Radio communication is organized between two radio stations: a transmitting PS and a receiving PS (Fig. B.1). The primary electrical signals are fed through the connecting lines to the inputs α,β of the device A1, designed to combine the primary electrical signals into a single group signal (GS). This HS enters the radio transmitter.
Radio transmitter (RP) is called a device for generating a radio frequency signal to be emitted. The input group signal modulates the carrier frequency of the radio transmitter. At the output of the RP, a radio frequency signal is generated, which enters the transmitting antenna WA1. Transmitting (receiving) antenna called a device designed to emit (receive) radio waves. Thus, radio waves propagate between the transmitting WA1 and receiving WA2 antennas. radio waves called electromagnetic oscillations with frequencies up to 3 10 12 Hz, propagating in a medium without artificial guide lines. Antenna WA2 converts the received radio wave into a radio frequency signal that enters the Rpr radio receiver.
radio receiver called a device designed to separate the transmitted signal from the received radio frequency signal. The selected GS group signal is fed to the device A2, which divides it into. primary electrical signals so that each of these signals goes to its recipient. In A1 and A2, the combination and separation of the primary electrical signals can occur on the basis of frequency division channels (FDM), or time division channels (TDC).
Under radio relay understand radio communication based on the relaying of radio signals of decimeter and shorter waves by stations located on the surface of the Earth. The set of technical means and the environment for the propagation of radio waves to provide RRL communications form radio relay line .
Table 1
Terrestrial called a radio wave propagating near the earth's surface. Terrestrial radio waves shorter than 100 cm propagate well, as a rule, only within the line of sight. Therefore, a radio relay communication line over long distances is built in the form of a chain of receiving and transmitting radio relay stations(PPC), in which neighboring RPCs are placed at a distance providing line-of-sight radio communications, and are called radio relay line of sight(RRL). On fig. B.2 is PPC1-PPC2, PPC2-PPRS.
A tropospheric radio wave propagates between points on the earth's surface along a trajectory that lies entirely in the troposphere. The energy of a tropospheric radio wave shorter than 100 cm is scattered by irregularities in the troposphere. In this case, part of the transmitted energy falls on the receiving antenna of the RRS, located outside the line of sight at a distance of 250 ... 350 km. A chain of such RRS forms a tropospheric radio relay link (TRL) (Fig. B.3).
Depending on the modulation method used in the radio relay system, it is customary to distinguish between analog radio relay systems with FM (ARRS), digital radio relay systems (CRRS), etc.
Satellite radio communication is communication through a repeater installed on an artificial Earth satellite (AES). A satellite communication line (SLS) is formed by two stations located on the Earth and a station on a satellite. The first received the name of earth stations (ES), the second - space (CS). In contrast to ZS, RRL and TRL radio stations are called terrestrial. The satellite communication line consists of two sections: Earth - AES and AES - Earth.
Classification of radio relay communication lines. Usually they are classified according to a number of the most significant features. Depending on the mechanism of propagation of radio waves, there are: RRL and TRL.
Depending on the EASS primary network to which they belong, there are main, intrazonal and local RRLs (or TRLs).
Depending on the method adopted for the formation of the HS, there are analog and digital RRL (or TRL). In turn, analog radio relay communication lines are classified depending on the method adopted for combining (separating) primary electrical signals and the method of carrier modulation: RRL (or TRL) with FDM and FM and RRL with PIM-AM; depending on the number N of organized PM channels: low-channel - N≤24; with an average throughput - N=60 ... 300; with large bandwidth -N=600... 1920.
Digital RRL is classified according to the method of carrier modulation: PCM-FM, PCM-FM and others; depending on the bit rate B: low-B 5<10 Мбит/с, средней В =10...100 Мбит/с и высокой В>100 Mbps bandwidth.
4. Security questions
Glossary
Consider the structure of radio communication (Fig. 2.15).
