The application of the repeater is very common today. From the working principle, it is essentially a bidirectional power amplifier. It plays a role in filling the blank area of ​​the signal transmission of the cell in the mobile communication network, which is reflected in eliminating blind spots and improving coverage. Extend cell boundaries and other applications. In wireless transmission, it can also act as a relay to increase link margin and absorb traffic for a particular base station. Based on its small size, low price, simple structure and convenient installation, it is no longer a proprietary property of communication operators. Some factories, hotels, shopping malls, parking lots and other places will be installed privately according to needs.
While standing in the commercial communication network, the direct play station plays an active role. Because of its large number and poor management, it also brings many side effects. If it deteriorates the electromagnetic environment of the public mobile communication band, a large number of radio interferences are generated, and the investigation of these interferences is not easy.
Repeater interference check recordWe have received an interference complaint from China Unicom, saying: Ronggui Huabao GSM900 base station uplink signal is interfered, network statistics analysis shows that the call drop rate is very high. They believe that it is industrial interference generated by machine tools, and initially determined that the interference source is in the Guangdong Meizhi factory area that is separated from the base station. We dispatched the monitoring vehicle and used the ESMB/DDF190 monitoring/direction finding device on the vehicle. At the same time, the E4407B spectrum analyzer was turned on, and the omnidirectional and directional antennas were connected respectively, and the interference signals appeared around the base station and in the area of ​​Meizhi in Guangdong. With the strong support of its high-gain active antenna, the ESMB/DDF190 system received a signal but a false signal, and the spectrum analyzer did not move at all. However, network statistics analysis of the Unicom central computer room shows that interference still exists during this time.
When the monitoring vehicle passed through the gate of a well-known company's factory building, the spectrum analyzer showed a response, and the noise floor increased by nearly 20dB. We immediately replaced the directional antenna for simple direction finding, and the measured signal maximum direction pointed to the company's office building. So, we switched to the TekNet YBT250 base station maintenance tester and equipped with the EB200 handheld DF antenna to look inside, around the building for a week, and finally locked the suspect in the elevator room. The maximum value of the signal measured at the elevator engine room on the roof is about -70 dBm (the spectrum diagram is shown in Figure 1). We thought that the signal was sent by the video surveillance wireless transmission equipment in the elevator, but it was not found. Later, we accidentally discovered that there were two antennas downstairs standing on the side of the fiber awning at the entrance of the parking lot, and another one was found in the shed. The handheld antenna is then used to align one of the directional Yagi antennas, and the maximum amplitude of the signal is measured to be close to -50 dBm (see Figure 2 for the spectrogram). We walked along the feeder line and found that on the side of the corner of the parking lot, three amplifiers were installed up and down. The other end of the amplifier is connected to a whip antenna, which is fixed on the ceiling iron frame of the parking lot.
Figure 1 Signal spectrum measured next to the elevator room in the office building
Figure 2 Signal spectrum measured next to the antenna at the entrance to the parking lot
At this point, the truth is finally revealed: the interference is emitted by a public mobile indoor coverage system.
Interference reorganizationAccording to the field wiring, a schematic diagram of the underground parking lot signal coverage system can be drawn (as shown in Figure 3). As can be seen from Figure 3, the system is mainly composed of a repeater, a donor antenna and a retransmission antenna.
Figure 3 Underground parking lot indoor coverage system framework
The repeater is a bidirectional power amplifier, which amplifies the downlink signal of the base station received by the donor antenna and sends it to the mobile station via the retransmission antenna, and amplifies the uplink signal received by the mobile station and sends it to the base station antenna through the donor antenna. Figure 3 shows three independent systems. The three donor antennas point to the nearby mobile, Unicom G network and China Unicom's C network base station, respectively connecting the mobile, Unicom G network and China Unicom C network signals, covering the underground parking lot of the signal blind zone. .
When the repeater is operating, the thermal noise level it produces is equivalent to the receiver input at the base station:
NBTS-R=KTB+NFR+GR-EDoPL (1).
In equation (1): NFR is the repeater noise figure; GR is the repeater gain; EDoPL is the effective donor path loss, which consists of all the gains and losses of the base station's feeder connector to the repeater donor antenna port.
The NBTS-R is superimposed into the thermal noise level of the base station receiver itself. When it is greater than the maximum interference level allowed by the GSM system, it will cause interference; if it is too large, it will annihilate the normal uplink signals of other mobile stations in the cell, or even block the base station, so that the signal-to-noise ratio is reduced and the BTS receiving sensitivity is lowered. , there is a situation such as dropped calls. This case is due to the poor tuning of the Unicom G network repeater, which causes the noise floor to be too high, which interferes with the uplink signal and makes the mobile phone access in the nearby area difficult or dropped.
The GSM base station receiver has a noise level of -116 dBm, a carrier-to-interference ratio (C/I) of 9 dB, and the maximum allowable interference level is approximately -125 dBm. Therefore, we require the repeater to act on the base station receiving front-end noise. The level is less than -125 dBm. In order not to cause interference to the source station, in the case where the source station and the repeater position have been determined, the field test must be carried out before the repeater is officially put into use, and the coverage is systematically adjusted. According to equation (1), the adjustable parameters are GR and EdoPL. In order to reduce the thermal noise level arriving at the base station, the repeater gain GR or the ADDPL may be lowered to make the NBTS-R satisfy the condition of less than -125 dBm.
