CN110856262B - Authorization-free uplink transmission method based on joint time-frequency diversity - Google Patents

Authorization-free uplink transmission method based on joint time-frequency diversity Download PDF

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CN110856262B
CN110856262B CN201910913021.5A CN201910913021A CN110856262B CN 110856262 B CN110856262 B CN 110856262B CN 201910913021 A CN201910913021 A CN 201910913021A CN 110856262 B CN110856262 B CN 110856262B
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data packet
probability
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user equipment
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CN110856262A (en
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王熠晨
赵书玉
谢云聪
徐东阳
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Xian Jiaotong University
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W72/00—Local resource management
    • H04W72/50—Allocation or scheduling criteria for wireless resources
    • H04W72/54—Allocation or scheduling criteria for wireless resources based on quality criteria
    • H04W72/543—Allocation or scheduling criteria for wireless resources based on quality criteria based on requested quality, e.g. QoS
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W72/00—Local resource management
    • H04W72/04—Wireless resource allocation
    • H04W72/044—Wireless resource allocation based on the type of the allocated resource
    • H04W72/0446—Resources in time domain, e.g. slots or frames
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W72/00—Local resource management
    • H04W72/04—Wireless resource allocation
    • H04W72/044—Wireless resource allocation based on the type of the allocated resource
    • H04W72/0453—Resources in frequency domain, e.g. a carrier in FDMA
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W72/00—Local resource management
    • H04W72/50—Allocation or scheduling criteria for wireless resources
    • H04W72/54—Allocation or scheduling criteria for wireless resources based on quality criteria
    • H04W72/542—Allocation or scheduling criteria for wireless resources based on quality criteria using measured or perceived quality
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/0404—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas the mobile station comprising multiple antennas, e.g. to provide uplink diversity

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Abstract

The invention discloses an authorization-free uplink transmission method based on joint time-frequency diversity, which jointly applies time diversity and frequency diversity technology based on a Slotted-Aloha model, equally divides uplink transmission time into a plurality of mini time slots with equal length, and in each time slot, user equipment randomly selects a plurality of copies for transmitting the same data packet in all sub-channels provided by a system; in the transmission mode, the situation that two or even a plurality of user equipment try to send data packets on the same shared channel at the same time can occur, so that collision can occur between the data packets; based on the transmission scheme of the invention, under the condition of simultaneously meeting the requirements of ultra-reliability and low time delay of URLLC, the number of copies of the data packets in the time domain and the frequency domain is simultaneously optimized, so that the number of user equipment which can be supported by a system is maximized.

Description

Authorization-free uplink transmission method based on joint time-frequency diversity
Technical Field
The invention belongs to the technical field of uplink transmission of machine equipment communication services, and particularly relates to an authorization-free uplink transmission method based on joint time-frequency diversity.
Background
The 5G mobile communication technology is a new generation mobile communication technology that is being developed to meet the rapid popularization of smart terminals and the rapid development of mobile internet following 4G. Ultra-Reliable and Low-latency Communications (URLLC) is one of three major application scenarios of 5G, and the service quality requirements thereof include the following two aspects: one is ultra-short end-to-end (E2E) delay, requiring no more than 1 ms; the other is transmission reliability, which is required to be not less than 99.999%. According to the existing research results, in order to reduce the system delay, the uplink transmission process may adopt a contention-based authorization-free mode, in which the ue does not need to wait for resource allocation information or transmission authorization before sending a data packet, but can send the data packet immediately. In the contention-based unlicensed transmission mode, two or even more ues may attempt to transmit data packets simultaneously on the same shared channel, and thus collisions between data packets may occur.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides an authorization-free uplink transmission method based on joint time-frequency diversity, which improves the reliability of a system, improves the probability of successful transmission of data packets by repeatedly sending the same data packet, meets the requirements of ultra-reliable and low-delay service quality of URLLC service, and maximizes the number of user equipment which can be supported by the system.
In order to achieve the above object, the present invention adopts a technical solution that an authorization-free uplink transmission method based on joint time-frequency diversity includes the following steps:
s1, establishing a data packet transmission system, wherein the data packet transmission system comprises M user equipment with single antenna and a base station, and the base station is provided with NtN shared sub-channels are arranged in the system;
s2, based on the system established by S1, the total time delay of the uplink transmission process is equally divided into gamma time slots, and in each time slot, the user equipment randomly selects a plurality of copies for transmitting the same data packet from all sub-channels provided by the system; and the data packet adopts a Slotted-Aloha-based protocol and an authorization-free transmission mode;
s3, calculating the probability of data packet transmission failure of the target device in the time slot S2;
and S4, under the condition that the URLLC time delay requirement and the reliability requirement are met, maximizing the number of the devices which can be supported by the system in the S1, obtaining a series of optimal solutions of the time diversity and the frequency diversity times corresponding to the optimal solutions, and obtaining the maximum value of the supportable devices through the optimal solutions.
