How radio communication works

Radio (lat.radio- radiate, emit rays, radius- ray) is a type of wireless communication in which radio waves are used as a signal carrier, freely propagating in space.

Principle of operation
Transmission occurs as follows: on the transmitting side, a signal with the required characteristics (frequency and amplitude of the signal) is generated. The transmitted signal then modulates the higher frequency oscillation (carrier). The received modulated signal is emitted by the antenna into space. On the receiving side, the radio waves induce a modulated signal in the antenna, after which it is demodulated (detected) and filtered by a low-pass filter (thereby getting rid of the high-frequency component - the carrier). The received modulated signal is radiated by the antenna into space.
On the receiving side, the radio waves induce a modulated signal in the antenna, after which it is demodulated (detected) and filtered by a low-pass filter (thereby getting rid of the high-frequency component - the carrier).). Thus, a useful signal is extracted. The received signal may differ slightly from that transmitted by the transmitter (distortion due to interference and interference).

Frequency ranges
The frequency grid used in radio communications is conventionally divided into ranges:

• Long waves (LW) - f = 150-450 kHz (l = 2000-670 m)
• Medium waves (MW) - f = 500-1600 kHz (l = 600-190 m)
• Short waves (HF) - f = 3-30 MHz (l = 100-10 m)
• Ultrashort waves (VHF) - f = 30 MHz- 300 MHz (l = 10-1 m)
• High frequencies (HF-centimeter range) - f = 300 MHz - 3 GHz (l = 1-0.1 m)
• Extremely high frequencies (EHF-millimeter range) - f = 3 GHz - 30 GHz (l = 0.1-0.01 m)
• Hyperhigh frequencies (HHF - micrometer range) - f = 30 GHz - 300 GHz (l = 0.01-0.001 m)

Depending on the range, radio waves have their own characteristics and propagation laws:

• LWs are strongly absorbed by the ionosphere; surface waves, which propagate around the earth, are of primary importance. Their intensity decreases relatively quickly with distance from the transmitter.
• SW are strongly absorbed by the ionosphere during the day, and the area of ​​action is determined by the surface wave, in the evening they are well reflected from the ionosphere and the area of ​​action is determined by the reflected wave.
• HF propagates exclusively through reflection by the ionosphere, so there is a so-called radio silence zone around the transmitter. During the day, shorter waves (30 MHz) propagate better, at night, longer ones (3 MHz). Short waves can travel long distances with low transmitter power.
• VHF propagates in a straight line and, as a rule, is not reflected by the ionosphere. They easily bend around obstacles and have a high penetrating power.
• HF does not go around obstacles, spreads within the line of sight. Used in WiFi, cellular, etc.
• EHF does not bend around obstacles, is reflected by most of the obstacles, and spreads within the line of sight. Used for satellite communications.
• Hyper-high frequencies do not bend around obstacles, are reflected like light, and propagate within the line of sight. Limited use.

Propagation of radio waves
Radio waves propagate in emptiness and in the atmosphere; the earthly firmament and water are opaque for them. However, due to the effects of diffraction and reflection, communication is possible between points on the earth's surface that do not have a line of sight (in particular, those located at a great distance).
The propagation of radio waves from a source to a receiver can occur in several ways simultaneously. This spread is called multipath. Due to the multipath and changes in the parameters of the environment, fading occurs - a change in the level of the received signal over time. With multipath, the change in the signal level occurs due to interference, that is, at the point of reception, the electromagnetic field is the sum of time-shifted radio waves of the range.

Radar

Radar- the field of science and technology, combining methods and means of detection, measuring coordinates, as well as determining the properties and characteristics of various objects based on the use of radio waves. A close and somewhat overlapping term is radio navigation, however, in radio navigation, an object whose coordinates are being measured plays a more active role, most often this is the determination of its own coordinates. The main technical device for radar is a radar station.

Distinguish between active, semi-active, active with a passive response and passive RL. They are subdivided according to the used range of radio waves, by the type of the probing signal, the number of channels used, the number and type of measured coordinates, and the location of the radar.

