In the cellular mobile communication system, at the initial stage of network operation, the main goal of the operator is to build a large-scale macro cell to obtain the greatest possible regional coverage. As the number of users increases, the macro cell undergoes cell division and becomes smaller and smaller. When the cell is small to a certain degree, the cost of building a site will increase sharply, and the narrowing of the cell radius will also cause serious interference. On the other hand, the blind spot still exists, and the high traffic volume in the hot spot area cannot be well absorbed. Microcellular technology was created to solve the above problems.
Compared with the macro cell, the main characteristics of the micro cell are:
1. Small coverage, generally 100m ～ 1km;
2. Low transmission power, generally 10mW ～ 100mW;
3. Generally installed on buildings, wireless transmission is greatly affected by the environment.
4. Small size, convenient and flexible installation.
Because of the above characteristics, microcells can be used as supplements and extensions to macrocells. The application of microcells is mainly in two aspects: one is to increase coverage and apply to some blind spots that macrocells are difficult to cover, such as subways and basements; two It is to increase capacity, mainly used in areas with high traffic volume, such as bustling commercial streets, shopping centers, stadiums, etc. Microcells generally form multi-layer networks with macrocells as applications to increase network capacity. Macro cells cover a large area. As the bottom layer of a multi-layer network, micro cells continuously cover a small area and overlay on the macro cells to form the upper layer of the multi-layer network. Micro cells and macro cells are different cells in system configuration and have independent Broadcast channel.
In the multi-layer network structure, the main service object of the microcell is a low-speed mobile station. For high-speed mobile stations, in order to avoid frequent handovers caused by dropped calls, the macrocell should serve. The system should have an algorithm to detect the mobile speed. The quality of this switching algorithm directly affects the ability of micro-cells to increase capacity.
In the initial stage, microcells are generally scattered in hot spots. The traffic volume is relatively concentrated, the coverage area is small, and the increase in capacity is limited. With the development of users, the hot spots have been gradually connected from spots to pieces. An independent microcell layer is formed, and each microcell is connected to continuously cover within a certain range. At this time, the network capacity can be greatly improved. Generally, for a cell with a radius of about 1km, if each fan In the hotspot area of ​​the district, 6-8 microcells with a radius of about 0.1km are used to form a microcell layer, which can increase the network capacity by 3 to 4 times.
In a multi-layer network where microcells are connected into pieces, the handover of the mobile station is preferentially performed between the microcell and the microcell. When the mobile station reaches the edge of the microcell layer, or the microcell is heavily congested, or the mobile station moves at a high speed Only the handover between microcell and macrocell is carried out, and the improvement of the network capacity of the multi-layer network is based on the complex handover algorithm.
Two schemes can be used for frequency planning of multi-layer networks: one is that microcells and macrocells use the same frequency band, and microcells use the frequency stealing method to configure frequency points. The advantage of this scheme is that there is no need to allocate additional frequencies for microcells. , Mainly used in the case of a small number of microcells; another solution is to independently allocate 2M ~ 3M frequency bandwidth (10 ~ 15 carrier frequency) for microcells, the microcell layer adopts frequency reuse mode, which is used in conjunction with macrocells. The intersecting frequency avoids adjacent frequency interference, reduces the complexity of frequency planning, and is easy to expand the network. This scheme is suitable for the situation where a large number of microcells form a microcell layer.
The number of sites on the micro-cell layer is large, and the transmission cost accounts for a larger proportion of the overall equipment investment than the macro-cell base station. It is very important to choose a reasonable network structure and transmission method according to the actual situation. Microcells generally have a carrier frequency of 1-2. When PCM transmission is adopted, if the star connection is used, the transmission line occupancy rate is very low. Generally, the microcells adopt a chain mode, so that 4 to 5 microcells can use a pair of transmissions. Connecting the line to the computer room can effectively save costs. Compared with the PCM method, the HDSL transmission method using existing telephone lines for transmission is currently a very economical transmission method, and microcellular equipment also tends to have built-in HDSL transmission equipment. When microcells cannot be transmitted in the above manner due to special reasons, optical fibers and microwaves are also more commonly used options.
The transmission power of the micro-cell equipment provided by most manufacturers of micro-cell equipment is not limited to the power range of the micro-cell. Some micro-cell equipment have a maximum transmission power comparable to that of the macro-cell, up to 4W-5W, which provides another Realistic applications, that is, using microcellular devices as macrocells. In some areas where the construction of a site is difficult and costly, such as areas with complex terrain or high-value real estate blocks, the entire network can be implemented with microcellular devices first. The coverage of some micro-cell base stations provided by some manufacturers can form a maximum of 6 + 6 + 6 cells, which can fully meet the needs of various network planning from macrocells to microcells.
In short, microcells can be used as both basic coverage and supplementary coverage. They can also be used for multi-layer coverage from point to surface with the distribution and growth of traffic, and gradually increase the capacity of the network. Although the use of microcells increases network complexity, planning difficulty, and economic costs, in order to make full use of limited frequencies and achieve continuous business growth, the use of microcells is an inevitable result of network development.
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