S1 instantaneous channel gains of the N sub-channels are independent of each other, i.e. the frequency spacing of the N sub-channels exceeds the channel coherence bandwidth Wc(ii) a Total system bandwidth of WmaxThe bandwidth allocated to each shared sub-channel is
Figure BDA0002215260440000021
Each shared subchannel is flat fading.
The data packet can be successfully transmitted on the sub-channel with the instantaneous channel gain greater than or equal to the set threshold value, and on the channel with the instantaneous channel gain smaller than the threshold value, the data packet is considered to be lost, namely, the data packet is successfully transmitted under the condition that the data packet does not collide with other packets, and the instantaneous channel gain of the selected sub-channel is not smaller than the required channel gain threshold value.
In S3, the probability that any other user equipment does not collide with the target packet is:
P0＝(1-Pran)+PranPacc
wherein, PaccProbability that the data packet sent by any other user equipment does not select the same subchannel with the target data packet; pranIs the packet sending probability of each device in the total time delay of the uplink transmission process;
Pran＝1-e-λ
Figure BDA0002215260440000022
wherein λ represents DuThe number of data packets arriving internally, the average inter-arrival time of the data packets is mu, then
Figure BDA0002215260440000031
The probability that any other user equipment does not collide with the target data packet is as follows:
P0＝(1-Pran)+PranPacc
when all other user equipment send data packets without collision with the target data packet, the target data packet does not collide; the probability of no collision is:
Psucc＝P0 M-1＝[(1-Pran)+PranPacc]M-1。
then, the probability that the target data packet collides in one time slot is obtained, which is:
Figure BDA0002215260440000032
in repeating Γ slots, the probability of a target UE transmission failure is:
Figure BDA0002215260440000033
wherein, PFDThe probability of transmission failure of a data packet sent by a target device UE in one time slot,
Figure BDA0002215260440000034
probability of collision of target data packets in a time slot, PtThe packet loss probability of a data packet on a certain channel is shown.
In S4, the optimal solution of the corresponding times of time diversity and frequency diversity is obtained as follows:
giving the value of the frequency diversity times, optimizing the length of a time slot by applying a particle swarm algorithm, solving according to the relation between the time slot length and the time diversity times to obtain the optimal solution of the time diversity times, and solving the maximum value of the corresponding supportable equipment;
converting the time diversity times and the frequency diversity times of the joint optimization into a calculation model, which comprises the following specific steps:
Figure BDA0002215260440000035
t is the length of each time slot, DuIs the total time delay of the uplink transmission process, beta represents the frequency diversity times,
let St.T ∈ [ T ∈ [ ]min,Du]As a constraint condition, the time slot length is ensured not to exceed the value interval in the solving process,
Figure BDA0002215260440000036
Figure BDA0002215260440000037
shows the relation between the time diversity times and the time slot length, restrains the time diversity times by optimizing the length of TTI,
Figure BDA0002215260440000041
P≤1-Prel
finally, the optimal solution T is obtained*The obtained optimal solution of the corresponding time diversity order is expressed as Γ*The maximum value of the user equipment corresponding to the optimal solution of the time diversity times is expressed as M*；
Optimizing frequency diversity times, wherein beta belongs to {1,2,3, …, N }; one-dimensional search is carried out on all possible values of the beta value interval to obtain N groups of optimal solutions about the time slot length and the equipment number, namely { T* 1，M* 1}，{T* 2，M* 2}，{T* 3，M* 3}，…，{T* N，M* NFinding out the maximum user equipment number M from the optimal solutionsopt。
Compared with the prior art, the invention has at least the following beneficial effects:
the invention adopts a competition-based authorization-free transmission mode, and under the mode, the user equipment does not need to wait for resource allocation information or transmission authorization before sending the data packet, but can send the data packet immediately, thereby effectively reducing the system time delay; the invention jointly applies time diversity and frequency diversity technology based on a Slotted-Aloha model, namely, the uplink transmission time is equally divided into a plurality of mini time slots with equal length, the length of each time slot is equal to the length of a system TTI (transmission time interval), and in each TTI, user equipment randomly selects a plurality of copies for transmitting the same data packet in all sub-channels provided by the system; in this transmission mode, two or even a plurality of user equipments may try to send data packets on the same shared channel at the same time, so that collision may occur between the data packets, and by randomly selecting a channel and repeatedly transmitting the data packets for a plurality of times, the successful transmission probability of the data packets is greatly improved, thereby being beneficial to improving the reliability of the system; based on the transmission scheme, under the condition of simultaneously meeting the requirements of ultra-reliability and low time delay of URLLC, the invention simultaneously optimizes the transmission times of the data packet copies on the time domain and the frequency domain so as to maximize the number of user equipment which can be supported by the system.
