Mobile cellular

cellular- one of the types of mobile radio communications, which is based on cellular network. Key Feature is that the total coverage area is divided into cells (cells) determined by the coverage areas of individual base stations (BS). The cells partially overlap and together form a network. On an ideal (flat and undeveloped) surface, the coverage area of ​​one BS is a circle, so the network composed of them looks like honeycombs with hexagonal cells (honeycombs).

It is noteworthy that in the English version the connection is called "cellular" or "cellular" (cellular), which does not take into account the hexagonal cells.

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

History

The first use of mobile telephone radio in the United States dates back to 1921 when the Detroit police used one-way dispatch communication in the 2 MHz band to transmit information from a central transmitter to vehicle-mounted receivers. In 1933, the NYPD began using a two-way mobile telephone radio system, also on 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 had completely supplanted amplitude modulation. The first public mobile radiotelephone appeared in 1946 (St. Louis, USA; Bell Telephone Laboratories), it used the 150 MHz band. In 1955, an 11-channel system began operating in the 150 MHz band, and in 1956, a 12-channel system in the 450 MHz band. 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 wearable 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 1960s, Christo Bochvarov demonstrated his prototype of a pocket mobile radiotelephone in Bulgaria. At the exhibition "Interorgtekhnika-66" Bulgaria presents a kit for organizing a local mobile communications from PAT-0.5 and ATRT-0.5 pocket mobile phones and base station RATC-10, providing connection of 10 subscribers.

At the end of the 50s, the development of the Altai car 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 developed in other countries. Thus, in Norway, public telephone radio has been used as maritime mobile communications since 1931; in 1955 there were 27 coastal radio stations in the country. Land mobile communications began to develop after World War II in the form of private hand-switched 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 pace with rapidly growing needs, with a limited number of channels in strictly defined frequency bands. The way out was found in the form of a system cellular communication, which made it possible to dramatically increase the capacity due to the reuse of frequencies in a system with a cellular structure.

Of course, as is usually the case in life, individual elements of the cellular communication system existed before. In particular, some resemblance 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 a cellular communication system was only outlined in a technical report by the Bell System company, submitted to the US Federal Communications Commission in December 1971. And from that time, the development of cellular communication proper began, which became truly triumphant since 1985 in the last ten plus years.

In 1974, the US Federal Communications Commission decided to allocate a 40 MHz frequency band for cellular communications in the 800 MHz band; in 1986 another 10 MHz was added to it in the same range. In 1978, Chicago began testing the first experimental cellular communication system for 2,000 subscribers. Therefore, 1978 can be considered the year of the beginning of the practical application of cellular communications. The first automatic commercial cellular communication system was also put into operation in Chicago in October 1983 by American Telephone and Telegraph (AT&T). Cellular communication has been used in Canada since 1978, in Japan since 1979, in Scandinavian countries (Denmark, Norway, Sweden, Finland) since 1981, in Spain and England since 1982. As of July 1997 Cellular communications operated in more than 140 countries on 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 reached 100% coverage of the territory of Finland in. The size of the cell was about 30 km, in the city it had more than 30 thousand subscribers. She worked at a frequency of 150 MHz.

The principle of operation of cellular communication

The main components of the cellular network are cell phones and base stations. Base stations are usually located on the roofs of buildings and towers. When turned on, the cell phone listens to the air, finding a signal from the base station. The phone then sends its unique identification code to the station. The telephone and the station maintain constant radio contact, periodically exchanging packets. Communication between the phone and the station can go on 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 the network 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 enter into roaming agreements. Thanks to such contracts, the subscriber, while abroad, can make and receive calls through the network of another operator (though 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 in St. Petersburg, Delta Telecom launched the first cellular network in Russia (it worked in the NMT-450 standard) and a symbolic cellular call by the mayor of St. Petersburg, Anatoly Sobchak. By July 1997, the total number of subscribers in Russia was about 300,000. For 2007, the main cellular communication protocols used in Russia are GSM-900 and GSM-1800. In addition, UMTS also 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 MegaFon. In the Sverdlovsk region, the DAMPS standard cellular communication network, owned by the Motiv Mobile Communications company, continues to operate.

