0044] (1) The present invention is different from previous work on average AoI of Aloha network in V2X communication. It uses randomly generated data packets, has a large buffer that can accommodate any number of data packets, and uses a limited number of retries to improve the average AoI performance.

[0045] (2) The present invention considers V2X communication as a dual-server queue system and derives the average AoI of the general dual-server queue system as the probability generation function (PGF) of the service time. In order to further determine the PGF of the service time, a special Markov chain is constructed to simulate the behavior of HOL packets that are subject to a bounded retry limit strategy, which follows different rules compared to the arbitrary generation strategy and the threshold-based strategy.

[0046] (3) The present invention uses a special Markov chain to determine the PGF of the service time and the steady-state point. Compared with the bistable Aloha network with infinite retry limit, the V2X Aloha network with limited retry limit has two or three steady-state points. According to the PGF of the steady-state point and the service time, the expression of the average AoI of the V2X Aloha network with retry limit is further obtained, and the optimal transmission scheme is obtained based on the expression.

[0047] (4) When the input rate is small, the transmission scheme proposed in the present invention is better than the Aloha network with arbitrary generation strategy. In addition, when the packet loss probability is small, the transmission scheme proposed in the present invention is better than the Aloha network based on threshold retention, indicating that the Aloha network with retry limit is more suitable for cellular-V2X communication with the best AoI and packet loss probability trade-off. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 A schematic diagram of the flow of an Aloha network transmission method for optimizing information age according to an embodiment of the present invention;

[0049] Figure 2 A schematic diagram of a dual-server system of a time-slot V2X Aloha network with limited retry restrictions according to an embodiment of the present invention;

[0050] Figure 3 A state transition diagram of a HOL data packet in a slotted Aloha network with retransmission restriction according to an embodiment of the present invention;

[0051] Figure 4 A schematic diagram of the relationship between the minimum average age, the maximum throughput and the total input rate according to an embodiment of the present invention;

[0052] Figure 5 A schematic diagram of the relationship between the maximum throughput and the total input rate of an embodiment of the present invention;

[0053] Figure 6 Schematic diagram of the relationship between the minimum average AoI and the packet loss probability according to an embodiment of the present invention. DETAILED DESCRIPTION

[0054] In order to make the objectives, technical solutions and advantages of the present application more clearly understood, the present application is further described in detail below with reference to the accompanying drawings and examples.

[0055] The embodiment of the present application discloses an Aloha network transmission method for optimizing information age, which is aimed at a random access network based on slotted V2X Aloha with limited retry limit, and aims to solve the following problems: for a given retry limit M, how to adaptively adjust system parameters to optimize the average AoI performance of the random access network; and how the optimal average AoI performance varies with the limited retry limit M. Specifically, by modeling the behavior of each head-of-line data packet as a Markov chain, the network steady-state probability of successful transmission can be obtained. Existing analysis shows that for a slotted V2X Aloha network M with limited retry limit, the network either has three steady-state points or one steady-state point. On this basis, it is concluded that the average AoI of the slotted V2X Aloha network is a function of the transmission probability of each vehicle node, the packet input rate of each vehicle node, and the retry limit M, and is proved to be a monotonically decreasing function of the transmission probability.

[0056] like Figure 1 As shown, in one embodiment, the steps specifically include:

[0057] S1. For the slotted V2X Aloha network with bounded retry constraints, the queuing system is modeled as a dual-server system.

[0058] In a slotted V2X Aloha network, n vehicles send information to one receiver. It is assumed that each vehicle node has a Bernoulli arrival process with parameter λ, where λ is the input rate of each vehicle node, defined as the long-term average number of packets arriving at each vehicle node buffer in each time slot. Each vehicle node is equipped with a buffer to store received packets. In each time slot, as long as the buffer is not empty, each vehicle node sends its HOL packet with probability q∈(0,1], and all HOL packets are provided in the order of arrival, that is, first come first served (FCFS). At the receiver, the classic collision model rule is adopted, that is, the HOL packet can be successfully received if and only if there is no concurrent transmission from other nodes.

[0059] Each vehicle node adopts a retransmission limit M, that is, the HOL packet will be discarded only when the number of unsuccessful transmissions reaches M. When M is infinite, the HOL packet always stays in the vehicle node queue until the transmission is successful. When M is a finite value, the HOL packet of each vehicle node is discarded with a certain probability, where the probability of each HOL packet being discarded in each time slot is p d , otherwise the probability of successful transmission is 1-p d . Considering the queue of each vehicle node as a dual-server queue system, each vehicle node can be modeled as a dual-server queue system, such as Figure 2 shown.

[0060] S2. Characterize the average AoI of the dual-server queue system modeled in S1.

[0061] Based on queuing, for a two-server queue system, if The average AoI is characterized as:

[0062]

[0063] Among them, λ s =λ(1-p d ) is the input rate of successfully sent HOL packets, is the average service time of a HOL data packet, is the average service time of a successfully sent HOL packet, function Service time D T The probability generating function of The second derivative of at z=1.