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FIG. 1 is a diagram of a system model contemplated by the present invention.
Fig. 2 is a view showing a structure of a time frame considered by the present invention.
Fig. 3 is a diagram of a data packet transmission model according to the present invention.
FIG. 4 shows the value of the optimal solution of the time domain multiple-etching times and the optimal solution of the frequency domain multiple-etching times of the data packet along with the lower bound T of the TTI lengthminThe change curve of (2).
FIG. 5 shows a lower bound T on the TTI lengthminWhen the time domain multiple engraving times and the frequency domain multiple engraving times of the data packet respectively take the optimal solutions, the system can support the variation curve of the maximum value of the number of the user equipment.
FIG. 6 shows the time TminAnd when the time is 0.1ms, the system can support a three-dimensional relationship image between the number of the devices and the time domain multiple engraving times and the frequency domain multiple engraving times.
FIG. 7 shows the time TminAt 0.1ms, the relationship between the number of supportable devices and the packet arrival rate is plotted under three schemes of joint time-frequency diversity (Γ ═ 10, β ═ 2), time-domain-only complex imprinting (Γ ═ 10, β ═ 1), and frequency-domain-only complex imprinting (Γ ═ 1, β ═ 16).
FIG. 8 shows the time TminAt 0.1ms, under three schemes of joint time-frequency diversity (Γ ═ 10, β ═ 2), time-domain-only complex imprinting (Γ ═ 10, β ═ 1), and frequency-domain-only complex imprinting (Γ ═ 1, β ═ 16), a relationship curve between the number of devices and the total number of shared subchannels can be supported.
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings.
Referring to fig. 1 to fig. 3, an authorization-free uplink transmission method based on joint time-frequency diversity includes the following steps:
s1, establishing a data packet transmission system, wherein the data packet transmission system comprises M user equipment with single antenna and a base station, and the base station is provided with NtN shared sub-channels are arranged in the system;
s2, based on the system established by S1, the total time delay of the uplink transmission process is equally divided into gamma time slots, and in each time slot, the user equipment randomly selects a plurality of copies for transmitting the same data packet from all sub-channels provided by the system; and the data packet adopts a Slotted-Aloha-based protocol and an authorization-free transmission mode;
s3, calculating the probability of data packet transmission failure of the target device in the time slot S2;
and S4, under the condition of simultaneously meeting the URLLC delay requirement and the reliability requirement, maximizing the number of the devices which can be supported by the system in the S1, and obtaining the optimal solution of the time diversity and the frequency diversity times corresponding to the optimal solution.
The ultra-reliable low-delay communication is one of three application scenes of a fifth generation mobile communication system, and is required to have millisecond-level time delay and ultra-high reliability; in order to meet the requirements of ultra-reliability and low time delay of URLLC at the same time, the invention provides an authorization-free uplink transmission method based on joint time-frequency diversity.
First, contention-based unlicensed transmission: aiming at the low delay requirement of URLLC, the invention adopts a competition-based authorization-free transmission mode, and under the mode, the user equipment does not need to wait for resource allocation information or transmission authorization before sending the data packet, but can send the data packet immediately, thereby effectively reducing the system delay.
Secondly, joint time-frequency diversity: aiming at the ultra-high reliability requirement of URLLC, the invention jointly applies the time diversity and frequency diversity technology based on the Slotted-Aloha model, namely, the uplink transmission time is equally divided into a plurality of mini-slots (mini-slots) with equal length, and in each slot, the user equipment randomly selects a plurality of copies for transmitting the same data packet in all sub-channels provided by the system. In this transmission mode, it may happen that two or even more ues try to send data packets simultaneously on the same shared channel, and therefore collisions between data packets may occur. The scheme greatly improves the success probability of data packet transmission by randomly selecting channels and repeatedly transmitting the data packet copies for a plurality of times, thereby being beneficial to improving the reliability of the system.
