CN108600997A - A kind of extensive D2D communication means assisting NOMA based on HARQ - Google Patents
A kind of extensive D2D communication means assisting NOMA based on HARQ Download PDFInfo
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Abstract
The invention discloses a kind of extensive D2D communication means assisting NOMA based on HARQ, this method analyzes influence of the superposition interference to outage probability in extensive D2D networks first with random geometry method, then the relationship between outage probability and spectrum efficiency is utilized, derives the expression of spectrum efficiency；It is finally based on the optimization problem of these theoretical analysis results structure resource allocation：The rate of information throughput by the way that different D2D users are rationally arranged even occupies the distribution density of identical running time-frequency resource D2D terminals come maximum spectral efficiency (handling capacity or spatial frequency spectrum utilization rate) and ensures the reliability of communication simultaneously.Emulation and numerical value are the results show that cooperation mode application scheme ratio non tie up system provided by the present invention reduces 23% on outage probability, in addition, improving 17% in spectrum efficiency using orthogonal multiple access technique using non-orthogonal multiple access technology ratio.
Description
Technical Field
The invention relates to the technical field of wireless communication, in particular to a large-scale D2D communication method based on HARQ auxiliary NOMA.
Background
According to a white-skin document ITU-R M.2083-0 published by the International Union of telecommunication, the target of a next generation mobile cellular network (5G) is to enhance mobile broadband services, provide wireless links supporting a large number of terminals for a future Internet of things mode, and ensure communication service with ultra-reliability and low time delay. Support a large number of machine-form connections in the internet of things, and provide ultra-reliable and low-latency communication for certain services. These new requirements present unprecedented challenges to traditional Orthogonal Multiple Access (OMA) based approaches and base station centric communications. Therefore, 3GPP considers more greedy high spectrum utilization schemes, such as D2D communication and non-orthogonal multiple access technology (NOMA), to support a large number of connections. Although the D2D communication will increase the interference level of the original network, the D2D can still significantly improve the spatial spectrum utilization of the whole communication system due to the performance advantages of low power consumption and short-range direct communication. The NOMA technology provides services for a plurality of users simultaneously by using the same time-frequency resources, and compared with the traditional mode, the NOMA technology can significantly improve the spectrum utilization rate, and the core of the NOMA technology is to provide multi-user diversity by using two technologies of Superposition Coding (SC) and Successive Interference Cancellation (SIC), thereby effectively improving the spectrum utilization rate. Furthermore, NOMA techniques may achieve a very good balance between system throughput and user fairness by allocating more transmit power to users with poor channel conditions.
In order to further provide spectrum utilization, the present invention applies the NOMA technology to D2D communication, but when D2D communication is applied in a large scale, a very fatal technical bottleneck is encountered, that is, a contradiction between limited spectrum resources and excessive spectrum reuse occurs, so that strong network superposition interference exists in the terminal device, and finally, the receiver receiving performance is drastically reduced, such as the spectrum efficiency is reduced and the interruption probability is increased. However, when a great deal of research work is done in the discussion of large-scale D2D networks, how to improve the negative effect of strong network superposition interference is not considered. In order to ensure the reliability of transmission and further improve the spectrum efficiency, the invention combines the hybrid automatic repeat request (HARQ) technology to improve the reliability of transmission, and it is noted that the HARQ is a technology for effectively ensuring the reliability of transmission, and its technical core is the retransmission mechanism and forward error correction coding and decoding. In addition, when the HARQ technology is applied, the sending device is not required to know perfect instantaneous channel state information, and only needs to know partial channel state information or some statistical characteristics of the channel, which can effectively reduce frequent channel measurement and instantaneous snr report feedback, thereby greatly reducing system signaling overhead, which in turn makes it possible to apply D2D communication in a large-scale network. Because the simultaneous application of the NOMA technology and the HARQ scheme also prompts the invention to further improve the reliability of transmission through cooperative communication, because each receiving end receives the superposition coded signal, the superposition signal contains the information of all users, and therefore, the user who succeeds in decoding first can be used as a relay to forward the information of other users which have already been decoded to the users. Based on these considerations, it is highly desirable to propose an application scheme of the non-orthogonal multiple access technology in combination with cooperative hybrid automatic repeat request in the large-scale D2D network.
