CN114980144B - Method for evaluating probability of multi-channel unlicensed retransmission scheme under low time delay - Google Patents
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Abstract
The invention discloses a method for evaluating the probability of a multichannel unlicensed retransmission scheme under low time delay, which comprises the following steps: 1) Constructing a system model, wherein a user adopts a short packet form to transmit a data packet, and accesses a base station through an unlicensed retransmission technology; establishing a single-channel outage probability problem model by combining time delay constraint; 2) Given transmitting power, the original short packet is averagely divided into a plurality of blocks based on a single-channel model, the blocks are transmitted on a plurality of mutually independent channels, a receiving end is combined and then decoded, the maximum round-trip transmission process times are obtained by combining time delay constraint, and a Monte Carlo solution of the outage probability is established by utilizing a probability theory; 3) And selecting channels with the maximum channel gain for transmission, and obtaining the Monte Carlo solution of the outage probability based on a single-channel transmission model. The invention can improve the interruption probability of the single-channel 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 through the comparison of the two multi-channel schemes.
Description
Technical Field
The invention belongs to the field of unlicensed transmission system design, and particularly relates to a method for evaluating probability of a multichannel unlicensed retransmission scheme under low time delay.
Background
Uplink transmissions in cellular networks are typically grant-based, i.e. a User Equipment (UE) may transmit its payload from a serving Base Station (BS) through a Scheduling Grant (SG). GF unlicensed operation, i.e. the terminal initiates transmission without requesting SG, is considered as a promising solution for critical delay services aimed at by fifth generation (5G) radio access technology. Semi-persistent scheduling (SPS) with pre-allocated radio resources is the best known unlicensed operation and is considered to be an efficient method of handling periodic traffic. In case of sporadic packet arrivals, we advocate the use of shared radio resources for a group of UEs without uplink grants as a necessary solution to avoid forbidden resource wastage.
Recently, due to the emergency of a large number of new use cases of the internet of things (LoT), the research community has been paid attention again to the propagation of collision tendency based on shared resources, mainly focusing on large-scale access. Collisions are considered as destructive events in the basic aloha and slotted aloha protocol known limitations with respect to cell throughput. However, simultaneous transmissions may still be resolved in practice, especially if the interference suppression receiver is in place. An uncoordinated strategy is presented to maximize the slot random channel throughput. Enhanced non-orthogonal schemes, such as Sparse Code Multiple Access (SCMA) or interleaved multiple access (IDMA), achieve robustness to collisions by using user-specific signatures and aim to increase the unit capacity relative to conventional orthogonal resource allocation. The fundamental limits of the easy-to-collide random access communication under noise-limited and interference-limited conditions are derived. Most of the work in existence has focused on maximizing the number of supported users for a given resource, as well as limiting the energy consumption of the device. However, the constraint of low latency is typically ignored.
The third generation partnership project (3 GPP) standardization body for the upcoming 5G New Radio (NR) discusses different options for delay-critical uplink non-dial-up transmissions. The latency and throughput schemes of performance have recently been empirically evaluated in large networks where extensive system level simulation is provided. Consider an ultra-reliable low latency communication (URLLC) service with a 1ms delay with a challenging outage probability targeted at 10-5. A recent contribution has studied collision probability in shared resources, considering the possibility of solving them through repetition mechanisms and multi-user detection; the analysis is based on experience from previous simulation studies to perform threshold detection of signal to interference plus noise ratio (SINR). In general, extensive system-level simulation to address reliability performance requires more snapshots to capture a low percentile of failure probabilities of URLLC service targets than traditional broadband transmission studies.
However, the limited single channel transmission resources can lead to a large collision probability, so that the reliability is reduced, and through continuous research, students find that the multi-channel technology can reduce the collision probability, shorten the transmission delay and improve the reliability of communication. Some authors propose a multi-channel alohaGF access scheme for heterogeneous networks with sporadic traffic and investigate the performance index of delay violation probability. Effective capacity and QoS index in multi-channel fading in low-delay communication are studied, and an upper bound of error probability and a lower bound of QoS index are derived. There are also authors studying the optimal channel switching (time sharing) strategy under average power and cost constraints to maximize the average number of correctly received symbols between transmitters and receivers connected by multiple additive gaussian noise channels, and also proposed to consider multi-channel selection diversity where the user selects one channel with the greatest channel gain, however none of the above documents focuses on outage probability assessment of a multi-channel GF access scheme under URLLC, and makes a comparison of multiple schemes to analyze the trend of outage probability in different scenarios.