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

Market share of the largest mobile operators as of December 2008, it was: 34.4% for MTS, 25.4% for VimpelCom and 23.0% for MegaFon.

In December 2007, the number of cellular communication users in Russia grew to 172.87 million subscribers, in Moscow - up to 29.9, in St. Petersburg - up to 9.7 million. The level of penetration in Russia - up to 119.1%, 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 data of 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 Research Russian market cellular communication concluded that in 2005 the total duration of conversations on the cell phone of the inhabitants of the Russian Federation reached 155 billion minutes, and text messages 15 billion pieces were shipped.

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

see also

Sources

Links

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

It is hardly possible today to find a person who would never use a cell phone. But does everyone understand how cellular communication works? How is it arranged and how does what we all have long been accustomed to work? Are signals from base stations transmitted over wires, or does it all work in some other way? Or maybe all cellular communication functions only due to radio waves? We will try to answer these and other questions in our article, leaving the description of the GSM standard beyond its scope.

At the moment when a person tries to make a call from his mobile phone, or when they start calling him, the phone connects via radio waves to one of the base stations (the most accessible), to one of its antennas. Base stations can be observed here and there, looking at the houses of our cities, at the roofs and facades of industrial buildings, at skyscrapers, and finally at red-white masts specially erected for stations (especially along highways).

These stations look like rectangular boxes. gray color, of which various antennas stick out in different directions (usually up to 12 antennas). The antennas here work both for reception and for transmission, and they belong to the mobile operator. Base station antennas are directed in all possible directions (sectors) to provide “network coverage” to subscribers from all sides at a distance of up to 35 kilometers.

An antenna of one sector is able to serve up to 72 calls simultaneously, and if there are 12 antennas, then imagine: 864 calls can, in principle, be served by one large base station at the same time! Although usually limited to 432 channels (72 * 6). Each antenna is connected by cable to the control unit of the base station. And already blocks of several base stations (each station serves its own part of the territory) are attached to the controller. Up to 15 base stations can be connected to one controller.

The base station, in principle, is capable of operating on three bands: the 900 MHz signal penetrates better inside buildings and structures, spreads further, so this particular band is often used in villages and fields; the signal at a frequency of 1800 MHz does not spread so far, but more transmitters are installed in one sector, so such stations are more often installed in cities; finally 2100 MHz is a 3G network.

Of course, there may be several controllers in a settlement or district, so the controllers, in turn, are connected by cables to the switch. The task of the switch is to connect the networks of mobile operators with each other and with city lines of ordinary telephone communication, long distance communication and international communications. If the network is small, then one switch is enough; if it is large, two or more switches are used. The switches are interconnected by wires.

In the process of moving a person talking on a mobile phone along the street, for example: he walks, rides in public transport, or moves in a personal car, his phone should not lose the network for a moment, you cannot cut off the conversation.

Communication continuity is obtained due to the ability of the base station network to very quickly switch the subscriber from one antenna to another in the process of moving from the coverage area of ​​one antenna to the coverage area of ​​another (from cell to cell). The subscriber himself does not notice how he ceases to be connected with one base station, and is already connected to another, how he switches from antenna to antenna, from station to station, from controller to controller ...

At the same time, the switch provides optimal load distribution over a multi-layer network scheme in order to reduce the likelihood of equipment failure. A multilevel network is built like this: cell phone - base station - controller - switch.

Let's say we make a call, and now the signal has already reached the switch. The switch transfers our call towards the destination subscriber - to the city network, to the international or long-distance communication network, or to the network of another mobile operator. All this happens very quickly using high-speed fiber optic cable channels.

Further, our call arrives at the switchboard, which is located on the side of the receiving call (called by us) subscriber. The "receiving" switch already has data about where the called subscriber is located, in what network coverage area: which controller, which base station. And so, the network polling begins from the base station, the addressee is found, and a call “receives” on his phone.

The entire chain of the described events, from the moment of dialing the number to the moment the call is heard on the receiving side, usually lasts no more than 3 seconds. So we can now call anywhere in the world.

Andrey Povny

How many of us wonder what happens after we press the call button on a mobile phone? How do cellular networks work?