Disclosure of Invention
The application of the large-scale D2D communication is to share the load of the conventional mobile cellular network with the base station as the control center, however, due to the contradiction between the limited spectrum resources and the excessive frequency reuse, when the large-scale application of the D2D communication is performed, strong network superposition interference exists between the terminal D2D devices, and finally, the receiver receiving performance is sharply reduced, such as the spectrum efficiency is reduced and the interruption probability is increased. The present invention is directed to solve the above-mentioned drawbacks of the prior art, and provides a large-scale D2D communication method based on HARQ assisted NOMA, aiming to improve spectrum efficiency and reduce outage probability.
The purpose of the invention can be achieved by adopting the following technical scheme:
in order to reduce interference from the base station, the present invention assumes that the D2D transmitting device operates in an overlay mode, i.e., the base station and the D2D device use orthogonal spectrum resources. While assuming that all D2D transmitting devices obey the Homogeneous Poisson Point Process (HPPP), this is a very reasonable assumption whose validity has been verified by a number of experimental measurements. In addition, in order to reduce the requirement of hardware complexity, limit the interference level, and guarantee the requirements of low delay, the invention only considers the direct communication between the sending device and two D2D users, and simultaneously utilizes the NOMA technology in combination with the cooperative HARQ scheme to improve the spectrum efficiency and the interruption probability in the large-scale D2D communication application, and the performance improvement needs to be established on the basis of reasonable modulation and coding selection and optimized resource configuration, and the system model is as shown in fig. 1. The specific technical scheme implementation steps are shown in fig. 2 and comprise seven steps, and the details of each step are as follows:
s1, design of application scheme
In an application scheme based on non-orthogonal multiple access technology (NOMA) combined with cooperative hybrid automatic repeat request (HARQ), a transmitting device first transmits information s to be transmitted to two users according to the HARQ1And s2Respectively generating corresponding code words by forward error correction coding, and then adopting a non-orthogonal multiple access technology supporting multi-user simultaneous transmission to send information to D2D users by superposition codingwhere beta is the power allocation factor, in order to make more reasonable use of the non-orthogonal multiple access technique, it is necessary to useUsing the channel difference between D2D users to improve spectrum utilization, therefore, the decoding order is required to be decided according to the path loss of two users, and the path loss model can be expressed aswhere k represents a reference path loss value at a distance of 1m and α is a path loss exponent, it is assumed here that D2D user 1 is closer to the transmitting device than user 2, the distances between which are respectively defined as D1And d2And d is1＜d2. According to the non-orthogonal multiple access technology, a user 1 firstly decodes information of a user 2, then adopts an interference elimination technology to eliminate interference of the user 2, and then decodes the information of the user 1; and the user 2 directly decodes the information of the user without adopting an interference elimination technology. Once decoding of any user fails, the D2D user feeds back a NACK signal to the transmitting device, and requests the transmitting end to retransmit the information until both users decode successfully or the maximum allowed number of transmissions K is reached.
S2, representing the spectrum efficiency as an interruption probability function
The two most important performance indicators in spectral efficiency are throughputηthe former measures the spectrum efficiency of transmitting a piece of information, the latter measures the spectrum efficiency of the whole network, and the relation of the two is delta-lambda η, wherein lambda is the distribution density of D2D users occupying the same time-frequency resourceηCan be expressed as
The symbols in the formula have the following meanings:
Ri: indicating the preset information transmission rate of the D2D user i;
representing the probability of interruption of D2D user i after K transmissions;
indicating that two D2D users areκProbability of interruption after secondary transmission.
S3, decomposition of interruption probability
As described above, the spectrum efficiency is finally expressed as a function of many interruption probabilities, which are also the most critical evaluation indicators in the performance analysis. Therefore, the probability of interruption is calculated by the total probability formulaAndexpressed as:
the symbols in the formula have the following meanings:
representing the probability of an event occurring;
user 1 successfully decodes signal s after l transmissionsiThe event of (2);
union setComplement of, i.e. user 1 is inKInformation s still being unable to be decoded after sub-transmissioni；
After k transmissions, the user 2 successfully decodes the event of its own message;
union setThe complementary set of (2), i.e. user 2, cannot recover its own information after K transmissions.