Therefore, aiming at a 5G ultra-reliable low-delay communication scene, it is valuable to research an outage probability evaluation method of an unlicensed short packet retransmission technology under multiple channels.
Disclosure of Invention
The technical problem to be solved by the invention is to overcome the defects of the prior art, and provide a method for evaluating the probability of a multi-channel unlicensed retransmission scheme under low time delay, which not only can accurately obtain the performance index of a system, but also can reveal the influence of time delay constraint on the transmission interruption probability of the system.
The method comprises the following steps:
1) A network scene of single channel transmission is introduced, a system model is constructed, wherein a user adopts a short packet form to transmit a data packet, and accesses a base station through an unlicensed retransmission technology;
2) Establishing a single-channel outage probability problem model by combining time delay constraint;
3) And constructing a multi-channel transmission model, and obtaining a Monte Carlo solution of the outage probability.
Preferably, in step 1), the network scenario is: n users share a WHz frequency band to carry out uplink unlicensed transmission, and the bandwidth is divided into b frequency blocks with equal size; under the URLLC service, the data packet of the user equipment is transmitted in the form of a short packet, and the short packet is transmitted in a mode of being ready to go by combining with the uplink contention-based unlicensed access technology, so that a scheduling application is not required to be sent to a base station, and resource authorization from the base station is not required to be received; when a data packet arrives, a user equipment randomly selects one of b frequency blocks for transmission; irrespective of the link adaptation, each B-bit packet is mapped to a frequency block, N OFDM symbols corresponding to a transmission time interval of one duration TTI seconds; the packet arrival rate per transmission time interval is denoted by λ and the user is considered to be fully synchronized, assuming λt0< <1, where T0 represents the total time required to transmit the packet; the inter-packet arrival time is longer than the time required for transmitting the packet, and the queuing effect of each user equipment is ignored; an unlicensed transmission consists of a user equipment identifier; the channel is a flat rayleigh fading channel; the user's transmissions are received at the same average power; the receiving end is provided with M receiving antennas and an MRC receiver.
Preferably, the accessing the base station through the unlicensed retransmission technology in step 1) is: firstly, carrying out limited block length coding on a data packet transmitted by a real-time user, and accessing an access base station through competition; and decoding the short packet received by the base station, feeding back to the real-time user, and retransmitting if the receiving fails.
Preferably, the method for constructing the time delay constraint in the step 2) comprises the following steps: combining with the short packet transmission characteristics under URLLC, firstly obtaining the time delay of one round trip transmission process of the system as T RTT ＝gT tx +T bp +T fb +T up Wherein T is fb Representing base station feedback delay, T bp Representing the processing delay, T, of the base station up Representing user processing delay, T tx Represents the transmission time delay, and assume T fb 、T bp 、T up And T is tx The values are equal, and g represents the number of times of repeated HARQ packet sending; the sum of delays after m system scheme round trip transmissions is further obtained as: t (m) =t fa +mT RTT ＝[1+m(g+3)]T tx Wherein T is fa Representing frame alignment delay, also with T tx Equal in value.
