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 cell phone do not work simultaneously. Transmission occurs only for 1/8 of the frame duration. This significantly reduces battery power consumption and increases operating time in both transmit (talk) and receive (standby) modes. In addition, the requirements for the receiver's high-pass filter, made on a SAW, are noticeably reduced, which makes it possible to integrate the LNA with a 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 of a digital GSM radiotelephone

The received signal, after passing through the input bandpass filter, is amplified by the LNA and goes to the first input of the first mixer. The second input receives a local oscillator signal f direct from the frequency synthesizer. First intermediate frequency signal f etc., passes through a band-pass SAW filter and is amplified by the first intermediate frequency amplifier UPC1, after which it is supplied to the first input of the second mixer. Its second input receives a local oscillator signal f g from the frequency generator. Received second intermediate frequency signal f pr2 is filtered by a band-pass SAW filter, amplified by the amplifier UFC2, demodulated and supplied to an analog-to-digital converter (ADC), where it is converted into a signal necessary for the operation of a digital logic block implemented on the central processor CPU.

In transmission mode, the digital information signal generated in the logical block is supplied to the 1/O generator, where the modulating signal is generated. 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 transmitting from a frequency synthesizer. Received signal f c1, through a bandpass filter, enters a power amplifier (PA), controlled by the central processor CPU. Signal amplified to the required level f c1 passes through a bandpass ceramic filter to antenna A and is radiated into the surrounding space.

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

The digital part of the phone includes:

Digital signal processor (CPU) with its own RAM and permanent memory, which controls the operation of the cell phone. Phone CPUs are somewhat simpler than computer microprocessors, but nevertheless they are complex microelectronic products.

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

speech encoder, encoding a speech signal that is already in digital form according to certain laws using a compression algorithm to reduce signal redundancy. In this way, the amount of information that needs to be transmitted over a 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 when transmitting the signal over the communication line. For the same purpose, information is subject to certain repackaging. (interleaving). In addition, the channel encoder includes control information coming from the logical part into the transmitted signal.

channel decoder, separating 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 undergoes reverse repacking in relation to the encoder.

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

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

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

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 in some cases may be absent.

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

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

Message encryption and decryption block, designed to ensure confidentiality of information transfer.

Speech activity detector(voice activity detector), which turns on the transmitter to emit only during those time intervals when the subscriber is speaking. During a pause in the operation of the transmitter, so-called comfortable noise is additionally introduced into the path. This is done in the interests of economical consumption of power supply energy, as well as reducing the level of interference for other stations.

Terminal devices used for connection through special adapters using appropriate interfaces, fax machines, modems, etc.

SIM card(SIM - subscriber identification module, literally - subscriber identification module) - a plastic plate with a chip inserted into a special socket of the subscriber device. 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 identity (TMSI), location area identifier (LAI), encryption key (Ke), data on restricted mobile networks;

Service-related data: preferred language of communication, payment notifications and list of declared services.

One of the main purposes 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 in length is recorded on the SIM card, which the SIM card microprocessor, after turning on the station, checks with the number dialed by the user using the keyboard. If the wrong PIN number is dialed three times in a row, the SIM card is blocked until the subscriber enters the 8-digit Personal Unblocking Key (PUK).

If an incorrect 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, you can 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 your personal identification PIN number.

5.3 Cellular services. Communication confidentiality. Fraud in cellular communications. 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, fax communication. The emergency call service allows the subscriber station to establish voice communication with the nearest emergency service center. TO additional services connections include:

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

In the context of competition for subscribers, operators of large networks are trying to introduce new services. Recently, services such as prepaid subscriber connection, WAP service - access to the Internet 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).

Confidentiality of communication is 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-resistant coding and interleaving and consists of 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 (mixing, shuffling), which is a kind of encryption by rearranging sections of the spectrum or segments of speech, carried out in external software.

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

in relation to the mobile phone device with appropriate descrambling at the receiving end.

Fraud(from English fraud- deception, fraud) is one of the serious problems of cellular communications. Fraud can be defined as an illegal activity aimed at using cellular communication services without proper payment or at the expense of paying 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 fraud in the field of cellular communications. The most unpleasant thing is when a cell phone registered to someone falls into the hands of scammers who are able to deceive cellular providers and carry out large-scale negotiations without control. 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 communication networks. They practice altering cell phone numbers and all kinds of “chemistry” with codes and passwords.

