Discussion on the solution of signal coverage in villages in cities

Discussion on the solution of signal coverage in villages in cities

Abstract: The article mainly analyzes the problem of difficult coverage in urban villages, and determines coverage targets through road tests and spot tests, and then uses the idea of ​​indoor coverage systems to propose corresponding solutions.

Due to its special geographical location and cultural environment, the village in the city occupies an extremely important position in the city. Its low rent attracts a large number of migrant workers to live here, and the population is very dense. The area of ​​the village in the city is generally large, and the buildings in the village are dense, the laneways are narrow, and the height of the buildings in the village is generally 7 or 8 floors, which has a great impact on the propagation of wireless signals. In a nutshell, the entire village in the city is like a huge building. There are many places where the signal is weak within the coverage of the base station, and even signal blind spots appear. For villages in the city, on the one hand, there is a dense population, and the market potential is huge; on the other, dense buildings, the wireless signal is difficult to cover. There is no better solution for each operator. With the price reduction of mobile phones and the reduction of tariffs, the penetration rate of mobile phones is getting higher and higher, especially in the middle and low income class, the number of mobile phone users is growing rapidly, and mobile phone signal coverage in urban villages has become various operators The first problem in network coverage work. Unicom Shantou Branch started the experiment of using indoor coverage system to solve the mobile phone signal coverage of urban villages three years ago, and achieved certain results. Next, the author takes the indoor coverage project of H Village in Shantou City as an example to introduce.

1. Determine coverage goals

There are 3 Unicom base stations in Village H. According to the on-site outdoor road test and indoor sampling test analysis, the indoor reception field strength of the first to third floors is generally 15dBm ~ 30dBm lower than the outdoor reception field strength at the same location. Judging from the drive test results, the strong part of the GSM outdoor reception field is less than -80dBm, and the strong part of the indoor reception field on the first to third floors is -95dBm ~ -110dBm, and even the blind zone, the call is difficult, and the call quality is poor. Therefore, this plan mainly solves the indoor coverage problem of the first to third floors of buildings in the villages in the city.

2. Design plan ideas

2.1 Coverage

There are three dual-network base stations around Village H. Due to the large range of village H (0.5 square kilometers) and the high floors in the village (generally 7-9 floors), the rooms are dense, the channels are narrow, and the weak signal areas are many. More antennas are needed to effectively cover them. And the distribution area is large, and the main signal is inconsistent. Multiple installation points (a total of 13 installation points) and low installation positions (between 2 and 3 floors) can be used. The outdoor directional antenna is used to cover the roadway. The antenna should be Face the weak signal area as far as possible, facing away from the strong signal area, and at the same time, the antenna can be appropriately tilted down according to the specific situation to reduce the interference to the strong signal area and the remote base station and avoid the phenomenon of local lack of primary cells caused by different base station coverage. This design requires a total of 159 outdoor plate antennas.

2.2 Device selection and signal source selection

Because the Wireless Repeater has the problem of isolation, and it is easy to self-excite on a large output power, it is difficult to meet the coverage requirements with the wireless Repeater . In order to ensure the stability of the system and facilitate capacity expansion, the outdoor coverage of H village can be adopted by directly coupling the fiber repeater and the base station to the trunk amplifier.

In order to avoid the phenomenon of partial lack of primary cells and frequent handover after coverage. The distribution system coverage area should try to use the original coverage cell signal method. The village H source is selected as follows:

(1) In the southwest corner of Village H, it is mainly covered by the first sector (1031) of the Yuetong base station. Although the roadway direction is consistent with the base station antenna, the signal coverage is still poor due to the very narrow roadway and basically the same floor height. You can consider using a set of 10W fiber optic repeater + four sets of 5W trunk amplifiers for coverage.

(2) In the southeast corner of Village H, the second sector (1772) of the H village base station is mainly covered, and at the same time, part of the area is covered by the first sector (1031) of the Yuetong base station, because the direction of the roadway is inconsistent with the base station antenna The area covered by the first sector (1031) of Yuetong base station is farther away, and the coverage signal is poor. You can consider using a set of 10W fiber optic repeater + two sets of 5W trunk amplifier coverage.

