| High-speed railway, as a high-tech product for adapting modern civilization and social progress, is a kind of advanced transportation with the advantages of high velocity, large capacity, low pollution, safety and reliability. Since the birth of the high-speed railway, it is growing with the mobile communication demand of train to wayside. One part of traffic load demand is from the train control, which is an important part of the high-speed railway. And another part of the traffic load is from the passengers, which is one of the important key performance indexes that can satisfy the requirement of passenger service quality. With the rapid development of high-speed railway, future high-speed railway wireless communication systems aim to simultaneously achieve the train control, onboard surveillance for guaranteeing the security of railway operation and passenger services.Radio channel is the basis for design of a wireless communication system and accurate knowledge of radio channel characteristics is the prerequisite condition to design the system. Channel measurement is a direct way to acquire channel characteristics which can be described by channel models and then provide theoretical support for key technology evaluation, system and link simulation, prototype building, and network deployment. Although the high-speed railway radio channel has been largely investigated by domestic and foreign scholars in recent years, there are still some gaps as follows:1) In terms of propagation characteristics, existing researches on high-speed railway narrowband channel characteristics cannot provide sufficient reference for design of future high-speed railway wireless communication systems. There is still a lack of sufficient knowledge of wideband small-scale fading characteristics in some typical high-speed railway scenarios such as viaduct and U-shape cutting.2) In terms of channel modeling, current measurement-based high-speed railway channel models have not yet cover multiple input multiple output (MIMO) channel characteristics due to the limitation of test condition. The evaluation of MIMO performance in high-speed railway typical scenarios lacks the support of realistic channel models.3) In terms of channel measurement methods, existing channel sounder-based channel measurement approaches cannot be used to establish the database of channel impulse responses for future high-speed railway wireless communication systems. A highly efficient measurement method should be investigated to support the collection of high-speed railway radio channel data.In order to solve above problems, the dissertation deeply studies wideband propagation characteristics, MIMO channel modeling and high-efficiency channel measurement methods in high-speed railway scenarios. The main works are as follows:1) Based on channel measurement data collected by a standard channel sounder, we investigate large-scale and small-scale fading characteristics in viaduct and U-shape cutting scenarios. A large-scale propagation model covering path loss and shadow fading parameters is established. Moreover, we analyze Ricean K-factor from the perspective of narrowband, wideband and delay and derive the narrowband and wideband K-factor statistical models. According to extracted multipath and Doppler parameters, the time dispersion and frequency dispersion in viaduct and U-shape cutting scenarios are characterized.2) Based on time and frequency stochastic scattering theories, we research MIMO channel modeling for viaduct and U-shape cutting scenarios. According to the geometry-based stochastic scattering theory, we propose a reference model for the viaduct scenario, which is used to derive the close-form expressions of correlation function and Doppler power spectral density (DPSD). Environment-related parameters in the model are determined by the nonlinear fitting results depending on measured and theoretical DPSDs. The MIMO performance in the viaduct scenario is evaluated using the proposed semi-empirical MIMO channel model. On the other hand, we establish a propagation graph model for the U-shape cutting scenario based on the stochastic propagation graph theory, which can generate virtual channel impulse responses. The propagation graph model validated by comparing the measured data with the simulated data is extended to the MIMO case and is used to evaluate the MIMO performance in the U-shape cutting scenario.3) To solve the measurement restriction and measurement efficiency problems in high-speed railway scenarios, we investigate railway network-based high-efficiency channel measurement methods. A wideband code division multiple access (WCDMA)-based high-speed railway channel measurement method is proposed and the principle of using common pilot channel in WCDMA to perform sliding correlation-based channel sounding is described. A semi-physical simulation platform is built and is employed to validate the WCDMA channel sounding approach. In addition, we present a novel long term evolution (LTE)-based high-speed railway channel measurement method and specifically introduce how to measure the channel in frequency domain using the cell-specific reference signal (CRS) in LTE. The performance of LTE channel sounding method is analyzed and a new algorithm is proposed to compensate the timing offset that has a strong impact on the channel impulse response result. Finally, the LTE channel sounding principle is simply verified in MATLAB platform.4) To implement the LTE-based high-speed railway channel measurement method, we investigate a novel high-speed railway channel measurement system composed of existing LTE railway networks and a dedicated LTE sounder. The hardware platform of LTE sounder is established by hardware modules and the corresponding software application is developed to enable four functions involving data reading, data storage, result display and position marking. Especially, the LTE sounder is calibrated and verified by the standard vector signal generator. Based on the proposed system, we conduct the field channel measurements in plain viaduct scenarios on high-speed railway and analyze the strip-shaped coverage, cell combination, mobile relay, MIMO and coordinated multiple points transmission/reception (CoMP) measurement results.
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