Operating principle

Radar is based on the following physical phenomena:

• Radio waves are scattered by electrical inhomogeneities encountered along the path of their propagation (objects with other electrical properties that differ from the properties of the propagation medium). In this case, the reflected wave, as well as the actual radiation of the target, allows you to detect the target.
• At large distances from the radiation source, it can be assumed that radio waves propagate in a straight line and at a constant speed, due to which it is possible to measure the range and angular coordinates of the target (Deviations from these rules, which are valid only in the first approximation, are studied by a special branch of radio engineering - Radio wave propagation. these deviations lead to measurement errors).
• The frequency of the received signal differs from the frequency of the emitted oscillations with the mutual movement of the points of reception and emission (Doppler effect), which allows you to measure the radial speeds of the target relative to the radar.
• Passive radar uses the radiation of electromagnetic waves by the observed objects, it can be thermal radiation inherent in all objects, active radiation created by the technical means of the object, or spurious radiation created by any objects with working electrical devices.

cellular

cellular, mobile network- one of the types of mobile radio communication, which is based on cellular network... The key feature is that the total coverage area is divided into cells (cells), determined by the coverage areas of individual base stations (BS). The honeycombs partially overlap and together form a network. On an ideal (flat and without building) surface, the coverage area of ​​one BS is a circle, therefore, the network composed of them looks like honeycombs with hexagonal cells (honeycombs).

The network consists of spaced-apart transceivers operating in the same frequency range and switching equipment that allows determining the current location of mobile subscribers and ensuring continuity of communication when a subscriber moves from the coverage area of ​​one transceiver to the coverage area of ​​another.

The principle of cellular communication

The main components of a cellular network are cell phones and base stations, which are usually located on rooftops and towers. When turned on, the cell phone listens to the air, finding a signal from the base station. The telephone then sends its unique identification code to the station. The telephone and the station maintain constant radio contact, periodically exchanging packets. The phone can communicate with the station using an analog protocol (AMPS, NAMPS, NMT-450) or digital (DAMPS, CDMA, GSM, UMTS). If the phone leaves the range of the base station (or the quality of the radio signal of the service cell deteriorates), it establishes communication with another (eng. handover).

Cellular networks can consist of base stations of different standards, which allows you to optimize network performance and improve its coverage.

Cellular networks of different operators are connected to each other, as well as to the fixed telephone network. This allows subscribers of one operator to make calls to subscribers of another operator, from mobile phones to landlines and from landlines to mobiles.

Operators can conclude roaming agreements with each other. Thanks to such agreements, the subscriber, being outside the coverage area of ​​his network, can make and receive calls through the network of another operator. As a rule, this is done at higher rates. The possibility of roaming appeared only in 2G standards and is one of the main differences from 1G networks.

Operators can share network infrastructure, reducing network deployment and operational costs.

Cellular services

Cellular operators provide the following services:

• Voice call;
• Answering machine in cellular communication (service);
• Roaming;
• Caller ID (Automatic Caller ID) and AntiAON;
• Reception and transmission of short text messages (SMS);
• Reception and transmission of multimedia messages - images, melodies, video (MMS-service);
• Mobile bank (service);
• Access to the Internet;
• Video call and video conferencing

TV

TV(Greek τήλε - far away and lat. video- I see; from Novolatinsky televisio- far-sightedness) - a set of devices for transmitting a moving image and sound over a distance. In common use, it is also used to designate organizations involved in the production and distribution of television programs.

Basic principles

Television is based on the principle of sequential transmission of picture elements by radio signal or by wire. The decomposition of the image into elements occurs using a Nipkov disk, a cathode-ray tube or a semiconductor matrix. The number of image elements is selected in accordance with the radio channel bandwidth and physiological criteria. To narrow the bandwidth of transmitted frequencies and reduce the visibility of flickering on the TV screen, interlaced scanning is used. It also allows you to increase the smoothness of the transmission of motion.

The television path in general includes the following devices:

1. TV transmission camera. Serves for converting an image obtained with a lens on a target of a transmitting tube or semiconductor matrix into a television video signal.
2. Video recorder. Records and plays back the video signal at the right time.
3. Video mixer. Allows you to switch between multiple image sources: camcorders, VCRs and others.
4. Transmitter. The RF signal is modulated by a television video signal and transmitted by radio or wire.
5. Receiver - TV. With the help of sync pulses contained in the video signal, the television image is reproduced on the receiver screen (kinescope, LCD, plasma panel).

In addition, an audio path similar to a radio transmission path is used to create a television transmission. Sound is transmitted on a separate frequency, usually using frequency modulation, a technique similar to FM radio stations. In digital television, soundtrack, often multichannel, is transmitted in a common data stream with an image.

