GSM Network Quality Solutions Research On High-speed Railway

In April 2007 Chinese railway completed the sixth speed increase the speed reached 200 km / h, after high-speed railway speed increase GSM network indicators worsened. Coverage from 95 percent to 50 percent and only 30 percent access Rate, also occasionally get access the regular drop, unable to meet the basic communications needs. In order to find and formation of the root causes of these problems, and ultimately solve the problem, this papers of specific optimization process was meticulous in-depth analysis and practical to combine a practical solution.The GSM network's problems on High-speed railway faced by low coverage for the main performance, low accessed Rate, drop rate higher. Why would the network quality after such a deterioration of the railway speed increase, just what factors led to this result?First, high-speed train in order to meet the requirements of high-speed movement, tightness in both material and train the new changes Penetration loss increased significantly less than the existing network of high-shielding requirements under the conditions of coverage.CRH trains are used in confined design, particularly the materials used CRH5 compartment is large hollow aluminum profiles, train penetration loss have multiplied, after a lot of assessment test, comprehensive variety of factors that high-speed train running state The biggest loss for the penetration 25 dB. At the same time high-speed movement under the Doppler frequency shift will result in the mobile phone base stations and coherent demodulation performance degradation, direct impact on the choice of district, community cell reselection, such as handover performance.We test a large number of the connected and drop not conducted a comprehensive statistical analysis, found in the compartment level higher than -85 dBm incident occurred when the lower probability. And train the greatest penetration loss for the 25 dB, so, in order to fully guarantee the quality of the train, the train, call the signal level should be higher than -60 dBm, the original network signal levels fall far short of this standard.Secondly, after the train speed in the same time through the gap has increased significantly, the original area network and handover cell reselection strategy failed to meet the demand for high-speed sport.GSM standard provisions of the adjacent area above the service area C2 and C2 CRH and the value of the last five seconds before launching cell reselection, the two-way meet the demand for cell reselection of the overlapping coverage areas at least 10 seconds to meet the train running time. Completed two times rapid handover about 10 seconds, this time with the requirements of cell reselection of the same district. Therefore, if the trains running at 200 km / h, while the smallest overlapping coverage areas is 556 meters.At the same time norms stipulated by the C2 value trigger the district re-elected twice the minimum time interval between 15 s. To meet this condition train in each district of the running time of not less than 25 seconds, which can calculate the speed of trains running in the 200 km / h, a single area of coverage from at least 1389 meters.Finally, the network is targeting SDCCH restrictions can not satisfy the high-speed railway to update the location of the outbreak of the demand.In location update the phone will not respond because the result is not connected to paging. A total length of over 200 meters of the CRH will be in less than four seconds of time through the location of the border areas, there will be 200 during the phone at the same time about the need to update position, a position occupied SDCCH required to update the time about 3.5 seconds, High-speed railway along the edge of sub-district location SDDCH Congestion is the most serious is the most difficult to solve. Serious SDCCH Congestion will can not connecting to the probability of increased substantially.Coverage issues arising from the high-speed railway is the fundamental reason for the deterioration of quality, such as the previously mentioned high-speed railway network coverage should meet the following criteria:1, the area edge of handover, signaling strength≥-60 dBm;2, the adjacent area of overlapping coverage areas≥556 meters;3, single cell district covering distance≥1389 meters.These standards are needed to increase or extend the signal level to achieve. Through the new BTS, the original base stations to increase quarter, adjusted and new repeater antenna to achieve. COST-231 model in accordance with the calculated in line with the requirements of the high-speed railway directional coverage area covering a radius of up to 1.82 km, so the high-speed railway stations along the best pitch no more than 3 km; attention should be paid at the same time in different geographical conditions have different cover design principles, For example, through the dense forest belt covering the high-speed railway, slightly away from the base station rather than from the base station near the better. Repeater due to the traditional needs of the community and the introduction of the new single district can not meet the requirements of distance covered so I do not suitable for high-speed railway coverage, you can use the digital repeater signal to the community to achieve the extension of coverage requirements. You can also back the merger and Community Design Chain quarter sequence to overlapping coverage areas and meet the single coverage from the district's request. You can also flexibility in the use of small antennas to resolve the scope of coverage inadequate.The most serious SDCCH Congestion in the area edge position, when SDCCH channel configuration to limit the number of still can not meet the needs of the community by increasing the same to resolve.To the same quarter in SDCCH Congestion is a serious increase in the base stations of a district, the new district will set up the antenna and SDCCH Congestion serious exactly the same area. The two cell's SDCCH are set to the largest number, and then adjusted through the parameters of the two district reasonable share Traffic update the number and location in order to achieve the objective of resolving SDCCH Congestion. It should also minimize the high-speed railway across the location of the number of areas, to avoid a short period of time across multiple locations in the situation. Location of the demarcation of the border area as far as possible choices Traffic was relatively small in the region, attention should not be two railway lines as the location of the border area. Perfect coverage, adequate capacity and stability of the equipment running the high-speed railway network is to guarantee the quality of the foundation, but only with those still far from enough, but also the need for high-speed railway along the parameters of the district with the high-speed railway characteristics of the adjustment and optimization of this Can effectively guarantee high-speed train on the network quality.Idle mode mobile phone service to complete the main area and adjacent area of broadcast news monitoring, measurement, location updates, and the choice of district re-elected missions. In order to meet the high-speed railway signal the rapidly changing characteristics, should speed up the process of cell reselection of district, so that the phone in the presence of idle time as possible in the strongest signal. Appropriate attention to simplify the idle BA list, optimize the C1, C2 algorithm, the largest increase in the number of re-issued to shorten the DRX cycle, reduce the district re-elected, such as retardation.Handover from cell phones and networks together to complete, high-speed movement circumstances require more timely handover, handover at the same time, ensure that accurate, to avoid the poor quality of community involvement. Appropriate attention to simplify the list of areas adjacent , shorten the time handover penalties, allowing SDCCH handover, reducing the wireless link overtime timer, close the downlink power control and DTX, and so on.To sum up, high-speed railway on the GSM network quality of the final analysis, because the original network coverage can not adapt to the new train high-shield and Doppler frequency shift of the decline, the original district re-elected and can not adapt to high speed handover strategy the requirements of these two fundamental reasons. So ,to solve high-speed railway GSM network quality problems to resolve coverage and handover from optimization of the two-pronged set of parameters, specific and detailed optimization. As long as described in this paper, in accordance with the principles and concrete solutions to implement and continuously adjusted, we can make high-speed train on the GSM network quality meet the required standards.