For the Ku and Ka bands, the C / N carrier-to-noise ratio is relevant prior to demodulation at the receiver. The S / N ratio matters after demodulation. Thus, the S / N ratio depends on both the C / N ratio and the modulation and coding characteristics.
The transmitted signal may be misinterpreted by the receiving device due to various interference and distortions arising from its transmission over a noisy communication channel. To increase the noise immunity, different methods coding. Therefore, the output of the information source is connected to the encoder of the communication channel, where redundancy is introduced into the signal in order to reduce the probability of occurrence of erroneous bits. This procedure is called preliminary error correction (FEC) and is the only method to provide error correction without requesting retransmission of data. The bit error rate is related to the bit error rate (BER) of the receiving decoder. An indicator of the quality of the received signal in digital systems transmission, as is known, is the ratio E b / N 0, at which a certain value of BER is achieved, and which is equivalent to the S / N ratio for digital systems.
The ratio between C / N and E b / N 0, expressed in decibels, is determined by the following formula:
E b / N 0 = C / N + 10 log (1 / R) + 10 logDf, dB (5.32)
Where E b / N 0 dB is the ratio of the amount of energy in the E b bit (J) to the noise power flux density N 0 (W / Hz); R - information transfer rate, bit / s; Df - frequency band occupied by the channel, Hz; C / N - carrier-to-noise ratio in the Df frequency band, dB.
A characteristic feature of practical digital systems is the following: for a given ratio of the bit rate of information to the channel bandwidth, there is a signal-to-noise ratio, above which it is possible to receive a signal without errors and below which reception is impossible. Unlike analog signals, which are progressively degraded by noise, digital systems are relatively immune to noise until the error correction system can no longer operate effectively. The result is a sudden deterioration or collapse of the system. This property of digital systems eliminates the need for quality gradations. The quality of the received signal will not suffer if the total degraded level of the ratio E b / N 0 is higher than some required level corresponding to the acceptable bit error probability() or a certain BER value. The relationship between and E b / N 0 depends on the specific features of the chosen digital modulation method, therefore, satellite operators usually stipulate the minimum required level of the ratio E b / N 0. BER = excellent quality. The BER at the demultiplexer input depends on two factors: the quality of the input signal and correcting ability of anti-jamming code FEC. The FEC number indicates the redundancy of the anti-jamming code.
The required signal-to-noise ratio for high-quality reception of a digital signal with a BER value equal to is determined from the table.
Communication system characterized by a set of parameters. Those of them that are associated with the quality of the system by monotonic dependence are called indicators of the quality of the system. The more (less) the value of the quality indicator, the better (worse) the system, other things being equal.
When designing the system, take into account a large number of quality indicators and parameters in accordance with a previously justified criterion of optimality. The best (optimal) system is considered to be the one that corresponds to the largest (smallest) value of a certain objective function of the quality indicators. Quality indicators and parameters of communication systems are conventionally divided:
- for information (noise immunity, speed, bandwidth and delay of information transmission);
- technical and economic (cost, dimensions, weight);
- technical and operational indicators (mean time of failure-free operation, operating temperature range, etc.).
Let's highlight indicators characterizing communication system in terms of information transfer.
Noise immunity is one of the main indicators of the quality of a communication system. Noise immunity for a given interference is characterized by transmission fidelity - the degree of correspondence of the received message to the transmitted message. When transmitting continuous messages, the measure of fidelity is the standard deviation between the received a "(t) and transmitted a (t) messages:
where T - the time during which the message is received.
Primary signal b(t) linked to post a(t) linear dependence, i.e.
b(t) = ka(t),
where k - conversion factor.
where an asterisk denotes a signal estimate that differs from this signal by the amount of error.
The smaller the standard deviation, the higher the noise immunity.
The measure of fidelity can also be the probability that the error ε does not exceed a predetermined valueε 0:
The greater this probability, the higher the noise immunity.
The measure of the fidelity of the transmission of discrete messages is probability of error. The less this probability, the greater the noise immunity.
The maximum noise immunity possible for a given transmission conditions is called potential noise immunity.
Another important indicator of the quality of a communication system is its throughput, those. the maximum baud rate R max allowed by the system. It is defined by the number N channels of this system and throughput C communication channel:
For discrete communication channel without interference
where T- the duration of the transmission of one symbol; m - volume of the alphabet. (Here and below, notation of the form logx denotes the binary logarithm operation log 2 x.)
For a continuous communication channel
WITH= Flog (l + P with / P NS) ,
where F - channel bandwidth; Rс - signal strength; R NS - noise power.
The transmission rate (as well as the throughput) is measured in bits per second.
Transmission delay- this is the time from the moment of the beginning of the message transmission in the transmitter until the moment of the output of the restored message at the output of the receiver. It depends on the length of the communication channel and the duration of signal transformations in the transmitter and receiver. Transmission delay is one of the most important indicators of the quality of a communication system.