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Date the page was created: 2016-04-11

People have long learned to communicate at a distance. In ancient times, a messenger was sent with news, later they wrote letters. Now, to say a few words to a distant friend, you can just call him. The main thing is to have a cell phone with you. But how do they connect to each other if they don't even have wires? In this story I will tell you how the phone works.

What it is?

The mobile phone looks more like a walkie-talkie than a regular wired telephone. Radio waves are used to transmit the signal.

The difference is that the walkie-talkies are connected to the same antenna, and can only be connected by picking up a signal from it. Cell phones are not tied to a specific station. While moving, they are connected to the antenna from which the strongest signal comes, so we can use communication almost all over the world without changing the SIM card. Antennas, or base stations, are built all over the world, hiding in billboards, clocks, poles, and even trees. Each of them is responsible for its own hexagon-shaped zone. In the diagrams, these bordering territories resemble a honeycomb. Hence the name - cellular communication.

Who was the first?

Who do you think was the first person to speak on a mobile phone? Of course, it was an employee of Motorola, which released them. In 1973, while on one of the streets of New York, he called and boasted of a call from an unusual phone at that time to his main competitor. This phone became the prototype of the first mobile phone to hit store shelves 10 years later.

In order for the phone to work, you need to insert a SIM card into it. It contains information about the subscriber, that is, about the person who uses it. The mobile phone starts checking all the frequencies available to it, there are about 160 of them. The six best signals are recorded on the SIM card, these are the signals of your network.

After you have dialed your friend's number, your phone sends information about you to the antenna with the strongest signal. Your operator (for example, MTS or Beeline) recognizes you, finds a free channel on which your conversation can take place, and connects you. All this takes just a few seconds.

The conversation itself is a rather complicated technical process. Our voice is split into 20 millisecond chunks and converted to digital format, then encoded by a special system. The encrypted signals are processed again to remove extraneous noise.

Now cellular telephone serves not only for conversations. One small device accommodates such simple mechanisms as a simple clock, an alarm clock, a calculator, a calendar, a flashlight, as well as a complex camera, Internet access, a player and much more.

Mobile cellular communication

cellular- one of the types of mobile radio communications, which is based on cellular network... The key feature is that the total coverage area is divided into cells (cells), determined by the coverage areas of individual base stations (BS). The honeycombs partially overlap and together form a network. On an ideal (flat and without building) surface, the coverage area of ​​one BS is a circle, therefore, the network composed of them looks like honeycombs with hexagonal cells (honeycombs).

It is noteworthy that in the English version the communication is called "cellular" or "cellular" (cellular), which does not take into account the hexagonal nature of the honeycomb.

The network consists of spaced-apart transceivers operating in the same frequency range and switching equipment that allows determining the current location of mobile subscribers and ensuring continuity of communication when a subscriber moves from the coverage area of ​​one transceiver to the coverage area of ​​another.

History

The first use of mobile telephony in the United States dates back to 1921: Detroit police used one-way dispatcher communication in the 2 MHz band to transmit information from a central transmitter to receivers installed in vehicles. In 1933, the New York police began using a two-way mobile telephone radio system, also in the 2 MHz band. In 1934, the US Federal Communications Commission allocated 4 channels for telephone radio communications in the range of 30 ... 40 MHz, and in 1940, about 10 thousand police vehicles were already using telephone radio communications. All of these systems used amplitude modulation. Frequency modulation began to be used in 1940 and by 1946 completely replaced amplitude modulation. The first public mobile radiotelephone appeared in 1946 (St. Louis, USA; Bell Telephone Laboratories) using the 150 MHz band. In 1955, an 11-channel system began operating in the 150 MHz range, and in 1956 - a 12-channel system in the 450 MHz range. Both of these systems were simplex and used manual switching. Automatic duplex systems began operating in 1964 (150 MHz) and 1969 (450 MHz), respectively.

In the USSR In 1957, a Moscow engineer L. I. Kupriyanovich created a prototype of a portable automatic duplex mobile radiotelephone LK-1 and a base station for it. The mobile radiotelephone weighed about three kilograms and had a range of 20-30 km. In 1958, Kupriyanovich created improved models of the apparatus weighing 0.5 kg and the size of a cigarette box. In the 60s, Hristo Bochvarov in Bulgaria demonstrates his prototype of a pocket mobile radiotelephone. At the exhibition "Interorgtechnika-66" Bulgaria presents a set for organizing local mobile communications from pocket mobile phones RAT-0.5 and ATRT-0.5 and a base station RATTs-10, which provides connection of 10 subscribers.

