The principle of radio communication

Radio (lat.radio- emit, emit rays radius-beam) - a variety wireless communication, in which radio waves freely propagating in space are used as a signal carrier.

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

Frequency bands
The frequency grid used in radio communications is conditionally 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 \u003d 3-30 MHz (l \u003d 100-10 m)
• Ultrashort waves (VHF) - f = 30 MHz - 300 MHz (l = 10-1 m)
• High frequencies (HF - centimeter range) - f \u003d 300 MHz - 3 GHz (l \u003d 1-0.1 m)
• Extremely high frequencies (EHF-millimeter range) - f \u003d 3 GHz - 30 GHz (l \u003d 0.1-0.01 m)
• Hyper high 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:

• DWs are strongly absorbed by the ionosphere; ground waves, which propagate around the earth, are of primary importance. Their intensity decreases relatively quickly with increasing distance from the transmitter.
• SWs 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 propagate exclusively by reflection by the ionosphere, so there is a so-called radio silence zone around the transmitter. Shorter waves (30 MHz) propagate better during the day, longer ones (3 MHz) at night. Short waves can propagate over long distances with low transmitter power.
• VHF propagate rectilinearly and, as a rule, are not reflected by the ionosphere. Easily bend around obstacles and have a high penetrating power.
• HF do not go around obstacles, spread within the line of sight. Used in WiFi, cellular communications, etc.
• EHF do not go around obstacles, are reflected by most obstacles, and propagate within the line of sight. Used for satellite communications.
• Hyper-high frequencies do not go around obstacles, are reflected like light, and propagate within the line of sight. Use is limited.

Propagation of radio waves
Radio waves propagate in the void and in the atmosphere; the earthly firmament and water are opaque to them. However, due to the effects of diffraction and reflection, communication is possible between points on the earth's surface that do not have direct visibility (in particular, located on long distance).
The propagation of radio waves from a source to a receiver can occur in several ways simultaneously. This propagation is called multipath. Due to multipath and changes in the parameters of the environment, fading occurs - a change in the level of the received signal over time. With multipath, a change in the signal level occurs due to interference, that is, at the receiving point, the electromagnetic field is the sum of time-shifted radio waves in 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 related and somewhat overlapping term is radio navigation, but in radio navigation a more active role is played by the object whose coordinates are being measured, most often this is the determination of own coordinates. The main technical device of radar is a radar station (eng. Radar).

Distinguish between active, semi-active, active with a passive response and passive RL. They are divided according to the used range of radio waves, according to the type of 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 on the electrical inhomogeneities encountered on the path of their propagation (objects with other electrical properties that are different 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 source of radiation, it can be assumed that radio waves propagate rectilinearly 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 - Propagation of radio waves. In radar these deviations lead to measurement errors).
• The frequency of the received signal differs from the frequency of the emitted oscillations when the points of reception and radiation are mutually moved (Doppler effect), which makes it possible to measure the radial velocities of the target relative to the radar.
• Passive radar uses the radiation of electromagnetic waves by 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. 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).

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

The principle of operation 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 phone then sends its unique message to the station. identification code. 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 goes out of 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 the network and improve its coverage.

Cellular networks different operators connected to each other, as well as to the stationary 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 contracts, 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 carried out at increased 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 operating costs.

Cellular services

Cellular operators provide the following services:

• Voice call;
• Answering machine in cellular communication (service);
• Roaming;
• AON (Automatic Caller ID) and AntiAON;
• Receiving and sending short text messages (SMS);
• Reception and transmission of multimedia messages - images, melodies, videos (MMS service);
• Mobile bank (service);
• Internet access;
• Video call and video conference

TV

TV(Greek τήλε - far and lat. video- I see; from New Latin televisio- far vision) - a set of devices for transmitting a moving image and sound over a distance. In everyday life, it is also used to refer to organizations involved in the production and distribution of television programs.

Basic principles

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

The television path in general includes the following devices:

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

In addition, to create a television transmission, an audio path is used, similar to the radio transmission path. Sound is transmitted on a separate frequency, usually using frequency modulation, using a technology similar to FM radio stations. In digital television, sound, often multi-channel, is transmitted in a common data stream with the image.

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In the theoretical part, I will not delve into the history of the creation of cellular communications, about its founders, the chronology of standards, etc. To whom it is interesting - there is plenty of material both in print publications and on the Internet.

Consider what a mobile (cellular) phone is.

