Some issues in GSM network optimization

With the development of the mobile communications industry, the network scale continues to grow, and mobile users are increasing day by day. Wireless transceiver base stations have evolved from a large-area system in the early stages of development to cellular networks spread throughout streets, alleys, and rural corners. This makes the optimization of wireless networks increasingly complex and arduous. At the same time, mobile users are increasingly sensitive to wireless network service quality. The introduction of mobile communication competition mechanisms has made wireless network service quality more of an issue for operators and has become an important bargaining chip for business success. Wireless networks that developed earlier and are larger in scale have factors such as engineering legacy problems and complex network structures. To dominate the market competition, network optimization is even more important.

1. Scope of network optimization

Network optimization is a high-level maintenance work that uses new technology and optimization tools to reasonably adjust network parameters and network resources, thereby Maintenance work to improve network quality. Indoor distribution, frequency hopping, concentric circle technology, DTX, power control and other means can be used to reduce interference, increase network capacity, and improve the wireless environment; by adjusting the antenna angle, gain, azimuth angle, pitch angle and power level, the best station can be selected address, adjust carrier frequency configuration, balance traffic distribution, improve network quality, obtain the best coverage effect, etc.

2. Network optimization is the sublimation of basic maintenance work.

Good basic maintenance can ensure equipment integrity; but to improve network quality, network parameters must be optimized, that is, network optimization. Only by doing a good job in network optimization can the effectiveness of basic maintenance be fully reflected.

Maintenance serves operations, and operations serve users. The ultimate goal of maintenance is to provide high-quality network services to online users. Only through network optimization can the ultimate goal of maintenance be achieved and maintenance work be practical. significance.

3. Network optimization is a continuous work

1. Because the factors that affect network quality are not static, network optimization should be continuously carried out as network parameters and environment change. In various regions, especially in recent years, the economy has been booming, and high-rise buildings have continued to emerge in cities, which has changed the propagation environment of wireless signals, and new blind spots and interference from within the system may appear. Moreover, the distribution of traffic is also changing. Higher traffic demand will appear in areas where there was no traffic or where the traffic was small. The network needs to be adjusted in time to absorb the traffic.

2. Project construction will seriously change network parameters. Although project planning strives to be perfect, it is difficult for planners to adjust the parameters to the optimal state, which will inevitably cause interference and uneven traffic. This requires network optimization to solve.

3. The upgrade of wireless network software and hardware versions will also change some parameters in the BSC database, and parameter settings also need to be adjusted to implement network optimization.

Therefore, network optimization does not happen overnight, but is a long-term, long-lasting, and arduous maintenance work. Simply put, as long as the network operates for a day, network optimization is required. The importance and durability of network optimization determine that network optimization work must be carried out continuously by various cities and towns according to local actual conditions. Any short-term, sudden optimization will have little effect in the long run. Below we will discuss several hot issues such as indoor coverage in optimization, the role of antennas in network optimization, call drops and network virtual layering, in order to achieve the purpose of mutual learning.

Part 2, Optimization of Indoor Coverage

1. Significance of Indoor Coverage Optimization

As the density of base stations in urban areas increases, optimization work deepens. Outdoor coverage in cities has been basically seamless, and voice quality has been further improved. Due to the increasing traffic generated by users using mobile phones in large buildings (especially hotels, business and commercial centers, large shopping malls, parking lots, etc.), users are no longer satisfied with only mobile communication services with good outdoor coverage. , and also require network operators to provide good indoor coverage services. However, such places are often blind spots or blind spots in network coverage due to their own building reasons (such as thicker walls, larger areas, higher floors, etc.) The signal is extremely poor.

In particular, the GSM system currently used by most users has weaker signal penetration than the analog system, and the phenomenon is even more obvious. Therefore, solving indoor coverage, meeting user needs, and improving network communication quality have become an important part of engineering construction and network optimization work.

In a narrow sense, the indoor coverage problem is just an improvement of the indoor coverage blind area to solve the problem of unable to make calls. Broadly speaking, indoor coverage issues include improving indoor mobile communication voice quality, network quality, and system capacity. In addition to providing coverage for communication blind areas such as basements, first and second floors, etc., we should also improve the high-rise parts of the building that may cause call drops, intermittents, and unsuccessful handovers due to the reception of messy and unstable signals from multiple directions. . At the same time, indoor coverage, as a means of capacity expansion, also plays a significant role in sharing outdoor base station traffic in high traffic areas, increasing network capacity, allowing indoor traffic to be absorbed indoors, and reducing co-channel interference. In addition, good indoor coverage is of great significance for improving the image of network operators, providing users with better and more complete communication services anytime and anywhere, and improving corporate competitiveness.