At the end of the 50s, the development of the Altai automobile radiotelephone system began in the USSR, which was put into trial operation in 1963. The Altai system initially operated at a frequency of 150 MHz. In 1970 the Altai system operated in 30 cities of the USSR and a 330 MHz band was allocated for it.

Similarly, with natural differences and on a smaller scale, the situation has developed in other countries. Thus, in Norway, public telephone radio communications have been used as maritime mobile communications since 1931; in 1955 there were 27 coastal radio stations in the country. Terrestrial mobile communications began to develop after the Second World War in the form of hand-switched private networks. Thus, by 1970, mobile telephone radio communication, on the one hand, had already become quite widespread, but on the other hand, it clearly did not keep up with the rapidly growing needs, with a limited number of channels in strictly defined frequency bands. A solution was found in the form of a cellular communication system, which made it possible to dramatically increase capacity by reusing frequencies in a cellular system.

Of course, as is usually the case in life, individual elements of the cellular communication system existed before. In particular, some semblance of a cellular system was used in 1949 in Detroit (USA) by a taxi dispatch service - with the reuse of frequencies in different cells with manual channel switching by users at predetermined locations. However, the architecture of the system that is today known as the cellular communication system was only outlined in the Bell System technical report submitted to the FCC in December 1971. And from that time the development of cellular communication itself begins, which has become truly triumphant since 1985. g., in the last ten years and a little.

In 1974, the US Federal Communications Commission decided to allocate a 40 MHz frequency band for cellular communications in the 800 MHz range; in 1986, another 10 MHz was added to it in the same range. In 1978, tests of the first prototype cellular communication system for 2,000 subscribers began in Chicago. Therefore, 1978 can be considered the year of the beginning of the practical application of cellular communications. The first automatic commercial cellular system was also commissioned in Chicago in October 1983 by American Telephone and Telegraph (AT&T). In Canada, cellular communication has been used since 1978, in Japan - since 1979, in the Scandinavian countries (Denmark, Norway, Sweden, Finland) - since 1981, in Spain and England - since 1982. As of July 1997 g. cellular communication operated in more than 140 countries of all continents, serving more than 150 million subscribers.

The first commercially successful cellular network was the Finnish Autoradiopuhelin (ARP) network. This name is translated into Russian as "Car radiotelephone". Launched in the city, it has reached 100% coverage of the territory of Finland c. The size of the cell was about 30 km, in the city there were more than 30 thousand subscribers. She worked at a frequency of 150 MHz.

The principle of cellular communication

The main components of a cellular network are cell phones and base stations... Base stations are usually located on rooftops and towers. When turned on, the cell phone listens to the air, finding a signal from the base station. The telephone then sends its unique identification code to the station. The telephone and the station maintain constant radio contact, periodically exchanging packets. The phone can communicate with the station using an analog protocol (NMT-450) or digital (DAMPS, GSM, eng. handover).

Cellular networks can consist of base stations of different standards, which allows you to optimize network performance and improve its coverage.

Cellular networks of different operators are connected to each other, as well as to the fixed telephone network. This allows subscribers of one operator to make calls to subscribers of another operator, from mobile phones to landlines and from landlines to mobiles.

Operators from different countries can conclude roaming agreements. Thanks to such agreements, a subscriber, while abroad, can make and receive calls through the network of another operator (albeit at higher rates).

Cellular communication in Russia

In Russia, cellular communication began to be introduced in 1990, commercial use began on September 9, 1991, when the first cellular network in Russia was launched by Delta Telecom in St. Petersburg (it worked in the NMT-450 standard) and the first a symbolic call by cellular communication by the mayor of St. Petersburg Anatoly Sobchak. By July 1997, the total number of subscribers in Russia was about 300 thousand. For 2007, the main cellular communication protocols used in Russia are GSM-900 and GSM-1800. In addition, UMTS works. In particular, the first fragment of the network of this standard in Russia was put into operation on October 2, 2007 in St. Petersburg by the MegaFon company. The Sverdlovsk Region continues to operate a DAMPS standard cellular network owned by the MOTIV Cellular Communications Company.