Preferably, step 2) establishes a single channel outage probability model package in combination with a time delay constraintThe method comprises the following steps: the goal of UL unlicensed transmission for URLLC is to ensure that a large number of user equipments can successfully complete their payload delivery in a limited time, and that the outage probability is not higher than a certain target P out,t The method comprises the steps of carrying out a first treatment on the surface of the To calculate the outage probability, the following variables are defined:
ε 1 : missing the probability of user equipment identification;
P s (x) The method comprises the following steps Probability of x users selecting the same frequency block for transmission, x=0, …, N-1;
ε 2 (x) The method comprises the following steps When x user interference is active on the same frequency block transmitted by the user equipment, the data decoding error probability of the user equipment of interest is received with average signal-to-noise ratio (SNR) gamma;
according to the definition above, the probability of correctly decoding the payload is as follows:
the probability of failure of 1 transmission is: p (P) f1 ＝ε 1 +(1-ε 1 )(1-P d ) Indicating that the preamble error is identified or that the identification is correct but the decoding is wrong;
the probability of success of 1 HARQ is: p=1- (P) f1 ) g Indicating that the transmission failed only if each of the g transmissions failed;
the probability that m HARQ would be needed to succeed is: p (P) m ＝(1-P) m-1 P represents that the transmission fails m-1 times before and the last time succeeds;
the single channel outage probability problem model is: p (P) out.t ＝1-P m 。
Preferably, the method is characterized in that,r is the transmission rate, B is the packet length;
wherein Q is -1 Representing the inverse of the Gaussian function, ε represents the bit error rate generated by the short packet, and O (log B/B) represents an infinitesimal fraction of log B/B; c represents shannon capacity and has the expression that N represents the number of users:
C(η)＝log 2 (1+η)
v is the channel dispersion, which represents the characteristics of the channel, expressed as follows:
where η represents the noise ratio.
Preferably, the step 3) of constructing a multi-channel block transmission model, and the step of obtaining the monte carlo solution of the outage probability includes: given transmitting power, the original short packet is averagely divided into a plurality of blocks based on the system model, the short packet is transmitted on a plurality of mutually independent channels, and each channel obeys Rayleigh flat fading; combining time delay constraint to obtain the maximum round trip transmission process times, and establishing a Monte Carlo solution of the interrupt probability by utilizing a probability theory; the method comprises the following specific steps:
assuming q channels, each transmitting B/q bytes
R＝B/(a*q)
γ i Representing the signal-to-noise ratio of the receiving end byCalculating; since the power of the transmitting end is assumed to be the same, and the signal-to-noise ratio is snr=hp t /σ 2 The ratio of the signal to noise ratio to the channel gain to the noise gain, i.e. h/sigma 2 Proportional, sigma i Representing the noise power of the ith channel, the noise being Gaussian white noise, i.e. σ, due to the use of Rayleigh fading channels 2 ＝N 0 /2，γ i Can be simplified into gamma i ＝(h i /h 1 )*γ 1 The method comprises the steps of carrying out a first treatment on the surface of the As a result of: />H is the channel gain, so that the signal-to-noise ratio of the receiving end is in direct proportion to the channel gain under the condition of certain transmitting power;
the probability of failure of 1 transmission is: p (P) f1 ＝ε 1 +(1-ε 1 )(1-P d ) Indicating that the preamble error is identified or that the identification is correct but the decoding is wrong;
the probability of success of 1 HARQ is: p=1- (P) f1 ) n Meaning that only every n transmissions failed. The transmission time of the data packet in each HARQ is shortened due to the block transmission, so that the retransmission times n and the HARQ times m are changed, and the value of m can be determined after the value of n is selected.
T(m)＝T fa +mT RTT ＝[1+m(n+3)]T tx ，
The probability that m HARQ would be needed to succeed is: p (P) m ＝(1-P) m-1 P represents that the transmission fails m-1 times before and the last time succeeds;
interrupt probability: p (P) out.t ＝1-P m 。
Preferably, the step 4) of constructing a multi-channel selective transmission model, and the specific step of obtaining the monte carlo solution of the outage probability includes: a plurality of channels select a channel with the largest channel gain for low transmission; based on the single-channel transmission model, obtaining a Monte Carlo solution of the interruption probability;
γ i ＝(h max /h 1 )*γ 1
h max for maximum channel gain in q channels, gamma i Representing the signal-to-noise ratio of the receiving end, wherein the signal-to-noise ratio of the receiving end is calculated and changed according to the selected channel, but the signal-to-noise ratio is proportional to the single channel model, and the rest of the interruption probability solving formulas are the same as the single channel model, and are unchanged, and only the decoding failure probability epsilon is obtained 2 A change occurs.