2. Methods and means to improve indoor coverage

There are two basic methods to improve indoor coverage: one is to increase outdoor signals to solve indoor coverage; the other is to use indoor Signal distribution system approach.

1. Increase outdoor signals to solve indoor coverage

Place repeaters near places with indoor blind spots, or increase the transmit power of base stations covering the area to increase outdoor signal strength. The penetration ability of electromagnetic waves is used to solve the indoor coverage problem. The advantages of this method are: it is simple and fast, does not require a large investment, the engineering workload is small, no wiring is required in the building, and the construction speed is faster. This method not only solves the problem of indoor coverage in some places where the network is not very complete, but also solves the coverage and traffic absorption in surrounding areas. It is a kind of thing that kills two birds with one stone. However, in places where the network is already relatively complete and base stations are densely packed, it is not a wise move to use this method, especially the use of repeaters, which may have more impact on the system than solving indoor coverage in these areas. The disadvantage of this method is that it requires frequency planning, and sometimes even major frequency adjustments must be made to the network. At the same time, this method is not a comprehensive way to solve the problem. For basements, large buildings and buildings with metal glass curtain walls, there may be considerable areas in the interior that are still blind spots. Therefore, this method is no longer satisfactory. Coverage needs of large indoor buildings.

2. Indoor signal distribution system method

Building an indoor distribution system is currently the most effective method to solve the indoor coverage problem. The most fundamental difference between it and the previous solution is that the wireless signal It is directly introduced to every area indoors through wired methods, eliminating indoor coverage blind spots, suppressing interference, and providing stable and reliable signals for indoor users, so that users can enjoy high-quality communication services indoors. When designing this solution, we must consider that the signal does not leak outside the building and cause interference to the network.

3. Indoor distribution system composition

The indoor distribution system mainly consists of three parts: signal source equipment (microcell, macrocell base station or indoor repeater); indoor wiring and its Related equipment (coaxial cables, optical cables, leaky cables, electrical terminals, optical terminals, etc.); trunk amplifiers, power splitters, couplers, indoor antennas and other equipment.

The basic factors to be considered for indoor coverage in buildings include: 5-20dB blocking by partition walls, 20dB blocking by floors, 2-15dB blocking by furniture and other obstacles, multipath fading and The "island effect" and "ping-pong effect" on high-rise buildings. The impact of various indoor environments on the wireless environment is very significant, which must be comprehensively considered in engineering design and optimization.

IV. Comparison of different signal sources

The most commonly used signal sources mainly include the following two types: macrocell + repeater and microcell + indoor coverage.

1. Macrocell + repeater

This uses outdoor antennas to receive signals from nearby macrocell base stations, amplifies them, and then distributes them to the required coverage areas by indoor antennas. Location. This method of wireless coupling requires strict debugging in urban areas with high frequency reuse to avoid interference to the network. Since the repeater itself does not increase the channel resources, but only extends the signal, repeaters are generally used in places with low traffic volume and have small coverage. They can generally only be used to fill blind spots. Such as small restaurants, underground parking lots, etc.

2. Microcell + indoor coverage

A microcell is a base station, but the transmitting antennas of the base station are placed indoors. Microcells increase network channel resources, improve network capacity and call quality, and are suitable for large-scale indoor coverage. It is generally used in places with intensive call traffic (such as star-rated hotels, large entertainment venues, commercial and commercial centers, etc.), which not only ensures excellent coverage, but also shares the call traffic of surrounding base stations.

5. Optimization of Indoor Coverage System

For the built indoor coverage system, the most important thing is daily maintenance and optimization. The following is explained with examples from actual work.

1. Determination of adjacent cells

In the central area of ??the city, the density of base stations is relatively high, and the average station distance is less than 1km, so the indoor signals are usually messy and unstable. . Especially in the middle and upper floors of some high-rise buildings that are not completely enclosed, there are many signals entering the room. The signals from adjacent base stations are directly transmitted, and the signals from distant base stations enter the room through direct reflection, refraction, reflection, diffraction, etc., and the signal is suddenly strong. It is suddenly weak and unstable, and the same frequency and adjacent frequency interference is serious. When mobile phones are used in this environment, cell reselection is frequent when not making calls; frequent switching during calls can easily lead to poor voice quality and serious call drops.

