As you know, the coverage of any cellular network is a mesh (honeycomb) structure formed with the help of (BS). Each base station can serve one or more depending on the network configuration and the need for capacity and quality of coverage in a given area. Base station equipment in the most general case can be divided into 3 components: transceivers, antenna feeder device (AFD) and auxiliary equipment (air conditioning, power supply, fire extinguishing systems, security complex, etc.). There are countless ways to implement it. Depending on the generation, capacity, standard used, coverage area BS can be made both in a free-standing container in combination with a 72-meter mast, and in the form of a small compact case for covering inside buildings -. Let us consider the most typical cases of the implementation of full-scale BS for coverage both in urban conditions and outside the city.
The most typical way to locate BS equipment is to install a special or, at the foot of which one or more containers for transceiving equipment are located. The main purpose of installing an antenna mast structure is to locate the antenna feeder device. It includes a complex of antennas for creating an omnidirectional radio coverage, but more often of a sector type, as well as feeders that connect the antennas to the transmitting and receiving equipment. In addition, in suburban areas, signal amplifiers in the uplink direction (low noise amplifiers) are often used along with antennas, which expand the coverage area. Also, the tower is necessary to accommodate transport equipment if radio relay links (radio relay lines) are used. They usually include a directional parabolic antenna, a radio module that converts a low-frequency signal into a high-frequency signal for transmission to the remote side, and a separate feeder that transmits a low frequency signal from the BS equipment or a separate transport module inside the control room.
The container houses transceivers, transport equipment, and equipment designed for business continuity and safety. The transceiver equipment usually combines a control unit, transceivers (TRX) and combiners, which combine the radio signal from different antennas and TRX in different configurations. The control room may contain equipment operating in multiple frequency bands or even different standards and generation (s). At BSs located far from large settlements, they are usually used to form transport channels to the base station controller. However, in some cases, electrical wire lines or satellite communications are used. The power supply system is an integral part of BS equipment. Usually this is a special 48V DC source, entangled by 220 or 380V alternating voltage. It also switches to storage batteries (batteries) in the event of an external power failure and ensures their recharging - after resumption. Any BS equipment room is equipped with a system for maintaining the operating temperature and humidity values. Usually this is a split system, one or two operating either alternately or as active / standby. Typically, any high-rise structure should be marked with special obstruction lights to ensure its detection by aviation pilots in poor visibility conditions or at night. Therefore, in the control room, you can also find an additional power supply and a battery kit to power the tower lighting system.
For placement in cities, separate towers are rarely installed, because it is both expensive and ineffective. Therefore, antennas are usually installed on residential and industrial buildings and structures, as well as chimneys and other existing tower-type structures. The main requirement is that the location meets all hygienic standards for the installation of such facilities. This type of equipment placement usually does not change the composition of the BS. In this case, the container is usually replaced by a partition in the attic or technical floor or a separate room in the building, and antennas and feeders are often disguised as the appearance of the building so as not to spoil its appearance.
In addition to container placement of equipment, many manufacturers offer to install special outdoor equipment. For their placement, no separate room is required, and all equipment is placed in special thermoboxes and can be mounted in any convenient place: wall, roof, attic, etc. This significantly saves the company's operating costs. However, the main disadvantage of such BS is the low capacity and the difficulty of expanding their capacity. Therefore, they are not as widely used as with the container.
Also recently, many manufacturers have proposed the so-called. In this case, the transceiver equipment is divided into two parts: one is installed in the container and serves as the main unit for control and signal processing, and also provides interfaces to the base station controller. The other part is installed in the immediate vicinity of the antennas and converts the signal received from the control unit into a high-frequency radio signal transmitted to the antennas via feeders. Both parts are usually connected to each other using an optical patchcord or less often a twisted pair. At the same time, the savings in the length of the feeder can reach tens of times, which, accordingly, significantly reduces the attenuation and simplifies the installation. This scheme is especially widespread for the implementation of BS.
High-quality mobile communication is important both in the everyday life of citizens and in the activities of most organizations.