The microphone (M) converts the sound vibrations of speech into electrical current vibrations of sound (low) frequency. One of the main blocks of the radio transmitter is a master oscillator (MG) (or a high frequency generator), which converts direct current energy (a special power source) into the energy of high frequency (HF) current fluctuations. The audio frequency current amplified in the low frequency amplifier (ULF) enters the modulator (Mod), affecting one of the parameters (amplitude, frequency or phase) of the high frequency current. generated by the master oscillator. As a result, high frequency (radio frequency) currents are fed into the transmitter antenna, varying in amplitude, frequency, or phase in accordance with the transmitted sound vibrations (transmitted by the original message). The process of influencing one of the parameters of the RF signal according to the law of changing the transmitted initial message is called modulation , respectively amplitude, frequency or phase.
Figure 2.15 - Structural diagram of radio communication
High-frequency currents, passing through the transmitter antenna, form an electromagnetic field around it. Electromagnetic waves (radio waves) are separated from the antenna and propagate in space at a speed of 300,000 km/s.
In the receiving antenna, radio waves (an electromagnetic field) induce an EMF of radio frequency, which creates a modulated RF current that exactly repeats all changes in the current in the transmitting antenna. The high frequency currents from the receiving antenna are transmitted via the feeder line to a selective high frequency amplifier (UHF). Selectivity is provided by a resonant circuit, most often consisting of an inductor and a capacitor connected in parallel, forming a parallel oscillatory circuit having a current resonance at the frequency of the electromagnetic oscillations transmitted by the transmitter. To transmitters of radio stations operating at other frequencies, this radio receiver is practically insensitive.
The amplified signal is fed to the detector (Det), which converts the received RF signals into sound vibration currents, which change like the sound frequency currents created by the microphone at the transmitter. Such a transformation is called detection (demodulation). The audio or low frequency (LF) current received after detection is usually still amplified in the ULF and transmitted to the loudspeaker (speaker or headphones), which converts this LF current into sound vibrations.
Radio communication is one-way and two-way. With one-way radio communication, one of the radios only transmits, and the other (or others) only receives. In two-way radio communication, radios transmit and receive at the same time.
Simplex radio- this is a two-way radio communication, in which each subscriber only transmits or only receives in turn, turning off his transmitter for the duration of the reception (Fig. 2.16). For simplex communication, one radio frequency is sufficient (single-frequency simplex radio communication). Each radio station has one antenna, which, when receiving and transmitting, switches respectively to the input of the radio receiver or to the input of the radio transmitter.
Figure 2.16 - Structural diagram of simplex radio communication
Simplex radio is usually used when there are relatively small traffic flows. For radio networks with a large load, duplex communication is typical.
duplex radio- This is a two-way radio communication in which reception and transmission are carried out simultaneously. Two-way radio communication requires two different carrier frequencies, and transmitters and receivers must have their own antennas (Figure 2.17). In addition, a special filter is installed at the input of each receiver ( duplexer), which does not transmit vibrations of the radio frequency of its own transmitter. The advantages of duplex radio communication are its high efficiency and bandwidth of the radio network.
Figure 2.17 - Structural diagram of duplex radio communication
Radio communication has the following advantages over wired communication:
Ø rapid deployment on any terrain and in any conditions;
Ø high efficiency and survivability of radio communications;
Ø the ability to send various messages to any number of subscribers circularly, selectively or to a group of subscribers;
Ø the ability to communicate with moving objects.
Radio transmitting devices
In a functional sense, a radio transmitting device is understood as a set of equipment designed to form and emit a radio frequency signal (radio signal). As functional units, the radio transmitter includes a carrier generator and a modulator. In addition, radio transmitting devices (especially powerful ones) contain a lot of other equipment: power supplies, means of cooling, automatic and remote control, signaling, protection and blocking, etc.
The main indicators of radio transmitting devices can be conditionally divided into 2 groups: energy and electromagnetic compatibility indicators.
