How to achieve wireless coverage of high-speed rail?
China Railway has entered the era of high-speed. It is estimated that by 2020, China's high-speed railways with a speed of over 200km / h will exceed 18,000 kilometers. How to provide users with good mobile communication network services in a high-speed mobile environment is a problem that mobile communication operators have been researching and solving since the high-speed railway. Compared with the conventional CDMA network coverage plan, the high-speed railway coverage plan has certain characteristics. Only by identifying the key issues of high-speed rail coverage can the depth coverage and fine optimization along the high-speed rail be more precise. As shown in Figure 1. Figure 1 Research significance of high-speed rail coverage 4 iconic factors covered by high-speed rail High-speed rail network coverage is mainly reflected in four iconic factors: Doppler frequency shift, vehicle body penetration loss, switching problems and terrain diversity in the coverage area. Doppler shift The Doppler frequency shift problem refers to that the signal frequency of the receiving end will shift under the condition of high-speed movement, resulting in the degradation of the coherent demodulation performance of the base station and receiver. ◆ Impact on the receiver CDMA uses coherent demodulation, which requires the receiver to demodulate the carrier at the same frequency as the carrier of the received signal. Carrier frequency jitter affects the receiver's demodulation performance. At the same time, 1X uses the Qualcomm CSM6700 chip, and DO RevA uses the Qualcomm CSM6800 chip. The maximum values ​​of frequency shift are 1440Hz and 960Hz, respectively. According to the Doppler frequency shift formula, the maximum allowable moving speed of the receiver is 972km / h and 648km / h. This speed is much higher than the maximum operating speed of the high-speed railway, and the chip demodulation tolerance meets the requirements. ◆ Impact on the base station Due to the existence of Doppler frequency shift, the coherent demodulation performance of the base station and mobile phone is reduced. It can be considered to increase the original Eb / Nt value by 3dB when the link budget is reached. Switching problem The handover problem refers to the fact that high-speed movement causes calls to be dropped more easily during handover, and the requirements for handover are much higher than ordinary scenes. On the premise that the size of the switching area is unchanged, the faster the terminal, the shorter the time for the terminal to cross the switching area. Therefore, when the moving speed of the terminal is fast enough, the time to pass through the switching area is less than the minimum delay of the soft handover processed by the system, which may cause a call drop. When setting the parameters of the high-speed railway coverage base station, it is necessary to consider setting the minimum switching area to meet the switching needs of the high-speed train when running at high speed. The size of the minimum switching area can be calculated based on the train speed and the soft handoff delay. The soft handoff delay is generally 300ms. Therefore, the minimum handoff zone size mainly depends on the moving speed of the train. At present, the maximum speed of high-speed rail design is 350km / h, and the minimum soft handover area is calculated to be 29m according to the formula. Shenda High Speed ​​Rail CDMA Coverage Case We conduct research on the CDMA wireless network coverage of Shenda High Speed ​​Rail, which is mainly used to cover users of high-speed railway passenger dedicated lines, including passengers waiting at stations and high-speed trains. Overview of Shenda High Speed ​​Rail The Shenda high-speed railway runs through 6 cities including Tieling, Shenyang, Liaoyang, Anshan, Yingkou and Dalian in Liaoning Province, with a total length of 553km. It is a dual-line electrified railway with an average speed of 200km / h or more. It was completed and opened to traffic in 2011. There are 15 stations in Shenda High Speed ​​Rail. The high-speed train designed by Shenda High-speed Railway is a CRH5 model jointly manufactured by Changchun Rail Bus Company and Alstom Company of France. It consists of 8 units with a total length of 211.5m, a maximum width of 3200mm and a height of 4270mm.
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