Based on the transmission scheme, under the condition of simultaneously meeting the requirements of ultra-reliability and low time delay of URLLC, the invention simultaneously optimizes the number of copies of the data packet transmitted in time domain and frequency domain so as to maximize the number of user equipment which can be supported by the system.
And (3) system model: considering the contention-based unlicensed uplink transmission scenario, as shown in fig. 1, the data packet transmission system of the present invention includes M single-antenna User Equipments (UEs) and a base station, where the base station is equipped with NtAn antenna. The user equipment transmits data by randomly using resources; for each user equipment, a packet arrival process is modeled as a poisson process, M poisson processes are independent from each other, and since an uplink transmission process of the user equipment is license-free, a device which needs to send a packet does not need to send a scheduling request and does not need to wait for transmission of a grant or resource allocation information, a Slotted-Aloha protocol is a typical contention-based transmission model, and the contention model is used herein.
In order to apply frequency diversity, the system provides N shared subchannels for M user equipments; in each time slot, the device randomly selects beta pieces in the shared sub-channel to transmit the data packet copies, and instantaneous channel gains on N sub-channels are independent in order to maximize the gain of frequency diversity, namely, the frequency interval of the N sub-channels should exceed the coherence bandwidth W of the channelc(ii) a Let total bandwidth of system be WmaxThe bandwidth allocated to each shared sub-channel is
Figure BDA0002215260440000071
It is assumed that each shared sub-channel is allocated a bandwidth B less than the effective bandwidth W of the channelcThus, each shared subchannel is flat fading.
Let m be 1,2 … for the N devices, the large scale fading coefficient is αmInstantaneous channel gain of gi(ii) a Since the channel state information is unknown at the device due to the unlicensed transmission, the maximum transmission power of each user equipment is divided into β subchannels, and the maximum transmission power of each device is set to PmaxThen the transmission power allocated to each sub-channel is
Figure BDA0002215260440000072
Data packet transmission scheme based on joint time-frequency diversity
The invention discloses a transmission mechanism of a Sooka book data packet: in the contention-based authorization-free uplink transmission scenario, the user equipment can transmit the data packet in an 'instant-to-instant' manner without sending a scheduling request and receiving transmission authorization and resource allocation information; in order to improve the reliability of the system, time diversity and frequency diversity techniques are applied simultaneously in the data packet transmission process.
Total time delay D of uplink transmission processuThe total delay of the uplink transmission process is equally divided into Γ slots, and the time frame structure is shown in fig. 2. The length of each time slot is
Figure BDA0002215260440000073
Where Γ represents the number of time diversity, i.e., the number of copies of a packet transmitted in the time domain. In addition, the system provides N shared sub-channels for M user equipments, and in each time slot, the user equipment that needs to send packets randomly selects β (β ≦ N) channels from the N shared sub-channels provided by the system for transmitting β data packet copies, where β represents the frequency diversity times, i.e., the number of data packet copies transmitted in the frequency domain. If two or more packets select the same subchannel, then the selection is the sameIf the data packets of one sub-channel collide with each other, all the data packets of the selected channel fail to be transmitted; successful transmission of a packet on a channel is possible only if the channel is selected by only one packet. Each ue sends a total of f × β data packet copies, and as long as at least one of the data packet copies is guaranteed not to collide with any other data packet, it can be considered that the data packet that has not collided with the other data packet transmission is likely to be successfully sent, i.e. the ue sending the data packet is likely to be successfully transmitted. The packet transmission model is shown in fig. 3.
Even if a packet is fortunately enough to select a sub-channel that will not collide, the packet may fail to be sent due to poor channel performance. Applying a two-state channel transmission model; the meaning of the two states is: the data packet can be successfully transmitted only on the sub-channel with the instantaneous channel gain greater than or equal to the set threshold value, and the data packet is considered to be lost on the channel with the instantaneous channel gain less than the threshold value. Thus, a data packet can only be successfully transmitted without colliding with other packets and without the instantaneous channel gain of the selected sub-channel being less than the desired channel gain threshold.
