General principles of radio communication organization. Classification of radio transmission systems. Radio transmission systems
General principles radio communication organizations. Classification of radio transmission systems
The set of technical means and radio wave propagation medium that ensures the transmission of signals from a source to a receiver of information is called radio channel (channel radio communications). Radio channel providing radio communication in one azimuthal direction is called radio link.
A simplified block diagram of a single-channel radio link is shown in Fig. 2.
Rice. 2. Block diagram of a radio link
The radio link operates as follows. The transmitted message enters a converter (microphone, television transmitting camera, telegraph or fax machine, etc.), which converts it into a primary electrical signal. The latter enters the radio transmitting device of the radio station, which consists of a modulator (M), a carrier frequency synthesizer (MF) and a modulated oscillation amplifier (UMK). Using a modulator, one of the parameters of the carrier frequency (high-frequency oscillation) changes according to the law of the primary signal. Using the antenna (A), the radio frequency energy of the transmitter is radiated into the radio wave propagation path.
At the receiving end, radio waves induce an emf in the receiving antenna (A). The radio receiver of the radio station, using selective circuits (SC), filters out signals from interference and other radio stations. In the detector (D), the reverse process of modulation occurs - the separation of the original electrical signal from the modulated oscillations. Next, in the converter, this signal is converted into a message, which is sent to the subscriber.
The considered radio link circuit provides one-sided radio communication, in which one of the radio stations transmits messages, and the other or others only receive. For organization bilateral radio communications, in which radio stations receive and transmit, at each point it is necessary to have both a transmitter (Per) and a receiver (Pr). If transmission and reception at each radio station are carried out alternately, then such radio communication is called simplex (Fig. 3, a). Simplex radio communication is used, as a rule, in the presence of relatively small information flows. Such radio communications can be single-frequency (reception and transmission on the same frequency) and dual-frequency (reception and transmission on different frequencies).
Rice. 3. Block diagram of radio communication organization: a - simplex; b- duplex
Two-way radio communication, in which communication between radio stations is realized simultaneously, is called duplex (Fig. 3, b).
With duplex radio communications, transmission in one direction and the other is usually carried out on different carrier frequencies. This is done so that the radio receiver receives signals only from the radio transmitter of the opposite point and does not receive signals from its own radio transmitter.
If it is necessary to have radio communication with a large number of points, then it is organized radio network, representing a set of radio links operating on one general for all subscribers, frequency or group of frequencies. Block diagrams of radio networks of various complexity
Details are shown in Fig. 4 for simplex radio communication and in Fig. 5 for full-duplex radio communication.
Rice. 4. Radio network based on complex simplex radio communications
Rice. 5. Radio network based on complex duplex radio communications
The essence of the functioning of the radio network is as follows. One radio, called the master (MR), can transmit messages for both one and several slave radios. The radio operator-operator GR monitors order in the radio network and sets the order of work for transmission to subordinate radio stations (SR). The latter, with appropriate permission, can exchange messages (information) not only with the GR, but also with each other. Such a communication organization can be implemented either on the basis of a complex simplex (Fig. A), and complex duplex (Fig. 5). In the first case, it is possible to use combined transceiver radio devices and a common operating radio wave (frequency). In the second case, the GR transmits on one frequency and receives on several (according to the number of subordinate radio stations). Note that a radio network can be organized based on half duplex radio, in which on one radio station (usually
main) transmission and reception are carried out simultaneously, and on other radio stations - alternately.
The centers of large industrial regions are connected by radio communication lines to many points. Why are radio transmitters and transmitting antennas located in the so-called transmitting radio center, and the radio receiver and receiving antennas are located on reception radio center. To connect message sources with radio transmitters and radio receivers and control the quality of radio communications in cities, they will equip radio bureau.
On long-distance radio networks, to increase the communication range, switch on relay stations(repeaters). A generalized block diagram of the repeater is shown in Fig. 6. To the already known notations and concepts, a new one is added here - feeder tract, representing a set of devices for transmitting electromagnetic energy from the antenna to the receiver (R) and from the transmitter (Per) to the antenna, containing a feeder and a number of auxiliary elements.