The figure shows the principle of operation in a very simplified way:

Fig.1 The principle of operation of a cell phone

A cell phone is a transceiver operating at one of the frequencies in the range of 850 MHz, 900 MHz, 1800 MHz, 1900 MHz. Moreover, the reception and transmission are separated by frequencies.

GSM system consists of 3 main components, such as:

Base station subsystem (BSS - Base Station Subsystem);

Switching/switching subsystem (NSS – NetworkSwitchingSubsystem);

Operation and Maintenance Center (OMC)

In a nutshell, it works like this:

Cellular (mobile) phone interacts with a network of base stations (BS). BS towers are usually installed either on their ground masts, or on the roofs of houses or other structures, or on rented existing towers of all kinds of radio / TV repeaters, etc., as well as on high-rise pipes of boiler houses and other industrial structures.

The phone, after turning on and the rest of the time, monitors (listens, scans) the air for the presence of a GSM signal from its base station. The phone determines the signal of its network by a special identifier. If there is one (the phone is in the network coverage area), then the phone selects the frequency that is best in terms of signal strength and sends a request to the BS to register with the network at this frequency.

The registration process is essentially an authentication (authorization) process. Its essence lies in the fact that each SIM card inserted into the phone has its own unique identifiers IMSI (International Mobile Subscriber Identity) and Ki (Key for Identification). These same IMSI and Ki are entered into the Authentication Center (AuC) database upon receipt of the manufactured SIM cards by the telecom operator. When registering a phone in the network, the identifiers are transmitted by the BS, namely AuC. Then AuC (Identification Center) sends a random number to the phone, which is the key to perform calculations using a special algorithm. This calculation takes place simultaneously in the mobile phone and AuC, after which both results are compared. If they match, then the SIM card is recognized as genuine and the phone is registered on the network.

For a phone, the identifier on the network is its unique number IMEI (International Mobile Equipment Identity). This number usually consists of 15 digits in decimal notation. For example 35366300/758647/0. The first eight digits describe the model of the phone and its origin. Remaining - serial number phone and check number.

This number is stored in the phone's non-volatile memory. In older models, this number can be changed using special software (software) and the corresponding programmer (sometimes a data cable), and in modern phones it is duplicated. One copy of the number is stored in the memory area that can be programmed, and the duplicate is stored in the OTP (One Time Programming) memory area, which is programmed by the manufacturer once and cannot be reprogrammed.

So, even if you change the number in the first memory area, then the phone, when turned on, compares the data of both memory areas, and if different numbers IMEI - the phone is blocked. Why change all this, you ask? In fact, the laws of most countries prohibit this. Phone by IMEI number is tracked on the network. Accordingly, if the phone is stolen, it can be tracked and seized. And if you have time to change this number to any other (working) number, then the chances of finding a phone are reduced to zero. These issues are dealt with by special services with the appropriate assistance of the network operator, etc. Therefore, I will not delve into this topic. We are interested in the purely technical moment of changing the IMEI number.

The fact is that under certain circumstances this number can be damaged as a result of a software failure or incorrect update, and then the phone is absolutely unusable. This is where all the means come to the rescue to restore the IMEI and the device's performance. This point will be discussed in more detail in the software repair section of the phone.

Now briefly about voice transmission from subscriber to subscriber in the GSM standard. In fact, this is a technically very complex process, which is completely different from the usual voice transmission over analog networks, such as a home wired / radio telephone. Digital DECT radiotelephones are somewhat similar, but the implementation is still different.

The fact is that the subscriber's voice, before it is broadcast, undergoes many transformations. The analog signal is divided into segments with a duration of 20ms, after which it is converted to digital, after which it is encoded using encryption algorithms with the so-called. public key– EFR system (Enhanced Full Rate - an advanced speech coding system developed by the Finnish company Nokia).

All codec signals are processed by a very useful algorithm based on the principle of DTX (Discontinuous Transmission) - discontinuous speech transmission. Its usefulness lies in the fact that it controls the phone's transmitter, turning it on only at the moment when speech begins and turning it off in the pauses between conversations. All this is achieved with the help of the VAD (Voice Activated Detector) included in the codec - a speech activity detector.

At the received subscriber, all transformations occur in the reverse order.

Mobile phone device and its main functional units (modules).

