CN113852449A - Method for evaluating interruption probability of authorization-free improved retransmission system under URLLC - Google Patents
Method for evaluating interruption probability of authorization-free improved retransmission system under URLLC Download PDFInfo
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Abstract
The invention discloses a method for evaluating interruption probability of an authorization-free improved retransmission system under URLLC, which comprises the following steps: 1. real-time users transmit data packets in a short packet mode and transmit the data packets by an authorization-free hybrid automatic repeat request technology combined with auxiliary packet transmission; 2. the system combines the time delay constraint to establish an interruption probability problem model: the method comprises the steps of giving transmitting power, combining a flat Rayleigh fading channel model to obtain a signal-to-interference-and-noise ratio of a receiving end, giving a threshold value of the signal-to-interference-and-noise ratio and the maximum time delay delivered under the ultra-reliable low-delay constraint, and establishing an interruption probability problem model by utilizing probability theory and random geometry; 3. the system transmission interruption probability: and calculating to obtain a closed solution of the interruption probability of the authorization-free retransmission scheme according to the model. The invention can accurately evaluate the interruption probability of the system under the ultra-reliable low-delay service requirement and reveal the influence of the system parameters of the network on the information transmission interruption probability.
Description
Technical Field
The invention belongs to the field of design of an authorization-free transmission system, and particularly relates to a method for evaluating the interruption probability of an authorization-free hybrid automatic repeat request scheme combined with secondary packet transmission under ultra-reliable low delay.
Background
The 5G ultra-reliable low-delay has become a main target of many application scenarios, such as intelligent transportation systems, industrial automation, smart power grids and the like. The most stringent reliability requirement of current URLLC standardized in 3GPP is to achieve 99.999% reliability within 1ms of the radio delay constraint, defined as the percentage of correctly received data packets within the delay constraint, which presents a myriad of challenges in terms of air interface design, network deployment, resource allocation. Under new communication requirements, the traditional authorization-based access scheme is no longer applicable, and the authorization-free access scheme becomes a promising technology. Compared with the access technology based on authorization, the authorization-free technology can effectively shorten the end-to-end transmission delay. Therefore, a contention-based access mode in the unlicensed uplink transmission becomes the focus of research of scholars. The authorization-free transmission technology comprises two schemes of resource reservation and competitive access, wherein the former scheme is more suitable for a periodic traffic scene, and the latter scheme is more suitable for a sudden traffic scene, and is more beneficial to the development of large-scale machine type communication.
The uplink authorization-free transmission technology comprises two access modes, wherein one mode is resource reservation, namely that user equipment transmits a data packet by using spectrum resources pre-allocated by a base station; and secondly, access based on competition is realized, and when a data packet needs to be sent, the user equipment transmits the data packet through the resources of the competition spectrum resource pool. The former is more suitable for the scenario of periodic packet arrival, which arrives in a fixed or cyclic pattern, while the latter is suitable for the scenario of packet arrival in bursts, which creates a contention problem. Based on the fact that a large number of users and sporadic packet arrival are typical characteristics of the 5G system, the burst traffic arrival scenario will be more researched.
The unlicensed transmission technology is used as a key technology in 5G ultra-reliable low-delay communication, and the performance of a wireless system needs to be researched under the constraint of limited block length coding by combining the characteristics of URLLC short packet transmission. In the unlicensed uplink access, retransmission is a solution for improving reliability in the unlicensed contention access transmission scheme, but the conventional hybrid automatic repeat request technique also introduces additional delay, so that further improvement of the retransmission technique is needed. For the extra time delay caused by retransmission, scholars further provide a transmission scheme combined with the transmission of the secondary packet, so that the reliability is improved, and the retransmission probability can be reduced. Many scholars have studied the problem of outage probability under two schemes of resource reservation and contention access in the unlicensed access technology, but have failed to analyze the problem of outage probability under limited block length coding. For the improved unlicensed hybrid automatic repeat request technology, students studied the problem of the outage probability of the transmission scheme, but failed to consider shannon capacity under limited block length coding. Currently, the problem of radio system outage probability due to the combination of the unlicensed combining with the improved retransmission technology and the limited block length coding has not been specifically studied and analyzed.