Rice. 6. Generalized block diagram of the repeater
The following requirements are imposed on the feeder path: energy transmission must be carried out with low losses; the transmitting feeder should not emit, and the receiving feeder should not receive, extraneous electromagnetic vibrations; reflections in paths that create passing flows should be minimal; Waves of other (higher) types should not propagate.
In modern radio transmission systems, the difference in the levels of radio signals emitted and received by antennas is very large (150 dB or more).
A diagram of a radio communications complex serving an administrative or economic center is shown in Fig. 7. Here: 1 - transmitting radio center with radio transmitters Per 1, Per 2, ....., Per N; 2- receiving radio center with radio receivers Pr 1, Pr 2,..., Pr N; 3 - a city that is connected to radio centers by connecting (wire) communication lines 4 and 5. Along the lines 4 to the radio center 1 transmitted signals arrive, and along the lines 5 V signals received by the radio center are transmitted to the city 2 signals for remote control of the operation of radio centers and signals for remote control of the operation of radio centers; signals for remote control of equipment are transmitted via the same lines. Radio Bureau 6 connected by communication lines to the telegraph and phototelegraph (fax) equipment rooms of the central telegraph 7 and 8 long distance telephone exchange 9, as well as broadcasting control room 10. The broadcasting control room is used to exchange broadcast programs with other cities or countries. Hardware are connected to the sources of transmitted messages, such as subscriber telegraph networks, telephone networks, etc.
There are many different classifications of radio transmission systems (RTS), depending on the characteristics underlying them. Here is a classification of RSPs according to the most important characteristics:
By accessories To various services in accordance with the Radio Regulations, RSPs are distinguished fixed services(radio communication between fixed points), broadcasting service (transmission of signals for direct reception by the population), RSP mobile services (radio communication between objects moving relative to each other);
Rice. 7. Diagram of a radio communication complex
By purpose differentiate international, main, intrazonal, local RSP, military RSP, technological RSP (for servicing railway transport facilities, power lines, oil and gas pipelines, etc.), space RSP (providing radio communications between spacecraft or between earthly points and spacecraft);
By range used radio frequencies or radio waves (see Table 1);
By mind transmitted signals distinguish RSP analog signals (telephone, radio broadcasting, fax, television, telemetry and telecontrol signals), RSP digital signals (telegraph, computer signals) and combined RSP;
By way separation channels (channel signals) distinguish between multi-channel RSPs With frequency division, temporary, phase And combined channel separation; there are also special RSPs with separation of channel signals according to form (for example, asynchronous-address systems with code-address separation of signals);
By mind linear signal differentiate analog, digital And mixed (hybrid) RSP. In analog RSPs, an analog signal is received at the input of the radio channel (trunk), and accordingly the radio signal is analog; Analog RSPs also include pulse RSPs, i.e. systems with pulse modulation (and time division of channels); in digital RSPs, a digital signal is received at the input of the radio channel and the propagation path (see Fig. 1); in mixed RSPs, the linear signal consists of an analog linear signal and a subcarrier modulated by a digital signal;
By mind modulation carrier analog RSPs are divided into systems with frequency, single lane And amplitude modulations, and digital RSPs - to systems with amplitude, frequency, phase And amplitude- phase manipulation;
By pass capabilities distinguish between RSP and small, average And high pass ability; The most commonly used bandwidth limits for various analog and digital RSPs are given in Table. 2.
table 2
"Or a television image transmission channel with one or more television and audio broadcasting sound transmission channels.
By character used physical process V propagation path radio waves distinguish: systems radio communications And radio broadcasting on long, average And short radio waves without repeaters; radio relay systems transfers straight visibility (RRSP), where radio waves propagate within line of sight; tropospheric radio relay systems transfers (TRSP), where long-distance tropospheric propagation of radio waves is used due to their scattering and reflection in the lower region of the troposphere with the relative position of radio relay stations beyond the line of sight; satellite systems transfers(SSP), using straight-line propagation of radio waves with their retransmission by an on-board repeater of an artificial Earth satellite (AES), located within the radio visibility of earth stations between which radio communication is carried out; ionospheric RSP on decameter waves (long-range propagation of decameter waves due to reflection from the layers of the ionosphere); space RSP (rectilinear propagation of radio waves in outer space and the Earth’s atmosphere); ionospheric RSP on meter waves (long-range propagation of meter waves due to their scattering by inhomogeneities of the ionosphere), etc.