In December 2008, there were 187.8 million mobile users in Russia (based on the number of SIM cards sold). The penetration rate of cellular communication (the number of SIM-cards per 100 inhabitants) as of this date was, thus, 129.4%. In the regions, excluding Moscow, the penetration rate exceeded 119.7%.

The market share of the largest cellular operators as of December 2008 was 34.4% for MTS, 25.4% for VimpelCom and 23.0% for MegaFon.

In December 2007, the number of mobile users in Russia increased to 172.87 million subscribers, in Moscow - to 29.9, in St. Petersburg - to 9.7 million.The penetration rate in Russia - up to 119.1%, in Moscow - 176% , St. Petersburg - 153%. The market share of the largest cellular operators as of December 2007 was: MTS 30.9%, VimpelCom 29.2%, MegaFon 19.9%, other operators 20%.

According to the British research company Informa Telecoms & Media for 2006, the average cost of a minute of cellular communication for a consumer in Russia was $ 0.05 - this is the lowest figure among the G8 countries.

IDC, based on a study of the Russian cellular market, concluded that in 2005 the total duration of conversations on a cell phone of residents of the Russian Federation reached 155 billion minutes, and 15 billion text messages were sent.

According to a study by J "son & Partners, the number of SIM cards registered in Russia as of the end of November 2008 reached 183.8 million.

see also

Sources of

Links

• Information site about generations and standards of cellular communication.
• Cellular communications in Russia 2002-2007, official statistics

cellular in Russia
Cellular operators	Utel Akos Altaysvyaz Baikalwestcom Beeline ETK MegaFon Motive MTS NSS NTK SMARTS Sky Link Sonnet Sotel SSB STEC GSM TomskTeleCom UUSS Digital expansion
Cell phone sales networks	Divizion Simphony AltTelecom Banzai Betalink Euroset ION Communication Spectrum Telefon.Ru Teleforum Point Ultra Tsifrograd Eldorado
Cellular standards	D-AMPS GSM IMT-MC-450 IS-95

How cellular communication works

The basic principles of cellular telephony are pretty simple. The FCC originally established geographic coverage areas for cellular radio systems based on modified data from the 1980 census.The idea behind cellular communications is that each area is subdivided into hexagonal cells that combine to form a honeycomb-like structure, as shown. 6.1, a. The hexagonal shape was chosen because it provides the most efficient transmission, approximating the circular pattern while eliminating the gaps that always occur between adjacent circles.

A cell is determined by its physical size, population and traffic patterns. The FCC does not regulate the number of cells in the system and their size, giving operators the ability to set these parameters in accordance with the expected traffic pattern. Each geographic area is allocated a fixed number of cellular voice channels. The physical dimensions of a cell depend on the subscriber density and call structure. For example, large cells (macro cells) typically have a radius of 1.6 to 24 km with a base station transmitter power of 1 W to 6 W. The smallest cells (microcells) usually have a radius of 460 m or less with a base station transmitter power of 0.1 W to 1 W. Figure 6.1b shows a cellular configuration with two cell sizes.

Figure 6.1. - Cellular structure of cells a); honeycomb structure with cells of two sizes b) classification of cells c)

Microcells are most commonly used in regions with a high population density. Due to their small range, microcells are less susceptible to transmission degradation effects such as reflections and signal delays.

A macro cell can overlap with a group of micro cells, with the micro cell serving slow moving mobile devices and the macro cell serving fast moving devices. The mobile device is able to determine the speed of its movement as fast or slow. This makes it possible to reduce the number of hops from one cell to another and the correction of location data.

The transition algorithm from one cell to another can be changed when the distance between the mobile device and the base station of the microcell is small.

Sometimes radio signals in a cell are too weak to provide reliable indoor communications. This is especially true for well-shielded areas and areas with high levels of interference. In such cases, very small cells are used - pico cells. Indoor picocells can use the same frequencies as conventional cells in a given region, especially in favorable environments such as underground tunnels.

When planning systems using hexagonal cells, base station transmitters can be located at the center of the cell, at the edge of the cell, or at the top of the cell (Figure 6.2 a, b, c, respectively). Cells with a transmitter in the center usually use omnidirectional antennas, and cells with transmitters at the edge or at the top use sector directional antennas.

Omnidirectional antennas emit and receive signals the same way in all directions.

Figure 6.2 - Placement of transmitters in cells: in the center a); on the edge b); at the top c)

In a cellular system, one powerful fixed base station located high above the city center can be replaced by multiple, identical, low-power stations that are installed in coverage at sites closer to the ground.