The method of complex statistical estimation is based on calculating statistics of temporal and spectral parameters and their changes based on a comparison of the distorted in the channel and the original signals. This technique combines the ability to assess by subjective criteria and hardware objectivity of measurements. It is based on the laws of human perception of external stimuli.
The high correlation of changes in signal properties and a subjective assessment of the transmission quality by the listener makes it possible to form an assessment according to the following criteria:
- conspicuousness signal changes;
- score transmission quality;
- preference transmitted signal.
When assessing according to the criterion of "noticeable signal changes", the maximum permissible distortions correspond to 50% visibility. This is the limit for 6300 Hz channels. For channels with a bandwidth of 10 kHz - P max = 30%, and for channels with a bandwidth of 15 kHz - P max = 15%.
The value integral deviation ∆S (the numerical value of the sum of the absolute deviations of the functions - IO) from two distributions of the OCM for the original and distorted signals. In fig. 7.5 are given (LCHP) OCM at analysis intervals of 200 ms for two types of distortions: compact representation using the MP-3 algorithm and lower band limiting.
AND ABOUT ∆S allows you to estimate the degree of divergence of the two distributions... When introducing distortions into the signal, the EUT repeats the change in the average value of the LCHP, being determined by the change in the RMS and the moments of higher orders. To confirm the correlation relationship, the quantity ∆S with noticeable distortion R, introduced by the ZVS transmission path, in Fig. 7.14 shows the integral curve of the visibility of distortions of the “lower band limiting” type (dashed curve), obtained when real signals are perceived. The solid lines in the same figure show the estimates of the integral deviations of the NCHPZ OCM broadcast signals subjected to the same distortions. The measurements were carried out on the hour programs of the RVS "Mayak", "Orpheus" and "Echo of Moscow". The high correlation of dependences is obvious. The plots for all other characteristic distortions are of a similar nature, which makes the widespread use of this criterion in various applications justified.
 |
 |
The nature and type of distortion can be determined using statistics of a number of other parameters of the DRS. The most informative of them turned out to be, in addition to the energy parameter of the OCM, form parameters, reflecting changes in the envelope in the areas of non-stationarity of the ZVS, i.e., the attacks and decays of the signal. In fig. 7.15 shows the change in EUT NCHPZ steepness of attacks for the same as in Fig. 7.14, signals with the same distortion.
The use of other sections of the non-stationarity of the ZVS - signal decays - is also promising, since changes in the sections of the audio signal decays are perceived as changes in the reverberation time, i.e., the acoustic environment, which negatively affects the subjective assessment of the sound.
In fig. 7.16 and 7.17 show the change in EUT NCHPZ OCM and attack steepness respectively, for the same as in Fig. 7.14 and 7.15, signals, but when introducing nonlinear distortions.
 |
 |
Highly informative for channels eliminating redundancy are cepstrap parameters. In fig. 7.20 shows the dependence of the cepstrum crest factor on the transmission rate using the MP-3 algorithm. for two broadcast programs of one hour duration. A good correlation of the peak factor of the cepstrum as an objective parameter with the subjective visibility of distortions is noticeable (dashed curve).
In fig. 7.21 shows the graphs of the correspondence between the scoring of changes in the signal with a decrease in the coding rate from 256 kbit / s to 64 kbit / s (curve 1) and the percentage noticeability of similar changes obtained with the FID (curve 2).
The curves are identical, which indicates the equivalence of the point and percentage scales of visibility.
- ADC of the initial and passed signal transmission channel;
- normalization of signals at the level exceeded for 98% of the time;
- signal synchronization;
Signal analysis in accordance with MKSO, which involves calculating statistics of a number of parameters and their changes based on a comparison of the processed and original signals;
Formation of a comprehensive assessment of signal changes in the process of transmission, as well as the frequency response of the channel on a real broadcast signal. Data output to the screen, printing and saving in the database.
In accordance with the ICSO, a group of statistical characteristics is analyzed, namely: energy characteristics(relative average power in two varieties, differing in the standardization method - OSMs and OSMk); shape characteristics(analysis on the intervals of rise and fall of the filtered Hilbert envelope of the signal); spectral and cepstral characteristics(based on instantaneous amplitude spectra).
The result of the analysis of each group of parameters is normalized statistical frequency of occurrence of values(NCHPZ) parameter. On the basis of NCHPZ there is integral deviation(IO) NCHPZ as the averaged value of the absolute deviations of the frequencies of occurrence of the values of the signal parameters before and after the passage of the channel. For the case of spectral characteristics, a graphical representation of the channel frequency response, found from instantaneous amplitude spectra, is additionally carried out, and data on the root-mean-square deviation (RMS) from the ideal frequency response is also formed.