To build multi-channel telecommunication systems, the most widely used are radio relay and satellite transmission systems using the decimeter, centimeter and millimeter ranges of radio waves. Modern mobile radio communication systems for various purposes are also being built in the same range. Earlier mobile radio systems used discrete sections of meter waves. Therefore, it seems necessary to consider the features of the propagation of these types of radio waves.
Radio transmission system- is a transmission system in which telecommunication signals are transmitted via radio waves in open space.
The simplest scheme radio communication is shown in Fig. 73. To ensure one-way radio communication, a radio transmitter And transmitting antenna, and at the reception point - radio receiver containing receiving antenna and radio. The message being transmitted comes from source information in the form of sound, alphabetic text, still image on converter (Pr.S), where it is converted into low frequency electrical signals. Depending on the type transmitted message the transducer is a microphone transmitting a telegraph or phototelegraph device, etc. The signal then goes to radio transmitting device, consisting of modulator M , midrange carrier purity synthesizer and modulated oscillation amplifier UMK. Using a modulator, one of the parameters of a high-frequency oscillation changes according to the law of the transmitted message. Modulated high frequency oscillations ( radio signals) are fed into the transmitting antenna, the energy of radio frequency oscillations of the transmitter is radiated into the radio wave propagation path.
Radio transmitting Radio receiving
device device
Rice. 73. Block diagram of radio communication
At the receiving station, radio waves, crossing the receiving antenna, induce an alternating emf in it . Radio receiver with the help of selective circuits (SC), interference signals from other radio stations are filtered out. IN detector (D) the original electrical signal that controlled the radio transmitter is isolated from the modulated oscillations. Converter (Pr.S) converts an electrical signal into a message, which enters a reproducing device - a loudspeaker, a direct-printing machine, etc., after which the received information reaches the recipient.
2.10. Construction principles radio relay lines communications
On RF networks they are used various systems radio communications: radio relay line of sight, tropospheric, decameter waves, ionospheric, satellite, etc.
A radio transmission system in which telecommunication signals are transmitted using terrestrial relay stations (Fig. 37) is called radio relay transmission system(RRSP).
For radio relay communication The International Radio Advisory Committee (ICRC) has allocated limited frequency bands of 0.4 in the decimeter, centimeter, and millimeter bands; 2; 4; 6; 8; eleven; 13 GHz.
50…70…100 km
Rice. 74. Construction of radio relay communication
At VHF and microwave frequencies used in radio linear joint ventures, reliable connection With low level interference should only be received under conditions of direct visibility between the antennas emitting radio waves. The distance between antennas of radio relay systems depends on the structure earth's surface and the height of the antennas above it. Typical distances reach 40...50 km with the heights of towers and masts on which the antennas are installed being about 100 m. In these ranges there is practically no atmospheric and industrial interference, there is no transient mutual interference between radio relay systems within one country and different countries due to the impossibility of free propagation of radio waves over long distances.
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Radio transmission system - an EASC transmission system in which telecommunication signals are transmitted via radio waves to open space.
The first class consists of radio information transmission systems. In these systems, the messages to be transmitted come from outside, from some sources, and the role of the radio system is to transmit them to the recipient. All systems in this class begin with converters, whose task is to convert incoming messages into electrical signals that are most suitable for transmission. At the output of the system there are also usually converters that produce messages in a form convenient for the recipient. This class includes communications, command, telemetry, broadcasting, television and phototelegraph radio systems.
When analyzing the effectiveness of digital radio systems for transmitting information from a test feedback calculate the residual probability Growth, i.e. the probability of those errors that are not detected and corrected as a result of multiple retransmission sessions. The values of Growth and rmax depend on the properties of both the forward and reverse RSPI channels and on the characteristics of the interference operating in these channels.
Radio relay transmission system - an EASC radio transmission system in which telecommunication signals are transmitted using terrestrial relay stations.
Space transmission system - an EASC radio transmission system that uses space stations, passive satellites or other space objects.
Meteor transmission system is an EASC radio transmission system that uses the reflection of radio waves from ionized meteor trails.