S4, accurate calculation of interruption probability by using random geometry
In order to deduce the probability in step S2 Andtheoretical analysis is here performed by shannon's theorem from an information theory perspective. In addition, the distribution of interference D2D users is accurately simulated through a two-dimensional Homogeneous Poisson Point Process (HPPP), and specific expressions of the probabilities are deduced by utilizing correlation theory in random geometry,the following were used:
the symbols in the formula have the following meanings:
τa＝(τ1+1,τ2,τ3),τb＝(τ1,τ2+1,τ3),τc＝(τ1+1,τ2),τd＝(τ1,τ2+1),τe＝(τ3,τ4),τf＝(τ1+1,τ2+1),τg＝(τ1,τ2+2),
the function Ψ (U, τ; U, τ) is defined asWhere U is (U)1,…,UN)，τ＝(τ1,…,τN)， Can be expressed in the form of double integral
S5, approximate calculation of interruption probability under precondition of short-distance communication
As described in step S4, all probability calculations eventually require the calculation of the corresponding Ψ (U, τ; U, τ) function, however, the calculation process involves double integration with high computational complexityIn order to reduce the computational complexity and facilitate the later optimization design, it is necessary to provide an approximate calculation method. This assumption is reasonable considering that the distance D between D2D users is generally relatively close, and due to cooperative communication. Let D be approximately equal to 0, double integralCan be approximated as
Wherein,δsa dirichlet function is defined which,a fourth class of Laureicella functions is defined, and B (a, B) defines a Beta function. If N + M is equal to 1,the above approximation will become an equation. The result of the approximation is not only low in computational complexity, but also shows very good approximation performance in numerical analysis.
S6, maximizing the spectrum efficiency, and reasonably selecting the information transmission rate R of the user1And R2To maximize throughputηWhile ensuring reliability of service, the mathematical form of the optimization problem is expressed as
The symbols in the formula have the following meanings:
εi: representing D2D usersiThe maximum allowed probability of interruption of the system,
indicating the probability of user i having an interruption after the kth transmission,
β2: indicating a pre-set power allocation factor,
the optimization problem can be solved by utilizing an interior point method. In addition, in order to further optimize the distribution density of D2D users, the service quality can be simultaneously guaranteed by maximizing the spatial spectrum utilization Δ, so the mathematical form of the optimization problem is as follows
λ＞0,
The symbols in the formula have the following meanings:
λ: indicating the distribution density of the interfering users,
εi: representing the maximum allowed outage probability for user i of D2D,
indicating the probability of user i having an interruption after the kth transmission,
β2: representing a preset power allocation factor.
S7, setting of modulation mode and coding scheme and radio resource allocation
Transmission rate R obtained by optimization1 *And R2 *And D2D user distribution densityλ *The radio resource is configured by the value, which is as follows: by optimised transmission rate R1 *And R2 *Setting a reasonable modulation mode and a coding scheme at a transmitting end; by optimization ofλ *And guiding wireless resource allocation, and performing spectrum reuse rate configuration on the result spectrum resource.
Compared with the prior art, the invention has the following advantages and effects:
simulation and numerical results show that the interruption probability of the cooperative mode application scheme provided by the invention is reduced by 23% compared with that of a non-cooperative mode, and in addition, the spectrum efficiency of the application of a non-orthogonal multiple access technology is improved by 17% compared with that of the application of an orthogonal multiple access technology. When the method is applied to a large-scale D2D network, the method provides very meaningful application guidance and reference values for the setting of the modulation mode and the coding scheme of the D2D terminal and the resource allocation of the whole network.