Transmission failure probability analysis and calculation:
at DuAnd the packet sending probability of each device is as follows:
Pran＝1-e-λ (4)
wherein λ represents DuThe number of data packets arriving in. Let the average inter-arrival time of a packet be mu, then
Figure BDA0002215260440000081
For any user equipment except the target equipment, the target data packet does not collide with the data packet sent by any user equipment, and the following two conditions exist: firstly, the arbitrary user equipment does not send a packet, only the equipment to which the target data packet belongs can send the data packet, and the data packets from the same equipment cannot collide with each other, so that the target data packet cannot collide under the condition that the equipment to which the target data packet belongs can send the data packet; the second situation is that although any user equipment also sends a data packet, the data packet sent by the user equipment does not select the same subchannel as the target data packet, so that the target data packet is ensured not to collide; therefore, the probability that any other ue will not collide with the target packet is:
P0＝(1-Pran)+PranPacc (5)
wherein, PaccProbability that the data packet sent by any other user equipment does not select the same subchannel with the target data packet; then, when all other user equipments send data packets without collision with the target data packet, the target data packet will not collide; the probability of no collision is:
Psucc＝P0 M-1＝[(1-Pran)+PranPacc]M-1 (6)
the probability P is as followsaccThe solving process of (1).
Firstly, a target data packet is set to select a certain sub-channel, and all data packets sent by other user equipment are not allowed to select the sub-channel selected by the target data packet in order to ensure that the data packet does not collide with other data packets; therefore, in a timeslot, the probability that a data packet sent by any other ue and a target data packet do not select the same subchannel is:
Figure BDA0002215260440000091
in a timeslot, the probability of collision of the target packet is:
Figure BDA0002215260440000092
in the data packet transmission scheme of joint time-frequency diversity, each user equipment transmits gamma multiplied by beta data packet copies together; for any user equipment, as long as at least one data packet in the data packet copy is guaranteed to be successfully transmitted, the user equipment which transmits the data packet can be considered to be successfully transmitted.
The successful transmission of a data packet needs to satisfy two conditions: the first condition is that when randomly selecting a sub-channel, the selected sub-channel is selected by only one data packet of a target data packet, namely the data packets cannot collide; the second condition is that the channel performance of the selected sub-channel of the data packet needs to meet the transmission condition on the premise of no collision, i.e. the instantaneous channel gain of the selected channel should not be less than the required channel gain threshold; if the instantaneous channel gain is greater than or equal to the channel gain threshold (g)i≥gth) Then the data packet can be transmitted to
Figure BDA0002215260440000093
Is successful, wherein
Figure BDA0002215260440000094
Representing the bit error rate; if the instantaneous channel gain is less than the channel gain threshold (g)i<gth) The packet is considered lost.
For a single-input multiple-output system, in each time slot, the maximum number of bits transmitted on the ith subchannel by the mth device is,
Figure BDA0002215260440000101
where i is 1,2 …, β, T is the length of the time slot, B is the bandwidth allocated to each subchannel, αmIs the large-scale fading coefficient, P, of the device mmaxFor maximum transmission power per device, giFor instantaneous channel gain, N0Is a single-sided power spectral density, em,iIn order to be an error rate,
Figure BDA0002215260440000102
representing the inverse of the Q function. From the calculation formula (1), the maximum number of bits to be transmitted increases with the increase of the instantaneous channel gain, and the size of the data packet to be transmitted is set to b bits when R ism,iWhen b is equal, the channel gain threshold can be obtained by the calculation formula (1)
Figure BDA0002215260440000103
Comprises the following steps:
Figure BDA0002215260440000104
if the instantaneous channel gain is greater than or equal to the channel gain threshold (g)i≥gth) Then the data packet can be transmitted to
Figure BDA0002215260440000105
The probability of being successful; if the instantaneous channel gain is less than the channel gain threshold (g)i<gth) The packet is considered lost. Then the packet loss probability on the ith channel is:
Figure BDA0002215260440000106
wherein the content of the first and second substances,
Figure BDA0002215260440000107
Ntthe number of antennas.