Determining the delay time and frequency of the radio signal turns out to be necessary in radio information transmission systems to implement in various ways synchronization For example, the quality of determining the carrier frequency characterizes the possibility of obtaining a reference voltage for a synchronous detector, and the determination of the delay time characterizes the accuracy of synchronization of symbols or words when receiving signals from a CMM. Measuring delay and frequency is the basis of radio systems that determine the range and speed of various emitting objects.
The information provided in the monograph may be useful for developers new space systems control of spacecraft, as well as students of higher education educational institutions in the specialties of Radio Engineering and Radio Information Transmission and Control Systems and other specialties.
Signals practically used in radio systems usually have a very complex structure. Thus, in multi-channel radio information transmission systems, the signal carries big number different messages, each of which modulates its own subcarrier. Designing a high-frequency radio link path is impossible without an idea of the structure of the signal passing through the path. It is usually necessary to evaluate general character spectrum, its position on the frequency axis and the occupied band. Often it is also necessary to know the finer structure.
A special message certifying the authenticity of the transmitted information is called an authenticator. Authenticators such as a signature and seal attached to a message to verify its authenticity are good if the message is transmitted on paper and cannot be changed without damaging the medium. When transmitting a message using signals used by radio-electronic systems in general and radio information transmission systems in particular, simply attaching a group of characters to the main text cannot reliably verify its authenticity.
A special area in the theory of radio systems (not touched upon here for many reasons) are problems associated with the conversion of electrical signals into an electromagnetic field and vice versa. It is here that the difference between radio systems for extracting and transmitting information most often and to a strong extent affects. If in radio information transmission systems the route and antennas determine mainly the energy potential and the nature of the interference, then in information retrieval systems they also determine the useful modulation of the received signal. Roughly speaking, in the first case information properties signals are determined only by the time structure electromagnetic field, and in the second case - spatiotemporal. The characteristics of the antenna system greatly influence the quality of the radio system as a whole, so the question of finding the best (optimal) way to convert the field into a signal is very important.
Transferring information using various types radio signals is always based on the fact that the message is embedded in some parameter of the signal. At the receiving end of the radio link, this parameter is measured and thus the transmitted message is determined. Since radio links always contain all kinds of interference, an error is introduced into the measurements, distorting the message. Depending on how the message is embedded in the signal, it will be distorted differently by interference. In this regard, when designing radio information transmission systems, the question arises about the most appropriate method of signal modulation. In radio systems with external modulation, it is necessary to select the shape of the emitted (probe) signal.
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Radio communication is a type of telecommunication carried out using radio waves. Radio waves are usually understood as electromagnetic waves, the frequency of which is above 30 kHz and below 3000 GHz, propagating in a medium without artificial guiding media (lines). The concept of radio frequency is closely related to the concept of a radio wave, i.e. radio wave frequencies.
The speed of propagation of electromagnetic waves in any medium is equal to
where c is the speed of light propagation in vacuum; ε- dielectric, μ - magnetic permeability of the medium. For air μ ≈ ε ≈ 1, and the speed of propagation of electromagnetic waves is close to the speed of light in vacuum, i.e. v≈ 3 · 10 8 m/s.
Electromagnetic waves are created by a source of periodically changing EMF with a period T. If at some moment the electromagnetic field (EMF) had a maximum value, then it will have the same value after time T. During this time the EMF will move a distance
The minimum distance between two points in space in which the field has same value, is called the wavelength. The wavelength depends on the speed of its propagation and
period of the TEDS transmitting this field. Since the current frequency is f = 1/T, then the wavelength
Wavelength λ is related to the oscillation frequency f by the known relation
Radio frequency spectrum is the frequency region occupied by radio waves. Frequency band - a frequency region limited by lower and upper limits. Frequency range - a frequency band to which a conventional name has been assigned.
In accordance with the Radio Regulations, the entire radio frequency spectrum is divided into 12 ranges, which are defined as radio frequency areas equal to (0.3...3) x 10N Hz, where N is the range number. For radio communication purposes, nine bands are used and, therefore, N = 4...12.
Radio wave range is a certain continuous section of radio wave lengths, which is assigned a conventional metric name. Each range of radio waves corresponds to a specific range of radio frequencies.
The classification of radio frequency ranges or radio waves is given in table. 1.1. This classification is primarily related to the characteristics of the propagation of radio waves and their use.