Any mobile phone- it's complicated technical device, consisting of many functionally complete modules that are interconnected and generally ensure the normal operation of the device. The failure of at least one module entails a minimum - a partial malfunction of the device, a maximum - the phone is completely inoperative.

Schematically, a mobile phone looks like this:

Fig.2 Cell phone device

Purpose and operation of individual nodes.

1. Rechargeable battery (battery)- the main (primary) power source of the phone. During operation, it has one unpleasant property - aging, i.e. loss of capacitance, increase in internal resistance. This is an irreversible process and the rate of battery aging depends on many factors, the key of which is proper operation and storage.

Previously, the bulk of batteries for phones were produced using NiCd (nickel and cadmium-based), NiMH (nickel metal hydride) technologies. These batteries are currently discontinued. With the spread of batteries based on Li-Ion (lithium-ion) technology, the latter have shown best ratio price-quality, and also had a number of advantages, in particular the absence of the so-called. "memory effect". The service life is approximately 3-4 years. Not so long ago, Li-Pol (lithium polymer) batteries appeared on the market. They are cheaper than lithium-ion ones, but they also have a shorter service life - about 2 years.

Modern batteries are recognized as efficient if they have retained at least 80% of the nominal capacity. In practice, there are batteries with 50% or less. That is, many users try to “squeeze” the last milliamps out of the battery, which is why they themselves then suffer, since a worn-out battery often begins to swell, which can lead to breakdowns of the phone case, and sometimes even to failure of the network charger, circuits phone charging, power controller. So, it's not worth saving money on the battery. The phone needs good power too

Batteries do not require special care. The main thing is to prevent hypothermia in winter (up to -10 ° C), because. accelerated discharge and aging. As well as heating up to 50-60 ° C and above. This is dangerous - the battery can simply swell and even explode (this is critical for lithium batteries) !!!

The battery of a mobile phone consists of 2 parts: the battery itself and a small electronics-automatic board.

Fig.3 Device battery

In the figure, for clarity, I showed an already damaged swollen battery. Most often this happens as a result of using cheap chargers, in case of malfunctions in the phone charging circuit, as well as in case of high charging currents selected by the manufacturer (to reduce the battery charge time). And, of course, cheap non-original batteries “get fat” very quickly.

As for the electronics board, it performs protective function, preventing both the battery itself and the phone from emergency situations such as:

Short circuit (short circuit) of the battery supply terminals;

Battery overheating during charging and operation;

Battery discharge below the established minimum allowable rate;

Battery recharge;

When one of them occurs, the so-called. the electronic relay and the battery output terminals are de-energized.

As a rule, a modern battery has at least 3 contact terminals for connecting to a mobile phone's battery connector. These are respectively “+”, “-”, and “TEMP” (temperature sensor, with which the battery controller, together with the phone’s power controller, control the battery charging process, reducing or increasing the charging current, and in case of overheating or short circuit, disconnect the battery from the board terminals altogether electronics).

Fig.4 Location of battery contacts

It should be noted that at different manufacturers Contact arrangement may vary!

The main characteristics of the battery are:

Rated voltage- usually 3.6 - 3.7 volts. For a fully charged battery 4.2 - 4.3 volts.

- capacity - for modern phones from about 700mA to 2000mA or more.

Internal resistance - the lower the better (up to about 200 milliohms)

2. Power controller- serves to convert the battery voltage into several types of voltage to power individual components and devices of the phone, such as the CPU (central processing unit), RAM and ROM (memory chips), all kinds of amplifiers, sometimes keyboard and display backlights, etc., and also controls the battery charging process. Together with the processor, it activates the built-in or external amplifiers the sound of the conversational speaker, microphone, buzzer (polyphonic loudspeaker). Plus, it provides data exchange with a SIM card.

Structurally made in the form of a separate chip. Sometimes it can be combined with a processor (Chinese fakes of famous brands like Nokia N95, etc.)

During normal use of the phone, the power controller rarely fails. Most often this happens during overheating charging or when using a non-original or faulty charger (charger). Less often - if the phone was exposed to moisture, it was hit hard.

Appearance shown in Figure 2 and may differ (depending on specific model phone and its manufacturer).