The random geometric theory can effectively depict the random distribution of base stations and users in the network, and the Poisson Point Process (PPP) can not only accurately describe the random distribution characteristics of nodes, but also provide conditions for obtaining the mathematical analysis of network performance indexes.
Disclosure of Invention
The invention aims to provide a method for evaluating the interruption probability of an authorization-free improved retransmission system under a URLLC (universal resource level control Link control) so as to solve the technical problem that the influence of delay constraint on the interruption probability of short packet transmission cannot be revealed.
In order to solve the technical problems, the specific technical scheme of the invention is as follows:
a method for evaluating interruption probability of an authorization-free improved retransmission system under a URLLC (universal resource level link control) condition comprises the following steps:
step 1, a real-time user transmits a data packet in a short packet mode and accesses a base station through a system transmission scheme;
step 2, establishing a system model by combining a network scene, analyzing the transmission delay of the system, and establishing an interruption probability problem model according to delay constraint;
and 3, calculating to obtain a closed solution of the interruption probability of the system transmission scheme according to the model.
Further, the step 1 specifically includes the following steps:
firstly, under URLLC service, limited block length coding is carried out on a data packet transmitted by a real-time user, an uplink contention-based authorization-free access technology is adopted, a short packet is transmitted in a manner of arriving and going without sending a scheduling application to a base station and receiving resource authorization from the base station; meanwhile, the user can transmit a plurality of auxiliary packets at continuous time intervals, and the base station processes and decodes the received auxiliary packets in a soft combining mode; and finally, giving decoding feedback to the user. A successful transmission is considered only if the user has not collided and the received signal to interference plus noise ratio is greater than a threshold. If the transmission fails, the system transmission scheme is adopted to carry out retransmission on the next time slot.
Further, the relational expression of the transmission rate R and the packet length L under the finite block length coding in step 1 is:
wherein Q-1Represents the inverse of the gaussian function, O (logL/L) represents infinitesimal of logL/L, γ represents the signal-to-noise ratio, ε represents the bit error rate, and V represents the channel dispersion and is characteristic of the channel, as follows:
further, the step 2 specifically includes the following steps:
firstly, a system model is established according to a network scene, then a time interval (TTI) is defined according to a transmission rate R and a data packet length L under finite block length coding in short packet transmission, and finally, the total time delay of one round-trip transmission process of the system is obtained according to transmission time interval analysis
And secondly, giving a transmitting power, establishing a channel model to obtain a signal-to-interference-and-noise ratio at a base station, finally giving a signal-to-interference-and-noise ratio threshold value and a time delay delivered under an ultra-reliable low-delay constraint, obtaining the maximum round-trip transmission process times by combining the time delay constraint, and establishing an interruption probability problem model by utilizing a probability theory and random geometry based on the analysis result.
Further, the specific steps of step 2 include:
step 2.1, the network scene is as follows: the base station and the user obey two independent poisson point processes phi on the space distributionBAnd phiDAnd the intensity is respectively lambdaBAnd λD(ii) a Each user forms a Thiessen polygon in association with the base station closest to its geographical location and is synchronously connected to the serving base station; in a data packet buffer area, under the condition of considering the arrival of a single packet sequence, each data packet buffer area obeys an independent and identically distributed Bernoulli flow generation model with probability pa∈[0，1](ii) a Based on the cache model, each user will have a probability paReceiving a data packet from a higher layer, when the data packet is successfully transmitted and no new packet arrives, the buffer area has no data packet sequence, otherwise, the buffer area waits for retransmission; in a single-layer cellular network and buffer model, N users are pre-allocated with S orthogonal pilots in a transmission time interval, a user needing to transmit a short packet is defined as a real-time user, the real-time user randomly selects one transmission data from the S orthogonal pilots, and the real-time user density for selecting the same pilot is defined as lambdaa＝paλD/S；
Step 2.2, the system model is as follows: a real-time user transmits K identical auxiliary packets through an authorization-free hybrid automatic repeat request technology of auxiliary packet transmission at continuous transmission time intervals, a receiving end combines the received auxiliary packets, decodes the auxiliary packets and feeds the decoded auxiliary packets back to the real-time user, if at least one auxiliary packet is successfully transmitted, retransmission cannot occur, and otherwise, retransmission is performed;