The beneficial effects are that: compared with a system transmission scheme of single-channel transmission, the two multi-channel transmission schemes in the invention are more in line with the transmission characteristics of 5G URLLC, are more in line with an outage probability analysis model in a scene of short packet transmission and low delay correlation, verify the accuracy of outage probability analysis in Monte Carlo simulation analysis, and can reduce outage probability.
Drawings
FIG. 1 is a system block diagram of an embodiment of the present invention.
Fig. 2 is a graph of outage probability for three schemes with respect to packet arrival rates for a multi-channel case.
Fig. 3 is a graph of outage probability for two multi-channel schemes with respect to the number of channels in a multi-channel case.
Detailed Description
The invention comprises the following steps:
a method for evaluating the probability of a multi-channel unlicensed retransmission scheme under low time delay comprises the following steps:
1) Introducing single channel transmissionNetwork scenario of the transport: wherein, the user adopts short packet form to transmit data packet, and accesses the base station through the unauthorized retransmission technique: firstly, carrying out finite block length coding on data packets transmitted by a real-time user, defining a Transmission Time Interval (TTI) by using a transmission rate and a packet length, accessing a base station through competition, and analyzing and obtaining the total time delay of a system in a round trip transmission process according to a given transmission time intervalAnd calculating the competitive probability and the error rate by utilizing the probability theory related knowledge to obtain an outage probability model.
2) Multi-channel block transmission model: given the transmitting power, the original short packet is divided into a plurality of blocks on the basis of a single-channel model, the blocks are transmitted on a plurality of mutually independent channels, each channel obeys Rayleigh flat fading, and the corresponding channel gains are different, so that the error rates are also different. Combining time delay constraint to obtain the maximum round trip transmission process times, and establishing a Monte Carlo solution of the interrupt probability by utilizing a probability theory; the method comprises the steps of carrying out a first treatment on the surface of the
3) Multi-channel selective transmission model: the channels with the largest channel gain are selected for transmission, so that the signal-to-noise ratio of a receiving end is increased and the error rate response is reduced due to the fact that the largest channel gain is selected. And obtaining the Monte Carlo solution of the outage probability based on the single-channel transmission model.
The specific steps of the step 2) comprise:
21 The network scenario is: n users share a WHz frequency band, and uplink unlicensed transmission is carried out on the N users, and the bandwidth is divided into b frequency blocks with equal size; under the URLLC service, the data packet of the user equipment is transmitted in the form of a short packet, and the short packet is transmitted in a mode of being ready to go by combining with the uplink contention-based unlicensed access technology, so that a scheduling application is not required to be sent to a base station, and resource authorization from the base station is not required to be received; when a packet arrives, a UE randomly selects one of the b frequency blocks for transmission. Irrespective of the link adaptation, each B-bit packet is mapped to a frequency block, and N OFDM symbols correspond to a transmission time interval of one duration TTI seconds. We denote the packet arrival rate per TTI by λ and the user is considered to be fully synchronized. We further assume λt0< <1, where T0 represents the total time required to transmit the packet (including the final retransmission); the inter-packet arrival time is greater than the time required to deliver the packet, so the queuing effect of each UE can be ignored. An unlicensed transmission consists of a UE identifier that is mapped onto, e.g., a preamble followed by a payload. Consider UEs operating on a flat rayleigh fading channel, i.e., the channel response is constant over a selected frequency block, however, it may change every time it is transmitted or retransmitted. Users are power controlled so that their transmissions are received at the same average power, although their instantaneous received power may vary at each transmission due to rayleigh fluctuations, the receiving end being equipped with M receive antennas and MRC receivers.