3. SIM-holder (sim - connector) - SIM card holder. As the name suggests, it serves SIM connections- cards to the phone. The design is almost the same for all phones, since modern SIM cards are brought to the same standard. It has 6 (rarely 8) spring-loaded contacts, through which the electrical connection of the SIM card and the power controller or processor is carried out. They differ only in the design of fastening (holding) the SIM card. Breakdowns include breaking contacts when frequently changing SIM cards or inept (incorrect) removal of them, when the user begins to use improvised means to pick a SIM card for further capture with fingers and removal from the holder. Often our beautiful ladies resort to this, using their long, expensively manicured nails. As a result, both the phone and manicure suffer.

The connector does not require special care. But there are cases (again, it depends on the user) when the contacts are oxidized, clogged, lose their springy properties. In this case, be VERY CAREFUL!!! wipe them with an eraser (eraser) and VERY CAREFULLY !!!, slightly bend the contacts up with a needle or a wooden toothpick.

With the SIM-holder (holder) malfunctions described above, the phone will not “see” your SIM-card and will constantly display a message like: “Insert SIM-card”. Broken holders cannot be repaired and must be replaced with new ones.

4. Microphone- serves to convert the user's voice into weak electrical signals in order to further amplify, convert and send them on the air. There are two types of cell phones: analog and digital. The latter have a more complex design and require more labor during dismantling and replacement.

Microphones lose their performance or break down mainly when they get dirty, get water, or get hit by the phone (this is especially true for digital microphones, because they themselves are very fragile).

If the microphone malfunctions in the phone, there may be such defects:

The second subscriber does not hear the user at all;

The second subscriber hears the user very weakly;

In the auditory (conversational) speaker, a crackling is heard (the so-called pickup of the GSM signal). The same noise can be heard by bringing a cell phone in talk mode or sending sms to a working radio, amplifier, computer speakers, etc. As a rule, microphones are not repairable and must be replaced (except in cases of clogging of holes, sound ducts of a mobile phone case. They should simply be cleaned of dust, dirt, etc.)

5. Speaker ( speaker) - serves to convert electrical signals into sound vibrations. That is, it works in the reverse order of the microphone. One caller speaks into a microphone that converts the voice to email. signals, then these signals are converted (see the description above), radiated into the air. The other party receives these signals on the phone and hears them on the phone's speaker.

Most phones have several speakers - separately conversational and separately polyphonic. The polyphonic speaker plays a melody when incoming call, sms, etc. But there are telephones (mostly Samsung), where the role of colloquial and polyphonic is performed by the same speaker. Only when playing a melody or other signals, the auxiliary audio power amplifier is activated. Speaker failures include partial failure and complete failure. Partial is the reproduction of speech or music very quietly, with wheezing and an unpleasant ringing. This can be eliminated, but only in those cases when, after an external examination, it will be seen that the speaker is clogged with foreign objects. For example, such as very small metal shavings that like to penetrate through specially designated holes for the sound of the speaker to exit. This is due to the fact that the speaker in its design contains a permanent magnet. So he magnetizes small metal objects to himself. Personally, I am a supporter of replacing such speakers with new ones. Firstly, it will save you time that you will spend on cleaning, and you will need a lot of it. Secondly, it rarely happens that after cleaning the speaker works just as cleanly, without distortion and just as loud. So, do not think - immediately change to a new one. Especially if this phone is not yours, but came in for repair.

Complete - no sound at all. The reason is a break in the wire of the voice coil of the speaker. The only solution is to replace the speaker. I will write about how to check the speaker for serviceability (integrity) below.

6. Speaker (buzzer, bell, polyphonic speaker - it's all the same)- the same speaker, only in most cases it is designed to play ringtones, sms, MP3, etc. But, as mentioned above, it can also be used for conversation. Faults and troubleshooting are the same as for the conversational speaker.

7. Central processing unit (CPU)- is the main device of the mobile phone. This is the same processor that is present in any personal computer, laptop, etc., only a little smaller and more primitive. Designed to execute machine commands, instructions and operations provided for software(firmware - coll.) of the phone, as well as clear interaction with other modules and devices and their subsequent control. In a word, the processor is the “brain” that completely controls the operation of the mobile phone. Structurally made in the form of a separate chip. Responsible for many processes that occur during the normal operation of the phone. The main ones are: displaying an image on the display, receiving and processing cellular network signals, receiving and processing keyboard module signals, controlling the operation of the camera, devices for receiving / transmitting information, the process of charging the battery (together with the power controller) and much more.

Under the condition of normal operation of the phone, the processor almost never fails and does not require any maintenance.