step 2.3, analyzing the transmission delay by combining the short packet transmission characteristics under the URLLC;
obtaining the transmission time delay T according to the finite block length transmission rate and the packet lengthtxThe expression is as follows:
obtaining transmission time delay of one round trip transmission process
Comprises the following steps:
frame alignment delay TfaBase station feedback delay TfbBase station processing time delay TbpAnd user processing delay TupAre all delayed by a transmission time TtxEquality, further obtaining the transmission time delay T after m round-trip transmission processesK(m) is:
step 2.4, the system combines the time delay constraint to establish an interruption probability problem model: obtaining the SINR gamma at the base station according to the path loss modelmGiven a SINR threshold gammathTime delay T of delivery under ultra-reliable low-delay constraintmaxObtaining the maximum round-trip transmission process times M by combining time delay constraint, and then establishing an interruption probability problem model by utilizing probability theory and random geometry;
obtaining a signal to interference and noise ratio of
Wherein
Representing the signal-to-interference-and-noise ratio of the q-th secondary packet in the mth system round-trip transmission process, wherein rho represents a receiving power threshold, namely, full path loss inversion power control is adopted, namely, full path loss inversion power control is applied to all users, wherein each user compensates the path loss of the user to keep the average receiving signal power equal to the same threshold, the density of the base station is high enough, and no user has truncation interruption, namely, the transmitting power of the user is large enough to perform uplink path loss inversion without violating the maximum transmitting power constraint of the user;
representing the channel power gain from the qth sub-packet to the serving base station in the mth round-trip transmission process of the system, and obeying the exponential distribution with the average value of 1, namely h-Exp (1); sigma2Representative is the noise power, IintraAnd IinterRepresentative are aggregated intra-zone interference and inter-zone interference;
based on the model scene, the URLLC reliability index of system transmission is the percentage of data packets which are not successfully transmitted and transmitted within a certain time limit, namely the interruption probability is PF＝Pr{Ts≤Tmax}; the problem is described as the delay constraint T at the contracted QoS deliverys≤TmaxThe outage probability is guaranteed to be lower than epsilon; the reliability problem model of the system under URLLC is modeled as the following expression:
PF＝Pr{Ts≤Tmax}≤ε
wherein T issRepresenting the actual delay, T, incurred by the successful transmission of the data packetmaxRepresenting the maximum delay constraint for QoS delivery under URLLC, epsilon is an infinitesimal value.
Further step 2.4 in-zone interference power IintraThe expression is as follows:
wherein h isijRepresenting the channel power gain of the jth secondary packet in the ith intra-cell interference user;
inter-cell interference power IinterThe expression is as follows:
where Ω represents the set of inter-sector interfering users from different base station service areas, PtRepresenting the transmission power, h, of the interfering users in the t-th intervaltjChannel representing jth secondary packet in t interval interference userPower gain, rtThe distance from the t-th inter-cell interfering user to the serving base station is represented, and α represents the path loss.
Further, the step 3 specifically comprises the following steps:
calculating the interruption probability P of a system transmission schemeFComprises the following steps:
wherein, M ═ 0 represents that there is no retransmission within the delay constraint, the user cannot receive the feedback information, and the transmission fails; m ≧ 1 represents at least one round-trip transmission within the delay constraint, the expression for M is:
wherein
Representing the largest integer smaller than this
Representing the probability that the user needs the mth round trip transmission of the system scheme in the system transmission scheme, i.e. the probability that the first m-1 round trip transmission of the system scheme fails, i.e. under the maximum delay constraint TmaxTransmitting a total delay T for the first m-1 systematic round trip procedureKThe probability of interruption at (m-1), described as:
in the formula
Representing the probability that the number N of the interfering users in the middle area is N in the transmission of the mth system scheme round trip process; thetaK[n，m，K]Representing the transmission success rate of the user under the conditions that the number N of interference users in the region is equal to N and the number of auxiliary packets is K in the mth round-trip process of the system scheme in the system transmission scheme; (1-theta)K[n，m，K])nRepresents the non-collision rate of the mth round trip process of the system scheme in the system transmission scheme, i.e. the probability that the rest n intracell interference users are not successfully received by the base station.