Drawings
FIG. 1 is an application model of the communication method disclosed in the present invention;
fig. 2 is a flowchart illustrating implementation steps of a large-scale D2D communication method based on HARQ assisted NOMA according to the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Examples
Non-orthogonal multiple access (NOMA) is an important technology for next generation mobile communications, for example, a transmission technology that employs NOMA's multi-user superposition transmission as a downlink of LTE-a has been proposed to the 3GPP organization. In these scenarios, some D2D users may act as relays to assist the base station in forwarding messages, thereby extending the coverage of the wireless network. Furthermore, to mitigate severe interference from base stations, we assume that the D2D network operates in overlay mode, in which the D2D network uses dedicated spectrum resources for communication. Bits for all D2D devices in a D2D networkRandom distribution, a widely adopted modeling method is adopted, namely that all D2D sending devices occupying the same time-frequency resource are assumed to obey the homogeneous Poisson point processThe intensity of which is defined asλ. All D2D devices were equipped with backlog buffers and transmitted over a shared frequency channel dedicated to D2D communications. Without loss of generality, the present embodiment focuses on one transmitting device communicating with its neighboring two D2D users by combining NOMA and cooperative HARQ, as shown in fig. 1. It is assumed here that only two D2D receiving terminals are based on the following considerations: the assumption of limiting the level of interference between multiple users, reducing hardware costs and meeting low latency processing is also widely adopted in research and practical applications.
Let z be the location of the sending device and define the sending device and the user i (at location o)iOf a user) is di＝||z-oiAnd | represents, where i ∈ {1,2 }. Suppose user 1's location is o1Position of user 2 is o and ═ 0,02Because of the stationarity of PPP, (D,0) this assumption does not affect the results of the analysis. Unlike conventional Time Division Multiple Access (TDMA), the core idea of NOMA is to exploit the differences between fading channels, so to avoid loss of generality, user 1 is here specified to be closer to the transmitting device than user 2, i.e. d1＜d2. Note that HARQ can effectively combat channel uncertainty and measurement errors for reliable transmission, so the proposed NOMA scheme in combination with cooperative HARQ does not need to know perfect instantaneous channel state information. For this reason, the NOMA user information decoding order is based on the magnitude of their average fading gains (path losses). It should be noted that the average channel gain/path loss does not typically change rapidly or drastically in a short time compared to small scale fading, and therefore does not require frequent channel measurements and instantaneous snr reporting. Considering that the transmission distance determines the average fading gain/path loss, the decoding order of two NOMA users is determined according to the distance between the NOMA users and the transmitting device. For ease of illustration, the proposed scheme can be divided into two phases during application. In a first phase, the transmitting device uses superposition coding for the signal s to be transmitted to the users 1 and 21And s2Power domain multiplexing is performed. Based on the received superposition coded information, user 1, which is closer to the transmitting device, first decodes user 2's information s2(this is because more transmit power is allocated to user 2's information, so s is transmitted2The signal-to-noise ratio of the information tends to be large, and therefore s is successfully decoded2Is more likely) and the information s of the user 22Information s actually for user 11An interference is formed and is therefore referred to hereinafter as a NOMA interference. Once s has occurred2Being successfully decoded, user 1 can first cancel the interfering signal s in the current and subsequent transmissions using interference cancellation techniques2And further decode the self information s1. Unlike user 1, user 2, who is further away, sends signal s directly1Seeing as noise and decoding s directly2. The superposition message is repeatedly sent until user 1 or 2 acknowledges successful reception or the maximum number of retransmissions K is reached. If only any one of the users confirms successful reception of the information, the second phase begins, in which the transmitting device transmits only user signals that were not successfully received. It should be emphasized that when user 1 confirms successful decoding of the information, it will assist the transmitting device as a relay device to transmit signal s2And forwarded to user 2. When both users 1 and 2 confirm successful reception of the information, the transmitting device will take the next set of information from the transmission queue and transmit it by the same operation as described above. If the maximum number of transmissions K is reached and the expected information is not successfully decoded, the transmitting end deletes the set of signals from the transmit queue and declares an interruption in the transmission of the information. For simplicity, it is assumed that the feedback channel is error-free and delay-free, which can be evidenced by low transmission rates and short acknowledgement messages.
A. Signal transmission model
The channel is assumed to be subject to rayleigh block fading (i.e., the channel impulse response coefficients remain unchanged during each round of HARQ transmission) and to have statistical knowledge of the channel state information at the transmitting device. In addition, the channel gain of each transmission process is assumed to be random variables which are independent and distributed, the position of an interference source does not change greatly in a short time interval, and the assumption is very reasonable especially for the interference source with medium and low speed mobility. Therefore, a static interferer model is used here, i.e. the approximation that the source of the interferer is fixed during the HARQ transmission process is very reasonable, since HARQ has a limited maximum number of transmissions in practical applications, e.g. the maximum number of transmissions is usually selected to be 5, and the time interval for each round of HARQ is about 8 ms. The signal received by the NOMA user during the different transmission phases may be represented in the following form.