Cells using the same radio channel group can avoid interference if they are properly spaced. In this case, frequency reuse is observed. Frequency reuse is the allocation of the same group of frequencies (channels) to multiple cells, provided that these cells are separated by significant distances. Frequency reuse is facilitated by a reduction in the coverage area of ​​each cell. The base station of each cell is assigned a group of working frequencies that are different from those of neighboring cells, and the base station antennas are selected to cover the desired coverage area within its cell. Since the coverage area is limited to the boundaries of one cell, different cells can use the same group of working frequencies without mutual influences, provided that two such cells are at a sufficient distance from each other.

The geographical coverage area of ​​the cellular system containing several groups of cells is divided into clusters (Figure 6.3). Each cluster consists of seven cells, which are allocated the same number of full duplex communication channels. Cells with the same letter designations use the same operating frequency group. As can be seen from the figure, the same frequency groups are used in all three clusters, which makes it possible to triple the number of available mobile communication channels. Letters A, B, C, D, E, F and G denote seven groups of frequencies.

Figure 6.3 - Principle of frequency reuse in cellular communication

Consider a system with a fixed number of full duplex channels available in an area. Each service area is divided into clusters and receives a group of channels that are distributed between N cells of the cluster, grouping into non-repeating combinations. All cells have the same number of channels, but they can serve one-size zones.

Thus, the total number of cellular communication channels available in the cluster can be represented by the expression:

F = GN (6.1)

where F- the number of full-duplex cellular channels available in the cluster;

G- the number of channels in the cell;

N- the number of cells in the cluster.

If the cluster is "copied" within the specified service area m times, then the total number of full duplex channels will be:

C = mGN = mF (6.2)

where WITH- the total number of channels in a given zone;

m- the number of clusters in a given zone.

From expressions (6.1) and (6.2) it can be seen that the total number of channels in the cellular telephone system is directly proportional to the number of "repetitions" of the cluster in a given service area. If the cluster size decreases, but the cell size remains unchanged, then more clusters will be required to cover a given service area, and the total number of channels in the system will increase.

The number of subscribers who can simultaneously use the same group of frequencies (channels), being not in neighboring cells of a small service area (for example, within a city), depends on the total number of cells in this area. Usually the number of such subscribers is four, but in densely populated regions it can be much higher. This number is called frequency reuse factor or FRF – Frequency reuse factor... Mathematically, it can be expressed by the relation:

(6.3)

where N- the total number of full duplex channels in the service area;

WITH- the total number of full duplex channels in the cell.

With the projected increase in cellular traffic, the increased service demand is met by reducing the size of the cell, dividing it into several cells, each with its own base station. Efficient cell division allows the system to handle more calls, provided the cells are not too small. If the cell diameter becomes less than 460 m, then the base stations of neighboring cells will influence each other. The relationship between frequency reuse and cluster size determines how scale cellular system in the event of an increase in subscriber density. The fewer cells in a cluster, the greater the likelihood of mutual influences between channels.

Since the cells are hexagonal, each cell always has six equidistant neighboring cells, and the angles between lines connecting the center of any cell to the centers of neighboring cells are multiples of 60 °. Therefore, the number of possible cluster sizes and cell layouts is limited. To connect cells to each other without gaps (in a mosaic way), the geometric dimensions of the hexagon must be such that the number of cells in the cluster satisfies the condition:

(6.4)

where N- the number of cells in the cluster; i and j- non-negative integers.

Finding a route to the nearest cells with a co-channel (the so-called cells of the first tier) is as follows:

Moving to i cells (through the centers of neighboring cells):

Moving to j cells forward (through the centers of neighboring cells).

For example, the number of cells in the cluster and the location of the cells of the first tier for the following values: j = 2.i = 3 will be determined from the expression 6.4 (Figure 6.4) N = 3 2 + 3 2 + 2 2 = 19.

Figure 6.5 shows the six nearest cells using the same channels as the cell A.

A handover process from one cell to another, i. E. When the mobile device moves away from base station 1 to base station 2 (Figure 6.6), there are four main steps:

1) initiation - the mobile device or network detects the need for a handover and initiates the necessary network procedures;

2) resource reservation - with the help of appropriate network procedures, network resources are reserved, which are necessary for the transfer of service (voice channel and control channel);

3) execution - direct transfer of control from one base station to another;

4) termination - excess network resources are freed, becoming available to other mobile devices.