Further, the specific step of calculating the closed solution includes:
step 3.1, solving the probability that the number N of the interfering users in the middle area is N in the transmission of the mth system scheme round trip process
The expression is as follows:
wherein c is a constant of 3.5;
step 3.2, obtaining the transmission success rate theta of the user under the condition that the number N of the interference users in the region is N in the mth round-trip process of the system scheme in the system transmission scheme without considering the collision conditionK[n，m，K]The expression is:
step 3.3, according to the interference power I in the regionintraThe expression and channel gain h are calculated by following an exponential distribution probability density function with a mean value of 1, part1 can be obtained to the desired value:
step 3.4, according to the interval interference power IinterThe expression and the channel gain h follow an exponential distribution probability density function with a mean value of 1 to the desired values:
by using the method of changing elements
Further obtained in the formula:
wherein EP(P2/α) Probability density function f according to power PP(p) solving a probability density function of
Thus obtaining EP(P2/α)＝ρ2/α/πλBAnd finally, obtaining:
step 3.5, finally obtaining the transmission success rate theta of the mth system scheme in the system transmission scheme in the round-trip processK[n，m，K]The expression is as follows:
the method for evaluating the interruption probability of the authorization-free improved retransmission system under the URLLC has the following advantages:
the transmission rate under the combination of the packet length and the limited block length coding better accords with the transmission characteristic of 5G URLLC, and is closer to an interrupt probability analysis model under the scenes of short packet transmission and low time delay correlation. The invention combines the authorization-free competition-based access technology and the improved retransmission technology to solve the problem of the interruption probability of the real-time user in the short packet transmission system, and verifies the accuracy of the interruption probability analysis of the invention and also verifies that the improved retransmission scheme has higher reliability in the short delay constraint under the Monte Carlo simulation analysis.
Drawings
FIG. 1 is a diagram of a system transport framework of the present invention;
fig. 2 is a schematic diagram illustrating a relationship between a success rate without considering collision probability and a signal to interference plus noise ratio threshold of the ue according to the present invention;
Detailed Description
In order to better understand the purpose, structure and function of the present invention, a method for evaluating the interruption probability of the unlicensed modified retransmission system in the URLLC according to the present invention is described in further detail below with reference to the accompanying drawings.
As shown in fig. 1, a method for evaluating an interruption probability of an unlicensed modified retransmission system in a URLLC according to the present invention includes the following steps:
step 1, firstly, under URLLC service, limited block length coding is carried out on a data packet transmitted by a real-time user, an uplink contention-based authorization-free access technology is adopted, a short packet is transmitted in a manner of arriving and walking immediately without sending a scheduling application to a base station and receiving resource authorization from the base station; meanwhile, the user can transmit a plurality of auxiliary packets at continuous time intervals, and the base station processes and decodes the received auxiliary packets in a soft combining mode; and finally, giving decoding feedback to the user. A successful transmission is considered only if the user has not collided and the received signal to interference plus noise ratio is greater than a threshold. If the transmission fails, a system transmission scheme is adopted to carry out retransmission on the next time slot;
the relational expression of the transmission rate R and the data packet length L under the finite block length is as follows:
wherein Q-1Represents the inverse of the gaussian function, O (logL/L) represents infinitesimal of logL/L, γ represents the signal-to-noise ratio, ε represents the bit error rate, and V represents the channel dispersion and is characteristic of the channel, as follows:
step 2, establishing a system model, wherein the system establishes an interruption probability problem model by combining time delay constraint: the method comprises the steps of giving transmitting power, establishing a channel model, obtaining a signal to interference plus noise ratio at a base station, giving a signal to interference plus noise ratio threshold value and a time delay delivered under an ultra-reliable low-delay constraint, obtaining the maximum round-trip transmission process times by combining the time delay constraint, and then establishing an interruption probability problem model by utilizing a probability theory and random geometry.