22 The system model: the real-time user transmits the short packet to the service base station through an unauthorized technology in a short packet form, the short packet received by the service base station is decoded and fed back to the real-time user, and if the receiving fails, retransmission is carried out;
23 Combining with the short packet transmission characteristics under URLLC, firstly obtaining the time delay of one round trip transmission process of the system transmission scheme as T RTT ＝gT tx +T bp +T fb +T up Wherein T is fb Representing base station feedback delay, T bp Representing the processing delay, T, of the base station up Representing user processing delay, T tx Represents the transmission time delay, and assume T fb 、T bp 、T up And T is tx The values are equal, and g represents the number of times of repeated HARQ packet sending; further obtaining the sum of delays after m system scheme round trip process transmissions in the system transmission scheme as follows: t (m) =t fa +mT RTT ＝[1+m(g+3)]T TTI ＝[1+m(g+3)]B/R, wherein T fa Representing frame alignment delay, also with T tx Equal in value, B represents the data packet length, R represents the data rate under the finite block length coding;
24 The system combines time delay constraint to establish a single channel outage probability problem model: the goal of UL unlicensed transmission for URLLC is to ensure that a large number of UEs can be inSuccessful completion of its payload delivery within a limited time and with a outage probability not higher than a certain target P out,t . To calculate the outage probability, the following variables are defined:
ε 1 : probability of missing UE identification
P s (x) The method comprises the following steps Probability of x users selecting the same frequency block for transmission, x=0, …, N-1;
ε 2 (x) The method comprises the following steps When x user interference is active on the same frequency block transmitted by the UE, the data decoding error probability of the UE of interest is received with an average signal-to-noise ratio (SNR) γ.
According to the definition above, the probability of correctly decoding the payload is as follows:
and calculating outage probability, wherein the probability of failure of one transmission is as follows:
P f1 ＝ε 1 +(1-ε 1 )(1-P d )
indicating that the preamble error is identified or that the preamble error is identified but the decoding error is correct.
The probability of success of 1 HARQ is:
P＝1-(P f1 ) g indicating that the transmission failed only if each of the g transmissions failed.
The probability that m HARQ would be needed to succeed is:
indicating that the previous m-1 transmissions failed and the last successful.
Interrupt probability:
P out.t ＝1-P m
the specific steps of the step 1) comprise:
31 Calculating probability P of correct decoding d
P a ＝1-e -λ
P s (x) Indicating the probability that the users have no collision, p (u, x) indicates the probability that x users out of the u users have a collision.
P a Representing the probability of arrival of the data, subject to the probability distribution.
32 Calculating error rate epsilon 2 (x)
C(η)＝log 2 (1+η)
R=b/a. b is the number of blocks divided by a channel, x is the number of competing users, and M is the number of antennas at the receiving end. Beta represents the instantaneous SNR, gamma represents the received signal-to-noise ratio, which is constant for one TTI time, but re-transmission is performedAnd changes will occur. Note that the received signal-to-noise ratio gamma changes at each new transmission due to the change in the rayleigh fading channel gain, and thus epsilon 2 (x) Also in the variation, the value is updated every time it is calculated.
In step 1): the relation expression of the transmission rate R and the data packet length B under the finite block length is as follows:
wherein Q is -1 Representing the inverse of the Gaussian function, ε represents the bit error rate generated by the short packet, and O (log B/B) represents an infinitesimal fraction of log B/B; c represents shannon capacity and has the expression:
C(η)＝log 2 (1+η)
v is the channel dispersion, which represents the characteristics of the channel, expressed as follows:
wherein η represents the noise ratio; obtaining transmission time delay T according to finite block length transmission rate and packet length tx The expression is:
the transmission delay T (m) of one round trip transmission process is obtained according to the system frame principle:
since the transmission time in short packet transmission is short, a frame alignment delay T can be assumed fa Base station feedback delay T fb Base station processing delay T bp And user processing delay T up Are all delayed from the transmission time T tx Equality, further obtain the transmission time after m round trip transmission processesThe delay T (m) is as follows:
T(m)＝T fa +mT RTT ＝[1+m(g+3)]T TTI ＝[1+m(g+3)]B/R
the multi-channel block transmission model in step 2) is:
assuming q channels, each transmitting B/q bytes
R＝B/(a*q)
γ i Representing the signal-to-noise ratio at the receiving end can be achieved byCalculating; since the power of the transmitting end is assumed to be the same, and the signal-to-noise ratio is snr=hp t /σ 2 The ratio of the signal to noise ratio to the channel gain to the noise gain, i.e. h/sigma 2 Proportional, sigma i Representing the noise power of the ith channel, the noise being Gaussian white noise, i.e. σ, due to the use of Rayleigh fading channels 2 ＝N 0 /2，γ i Can be simplified into gamma i ＝(h i /h 1 )*γ 1 The method comprises the steps of carrying out a first treatment on the surface of the As a result of: />Where h is the channel gain, so that the signal-to-noise ratio of the receiving end is proportional to the channel gain when the transmit power is constant.