In modern phones, and especially smartphones (translated from English, smartphone - smart phone. The same phone, only it resembles a computer due to the presence operating system and many installed programs to perform certain tasks) often 2 processors are installed. One of them performs the same functions as in regular phone, and the second is for the operation of the operating system and the execution of its programs.

If the central processor fails, the phone is completely inoperative.

8. Flash - memory. A separate chip (microcircuit), which is designed to store the phone software (firmware, firmware), as well as user data (contacts, melodies, photos, etc.). Software (firmware, firmware) is a program developed by the phone manufacturer, which is processed and executed by the processor. For the user, this is what he sees on the screen of a mobile phone and the functions that are available to him in a particular phone model.

Flash memory also rarely fails under normal use. But it should be remembered that these chips have, though large, but still limited quantity cycles of reading/writing information.

Flash memory is non-volatile and retains all data written to it even after disconnecting the power source (eg battery).

9. RAM - memory (RAM). Serves for temporary storage of data. It performs all processor calculations program code, as well as the results of calculations and information processing at a specific current moment (for example, listening to music, playing videos, running applications, games, etc.) are stored. As unnecessary, the memory is cleared of some data and loads new ones, and so constantly.

It should be remembered that RAM memory (random access memory) is volatile and in the event of a power outage, all data stored in RAM will be lost !!!

10. Keyboard module- a standard numeric keypad for dialing a subscriber's number, texting SMS messages + a set of additional buttons that perform functions defined by the phone software, such as adjusting the volume level, launching programs, a camera, a voice recorder, etc. For the normal operation of the keyboard module, the main task of the user is to keep the keyboard clean and prevent moisture, dirt and other objects from entering. Otherwise, the buttons have to be pressed with great effort, or the phone does not respond to pressing at all. You can restore the operation of the keyboard module by cleaning it from dirt. If the contact pads and the conductors connecting them were exposed to moisture or other liquids and were damaged, then such a keyboard module must be replaced with a new one.

11. LCD display- the actual display (screen) of the phone. The purpose is clear to everyone, so I will not delve into this. The main characteristics are parameters such as:

Resolution, that is, the number of reproduced pixels (dots). The higher this parameter, the clearer and better the picture will be. More or less modern phones are characterized by such screen resolutions: 220X176 pixels, 320X240. For phones with large touch screens: 400X240, 640X360, 800X400.

The number of reproduced (displayed) colors. Same thing, the more the better. In older phones with color displays, this value is mostly 4096 colors. With the improvement, this parameter increased to 65 thousand, then reached 262 thousand. Now all modern expensive phones are equipped with displays with a color depth of 16 million.

When used properly, the display does not require any maintenance. In some cases, when the phone is used in a dusty environment, or just over time, a lot of dust and debris have accumulated in the case, then the display must be CAREFULLY wiped with microfiber (a special cleaning cloth that cleans well and leaves no marks and streaks. It can be purchased at sales outlets Some types of glasses are equipped with such cleaning microfiber.) When using the phone, do not allow physical impact on the display (shocks, squeezing, strong bends), as well as expose it to direct sunlight and high temperatures. This will cause it to fail.

12. Transceiver- serves to receive and transmit a cellular GSM signal. It contains many functional elements (generators controlled by the voltage of the receiver and transmitter, bandpass filters, decoupling capacitors, inductances, etc.). It is controlled by a processor and a 26 MHz quartz resonator.

If the transceiver malfunctions, the phone will not be able to register in the cellular network and there will be no indicator of the GSM signal strength on the display.

13. Power amplifier– designed to amplify the signal generated by the transceiver to the power level required for the antenna to radiate on the air.

If the power amplifier malfunctions, the phone will receive a cellular network signal, but will not be able to register in it, since it will not be able to transmit a GSM signal.

14. Antenna switch (switch)– designed to interface (connect) the receiving and transmitting paths of the GSM module to the telephone antenna. This ensures that the phone has one common antenna for receiving and transmitting, and also excludes the influence of the power amplifier on the receiving path.

People have long since learned to communicate at a distance. In ancient times, a messenger was sent with news, later letters were written. 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?

A mobile phone is more like a walkie-talkie than a regular corded phone. Radio waves are used to transmit the signal.

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

Who was first?

Who do you think was the first to talk on a cell phone? Of course, it was an employee of Motorola, which released them. In 1973, while on 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 that hit the stores 10 years later.

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 buddy's number, your phone transmits 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 only a few seconds.

