Research On Multiple Access Technology For Massive Machine Type Communication System

With the development of communication technology,more emphasis will be placed on communication between objects in the future.Massive machine type communication(mMTC)scenarios,as one important application of 5G mobile communication,can provide communication services to a large number of terminal devices and meet the communication needs of future Internet of Things.While faced with huge number of terminal devices and short and frequent data packets in the mMTC scenario,traditional multiple access technologies are not able to meet communication needs.Therefore,it is necessary to design a multiple access technology suitable for the mMTC scenario.This paper focuses on mMTC multiple access technology,and optimizes and improves the grant-free multiple access(GFMA) technology and traditional random access technology.First of all,this paper introduces the details and advantages of the application of GFMA technology and random access technology in the mMTC scenario,and analyzes the possible asynchronous problems and pilot conflict problems.Then,in view of the asynchronous problem in the GFMA,a detection method of widen the time window is proposed,and a mathematical model containing delay is established to ensure that every possible active user can be detected.On the other hand,the mathematical model is modified to make it a standard multiple measurement vector(MMV)problem of compressive sensing.And an enhanced grouping algorithm based on the orthogonal matching pursuit(OMP)algorithm is proposed.This algorithm can use the grouping characteristics of the model to jointly perform active user detection and channel estimation,and at the same time,it can estimate the discrete delay of active users.Simulation results show that the proposed model and algorithm has better performances of active user detection and channel estimation performance than the traditional OMP algorithm,and it can apply to asynchronous mMTC scenarios.Next,aiming at the problem of pilot conflicts in the traditional random access technology,this paper uses time-shifted pilots to solve the inter-cell interference and cell partitions and to increase the spatial detection dimension.These methods can increase the number of terminal devices wanting to communication in time slots.This paper proposes an Enhanced Random Access with Time-shifted Pilot(ERA-TSP)scheme for centralized scenario and details the flow of the scheme and pilot detection method.The performance of the scheme in the centralized mMTC scenario,such as collision probability,average collision probability,and average access delay,is analyzed.Simulation results show that compared with the traditional random access scheme,the ERA-TSP scheme has lower probability of pilot collision and shorter average access delay,and can be applied in centralized mMTC scenarios.Finally,aiming at the characteristic that antenna units distributed in the cell in the distributed scenario,a Distributed Enhanced Random Access with Time-shifted Pilot(D-ERA-TSP)scheme is proposed.It takes advantage of time-shift pilot and many antenna units which can increase the number of access terminal equipment in the same time slot.This paper introduces details the flow of the scheme and analyzes performance of the scheme,such as collision probability and average access delay.The simulation results show that the D-ERA-TSP scheme can effectively reduce the pilot collision probability and access delay in the distributed scene,and can be applied in the distributed mMTC scenario.To sum up,this paper mainly studies the application of GFMA technology and random access technology in mMTC scenarios.It focuses on the situation of asynchronous delay and pilot conflict,and puts forward the improvement scheme.The research shows that the asynchronous GFMA technology proposed in this paper and the random access technology based on time-shifted pilot can solve the corresponding problems.