The method comprises the following specific steps:
step 2.1, the network scene is as follows: the base station and the user obey two independent poisson point processes phi on the space distributionBAnd phiDAnd the intensity is respectively lambdaBAnd λD(ii) a Each user forms a Thiessen polygon in association with the base station closest to its geographical location and is synchronously connected to the serving base station; in a data packet buffer area, under the condition of considering the arrival of a single packet sequence, each data packet buffer area obeys an independent and identically distributed Bernoulli flow generation model with probability pa∈[0,1](ii) a Based on the cache model, each user will have a probability paReceiving a data packet from a higher layer, when the data packet is successfully transmitted and no new packet arrives, the buffer area has no data packet sequence, otherwise, the buffer area waits for retransmission; in the single-layer cellular network and buffer model, N users are pre-allocated with S orthogonal pilots in a transmission time interval, and the user needing to transmit a short packet is defined as a real-time user, and the real-time user is randomly positionedSelecting one of S orthogonal pilot frequencies for transmission data, thereby defining the real-time user density of the same pilot frequency as lambdaa＝paλD/S；
Step 2.2, the system model is as follows: a real-time user transmits K identical auxiliary packets through an authorization-free hybrid automatic repeat request technology of auxiliary packet transmission at continuous transmission time intervals, a receiving end combines the received auxiliary packets, decodes the auxiliary packets and feeds the decoded auxiliary packets back to the real-time user, if at least one auxiliary packet is successfully transmitted, retransmission cannot occur, and otherwise, retransmission is performed;
step 2.3, analyzing the transmission delay by combining the short packet transmission characteristics under the URLLC;
obtaining the transmission time delay T according to the finite block length transmission rate and the packet lengthtxThe expression is as follows:
obtaining transmission time delay of one round trip transmission process
Comprises the following steps:
frame alignment delay TfaBase station feedback delay TfbBase station processing time delay TbpAnd user processing delay TupAre all delayed by a transmission time TtxEquality, further obtaining the transmission time delay T after m round-trip transmission processesK(m) is:
step 2.4, the system combines the time delay constraint to establish an interruption probability problem model: obtaining the SINR gamma at the base station according to the path loss modelmGiven a SINR threshold gammathThe maximum round-trip transmission process times M are obtained by the delivery time delay Tmax under the ultra-reliable low-delay constraint and the time delay constraint, and then an interruption probability problem model is established by utilizing probability theory and random geometry;
obtaining a signal to interference and noise ratio of
Wherein
Representing the signal-to-interference-and-noise ratio of the q-th secondary packet in the mth system round-trip transmission process, and p representing a receiving power threshold, namely adopting full path loss inversion power control, namely applying the full path loss inversion power control on all users, wherein each user compensates the path loss of the user to keep the average receiving signal power equal to the same threshold, and the density of the base station is high enough, and no user has truncation interruption (namely the transmitting power of the user is large enough, the uplink path loss inversion can be carried out, and the maximum transmitting power constraint of the user is not violated).
Representing the channel power gain from the qth sub-packet to the serving base station in the mth round-trip transmission process of the system, and obeying the exponential distribution with the average value of 1, namely h-Exp (1); sigma2Representative is the noise power, IintraRepresented by the aggregated intra-zone interference power, IinterRepresentative is the aggregated inter-zone interference power;
interference power I in areaintraThe expression is as follows:
wherein h isijRepresenting the channel power gain of the jth secondary packet in the ith intra-cell interference user;
inter-cell interference power IinterThe expression is as follows:
where Ω represents the set of inter-sector interfering users from different base station service areas, PtRepresenting the transmission power, h, of the interfering users in the t-th intervaltjRepresenting the channel power gain, r, of the jth secondary packet in the t-th inter-cell interfering usertThe distance from the t-th inter-cell interfering user to the serving base station is represented, and α represents the path loss.
Based on the model scene, the URLLC reliability index of system transmission is the percentage of data packets which are not successfully transmitted and transmitted within a certain time limit, namely the interruption probability is PF＝Pr{Ts≤Tmax}; the problem may be described as a delay bound T at the contracted QoS deliverys≤TmaxThe outage probability is guaranteed to be lower than epsilon; the reliability problem model of the system under URLLC can be modeled as the following expression:
PF＝Pr{Ts≤Tmax}≤ε
wherein T issRepresenting the actual delay, T, incurred by the successful transmission of the data packetmaxRepresenting the maximum time delay constraint of QoS delivery under URLLC, wherein epsilon is an infinitesimal value;
step 3, system transmission interruption probability: and calculating to obtain a closed solution of the interruption probability of the system transmission scheme according to the model.