1 time of transmissionThe probability of failure of the infusion is: p (P) f1 ＝ε 1 +(1-ε 1 )(1-P d ) Indicating that the preamble error is identified or that the identification is correct but the decoding is wrong;
the probability of success of 1 HARQ is: p=1- (P) f1 ) n Meaning that only every n transmissions failed. The transmission time of the data packet in each HARQ is shortened due to the block transmission, so that the retransmission times n and the HARQ times m are changed, and the value of m can be determined after the value of n is selected.
T(m)＝T fa +mT RTT ＝[1+m(n+3)]T tx
The probability that m HARQ would be needed to succeed is: p (P) m ＝(1-P) m-1 P represents that the transmission fails m-1 times before and the last time succeeds;
interrupt probability: p (P) out.t ＝1-P m 。
The multi-channel selective transmission model in step 3) is:
γ i ＝(h max /h 1 )*γ 1
h max for maximum channel gain in q channels, gamma i Representing the signal-to-noise ratio of the receiving end, wherein the signal-to-noise ratio of the receiving end is calculated and changed according to the selected channel, but the signal-to-noise ratio is proportional to the single channel model, and the rest of the interruption probability solving formulas are the same as the single channel model, and are unchanged, and only the decoding failure probability epsilon is obtained 2 A change occurs.
Examples: in the simulation test environment, b=32 bytes. Let n=20 users share a bandwidth of w=10 MHz. UEs are power controlled and the average signal-to-noise ratio per antenna is γ=3 dB when measured over the entire bandwidth. We assume that the same UE power is present in different bandwidth allocation cases, i.e. the values of b are different; after operation on a single frequency block, its signal-to-noise ratio is equal to bγ due to the higher power spectral density. Consider a TTI duration of n=2 OFDM symbols, corresponding to a small segment in the most recently defined NR terminology. For a sub-carrier spacing of Δf=15 kHz, this will result in tti=0.143 ms when considering the same cyclic prefix duration (short configuration) as the long-term evolution standard. For simplicity, the processing time and the feedback transmission time have similar TTI durations, assuming negligible propagation delay, and the number of channels q is 4. When not specified, epsilon is caused I ＝ε S ＝min(10 -3 ,1-P d ) I.e. the identification and signal errors are not larger than the payload decoding errors. The rate per TTI is r=0.192·b.
By adopting the interrupt probability deducing method in the invention, the simulation effect is as follows:
as can be seen from fig. 2, the outage probability given by the two multi-channel methods is much lower than that given by the single channel, and the block transmission performance of the data packet is better, because the data packet has a diversity effect when transmitted by different channels, the possible error rate is lower because of the block than the transmission on one channel, and the signal-to-noise ratio of the receiving end is larger and the error rate is lower because of the gain of the selected channel when the transmission on one channel is selected. However, as the packet arrival rate increases, the outage probability increases, as the collision probability increases, resulting in an increase in bit error rate.
Fig. 3 shows that as the number of channels increases, the interruption probability of block transmission increases, the selective transmission interruption probability decreases, and the stability is maintained when the number of channels increases to a certain level. Because the more the number of channels, for block transmission, some channels may have poor performance, and the bit error rate is particularly high, resulting in higher bit error rate during combination; for selective transmission, the optimal channel is always selected, the more the number of channels is, the better the selection is made, and therefore the better the performance is, but when the number of channels is increased to a certain extent, the fluctuation degree of the channel gain reaches a stable state due to the fact that the channel is subjected to Rayleigh flat fading, and therefore the outage probability is not reduced greatly.
The invention firstly introduces a network scene of single-channel transmission, and then proposes a multi-channel block transmission model and a multi-channel selective transmission model. The invention can improve the interruption probability of the single-channel 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 through the comparison of the two multi-channel schemes. The foregoing is only a preferred embodiment of the invention, it being noted that: it will be apparent to those skilled in the art that various modifications and adaptations can be made without departing from the principles of the present invention, and such modifications and adaptations are intended to be comprehended within the scope of the invention.
