1 Introduction
In January 2004, the State Council of China's railway history through the first "long-term railway network plan," to determine the 2020 railway construction blueprint. Fourth Session of the Tenth National People's Congress passed the "National Economic and Social Development Five-Year Plan" proposed to give priority to the development of transport, accelerate the development of rail transport. "Eleventh Five-Year" is building a moderately prosperous society in our country critical period is the golden opportunity for railway construction.
In the next two years, China's railway will form about 10 million kilometers of operation mileage of which 200 to 350 kilometers per hour high-speed railway will reach 8,000 km, and over time high-speed railway mileage will continue to grow. 3G high-speed railway will be an important business application scenario, high-speed rail sealing, determine the characteristics of speed TD-SCDMA network construction requires full use of its technical characteristics, choose a reasonable way to ensure wireless network coverage greatly increase the speed Railway communication needs.
Compared to static or low speed communications, high-speed mobile communication state is facing more problems, such as Doppler shift, cell switching and re-election, and then make the user experience a sense of deterioration.
(1) the Doppler effect, part of the body through loss of signal over 20dB, common scenarios set overlap can not meet the requirements of switching and re-election;
(2) KPI worse: the success rate dropped to switch, switch rate decreased drop rate increase;
(3) The user experience is poor: often out of network, voice quality is poor, lower quality data services, reduced throughput, or even dropped calls;
(4) operator revenue and brand affect: a substantial increase in customer complaints, serious impact on the brand, leading to lower earnings to reduce traffic.
2 high-speed railway, covering strategy
For TD-SCDMA coverage of high-speed railway, proposed Doppler shift compensation through loss to overcome, switching and optimization settings, three re-election strategy.
2.1 Doppler shift compensation strategy
TD-SCDMA band within the typical range of Doppler shifts in Figure 1, Table 1 below:
Figure 1 Typical Doppler shift
Table 1 Typical Doppler shift
Standard protocol defined in 3GPP TD-SCDMA is the maximum speed of 120 km / h, significantly lower than the demand for high-speed railway, whether terminal or base station needs to do frequency compensation.
(1) terminal
TD-SCDMA terminals by AFC (Automatic Frequency Control, automatic frequency control) technology carrier frequency tracking, while the base station side of the fixed frequency of the carrier frequency signal reception, making the largest high-speed rail station along the Doppler shift of up to 1400Hz , compared to Table 1, has more than 500 km per hour, to meet the requirements. But in order to ensure data demodulation performance, the need for test data FOE (Frequency Offset Estimation, Frequency Estimation) and FOC (Frequency Offset Calibration, offset calibration).
(2) Base Station
In the case of high-speed mobile business, TD-SCDMA base stations to do a one-time shift estimation, calibration is extremely difficult, requires the use of two offset compensation. First, according to the channel estimation has been used to estimate the channel response offset, and then use the value of combined detection of the initial calibration data, according to preliminary data symbol offset correction value estimation method using the second offset value is calculated, and the offset compensation. According to the analysis, the first frequency offset estimation precision somewhat less than about 2000Hz, while the second frequency offset in the 800Hz or so. In high-speed environment, after two frequency offset estimation, the estimated frequency offset can be controlled in the range 200Hz or less.
2.2 Strategies to overcome the penetration loss
In the 2GHz band TD-SCDMA, for different high-speed train, the penetration loss is not the same, see Table 2:
Table 2 train penetration loss (dB)
Penetration loss in a typical train 14dB ~ 24dB. Base station site, as far as possible sites for the track line with a certain distance (greater than 50 meters), making the antenna main lobe direction of the rail line as far as possible with a certain angle (grazing angle), to reduce the penetration loss.
Figure 2 grazing angle of penetration loss of
As the grazing angle decreases, the trains through the increased rate of loss increases when the grazing angle of 10 degrees or less, the rate has accelerated noticeably. Therefore, when TD-SCDMA network planning, try to control the grazing angle of 10 degrees.
Considering the track line from the base station location and the relationship between the grazing angle, given "the" distribution and ")"-shaped distribution:
(1) "the" distribution: that is, in turn netting on both sides of the tracks. Given the current high-speed double-track railway track approach, using "the" word distribution can better take into account "from" Train the coverage requirements.
(2) If you need to bypass the important facilities, valleys, etc., recommended ")" curved track in the form, consider the base station in the ")" shaped curve inside of the curved track to ensure good coverage.
Li with the slope, Xu equality were also raised in laying high-speed trains and other ways to reduce repeater penetration loss, is a better solution [1,2].
2.3 switching and re-elected to set policies
Under normal circumstances, TD-SCDMA terminal switching time of 0.3 to 2 seconds. When the terminal moves fast enough so that switching zone system processing time is less than the minimum switching time, the switch will not complete the process, leading to dropped calls.
Taking into account the district's two-way switch, overlapping coverage should be switched from the more than 2 times the distance that the overlapping coverage from the train speed = 2 * * switching time. Overlapping areas of coverage distance can refer to Table 3:
Table 3 switching time and distance
Set the trigger for cell switching, the following strategies:
(1) sub-sector of power: The power splitter, the power of the district are divided into two parts, then use two high-gain antenna radiation out to solve the same base station to switch between different cells within the problem.
