Cellular V2X Massive MIMO Transmission

Vehicle-to-Everything(V2X)communications have achieved rapid development in recent decades.The number of vehicle terminals and the corresponding demand for mobile services is growing,which puts forward higher requirements for the performance gains of energy and spectral efficiencies of the V2 V wireless communication system.The massive multiple-input multiple-output(MIMO)system uses large-scale antenna array and serves a plurality of users simultaneously,which becomes quite attractive in V2 V communications.Moreover,cellular based vehicular communications introduced by 3GPP can support the communication connection with large bandwidth demand and wide network coverage,and have the potential to create lower latency and more reliable vehicular communications,V2 V underlay massive MIMO transmission has become a promising technology for future cellular networks.Massive MIMO wireless transmission performance depends on the acquisition of channel state information.However,in actual V2 X scenarios,the obtained channel statistical information(CSI)is much easier to be outdated due to the high mobility and fast channel fading,which contributes to a huge pilot overhead and consequently decreases the system performance.As a result,this dissertation investigates the transmission scheme based on the non-orthogonal pilot for the single-cell and multi-cell V2 V underlay massive MIMO system,respectively,the major work of this dissertation is listed as follows.Firstly,a pilot reuse scheme for the single-cell V2 V underlay massive MIMO cellular system is investigated.Based on the spatially correlated Rayleigh fading Massive MIMO channels,a large-scale MIMO channel model for cellular V2 V is established.Based on the channel model,the mean square error(MSE)of the channel estimation for cellular and V2 V links can both be minimized when the channels of users reusing the pilot lie in approximately orthogonal spatial directions and the severe pilot interference caused by pilot reuse can be eliminated.The feasibility of pilot reuse in large-scale spatial correlated cellular V2 V MIMO channels with limited angle spread is proved theoretically.Moreover,the channel angle domain conditions,which can minimize the MSE of channel estimation for cellular and V2 V links,are derived respectively,and an angle information assisted pilot assignment algorithm is developed.The proposed algorithm allocates pilots for cellular users(CUEs)and V2 V transmitters(VTs)based on the statistics CSI with low complexity.What’s more,regarding that channel estimation performance might degrade due to pilot reuse,the robust receiver design of BS and each V2 V receiver(VR)under pilot reuse is studied.Numerical results indicate that the proposed angle information assisted pilot reuse scheme can significantly improve the performance of spectral efficiency compared with the conventional orthogonal pilot scheme for both cellular and V2 V links.Secondly,a pilot reuse scheme for the multi-cell V2 V underlay massive MIMO cellular system is studied,and the multi-cell cooperative pilot allocation is implemented for CUEs and VTs.Based on the spatially correlated Rayleigh fading Massive MIMO channels,a MIMO channel model for multi-cell cellular V2 V is established.Based on this model,the pilot interference between users can almost be eliminated when the channels of users sharing the pilot lie in orthogonal spatial directions,then the channel estimation MSE for cellular and V2 V links in each cell can both be minimized.Different from the conventional method of a completely orthogonal pilot in the same cell and a fully multiplexing pilot between cells.In this method,users within or between cells implement pilot reuse according to their channel correlation,which can effectively solve the inter-cell interference caused by fully pilot reuse between cells without increasing the pilot overhead.Moreover,the optimal conditions for joint pilot scheduling of multi-cell CUEs and VTs are derived,and a multi-cell cooperation pilot assignment(MCCPA)algorithm is developed.Numerical results indicate that the proposed MCCPA algorithm can significantly reduce the pilot interference within and between cells and improve the performance of spectral efficiency in multi-cell V2 V scenarios.Finally,a transmission approach based on the bipartite graph is investigated for FDD V2 V underlay massive MIMO system to establish the equivalent reduced-dimensional channels for CUEs and VRs.Based on the statistical CSI,a bipartite graph model for V2 V underlay massive MIMO system is formulated.Due to the inherent sparsity of massive MIMO channels in the beam domain,most of the channel energy is concentrated in the relatively limited beam directions.Constraints are introduced to schedule the users with channel dimensions no larger than the desired pilot dimension and beam directions with strong channel power as well as weak inter-user interference.Moreover,an optimization problem is developed to maximize the rank of the effective channel matrix subject to the introduced constraints.The optimization problem can be efficiently solved by casting it as a mixed-integer linear program.Further,a channel estimation and precoding scheme is provided for the BS and each VT over the equivalent reduced-dimensional channels.Simulation results demonstrate that the proposed bipartite graph approach can support a larger dimensional V2 V underlay massive MIMO cellular system and significantly improve the spectral efficiency compared with the conventional all orthogonal pilot approach under the same pilot overhead.