Most likely no. Most often, we dial the federal number of the interlocutor on the machine, as a rule, on business, so what is there and how it works does not interest us at a particular point in time. But these are amazing things. How can you call a person who is in the mountains or in the middle of the ocean? Why during a conversation we can hear each other badly, or even completely interrupt. Our article will try to shed light on the principle of cellular communications.

So, most of the densely populated territory of Russia is covered by the so-called BS, which, without abbreviation, are called Base Stations. Many could turn their attention to them, traveling between cities. In the open field, base stations are more like towers that have red and White color. But in the city, such BS are thoughtfully placed on the roofs of non-residential skyscrapers. These towers are able to pick up a signal from any cell phone located territorially within a radius of no more than 35 kilometers. "Communication" between the BS and the phone occurs through a special service or voice channel.

As soon as a person dials the number he needs on a mobile device, the device finds the Base Station closest to him, therefore, a special service channel and asks her to allocate a voice channel. The tower, after receiving a request from the device, sends a request to the so-called controller, which we will call BSC for short. This same controller redirects the request to the switch. The smart switch MSC will determine which operator the called subscriber is connected to.

If it turns out that the call is made to a phone within the same network, for example, from a Beeline subscriber to another subscriber of this operator, or within MTS, within Megafon, and so on, the switch will begin to find out the location of the called subscriber. Thanks to the Home Location Register, the switch will find where the right person is located. It can be anywhere, at home, at work, in the country or even in another country. This will not prevent the switch from transferring the call to the appropriate switch. And then the "tangle" will begin to "unwind". That is, the call from the switch - "answerer" will go to the controller - "responder", then to its Base Station and to the mobile phone, respectively.

If the switch finds out that the called subscriber belongs to another operator, it will send a request to the switch of another network.
Agree, the scheme is quite simple, but it is difficult to imagine. How the "smart" Base Station finds the phone, sends a request, and the switch itself determines the operator and the other switch. What is a base station, really? It turns out that these are several iron cabinets that are located either under the very roof of the building, in the attic or in a special container. The main condition is that the room must be perfectly air-conditioned.

It is logical that the BS has an antenna, which helps it "catch" the connection. The BS antenna consists of several parts (sectors), each of which is responsible for the territory. The part of the antenna that is located vertically is responsible for communication with mobile phones, and the round one is for communication with the controller.

One sector is able to simultaneously receive calls from seventy telephone sets. If we take into account that one BS can consist of six sectors, then at the same time it will easily serve 6 * 72 = 432 calls.

As a rule, such power of the Base Station is enough "with a head". Of course, there are situations when the entire population of our country begins to call each other at the same time. This New Year. Some just need to say the cherished phrase “Happy New Year!” into the phone, while others are ready to pronounce hours with an unlimited tariff from Communications Corporation, discussing guests and plans for the whole night.

However, regardless of the duration of the call, the Base Stations cannot cope, and it can be very difficult to get through to the subscriber. But on weekdays, for most of the year, BS from six sectors is quite enough, especially for optimal workload, the operator selects Stations in accordance with the population of the territory. Some operators give their preference to large BS in order to improve the quality of the communication provided.

There are three ranges in which the BS can operate and which determine the number of supported devices and the distance covered. In the 900 MHz band, the station is able to cover a large area, but in the 1800 MHz band, the distance will be significantly reduced, but the number of connected transmitters will increase. The third band at 2100 MHz already assumes a new generation connection - 3G.
It is clear that in sparsely populated areas it is more expedient to set the Base Station at 900 MHz, but in the city 1800 MHz is suitable in order to better penetrate through thick concrete walls, and these BSs will need ten times more than in the village. Note that one BS can support three bands at once.

Stations in the 900 MHz mode cover an area with a radius of 35 km, but if this moment since it serves few phones, it can "break through" up to 70 km. Naturally, our mobile phones can "find" the BS even at a distance of 70 km. Base Stations are designed to cover the earth's surface as much as possible and provide a large number of people with communication on the ground, therefore, if it is possible to catch signals at a distance of at least 35 kilometers, at the same distance, but into the sky, Base Stations do not "break through".

