| Platooning,as an important application scenario in vehicle to everything(V2X),allows a fleet of vehicles to connect in a tight manner,and to perform cooperative driving through information exchange between vehicles.In this scenario,there are a large number of user connections and frequent exchanges of information,which also put forward higher requirements on delay and reliability.Therefore,it is necessary to improve spectrum efficiency.Non-orthogonal multiple access(NOMA)technology can not only improve the spectrum efficiency,but also solve the resource collision problem between users and avoid the delay overhead caused by retransmission.Therefore,this paper believes that the NOMA technology can be applied to the team arrangement scenario.In this paper,a time resource allocation scheme is designed for platooning communication.In addition,the relay strategies under periodic multicasting transmission and the active user detection algorithm under aperiodic grant-free transmission are discussed respectively.Firstly,this paper describes the research status and standardization process of V2 X and NOMA,and investigates the application direction and research status of NOMA in V2 X system.Secondly,this paper introduces the communication mode and communication flow under the platooning scenario.A time resource allocation scheme is designed,which divides a communication interval into periodic communication interval and aperiodic communication interval.The self-contained subframe structure is introduced in the periodic communication interval to achieve fast feedback,and it is proposed that NOMA technology can be introduced to save symbol resources by superimposing a variety of reported information on one symbol.In the aperiodic communication interval,a grantfree transmission scheme based on NOMA is introduced,and the signature allocation issues in this scheme are analyzed.Then,this paper studies the scenario of PL multicasting in periodic communication intervals,and proposes a sparse code multiple access(SCMA)-based multicasting scheme.In order to extend the coverage range of the PL,we consider that the PMs can be utilized as relays.Thus,the single relay scheme and the multi-relay scheme are proposed in this paper.The average outage probabilities under the two relay strategies and direct transmission are analyzed,and the explicit expressions are derived.This paper also analyzes the total communication cycle of the two relay strategies in a multi-hop transmission scenario and compare their delay performances.Then,this paper simulates the theoretical derivation results of outage probability and the performance under actual channel scenarios.The simulation results show that the theoretical derivation results are basically consistent with the simulation results under the actual channel,and the utilization of relay can greatly reduce the outage probability of the system.The single relay scheme has the best outage performance among three transmission schemes since the optimal relay is selected.Moreover,although the multi-relay scheme sacrifices the outage performance by introducing inter-user interference,it can reduce the total communication cycle time and meet the low-latency communication requirement.Finally,this paper proposes a grant-free asynchronous access model for emergency safety communication in aperiodic communication intervals.A receiver framework based on asynchronous sparse Bayesian learning(ASBL)algorithm and support vector machine(SVM)classifier is proposed.That is,firstly,the ASBL algorithm is used to perform timedomain channel estimation with the received pilot signal.Then,the SVM classifier is used to classify the channel estimation results obtained by the ASBL algorithm to identify the active users,and obtain a more accurate active user index.Simulation results show that compared with compressed sensing(CS)-based algorithms,the proposed ASBL algorithm can achieve better channel estimation performance.Moreover,the introduction of the SVM classifier can greatly reduce the false alarm probability without affecting the channel estimation results,and improve the performance of active user detection. |
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