The most important way to solve this type of problem is to select appropriate adjacent cells for the microcell according to the actual situation. The restriction of adjacent cell measurement frequencies can effectively control the contact between microcells and other cells.

For example, Hongda Hotel in a prosperous area of ??Xiangtan has installed a microcell indoor coverage system. Due to the high density of base stations in the area, indoor atrium signals are complex. Since there are many neighboring cells operating on microcells, handovers are frequent, and the indicators are reflected in low handover success rates and more dropped calls. Through field measurements, the three most important 900M macro cell service cells were determined: 9141, 9142, and 9143, and a two-way handover relationship was established. And because the strong 1800M macro cell 63141 signal was measured at the elevator entrance on the third floor, considering the high possibility of users occupying this community and entering the micro cell, a one-way switching relationship between 62141 and micro cell was pretended. After the adjacent cells are streamlined, the indicators show that the handover success rate is significantly improved and the call drop rate is reduced.

From this typical case, it can be seen that the neighboring cells of microcells must be adapted to local conditions. The number is not small, but accurate. Generally, it is enough to determine two or three main serving cells, but at the same time, if there are too few neighboring cells, the problem of macro cell outage and inability to switch from outside to indoor must be considered. Therefore, there must be at least two adjacent communities.

2. Optimization of reselection and switching

Modern buildings are mostly made of reinforced concrete as the skeleton, and coupled with fully enclosed exterior decoration, the shielding and attenuation of wireless signals are particularly severe. There are many elevators in high-rise buildings, and most of them are metal fully enclosed structures, which results in very strong signal changes when entering and exiting the building and elevators. This requires detailed settings and adjustments to the relevant reselection and switching parameters of the microcell. For example, the lobby and lower floors of a hotel in Wuhan are covered by microcell A, and the elevators and upper floors are covered by microcell B. When entering the elevator from the lobby and reselecting the mobile phone from A to B, it is normal. However, when entering the lobby from the elevator, the reselection of the mobile phone from B to A is obviously slow, and there may even be a temporary no signal. Through cell parameter query, it is found that the settings of cell reselection bias parameters for cells A and B are obviously inconsistent, and B is much larger than A.

The designer's original intention was to make it easier for B to absorb traffic and make it easier for the mobile phone to reselect into the cell when it is idle. However, the difference is too big, resulting in the mobile phone still being unable to access the cell even when the signal in cell B is very weak and the signal in cell A is already strong. Reselect. By adjusting, the above situation disappears and the phone reselects normally.

3. Carrier frequency adjustment and optimization

For many large hotels and shopping malls that use multiple micro-cell coverage to share traffic, we recommend that we try our best to adjust the carrier frequency. Frequency distribution, multiple cells are merged into one cell, because that often leads to problems such as unbalanced or even disparity in traffic volume and low handover success rate between cells. By optimizing and adjusting multiple cell coverage to one cell coverage, users can make calls without switching, eliminating potential instability factors.

In addition, the process quality of the distribution system will also affect the microcell signal. For example, the mismatch of uplink and downlink power leads to uplink interference or weak signals, causing intermittent or dropped calls. These must be optimized with the cooperation of distribution system manufacturers.

Part 3. The role of antennas in network optimization

Antenna technology is the basis of mobile communication technology. Base station antennas are equipment that wirelessly connect the mobile communication network and user mobile terminals in the air. The main function is to radiate or receive radio waves. When radiating, it converts high-frequency current into electromagnetic waves and electrical energy into electromagnetic energy; when receiving, it converts electromagnetic waves into high-frequency current and magnetic energy into electrical energy. The performance quality of the antenna directly affects the coverage and service quality of the mobile communication network; different geographical environments and different service requirements require the use of different types and specifications of antennas. Antenna adjustment plays a big role in mobile communication network optimization.

1. Main performance indicators of the antenna

The main parameters characterizing the performance of the antenna include pattern, gain, input impedance, standing wave ratio, polarization mode, and isolation of dual-polarized antennas. degree, and third-order intermodulation, etc.