The most important energy indicators of a radio transmitting device are the rated power and industrial efficiency. Under rated power (P) understand the average value of the energy supplied to the antenna over the period of the radio frequency oscillation. Industrial efficiency factor (COP) is the ratio of the rated power P to the total P total consumed from the AC mains by the radio transmitter: η = P/P total 100%.
The main indicators of electromagnetic compatibility are the operating frequency range, oscillation frequency instability and out-of-band emissions.
Operating frequency range called the frequency band in which the radio transmitting device provides operation in accordance with the requirements of the standard.
Under frequency instability radio transmitter understand the deviation of the oscillation frequency at its output for a certain period of time relative to the set frequency. Small instability (high stability) of the frequency allows you to reduce interference to radio reception.
out of band call such radiation, which are located outside the band allocated for the transmission of useful messages. Out-of-band emissions are a source of additional interference to radio reception. When suppressing out-of-band emissions, the signal transmission quality does not deteriorate.
According to their purpose, radio transmitting devices are divided into communication devices. Broadcasting and television. According to the operating frequency range, radio transmitting devices are divided in accordance with the classification of types of radio waves. Depending on the rated power, radio transmitting devices are divided into low-power (up to 100 W), medium power (from 100 to 10000 W), powerful (from 10 to 500 kW) and heavy-duty (over 500 kW).
The specifics of operation makes it possible to distinguish between stationary and mobile radio transmitting devices (automobile, aircraft, portable, etc.).
radio receivers
radio reception is the separation of signals from radio emission. In the place where radio reception is conducted, there are simultaneously radio emissions from many natural and artificial sources. The power of a useful radio signal is a very small fraction of the power of the total radio emission at the place of radio reception. The task of the radio receiver is to isolate a useful radio signal from a variety of other signals and possible interference, as well as to reproduce (restore) the transmitted message.
The main (in the sense of universality) indicators of radio receivers are: operating frequency range, sensitivity, selectivity, noise immunity.
Operating frequency range determined by the range of possible tuning frequencies. In other words, this is the tuning frequency range within which the radio receiver can smoothly or hop from one frequency to another.
Sensitivity is a measure of the ability of a radio receiver to receive weak radio signals. It is quantitatively estimated by the minimum value of the electromotive force (EMF) of the signal at the input of the radio receiver, at which the required signal-to-noise ratio at the output takes place in the absence of external interference.
Selectivity is called the property of a radio receiver that allows you to distinguish a useful radio signal from radio interference according to certain features inherent in the radio signal. In other words: it is the ability of the radio receiver to isolate the desired radio signal from the spectrum of electromagnetic waves at the receiving site, reducing interfering radio signals. Distinguish between spatial and frequency selectivity. Spatial selectivity It is achieved by using an antenna that provides reception of the desired signals from one direction and attenuation of radio signals from other directions from extraneous sources. Frequency selectivity quantitatively characterizes the ability of the radio receiver to select from all radio frequency signals and radio interference acting at the input, the signal corresponding to the tuning frequency of the radio receiver.
Noise immunity radio receiver is called its ability to counteract the interfering effect of interference. Quantitatively, the noise immunity is estimated by the maximum value of the interference level in the antenna, at which the reception of radio signals is still ensured.
Radio receivers can be classified according to various criteria. According to the purpose, broadcasting (usually called radio receivers or receivers), television (TV sets), professional, special radio receivers can be distinguished. The professional ones include trunk radio receivers of the decameter range, radio relay and satellite communication lines. Among special-purpose radio receivers, for example, radar, radio navigation, aircraft, etc. should be mentioned.
Antennas and feeders
Antenna represents an interface element between the transmitting or receiving equipment and the radio wave propagation medium. Antennas, in the form of wires or surfaces, emit electromagnetic waves when transmitting, and when receiving, they "collect" the incident energy. Antennas consisting of wires with a small cross-section compared to the wavelength and longitudinal sections are called wire. Antennas that radiate through their aperture are called aperture. Sometimes they are called diffraction, reflex, mirror. The electric currents of such antennas flow along conductive surfaces having dimensions commensurate with the wavelength or much larger than it.
