Challenges facing high-speed rail coverage
According to the future development trend of high-speed rail and the current operating conditions of European and American countries, the high-speed rail coverage plan should be able to meet the speed of 350km / h or more, and the highest speed of 450km / h. The attenuation of the new fully enclosed car to the mobile phone signal is above 24dB. According to the GSM-R private communication network of the Beijing-Tianjin High-speed Railway after completion, the high-speed rail coverage plan should meet the traffic needs of about 300 passengers at the shortest departure interval (3 minutes). The network connection rate exceeds 95%, and the coverage rate It is 99.5%, the call drop rate is not higher than 5%, and the handover success rate is above 90%.
The network coverage of high-speed train scenarios faces the following challenges.
Large body penetration loss
The high-speed train adopts a closed-body design, which increases the loss of the car body. Various types of CRH trains have different penetration losses. ZTE conducted detailed tests on the losses of various major passenger vehicle models. The comprehensive attenuation values ​​are shown in Table 1.
Doppler frequency offset
The Doppler effect is the biggest impact of high-speed coverage scenarios on the performance of FDD LTE systems. The wavelength of the received signal changes due to the relative movement of the signal source and receiver, called the Doppler effect. In mobile communication systems, especially in high-speed scenarios, this effect is particularly evident.
High speed affects performance
In the high-speed scenario of the UE (user equipment), the handover performance will be greatly affected. To ensure the seamless mobility and QoS of users, the most basic requirement is that the time for users to pass through the switching area is greater than the processing time of the switching, otherwise the switching process cannot be completed, which will cause the user's QoS to drop or even drop calls. In high-speed scenarios, because the UE residence time is shorter than the cell selection process, network problems such as off-network and cell selection failure are also prone to occur.
Public network and high-speed rail coverage private network interact with each other
As part of FDD LTE public network coverage, high-speed rail coverage must consider the interaction between the high-speed rail coverage private network and the public network. Avoid the formation of voids and excessive overlap between the private network and the public network, especially to avoid the coverage of large sites over the high-speed rail track. It is necessary to do a good job of switching between public networks and private networks, reselection, etc., to ensure a normal transition between each other.
FDD LTE high-speed rail coverage solution
In response to technical difficulties such as large frequency deviation and frequent handovers faced by high-speed rail coverage, ZTE provides professional LTE high-speed rail coverage solutions.
Adaptive frequency offset correction algorithm
For high-speed mobile users, the Doppler frequency offset is often very large. The base station receiver must estimate the frequency error between the transmitter and the frequency error correction, otherwise it will have a great impact on the link performance. In addition, the base station receiver also needs to deal with the problem of rapid changes in frequency offset, that is, to ensure that it can quickly keep up with the speed of frequency offset changes and perform effective compensation. For the scenario where the train speed is up to 300km / h, if the frequency deviation tracking speed is too slow, a huge estimation error will occur when the frequency deviation changes rapidly, resulting in a serious deterioration in performance. ZTE's self-developed adaptive frequency offset correction algorithm can detect the relevant information of the current subframe frequency offset at the baseband level in real time, correct the phase offset of the baseband signal caused by the frequency offset, and improve the demodulation of baseband performance.
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