In order to provide their passengers with cellular communications, some airlines are starting to place small base stations on board aircraft. The connection of the "heavenly" Base Station with the "terrestrial" is carried out using a satellite channel. Since work mobile devices can interfere with the flight process, onboard BS can easily be turned on / off, they have several modes of operation, up to complete shutdown transmission of voice messages. During the flight, the phone may accidentally be transferred to a base station with a poorer or no signal. free channels. In this case, the call will be terminated. All these are the subtleties of cellular communication in the sky in motion.

In addition to aircraft, some problems arise for residents of penthouses. Even unlimited tariff and VIP - the conditions of the mobile operator will not help in the case of different BS. A resident of an apartment on a high floor, moving from one room to another, will lose connection. This may be due to the fact that the phone in one room "sees" one BS, and in another it "discovers" another. Therefore, during a conversation, the connection is interrupted, since these BSs are at a relative distance from each other and are not even considered "neighboring" by one operator.

Structural scheme GSM cellular phone

The block diagram of a cellular radiotelephone operating in the GSM digital standard (Fig. 5.3) consists of analog and digital parts, which are usually located on separate boards. The analog part includes receiving and transmitting devices, which, in their characteristics and construction, resemble those described above.

In GSM systems, the transmitter and receiver of a cell phone do not operate simultaneously. Transmission occurs only for 1/8 of the frame duration. This significantly reduces battery consumption and increases the operating time in both transmit (talk) and receive (standby) modes. In addition, the requirements for the SAW receiver RF filter are significantly reduced, which makes it possible to integrate the LNA with the mixer. The transmit-receive interface unit is an electronic switch that connects the antenna either to the output of the transmitter or to the input of the receiver, since a cell phone never receives and transmits at the same time.

Rice. 5.3. Functional diagram radiotelephone digital standard GSM

The received signal after passing through the input band-pass filter is amplified by the LNA and fed to the first input of the first mixer. The second input receives a local oscillator signal f prm from the frequency synthesizer. First intermediate frequency signal f pr, passes through a SAW bandpass filter and is amplified by the amplifier of the first intermediate frequency UPCH1, after which it enters the first input of the second mixer. Its second input receives a local oscillator signal f g with a frequency generator. Received signal of the second intermediate frequency f pr2 is filtered by a SAW bandpass filter, amplified by the UPCH2 amplifier, demodulated and fed to an analog-to-digital converter (ADC), where it is converted into a signal necessary for the operation of a digital logic block made on the CPU.

In the transmission mode, the information digital signal generated in the logic block is fed to the 1/O generator, where the modulating signal is formed. The latter enters the phase modulator, from which the signal f fm enters the mixer. The second input of the mixer receives a signal f prd from a frequency synthesizer. Received signal f c1 through a band-pass filter enters the power amplifier (PA), controlled by the CPU. Signal amplified to the required level f c1 through a band-pass ceramic filter enters antenna A and is radiated into the surrounding space.

The digital logic part of a cell phone (Fig. 5.4) provides the formation and processing of all necessary signals. The core of this important part of the digital phone is CPU CPU. It is made in the form of VLSI on micropower field effect transistors with the structure "metal-dielectric-semiconductor" (MIS or MOS).

The digital part of the phone includes:

Digital Signal Processor (CPU) with its operational and permanent memory, which controls the operation of a cell phone. Phone CPUs are somewhat simpler than computer microprocessors, but nevertheless they are the most complex microelectronic products.

Analog-to-digital converter (ADC), which converts the analog signal from the microphone output to digital form. In this case, all subsequent processing and transmission of the speech signal is carried out in digital form, up to the reverse digital-to-analog conversion.

speech encoder, which encodes a speech signal, which is already digital, according to certain laws using a compression algorithm to reduce signal redundancy. Thus, the volume of information that must be transmitted over the radio communication channel is reduced.

channel encoder, adding additional (redundant) information to the digital signal received from the output of the speech encoder, designed to protect against errors during signal transmission over the communication line. For the same purpose, information is subjected to a certain repackaging. (interleaving). In addition, the channel encoder adds control information from the logic part to the transmitted signal.

channel decoder, extracting control information from the input data stream and directing it to the logical block. The received information is checked for errors, which are corrected if possible. For subsequent processing, the received information is repackaged inversely with respect to the encoder.