The electrical circuit and accessories by which RF signal energy is conducted from a radio transmitter to an antenna or from an antenna to a radio receiver is called feeder. The following requirements are imposed on feeders: energy losses of high-frequency signals in it must be minimal; they should not have an antenna effect, i.e. must not emit or receive electromagnetic waves; have sufficient electrical strength, i.е. transmit the required power without the danger of electrical breakdown of the insulation.
Transmitting antennas used in the kilometer and hectometer radio wave bands are connected to the radio transmitter using multi-wire coaxial feeders. In the decameter range, feeders are usually made in the form of wire two- or four-wire lines. To antennas of meter radio waves, energy, as a rule, is conducted using a coaxial cable. At shorter wavelengths, in particular in the centimeter range, the feeder is made in the form of a hollow metal pipe - a waveguide of rectangular, elliptical or circular cross section.
The classification and propagation methods of radio waves are given in the tables below.
Consider the structure of radio communication (Fig. 2.15).
The microphone (M) converts the sound vibrations of speech into electrical current vibrations of sound (low) frequency. One of the main blocks of the radio transmitter is a master oscillator (MG) (or a high frequency generator), which converts direct current energy (a special power source) into the energy of high frequency (HF) current fluctuations. The audio frequency current amplified in the low frequency amplifier (ULF) enters the modulator (Mod), affecting one of the parameters (amplitude, frequency or phase) of the high frequency current. generated by the master oscillator. As a result, high frequency (radio frequency) currents are fed into the transmitter antenna, varying in amplitude, frequency, or phase in accordance with the transmitted sound vibrations (transmitted by the original message). The process of influencing one of the parameters of the RF signal according to the law of changing the transmitted initial message is called modulation , respectively amplitude, frequency or phase.
Figure 2.15 - Structural diagram of radio communication
High-frequency currents, passing through the transmitter antenna, form an electromagnetic field around it. Electromagnetic waves (radio waves) are separated from the antenna and propagate in space at a speed of 300,000 km/s.
In the receiving antenna, radio waves (an electromagnetic field) induce an EMF of radio frequency, which creates a modulated RF current that exactly repeats all current changes in the transmitting antenna. The high frequency currents from the receiving antenna are transmitted via the feeder line to a selective high frequency amplifier (UHF). Selectivity is provided by a resonant circuit, most often consisting of an inductor and a capacitor connected in parallel, forming a parallel oscillatory circuit having a current resonance at the frequency of the electromagnetic oscillations transmitted by the transmitter. To transmitters of radio stations operating at other frequencies, this radio receiver is practically insensitive.
The amplified signal is fed to the detector (Det), which converts the received RF signals into sound vibration currents, which change like the sound frequency currents created by the microphone at the transmitter. Such a transformation is called detection (demodulation). The audio or low frequency (LF) current received after detection is usually still amplified in the ULF and transmitted to the loudspeaker (speaker or headphones), which converts this LF current into sound vibrations.
Radio communication is one-way and two-way. With one-way radio communication, one of the radios only transmits, and the other (or others) only receives. In two-way radio communication, radios transmit and receive at the same time.
Simplex radio - This is a two-way radio communication, in which each subscriber only transmits or only receives in turn, turning off his transmitter for the duration of the reception (Fig. 2.16). For simplex communication, one radio frequency is sufficient (single-frequency simplex radio communication). Each radio station has one antenna, which, when receiving and transmitting, switches respectively to the input of the radio receiver or to the input of the radio transmitter.