The method comprises the following specific steps:
calculating the interruption probability P of a system transmission schemeFComprises the following steps:
wherein, M ═ 0 represents that there is no retransmission within the delay constraint, the user cannot receive the feedback information, and the transmission fails; m ≧ 1 represents at least one round-trip transmission within the delay constraint, the expression for M is:
wherein
Represented is the largest integer smaller than x,
representing the probability that the real-time user still needs the mth round trip transmission, i.e. the probability that the previous m-1 round trip transmission fails, i.e. the maximum delay constraint TmaxTransmitting a total delay T for the first m-1 round tripsKThe probability of interruption at (m-1), described as:
in the formula
Representing the probability that the number N of the interfering users in the middle area is N in the transmission of the mth system scheme round trip process; thetaK[n，m，K]Representing the transmission success rate of the user under the conditions that the number N of interference users in the region is equal to N and the number of auxiliary packets is K in the mth round-trip process of the system scheme in the system transmission scheme; (1-theta)K[n，m，K])nRepresents the non-collision rate of the mth round trip process of the system scheme in the system transmission scheme, i.e. the probability that the rest n intracell interference users are not successfully received by the base station.
The specific steps of calculating the closed solution include:
step 3.1, solving the mth system scheme round trip processProbability of number N of interfering users in transmission middle area
The expression is as follows:
wherein c is a constant of 3.5;
step 3.2, obtaining the transmission success rate theta of the user under the condition that the number N of the interference users in the region is N in the mth round-trip process of the system scheme in the system transmission scheme without considering the collision conditionK[n,m，K]The expression is:
step 3.3, according to the interference power I in the regionintraThe expression and channel gain h are calculated by following an exponential distribution probability density function with a mean value of 1, part1 can be obtained to the desired value:
step 3.4, according to the interval interference power IinterThe expression and the channel gain h follow an exponential distribution probability density function with a mean value of 1 to the desired values:
by using the method of changing elements
Further obtained in the formula:
wherein EP(P2/α) Probability density function f according to power PP(p) solving a probability density function of
Thus obtaining EP(P2/α)＝ρ2/α/πλBAnd finally, obtaining:
step 3.5, finally obtaining the transmission success rate theta of the mth system scheme in the system transmission scheme in the round-trip processK[n,m,K]The expression is as follows:
example (b): in a simulated test environment at 1600km in the network2The circular area of the user equipment randomly arranges a plurality of base stations and user equipment which are in accordance with the distribution of the poisson point process, the user equipment selects the base stations with close distance to transmit data packets, and Monte Carlo simulation is adopted to analyze the success probability of the user. Suppose the densities of the base station and the user equipment are λ respectivelyB＝1BSs/km2，λD＝10UEs/km2The power threshold ρ is 20dB and the noise power is σ2The path loss α is 4, the number of sub-packets is 4,6,8, the channel gain follows an exponential distribution h to exp (1), the number of pilots S is 2, pa＝0.1。
By adopting the interrupt probability derivation method, the simulation effect is as follows:
the SINR threshold is increased gradually from-10 dB to 10dB, and fig. 2 shows that the transmission success rate based on the method without considering collision probability is the same as the result of numerical solution obtained through a large number of Monte Carlo simulations. In the figure, along with the increase of the signal-to-interference-and-noise ratio threshold value, when the invention is applied to data transmission, the success rate of the system is obviously reduced compared with the traditional authorization-free retransmission system, but the success rate is gradually reduced along with the reduction of the number of the auxiliary packets.
The invention applies the limited block length coding requirement in short packet transmission in ultra-reliable low-delay communication to the improved authorization-free retransmission technical scheme, and deduces the time delay constraint T delivered by the URLLC based on the packet length, the data transmission rate and the time delay analysismaxLower, the interruption probability of the system transmission closes the solution PF。
It is to be understood that the present invention has been described with reference to certain embodiments, and that various changes in the features and embodiments, or equivalent substitutions may be made therein by those skilled in the art without departing from the spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
Claims (8)
1. A method for evaluating interruption probability of an authorization-exempt improved retransmission system under URLLC (universal resource level link control), comprising the following steps of:
step 1, a real-time user transmits a data packet in a short packet mode and accesses a base station through a system transmission scheme;
step 2, establishing a system model by combining a network scene, analyzing the transmission delay of the system, and establishing an interruption probability problem model according to delay constraint;
and 3, calculating to obtain a closed solution of the interruption probability of the system transmission scheme according to the model.
