Solution to blind area communication of intercom
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In conventional ultra-short wave wireless communications, the problem of short communication distance of handheld devices is often encountered. Because the handheld device is restricted by many factors such as size, weight and power consumption, it is impossible to significantly increase the power and use high-gain antennas to expand the communication range. The usual solution to this problem is to use a repeater to expand the communication distance of the handheld device. Since ultra-short waves are line-of-sight transmission, as long as the repeater is placed at a high place,
In conventional ultra-short wave wireless communications, the problem of short communication distance of handheld devices is often faced. Because the handheld device is restricted by many factors such as size, weight and power consumption, it is impossible to significantly increase the power and use high-gain antennas to expand the communication range. The usual solution to this problem is to use a repeater to expand the communication distance of the handheld device. Since ultra-short waves are line-of-sight transmissions, as long as the repeater is placed at a high place, a high-gain antenna is used and the transmission power is appropriately increased, the communication distance between handheld devices can be effectively extended. The repeater receives information at a specified frequency, and after processing, transmits the received information as it is at another specified frequency. The repeater is often placed on the top floor of a building, on a tower or on the top of a mountain. The principle is shown in Figure 1.
The blind area discussed in this article is the blind area that exists within the radio wave coverage area of the repeater station. It is caused by obstacles that cause the radio wave attenuation to be very large within a certain range, making it impossible to communicate. For example, in a city, due to the obstruction of a certain building, the signal attenuation is very large in the area close to the building, and the radio wave signal near the building (especially against the direction of radio wave propagation) is very weak. However, away from the building, due to the enhanced diffraction of the radio wave, the radio wave signal is increased again, and the handheld device can communicate again. (See Figure 2).
For example, in a building, the radio wave signal in the basement and garage is very weak, and the handheld device cannot communicate, but the handheld device can still communicate inside the building. The so-called blind area is that even within the effective communication range, there are some areas that cannot communicate due to various reasons. To solve the handheld device communication in these blind areas, the following methods can be used:
1. Use of repeaters
(1) What is a repeater?
A repeater is a frequency-selective amplifier that directly amplifies signals. Its structure is shown in Figure 3. It is a bidirectional frequency-selective amplifier, with an uplink amplifier and a downlink amplifier. The uplink amplifier amplifies the signal transmitted by the handset, and after amplification, it is sent to the repeater through the uplink antenna. The downlink amplifier amplifies the signal sent by the repeater, and after amplification, it is sent to the handset through the downlink antenna. The frequency-selective filters at the input and output of the uplink amplifier only allow the signal of the working frequency transmitted by the handset to pass. The frequency-selective filter of the downlink amplifier also only allows the signal of the working frequency transmitted by the repeater to pass. The repeater also has a duplexer and a power supply. Repeaters are very effective in solving blind area communication problems in public networks (GSM and CDMA) for network optimization.
(2) Application of repeaters
In the bustling city, there are many high-rise buildings. Due to the obstruction of these high-rise buildings, the received signal is too weak and the handheld device cannot communicate. In this case, a repeater can be used to solve the handheld device communication in this blind area. Choose a suitable top floor of a building, and aim the uplink antenna (directional gain antenna) of the repeater in the direction of the repeater station. The downlink antenna can be a directional or omnidirectional antenna according to the actual situation. As shown in Figure 4. The uplink and downlink amplification gain of the repeater can be designed according to the actual situation. According to the author's experience, the uplink output power does not exceed 2W, and the downlink output power does not exceed 10W. When installing and using a repeater, the key is to ensure that there is sufficient air isolation between the uplink and downlink antennas, that is, the signal transmitted by the uplink antenna cannot be fed back to the downlink antenna, and the signal transmitted by the downlink antenna cannot be fed back to the uplink antenna. Otherwise, it will cause the self-excitation of the repeater amplifier and damage the amplifier. Usually, the spatial isolation between the uplink and downlink antennas is better to be above 40dB. Since conventional wireless communications are below the 450MHz frequency band, the antennas are large and their directivity is not as good as in the 800MHz and above frequency bands, so it is more difficult to spatially isolate the uplink and downlink antennas.
2. Power Distribution Method
The power distribution method is mainly applicable to the communication between the floors of a building and the basement, or underground shopping malls, underground multi-storey parking lots, etc., especially the communication between the handsets of building security, underground shopping malls, and underground parking lot security.
Power allocation is to distribute the power of a transmitter to several spaces to solve the problem that users in different spaces cannot communicate due to the obstruction of walls and floors. For example, there are two underground floors in a 30-story building. How can the building security handsets communicate with the control center and with each other when they are on different floors and in the basement? To solve the communication problem of building security handsets, power allocation method needs to be adopted. Its principle is shown in Figure 5. The building selects a fixed repeater with an output power of 50W, and then divides the power into three parts through a power divider, and transmits it to the basement, the shopping malls on the 1st to 5th floors of the building, and the corresponding floors on the 6th to 30th floors through feeders, and then selects appropriate antennas to solve the communication between handsets in the corridors, hallways, basements and other spaces on each floor. The electromagnetic wave attenuation of the floor can be calculated using the formula: L=30+35log(d)+15×N……dB, where d=N×h, N is the number of floors, and h is the height of each floor.
If the signal power of the distributor is not enough, you can add a low-power repeater to amplify the signal before transmitting it.
There are basically two methods to solve the blind area communication. When undertaking such communication projects, the signal field strength should be measured and engineering calculations should be performed to determine the location of the main turntable, the power distribution of each layer, whether to add a repeater, how to install the repeater, and a series of other issues, and then the equipment should be selected for installation and debugging. In short, these two methods are usually used to solve such blind area communication. The important thing is how to apply them in combination with specific projects.
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提升卡
变色卡
千斤顶