Figure 2.16 - Structural diagram of simplex radio communication
Simplex radio is usually used when there are relatively small traffic flows. For radio networks with a large load, duplex communication is typical.
duplex radio - This is a two-way radio communication in which reception and transmission are carried out simultaneously. Two-way radio communication requires two different carrier frequencies, and transmitters and receivers must have their own antennas (Figure 2.17). In addition, a special filter is installed at the input of each receiver ( duplexer), which does not transmit vibrations of the radio frequency of its own transmitter. The advantages of duplex radio communication are its high efficiency and bandwidth of the radio network.
Figure 2.17 - Structural diagram of duplex radio communication
Radio communication has the following advantages over wired communication:
rapid deployment on any terrain and in any conditions;
high efficiency and survivability of radio communications;
the ability to send various messages to any number of subscribers circularly, selectively or to a group of subscribers;
the ability to communicate with moving objects.
Introduction
In control systems for various purposes, various types of electrical communication are widely used for the transmission of messages, including radio communication carried out by means of radio waves.
Fig.1 Generalized block diagram of a radio communication system.
The sender and recipient of messages can be both a person and technical devices. Messages can be in the form of speech, alphanumeric text, images, etc.
By their nature, messages can be discrete-valued or discrete and continuous-valued or continuous.
Discrete-valued messages are those that take on a finite or countable number of values. For example: alphanumeric text, letters, numbers, punctuation marks. The set of possible messages with their probabilistic characteristics form an ensemble of messages. The choice of specific messages from the ensemble is carried out by the message sender.
Continuous-valued messages are those whose possible values are inseparable and continuously fill a certain range of values. For example: speech, music, moving images, etc. They are characterized by a probability density.
For transmission over a communication channel, any kind of message must be converted into a primary electrical signal. There must be a one-to-one correspondence between the message and the signal, so that the transmitted message can be obtained by inverse transformation at the receiving point.
Sound pressure during the transmission of speech messages is converted by a microphone into electrical voltage. Electrical signals that are analogues of continuous-valued messages are called analog.
Primary electrical signals corresponding to discrete-valued messages are called digital.
The process of converting discrete-valued messages into digital signals is called encoding.
The correspondence system between discrete-valued messages and code combinations of single elements is usually called the primary code.
The transmission system generally uses binary codes. This makes it possible to widely use standard elements of digital technology in communication equipment. Symbols of unit elements of code combinations "1" and "0" are called bits.
Analog signals can be converted to digital signals. The conversion of an analog signal into a digital one is achieved by its time sampling and level quantization.
With pulse code modulation, an analog signal is converted into a digital signal by sampling, quantization of samples and their coding.
Since the transmission of the primary electrical signal over long distances is impossible, it is converted into a radio signal in the radio transmitting device (TXD) using modulation or manipulation. This radio signal is transmitted through a space-line of communication to a radio receiver (RRP).
Modulation is the process of changing one or more parameters of a radio frequency oscillation in accordance with the representative parameter of the primary electrical signal.
The parameters changed in this case are called informational, the rest - related.
The modulation of radio frequency oscillations by a primary digital signal is called keying.
The modulation of radio frequency oscillations by a primary pulse signal (a sequence of pulses) is called pulse modulation.
In a radio receiver (RRP), a primary electrical signal is extracted from the received radio signal, which is then used to restore the message.
The set of TX, communication lines, PFP is called a radio channel.
A sender, a radio communication channel, a receiver form a radio communication system.
The presence of interference, distortion in the communication line and the equipment itself distinguishes the message at the output of the PRM from the transmitted one. The ability of a radio communication system to withstand the harmful effects of radio interference and distortion is characterized by noise immunity.
Noises are divided into additive n(t) and multiplicative .
If the received message can be represented as the sum of the signal S(t) and noise n(t): 
, then this noise is called additive.
Additive interference can be: fluctuation, impulse, stationary.
Fluctuation interference has a uniform energy spectrum, the width of which exceeds the spectrum of the radio signal (this may be the inherent noise of the PFP).
Impulse interference is a regular or random sequence of impulses, the duration of which is much less than the period of their repetition (lightning discharges, ignition of cars).