Losses from fraud, even after many years of fighting it, reach several percent of the total volume of cellular services. For example, in 1996 in the USA they amounted to just over $1 billion, with total revenue from cellular communications being $21 billion. Most operators try not to publish data on such losses, and they become known to the public years after major “mistakes” .

If you suspect that someone is using (explicitly or indirectly) your device, you must immediately notify your cellular service provider. For example, such a suspicion may be based on a noticeable increase in the volume of payments for cellular services compared to your usual level. If you don't control what happened, you could 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 “grey” phones causes enormous damage to cellular communications. These can be rejected devices purchased on the cheap, which are then handicraftally brought to working condition - often not for all functionality. Such devices cause a lot of trouble not only to their owners who are looking for cheap prices, but also to cellular operators. Because, performing many functions poorly (or not performing at all), they cause a flurry of calls to customer service.

Eavesdropping on conversations on cell phones is also far from a harmless thing. Analog networks are especially vulnerable to this. But even in digital networks, even with the appropriate equipment for encoding and decoding conversations, eavesdropping on them is also quite possible. It’s worth remembering this when having conversations.

The methods of illegal use of cell phones are varied, although there is an opinion that you need to be aware of it. But to what extent? For example, it is clear to everyone that a cell phone can be used as a very simple radio fuse. 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 presence of gaps in the legal use of cell phones, we will end the description of these subtle points in the use of cell phones.

Biological safety.

From time to time, sensational news appears about the development of cancerous tumors from the use of a cell phone. Somewhere in the USA there seemed to be even lawsuits about this. There are also reports of parking lots exploding while cars are being refueled, of planes going astray, of nuclear power plant reactors stopping due to the fault of cell phones, etc. In the vast 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 cell phone radiation energy is absorbed by the tissues of the human head. True, only part of the microwave radiation energy gets deep into the head. Most of it is absorbed by the skin and bones of the skull.

Meanwhile, there is no official data on any effect of cell phone radiation on the human body. And not because the relevant research has not been carried out. But because the standards for radiation power are much lower than the standards that were established for people by the relevant authorities.

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

It is not difficult to understand that such a purely “thermodynamic” approach does not at all help to calm people down. For one does not need to have extensive medical knowledge to believe that the effect of radiation is not limited to heating the tissues of the body. It cannot be ignored that at the genetic level, much less powerful radiation can cause disruption 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 radically reduce the degree of exposure to radio radiation mobile phones on the human body, and above all on his head. This is the use of a special hands free headset. This headset consists of a head-mounted earphone 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 types of danger associated with cell phones, the first place is distracting the user from his main job. For example, car accidents are very common when the driver picks up a phone while driving, and especially when he dials a number. 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.

Control questions

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

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

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

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. The main symptoms of the disease caused by radio radiation?

7. List the main services provided by cellular communications?

8. How is communication confidentiality ensured in mobile networks?

To do this, we suggest you go to the Beeline company.

A huge number of BS base stations are installed on the territory of Russia. Probably, many of you yourself have seen red and white structures rising in the fields or structures installed on the roofs of non-residential buildings. Each such base station is capable of picking up a signal from a cell phone at a distance of up to 35 km, communicating with it via service or voice channels.

After you have dialed the number of the desired subscriber on your phone, the following happens: the mobile phone finds the nearest BS, contacts it via a service channel and requests a voice channel. After this, the BS sends a request to the controller (BSC), which is then sent to the communicator. If the person you are calling is on the same operator as you, the communicator will check the Home Location Register (HLR) database to find out where exactly the person you are calling is located and will route the call to the correct switch, which will then transfer the call to the controller and then to the Base Station. And finally, the Base Station will contact the mobile phone of the desired person and connect you with him. And if the person you want to talk to is a subscriber of another cellular operator, or you are calling a landline number, then the switch will “find” the corresponding switch of the other network and contact it. Sounds quite confusing, right? Let's try to analyze this issue in more detail.

But let's get back to the equipment. As we have already said, the call is transferred from the BS to the controller (BSC). Externally, it is not much different from the Base Station:

The number of BSs that the controller is able to service can reach six dozen. The controller and the BS communicate via optical or radio relay channels. The controller controls the operation of radio channels.