Claims (7)
1. The method for evaluating the probability of the multi-channel unlicensed retransmission scheme under the condition of low time delay is characterized by comprising the following steps:
1) A network scene of single channel transmission is introduced, a system model is constructed, wherein a user adopts a short packet form to transmit a data packet, and accesses a base station through an unlicensed retransmission technology;
2) Establishing a single-channel outage probability problem model by combining time delay constraint;
3) Constructing a multi-channel transmission model, and obtaining a Monte Carlo solution of the outage probability;
the network scenario in step 1) is: n users share a WHz frequency band to carry out uplink unlicensed transmission, and the bandwidth is divided into b frequency blocks with equal size; under the URLLC service, the data packet of the user equipment is transmitted in the form of a short packet, and the short packet is transmitted in a mode of being ready to go by combining with the uplink contention-based unlicensed access technology, so that a scheduling application is not required to be sent to a base station, and resource authorization from the base station is not required to be received; when a data packet arrives, a user equipment randomly selects one of b frequency blocks for transmission; irrespective of the link adaptation, each B-bit packet is mapped to a frequency block, N OFDM symbols corresponding to a transmission time interval of one duration TTI seconds; the packet arrival rate per transmission time interval is denoted by λ and the user is considered to be fully synchronized, assuming λt0< <1, where T0 represents the total time required to transmit the packet; the inter-packet arrival time is longer than the time required for transmitting the packet, and the queuing effect of each user equipment is ignored; an unlicensed transmission consists of a user equipment identifier; the channel is a flat rayleigh fading channel; the user's transmissions are received at the same average power; the receiving end is equipped with M receiving antennas and an MRC receiver.
2. The method for evaluating the probability of a multi-channel unlicensed retransmission scheme under low latency according to claim 1, wherein the accessing the base station through the unlicensed retransmission technique in step 1) is: firstly, carrying out limited block length coding on a data packet transmitted by a real-time user, and accessing an access base station through competition; and decoding the short packet received by the base station, feeding back to the real-time user, and retransmitting if the receiving fails.
3. The method for evaluating the probability of a multi-channel unlicensed retransmission scheme under low latency according to claim 1, wherein the method for constructing the latency constraint in step 2) is as follows: combining with the short packet transmission characteristics under URLLC, firstly obtaining the time delay of one round trip transmission process of the system as T RTT ＝gT tx +T bp +T fb +T up Wherein T is fb Representing base station feedback delay, T bp Representing the processing delay, T, of the base station up Representing user processing delay, T tx Represents the transmission time delay, and assume T fb 、T bp 、T up And T is tx The values are equal, and g represents the number of times of repeated HARQ packet sending; the sum of delays after m system scheme round trip transmissions is further obtained as: t (m) =t fa +mT RTT ＝[1+m(g+3)]T tx Wherein T is fa Representing frame alignment delay, also with T tx Equal in value.
4. A low latency assessment according to claim 3The method for establishing the probability of the multi-channel unlicensed retransmission scheme is characterized by comprising the following steps of: the goal of UL unlicensed transmission for URLLC is to ensure that a large number of user equipments can successfully complete their payload delivery in a limited time, and that the outage probability is not higher than a certain target P out,t The method comprises the steps of carrying out a first treatment on the surface of the To calculate the outage probability, the following variables are defined:
ε 1 : missing the probability of user equipment identification;
P s (x) The method comprises the following steps Probability of x users selecting the same frequency block for transmission, x=0, …, N-1;
ε 2 (x) The method comprises the following steps When x user interference is active on the same frequency block transmitted by the user equipment, the data decoding error probability of the user equipment of interest is received with average signal-to-noise ratio (SNR) gamma;
according to the definition above, the probability of correctly decoding the payload is as follows:
the probability of failure of 1 transmission is: p (P) f1 ＝ε 1 +(1-ε 1 )(1-P d ) Indicating that the preamble error is identified or that the identification is correct but the decoding is wrong;
the probability of success of 1 HARQ is: p=1- (P) f1 ) g Indicating that the transmission failed only if each of the g transmissions failed;
the probability that m HARQ would be needed to succeed is: p (P) m ＝(1-P) m-1 P represents that the transmission fails m-1 times before and the last time succeeds;
the single channel outage probability problem model is: p (P) out.t ＝1-P m 。
5. The method for evaluating the probability of a multi-channel unlicensed retransmission scheme with low latency according to claim 3,r is the transmission rate, B is the packet length; /> Wherein Q is -1 Representing the inverse of the Gaussian function, ε represents the bit error rate generated by the short packet, and O (log B/B) represents an infinitesimal fraction of log B/B; c represents shannon capacity and has the expression: c (η) =log 2 (1+η); v is the channel dispersion, which represents the characteristics of the channel, expressed as follows:
where η represents the noise ratio.