2. The method according to claim 1, wherein the step 1 specifically comprises the following steps:
firstly, under URLLC service, limited block length coding is carried out on a data packet transmitted by a real-time user, an uplink contention-based authorization-free access technology is adopted, a short packet is transmitted in a manner of arriving and going without sending a scheduling application to a base station and receiving resource authorization from the base station; meanwhile, the user can transmit a plurality of auxiliary packets at continuous time intervals, and the base station processes and decodes the received auxiliary packets in a soft combining mode; finally, giving decoding feedback to the user; the transmission is considered to be successful only when the user is not collided and the received signal-to-interference-and-noise ratio is greater than a threshold value; and if the transmission fails, retransmitting on the next time slot by adopting a system transmission scheme.
3. The method according to claim 2, wherein the relation expression of the transmission rate R and the packet length L in the limited block length coding in step 1 is:
wherein Q-1Represents the inverse of the gaussian function, 0(logL/L) represents infinitesimal of logL/L, γ represents the signal-to-noise ratio, ε represents the bit error rate, and V is the channel dispersion representing the channel characteristics, as expressed below:
4. the method according to claim 1, wherein the step 2 specifically comprises the following steps:
firstly, establishing a system model according to a network scene, defining a time interval according to a transmission rate R and a data packet length L under finite block length coding in short packet transmission, and finally, analyzing and obtaining the total time delay of the system in a one-time round-trip transmission process according to the transmission time interval
And secondly, giving a transmitting power, establishing a channel model to obtain a signal-to-interference-and-noise ratio at a base station, finally giving a signal-to-interference-and-noise ratio threshold value and a time delay delivered under an ultra-reliable low-delay constraint, obtaining the maximum round-trip transmission process times by combining the time delay constraint, and establishing an interruption probability problem model by utilizing a probability theory and random geometry based on the analysis result.
5. The method of claim 4, wherein the step 2 comprises the following steps:
step 2.1, the network scene is as follows: the base station and the user obey two independent poisson point processes phi on the space distributionBAnd phiDAnd the intensity is respectively lambdaBAnd λD(ii) a Each user forms a Thiessen polygon in association with the base station closest to its geographical location and is synchronously connected to the serving base station; in a data packet buffer area, under the condition of considering the arrival of a single packet sequence, each data packet buffer area obeys an independent and identically distributed Bernoulli flow generation model with probability pa∈[0,1](ii) a Based on the cache model, each user will have a probability paReceiving a data packet from a higher layer, when the data packet is successfully transmitted and no new packet arrives, the buffer area has no data packet sequence, otherwise, the buffer area waits for retransmission; in a single-layer cellular network and buffer model, N users are pre-allocated with S orthogonal pilots in a transmission time interval, a user needing to transmit a short packet is defined as a real-time user, the real-time user randomly selects one transmission data from the S orthogonal pilots, and the real-time user density for selecting the same pilot is defined as lambdaa＝paλD/S；
Step 2.2, the system model is as follows: a real-time user transmits K identical auxiliary packets at continuous transmission time intervals, a receiving end combines the received auxiliary packets, decodes and feeds back the auxiliary packets to the real-time user, if at least one auxiliary packet is successfully transmitted, retransmission cannot occur, and otherwise retransmission is performed;
step 2.3, analyzing the transmission delay by combining the short packet transmission characteristics under the URLLC;
obtaining the transmission time delay T according to the finite block length transmission rate and the packet lengthtxThe expression is as follows:
obtaining transmission time delay of one round trip transmission process
Comprises the following steps:
wherein the frame alignment is delayed by TfaBase station feedback delay TfbBase station processing time delay TbpAnd user processing delay TupAre all delayed by a transmission time TtxEquality, further obtaining the transmission time delay T after m round-trip transmission processesK(m) is:
step 2.4, the system combines the time delay constraint to establish an interruption probability problem model: obtaining the SINR gamma at the base station according to the path loss modelmGiven a SINR threshold gammathTime delay T of delivery under ultra-reliable low-delay constraintmaxObtaining the maximum round-trip transmission process times M by combining time delay constraint, and then establishing an interruption probability problem model by utilizing probability theory and random geometry;
obtaining a signal to interference and noise ratio of
Wherein
Representing the signal-to-interference-and-noise ratio of the q-th secondary packet in the mth system round-trip transmission process, wherein rho represents a receiving power threshold, namely, full path loss inversion power control is adopted, namely, full path loss inversion power control is applied to all users, wherein each user compensates the path loss of the user to keep the average receiving signal power equal to the same threshold, the density of the base station is high enough, and no user has truncation interruption, namely, the transmitting power of the user is large enough to perform uplink path loss inversion without violating the maximum transmitting power constraint of the user;
representing the channel power gain from the qth sub-packet to the serving base station in the mth round-trip transmission process of the system, and obeying the exponential distribution with the average value of 1, namely h-Exp (1); sigma2Representative is the noise power, IintraRepresented by the polymeric intrazone interference, IinterRepresentative is aggregated inter-zone interference;
based on the model scene, the URLLC reliability index of system transmission is the percentage of data packets which are not successfully transmitted and transmitted within a certain time limit, namely the interruption probability is PF＝Pr{Ts≤Tmax}; the problem is described as the delay constraint T at the contracted QoS deliverys≤TmaxThe outage probability is guaranteed to be lower than epsilon; the reliability problem model of the system under URLLC is modeled as the following expression:
PF＝Pr{Ts≤Tmax}≤ε
wherein T issRepresenting the actual delay, T, incurred by the successful transmission of the data packetmaxRepresenting the maximum delay constraint for QoS delivery under URLLC, epsilon is an infinitesimal value.
