With the rapid development of economy and the acceleration of urbanization, many large high-rise buildings cover more and more areas. These high-rise buildings have a high density and complex structure, which cause a lot of shielding for wireless intercom signals. Especially in the elevators and fire stairs in the core areas of the buildings, outdoor signals cannot be transmitted to these important areas, resulting in many communication blind areas. Therefore, how to solve the indoor coverage of wireless communication signals in large buildings and improve the communication quality has become more and more important, becoming a key problem of wireless intercom system communication.

  1  The significance of wireless intercom system in large buildings

Wireless intercom system communication is an important means to ensure the communication command of public security and other emergency departments. In the large buildings with wireless intercom indoor blind areas, the security and evacuation of rescue workers within the signal blind areas will not be able to communicate through interphones, resulting in unimaginable consequences. In addition, most of these units belong to the key security units. In case of disasters, there will be multiple forces involved in the war and complex organizational combination, so as to ensure smooth wireless intercom system and clear sound quality. It is of great significance for the rear commanders to grasp the situation of disasters and accidents at any time, real-time dynamics of the scene and reinforcements, and to issue operational orders.

  2  Design principles of wireless intercom systems for large buildings

In a broad sense, indoor wireless coverage is not only the coverage of indoor blind areas, but also the improvement of indoor wireless communication quality. Indoor coverage design of wireless intercom system must follow the principles of conforming to the characteristics of building structure, reducing the implementation cost and construction difficulty, achieving better communication effect and avoiding interference as much as possible. Specifically, the following three points should be achieved:

(I) on the premise of ensuring the quality of coverage signals of the wireless intercom system, reduce the engineering cost as far as possible, and adopt appropriate cables and devices.

(ii) the design scheme of wireless intercom system shall minimize the difficulty of construction. In the scheme design, fully consider the construction process of the feeder laying simple, easy, firm and other factors, to ensure the construction of the minimum degree of damage to the structure and beauty of the building, determine a reasonable way of wiring.

(iii) the indoor field strength should be even as far as possible in the design, and there should be enough edge signal strength. Reasonable choice of antenna type and antenna output power and placement position, to meet the design requirements, achieve good uniform coverage at the same time, the use of the minimum number of antennas, and to reduce the same or different frequency interference or even no interference.

(iv) considering the viability of the walkie-talkie coverage system used by the public security department, the cables are selected to avoid the volatilization of inflammable and toxic gases, so as to play a real role in disaster relief.

  3  Methods and analysis of wireless intercom coverage in buildings

(1) building self-built base station mode

In many large construction projects, in order to ensure the operation of the buildings, indoor walkie-talkie coverage system is usually added in the design and construction. The self-built n-g-source is used to cover all the locations of the whole building through a set of antenna feed system. The indoor distribution system can well cover the partition area and machine room inside the building by virtue of its multiple distribution points. At the same time, the outdoor antenna located on the top of the building can cover the surrounding area to form a communication network centered on the building.

Advantages:

1. Indoor antenna distribution signals are uniform, without dead Angle and blind area;

2. Good signal quality can be obtained both indoors and outdoors;

3. No communication interference and self-excitation.

Disadvantages:

1. Limited outdoor coverage area;

2. No networking function with external base stations;

3. The frequency is generally used for civil use, but cannot be used by emergency departments such as public security;

4. Large number of indoor antennas, high cost and large construction.

(2) the same frequency (different frequency) same broadcasting mode within the building is very effective for the outdoor signal long-distance transmission, especially for the cross-region, two base station signals do not overlap, can effectively increase the coverage of the master and slave base station, and do not need to switch channels after cross-region. When the building needs to communicate with the same channel of the external base station, the same frequency and same broadcast mode can usually be used to achieve. The signal source of the external base station is transmitted through the network or other carriers to the slave base station inside the building, which is then transmitted through the internal antenna from the base station.

Advantages:

1. Indoor signal and main base station are at the same frequency, which can achieve a wide range of network coverage;

2. Reduce the same frequency interference through signal synchronization.

Disadvantages:

1. If the distributed antenna system is not used indoors, signal quality in blind areas such as stairs and basement cannot be guaranteed;

2. Although synchronous measures can further reduce the area of same-frequency interference, they cannot be eradicated;

3. External channels are required to transmit signals between the master and slave base stations; 4. Different frequency simultaneous broadcasting requires high frequency number.

(3) introduction of wireless coupled repeater for base station signals when wired routing is not available, the method of wireless coupled repeater can also be adopted to realize cofrequency forwarding. The donor antenna located at the top of the building is used to collect the signals emitted by the external base station. As long as the signal intensity reaches a certain value, the signal can be lifted through repeater and amplified inside the building to achieve the same frequency communication between indoor signals and external base station signals.

Advantages:

1. Simple project implementation through outdoor space reception;

Working at the same frequency.

Disadvantages:

L. Indoor signals are easy to be received by the donor antenna, resulting in self-excitation of the repeater;

2. The donor antenna may interfere with the outdoor main base station to some extent, which may reduce the receiving sensitivity of the host.

(iv) adopt the solution of introducing external signal fiber into the room. From the outdoor covered main base station antenna, very small signals are separated for long-distance radio frequency transmission through single-mode optical fiber, and then the signal strength is improved in the building and the indoor distribution system is combined to cover important blind areas in the building. Since the optical fiber transmits radio frequency signal, the amplifier in the building does not change the signal phase and other indicators while amplifying the signal, and the influence of optical fiber on signal transmission time delay is very small. Therefore, even if the indoor antenna and outdoor signal penetrate into the overlap of the building interior, the problem of same-frequency interference will not be generated.

Advantages:

1. Indoor signal and main base station are at the same frequency, which can achieve a wide range of network coverage;

2. The method of re-amplification of rf signals transmitted by optical fiber completely overcomes the same frequency interference and other problems;

Disadvantages: external fiber routing conditions are required for implementation.

To sum up, the introduction of base station signal fiber in large buildings with conditions can better realize the coverage of the indoor blind area of wireless intercom system, improve the indoor communication effect of interphone, and minimize the problem of same-frequency interference.

At the same time, combined with the indoor antenna distribution system inside the building, the signal is evenly distributed in different functional areas of the building, so as to ensure that the walkie-talkie keeps the signal unimpeded during the movement of the building.

Each large building is located in a different geographical location, the architectural structure is not the same, different routing conditions, so the selection of appropriate indoor signal coverage mode is the most important.