I. Introduction
With the development of network, the problem of indoor coverage is becoming more and more prominent. Taking GSM network of China Mobile as an example, outdoor coverage in medium-sized cities is no longer a problem. Moreover, with the strengthening of planning optimization in various places, the problems easily found in outdoor testing have been basically solved, and the focus of work has gradually shifted to the indoor direction. There are three main ways to achieve indoor coverage:
(1) Indoor coverage in the coverage area is provided by outdoor Hong Fengwo. This way is still the most important way in China at present.
(2) When the outdoor station has surplus capacity, the outdoor signal is introduced into the indoor coverage blind area through the repeater;
(3) Set up indoor microcells where traffic is concentrated, and solve the problems of coverage and capacity at the same time.
Because there are many high-rise buildings in China, indoor coverage provided by outdoor stations is very limited. If we want to ensure the quality of indoor coverage, outdoor interference will become difficult to control, affecting the overall planning and capacity of the network. In addition, outdoor stations cannot cover shopping malls and entertainment centers with great depth. Therefore, the main methods to solve indoor coverage are to establish microcells and repeaters. The following is a concrete analysis and comparison of these two main technologies.
Second, microcellular technology.
Micro-cellular technology is developed on the basis of Hong Fengwo, and it is one of the effective methods to solve the capacity problem in high-traffic areas. The coverage radius of microcells is about 30m-300m；; ; The transmission power is small, generally below 1W; The antenna of the base station is placed in a relatively low place (generally 5m- 10m higher than the ground), and the transmission is mainly along the line of sight, so the signal leakage on the roof is small. Therefore, in Hong Fengwo, cellular phones can be used to expand radio coverage and eliminate "blind spots". At the same time, because the microcellular base station with lower transmission power allows shorter frequency reuse distance and more channels in the unit area, the service density is greatly improved and the radio frequency interference is very low. By placing it in a "hot spot" in Hong Fengwo, it can meet the quality and capacity requirements of the microcellular area.
In the early days, microcells were generally used to improve network coverage and applied to scattered "hot spots" areas, that is, areas with relatively concentrated telephone traffic and small areas. At this time, the increase in capacity is very limited. With the increase of capacity demand, when the area with high traffic volume has gradually changed from point to area, and Hong Fengwo can no longer meet it, microcells can continuously cover a certain range, and the effect is obvious at this time.
In practical design, microcells, as a supplement to wireless coverage, are generally used in places with large traffic that are not covered by Hong Fengwo, such as underground conference rooms, recreation rooms, subways and tunnels. As a popular application, it is generally in areas with relatively concentrated telephone traffic, such as shopping centers, entertainment centers, conference centers, business buildings, parking lots and other places.
With the development of microcells and picocells, layered cellular technology was put forward rapidly. It provides more "embedded" cells and forms a hierarchical cell structure, which mainly solves the problems of "blind spots" and "hot spots" in the network and improves the network capacity. In the hierarchical cell structure, cells with different sizes overlap each other, and base stations with different transmission powers coexist closely, so the whole communication network presents a multi-level structure. Each layer is assigned different frequency bands to ensure that each layer works independently and will not interfere with each other. The handover of adjacent microcells all returns to Hong Fengwo where they are located. Hong Fengwo's wide-area high-power coverage can be regarded as the upper network of Hong Fengwo, which acts as a "safety net" for mobile users when they move between two microcells, and a large number of microcells constitute the lower network of microcells. When the user accesses, the system selects the appropriate cell (Hong Fengwo, microcell or picocell) for the call according to the measured signal strength and the capacity of each cell. The switching point is determined by the system and automatically assisted by the GSM mobile station. The handover process also depends on the capacity of all levels at that time. If microcells and picocells are saturated, the service will be switched to a higher cell.
Hierarchical cellular network is usually a multi-cellular system consisting of an upper Hong Fengwo network and several lower microcellular networks. It includes Hong Fengwo, microcellular and microcellular. Each unit performs different functions that have been defined. Generally speaking, Hong Fengwo is used to deal with fast-moving vehicles, microcells are used to deal with slow-moving vehicles, focusing on walking or traffic jams, and microcells are used to cover indoor areas such as shopping malls and office areas. The reason why the layered bearer is like this is related to the handover function, because the rapid movement of the car phone between microcells will cause frequent handover and increase the network burden. From the point of view of network management, the services that lead to frequent handover will be transferred to Hong Fengwo, which will improve the network efficiency. Because it takes a long time for slow-moving vehicles to cross the cellular boundary and the possibility of switching is small, microcells handle this kind of business.
Micro-cellular networking is simple and can be directly added to the existing system without changing the existing network structure. Its equipment is small in size, easy to install and flexible in application. It can be directly built where it is needed to quickly solve the communication problem covering blind areas and hot spots. It obviously improves the capacity, but it needs a lot of investment.
Third, set up a repeater.
As GSM is the first standard (ETSI GSM 05.05) that incorporated the repeater specification into its equipment specification, and was accepted by SMG in 1994, the repeater system has not been used for a long time in cellular networks. Although the use of repeater in cellular mobile communication system is a little earlier than this time, the large-scale adoption of repeater technology is still after the emergence of a new generation of products.
The types of repeaters are analog repeaters, channel selection repeaters and relay repeaters. Now the most widely used channel selection repeater is.
The main components of GSM channel selection repeater: low noise amplifier (LNA), combiner (CMB), channel board (up-down converter, surface acoustic wave filter (SWA), power amplifier), duplexer, donor antenna and service antenna. The downlink carrier signal of the base station received by the donor antenna is first processed by a low noise amplifier, then down-converted from a 900M RF signal to an intermediate frequency of 765,438+0 m, filtered and amplified by an intermediate frequency with a bandwidth of 200KHz, and then converted to a 900M RF signal for power amplification. Finally, it is transmitted through the service antenna to cover the area to be covered. The uplink signal processing process is exactly the same as the downlink signal processing process. The selection of antenna and feeder system is very important when installing repeater. The following points should be noted:
Gain of (1) antenna. According to the specific signal situation and the need of coverage, choose the appropriate gain;
(2) the directivity of the antenna. Because the repeater belongs to the same frequency relay system, it can't use omni-directional antenna, otherwise it may cause system self-excitation. The main lobe width of the donor antenna should be as narrow as possible to reduce the introduction of noise; The transmitting direction of the repeater antenna should be strictly controlled to ensure that the repeater signal will not be fed into the donor antenna;
(3) Selection of signal source base station: the base station with good signal quality should be selected as the feed, and the capacity of the base station should be ensured to be sufficient, otherwise congestion will be introduced; From the above analysis, it can be seen that the construction of repeater must involve the operation department, otherwise the quality is difficult to guarantee. In practical application, because the repeater has the characteristics of simple installation and debugging, fast opening speed, low requirements for installation environment and low capital investment, as long as the repeater is well designed and installed, it will be more and more welcomed by operators.
Four. conclusion
This paper discusses the main method of GSM mobile communication system to realize indoor coverage & designing repeater or establishing micro-cell. Repeater has become an important way to solve simple problems because of its flexibility and simplicity. Repeater does not need base station equipment and transmission equipment, its installation is simple and flexible, and its equipment models are also rich and varied, so it is playing an increasingly important role in mobile communication. Micro-cellular technology has the advantages of small coverage, low transmission power, convenient and flexible installation, and can be used as a supplement and extension of Hong Fengwo. The two technologies have their own advantages, and the specific implementation can be flexibly adopted according to the actual situation, so as to obtain the best coverage effect with the minimum investment.
Author: Li Jing