Below you can see what the switch is:

The number of controllers served by the switch varies from two to thirty. Switches are placed in large rooms filled with metal cabinets containing equipment.

The switch's job is to control traffic. If previously, in order to talk to each other, subscribers first had to contact the telephone operator, who then manually rearranged the necessary wires, but now the switch copes with her role perfectly.

Inside cars there are devices designed for collecting and processing data:

Controllers and switches are monitored 24 hours a day. Tracking is carried out in the so-called TsKS (Flight Control Center of the Network Control Center).

Cellular communication is considered one of the most useful inventions of mankind - along with the wheel, electricity, the Internet and the computer. And in just a few decades, this technology has gone through a number of revolutions. Where did wireless communication begin, how do cells work, and what opportunities will the new mobile standard open up? 5G?

The first use of mobile telephone radio dates back to 1921 - then in the United States, the Detroit police used one-way dispatch communication in the 2 MHz band to transmit information from a central transmitter to receivers in police cars.

How did cellular communication come about?

The idea of ​​cellular communications was first put forward in 1947 by Bell Labs engineers Douglas Ring and Ray Young. However, real prospects for its implementation began to emerge only in the early 1970s, when company employees developed a working architecture for the cellular hardware platform.

Thus, American engineers proposed placing transmitting stations not in the center, but in the corners of the “cells,” and a little later a technology was invented that allowed subscribers to move between these “cells” without interrupting communications. After this, it remains to develop operating equipment for such technology.

The problem was successfully solved by Motorola - its engineer Martin Cooper demonstrated the first working prototype of a mobile phone on April 3, 1973. He called the head of the research department of a competitor company straight from the street and told him about his own successes.

Motorola management immediately invested $100 million in the promising project, but the technology entered the commercial market only ten years later. This delay is due to the fact that it was first necessary to create a global infrastructure of cellular base stations.

The first cell phone went on sale on March 6, 1983. It weighed almost 800 grams, could work on a single charge for 30 minutes of talk time and could be charged for about 10 hours. Moreover, the device cost $3,995 - a fabulous sum at that time. Despite this, the mobile phone instantly became popular.

Why is the connection called cellular?

Principle mobile communications simple - the territory in which the connection of subscribers is provided is divided into separate cells or “cells”, each of which is served by a base station. At the same time, in each “cell” the subscriber receives identical services, so he himself does not feel the crossing of these virtual boundaries.

Typically, a base station in the form of a pair of iron cabinets with equipment and antennas is placed on a specially built tower, but in the city they are often placed on the roofs of high-rise buildings. On average, each station picks up signals from mobile phones at a distance of up to 35 kilometers.

To improve the quality of service, operators are also installing femtocells - low-power and miniature cellular stations designed to serve a small area. They can dramatically improve coverage in places where it is needed. Cellular communications in Russia will be combined with space

A mobile phone located on the network listens to the air and finds a signal from the base station. In addition to the processor and RAM, a modern SIM card contains a unique key that allows you to log in to the cellular network. Communication between the phone and the station can be carried out using different protocols - for example, digital DAMPS, CDMA, GSM, UMTS.

Cellular networks of different operators are connected to each other, as well as to the landline telephone network. If the phone leaves the range of the base station, the device establishes communication with others - the connection established by the subscriber is quietly transferred to other “cells”, which ensures continuous communication while moving.

In Russia, three bands are certified for broadcasting - 800 MHz, 1800 MHz and 2600 MHz. The 1800 MHz band is considered the most popular in the world, as it combines high capacity, large radius action and high penetrating ability. This is where most mobile networks now operate.

What mobile communication standards are there?

The first mobile phones worked with 1G technologies - this is the very first generation of cellular communications, which was based on analog telecommunications standards, the main one of which was NMT - Nordic Mobile Telephone. It was intended exclusively for transmitting voice traffic.

The birth of 2G dates back to 1991 - GSM (Global System for Mobile Communications) became the main standard of the new generation. This standard is still supported today. Communication in this standard has become digital, and it has become possible to encrypt voice traffic and send SMS.