(3) In the northeast corner of village H, it is mainly covered by the first sector (1771) and the third sector (1773) of the village H base station, and a small part of the area is covered by the second sector (1772) of the village H base station and the Yangtze River The first sector (691) of Shanghai is covered. The area covered by the first sector (1771) of the village H base station is close to Tianshan South Road. The space between the buildings is large and the floors are low. The coverage is good, and no additional coverage is considered. The area covered by the third sector (1773) of the H village base station is inconsistent with the antenna of the base station, and the roadway is narrow, and the coverage is poor. You can consider directly coupling the third sector (1773) of the H village base station to a group of 5W Trunk amplifier to cover. A small part of the area near the Yangtze River Commercial Sea is covered by the first sector of the Yangtze River Commercial Sea (691).

(4) In the northwest corner of Village H, it is mainly covered by the first sector (691), the second sector (692), and the third sector (693) of the Yangtze River Shanghai, and a small part of the area is covered by the first Sector (1031) covers. The area covered by the third sector (693) of the Yangtze River Commercial Sea is close to the Yangtze River Road, and the distance between the buildings is large. The coverage is good, and no additional coverage is considered. The area covered by the first sector (691) and the second sector (692) of the Yangtze River Shanghai is not consistent with the base station antenna and the roadway is narrow, and the dual-network coverage is poor (signal coverage in a small part of open areas is relatively poor) Ok, no additional coverage is considered). It can be considered that the area covered by the first sector (691) is covered with a set of 10W optical fiber repeaters + a set of 5W trunk amplifiers, and the area covered by the second sector (692) can be directly coupled to two sets of 5W trunk amplifiers cover.

3. Analysis of design plan

The prediction and analysis of the field strength of the coverage area can be calculated by formula (1):

3.1 Field strength prediction and analysis

Ls (dB) = 32.45 + 20lgf (MHz) + 20lg d (km)

Ls: the value of indoor space propagation loss;

f: system frequency;

d: is the distance.

Taking the GSM system as an example, f takes 900MHz and substitutes into formula (1) to calculate:

Ls (dB) = 32.45 + 20lg900 (MHz) + 20lg d (km)

Available: 50m space loss is about 65.51dB; multipath fading margin is about 15dB; wall loss is 25dB; antenna gain is 7dB; average output power of the antenna port is 13dBm.

The strength of the airport at 50 meters away from the antenna is estimated as follows:

Mobile phone received power = antenna port power + antenna gain-space loss-wall occlusion loss-multipath loss = 13dBm + 7dB-65.51dB-25dB-15dB = -85.51dBm. The relationship between distance and loss is shown in Table 1:

Table 1 Relationship between distance and loss

The field strength of mobile phones within 50 meters is higher than this value.

It can be seen from the simulation test that when the power of the antenna port is 13 dBm, the strength of the hand field in the coverage area farthest from the antenna is basically ≥-85 dBm.

3.2 Type analysis of repeater

When covering the village in the city, the use of radio frequency repeaters cannot guarantee the isolation between the donor antenna and the coverage antenna. It is easy to cause the repeater to self-excite due to the problems caused by the isolation, but the use of fiber optic repeaters or direct coupling does not There are the above problems, and it is convenient to replace the optical fiber repeater with microcell when the traffic volume is high.

3.3 Upstream and downstream balance analysis

Since the devices and feeders used in the urban village distribution system have the same loss on the upstream and downstream signals, and the signal loss during spatial propagation is also the same, it can be considered that the device and the feeder have no effect on the upstream and downstream balance of the signal. Only the repeater used by the system will have a certain impact on the system's uplink and downlink balance, but this can be adjusted by adjusting the uplink and downlink gain of the repeater to make the uplink and downlink gains consistent to ensure the uplink and downlink Balanced stability.

3.4 System upstream noise analysis

Design the system upstream noise level and gain according to the reception level, assuming:

LNT-system upstream noise level

LNR——Noise level reaching the source base station port

L——Space loss from the system host to the source base station port

GUP-system upstream gain

NF-system upstream noise figure

PC-the output power of the source base station

PO——output power of repeater

∵LNR≤-120dBm means LNT-L≤-120dBm

∴LNT≤-120dBm + L take LNT1 = -120dBm + L (LNT1 is different from LNT)

For GSM networks:

LNT1 = -121dBm + NF + GUP, L = PC-LRX

That is -121dBm + NF + GUP1 = -120dBm + (PC-LRX)

∴GUP1 = 1-NF + PC-LRX

= 1-4dB + 40dBm + 50dBm = 87dB

GDOWN = P0-LRX = 34dB + 50dBm = 84dB

According to the principle of uplink and downlink balance, GUP = min (GUP1, GDOWN) = 84dB

Available: LNT = -121dBm + NF + GUP = -121dBm + 4dB + 84dB = -33dBm

That is, the system upstream noise level is less than or equal to -33dBm and will not interfere with the source base station.