Analysis of Interference Signal Search Process(1) Why did we not get the interference signal for the two days after the ESMB/DDF190 monitoring/direction finding device or the E4407B spectrum analyzer?
Analysis: Figure 4 is a schematic diagram of the surrounding environment of the Unicom base station. In the figure, the donor antenna is a directional Yagi antenna with good directivity. When it points to the corresponding base station, the signal transmission has a substantially fixed spatial path, and the range is narrow. In addition, during the transmission, the signal is attenuated by the adjacent hills and the buildings in front of the building, and the path is certainly outside the path. The signal is not received or the signal is rather weak; even if the path passes, if the receiving point and the donor antenna are not within the line of sight, the signal amplitude will be greatly reduced or even completely disappeared, so it will not be received.
Figure 4 Schematic diagram of the surrounding environment of the base station
(2) From the spectrum diagram, the operating frequency range of the repeater is 885 MHz to 915 MHz (upstream). As a wideband amplifier, it covers the uplink signal band of China Unicom and mobile. But why is the base station of China Mobile not interfered?
Analysis: The repeater is a broadband amplifier, and the frequency range covers the uplink working frequency bands of the two major operators of China Mobile and China Unicom. When the two G networks are co-sited, the possibility of simultaneous interference is very high. The orientation of the antenna is different unless the donor antenna is not aligned with the base station antenna, or the coverage areas of the two are different. When the donor antenna just faces the side lobes of the base station antenna instead of the main lobe, the receiving gain decreases, which can be converted into EDoPL increase, and NBTS-R decreases, so the interference level is weakened.
In addition, the underground parking lot coverage system consists of three independent sub-systems. If the two G-networks are co-sited, there is no need to set up three subsystems, two of which are sufficient. As long as the system is tuned, the signal transmission between the repeater and the base station can be better achieved. It can be seen that the two are not co-sites (the information is checked afterwards, which is also confirmed). Since the common antenna is not contiguous, and the donor antenna has directionality, when the mobile base station deviates from its transmission path, the signal is weak or even absent, and thus the repeater of the Unicom does not interfere with the mobile base station. The Unicom C line frequency is 825 MHz ~ 835 MHz, which is far from the G network, and it does not interfere.
Summary and experience(1) When interfering with the investigation, it is best to work closely with the technical staff of the communication operator to strengthen communication. Because only they can communicate well with the central computer room, and timely grasp the current status of some base station operating parameters and statistical data, such as RSSI, etc., these data can better guide the interference investigation work, and can understand the specific disturbed sectors, Narrow your search to make your work more targeted and targeted. In addition, they can more easily lead us to some base station rooms - many of which are leased private properties.
(2) When no signal is detected around the base station but the interference still exists, the test near the rooftop of the base station antenna should be considered, because even if an interference signal is received somewhere below, the interference signal does not necessarily affect the base station. Sector. This aspect is due to path attenuation. On the other hand, the interference signal may be a directional signal, and there may be a large directional attenuation for the base station receiving antenna. Whatever the reason, as long as the amount of attenuation is large enough that the signal is less than the maximum allowable interference level of the BTS, it will not have an effect. Therefore, in this case, the signal received near the antenna of the base station shall prevail. When a specific sector is involved in interference, the antenna corresponding to the sector and its orientation should be clarified, and the direction should be selected to measure the interference signal.
(3) Establish a base station and repeater data database. In view of the narrow width of the main lobe of the donor antenna, it may be difficult to find a commanding point in the main lobe and then re-orientation cross-position, except beside the antenna of the sector of the victim base station. In addition, since the frequency bandwidth and the direction of the handheld direction finding antenna are not sharp enough, the direction finding accuracy is not very high. Even if another commanding height can be found in the main lobe, the point error of the surfacing may be larger due to the closer distance between the two points and the smaller angle of the pointing line. Therefore, when looking for interference, it may be done with a point direction and then along the direction-finding line. Obviously, this is not the best way. If there is a huge database such as the distribution of the station and the working parameters, it can reduce the blindness of the work and greatly shorten the time required for troubleshooting.
(4) Establish a system for detecting equipment use cycles. The search for interference is actually a far-field test of the emission characteristics of an unknown source (transmitter). To be aware of the wireless parameters of a particular device, it is necessary to implement a transmitting device that is in use or intended to be used (here Repeater) A system for periodic testing. In this respect, it is possible to prevent inferior and unqualified equipment from being put into use or to continue to use; on the other hand, collecting the original data for the above-mentioned station database, and also necessary measures to prevent interference from the source.
ConclusionThe investigation of interference is superficially a technical work, but the actual situation is not exactly the same, and there are many things outside the technology that play a leading role. Fine management and perfect database of station data can make it possible to theoretically infer the location of the interference source based on the phenomenon combined with the data clues. The technical search is only a part of the preliminary investigation of the general direction after the interference process, and plays a role of verification and verification. Although it is still down to the conclusion, if there is strong database support, interference search can often get twice the result with half the effort, and the time spent on the whole work technology will be greatly reduced.
ShenZhen JunJin Technology Co.,Ltd , https://www.jjtcl.com