1. Directional pattern

The antenna pattern is a graph that represents the spatial angle relationship of the antenna radiation characteristics. Taking a transmitting antenna as an example, the power or field strength radiated from different angles forms a pattern. Generally, the three-dimensional pattern of an antenna is represented by two mutually perpendicular plane patterns including the maximum radiation direction, which are divided into horizontal plane patterns and vertical plane patterns. The figure cut parallel to the ground at the position of the maximum field strength of the beam is called the horizontal plane pattern; the figure cut perpendicular to the ground at the position of the maximum beam field strength is called the vertical plane pattern.

Another important parameter describing the radiation characteristics of the antenna is the half-power width. When the antenna radiation power is distributed on both sides of the main lobe maximum, the power intensity drops to half of the maximum value (the field strength drops to the maximum value). 0.707 times, 3dB attenuation), represents the degree of concentration of the antenna's radiated power in a specified direction. Generally, the half-power beam width of a GSM directional base station in the horizontal plane is 65o. At the cell edge of 120o, the antenna radiation power is 9-10dB lower than in the maximum radiation direction.

2. Directivity parameters

Different antennas have different directional patterns. To express the degree of their concentrated radiation and the sharpness of the directional patterns, we introduce directivity parameters. The radiation of an ideal point source antenna has no directionality, the radiation intensity is equal in all directions, and the direction is a sphere. We use the ideal point source antenna as a standard to compare with the actual antenna. With the same radiation power, the ratio of the square E2 of the electric field intensity generated by an antenna at a certain point to the square E02 of the electric field intensity generated by the ideal point source antenna at the same point is called is the directional parameter D=E2/E02 of this point.

3. Antenna gain

Gain and directivity coefficient are both parameters that represent the concentration of radiated power, but they are not the same. The gain is discussed under the same output power condition, and the directivity coefficient is discussed under the same radiated power condition. Since the radiation intensity of the antenna in all directions is not equal, the directivity coefficient and gain of the antenna change with different observation points, but their changing trends are consistent. Generally, in practical applications, the directivity coefficient and gain in the maximum radiation direction are taken as the directivity coefficient and gain of the antenna.

In addition, the parameters characterizing the antenna gain include dBd and dBi. DBi is the gain relative to the point source antenna, and the radiation is uniform in all directions; dBd is relative to the gain of the symmetrical array antenna dBi=dBd 2.15. Under the same conditions, the higher the gain, the farther the radio wave propagates. It is customary for us to use dBi to characterize the gain of the antenna.

4. Input impedance

Input impedance refers to the high-frequency impedance of the antenna in the working frequency band, that is, the ratio of high-frequency voltage to high-frequency current at the feed point, which can be tested with a vector network When measured by the analyzer, its DC impedance is 0Ω. Generally, the input impedance of mobile communication antennas is 50Ω and 75Ω. In Xiangtan’s mobile network, we all use antennas with an input resistance of 50Ω.

5. Standing wave ratio

Since the input impedance of the antenna cannot be completely consistent with the characteristic impedance of the feeder, partial signal reflection will occur, and the reflected wave and the incident wave will be superimposed on the feeder. A standing wave is formed, and the ratio of its adjacent maximum voltage value to its minimum value is the voltage standing wave ratio VSWR. Generally speaking, the voltage standing wave ratio of mobile communication antennas should be less than 1.4, but in practical applications we all require that the VSWR should be less than 1.2.

6. Polarization mode

According to the orientation of the electric field vector of the antenna in the maximum radiation (or reception) direction, the antenna polarization mode can be divided into linear polarization, circular polarization and Elliptical polarization. Linear polarization is divided into horizontal polarization, vertical polarization and ±45o polarization. The transmitting antenna and the receiving antenna should have the same polarization method. Generally, vertical polarization or ±45o polarization is used in mobile communications. In fact, the use of vertical polarization is a mistake caused by history, because vertical polarization waves are greatly affected by weather, especially rain, so in future work, this type of antenna should be used as little as possible.

7. Dual-polarized antenna isolation

The dual-polarized antenna has two signal input ports. The power signal P1dBm is input from one port, and the same signal is received from the other port. The difference in power P2dBm is called isolation, that is, isolation = P1-P2.

Mobile communication base stations require polarization isolation greater than 28dB within the operating frequency band. The ±45o dual-polarized antenna uses the polarization orthogonal principle to integrate the two antennas together, and through other special measures, the isolation of the antenna is greater than 30dB.