The data transfer rate within GSM did not exceed 9.6 kbit/s, which made it impossible to transmit video or high-quality audio. The GPRS standard, known as 2.5G, was designed to solve the problem. For the first time, it allowed mobile phone owners to use the Internet.

Another difference of 3G was the assignment of an IP address to each subscriber, which made it possible to turn mobile phones into small computers connected to the Internet. The first commercial 3G network was launched on October 1, 2001 in Japan. Subsequently, the throughput of the standard was repeatedly increased.

The most modern standard is fourth-generation 4G communications, which is intended only for high-speed data services. The throughput of the 4G network can reach 300 Mbit/s, which gives the user almost unlimited possibilities for surfing the Internet.

Cellular communications of the future

The 4G standard is designed for continuous transmission of gigabytes of information; it does not even have a channel for voice transmission. Due to extremely efficient multiplexing schemes, downloading a high-definition movie on such a network will take the user 10-15 minutes. However, even its capabilities are already considered limited.

In 2020, the official launch of the new generation of 5G communications is expected, which will allow the transfer of large amounts of data at ultra-high speeds of up to 10 Gbit/s. In addition, the standard will allow up to 100 billion devices to be connected to high-speed Internet.

It is 5G that will allow the true Internet of Things to emerge - billions of devices will exchange information in real time. According to experts, network traffic will soon increase by 400%. For example, cars will begin to constantly be in global network and receive traffic data.

Low latency will enable real-time communication between vehicles and infrastructure. Reliable, always-on connectivity is expected to open the door to fully autonomous vehicles on the road for the first time.

Russian operators are already experimenting with new specifications - for example, Rostelecom is working in this direction. The company signed an agreement on the construction of 5G networks in the Skolkovo innovation center. Project implementation is included in state program"Digital Economy", recently approved by the government.

How cellular communication works

The basic principles of cellular telephony are quite simple. The Federal Communications Commission originally established geographic coverage areas for cellular radio systems based on modified 1980 Census data. The idea behind cellular communications is that each area is subdivided into hexagonal-shaped cells that fit together to 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, approximately matching the circular radiation pattern while eliminating the gaps that always appear between adjacent circles.

A cell is defined by its physical size, population, and traffic patterns. The Federal Communications Commission does not regulate the number of cells in a system or their size, leaving operators to set these parameters in accordance with expected traffic patterns. Each geographic area is allocated a fixed number of cellular voice channels. The physical size of a cell depends on subscriber density and call structure. For example, large cells (macrocells) typically have a radius of 1.6 to 24 km with a base station transmitter power of 1 W to 6 W. The smallest cells (microcells) 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 cellular 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 often used in regions with high population densities. Due to their short range, microcells are less susceptible to interference that degrades transmission quality, such as reflections and signal delays.

A macro cell can be superimposed on a group of micro cells, with the micro cells serving slow moving mobile devices and the macro cell serving fast moving mobile devices. The mobile device is able to determine the speed of its movement as fast or slow. This allows you to reduce the number of transitions from one cell to another and the correction of location data.

The algorithm for moving from one cell to another can be changed at short distances between the mobile device and the microcell base station.

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

When planning systems using hexagonal-shaped cells, base station transmitters can be located 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). Cells with a transmitter in the center typically use omnidirectional antennas, while cells with transmitters on an edge or vertex typically use sectorial directional antennas.

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 group of radio channels can avoid interference if they are properly spaced. In this case, frequency reuse is observed. Frequency reuse is the allocation of the same group of frequencies (channels) to 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 differ from the frequencies of neighboring cells, and the base station antennas are selected in such a way as to cover the desired service area within its cell. Since the service area is limited to the boundaries of a single cell, different cells can use the same group of operating frequencies without interference, provided that two such cells are located at a sufficient distance from each other.

The geographic service area of ​​a cellular system containing several groups of cells is divided into clusters (Figure 6.3). Each cluster consists of seven cells, which are allocated the same number of full-duplex communication channels. Cells with the same letter designations use the same group of operating frequencies. As can be seen from the figure, the same groups of frequencies 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 frequency groups.

Figure 6.3 – 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 that are distributed between N honeycombs of the cluster, grouping into non-repeating combinations. All cells have the same number of channels, but they can serve single-sized areas.

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

F=GN (6.1)

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

G– number of channels in the cell;

N– number of cells in the cluster.

