Research On Grant-free Non-Orthogonal Multiple Acess For The Uplink

After satisfying the requirements of human to human(H2H)communications,the internet of thing(IoT)injects new vigor to the development of wireless communications.The mobile internet and the IoT have been the two driven powers to develop the fifth generation(5G)mobile communication system.To satisfy its communication requirements,the 5G needs a novel multiple access scheme.All the mobile communication systems from the first generation(1G)to the forth generation(4G)adopt orthogonal multiple access schemes for the uplink transmisstions.For the 5G communications,the insufficiency of these orthogonal schemes,including low spectrum efficiency,inefficient massive connectivities support,becomes more and more severe.As a consequence,the study and standardlization work of wireless communications attach great importance to non-orthogonal multiple access schemes,which can offer higher spectrum efficiency and support more conectivities.The existing non-orthogonal multiple access schemes for the uplink include sparse code multiple access(SCMA),multi-user shared access(MUSA),pattern division multiple access(PDMA),and so on.It has no doubt that these schemes can achive gains in spectrum efficiency and massive connectivites support ability.However,all these schemes have certain technique difficulties remained to solve.Besides,except the MUSA,when proposed,they basically pay no attention to the question how users get the non-orthogonal transmission resource,i.e.,access protocol.Performing the access protocol will produce access signaling overhead.If the 5G still adopts the access protocols of previous systems,it will face the problem of huge signaling overhead,even the signaling overhead storm.In this case,the advantages of the non-orthogonal schemes will be seriously weakened.To solve the above problem,this thesis proposes a non-orthogonal multiple access scheme that can save the access signaling overhead,i.e.,grant-free scheme.Some key aspects of the proposed scheme are studied.The main contents of the thesis include:1)Grant-free transmission scheme of non-orthogonalIn the proposed scheme,before transmission,each data packet will firstly be extended by an extension code.By this step,the data packet will have more symbols.The precoded packet will then be put into the precviously assigned time-frequency resource for transmission,and will be received by multiple antennas.In this process,the users get their time-frequency resource when they register the network,or the first time when they have data to send,or when the system reassigns the resource to balance the loading.The transmissions require no dynamic and frequent resource assignments.Therefore,the proposed scheme needs no access signaling overhead.At the same time,by non-orthogonal extension,the system allows many user share the same time-frequency resource.Therefore,compared with the orthogonal multiple access schemes,the proposed scheme has obvious advantages in spectrum efficiency and massive connectivites support ability.2)Data recovery of the grant-free schemeDue to the naturally sparse user transmission behavior,the thesis establishes the block-sparse multilple access model of non-orthogonal.Since no dynamic resource assignment is needed,the system has no apriori information about which users have sent data.Therefore,data recovery includes the user detection and the thereafter symbol demodulation.For this problem,the thesis proposes developed algorithm from the block orthogonal matching pursuit(BOMP)algorithm of sparse recovery,named PICNBOMP(perfect interference cancellation based and normalized BOMP)algorithm.The main idea of PICNBOMP is taking use of the error correction and detection structure of the communication signals to perform perfect interference cancellation.This approach contributes to a recovery of more accurate and lower-cost.Specially,this idea for improvement works well for controlling the error propagation of the iterative algorithm,and can be transplanted to many iterative recovery algorithms.As long as the data packets have the structure of error correction and detection and interference exists among them,their iterative recovery can take use of this improvement method.3)Extension codes design of of the grant-free schemeIn the proposed scheme,the extension codes are important,because they represent the identity of the users,directly influence the interference among different packets,and even directly influence the realization complexity of the proposed scheme.The extension code is a precoding matrix.Based on the PICNBOMP recovery,the thesis analyzes how the precoding matrices influence the recovery accuracy.The study indicates that,precoding matrices of column-orthogonal are favorable for the sparse recovery.Three kinds of column-orthogonal precoding matrix design examples are then given.Among them,the precoding matrices that are very sparse can even greatly reduce the realization complexity of the proposed access scheme.To further reduce the complexity of user detection by the PICNBOMP,a kind of structured precoding matrices is designed.Compared with the codebook/sequence/pattern designs of existing non-orthogonal multiple access schemes,the extension code design of our scheme is rather simple,randomly generated precoding matrices of column-orthogonal are able to offer favorable multiple access performance.