Rice. 5.4. Digital and logic part of a mobile cell phone

speech decoder, restoring the digital speech signal coming to it from the channel decoder, translating it into a natural form, with its inherent redundancy, but still in digital form. Note that for a combination of an encoder and a decoder located in the same package of an integrated circuit, the name is sometimes used codec(eg speech codec, channel codec).

Digital-to-analog converter (DAC), converting the received speech signal into analog form and feeding this signal to the speaker amplifier input.

Equalizer, serving to partially compensate for signal distortion due to multipath propagation. The equalizer is an adaptive filter that is adjusted according to the training sequence of symbols included in the transmitted information. This block, generally speaking, is not functionally necessary and may be absent in some cases.

Keyboard, which is a dialing field with numeric and function keys for dialing the number of the called subscriber, as well as commands that determine the mode of operation of the cell phone.

Display, display clerk various information provided by the device and operating mode of the station.

Block for encrypting and decrypting messages, designed to ensure the confidentiality of information transfer.

Speech activity detector(voice activity detector), which turns on the transmitter for radiation only for those time intervals when the subscriber speaks. For the duration of the pause in the operation of the transmitter, the so-called comfort noise is additionally introduced into the path. This is done in the interests of saving power from the power supply, as well as reducing the level of interference to other stations.

terminal devices, used to connect through special adapters using the appropriate interfaces, fax machines, modems, etc.

SIM card(SIM - subscriber identification module, literally - subscriber identification module) - a plastic plate with a microcircuit inserted into a special socket of the subscriber unit. The SIM card stores:

Data assigned to each subscriber: international mobile subscriber identity (IMSI), subscriber authentication key (Ki) and access control class;

Temporary Network Data: Temporary Mobile Subscriber Identity (TMSI), Location Area Identifier (LAI), Encryption Key (Ke), Denied Mobile Network Data;

Service-related data: preferred language of communication, billing notices, and list of claimed services.

One of the main tasks of a SIM card is to provide protection against unauthorized use of a cell phone. At the subscriber interface level, a personal identification number (PIN number) of 4 to 8 digits is recorded on the SIM card, which the SIM card microprocessor, after turning on the station, compares with the number dialed by the user using the keyboard. If an incorrect PIN number is dialed three times in a row, the use of the SIM card is blocked until the subscriber enters an 8-digit Personal Unblocking Key (PUK).

If an erroneous PUK is entered 10 times in a row, the use of the SIM card is completely blocked and the subscriber will be forced to contact the network operator.

In addition, thanks to SIM-cards, it is possible to make calls not only from your cell phone, but also from any other GSM phone, just insert the SIM-card into the device and dial a personal identification PIN-number.

5.3 Cellular Services. Communication privacy. Fraud in cellular communication. biological safety.

In second generation systems, the user can be provided with basic and additional communication services. Basic communication services: telephone communications, emergency calls, short message transmission, facsimile communication. The emergency call service allows the subscriber station to establish voice communication with the nearest center emergency service. TO additional services connections include:

number recognition services;
call forwarding and redirection;
· termination services (call on hold, call with waiting, etc.);
conference call;
services for accounting for the cost of negotiations;
group connection services;
call restriction services, etc.

In the context of competition for the subscriber, operators of large networks are trying to introduce new services. Recently, services such as prepaid subscriber connection, WAP service - Internet access directly from a mobile terminal, GPS global positioning system, video communication, etc. have been introduced. But such opportunities appeared with the advent of communicators (smartphones).

Communication Privacy provided with protection against unauthorized access to communication channels. Various encryption methods are used for this. For example, in the GSM standard, encryption is carried out by noise-correcting coding and interleaving and consists in bitwise addition modulo 2 of the information bit sequence and the pseudo-random bit sequence that forms the basis of the cipher. Repeated application of the modulo 2 addition operation with the same pseudo-random sequence to the encrypted information sequence restores the original information bit sequence, that is, it implements the decryption of the encrypted message (Fig.).