For CDMA networks:

LNT1 = 113dBm + NF + GUP, L = PC-LRX

That is -113dBm + NF + GUP1 = -120dBm + (PC-LRX)

∴GUP1 = -7-NF + PC-LRX

= -7-4dB + 40dBm + 50dBm = 79dB

GDOWN = PO-LRX = 34dB + 50dBm = 84dB

According to the principle of balance between uplink and downlink, GUP = min (GUP1, GDOWN) = 79dB

Available: LNT = -113dBm + NF + GUP = -113dBm + 4dB + 79dB = -30dBm

That is, the system upstream noise level is less than or equal to -30dBm and will not interfere with the source base station.

3.5 Switching analysis

In order to avoid the formation of new island cells after the signal coverage or conflict with the frequency plan resulting in high bit error quality, the selection of the donor base station of the repeater refers to the cell coverage distribution map in the test results.

Since the majority of H village GSM network coverage is still covered by the original coverage base station, there is no need to change the original neighboring cell settings, so normal handover can be guaranteed and no call drop will occur.

3.6 Signal leakage and interference control

Because the coverage antenna of the H village distribution system is placed outdoors, there is no external wall and building height (relative to the main road) of the ordinary indoor distribution system for isolation. Therefore, the interference control method for the coverage area of ​​the peripheral base station mainly uses the antenna selection, placement location, The height of the installation position is optimized to control.

For the coverage antenna located at the edge of the strong signal coverage area of ​​the base station, use a directional antenna facing the weak signal area, facing away from the strong signal area, and having good directional performance to reduce interference to the strong signal area and the remote base station; control of lateral radiation This is achieved by placing the antenna in the roadway. Try to choose the original coverage cell signal in the coverage area of ​​the distribution system to avoid the local lack of primary cells caused by different base station coverage.

For the place located in the center of the weak signal coverage area, a lower height antenna installation method can be used to reduce the interference to the distant place.

For coverage of open areas, the antenna downtilt angle can be appropriately increased to reduce interference to distant locations.

The forward control of the covering antenna also needs to be considered. First, when the repeater antenna cannot be aligned with the direction of the base station antenna, the signal coverage of the repeater will be too far, which will disrupt the original frequency plan, resulting in new handover and frequency interference problems. Secondly, when the repeater signal and the base station's direct coverage signal transmission path are different, the repeater's too far coverage (different direction from the base station antenna) will cause a larger path delay. When the delay difference reaches 15 microseconds, if the base station If the C / I of the repeater signal is less than 9dB, then the base station equalizer will be unable to equalize and become co-channel interference.

Regardless of the antenna type or direction, the control of the antenna coverage area can take advantage of the special propagation mode of the antenna in the tunnel below the surrounding buildings, that is, the signal breaks at a distance related to the wavelength and the height of the antenna:

Rb = 4 (HBS * Hm) / λ (Unit: meter)

Among them: HBS-the height of the transmitting antenna; Hm-the height of the mobile phone antenna; the different loss slopes (2 and 4) before and after.

In addition, the distance of the breakpoint can be adjusted by controlling the height of the covering antenna, so as to effectively reduce the long-distance radiation signal. Since the average height of village H is 4 to 7 floors, and the weak signal area is 1 to 3 floors, the antenna height is placed at a height of 3 to 4 meters on the second floor. In the 800M to 900M frequency band, the corresponding breakpoint distance is 36 ~ 72 meters, and then add the directional antenna down, you can achieve the purpose of controlling long-distance signals, while ensuring normal coverage.

4 Conclusion

Shantou Unicom has actively and beneficially explored the application of indoor coverage system to solve the problem of mobile phone signal coverage in urban villages, and achieved certain results. It is expected that after the system is opened, the field strength of the floors in the coverage area will be greater than -85dBm, and the laneway will be greater than -85dBm. Expansion provides strong support.

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