The conversation itself is a rather complicated technical process. Our voice is broken into segments lasting 20 milliseconds and converted to digital format, then encoded by a special system. Encrypted signals are processed again to remove extraneous noise.

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

It's a little sad that the vast majority of people answer the question: "How does cellular communication work?", Answer "over the air" or in general - "I don't know."

In continuation of this topic, I had one funny conversation with a friend on the topic of mobile communications. It happened exactly a couple of days before being celebrated by all signalmen and telecom operators radio day celebration. It so happened that, due to his ardent position in life, my friend believed that mobile connection works without wires at all via satellite. Exclusively due to radio waves. At first, I couldn't convince him. But after a short conversation, everything fell into place.

After this friendly "lecture", the idea came up to write in simple language about how cellular communications work. Everything is as it is.

When you dial a number and start to call, well, or someone calls you, then your mobile phone communicates via radio from one of the antennas of the nearest base station. Where are these base stations, you ask?

pay attention to industrial buildings, urban skyscrapers and special towers. On them are large gray rectangular blocks with protruding antennas of various shapes. But these antennas are not television or satellite, but transceivers mobile operators. They are directed to different sides to provide communication to subscribers from all directions. After all, we don’t know where the signal will come from and where it will bring the “unfortunate subscriber” from handset? Antennas are also called "sectors" in professional jargon. As a rule, they are installed from one to twelve.

From the antenna, the signal is transmitted via cable directly to the control unit of the station. Together they form the base station [antennas and control unit]. Several base stations, whose antennas serve a separate area, for example, a city area or a small town, are connected to a special unit - controller. Up to 15 base stations are usually connected to one controller.

In turn, the controllers, which can also be several, are connected by cables to the "think tank" - switch. The switch provides output and input of signals to city telephone lines, to other cellular operators, as well as to long-distance and international communications.

In small networks, only one switch is used, in larger networks serving more than a million subscribers at once, two, three or more switches can be used, again interconnected by wires.

Why such complexity? Readers will ask. It would seem that, you can simply connect the antennas to the switch and everything will work. And then there are base stations, switches, a bunch of cables ... But, not everything is so simple.

When a person moves along the street on foot or goes by car, train, etc. and at the same time talking on the phone, it is important to ensure communication continuity. Signalers handover process in mobile networks called the term handover. It is necessary to switch the subscriber's phone in time from one base station to another, from one controller to another, and so on.

If the base stations were directly connected to the switch, then all these switching would have to be controlled by the switch. And he "poor" and so there is something to do. A multi-level network scheme makes it possible to evenly distribute the load on technical means . This reduces the likelihood of equipment failure and, as a result, loss of communication. After all, we all interested in uninterrupted communication, right?

So, reaching the switch, our call is being translated further - to the network of another operator of mobile, urban intercity and international communications. Of course, this happens over high-speed cable communication channels. The call arrives at the switch another operator. At the same time, the latter “knows” in which territory [in the scope of which controller] the desired subscriber is currently located. The switch transmits phone call to a specific controller, which contains information about which base station the call recipient is in the coverage area of. The controller sends a signal to this single base station, and it in turn “polls”, that is, calls the mobile phone. A tube starts ringing strangely.

This whole long and complicated process actually takes 2-3 seconds!

In the same way, phone calls to different cities of Russia, Europe and the world take place. Contact switches of various telecom operators use high-speed fiber optic communication channels. Thanks to them, a telephone signal overcomes hundreds of thousands of kilometers in a matter of seconds.

Thanks to the great Alexander Popov for giving the world radio! If not for him, perhaps we would now be deprived of many of the benefits of civilization.

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 electronic switch, connecting the antenna to either the output of the transmitter or the input of the receiver, since a cell phone never receives and transmits at the same time.

Rice. 5.3. Functional diagram of the 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 transmission mode, information digital signal, formed in the logic block, goes 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 digital phone is the 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, converting 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 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, serving to display various information provided by the device and the 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 an identification number Mobile Subscriber Identifier (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. Service emergency call 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. For this, they are used various methods encryption. For example, in the GSM standard, encryption is carried out by error-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 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 segments of speech, 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 the documentation for cellular networks connections. 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 USA they amounted to a little more than $1 billion with a total income from cellular communications of $21 billion. Most operators try not to publish data on such losses, and they become known to the public years after major "punctures" .

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 possible to connect to it and external antenna, 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 communication in mobile networks ensured?