1) Stage I: in the k-th round HARQ transmission, the signal received by user i can be expressed as
where P denotes the transmit power, β denotes the power distribution coefficient, siIs the information sent to user i and follows a gaussian distribution of unit variance. siAt the transmitting device, independent coding and modulation are firstly carried out, and then power domain multiplexing is carried out to fix the transmission rateTransmitting information to user i, wherein biIs the signal siL defines the signal siNumber of symbols contained, sx,kRepresents the signal coming from the k-th transmission of the interferer, and obeys a gaussian distribution of unit variance,representing the path loss, α denotes the path loss exponent, where k represents the path loss per unit distance reference, the magnitude of which depends on the carrier frequency and antenna characteristics, x ∈ Φ \ z represents the set of interference sources, ni,kDenotes zero mean and variance as σ2Has high compound addition propertyWhite noise (AWGN), i.e. Represents the channel coefficients from interfering device x to user i during the kth round of HARQ transmission;andall obey a complex gaussian distribution with a mean of zero and a variance of 1, i.e.
According to the NOMA transmission protocol, after receiving the superposition coded signal, both user 1 and user 2 first decode user 2's information with a signal to interference plus noise ratio of
Wherein Ii,kRepresents the total interference received by the user i to the interference source set phi \ z, and the mathematical form can be expressed as
When user 1 successfully decodes user 2 message s2User 1 will recover the own signal s using interference cancellation techniques1After the NOMA interference signal of user 2 is eliminated, the obtained target is s1Has a signal to interference and noise ratio of
2) Stage II: according to the proposed scheme, if s2If the decoding is successful before the k-th HARQ transmission, the signal received by user 1 in the k-th HARQ can be expressed as
s1Corresponding to a signal to interference and noise ratio of
On the contrary, if s1Successfully decoded before the k-th round of HARQ transmission, then in the k-th round of HARQ transmission, user 1 will assist the sending device to relay the information of user 2, and the signal received by user 2 is expressed as
Wherein,representing the channel coefficients between two users in the k-th transmission. Similar to (6), user 2 has a received signal to interference and noise ratio of
Wherein,representing the equivalent channel coefficient of the kth transmission and obeying a complex Gaussian distribution of zero mean and unit variance, i.e.Andrepresenting the equivalent path loss.
Based on these signal transmission models, a specific spectral efficiency (throughput and spatial spectral efficiency) analysis and outage probability analysis are next performed.
B. Average throughput and outage probability analysis
Long Term Average Throughput (LTAT) is a commonly used performance indicator to evaluate the throughput of HARQ systems. For convenience, t is defined herein as the number of slots used,the number of total information bits to send to user i and accumulated up to time t, so LTAT can be expressed as
WhereinIndicating information bits successfully decoded by user i within a unit time of a unit bandwidth. An event in which user i stops the transmission of the current message is regarded as a recurring event. The occurrence of this recurring event is accompanied by a random reward being earned for both usersAndthus, with the update reward theorem, the LTAT of the proposed solution can be further written as
WhereinRepresenting the random transmission number (update interval time) between two consecutive retransmission events. Thus, if user i successfully recovers its own message, thenOtherwiseThen there are
Indicating the outage probability after k rounds of HARQ transmission for user i. In addition to this, the present invention is,is a discrete random variable with a sample space of {1,2, …, K } and obeying a probability distribution of
Wherein,the probability that the user 1 or the user 2 still has interruption after k times of transmission is shown, and the probability can be obtained by utilizing the principle of repulsion and capacitance
WhereinIndicating the probability that two users cannot decode their own information after the k round HARQ transmission. Therefore, by using (12) and (13), it can be deducedIs composed of
Therefore, substituting (11) and (14) into (10) results in
LTAT is therefore expressed as a function of outage probability, and it can be seen that outage probability is the most basic performance indicator. It should be noted, however, that the expression of the probability of interruption depends on the specific proposed application. Thus, cooperative HARQ combined with NOMA scheme determines outage probabilityAndan explicit expression of (2). For the purpose of continuing the analysis, their expressions are respectively derived as follows:
b.1 outage probability
According to the preceding system model, s1Depending on the successful decoding and cancellation of the NOMA interference s by user 12The number of transmissions consumed and, due to cooperative communication, its performance also depends on the number of transmissions experienced by the user 2 for successful decoding. To facilitate subsequent analysis, the following events are defined:
user 1 successfully decodes signal s after l transmissionsiThe event of (2);
union setComplementary set of (i.e. user 1 still cannot decode the information s after K transmissionsi；
After k transmissions, the user 2 successfully decodes the event of its own message;
union setThe complementary set of (2), i.e. user 2, cannot recover its own information after K transmissions.