To ensure a wide coverage area and establish a continuous stable signal, cellular operators are forced to place equipment as concentrated as possible, including in residential areas.
How safe is such a neighborhood for the population?
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Effects of radio signals on human health
Today, civilized human life proceeds under the continuous influence of electromagnetic radiation (EMR). Its sources are electronics and household appliances and, of course, wireless communications.
Radio communication is the transmission of high-frequency electromagnetic waves from a transmitter to a receiving device. Thus, every person using a mobile phone is constantly in the area of \u200b\u200bthe electromagnetic field (EMF).
At a certain level, electromagnetic radiation can have an adverse effect on the health of people and other living beings, cause malfunctions in the operation of navigation equipment and other devices.
Finding a person for a long time in an area with an increased level of EMF can cause:
- Physiological disorders (nausea, headache, increased fatigue);
- Psychological disorders (irritability, decreased self-control).
With a significant increase in the intensity of exposure to radio waves on the human body, the internal organs of the following systems can be affected:
- Endocrine;
- Nervous;
- Immune;
- Reproductive.
Such an impact can have extremely negative health consequences, expressed in the development of serious diseases in a person, up to oncology.
The intense exposure to EMR is especially dangerous for children, pregnant women, people suffering from diseases of the central nervous, cardiovascular systems, and allergies.
Are towers harmful?
Cellular communication is based on the principle of interaction between base stations and the direct receiving device (mobile phone, tablet, navigator).
The interaction is based on the transmission of an electromagnetic signal in the UHF (ultra-high frequency) range.
The propagation radius of the base station signal depends on:
- The cellular standard used by the operator;
- Loads;
- Building density;
- Equipment used by the operator.
The coverage area of \u200b\u200ba certain territory is carried out by installing cell towers according to the cell principle. Hence the name - cellular communication.
The technology of functioning of the cellular system assumes that the maximum radiation energy is concentrated and directed away from the structures on which the base station antennas are located.
The power of the station is not constant and is regulated depending on the load on the network.
Cellular base stations located outside cities are often equipped with signal amplifiers to increase the signal propagation radius. Accordingly, the EMR level near such objects will be higher.
The studies and measurements carried out in the area adjacent to the place where the base stations of cellular communication are installed confirm that the EMR level is within the standard values \u200b\u200band practically does not differ from the background radiation level typical for a particular area.
Thus, the residence of citizens in the immediate vicinity of the location of cell towers is safe if:
- The equipment is located above the nearby building area;
- The parameters of the equipment correspond to the established sanitary and hygienic standards.
If the base station signal is directly directed towards a nearby building, then such a neighborhood can be hazardous to health.
If you want to know the law banning smoking in the entrances of residential buildings, we advise you to read.
Cellular base station on the roof of the house
In densely built cities, mobile operators are often forced to install equipment on the roofs of high-rise buildings, including residential buildings.
Such actions are not prohibited by law (installation of industrial equipment on the territory of residential buildings is not allowed, and cellular communication equipment does not apply to such), but require a certain procedure.
Equipment placement parameters must comply with the established standards:
- The EMF level in the adjacent area should not exceed 10 mW / cm 2;
- Depending on the radiated power, the antenna should be erected at a level of 1.5 to 5 meters from the roof surface and at a distance of 10–25 meters from other buildings;
- The possibility of access of people to the roof must be excluded.
The telecom operator must obtain permission from the supervisory authority for the installation of equipment, as well as the consent of the owners of the premises located in the house on the roof of which the base station is supposed to be erected.
The decision of the owners to approve the installation of equipment is made at the general meeting in accordance with Article 44 of the RF LC, while at least two-thirds of all owners must vote for such a decision.
After that, the telecom operator develops design documentation containing all the characteristics of the installed equipment, which, in turn, must be certified.
The equipment is put into operation after the communications organization receives a sanitary and epidemiological conclusion. Further, at least 1 time in 3 years, control measurements of the EMF level are carried out.
Government regulation
At the state level, standards are established that reflect the safe limits of the EMI of radio-technical transmitters.