[0064] S3. Markov modeling based on the queue head state characterizes the average service time and the steady-state probability of successfully transmitting data packets.

[0065] A discrete-time Markov chain is established for each vehicle node to simulate the behavior of each HOL data packet. Figure 3 As shown, a new HOL data packet is initially in state T. If the transmission is successful, the HOL data packet will stay in state T for one time slot; if the transmission is suspended, the HOL data packet will enter state R0; if the transmission fails, the HOL data packet will enter state R1.

[0066] For the state R i (0≤i≤M-2) HOL packets, if the transmission is suspended, the HOL packets will be in state R i If the transmission is successful, the HOL packet will go to state T; if the transmission fails, the HOL packet will go to state R i+1 .

[0067] For the state R M-1 As long as the transmission probability reaches q, the HOL packet will move to the initial state T because the number of transmission attempts has reached the retry limit M.

[0068] according to Figure 3 The state transition diagram shown can be analyzed to obtain the service time D T The probability generating function of is:

[0069]

[0070] Probability Generating Function The second-order derivative of is:

[0071]

[0072] Among them, z is the input of the probability generation function, p is the probability of successfully transmitting a HOL data packet, and q is the node transmission probability.

[0073] The average service time is characterized by:

[0074]

[0075] The average service time of successfully transmitted HOL packets is characterized by:

[0076]

[0077] The steady-state probability of successfully transmitting a data packet is characterized as:

[0078]

[0079] Substituting equations (2) to (6) into equation (1), we can obtain the exact average AoI for M, λ, and q.

[0080] S4. Based on the obtained characterization information, the node transmission probability and the number of retransmissions that minimize the AoI are selected.

[0081] According to the monotonicity of AoI with respect to the transmission probability q, the node transmission probability of AoI is set to:

[0082]

[0083] Among them, p S is a solution that characterizes the intermediate values ​​of the steady-state probability p of successfully transmitting a data packet.

[0084] Minimizing the number of retransmissions is to make the maximum retry limit M that has a solution to equation (6). The specific value of the retry limit M is found through the bisection method.

[0085] In a specific embodiment, the first-come, first-served, limited retry limit (FCFS FRL) strategy proposed in this application is compared with four existing strategies, including the CFS unlimited retry limit (IRL) strategy, the last-in, first-out (LCFS) FRL strategy, the pure on-demand generation (GW) strategy, and the threshold-based strategy. In the pure GW strategy, each node is equipped with a single packet buffer and transmits it immediately after generating a new packet. The pure GW strategy corresponds to the case where M=1 and q=1 in the embodiment of the present application.

[0086] If the transmission discipline of the Aloha network is LCFS, the newly arrived packets will be prioritized and served first. In the FCFS FRL and LCFS FRL policies, the retry limit is set to the optimal retry limit M. * , as shown below:

[0087]

[0088] in, is the minimum average age.

[0089] Figure 4 Shows how the minimum average age (AoI) of different strategies varies with the total input rate of the node Changes. Figure 4 , the average AoI performance of the FCFS FRL strategy is the same as that of the LCFS FRL strategy. This is because when When q is small, the impact of service priority is not significant and q = 1. When is larger, the optimal M of the LCFS FRL policy is obtained under the condition of M = 1 and q = 1, which is exactly equivalent to M = 1 and q = 1 of the FCFS FRL policy. It should be noted that the LCFS policy may disrupt the order of data packets received by the receiver. Therefore, this embodiment focuses on the performance of the FCFS FRL policy. When , the minimum average AoI of the FCFSFRL strategy is smaller than that of the pure GW strategy. Therefore, in order to optimize the average AoI when the total traffic input rate of the node is small, a larger retry limit is required to store incoming packets to avoid dropping too many packets. In particular, when The IRL strategy is optimal when When , the FRL strategy is optimal. On the other hand, when the total input rate of the nodes is large, the minimum average AoI of the FRL strategy is similar to that of the pure GW strategy.

[0090] Figure 5 shows how the maximum throughput of different strategies varies with the total input rate of the node Changes. Figure 5 ,The maximum throughput performance of the FCFSFRL policy is the same as that of the FCFS IRL policy. That is, to obtain the optimal throughput, the optimal retry limit is always infinite, while the optimal retry limit that minimizes the average AoI is a finite value.

[0091] Figure 6 It shows how the average age (AoI) of different strategies varies with the packet drop probability. In the threshold-based strategy, each node can transmit its packet only when the AoI exceeds a certain threshold, and the threshold can be optimized to minimize the average AoI. Figure 6, when the probability of packet discard is low, the strategy proposed in this application outperforms the pure on-demand generation (GW) strategy and the threshold-based strategy. However, when the probability of packet discard is large, the threshold-based strategy can further improve the average AoI. In other words, the first-come, first-served limited retry limit (FCFS FRL) strategy proposed in this application performs better in terms of the trade-off between packet discard probability and AoI.