2. Selection of antennas in optimization

1. Areas with dense traffic in urban areas

In urban areas with highly dense traffic, the distance between base stations Generally, it is 500-1000 meters. In order to reasonably cover the range of about 500 meters around the base station, the antenna height should not be too high according to the surrounding environment. Choose an antenna with a general gain, and the antenna can be tilted down. The formula for calculating the antenna downtilt angle is: α=arctg(h/(r/2)), α is the beam inclination angle, h is the antenna height, and r is the distance between stations.

Choosing a dual-polarized directional antenna with built-in electrical downtilt, combined with mechanical downtilt, can ensure that the horizontal half-power width of the pattern changes little within the angle of main lobe downtilt.

(1) In urban areas with high traffic density and the distance between base stations is 300-500 meters, it can be calculated that the antenna inclination angle α is approximately between 10o and 19o. The original antenna simply uses mechanical downtilt. The downtilt angle is generally above 10o, and the half-power lobe width of the horizontal pattern will become wider, causing inter-station interference; if a ±45o dual-polarized antenna with a built-in electrical downtilt of 9o is used, the electrical downtilt plus the mechanical downtilt will be variable The inclination angle will reach 15o, which can ensure that the half-power beam width of the horizontal pattern does not change within 10o---19o of the main lobe downward tilt. At the same time, combined with the appropriate adjustment of the base station transmit power, it can fully meet the needs of high-traffic and dense urban coverage. Non-interference requirements.

(2) In urban areas with dense traffic and the distance between base stations is greater than 500 meters, the antenna inclination angle α can be calculated to be approximately between 6o and 15o. Polarized antenna, so that the electrical downtilt plus mechanical downtilt variable tilt angle will reach 10o, which can ensure that the half-power beam width of the horizontal pattern does not change within 6o---16o of the main lobe downtilt, which can meet the conversation traffic volume Requirements for coverage in denser urban areas without interference.

(3) In urban areas with low traffic volume, the distance between base stations may be larger. The antenna inclination angle α is about 3o-12o. A ±45o dual-polarized antenna with built-in electrical downtilt of 3o can be used. , in this way, the electrical downtilt plus mechanical downtilt variable tilt angle will reach 8o, which can ensure that the half-power beam width of the horizontal pattern does not change within 3o---12o of the main lobe downtilt, which can meet the needs of covering this area and Non-interference requirements. 2. In suburban or rural areas

In suburban or rural areas where the traffic is not too dense, the signal coverage should be appropriately large, and the distance between base stations is relatively large. Single polarization, spatial diversity, and gain can be selected. Higher 65o directional antennas, such as Xi'an Haitian's (17DB) 65o directional antenna HTDBS096517 model antenna, consider both capacity and coverage.

3. In rural areas

In rural areas where the traffic volume is very low, signal coverage is mainly considered, and most base stations are omnidirectional stations. The antenna can be considered to use a high-gain omnidirectional antenna. The antenna height can be set at 40-50 meters. At the same time, the base station transmit power can be appropriately increased to enhance the signal coverage. Generally, the coverage distance of -90dBm in plain areas can reach 5 kilometers.

4. Along railways or highways

Along railways or highways, the strip coverage distribution along the line should be mainly considered. Dual-sector base stations can be used, with each area 180o; the antenna should be A high-gain directional antenna with a single polarization 3dB beam width of 90o is used. The two antennas are placed opposite each other, and the maximum radiation direction is consistent with the direction of the highway.

In addition, if the traffic volume along the route is very low, considering both coverage and equipment cost, a bidirectional antenna with omnidirectional antenna deformation can be used. The bidirectional 3dB lobe width is 70o and the maximum gain is 14dBi, such as : Xi'an Haitian's omnidirectional antenna deformed bidirectional antenna HTSX-09-14 model antenna.

5. In some indoor or underground areas in the city

In some indoor or underground areas in the city, such as tall office buildings, underground supermarkets, lobby of large hotels, etc., signal coverage Poor, but the traffic volume is high. In order to meet the communication needs of users in this area, indoor microcells or indoor distribution systems can be used. The antennas use distributed low-gain antennas to avoid signal interference affecting communication quality.

In short, antennas play a very important role in the optimization of mobile communication networks. At the same time, the quality of feeders, feeder conversion heads and indoor and outdoor jumpers also greatly affects the coverage quality of mobile communication base stations. Most base stations with poor coverage are caused by the poor quality of feeders and connecting parts. The VSWR meter can be used to measure step by step to determine the parts with poor quality, and replace them in time to ensure the quality of the antenna feeder part of the entire base station and ensure the safety of the base station. Run quality and coverage quality.