Stationary interference is interference from neighboring radio stations and other radio devices, as well as targeted interference.
Under the influence of multiplicative interference, the received signal is represented as the product of the transmitted signal S(t) and interference : 
There may be other ways of interaction between the useful signal and interference. Multiplicative interference includes the fading of the radio signal, the arrival at the receiving point of radio signals shifted relative to each other in time.
In general, the received signal is affected by multiplicative and additive noise.
TRANSMISSION RADIO SYSTEMS.
1. Principles of organization of radio transmission systems.
Consider a simplified block diagram of a radio link.
Fig.1.
The transmitted message enters the converter (microphone, television camera, telegraph machine or key), which converts it into an electrical signal. The latter enters the RPDU, which consists of a modulator (M), a carrier frequency synthesizer (MF) and a modulated oscillation amplifier (UMA). Using an antenna (A), the energy of the radio frequency oscillations of the transmitter is radiated into the radio wave propagation path.
At the receiving end of the radio wave, an EMF is induced in the antenna. The radio receiver (RPU) with the help of selective circuits (SC) filters out signals from interference. In the detector (D), a process occurs, the reverse of modulation - the selection of the original electrical signal from the modulated oscillations, which controlled the radio transmitter. With the help of a converter (loudspeaker, telegraph apparatus, receiving television tube), the electrical communication signal is converted into a message delivered to the subscriber.
The considered radio link provides one-way message transmission, which is acceptable only in alert services. One-way radio communication is, in essence, broadcasting, although in this case the reception is carried out not in one, but in many points. Reception at many points is also carried out with a circular transmission: orders are transmitted to many executors; messages are transmitted from the press center to the editorial offices of many newspapers, etc.
To organize two-way radio communication at each point, it is necessary to have both a transmitter and a receiver. If, in this case, transmission and reception are carried out at each radio station in turn, then such radio communication is called simplex (Fig. 2, a). Two-way radio communication, in which communication between radio stations is realized simultaneously, is called duplex (Fig. 2, b).
With duplex radio communication, transmission in one direction and the other is carried out, as a rule, at different carrier frequencies. This is done so that the receiver receives only signals from the transmitter (TX) from the opposite point and does not receive signals from its own transmitter.
For radio communication over long distances, radio transmitters with a power of tens and hundreds of kilowatts are used. Therefore, although in duplex communication the receiver is tuned to a different frequency than its transmitter is tuned to, it is difficult to ensure its normal operation in the vicinity of a powerful TX. Based on this, the receiver and transmitter have to be placed at a distance of tens of kilometers from each other.
Simplex communication is used, as a rule, in the presence of relatively small information flows. For objects with a large load, duplex communication is typical.
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Fig. 2. Structural scheme of radio communication: a) simplex; b) duplex.
If it is necessary to have radio communication with a large number of objects, then a so-called radio network is organized (Fig. 3).
but)
Fig. 3. Structural diagrams of the radio network: a) a complex simplex; b) complex duplex.
One radio station, called the master (GR), can transmit messages for one or several slave objects. Its radio operator keeps order in the radio network and sets the order of work for the transmission of slave radio stations (PR). Slave radio stations, with the appropriate permission, can exchange information not only with the main radio station (GR), but also with each other. This version of the radio network organization can be built on the basis of both a complex simplex (Fig. 3a) and a complex duplex (Fig. 3b). In the first case, it is possible to use combined transceivers and a common working radio wave (frequency). In the 2nd case, the main radio station (GR) transmits on one frequency, and receives on several (according to the number of slave radio stations).
At the command of the GR, any slave radio station (PR) can be brought to the radio direction in especially important cases.
Despite the difference in receiving and transmitting frequencies, here, as with simple duplex, it is necessary to place the receiver and transmitter at a distance from each other. Otherwise, due to interference generated by the radio transmitting device (RPDU
