| High-speed railway (HSR) has become an important fulcrum of the economy development and the strategic layout of our country. Moreover, it is also an important part of the globalization strategy of our country. As the nerve center of high-speed railway system, the HSR communication network plays a protection role to the safety and reliability of train running. However, with the rapid development of broadband multimedia data services, the insufficient capacity of current HSR communication system has becoming a more and more obvious shortage. Hence many new type data services cannot be supported by the HSR communication system, such as real-time monitoring, passenger entertainment and business applications, etc. It has hindered the development of HSR towards safer and more comfortable. Therefore, more research is needed on the problem of multimedia services transmission in HSR communication system.In the high-speed railway scenario, high-speed train will remain a constant speed and running straight for a long period of time after it out of the station and accelerate since the adjacent station spacing for hundreds and even thousands of kilometers. Hence the position of the train at any moment can be predicted, which is the most significant difference between HSR system and other mobile communication systems. Based on this characteristic, the distance between high-speed train and the trackside station can be predicted at any moment and then the large scale path loss of radio signal propagation can be calculated. In addition, viaduct scene is the most common scene in the process of high-speed train running. In viaduct scenario, there is a direct path of wireless propagation channel between the trackside station and high-speed trains. Thus the rician fading channel can be used to describe the small scale fading of wireless signal. To combine the above two points, this paper formulates the wireless channel model and channel capacity model which are accordant with the practical situation of high-speed train running. As a result, we can provide a basic scene model of HSR communication system to establish optimization problems of multimedia services transmission.On the basis of the contents discussed above and the proposed mathematical models of each protocol layer, we study the transmission problems of delay-aware services and delay-constrained services respectively by using cross-layer optimization methods. To address delay-aware services, we set the goal to minimize current transmission energy consumption and introduce equivalent rate constraints method to transfer the packet delay of MAC layer into the resource allocation strategy of physical layer to meet the delay performance of differential services. By introducing Lyapunov drift theory, we formulate the optimization problem based on DBP algorithm which can maximize the system throughput and minimize the average queue length at the MAC layer. What’s more, the different delay constraints should also be satisfied. The simulation results show the performance of these two methods. To address delay-constrained services, we set the goal to minimize global transmission energy consumption and introduce graphic modeling method to describe the accumulation capacity of services arrival and the accumulation capacity of minimum transmission requirement, respectively. On the basis of these results, we formulate the mathematical problem to optimize power control strategy. However, these problems cannot be solved directly since they are nonconvex. Therefore some heuristic algorithms are designed by using the graph characteristics and then we propose the online algorithm. The simulation results show the performances of different algorithms. |
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