Based on the above definition, the outage probability of user 1Can be obtained by using a total probability formula
In view ofAndis a mutually exclusive event, and similarly，Andare also mutually exclusive events. In addition, if l > k,andis also a mutual exclusion event because the transmitting device only transmits s after receiving the ACK signal fed back by the user 21After which the user 1 no longer needs to use interference cancellation techniques during transmission. Therefore, the temperature of the molten metal is controlled,can be expressed as
All terms to the right of equation (17) etc. are derived one by one as follows:
1)from the point of view of information theory, when the mutual information amount is smaller than the transmission rate, an interrupt event occurs. Here, theIndicating that user 1 successfully decoded s after a given user 1 transmitted through l times2And uses SIC to cancel NOMA interference and user 2 decodes s2Consume and use upkOutage probability of one HARQ round. Considering that the condition l ≦ k must be satisfied in this case, according to the previous signal model,watch capable of showingIs shown as
Wherein,representing the amount of information for a given signal to interference and noise ratio gamma. Using random geometry, (18) can be written as a closed form
Wherein,τa＝(τ1+1,τ2,τ3) And τb＝(τ1,τ2+1,τ3) It has to be pointed out here that the user 1 is able to cancel the NOMA interference s2Must satisfy the conditionsIn addition to this, the present invention is,the function is defined as follows
Wherein U ═ U (U)1,U2,...,UN)，τ＝(τ1,...,τN)，And
2)once user 2 successfully decodes s after k HARQ rounds2The transmitting device will transmit only s in subsequent retransmissions1User 2 does not need to decode directly by interference cancellation techniques when decoding. Therefore, the temperature of the molten metal is controlled,can be expressed as
Likewise, (22) can be derived
3)After l rounds of HARQ transmission, user 1 successfully decodes s2This means that user 1 can completely cancel NOMA interference to decode s in the current and subsequent HARQ rounds1This way, the outage probability can be improved effectively. Therefore, the temperature of the molten metal is controlled,can be written as
Similarly, it can be obtained from (24)
Wherein tau isc＝(τ1+1,τ2)，τd＝(τ1,τ2+1) and
4)after K transmissions, if user 1 cannot eliminate NOMA interference and user 2 cannot decode its own information, user 1 will be interrupted inevitably, and the interruption probability can be written as
Therefore, the temperature of the molten metal is controlled,can be finally deduced as
B.2 probability of interruption oK,o2
Similar to (17), according to the total probability formula, the probability of interruption of the reception of information by the user 2 is
Wherein the above formula holds true when k < lAnd isDue to the fact thatAndit has been deduced in the foregoing. Therefore, the remaining term is derived nextThe specific expression of (1).
