| The intelligent transportation system is the development direction of the future traf-fic system,and the high-speed mobile communication system,which aims at ensuring the efficient and reliable transmission of various transportation service information,is the "nerve center" of the intelligent transportation system.Vehicular network is a typ-ical application of intelligent transportation system in high-speed mobile environment.In general,vehicular networks includes the vehicular networks for highway traffic and high-speed railway(HSR)communication systems.With the vigorous development of vehicular networks,the contradictions between the growing demands for mobile commu-nication services and the insufficient transmission capabilities of mobile communication systems have become increasingly fierce.For example:considering the standard LTE systems,a 20 MHz bandwidth can provide a transmission rate of more than 100 Mbps under normal walking speed conditions,whereas under a high-speed movement condition(350 km/h),the target of the transmission rate design is only 100 kbps for business data throughput.It is conservatively estimated that this throughput is still less than one-tenth of the total throughput requirement for vehicle communication.The serious decline in the system transmission capacity is mainly caused by some characteristics brought by high-speed movement.These characteristics are:fast time-varying channel and Doppler effect.In addition,vehicular networks need to provide various communication services for the vehicle users with different moving speeds.Vehicular networks have some other characteristics,such as:differences of quality of service(QoS)requirements for various communication services and differences of channel characteristics for the users with dif-ferent moving speeds.The characteristics play a significant role in improving the perfor-mance of the communication system.Therefore,in view of these features of high-speed mobile communications,it is very significant to design the transmission strategies that can effectively improve the performance of the communication system.This thesis addresses the problem of insufficient transmission capacity for HSR com-munication systems and vehicular networks.The characteristics of fast time-varying channels,Doppler effects,differences of QoS requirements,and differences of user chan-nel quality are studied and investigated.Utilizing the temporal correlation of the fast time-varying fading channel,an adaptive transmission scheme is designed for high-speed mobile communication.With satisfying the QoS requirements of various services/users,the transmission capability of high-speed mobile communication systems is significant-ly improved.First,Chapter 2 considers the scenario of multi-carrier system communi-cates between base station and train in high-speed mobile environment.Considering fast time-varying channels and Doppler effect,it designs the cross-layer adaptive transmis-sion scheme with imperfect channel state information(CSI)for increasing the system throughput.Second,based on the system model of Chapter 2,Chapter 3 takes the advan-tage of the differences of QoS requirements for various communication services.It de-signs the hierarchical modulation(HM)multi-service multiplexing transmission scheme with imperfect CSI to improve the system performance.Then,considering the method of providing the multiplexing transmission with hierarchical modulation,Chapter 4 takes the advantage of the differences of channel characteristics for the user with different mo-bile speeds,and designs the dynamic cross-layer resource management strategy.Last,since bi-directional communication between vehicles can be regarded as a symmetrical full-duplex communication,Chapter 5,which is taking the advantage of full-duplex com-munication,designs the variable-rate variable-power transmission scheme with imperfect CSI to improve the spectrum efficiency(SE)of the bi-directional communication.The main innovations and contributions of this thesis are listed as follows:1)Chapter 2 investigates the throughput maximization problem of the multi-carrier communication between base stations and vehicles in high-speed mobile environ-ments.This chapter uses the data frame as the minimum transmitted unit.In fast time-varying fading channels,it is a serious issue to ensure that the data frame can be accurately received at the vehicle.Considering inter-carrier interference(ICI)caused by Doppler effect,using the time domain correlation of fast time-varying channels,the statistical characteristic of imperfect CSI is modeled,and an irre-versible and complex form of bit error rate(BER)expression is obtained.Then,an upper bound of BER is given,and a cross-layer adaptive transmission scheme with imperfect CSI is designed.This solution not only meets the packet loss rate(PLR)requirements of the service but also improves the throughput of the system.2)Chapter 3 studies the SE maximization problem of the multi-service simultaneous transmission system.Using the differences of QoS requirements for various ser-vices,BERs of different bits in HM symbols are obtained with the statistical char-acteristics of imperfect CSI.The expression of BER is very complicated.Using the"main term approximation method",the upper bound of BER is derived through scientific analysis and simulation verification.In addition,considering fast time-vary fading channel and ICI caused by Doppler effect,a HM-based multi-service multiplexing transmission scheme with imperfect CSI is designed,which not only meets the PLR requirements of the different services but also increase the SE of the system.3)Chapter 4 investigates the throughput maximization problem of joint communica-tions from the base station to vehicle users and static users in the vehicular network.Utilizing the differences of the statistical characteristics of CSI for two types users,a HM-based multi-user multiplexing transmission scheme is designed.In order to mdet the QoS requirements of two types users and maintain the stability of traf-fic data queues,the dynamic cross-layer resource management strategy that joint admission control,multi-user scheduling,and HM-based multi-user multiplexing transmission is designed.In addition,the optimal exhaustive search algorithm and suboptimal heuristic search algorithm are proposed for the multi-user scheduling.The simulation results show that the low-complexity heuristic search algorithm can achieve almost the same throughput as the exhaustive search algorithm.4)Chapter 5 studies the SE maximization problem of the bi-directional communica-tion between two vehicles in vehicular networks.The bi-directional communica-tion between two vehicles can be regarded as a symmetrical full-duplex communi-cation.Since the local transmitter cannot instantaneously obtain the transmit power of the opposite transmitter at the next time slot,the signal to interference and noise ratio(SINR)of the opposite receiver is unknown.Hence,the accuracy of the local service data transmission cannot be guaranteed.By analyzing the expression of BER,the upper bound of BER is derived.The simulation result verifies the validity of the upper bound expression.Based on the upper bound of BER,a lower bound of SE maximization problem is formulated.The variable-rate variable-power trans-mission scheme for full-duplex communication with imperfect CSI is designed.By maximizing the lower bound of SE,the proposed transmission scheme increases the achievable SE of the bi-directional communication between two vehicles. |
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