In Russia, there are Sanitary and Epidemiological Rules and Norms SanPiN 2.1.8 / 2.2.4.1383-03, approved by the Resolution of the Chief State Sanitary Doctor of the Russian Federation dated 09.06.2003 No. 135.
Rospotrebnadzor is the state body authorized to monitor the level of EMR from cellular base stations.
It is to this body that complaints about possible violations by telecom operators should be sent.
If, as a result of the inspection, the fact of exceeding the permissible level of EMR is confirmed, then through the court the officials of Rospotrebnadzor may demand to dismantle the equipment, the operation of which may threaten the health of citizens.
Protect the Earth's population from electromagnetic waves. About 200 scientists from all over the world made such an appeal to the UN. We invite you to watch the video.
Most cellular users have no idea what a huge system they provide for their comfort. Meanwhile, these are not only switch computers, special controllers, but also thousands, no - tens of thousands of base stations, sensitive antennas of which allow subscribers to always stay in touch.
More brains than hardware
The main element of a cellular network of any standard - this is the base station (BSS, Base Station System), which deals with call distribution and mobile phone authentication. Depending on the communication standard, base stations (BS) operate in the frequency range from 450 to 1880 MHz. BSs form the basis of macrocells, the so-called "honeycomb". Since the working radius of such stations is on the order of 10-12 km outside the city and about 5 km in the city, many BSs are built and are located relatively close to each other. Fully autonomous and automated base stations are small containers that are usually installed on the roof of buildings. There are usually several computers, an autonomous power supply and an air conditioner - all BS equipment is very sensitive to temperature changes. All this wealth is equipped with an automatic fire extinguishing system and alarm. It is imperative that there is a wireless or cable communication channel with the network control center, where a huge data stream is transmitted - incoming and outgoing calls from subscribers.
BS container.
Secure communication
There is an opinion that base stations are very harmful to health. According to domestic sanitary norms and rules, BS antennas are placed on existing buildings or on special masts. There are two types of antennas: transmitting (or transmitting-transmitting), and receiving, which are not sources of an electromagnetic field at all. The main radiation energy of the transmitting antenna is concentrated in a rather narrow "beam", which is always directed away from the structures and above the adjacent buildings. This is a necessary condition for the normal functioning of cellular communications and environmental safety.
Base station stuffing.
The radiation power of the BS antenna is not constant, it changes depending on the network load - the number of active cell phones in the service area. At the same time, for stations located in different parts of the city, the load varies. At night, it is practically zero, by the evening it rises sharply.
Studies of the electromagnetic environment in the area adjacent to the BS have been repeatedly carried out by specialists from Sweden, Hungary and Russia. If you study the results of these measurements, you can see that in 100% of cases the electromagnetic environment in the building where the BS is installed does not differ from the background one. On the adjacent territory, in 91% of cases, the recorded level of the electromagnetic field was 10 times less than the MPL (maximum permissible level) established for radio engineering facilities in Moscow. The maximum value recorded during the measurements was 2 times less than the installed remote control, near the building, on which three stations of different standards were installed at once. Thus, we can say with confidence that cellular base stations are not dangerous to public health.
Came, set, flew away
Base stations are mounted on high-rise buildings or on metal towers that are specially built. High-rise masts (over 50 meters) are usually mounted outside the city by helicopters. All structures are brought from special factories on long tractors and then assembled into 4 large sections, which the helicopter will have to hoist one on top of the other. The first section, 20 meters high, is erected with a heavy truck crane, and the rest - only with the help of a rotary-wing machine.
Helicopters are used for mounting special. It looks like an ordinary Mi-8, but in fact it is a radically modified machine - a flying laboratory worth about $ 2 million. Specially for the installation of complex structures, an external suspension is provided on it, on which a cable with tower blocks is attached. It is controlled by a computer, which takes into account all gusts of wind and keeps several tons of metal strictly vertically. There is also a special transparent rear cockpit, from which another pilot controls the installation. The whole process is carried out in radio silence mode - control is carried out only by visual commands of the "flag" engineer from the ground. It is this person who must make sure for himself that the flanges of the blocks are in contact, and only after securing the section, give the command to the helicopter pilot to unhook the cable from the external sling. The assembly process itself takes place very quickly - in just 40 minutes.