Therefore, the packet loss probability of the data packet on the ith channel is PtThen, the transmission failure probability of the target data packet on the channel is:
Figure BDA0002215260440000108
then, in one timeslot, the transmission failure probability of the data packet sent by the target device UE (i.e. the probability of all transmission failures of the selected β channels) is:
Figure BDA0002215260440000109
each UE also repeats the transmission of the packet in Γ slots, and then the probability of the target UE failing to transmit (i.e., all of Γ × β data packets failing) in the repeated Γ slots is:
Figure BDA0002215260440000111
2.3 optimization problem modeling
And constructing an optimization problem based on the derivation, jointly optimizing the time diversity times and the frequency diversity times, maximizing the number of devices supported by the system under the condition of simultaneously meeting the URLLC time delay requirement and the reliability requirement, and solving the optimal solution of the corresponding time diversity times and the frequency diversity times. The optimization problem is modeled as follows:
Figure BDA0002215260440000112
St.T∈[Tmin,Du] (13)
Figure BDA0002215260440000113
Figure BDA0002215260440000114
P≤1-Prel (16)
Figure BDA0002215260440000115
the constraint condition of the expression (13) ensures that the time slot length cannot exceed the value interval in the optimization process; approximation of expression (14)The beam condition embodies the relation between the time diversity times and the time slot length, wherein [ ·]Indicates rounding down the number in parentheses, pass; the constraint condition of expression (15) is to limit the frequency diversity order; the constraint of expression (16) ensures the URLLC reliability requirement, P, of the systemrelIs a constant; the constraint of expression (17) constrains the number of user equipments to a positive integer.
3 solving an optimization problem using a two-step method based on the PSO algorithm
A two-step approach is proposed to solve the above optimization problem;
step 1: optimizing time diversity order
Giving the value of the frequency diversity times, optimizing the length of a time slot by applying a particle swarm algorithm, solving according to a relational expression (14) of the time slot length and the time diversity times to obtain an optimal solution of the time diversity times, and solving the maximum value of corresponding supportable equipment; since the probability of packet transmission errors increases with the number of ues, we solve the maximum probability of packet transmission errors by directly substituting the maximum probability of packet transmission errors into the optimization problem (12) in order to obtain the maximum number of ues that can be supported. In addition, in order to further simplify the solving process of the optimization problem, the positive integer limiting condition of the equipment number M is not considered, and only the downward rounding operation is carried out on the obtained maximum value of M; thus, the original optimization problem can then be simplified as follows:
Figure BDA0002215260440000121
St.T∈[Tmin,Du] (19)
Figure BDA0002215260440000122
P＝1-Prel (21)
wherein, the constraint condition of the expression (19) is the whole search space of the time slot length solved by the particle swarm algorithm; the constraint condition of expression (20) representsThe relation between the time slot length and the time diversity times; the constraint condition of the expression (21) ensures the system reliability requirement; the optimal solution obtained by the optimization problem is represented as T*Expressing the obtained optimal solution of the corresponding time diversity times as Gamma*The maximum value of the user equipment corresponding to the optimal solution of the time diversity times is expressed as M*。
Step 2, optimizing frequency diversity times
Because the number of shared subchannels provided by the system is limited, all possible values of the frequency diversity times are also limited, i.e. β ∈ {1,2,3, …, N }; one-dimensional search is carried out on all possible values of the beta value interval, and N groups of optimal solutions about the time slot length and the equipment number, namely { T }* 1，M* 1}，{T* 2，M* 2}，{T* 3，M* 3}，…，{T* N，M* NThe subscripts of the optimal solutions correspond to the values of the corresponding frequency diversity times; then, the maximum user equipment number M is found out from the optimal solutionsoptAnd the corresponding optimal solution of the time slot length and the optimal solution of the frequency diversity times are the optimal solutions of the original optimization problem.
The method of the invention is adopted to carry out numerical simulation and result analysis
The system parameters set by the simulation are shown in table 1:
TABLE 1 simulation parameters
Figure BDA0002215260440000131
Lower bound T of value range of time slot length in optimization problemminThe value of the optimal solution is also affected.
FIG. 4 shows the lower bound T on the slot lengthminUnder different value conditions, optimizing the value condition of the optimal solution of the problem. As can be seen from the figure, as the lower bound of the time slot length increases, the optimal solution of the time domain multiple engraving times decreases progressively, and the optimal solution of the frequency domain multiple engraving times increases progressively.