There is also the possibility of protection against eavesdropping - this is scrambling (scrambling - mixing, shuffling), which is a kind of encryption by rearranging sections of the spectrum or speech segments, carried out in an external software

Fig.5.5. The principle of encryption and decryption of information in the GSM standard.

towards a mobile phone device with appropriate descrambling at the receiving end.

Fraud(from English. fraud- deceit, fraud) is one of the serious problems of cellular communications. Fraud can be defined as an illegal activity aimed at the use of cellular communication services without proper payment or at the expense of payment for these services by people who do not use such services.

From time to time, the world and our press are shocked by reports of cell phone scams. The most unpleasant thing is when a cell phone registered for someone falls into the hands of scammers who are able to deceive cellular providers and carry out large-scale negotiations uncontrollably. Sometimes primitive methods are used for this (for example, malicious non-payments), and sometimes very subtle methods based on excellent knowledge of documentation on cellular networks. Practiced alteration of cell phone numbers and all sorts of "chemistry" with ciphers and passwords.

Losses from fraud, even after many years of fighting against it, reach several percent of the total volume of cellular services. For example, in 1996 in the United States they amounted to just over $1 billion, with a total income from cellular communications of $21 billion. .

If you have a suspicion that someone is using (explicitly or implicitly) your device, you must immediately notify your mobile service provider. For example, such a suspicion may be based on a noticeable increase in the volume of payment for cellular services compared to the level you are used to. If you do not control what happened, then you can suddenly receive a bill for hundreds, if not thousands of dollars. And you will be embroiled in a long legal battle with an unclear outcome.

In addition to fraud, the sale of “gray” phones causes enormous damage to cellular communications. These can be defective devices purchased cheaply, which are then handicraft brought to working condition - often far from all functionality. Such devices cause a lot of trouble not only to their owners, who are looking for cheapness, but also to mobile operators. For, performing poorly (or not performing at all) many functions, they cause a flurry of calls to service departments.

Eavesdropping on cell phones is also far from a harmless thing. Analog networks are especially vulnerable to this. But in digital networks, even with the appropriate equipment for encoding and decoding conversations, eavesdropping on them is also quite possible. This is something to keep in mind when talking.

The methods of illegal use of cell phones are varied, although there is an opinion that it is necessary to know about it. Just to what extent? For example, it is clear to anyone that a cell phone can be used as a very simple radio detonator. However, a description of even a simple scheme for such an application can hardly be welcomed. The relevant authorities can instantly recognize this as a benefit for terrorists. Therefore, having warned the user about the gaps in the legal use of cell phones, we will end the description of these subtle points in the use of mobile phones.

biological safety.

From time to time there is sensational news about the development of cancerous tumors from cell phone use. Somewhere in the US, there were even lawsuits about this. There are also reports of explosions in car parks during refueling of cars, about planes that have gone astray, about reactors of nuclear power plants that have stopped due to cell phones, and so on. In the overwhelming majority of cases, such "news" is not documented.

In fact, cellular frequencies refer to the type of electromagnetic radiation that is easily absorbed by the tissues of our hands, head and brain. Studies have shown that up to 60% of the radiation energy of a cell phone is absorbed by the tissues of the human head. True, only part of the energy of microwave radiation enters deep into the head. Most of it is absorbed by the skin and bones of the skull.

Meanwhile, there are no official data on any effect of cell phone radiation on the human body. And not because the relevant studies have not been conducted. But because the norms for radiation power are much less than those norms that were established for people by the relevant authorities.

The degree of absorption of electromagnetic radiation energy by the human body is the SAR value (Specific Absorption Rates). It is expressed in the energy of absorbed radiation per unit mass (g or kg) of biological tissue. At the same time, in 20 minutes of exposure, the tissue heats up by 1 °C.

It is not difficult to understand that such a purely "thermodynamic" approach is by no means conducive to reassuring people. For one does not need to have extensive medical knowledge to believe that the effect of radiation is by no means limited to heating the tissues of the body. It must be taken into account that at the genetic level, much less powerful radiation can cause a violation of the cellular structure of the body or damage to genes. Therefore, in Europe, for example, the SAR standard is set at 2 mW/g.