Installation of a base station from a helicopter.
A parade of unusual projects
Exotics in the construction of sea base stations. It's no joke - each cellular company can have several thousand communication facilities, and not all of them are located in megacities. For example, in MTS there is a BS powered by a wind generator - they are installed in the Krasnodar Territory. There are BSs in portable containers - a box the size of a small TV, in which an uninterruptible power supply and an air conditioner are mounted. Such BSs can be mounted almost anywhere, for example, directly on poles. In particular, in Moscow, a similar BS is installed in Gostiny Dvor. There are BSs mounted on bell towers - in Suzdal and Sergiev Posad, for example. These are the highest places in the district, it is unprofitable to erect a mast there - so I had to ask permission from the church. But a completely unique system is the base station in the Lefortovo tunnel in Moscow. There, the antenna is a slotted radiating cable stretched in a three-kilometer tunnel.
VimpelCom has its own pride - to ensure coverage of the Bee Line GSM network on the busy section of the federal highway Krasnodar-Sochi, passing through the village Moldavanovka, a solar-powered base station was launched. On a mountain pass in the Moldavanovka area, it turned out to be difficult to provide quality coverage. One of the possible positions of the base station, providing an acceptable coverage area, was a natural site at an altitude of 711 meters on the mountain. It was decided to replace the expensive project for the organization of traditional power supply with an alternative solar-powered system. The peak power of solar panels is enough to power the station itself, two air conditioners and recharge the batteries overnight. In addition to solar panels, a wind turbine was also installed. IN " VimpelCom”Emphasize that this is the first base station on alternative energy sources in Russia, but not the first in the world.
Base station "Bee Line" on solar energy.
In April 2004, a solar-powered mobile base station was also launched in Kimberley County, Western Australia. In total, it uses 60 solar panels with a total weight of 7.4 tons.
Distinguished in the construction of "exotic" and mobile operator specialists SMARTS... For example, BS with a coverage area extended to 100 km are used in the Astrakhan region. In Samara, Penza and Volgograd regions, base stations are at high altitude. There are a lot of hills there. Such stations have to be mounted with the help of specially trained mountaineering units.
More bandwidth is required for mobile fBs that deploy in just a few hours. For example, such a BS works at the Grushinsky Festival.
It should be noted that exotic BS is not an invention of the "mysterious Russian soul". For example, the UK began to install "green" base stations for cellular communications. Finally, the English "greens" who opposed the construction of base stations of cellular communications in the protected areas of Foggy Albion can sleep peacefully. In the heart of the Aberdeenshire Forest, a second self-contained hydrogen fuel cell base station has been installed. The first such station was installed in Scotland, near the world famous ski center. The installation of a conventional station would require laying about five kilometers of electrical cable, which would cause irreparable damage to the nature in these places.
By the way, for amateurs and specialists: the cellular configuration of the base stations built in many domestic and foreign resorts is very interesting and unusual. At such stations, as a rule, there are only two cells, one "shoots" in one side of the coast, the second - in the opposite. You won't climb into the sea with your phone, but taking a picture of yourself on the beach and sending yourself to the snow-covered capital is a nice thing.
But African operators went farthest - they set up base stations on special rafts in the middle of large rivers. As a result, they "cover" both the nearby villages and the river itself. There are no roads, and the main transport routes are by water. Well, it is more difficult to steal equipment from there.
Total control
Constant monitoring of the cellular network coverage, the quality of voice communication is vital for any cellular operator. That is why mobile laboratories equipped with the latest technology go every day on the road. A typical example of such a car is a minibus Volkswagen Transporter or SUV Suzuki grand vitara, they are equipped with measuring equipment for testing AMPS / DAMPS or GSM900 / 1800 networks.