FIG. 5 shows the lower bound T on the slot lengthminAnd under the condition of different values, when the time domain multiple engraving times and the frequency domain multiple engraving times respectively take the optimal solutions, the number of the supportable user equipment is taken as a value. As can be seen from the figure, as the lower bound of the slot length increases, the maximum supportable number of ues decreases; in the following experimental simulation, T is takenmin＝0.1ms。
Fig. 6 shows the variation of the number of user equipments supportable by the system under different frequency domain multiple engraving times and time domain multiple engraving times. It can be seen from the figure that no matter what value the time domain multiple engraving times are, the number of the user equipments supportable by the system always increases and then decreases with the increase of the frequency domain multiple engraving times. On the one hand, the increase of the frequency domain duplication times means that the number of subchannels which can be selected by the user equipment for transmitting the data packet duplicates is increased in each time slot, which reduces the probability of collision between data packets; on the other hand, since the maximum transmission power allocated to each device is limited, when the number of frequency domain repetitions increases, the transmission power allocated to each subchannel decreases, resulting in an increase in the packet loss rate occurring on the channel. When the frequency domain repeated engraving times are smaller, the influence of the reduction of the collision probability is larger than the influence of the increase of the packet loss rate, so that the system can support the equipment number to be in an ascending trend; when the frequency domain repetition times increase to exceed a certain threshold, the influence of the increase of the packet loss rate becomes a dominant factor influencing the number of supportable devices, so that the number of user devices meeting the QoS requirement tends to decrease. Fig. 6 also shows the relationship between the number of supportable devices and the number of time domain multiple engraving. As can be seen from the figure, the more the time domain repetition times are, the more the number of user equipments can be supported by the system. This is because by increasing the number of time domain repetitions, the probability of collisions between packets is greatly reduced, thereby allowing a significant increase in supportable user equipment.
Fig. 7 shows a relationship between the number of supportable devices and an arrival rate of a data packet in a case where an optimal solution is respectively obtained for a joint diversity scheme, a time-domain-only complex etching scheme, and a frequency-domain-only complex etching scheme, where the time-domain complex etching frequency of the joint scheme is 10 and the frequency-domain complex etching frequency is 2; in the time domain only multiple engraving scheme, the frequency domain multiple engraving frequency is 1, and the time domain multiple engraving frequency is 10; in the frequency domain only repeated engraving scheme, the frequency domain repeated engraving frequency is 16, and the time domain repeated engraving frequency is 1; as can be seen, as the packet arrival rate increases, the number of supportable ues decreases. This is because the higher the arrival rate of the data packets, the higher the probability that the user equipment is activated, the greater the number of devices that need to transmit the data packets, the greater the probability that collisions occur between the data packets, and thus the fewer the number of devices that can be supported by the system.
Fig. 8 shows the relationship between the number of supportable devices and the total number of shared sub-channels in the case that the joint diversity scheme, the time-domain-only complex etching scheme, and the frequency-domain-only complex etching scheme respectively take optimal solutions. The more number of shared subchannels the system provides, the more number of devices that can be supported, while ensuring that the allocated bandwidth per subchannel remains the same. Since the more the number of shared channels, the greater the probability that a packet will select a different sub-channel, the lower the probability of collision between packets, and thus the number of supportable devices will increase.
As can be seen from the figure, the performance gain of the joint diversity scheme is the best, which is only a time domain multiple etching scheme, and the performance gain of the frequency domain multiple etching scheme is the smallest. It can be seen that the performance advantages of the joint diversity scheme proposed by the present invention mainly result from the optimization of the slot length and the random selection of different sub-channels for data packet transmission in each slot for the data packet copy.
In summary, the authorization-free URLLC uplink transmission scheme based on joint time-frequency diversity provided by the present invention not only can meet the delay and reliability requirements of URLLC service, but also can maximize the number of devices supportable by the system by selecting the optimal time domain multiple engraving times and frequency domain multiple engraving times of the data packet.
Simulation results show that: compared with the existing scheme, the scheme provided by the invention not only effectively guarantees the service quality requirement of the URLLC service, but also obviously improves the number of user equipment which can be supported by the system, and has very important practical significance and application prospect.
The foregoing is a detailed description of the invention and is not to be taken as limiting, since numerous simple deductions and substitutions may be made by those skilled in the art without departing from the spirit of the invention, which is to be construed as falling within the scope of the invention as defined by the appended claims.

Claims (6)

1. An authorization-free uplink transmission method based on joint time-frequency diversity is characterized by comprising the following steps:
s1, establishing a data packet transmission system, wherein the data packet transmission system comprises M user equipment with single antenna and a base station, and the base station is provided with NtN shared sub-channels are arranged in the system;
s2, based on the system established by S1, the total time delay of the uplink transmission process is equally divided into gamma time slots, and in each time slot, the user equipment randomly selects a plurality of copies for transmitting the same data packet from all sub-channels provided by the system; and the data packet adopts a Slotted-Aloha-based protocol and an authorization-free transmission mode;
s3, calculating the probability of the data packet transmission failure of S2 in a time slot;
and S4, under the condition that the URLLC time delay requirement and the reliability requirement are met, maximizing the number of the devices which can be supported by the system in the S1, obtaining a series of optimal solutions of the time diversity and the frequency diversity times corresponding to the optimal solutions, and obtaining the maximum value of the supportable devices through the optimal solutions.