By the way, there is a simple way to drastically reduce the impact of mobile phone radio emission on the human body, and above all on his head. This is the use of a special headset hands free (free hands). This headset is a head-mounted earpiece and microphone, as well as a radiotelephone control panel. The phone itself can be installed remotely. It is also possible to connect an external antenna to it, which can be installed outside the window or even on the roof of the car.

By the way, of all the dangers associated with cell phones, in the first place is the distraction of the user from their main work. For example, car accidents associated with the fact that the driver picks up the phone while driving, and especially when he dials a number, are very frequent. In many countries, including Russia, this is prohibited and punishable by fines. hands free headset and voice control telephone - these are the main means against this factor.

test questions

1. What are the typical blocks of a subscriber mobile station?

2. Tell us the device and the main purpose of the analog mobile phone nodes?

3. Tell us the device and the main purpose of digital mobile phone nodes?

4. Define "fraud" and why is it dangerous?

5. List the main measures aimed at reducing the impact of cellular radiation on the human body?

6. What are the main symptoms of the disease caused by radio emission?

7. List the main services provided by cellular communication?

8. How is the confidentiality of communications in mobile networks?

The principle of operation of cellular communication

The basic principles of cellular telephony are quite simple. Initially, the FCC established geographic coverage areas for cellular radio systems based on revised 1980 Census data. The idea behind cellular communications is that each area is subdivided into hexagonal cells that, when combined, form a honeycomb-like structure, as shown in the figure. 6.1, a. The hexagonal shape was chosen because it provides the most efficient transmission by approximately matching the circular radiation pattern while eliminating the gaps that always occur between adjacent circles.

A cell is defined by its physical size, population, and traffic pattern. The FCC does not regulate the number of cells in the system and their size, leaving operators 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 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 (micro cells) typically 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 honeycomb configuration with two cell sizes.

Figure 6.1. – Honeycomb structure of cells a); honeycomb structure with honeycombs of two sizes b) classification of honeycombs c)

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

A macro cell may overlap with a group of micro cells, with the micro cells 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 algorithm of transition from one cell to another can be changed at small distances between the mobile device and the base station of the microcell.

Sometimes the 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 level interference. In such cases, very small cells are used - pico cells. Indoor pico cells can use the same frequencies as regular cells in a given region, especially in favorable environments such as in underground tunnels.

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

Omnidirectional antennas radiate and receive signals equally in all directions.

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

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

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

The geographic service area of ​​a cellular system containing multiple cell groups 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 group of operating frequencies. 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 represent seven groups of frequencies.

Figure 6.3 – The principle of frequency reuse in cellular communications

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

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

F=GN (6.1)

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

G– number of channels in a cell;

N is the number of cells in the cluster.

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

C=mGN=mF (6.2)

where FROM– total number of channels in a given zone;

m is the number of clusters in a given zone.

It can be seen from expressions (6.1) and (6.2) that the total number of channels in a cellular telephone system is directly proportional to the number of cluster "repetitions" in a given service area. If the cluster size decreases while the cell size remains the same, 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) while not in neighboring cells of a small service area (for example, within a city) depends on the total number of cells in this area. Typically, 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 as:

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

FROM– total number of full duplex channels in the cell.

With the predicted increase in cellular traffic, the increased demand for service is met by reducing the size of the cell, dividing it into several cells, each of which has its own base station. Efficient cell separation allows the system to handle more calls as long as the cells are not too small. If the cell diameter becomes less than 460 m, then the base stations of adjacent cells will influence each other. The relationship between frequency reuse and cluster size determines how one can change scale cellular system in the event of an increase in subscriber density. The fewer cells in a cluster, the greater the likelihood of crosstalk between channels.

Because 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:

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

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

Move on i cells (through the centers of neighboring cells):

Move on 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 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 BUT.

The process of handover from one cell to another, ie. when the mobile device moves away from base station 1 to base station 2 (Figure 6.6) includes four main stages:

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, the network resources necessary for handover (voice channel and control channel) are reserved;

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

4) ending - redundant network resources released, becoming available to other mobile devices.

Figure 6.6 – Handover