Figure 6.6 - Handover

We all use mobile phones, but at the same time hardly anyone thinks - how do they work? In this article we will try to figure out how, in fact, communication is realized with respect to your mobile operator.

When you make a call to your interlocutor, or someone calls you, your phone connects over the radio channel to one of the antennas of the neighboring base station (BS, BS, Base Station).Each base station of cellular communication (in the common people - cell towers) includes from one to twelve transceiver antennas with directions in different directions in order to provide high-quality communication to subscribers within the radius of their operation. Such antennas are called by experts in their own jargon "Sectors", which are gray rectangular structures that you can see almost every day on the roofs of buildings or special masts.

The signal from such an antenna goes through a cable directly to the control unit of the base station. The base station is a collection of sectors and a control block. At the same time, a certain part of a settlement or territory is served by several base stations at once connected to a special block - local zone controller(abbreviated LAC, Local Area Controller or just "controller"). As a rule, one controller combines up to 15 base stations of a certain area.

For their part, controllers (there can also be several of them) are connected to the most important block - Mobile services Switching Center (MSC), which for simplicity of perception is usually called simply "Switch"... The switch, in turn, provides input and output to any communication lines - both cellular and wired.

If you display what is written in the form of a diagram, you get the following:
Small-scale GSM networks (usually regional) can use only one switch. Large ones, such as our operators of the "big three" MTS, Beeline or MegaFon, which simultaneously serve millions of subscribers, use several MSC devices connected to each other at once.

Let's see why such a complex system is needed and why it is impossible to connect base station antennas to the switch directly? To do this, you need to talk about another term, called in technical language handover... It characterizes the handover of service in mobile networks. In other words, when you move along the street on foot or in a vehicle and talk on the phone, so that your conversation is not interrupted, you should promptly switch your device from one BS sector to another, from the coverage area of ​​one base station or controller. local zone to another, etc. Therefore, if the sectors of the base stations were directly connected to the switch, he would have to carry out this handover procedure for all its subscribers himself, and the switch already has enough tasks. Therefore, to reduce the likelihood of equipment failures associated with its overloads, the scheme for constructing GSM cellular networks is implemented according to a multi-level principle.

As a result, if you and your phone move from the coverage area of ​​one BS sector to the coverage area of ​​another, then this movement is carried out by the control unit of this base station, without touching more "high-end" devices - LAC and MSC. If the handover occurs between different BSs, then the LAC is taken over, and so on.

The switch is nothing more than the main "brain" of GSM networks, so its operation should be considered in more detail. A cellular network switch undertakes approximately the same tasks as a PBX in the networks of wire operators. It is he who understands where you are making a call or who is calling you, regulates the work of additional services and, in fact, decides whether you can currently make your call or not.

Now let's see what happens when you turn on your phone or smartphone?

So, you pressed the "magic button" and your phone turned on. There is a special number on the SIM card of your mobile operator, which is called IMSI - International Subscriber Identification Number... It is a unique number for each SIM-card not only for your operator MTS, Beeline, MegaFon, etc., but a unique number for all mobile networks in the world! It is on it that operators distinguish subscribers from each other.

When the phone is turned on, your device sends this IMSI code to the base station, which transmits it further to the LAC, which, in turn, sends it to the switch. In this case, two additional devices come into play, connected directly to the switch - HLR (Home Location Register) and VLR (Visitor Location Register)... Translated into Russian, this, respectively, Home subscribers register and Guest subscriber register... HLR stores the IMSI of all subscribers on its network. The VLR contains information about those subscribers who currently use the network of this operator.

The IMSI number is transmitted to the HLR using an encryption system (another device is responsible for this process AuC - Authentication Center)... At the same time, HLR checks whether a subscriber with a given number exists in its database, and if the fact of its presence is confirmed, the system checks whether he can currently use communication services or, say, has a financial block. If everything is normal, then this subscriber goes to VLR and after that gets the opportunity to call and use other communication services.

For clarity, we will display this procedure using the diagram:

Thus, we have briefly described how GSM cellular networks work. In fact, this description is rather superficial, since if we delve into the technical details in more detail, then the material would have turned out to be many times more voluminous and much less understandable for most readers.

In the second part, we will continue our acquaintance with the operation of GSM networks and consider how and for what the operator debits funds from our account with you.