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

C = mGN = mF (6.2)

Where WITH– total number of channels in a given zone;

m– number of clusters in a given zone.

From expressions (6.1) and (6.2) it is clear that the total number of channels in a cellular telephone system is directly proportional to the number of “repetitions” of a cluster in a given service area. If the cluster size is reduced while the cell size remains the same, more clusters will be needed 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 being in neighboring cells of a small service area (for example, within a city), depends on the total number of cells in a given 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 by the relation:

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

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

With the projected increase in cellular traffic, the increased demand for service is met by reducing the size of the cell, dividing it into several cells, each with its own base station. Effective 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 neighboring cells will influence each other. Relationship between reuse frequencies and cluster size determines how you can change scale cellular system in case of increasing subscriber density. The fewer cells in a cluster, the greater the likelihood of mutual influences between channels.

Because cells are hexagonal in shape, each cell always has six equally spaced adjacent cells, and the angles between the 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– non-negative integers.

Finding a route to the nearest cells with a shared channel (the so-called first-tier cells) occurs as follows:

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

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

For example, the number of cells in the cluster and the location of the first tier cells 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 closest cells using the same channels as the cell A.

The process of handing over from one cell to another, i.e. when a mobile device moves 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 handover and initiates the necessary network procedures;

2) resource reservation - using appropriate network procedures, network resources necessary for service transfer (voice channel and control channel) are reserved;

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

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

Figure 6.6 – Handover

Do you know what happens after you dial a friend's number on your mobile phone? How does the cellular network find it in the mountains of Andalusia or on the coast of distant Easter Island? Why does the conversation sometimes suddenly stop? Last week I visited the Beeline company and tried to figure out how it works cellular…

A large area of ​​the populated part of our country is covered by Base Stations (BS). In the field they look like red and white towers, and in the city they are hidden on the roofs of non-residential buildings. Each station picks up signals from mobile phones at a distance of up to 35 kilometers and communicates with the mobile phone via service or voice channels.

After you have dialed a friend's number, your phone contacts the Base Station (BS) closest to you via a service channel and asks you to allocate a voice channel. The Base Station sends a request to the controller (BSC), which forwards it to the switch (MSC). If your friend is a subscriber to the same cellular network, then the switch will check the Home Location Register (HLR) to find out where in this moment the called subscriber is located (at home, in Turkey or in Alaska), and will transfer the call to the appropriate switch, from where it will forward it to the controller and then to the Base Station. The Base Station will contact your mobile phone and connect you to your friend. If your friend is on a different network or you are calling a landline, your switch will contact the corresponding switch on the other network. Difficult? Let's take a closer look. The Base Station is a pair of iron cabinets locked in a well-conditioned room. Considering that it was +40 outside in Moscow, I wanted to live in this room for a while. Typically, the Base Station is located either in the attic of a building or in a container on the roof:

2.

The Base Station antenna is divided into several sectors, each of which “shines” in its own direction. The vertical antenna communicates with phones, the round antenna connects the Base Station to the controller:

3.

Each sector can handle up to 72 calls simultaneously, depending on setup and configuration. A Base Station can consist of 6 sectors, so one Base Station can handle up to 432 calls, however, a station usually has fewer transmitters and sectors installed. Cellular operators prefer to install more BS to improve the quality of communication. The Base Station can operate in three bands: 900 MHz - the signal at this frequency travels further and penetrates better inside buildings 1800 MHz - the signal travels over shorter distances, but allows you to install a larger number of transmitters in 1 sector 2100 MHz - 3G Network This is what the cabinet looks like with 3G equipment:

4.

900 MHz transmitters are installed at Base Stations in fields and villages, and in the city, where Base Stations are stuck like hedgehog needles, communication is mainly carried out at a frequency of 1800 MHz, although any Base Station may have transmitters of all three ranges simultaneously.

5.

6.