[0092] Different from the previous average AoI of Aloha network in V2X communication, this embodiment models V2X communication as a dual-server queue system and derives the average AoI of the general dual-server queue system as the probability generation function (PGF) of the service time. This is achieved by considering the queuing process and establishing the relationship between the average AoI and the PGF of each time period length. To further determine the PGF of the service time, this embodiment constructs a dedicated Markov chain to simulate the behavior of HOL packets that are subject to a limited retry limit strategy. Compared with arbitrary generation strategies and threshold-based strategies, this behavior follows different rules. Using the dedicated Markov chain, the PGF of the service time and steady-state points is determined. Compared with the bistable Aloha network with unlimited retry limit, the V2X Aloha network with limited retry limit has two or three steady-state points. Based on the PGF of the steady-state point and service time, the average AoI of the V2X Aloha network with retry limit is further described.

[0093] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. An Aloha network transmission method for optimizing information age, characterized in that: The following steps are involved: S1. For the slotted V2X Aloha network with limited retry constraints, the queuing system is modeled as a dual-server queue system; S2, characterizes the average AoI of the dual-server queue system modeled in S1; S3, Markov modeling based on queue head state, characterizes the average service time and steady-state probability of successful packet transmission; S4. Based on the obtained characterization information, the node transmission probability and the number of retransmissions that minimize the AoI are selected.

2. The Aloha network transmission method for optimizing information age according to claim 1, characterized in that: The S1 comprises the following steps: In a slotted V2X Aloha network, n vehicles send information to one receiver, assuming that each vehicle node has a Bernoulli arrival process with parameter λ, where λ is the input rate of each vehicle node, defined as the long-term average number of packets arriving at each vehicle node buffer per time slot; In each time slot, as long as the buffer is not empty, each vehicle node will send its HOL data packet with probability q∈(0,1], and all HOL data packets are provided in the order of arrival, that is, first come first served; At the receiver, the classic collision model is adopted, that is, the HOL packet can be successfully received if and only if there is no concurrent transmission from other nodes; Each vehicle node adopts a retransmission limit M, that is, the HOL data packet will be discarded only when the number of unsuccessful transmissions reaches M. When M is infinite, the HOL data packet always stays in the vehicle node queue until the transmission is successful. When M is a finite value, the HOL data packet of each vehicle node is discarded with a certain probability; The queue of each vehicle node is regarded as a two-server queue system, where the probability of each HOL packet being dropped in each time slot is p d , otherwise the probability of successful transmission is 1-p d .

3. The Aloha network transmission method for optimizing information age according to claim 2, characterized in that: The S2 comprises the following steps: For a two-server queue system, if The average AoI is characterized as: Among them, λ s =λ(1-p d ) is the input rate of successfully sent HOL packets, is the average service time of a HOL data packet, is the average service time of a successfully sent HOL packet, function Service time D T The probability generating function The second derivative of at z=1.

4. The Aloha network transmission method for optimizing information age according to claim 3, characterized in that: The Markov modeling based on the queue head state includes the following steps: A discrete-time Markov chain is built for each vehicle node to simulate the behavior of each HOL packet: A new HOL packet is initially in state T. If the transmission is successful, the HOL packet will stay in state T for one time slot. If the transmission is paused, the HOL packet will enter state R0. If the transmission fails, the HOL packet will enter state R1. For the state R i (0≤i≤M-2) HOL packets, if the transmission is suspended, the HOL packets will be in state R i If the transmission is successful, the HOL packet will go to state T; if the transmission fails, the HOL packet will go to state R i+1 ; For the state R M-1 As long as the transmission probability reaches q, the HOL packet will move to the initial state T because the number of transmission attempts has reached the retry limit M.

5. The Aloha network transmission method for optimizing information age according to claim 4, characterized in that: The characterization of the average service time and the steady-state probability of successfully transmitting a data packet comprises the following steps: Service time D T The probability generating function of is: Probability Generating Function The second-order derivative of is: Among them, z is the input of the probability generation function, p is the probability of successfully transmitting a HOL data packet, and q is the node transmission probability; The average service time is characterized by: The average service time of successfully transmitted HOL packets is characterized by: The steady-state probability of successfully transmitting a data packet is characterized as: Substituting equations (2) to (6) into equation (1), we can get the exact average AoI.

6. The Aloha network transmission method for optimizing information age according to claim 5, characterized in that: The S4 comprises the following steps: According to the monotonicity of AoI with respect to the transmission probability q, the node transmission probability of AoI is set to: Among them, p S is the solution representing the intermediate value of the steady-state probability p of successfully transmitting a data packet; Minimizing the number of retransmissions is to make the maximum retry limit M that has a solution to equation (6). The specific value of the retry limit M is found through the bisection method.

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