Suppose user 1 successfully decodes s in the l-th round HARQ2And successfully decode s in the k round HARQ1Wherein k is more than or equal to l. In the following transmission s has been successfully decoded since user 12It is clear that user 1 can assist the sending device to deliver the message as a relay to user 2. In this case, the outage probability of user 2 after K rounds of HARQ transmissionThe derivation needs to be done in two cases. First, when k is l, user 1 cancellation of NOMA interference and decoding of self information occurs in the same round of HARQ transmission, and s is decoded on the same round of HARQ1. Therefore, the temperature of the molten metal is controlled,can be deduced as
Then applying random geometry to obtain
On the other hand, if k > l, i.e., user 1 successfully cancels the NOMA interference event and decodes self information occurs in two different HARQ rounds, thenCan be expressed as
Therefore, the temperature of the molten metal is controlled,can be expressed as
Wherein tau ise＝(τ3,τ4)，τf＝(τ1+1,τ2+1)，τe＝(τ3,τ4)，τf＝(τ1+1,τ2+1)，τg＝(τ1,τ2+2)，
B.3 outage probability
Similar to (17) and (28), according to the total probability formula,can be derived into
WhereinAndthese two expressions have been given in (25) and (27), respectively. Therefore, probability of interruptionAndcan be calculated by (17), (28) and (33), respectively. Substituting equation (15) may result in the LTAT of the proposed scheme. To calculate the outage probability, the double integration in equation (21) must be calculatedTo obtain the compound. However, the double integral in equation (21) is very complicated in calculation, and therefore it is very necessary to provide an algorithm with lower calculation complexity. Because of the need for cooperative communication, NOMA users are typically very short distances from each other, which can be simplified by the following theoremAnd (4) calculating.
Theorem 1: under the short-distance D → 0 condition,can be approximated as
δsA dirichlet function is defined which,a fourth class of Laureicella functions is defined, and B (a, B) defines a Beta function. If N + M is 1, the above approximation will become an equation. The above-mentioned approximate knotThe result is not only low computational complexity, but also very good approximation performance in numerical analysis.
According to the theoretical analysis of throughput and interruption probability, how to apply the results to carry out the optimization design of an application scheme is specifically analyzed, so that the throughput or the space spectrum utilization rate of the system is effectively improved.
C. Spectrum efficiency optimization
there are two key performance indexes for measuring the spectrum efficiency, i.e. throughput η and spatial spectrum efficiency Δ, the former measures the spectrum efficiency of transmitting a piece of information, and the latter measures the spectrum efficiency of the whole network, and the relationship between the two is Δ ═ λ η, where λ is the distribution density of D2D users occupying the same time-frequency resource.
C.1 throughput maximization
to overcome the negative effects of co-channel interference, an interference-aware optimization design is proposed to maximize LTAT, particularly by appropriate selection of system parameters while maintaining quality of service2The LTAT is maximized by optimally selecting the transmission rate. The mathematical form of the optimization problem can be expressed as
Wherein epsiloniRepresenting the maximum allowed outage probability for user i. It is worth mentioning that (35) due to the complexity of the interrupt probability expression and the non-convexity of the problem, it is difficult to solve the closed form optimization solution. However, numerical solutions by the interior point method can yield sub-optimal solutions.
C.2 spatial spectral efficiency maximization
Besides LTAT, space spectrum utilization (ASE) is another important performance index to measure the spectrum efficiency of the whole D2D network, and the specific expression is
Δ＝λη. (36)
Inspired by formula (35), in order to further optimize the distribution density of D2D users, the service quality can be ensured by maximizing the spatial spectrum utilization rate Δ, so the mathematical form of the optimization problem is as follows
Similar to (35), (37) can also be solved effectively by the interior point method.
In summary, due to limited spectrum resources, excessive frequency reuse is caused when large-scale D2D application is performed, thereby causing severe superposition interference to mobile terminals occupying the same time-frequency resources. Severe interference tends to cause drastic performance degradation of the communication system, such as a decrease in spectral efficiency and an increase in outage probability. In order to improve the spectrum efficiency and ensure reliable transmission, the present embodiment provides an application scheme combining a non-orthogonal multiple access technology (NOMA) with high spectrum efficiency and a cooperative hybrid automatic repeat request (HARQ) for ensuring transmission reliability in a large-scale D2D network.
The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such changes, modifications, substitutions, combinations, and simplifications are intended to be included in the scope of the present invention.