A signal with a frequency of 900 MHz can reach up to 35 kilometers, although the “range” of some Base Stations located along highways can reach up to 70 kilometers, due to the reduction in the number of simultaneously served subscribers at the station by half. Accordingly, our phone with its small built-in antenna can also transmit a signal over a distance of up to 70 kilometers... All Base Stations are designed to provide optimal radio coverage at ground level. Therefore, despite a range of 35 kilometers, a radio signal is simply not sent to the aircraft’s flight altitude. However, some airlines have already begun installing low-power base stations on their aircraft that provide coverage within the aircraft. Such a BS is connected to a terrestrial cellular network using satellite channel. The system is complemented by a control panel that allows the crew to turn the system on and off, as well as certain types of services, for example, turning off the voice on night flights. The phone can measure the signal strength from 32 Base Stations simultaneously. It sends information about the 6 best (in terms of signal strength) via the service channel, and the controller (BSC) decides which BS to transfer the current call (Handover) if you are on the move. Sometimes the phone may make a mistake and transfer you to a BS with a worse signal, in which case the conversation may be interrupted. It may also turn out that at the Base Station that your phone has selected, all voice lines are busy. In this case, the conversation will also be interrupted. They also told me about the so-called “upper floor problem.” If you live in a penthouse, then sometimes, when moving from one room to another, the conversation may be interrupted. This happens because in one room the phone can “see” one BS, and in the second - another, if it faces the other side of the house, and, at the same time, these 2 Base Stations are located at a great distance from each other and are not registered as “ neighboring" mobile operator. In this case, the call will not be transferred from one BS to another:

Communication in the metro is provided in the same way as on the street: Base Station - controller - switch, with the only difference being that small Base Stations are used there, and in the tunnel, coverage is provided not by an ordinary antenna, but by a special radiating cable. As I wrote above, one BS can make up to 432 calls simultaneously. Usually this power is enough, but, for example, during some holidays the BS may not be able to cope with the number of people wanting to call. This usually happens on New Year's Day, when everyone starts congratulating each other. SMS are transmitted via service channels. On March 8 and February 23, people prefer to congratulate each other via SMS, sending funny poems, and phones often cannot agree with the BS on the allocation of a voice channel. I was told an interesting case. In one area of ​​Moscow, subscribers began to receive complaints that they could not get through to anyone. Technical specialists began to figure it out. Most voice channels were free, but all service channels were busy. It turned out that next to this BS there was an institute where exams were going on and students were constantly exchanging text messages. The phone divides long SMS into several short ones and sends each one separately. Technical service staff advise sending such congratulations via MMS. It will be faster and cheaper. From the Base Station the call goes to the controller. It looks as boring as the BS itself - it’s just a set of cabinets:

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Depending on the equipment, the controller can serve up to 60 Base Stations. Communication between the BS and the controller (BSC) can be carried out via a radio relay channel or via optics. The controller controls the operation of radio channels, incl. controls the subscriber’s movement and signal transmission from one BS to another. The switch looks much more interesting:

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Each switch serves from 2 to 30 controllers. It occupies a large hall, filled with various cabinets with equipment:

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The switch controls traffic. Remember the old movies where people first dialed the “girl”, and then she connected them to another subscriber by switching the wires? Modern switches do the same thing:

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To control the network, Beeline has several cars, which they affectionately call “hedgehogs.” They move around the city and measure the signal level of their own network, as well as the level of the network of their colleagues from the Big Three:

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The entire roof of such a car is covered with antennas:

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Inside there is equipment that makes hundreds of calls and takes information:

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24-hour monitoring of switches and controllers is carried out from the Mission Control Center of the Network Control Center (NCC):

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There are 3 main areas for monitoring the cellular network: accident rates, statistics and feedback from subscribers. Just like in airplanes, all cellular network equipment has sensors that send a signal to the central control system and output information to dispatchers’ computers. If some equipment fails, the light on the monitor will begin to “blink.” The CCS also tracks statistics for all switches and controllers. He analyzes it, comparing it with previous periods (hour, day, week, etc.). If the statistics of any of the nodes began to differ sharply from previous indicators, then the light on the monitor will again begin to “blink”. Feedback accepted by operators subscriber service. If they cannot resolve the problem, the call is transferred to a technician. If he turns out to be powerless, then an “incident” is created in the company, which is resolved by the engineers involved in the operation of the relevant equipment. The switches are monitored 24/7 by 2 engineers:

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The graph shows the activity of Moscow switches. It is clearly visible that almost no one calls at night:

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Control over the controllers (forgive the tautology) is carried out from the second floor of the Network Control Center:

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