Part 4, Analysis and Solutions to Dropped Calls

The phenomenon of dropped calls is a problem that users often encounter when using mobile phones. It is also a hot spot reported by users. It is a system The comprehensive manifestation of various undesirable factors has a great impact on the operation quality of the system. Therefore, how to reduce the call drop rate of the system and improve the network operation quality is an important part of network optimization work.

1. Causes of call drops

System call drops are mainly reflected in SDCCH and TCH call drops. The main reasons are as follows:

1. Call drops due to handover

The so-called handover refers to when the mobile station moves from one base station coverage area to another base station coverage area during the call, or the call quality is degraded due to external interference. At this time, the original voice channel must be changed and transferred to a new idle voice channel to continue the call process.

Handover is a very important technology in mobile communication systems. Handover failure will cause call drops and affect the operation quality of the network. The GSM system uses a mobile station-assisted handover method, in which the mobile station monitors and makes decisions and is controlled by the switching center. During the handover process, both the base station and the mobile station participate in the handover process.

(1) The definition of handover parameters is unreasonable

For example: the uplink level switching threshold, downlink level switching threshold, handover margin and handover power control parameters are unreasonably defined. , resulting in handover failure and call drop.

(2) Improper setting of signal strength lag value

In some cells, because the signal strength lag value is set too small, the cell base station does not have enough time to process handover calls, causing many calls to be switched during handover lost. (But if the setting is too large, it will cause many unnecessary switching).

(3) The target base station does not switch channels during busy hours

In some cells, because adjacent cells are very busy, the target base station does not switch channels during busy hours or is missed in the topological relationship. Define handover conditions (including inter-BSC handover and over-office handover) so that the mobile phone user cannot occupy the idle voice channel of the adjacent cell during handover. At this time, the BSC will perform call reestablishment. If the signal from the calling base station cannot be used at this time, If the minimum working threshold is met or there is no idle voice channel, the call reestablishment fails and the call is dropped.

(4) Improper setting of network color code parameters

The allowed network color code parameters define the set of NCC codes of the cells that the mobile station needs to measure, providing feasible targets for mobile phone handovers community. If the data definition is wrong, it will cause handover failure and cell reselection failure, resulting in call drops.

(5) The signal strength is too weak

When the base station performs handover to share traffic, some handover requests will fail because the signal strength of the cell is too weak. Sometimes even if the handover If successful, calls will be dropped due to too weak signal strength. Because we have a minimum threshold for the received signal strength of mobile phone users in BSC. When it is lower than this threshold, the mobile phone cannot establish a call.

(6) There is a leaky coverage area or blind area in the network

When the mobile station enters the leaky coverage area or signal strength blind area of ??the network, the signal becomes too weak and a handover request is issued. Unsuccessful results in dropped calls.

(7) Island effect

The island effect is a base station coverage problem. When the base station covers a large water surface or a mountainous area and other special terrain, due to the reflection of the water surface or mountain peaks, the On the basis that the original coverage of the base station remains unchanged, an "enclave" appears far away, while the adjacent base stations that have a handover relationship with it are not covered due to terrain obstruction. This creates an "enclave" and adjacent base stations. There is no switching relationship between them, so the "enclave" becomes an isolated island. When a mobile phone occupies the signal in the "enclave" coverage area, it is easy to cause call drops because there is no switching relationship.

2. Call drops due to interference

The propagation characteristics of radio waves determine that they are susceptible to various external factors during the propagation process; due to internal reasons in the network, it is also It is affected by various factors within the network, such as same-channel and adjacent-channel interference, as well as intermodulation interference caused by nonlinearity of the equipment in the network and equipment failures. In the actual operation of the network, we often encounter the following types of interference:

(1) The nonlinearity of the equipment itself and the intermodulation interference caused by equipment failure. The lack of regular indicator testing and adjustment during equipment operation causes intermodulation interference to exist within a certain range. If the transmitting part, especially the repeater uplink transmitting spurious radiation is large, and the receiving part spurious response is large, causing interference to this channel and other channels, severe cases will make normal dialing and conversation impossible. During network operation, there have been cases where repeaters interfered with multiple frequency hopping base stations in urban areas, causing a large number of dropped calls