2. The joint time-frequency diversity based unlicensed uplink transmission method according to claim 1, wherein the instantaneous channel gains on the N subchannels of S1 are independent of each other, i.e. the frequency spacing of the N subchannels exceeds the channel coherence bandwidth Wc(ii) a Total system bandwidth of WmaxThe bandwidth allocated to each shared sub-channel is
Figure FDA0003229351440000011
Each shared subchannel is flat fading.
3. The method of claim 1, wherein in S2, the data packet can be successfully transmitted on the sub-channel with the instantaneous channel gain greater than or equal to the predetermined threshold, and on the channel with the instantaneous channel gain less than the threshold, the data packet is considered to be lost, that is, the data packet is successfully transmitted without colliding with other packets and the instantaneous channel gain of the selected sub-channel is not less than the required channel gain threshold.
4. The grant-free uplink transmission method based on joint time-frequency diversity according to claim 1, wherein in S3, the probability that any other ue will not collide with the target packet is:
P0＝(1-Pran)+PranPacc
wherein, PaccProbability that the data packet sent by any other user equipment does not select the same subchannel with the target data packet; pranIs the packet sending probability of each device in the total time delay of the uplink transmission process;
Pran＝1-e-λ
Figure FDA0003229351440000021
wherein D isuDenotes the total time delay of the uplink transmission process, and λ denotes DuThe number of data packets arriving internally, the average inter-arrival time of the data packets is mu, then
Figure FDA0003229351440000022
The probability that any other user equipment does not collide with the target data packet is as follows:
P0＝(1-Pran)+PranPacc
when all other user equipment send data packets without collision with the target data packet, the target data packet does not collide; the probability of no collision is:
Psucc＝P0 M-1＝[(1-Pran)+PranPacc]M-1；
Psuccthe probability that the data packet does not collide is obtained;
then, the probability that the target data packet collides in one time slot is obtained, which is:
Figure FDA0003229351440000023
n is the total number of subchannels provided by the system, and β is the frequency diversity order.
5. The grant-free uplink transmission method based on joint time-frequency diversity according to claim 4, wherein in repeating Γ slots, the probability of transmission failure of the target UE is:
Figure FDA0003229351440000024
wherein, PFDThe probability of transmission failure of a data packet sent by a target device UE in one time slot,
Figure FDA0003229351440000025
for the probability of a collision of a target packet in a time slot, PtThe packet loss probability of a data packet on a certain channel is shown.
6. The method of claim 5, wherein in step S4, the optimal solution of the time diversity and frequency diversity times corresponding to the optimal solution is obtained as follows:
giving the value of the frequency diversity times, optimizing the length of a time slot by applying a particle swarm algorithm, solving according to the relation between the time slot length and the time diversity times to obtain the optimal solution of the time diversity times, and solving the maximum value of the corresponding supportable equipment;
converting the optimal solution of the time diversity times and the frequency diversity times into a calculation model, which comprises the following steps:
Figure FDA0003229351440000031
t is the length of each time slot, DuIs the total time delay of the uplink transmission process, beta represents the frequency diversity times,
let St.T ∈ [ T ∈ [ ]min,Du]As a constraint condition, the time slot length is ensured not to exceed the value range [ T ] in the solving processmin,Du]，
Figure FDA0003229351440000032
Shows the relation between the time diversity times and the time slot length, restrains the time diversity times by optimizing the length of TTI,
Figure FDA0003229351440000033
P≤1-Prel
finally, the optimal solution T is obtained*The obtained optimal solution of the corresponding time diversity order is expressed as Γ*The maximum value of the user equipment corresponding to the optimal solution of the time diversity times is expressed as M*(ii) a M is the number of single antenna user equipments, PrelIs a constant, representing the reliability requirements that the device should meet;
optimizing frequency diversity times, wherein beta belongs to {1,2,3, …, N }; one-dimensional search is carried out on all possible values of the beta value interval to obtain N groups of optimal solutions about the time slot length and the equipment number, namely { T* 1，M* 1}，{T* 2，M* 2}，{T* 3，M* 3}，…，{T* N，M* N}，Finding the maximum number M of user equipments from these optimal solutionsopt。
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