Volkswagen Transporter
In each region, engineers develop specific routes to monitor the network. Alternatively, the crews set off on the road based on user complaints about poor communication quality or the inability to get through. The third option for detours is to thoroughly check the operation of the new base stations. The measuring complex not only checks the BS coverage area and verifies it with a plan, but also tests the quality of switching between the BS and its neighbors.
The special equipment of each car costs tens of thousands of dollars and takes up about a third of the passenger compartment. In total, the car can have two complexes, they are powered by the on-board network. The first task is to monitor the quality of the cellular network coverage in real time. It usually consists of a powerful laptop from Toshiba, several special devices and three phones Sagem, one of which works only in the GSM900 range, the second - GSM1800, the third - supports both communication standards. Each of the telephones continuously calls special technical numbers in the company's office with so-called “long calls” for 54-59 minutes. At this time, the measuring equipment analyzes the quality of the coverage, it is determined how successfully the cell call is transmitted from cell to cell, in which cases communication breaks may occur. All data is immediately entered into a computer for further analysis.
The second set consists of four Sagem phones. Two of them call each other with “short calls” lasting 59 seconds. Another one calls to a special service number at the office of the cellular company, another one receives calls from the office of the company. After connection, a short voice message is transmitted in a male and female voice. Since the standard of the text is known, the result obtained is instantly compared with it and a measure of the similarity of the message is established. The more similar they are, the better the connection quality. One of the important parts of the complex is the satellite navigation system (GPS). With its help, the computer determines the position in the city and enters into a special "black box" all the parameters of the cellular network with reference to the area. This allows you to pinpoint the locations of failures with high accuracy.
The terms "base station" and "cell tower" have long and firmly entered our lexicon. And if the average user remembers these things not so often, then the "cell phone", by habit, is clearly among the ten leaders. Hundreds of millions of people use cellular communications every day, but very few of them think about how this very connection is provided. And of this minority, very few really represent the complexity and subtlety of this communication tool.
From the point of view of most people, setting up a cellular base station is very straightforward. Just hang up a few antennas, connect them to the network - and you're done. But this view is fundamentally wrong. And so we decided to talk about how many subtleties and nuances arise when installing a base station in a metropolis.
Watch out for traffic!
To visually illustrate our story, we have documented in detail the process of installing a cell tower on the roof of a building in Moscow, at ul. Krasnodonskaya, 19, building 2. This is a two-storey detached administrative building. We chose this example because this base station not only has a small bracket for hanging antennas, but a 5-section tower 15 m high. But let's start in order.
Preparation and design
The work of installing a base station begins with finding a suitable object. When it is found, a lease is concluded with its owner. The required location of the antennas of the future station, the mass of the payload are determined, and on this basis the metal structures are designed. This takes into account the bearing capacity of the structural elements of the building itself.A set of documentation (almost 5 cm thick) is issued for each installed base station. Among other things, many parameters of the future structure are indicated here: its location on the object, overall dimensions, total weight, location of support points, voltage and power consumption, and so on.
This folder contains comprehensive information:
Project documentation,
Copies of statements, licenses, certificates and conclusions of conformity for all elements, up to nuts and paint,
Working documentation for equipment, metal structures, architectural and construction solutions, lightning protection.
Sanitary and epidemiological conclusion on the safety of the station for residents of surrounding houses.
Let's go back to our tower. After the agreement and approval of the project, the platform and five tower segments were manufactured separately at the plant. Since in this case it was a rather heavy structure, it had to be installed on the load-bearing walls of the building. For this, holes were cut in the roof and support beams were installed. They play the role of a pile foundation for the platform, on which the station equipment and a tower with antennas were later mounted. The total weight of the platform was 3857 kg.
The profile, dimensions and number of beams from which the platform is assembled, wall thickness, length of welds, hardware used - all these parameters are calculated based on the payload mass, the bearing capacity of the building walls, as well as possible wind loads in the region. Of course, these are far from the only criteria, first of all, the tower should provide the ability to install transmit-receive antennas at the required height in the visibility range of neighboring base stations. In addition, the structure must be rigid enough so that the relay communication beam does not get lost.