In addition, the following concepts are widely used in radio communication technology: operating range radio frequency band frequencies within which the radio station operates; grid of operating radio frequencies (frequency grid) - many following through specified intervals working radio frequencies; grid pitch of operating radio frequencies (frequency grid pitch) - the difference between adjacent discrete values of operating frequencies included in their grid; radio station - one or more transmitters and receivers or a combination thereof (including auxiliary equipment) necessary for radio communications; assigned radio frequency band - a frequency band within which a radio station is allowed to emit; working channel - frequency band that is used to transmit information (messages); assigned radio frequency - frequency, corresponding to the middle of the frequency band assigned to the radio station; operating radio frequency - a frequency intended for radio communications by a radio station.
Table 1.1
Classification of radio frequency bands or radio waves
| Range number | Wavelength range | Frequency range |
||
| Name | Name | |||
| Myriameter or ultra-long waves (VLW) | Very Low Frequencies (VLF) | |||
| Kilometer or long waves (LW) | Low Frequencies(LF) | 30... 300 kHz |
||
| Hectometric or medium waves (SW) | Mid frequencies (MF) | 300... 3000 kHz |
||
| Decameter or short waves(KB) | High frequencies (HF) | |||
| Meter or ultrashort waves (VHF) | Very high frequencies(VHF) | 30... 300 MHz |
||
| Decimeter waves (UHF) | Ultra high frequencies (UHF) | 300... 3000 MHz |
||
| Microwave | Ultrahigh frequencies (microwave) | |||
| Millimeter waves | Extremely high frequencies (EHF) | 30... 300 GHz |
||
| Decimillimeter waves | Hyper high frequencies (HHF) | 300... 3000 GHz |
||
To introduce other concepts and definitions, a generalized block diagram of a radio transmission system (RTS) should be considered. Under radio transmission system is understood as a totality technical means, providing the formation of standard channels and paths, as well as linear paths along which telecommunication signals are transmitted via radio waves in open space. Since the vast majority of RSPs are multi-channel, we present a generalized block diagram of a multi-channel RSP (Fig. 1.1), where the following notations are adopted:
KGO - channel-forming and group equipment that provides the formation of signals of standard channels and paths from a set of primary telecommunication signals to be transmitted at the transmitting end and reverse conversion of the signals of standard channels and paths into a set of primary signals at the receiving end.
SP - wire connecting lines that provide connection of channel-forming and group equipment to the distribution center in case of their territorial remoteness.
Rice. 1.1. Generalized structural scheme multi-channel radio communication system
To generate a radio signal and transmit it over distances via radio waves, various radio communication systems are used. A radio communication system is a complex of radio equipment and other technical means designed to organize radio communications in a given frequency range using a specific mechanism for propagating radio waves. Together with the medium (path) of radio wave propagation, the radio communication system forms linear path or a trunk consisting of trunk terminal equipment (TEE) and a radio trunk.
OOSper is the terminal equipment of the trunk of the transmitting end, where a linear signal is generated, consisting of an information group signal and auxiliary signals (service communication signals, signals for monitoring the performance of RSP equipment, etc.), which modulate high-frequency oscillations.
PCT is a radio channel whose purpose is to transmit modulated radio signals over distances using radio waves. A radio channel is called simple if it includes only two terminal stations and one radio wave propagation path, and composite if, in addition to two terminal radio stations, it contains one or more relay stations that provide reception, conversion, amplification or regeneration and retransmission of radio signals. The need to use composite radio channels is due to a number of factors, the main of which are the length of the radio line, its throughput and the mechanism of radio wave propagation. OOSpr is the terminal equipment of the receiving end trunk, where inverse transformations are carried out: demodulation of a high-frequency radio signal, selection of a group (multi-channel) signal and auxiliary service signals.
Radio receivers,
All radio systems are classified into the following groups:
1. radio information transmission systems
2. radio systems for information extraction (when information is not transmitted, but is extracted from a signal reflected from an object, for example, reflected from an object)
3. radio systems for information destruction (such radio systems must have a receiver)
Radio information transmission systems (RTIS)
The message source generates a signal x(t) - this can be a time process or a vector quantity. This message is encoded (modulated) and, using a transmitter and antenna, radiated through a radio channel. The receiver catches the message, processes, decodes (detects) and transmits the message to the recipient. Ideally, the fluctuation at the decoder output is proportional to the message x(t):
Where WITH- constant coefficient.
