Research On Key Technologies For Resource Optimization Based On Time-varying Vehicular Communication Channel

The aim of building intelligent transportation systems(ITS)with features of fast,green,efficient and safe is pursuing by most of the countries.In recent years,cellular vehicle-toeverything(C-V2X)communication technology has become a crucial key technology for achieving environmental awareness,information interaction and collaborative control in the ITS.In the C-V2 X communication system,vehicular users could establish the instant communication links in terms of vehicle-to-vehicle(V2V)communication and vehicle-toinfrastructure(V2I)communication by multiplexing the spectrum resources of cellular users.It makes up for the shortcomings of limited spectrum resources of vehicular ad hoc networks based on the 802.11 P standard,and improves road traffic safety and resource utilization.However,the spectrum multiplexing also brings co-channel interference,which affect system performance.In order to guarantee the improvement of the system performance,the co-channel interference can be managed effectively by adopting reasonable resource optimization methods.In fact,the vehicular communication channel behave as a timevarying feature due to the high-mobility of vehicle users,which brings new challenges to the optimization of communication resources.Therefore,this paper mainly focuses on the key technologies of resource optimization based on the vehicular time-varying channel.Firstly,a time-varying channel is modeled to characterize the non-stationary statistical properties.Secondly,the interference coordination scheme is performed by power control under the time-varying channel.Futhermore,the energy efficiency(EE)of the system can be optimized,thereby achieving the green communication.Thirdly,by utilizing the cochannel interference,a dynamic threshold-based access scheme is required to keep a desired trade-off balance between security protection efficiency and spectrum utilization.Finally,a relay cooperative transmission protocol is designed to further improve the reliability of transmission link and the robustness of resource optimization.The main contributions of this paper are summarized in follows:Firstly,with aim of solving the problem for the non-stationary statistical properties caused by the time-varying speed of vehicle nodes and the dynamic movement of the scatterers,a geometry-based cooperative scattering channel model for multiple input multiple output(MIMO)V2V communication systems is established.In this model,the scattering mobile cluster is equivalent to the relay cooperative node.A birth-death process is utilized to capture the appearance and disappearance dynamic properties of moving scatterers that reflect the time-variant time correlation and Doppler spectrum characteristics.Furthermore,the expressions of the second-order statistical characteristics such as space-time correlation function,Wigner-ville spectrum,ergodic capacity are derived based on the Von Mises angle distribution in the non-uniform scattering environment.The numerical results show that the dynamic changes of the scatterers and time-varying speed of the vehicle lead to the nonstationary statistical properties,which show the correctness of the proposed model.Moreover,the proposed model can provide theoretical basis for resource optimization and performance evaluation of the system.Secondly,to tackle the issue of the co-channel interference generated by device-to-devicebased V2V(D2D-V)links in C-V2 X system,an optimal power control scheme for EE maximization is presented with imperfect channel state information(CSI).In the scheme,the imperfect CSI is modeled as a first-order Gauss-markov process.The characteristics of dynamic vehicular communication are described by the variation of communication distance between D2D-V users,and the co-channel interference is modeled by stochastic geometry.Meanwhile,the feasible region of D2D-V user's transmission power is obtained under the constraints of guaranteeing outage probability of cellular user and maximizing transmission power of D2D-V users.Furthemore,the target function of EE maximization is formulated to solve the optimal power of D2D-V users.Numerical results show that the average sum rate(ASR)can be maximized by adjusting the access density of D2D-V users,and the EE can be reduced by channel estimation error.The EE performance can be improved in comparsion with that of the previous maximum transmit power allocation scheme under different maximum communication distances of D2D-V users.By optimizing the transmission power of D2D-V users and coordinating the co-channel interference,the EE performance is maximized.In other words,the scheme reduces the power loss and increases the information exchange rate of vehicular users.Thirdly,to address the problem that the computation-offloading information may suffer from an eavesdropping threat due to the broadcast nature of wireless links and high-mobility of vehicles,a dynamic threshold-based access scheme for security provisioning of C-V2 X computation-offloading network is proposed by considering an imperfect CSI.The proposed scheme can maximize the secrecy throughput under a connection outage constraint of the D2D-V links,with the total area spectral efficiency optimized under the security performance criterion for the offloading link.The optimal dynamic access thresholds are obtained by solving the above optimization problem.Finally,the effects of system parameters such as channel estimation error and eavesdropping user density on the access thresholds and security performances are analyzed.Numerical results show that the proposed scheme provides significant performance gains in terms of secrecy throughput compared with the previous access schemes.Furthermore,the proposed scheme is capable of implementing the interference control under variant channel conditions for the proposed network.Finally,to deal with the problem that the long distance and poor link quality may reduce the reliability and stability of the D2D-V links,and thus affect the robustness of resource optimization.A hybrid decode amplify forward(HDAF)relay cooperative transmission protocol based on the first-order autoregressive(AR1)model(AR1-HDAF)is designed under the time-varying channel.In the proposed protocol,the characteristics of time-varying channel are described by correlation coefficient of Doppler shift.The proposed protocol can be selected adaptively according to the variation of the channel gain.Furthermore,the moment generating function(MGF)is utilized to derive the closed-form expression for average symbol-error-rate(ASER)of M-ary quadrature amplitude modulation(M-QAM)in high SNR condition,and the influence of vehicular velocities and CSI estimation accuracy on ASER performance is also analyzed.Numerical results show that the improvement of the CSI estimation precision will reduce the error-floor-value caused by vehicle rapid movement and improve the system performance.Moreover,compared to AR1-based amplify and forward(AR1-AF)and AR1-based decode and forward(AR1-DF)protocols,the ASER performance of the proposed strategy can be increased about 3.6 d B and 1.5 d B,respectively.The research results can provide technical support for related research on relay selection and resource allocation under the time-varying channel.