Installation of metal structures
The building is small, it does not have a separate exit to the roof, so a team of installers has to climb the fire escape. Its lower part is cut off so that residents of the surrounding houses do not climb onto the roof. Unfortunately, this does not stop them too much, so something often disappears from the roofs - parts, cables, feeders, etc.Despite the fact that each station is equipped with an alarm, the security service does not always have time to arrive on time.
The base station of another cellular operator has already been installed on the roof, but its dimensions cannot be compared with ours.
After installing the platform, the sites are prepared for the installation of the first section of the tower:
After the section is installed, “tightening the nuts” begins:
The installation of the tower on the studs is done in order to compensate for deviations from the vertical during installation and further operation.
The verticality of the structure is constantly monitored from two points using theodolites. Moreover, measurements are taken separately for each section of the tower, and then the measurement log will be included in the set of documents. Subsequently, periodic measurements of the tower position are carried out, since under its own weight and the weight of the equipment, a slight spiral twisting of the structure can occur (up to 50 mm at 72 m in height).
Hardware cabinet prepared for platform installation:
So, the first section is installed and aligned. Installers are preparing to receive the second section:
Great attention is paid to the safety and comfort of work not only during installation, but also during further maintenance. The work sites are sized so that engineers have enough room to work. Ladder railings have been installed, the openings in the platforms on the tower are closed with hatches to prevent accidental falls. The platform is raised above the roof plane so that in winter time the equipment is not covered with snow and blocked by ice.
Installation of the remaining sections of the tower:
Hardware cabinet queue:
The tower has been mounted, the last measurements have been made using theodolites. Deviations are minimal and strictly within tolerances. The mass of the tower was 2827 kg, and the total weight of all metal structures was 6684 kg.
The colors of the sections are standard: the bottom and top are always red, the intermediate ones alternate with white. At the top, you can see 4 pins, which are a continuation of the edges of the tower - these are lightning protection elements.
Equipment
The next step was the installation of all necessary equipment and cabling. Complete list of installed equipment:
As a result, the station acquired a rather majestic appearance, especially in comparison with the building itself:
The station is supplied with a voltage of 380 V (3 phases), which is then converted into 48 V. The power is taken with a margin of up to 10 kW. Food is supplied in a separate locker.
Let's open the door of the hardware cabinet. It has built-in air conditioning (top) and heater (bottom).
The cabinet maintains a temperature of 18 ... 20 degrees Celsius throughout the year. This is necessary for the smooth operation of the equipment and the long life of the batteries (they are located at the bottom).
The accumulators are designed to ensure the operation of the station for about a day in the event of a power outage.
Above is the switching unit and voltage converter.
The transfer of information between the system modules and transceivers (about them below) is carried out via fiber optic cables. This is what the connector looks like in the junction box. In no case should you touch it with your hands, the fiber is very sensitive to damage and pollution.
All base stations of cellular communication are connected to a single information fiber-optic network, stretched throughout Moscow. The white bay under the equipment cabinet is just the cable through which this station is connected.
To the right of the cabinet are the GSM, CDMA and LTE system modules:
These modules are the heart of the base station; they receive the signal from the antennas and convert and compress it with further transmission. They are not afraid of precipitation, all connectors are sealed, and the operating temperature range is from +60 to -50.
Lightning arresters are located under the system modules that prevent equipment burnout in the event of a lightning strike:
On the right above the modules, there are fiber-optic cable coils, with which they are connected to the transceivers on the tower.
Let's move on to the tower. It has transceivers installed separately for each band (GSM, CDMA and LTE). They amplify the signal from extremely low values \u200b\u200bup to 115-120 dB. They are powered from the hardware cabinet:
Oblong vertical "boxes" are antennas. They are shielded at the back to protect operating personnel from electromagnetic radiation. Let's go up to the site.