6. The method for estimating a probability of a multi-channel unlicensed retransmission scheme under low-latency according to claim 4, wherein the multi-channel transmission model in step 3) is a multi-channel block transmission model; the step of constructing a multi-channel block transmission model and obtaining the Monte Carlo solution of the outage probability comprises the following steps: given transmitting power, the original short packet is averagely divided into a plurality of blocks based on the system model, the short packet is transmitted on a plurality of mutually independent channels, and each channel obeys Rayleigh flat fading; combining time delay constraint to obtain the maximum round trip transmission process times, and establishing a Monte Carlo solution of the interrupt probability by utilizing a probability theory; the method comprises the following specific steps:
assuming q channels, each transmitting B/q bytes
R＝B/(a*q)
γ i Representing the signal-to-noise ratio of the receiving end byCalculating; since the power of the transmitting end is assumed to be the same, and the signal-to-noise ratio is snr=hp t /σ 2 The ratio of the signal to noise ratio to the channel gain to the noise gain, i.e. h/sigma 2 Proportional, sigma i Representing the noise power of the ith channel, the noise being Gaussian white noise, i.e. σ, due to the use of Rayleigh fading channels 2 ＝N 0 /2，γ i Can be simplified into gamma i ＝(h i /h 1 )*γ 1 The method comprises the steps of carrying out a first treatment on the surface of the As a result of: />H is the channel gain, so that the signal-to-noise ratio of the receiving end is in direct proportion to the channel gain under the condition of certain transmitting power;
the probability of failure of 1 transmission is: p (P) f1 ＝ε 1 +(1-ε 1 )(1-P d ) Indicating that the preamble error is identified or that the identification is correct but the decoding is wrong;
the probability of success of 1 HARQ is: p=1- (P) f1 ) n Indicating that the transmission failed only if each of the n transmissions failed; the transmission time of the data packet in each HARQ is shortened due to the block transmission, so that the retransmission times n and the HARQ times m are changed, and the value of m can be determined after the value of n is selected by the user;
T(m)＝T fa +mT RTT ＝[1+m(n+3)]T tx
the probability that m HARQ would be needed to succeed is: p (P) m ＝(1-P) m-1 P represents that the transmission fails m-1 times before and the last time succeeds;
the outage probability is: p (P) out.t ＝1-P m 。
7. The method for estimating a probability of a multi-channel unlicensed retransmission scheme under low-latency according to claim 4, wherein the multi-channel transmission model in step 3) is a multi-channel selective transmission model; the specific steps of constructing the multi-channel selective transmission model to obtain the Monte Carlo solution of the outage probability include: a plurality of channels select a channel with the largest channel gain for low transmission; based on the single-channel transmission model, obtaining a Monte Carlo solution of the interruption probability;
γ i ＝(h max /h 1 )*γ 1
h max for maximum channel gain in q channels, gamma i Representing the signal-to-noise ratio of the receiving end, wherein the signal-to-noise ratio of the receiving end is calculated and changed according to the selected channel, but the signal-to-noise ratio is proportional to the single channel model, and the rest of the interruption probability solving formulas are the same as the single channel model, and are unchanged, and only the decoding failure probability epsilon is obtained 2 A change occurs.
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