6. The method of claim 5, wherein the intra-zone interference power I is a power of interference, wherein the power of interference is measured by the intra-zone interference power IintraThe expression is as follows:
wherein h isijRepresenting the channel power gain of the jth secondary packet in the ith intra-cell interference user;
inter-cell interference power IinterThe expression is as follows:
where Ω represents the set of inter-sector interfering users from different base station service areas, PtRepresenting the transmission power, h, of the interfering users in the t-th intervaltjRepresenting the channel power gain, r, of the jth secondary packet in the t-th inter-cell interfering usertThe distance from the t-th inter-cell interfering user to the serving base station is represented, and α represents the path loss.
7. The method according to claim 6, wherein the step 3 comprises the following steps:
calculating the interruption probability P of a system transmission schemeFComprises the following steps:
wherein, M ═ 0 represents that there is no retransmission within the delay constraint, the user cannot receive the feedback information, and the transmission fails; m ≧ 1 represents at least one round-trip transmission within the delay constraint, the expression for M is:
wherein
Represented is the largest integer smaller than x,
representing the probability that the real-time user still needs the mth round trip transmission, i.e. the probability that the previous m-1 round trip transmission fails, i.e. the maximum delay constraint TmaxTransmitting a total delay T for the first m-1 round tripsKThe probability of interruption at (m-1), described as:
in the formula
Representing the probability that the number N of the interfering users in the middle area is N in the transmission of the mth system scheme round trip process; thetaK[n,m,K]Representing the transmission success rate of the user under the conditions that the number N of interference users in the region is equal to N and the number of auxiliary packets is K in the mth round-trip process of the system scheme in the system transmission scheme; (1-theta)K[n,m,K])nRepresents the non-collision rate of the mth round trip process of the system scheme in the system transmission scheme, i.e. the probability that the rest n intracell interference users are not successfully received by the base station.
8. The method of claim 7, wherein the step of calculating the closed solution comprises:
step 3.1, solving the probability that the number N of the interfering users in the middle area is N in the transmission of the mth system scheme round trip process
The expression is as follows:
wherein c is a constant of 3.5;
step 3.2, obtaining the transmission success rate theta of the user under the condition that the number N of the interference users in the region is N in the mth round-trip process of the system scheme in the system transmission scheme without considering the collision conditionK[n,m,K]The expression is:
step 3.3, according to the interference power I in the regionintraThe expression and channel gain h are calculated by following an exponential distribution probability density function with a mean value of 1, part1 can be obtained to the desired value:
step 3.4, according to the interval interference power IinterThe expression and the channel gain h follow an exponential distribution probability density function with a mean value of 1 to the desired values:
by using the method of changing elements
Further obtained in the formula:
wherein EP(P2/α) Probability density function f according to power PP(p) solving a probability density function of
Thus obtaining EP(P2/α)＝ρ2/α/πλBAnd finally, obtaining:
step 3.5, finally obtaining the transmission success rate theta of the mth system scheme in the system transmission scheme in the round-trip processK[n，m，K]The expression is as follows:
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