Transceiver GSM:
CDMA transceiver:
LTE transceiver:
Fiber optic cables are connected to the transceiver at the edges, power supply in the center:
Grounding lead to the tower:
Cable connectors and their plugs on the antenna:
Basic circuit diagram of base station equipment switching:
We have already mentioned that designing and building a cellular base station is not at all as easy as it seems to the uninitiated. There are many nuances that are associated with the specific location of the station. For example, radio transmission over a large water surface is deteriorating, although it should be the other way around, because there are no obstacles. But the fact is that an electromagnetic field spreads over the surface of the earth, and a large volume of water works as a kind of condenser, over which interference to the radio signal is amplified. And there are many such subtleties, therefore the efficiency of the base station directly depends on the professionalism of the designers and installers. For example, from people like this foreman of installers, a high-class radio engineer, and just a wonderful person:
Plan:
1. Building cellular communications.
2. The structure of cellular communications.
3. The history of the development of cellular communications.
Cellular communications are the most modern and rapidly developing area of \u200b\u200btelecommunications. It is called cellular because the territory in which communication is provided is divided into separate cells or cells.
As a rule, in each cell the subscriber receives the same set of services and, within certain territorial boundaries, receives these services at the same price. Thus, moving from one cell to another, the subscriber does not feel territorial attachment and can freely use communication services. The continuity of the connection is also important.
While moving the connection established by the subscriber (voice call, packet data transmission) should not be interrupted. This is due to the so-called handover(Handover). The connection established by the subscriber is, as it were, grabbed by the neighboring cells on the relay, and the subscriber continues to unwittingly talk or travel across the Internet.
So, let's look at what a cellular network consists of. The entire network is divided into two subsystems: a base station subsystem and a switching subsystem.
The main elements of the base station subsystem (as it is not difficult to guess) are the base stations themselves ( Bts). They just create those honeycombs that were mentioned at the beginning. Each base station typically serves three cells. The radio signal from the base station is radiated through 3 sector antennas, each of which is directed to its own cell. Sometimes you can find a situation when several antennas of one base station are directed to one cell at once. This is due to the fact that the cellular network operates in several bands (900 and 1800). In addition, this base station may have equipment of several generations of communication at once ( 2Gand 3G).
The most common location for a base station is a tower or mast built specifically for it. However, in urban areas it is difficult to find a place for a massive structure. Therefore, in large cities, base stations are located on buildings. In addition, mobile versions of base stations placed on trucks have recently appeared. They are very convenient to use during natural disasters or in places of mass gatherings of people (football stadiums, central squares) during holidays, concerts, football matches. But, unfortunately, because of problems in the legislation, they have not yet found wide application.
Base station on the tower
Base station on the roof of the building
Mobile base station
Oddly enough, but cellular operators often allow their competitors to use their tower structures to accommodate antennas (Naturally, on mutually beneficial terms). This is due to the fact that building a tower or a mast is an expensive pleasure, and such an exchange saves a lot of money!
From the base station subsystem, the signal is transmitted towards the switching subsystem, where the connection is established with the direction desired by the subscriber. The switching subsystem has a number of databases that store information about subscribers. In addition, this subsystem is responsible for security.
We have covered the basic elements of a cellular network. The terms of the standard were specifically used here GSM... However, both in the previous and in the subsequent standards there are similar elements and functions, only under different names
Radio communication is organized not only with the help of fixed radio communication networks, but also with the use of networks with mobile objects (SRPO).
A radio communication network with mobile objects is a set of technical means with which it is possible to provide mobile objects with communication between themselves and with subscribers of the telephone network. It is designed to serve subscribers during international, national and regional movements (roaming) and allows communication between subscribers when they cross the borders of different geographic zones.
Radio communication networks with mobile objects are classified according to several criteria (Fig. 3.8). Technological SRPOs belong to certain departments and services (gas industry, railroad transport, ambulance, fire protection, etc.). They are intended to provide radio communication services to a limited contingent of individuals and legal entities.
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Classification of radio communication networks with mobile objects
Technological SRPOs are subdivided into dispatching, trunking and radio data transmission networks. Dispatching SRPOs are intended for radiotelephone communication of officials of control bodies with subordinate mobile objects, as well as subscribers among themselves.