Claims (4)
1. A large-scale D2D communication method based on HARQ assisted NOMA, the communication method comprises the following steps:
s1, designing the application scheme of combining NOMA and HARQ, the sending device will transmit the information S to two D2D users according to the HARQ1And s2Respectively generating corresponding code words by forward error correction coding, and then sending information to D2D users by superposition coding by using non-orthogonal multiple access technology supporting multi-user simultaneous transmissionD2D users decode information respectively, any D2D user fails to decode, D2D users feed back NACK signals to sending equipment to require a sending end to resend the information until two D2D users decode successfully or the maximum allowed transmission times K are reached;
s2, comparing the throughputηand spatial spectrum efficiency delta is expressed as an interruption probability function, wherein the throughput is the spectrum efficiency for transmitting a piece of information, the spatial spectrum efficiency delta is lambda η which is the spectrum efficiency for measuring the whole network, lambda is the distribution density of D2D users occupying the same time-frequency resource, and according to the theory of the updating process, the throughput is expressed as an interruption probability functionηIs shown as
The symbols in the formula have the following meanings:
Ri: indicating the preset information transmission rate of the D2D user i,
indicating the probability of interruption of user i of D2D after K transmissions,
represents the probability of two D2D users breaking after k transmissions;
s3, decomposing the interruption probability, and dividing the interruption probability by a total probability formulaAndis shown as
The symbols in the formula have the following meanings:
representing the probability of an event occurring;
user 1 successfully decodes signal s after l transmissionsiThe event of (2);
union setComplementary set of (i.e. user 1 still cannot decode the information s after K transmissionsi；
After k transmissions, the user 2 successfully decodes the event of its own message;
union setThe complementary set of (2), namely after K transmissions, user 2 can not recover its own information;
s4, accurately calculating the interruption probability by using random geometry, accurately simulating the distribution of interference D2D users by a two-dimensional homogeneous Poisson point process, and deducing a specific expression of the interruption probability by using a theory in the random geometry, wherein the specific expression comprises the following steps:
the symbols in the formula have the following meanings:
τa＝(τ1+1,τ2,τ3),τb＝(τ1,τ2+1,τ3),τc＝(τ1+1,τ2),τd＝(τ1,τ2+1),τe＝(τ3,τ4),τf＝(τ1+1,τ2+1),τg＝(τ1,τ2+2),
the function Ψ (U, τ; U, τ) is defined asWhere U is (U)1,…,UN)，τ＝(τ1,…,τN)，Expressed in the form of double integral
S5, approximate computation of probability of disruption under the premise of short-range communication, considering that the distance D between D2D users is very close, and due to cooperative communication, suppose D ≈ 0, double integralCan be approximated as
Wherein,δsa dirichlet function is defined which,defining a fourth class of Lauricella functions, B (a, B) defining a Beta function, if N + M ═ 1, then the above approximation will become an equation;
s6, maximizing the spectrum efficiency, and reasonably selecting the information transmission rate R of the user1And R2to maximize throughput η while ensuring reliability of service, the mathematical form of the optimization problem is expressed as
The symbols in the formula have the following meanings:
εi: representing the maximum allowed outage probability for user i of D2D,
indicating the probability of user i having an interruption after the kth transmission,
β2: representing a preset power allocation factor;
by optimizing the distribution density of D2D users, the spatial spectrum utilization Δ is maximized while guaranteeing quality of service, so the mathematical form of the optimization problem is as follows:
λ＞0,
the symbols in the formula have the following meanings:
λ: indicating the distribution density of the interfering users,
εi: representing the maximum allowed outage probability for user i of D2D,
indicating the probability of user i having an interruption after the kth transmission,
β2: representing a preset power allocation factor;
s7, setting modulation mode and coding scheme, configuring wireless resource, and utilizing optimized transmission rate R1 *And R2 *And D2D user distribution density λ*The value configures the radio resource.
2. The method of claim 1, wherein in the step S1 of designing the NOMA combined with HARQ, the decoding order is determined according to the path loss of two D2D users, and the path loss model is expressed asWhereina reference path loss value of 1m distance is indicated, α is the path loss exponent and d is the distance.
3. The HARQ-assisted NOMA-based large-scale D2D communication method as claimed in claim 1, wherein the step S6 is to optimize and solve the optimization problem by using an interior point method in the spectrum efficiency maximization to obtain the transmission rate R1 *And R2 *And D2D user distribution density λ*The value is obtained.
4. The method of claim 1, wherein the S7, the setting of modulation scheme and coding scheme, and the configuration of radio resources are as follows: by optimised transmission rate R1 *And R2 *Setting a reasonable modulation mode and a coding scheme at a transmitting end; by optimum lambda*And guiding wireless resource allocation, and carrying out spectrum reuse rate configuration aiming at the spectrum resources.
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