The source of the message, the transmitting and receiving parts of the radio system are spatially separated, which imposes certain conditions on the operation of the receiver. In this radio system, in addition to the useful signal, there is interference that can affect the encoder, transmitter, radio channel, and receiver. Any interference will distort both the signal and the message.
The problem arises: it is necessary to find such an optimal receiver structure in which the message would be reproduced the best way when there is a signal and noise at the input.
The main feature of a radio information transmission system: the transmitter and receiver are spatially separated.
Examples of RSPI: radio relay systems, radio telemetry systems.
Radio information retrieval systems (RSIS)
When the radio signal emitted by the transmitter reaches an object due to the propagation of electromagnetic waves, it is partially reflected, and information about the parameters of this object appears in the signal.
Examples of RSII: radar (radar) and radio navigation (RNS) systems.
If the receiver and transmitter are spatially combined, then this is a case of positional radar. This construction method gives great benefits for radar. If the receiver and transmitter are spatially separated, then this is bistatic radar.
Information destruction radio systems (RSIS)
The task of the transmitting part of the RSRI is to generate a signal and transmit it through a radio channel to the PRM in order to “destroy” the useful message x(t) in the signal at the demodulator output y(t).
1. Main characteristics and parameters
radio receiving devices (RPrU)
All RPRUs differ in frequency range operation of the radio system.
The RPRU includes: the receiver itself, the terminal device (load), and the power source.
Classification: 1) RPRU direct amplification;
2) RPRU of superheterodyne type.
1.1. RPRU direct amplification
Advantages: Simplicity.
Flaws: Low quality reproduced signal, limited reception range, etc.
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1.2. Superheterodyne type RPR
Advantages; a frequency converter is used, so the main amplification occurs at the intermediate frequency, high sensitivity, noise immunity, low linear and nonlinear distortions.
A variety of heterodyne type RPRUs are homodyne (synchronous) RPRU. In these receivers, the local oscillator frequency is equal to the carrier frequency input signal. As a result, such receivers combine the operations of heterodyning and signal demodulation.
According to the structure of construction, RPRUs are divided into analog, analog-digital and digital.
1.3. Main characteristics of RPRU
1. Sensitivity- ability of the PRM to accept weak signals in the presence of external interference. Quantification- this is the minimum signal power at the PRM input, at which the specified signal-to-noise ratio is ensured at the output of the linear part of the receiver.
2. Noise immunity- the ability of the receiver to ensure reception of signals with a given reliability at known method signal transmission and the presence of interference in the path.
3. Selectivity. First of all, this is frequency selectivity - the ability of the receiver to select useful messages within a given frequency band and attenuate the effect of signals outside this band.
Selectivity is divided into: mirror channel selectivity, selectivity direct channel(these types of selectivity are provided by the RPR preselector), selectivity on the adjacent channel (provided by the UPC).
4. Dynamic range - determined by the amplitude characteristic of the receiver.
Frequency distortion. Phase distortion.
 |
Nonlinear distortions are determined by the nonlinearity of the amplitude characteristic of the receiver and are estimated by the value:
6. Electromagnetic compatibility - the ability of the radio control unit to operate taking into account the impact of various nearby radio systems on each other.
2. Main types of signals and interference
All signals are divided into narrowband and broadband:
For narrowband signals the following condition is true: - where Df with- band,
f 0- carrier frequency.
For broadband, the following condition applies:
Narrowband signals can be represented as:
Where U0(t)- reflects the law amplitude modulation;
j(t)- angular modulation; j 0- initial phase.
All signals can be random or deterministic.
If the modulating function is random, then the signal is random.
All noise and interference can be divided into internal and external. The causes of internal interference are the receiving device itself, usually the internal noise of the RPR.
External interference is that which exists in the radio wave propagation path.
IN general case There is a mixture of signal and noise at the RPR input:
y(t) = U c (t)Ä U П (t)
Interference is divided into three types:
1. Additive(input mixture is the sum):
2. Multiplicative(multiplication):
Multiplicative noise modulates the signal itself:
Complex signal envelope: 
Complex interference envelope: 
3.Additive and multiplicative noise
Interferences are divided according to their temporary nature into: continuous, discrete And pulse. There may be interference narrowband And broadband.