(2) removal of the primary service area and adjacent areas: simplified network structure, and even set up special high-speed rail network, covering the formation of the main long-distance signal, will cover the short distance, fading fast clean-up of high-speed railway signal coverage, to avoid frequent re-election and switch.
(3) set the same area: The BBU + RRU series approach, a district with more than RRU is set with the same frequency, scrambling code, effectively expand the coverage of a single cell (up to 40 kilometers).
3 high-speed railway tunnel coverage strategy
Indoor distribution line covering the tunnel is covered by the scope of a leaky coaxial cable and antenna cover two. In comparison, for the highway tunnel, due to easy maintenance, the proposed approach to antenna coverage, the cost can be controlled even lower; for the railway tunnel, because of the difficulty to maintain, and require uniform coverage quality, coverage is recommended leaky coaxial cable, you can use fewer active devices, but the cost is higher.
3.1 Link Budget Model
Leakage cable coverage distance (m) model:
Ld = (Pin-(P + L1 + L2 + L3 + L4 + L5) / S (1)
Of which: Pin is leaking into the cable input power (dB), P is the edge of coverage required field strength (dBm), L1 to leak cable coupling loss (95% probability) (dB), L2 is the human loss (5dB), L3 = 20 * lg (d / 2) for the width factor (dB), d the distance to the terminal from the leaky coaxial cable (m), L4 is the attenuation margin (3dB), L5 is the body loss (see Table 2, the value of 22dB), S leak cable loss per meter (dB).
To common RFS-13 / 8 specification as an example of leaky coaxial cable tunnel cover uplink and downlink. Down into the power PCCPCH 26dB, the tunnel roof about 2.5 meters away from the user terminal, is calculated as follows:
Table 4 cover the distance down the tunnel
PATH uses a single host 1w,-90dBm according to the edge of the field strength, unilateral downward to cover 329 meters.
Table 5 covers the distance up the tunnel
From the above link budget results, the uplink coverage than the distance covered is far down in the downlink to meet the coverage under the premise up to meet the coverage requirements to ensure that on the downlink balance.
3.2 high-speed railway tunnel cover short
Short tunnel refers to the cable without the need for re-injection leakage power under the premise to solve the coverage of the tunnel, the TD-SCDMA, the general length of 500 meters. At this point, switch to reduce the number of inside and outside the tunnel should be set to the same area, the proposed laying outside in the tunnel roof antenna. Source base station not only provide cover tunnel, but also take into account the tunnel outside the coverage.
3 short tunnel coverage
In addition, to ensure the tunnel entrance level abrupt increase, the proposed base station side of the leak near the cable entrance of the tunnel about 10 meters out.
3.3 high-speed railway tunnel cover long
Long high-speed railway tunnel in the tunnel need additional coverage cascade RRU, to address the leaky coaxial cable into the power shortage problem. Additional cascade RRU outside the tunnel should be the source base station of cell, to reduce the switching zone.
Figure 4 long tunnel cover
Similarly, the tunnel will appear at level mutations, an increase in the exit of the tunnel roof antenna, so that the signal extends to the tunnel outside the tunnel, with the switch to move from the entrance of the tunnel inside the tunnel, to ensure a successful switch.
3.4 tunnel covering the source selection
For the independent short tunnel, the tunnel cover and tunnels together to consider the use of the same area with tunnels as a source of the RRU. For long tunnels, using a dedicated source (only used to cover the tunnel), to cover the use of BBU + RRU. For continuous tunnel group, the same dedicated source, the use of BBU + RRU, the area between the tunnel and the tunnel into the tunnel cover, avoiding switching.
4 Conclusion
Number of VIP users high-speed train, so need to focus on users (including voice and data users) business sense. In order to fully enhance the quality of coverage high-speed railway, this sub-high-speed railway, covering, cover the two most elaborate tunnel covering the essentials, details and strategies, in order to better guide the TD-SCDMA coverage of high-speed railway construction.
Of course, if the use of special high-speed railway network construction methods, still need to pay attention to the integration of the public network, especially in dense urban railway stations and along part of the private network and public network exit. Due to space limitations, this article will be introduced.
References
[1] Li with the slope. Datang Mobile TD-SCDMA high-speed rail solution [J]. Mobile Communications, 2009 (17): 70-73.
[2] Ping Xu. Future high-speed rail network mode TD-SCDMA wireless coverage Analysis [J]. Mobile Communications, 2009 (22): 52-55.
[3] Xiao Qinghua. TD-SCDMA indoor distribution system design and analysis [J]. Mobile Communications, 2007 (2): 38-42.
[3] Xiao Qinghua, Falcon House, Shu Jun, etc. and transform the environment of the new TD-SCDMA indoor distribution system [J]. Communication World, 2008 (18).
[4] Zhu Dong Zhao, Xiao Qinghua, and other. TD-SCDMA wireless network planning and design and optimization [M]. Beijing: People's Posts and Telecommunications Press, 2008. ★
Author: Xiao Qinghua
Source: Third generation mobile communication network |
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