Cellular SRPO refers to the public terrestrial radio communication networks with mobile objects, which provide subscribers with all types of conventional telephone services. They are built in the form of a set of networks covering the served territory, in which frequency reuse is used to ensure the efficient use of the allocated frequency resource and high network capacity.
Trunking (radial and radial-zone) networks are designed to provide communication services mainly to subscribers of departmental networks based on the implementation of multiple access to a small number of radio channels with limited or no access to the public telephone network. Trunking networks allow replacing radio communication networks with a fixed frequency distribution and integrating various groups of users within the same communication network in order to increase the efficiency of the use of the radio frequency spectrum.
From a topological point of view, a cellular network is built in the form of a collection of cells, or cells, covering the area served. The general structure of a cellular radio communication network with mobile objects is shown in Fig. 3.9.
The structure of the cellular radio network
The cellular network structure is based on the principle of frequency reuse - the main principle of the cellular network. In addition, the elements of the cellular network are:
- switching center;
- base stations;
- mobile stations, or subscriber radiotelephones.
A cellular base station (BS) serves all mobile stations within its cell, and the base station provides a resource for establishing connections upon request of mobile subscribers, usually on an equal footing.
When a subscriber moves from one cell to another, his service is transferred from one base station to another. All base stations of the network, in turn, are closed to the switching center, from which there is an access to the unified telecommunication network of the Russian Federation.
Currently, the pan-European standard GSM-900 is widely used. In this standard, mobile station transmitters operate in the frequency range 890-915 MHz, base station transmitters in the 935-960 MHz range. There is a constant separation of 45 MHz between the transmit and receive bands. Each of the sub-bands is divided into 124 frequency channels with a step of 200 kHz. The maximum communication range is 35 km.
The GSM standard provides a high degree of security for transmitted messages due to their encryption using an encryption algorithm with a public key. The functional interface of the system elements is carried out by a number of interfaces.
In the technology of building trunking communication, the principle is used, in which a specific channel is assigned for each communication session individually, depending on the distribution of the load in the system, and the load traffic is mainly closed within the networks. Subscribers' access to the public telephone network (PSTN) is limited.
Currently, radial and radial-zone trunking networks are used. Such a network includes:
- base station, consisting of antenna-feeder device, transceiver modules, controllers for each transceiver module and base controller;
- zone equipment (station), consisting of autonomous repeaters, connecting lines with a public network and controllers;
- control equipment, consisting of a system terminal "system manager", dispatcher consoles.
In trunking networks built on a radial principle, the entire channel resource is assigned to one central base station (CDS). The antenna of such a station is located at the highest point of the proposed service Fig. 3.10. An example of such an architecture is the Soviet Antey radio communication network, created in 1960.
The structure of a trunking network built on a radial principle
The considered scheme has a number of disadvantages, in particular, to expand the coverage area, it is necessary to increase the power of the subscriber station (SS), which accordingly increases the overall level of interference.
With a small number of subscribers, an increase in the service area can be achieved using the radial-zone principle. A so-called single-cell network with several antenna points and broadcasting on a common wave is being formed. In this case, along with the main antenna placement point (UKS), there is a number of auxiliary points (ZKS) connected by communication lines with the main one (Fig. 3.11).
The structure of a trunking network built on a radial-zone principle
In general, the technology for building trunking networks provides for the following:
- use of the method of free selection of an unoccupied radio access channel from the channel bundle allocated in each service area. This is achieved by creating a common service (signaling) channel for all users in each zone, through which call signals are received to the corresponding base station, including the identification of the called subscriber, as well as the number of the calling subscriber;
- they do not provide continuous communication when subscribers cross the borders of the radio coverage zones of base stations. "Handover" is replaced by the operation of re-entering the network when the quality of communication deteriorates due to the transition of the user from one zone to another;
- endowing base stations with the functions of local control of cells by directly connecting subscribers in the provisioning area through a local switch, as well as connecting mobile users to a local automatic telephone exchange (ATS), which has direct outputs to a local switch of a base station or through a control center.