In accordance with the MKSO, based on the results of the analysis of changes in the statistical characteristics of the signal, a generalized assessment of the noticeability of signal changes in percent and a "point estimate" of the transmission quality on a 5-point scale is formed. A hardware and software complex that calculates, constructs and analyzes the statistical characteristics of a number of parameters, as well as changes in these characteristics based on a comparison of the distorted in the channel and the original signals, can be used as a measuring tool for the MKSO.
The complexity of the formation of the assessment in accordance with the MKSO is significantly lower, and the accuracy and repeatability are much higher than when conducting the SIS. Moreover, this method of assessing the quality of transmission is not opposed to traditional changes in accordance with GOST 11515-91.
2. COMMUNICATION SYSTEMS AND THEIR MAIN CHARACTERISTICS
2.1. Basic concepts and definitions
The object of transmission in any communication system is a message that carries any information.
In message transmission systems, the semantic content of the concepts of information and message are very similar.
In general, information is understood as a set of information about any events, phenomena or objects. To transfer or store information, various signs (symbols) are used to express (represent) information in some form. These can be letters, numbers, gestures and drawings, mathematical or musical symbols, words and phrases of human speech, various realizations of forms of electrical oscillations, etc.
The message is understood as the form of information presentation. In other words, a message is something to be transmitted. The set of possible messages with their probabilistic characteristics is called ensemble of messages. The choice of messages from the ensemble is carried out by the source of the messages. The selection process is random; it is not known in advance which message will be transmitted. Distinguish between discrete and continuous messages.
Discrete messages are formed as a result of sequential issuance by the source individual elements- signs. Many different signs are called message source alphabet, and the number of characters is volume of the alphabet. In particular, characters can be letters of a natural or artificial language that satisfy certain rules of relationship.
Messages intended for processing in computer information systems, it is customary to call data.
The message is a sequence of states source of information, deployed in time. The sources are divided into
discrete and continuous (analog). Under discrete a source of information is understood to be some object, which at certain points in time receives one of the M states of a discrete set. A continuous source at any moment of time can take one of an infinite set of its states. The concept of a source of messages is introduced accordingly, and all possible sources can be divided into discrete and continuous.
To transmit a message over a distance, it is necessary to have some kind of carrier, a material carrier. As such, static or dynamic means, physical processes are used. Physical
the process used to carry the message and display the message being transmitted is called a signal.
The display of the message is provided by a change in any physical quantity that characterizes the process. This value is
informational parameter of the signal.
Signals, like messages, can be continuous and discrete. The information parameter of a continuous signal over time can take on any instantaneous values within certain limits. A continuous signal is often referred to as an analog signal. A discrete signal is characterized by a finite number of information parameter values. Often this parameter takes only two values.
In telecommunication systems, electrical signals are used as a carrier used to transmit messages over a distance, since they have the highest propagation speed (approaching the speed of light in a vacuum - 3 108 m / s).
Any physical process that changes in accordance with the transmitted message can be used as a signal. It is essential that the signal is not the physical process itself, but a change in individual parameters of this process. These changes are determined by the message that carries this signal.
In many cases, the signal reflects the temporal processes occurring in a certain system. Therefore, the description of a specific signal can be some function of time. Having defined this function in one way or another, we also define the signal. However, such Full description signal is not always required. To solve a number of tasks, more general description in the form of several generalized parameters characterizing the basic properties of the signal, similar to how it is done in transportation systems.
The information transmission technique is, in essence, the technique of transporting (transmitting) signals through communication channels. Therefore, it is advisable to determine the parameters of the signal, which are basic from the point of view of its transmission. These parameters are signal duration, dynamic range and spectrum width.
Any signal considered as a temporal process has a beginning and an end. That's why signal duration T is its natural parameter, which determines the time interval within which the signal exists.
The characteristics of the signal within the interval of its existence are the dynamic range and the rate of change of the signal.
Dynamic range is defined as the ratio of the highest instantaneous signal power to the lowest:
Д = 10 log P c max, (dB).
P cmin
The dynamic range of the speaker's speech is 25 ÷ 30 dB, vocal
ensemble - 45 ÷ 55 dB, symphony orchestra - 65 ÷ 75 dB.
V real conditions are always interfered with. Satisfactory transmission requires the lowest signal power to exceed the interference power. The signal-to-noise ratio characterizes the relative signal strength. Usually the logarithm of this ratio is determined, which is called the excess of the signal over the interference. This excess is taken as the second signal parameter. The third parameter issignal spectrum width F. This value gives an idea of the rate of change of the signal within the interval of its existence. The signal spectrum can extend over a very large frequency band. However, for most signals, you can specify the frequency band within which its main energy is concentrated. This band determines the width of the signal spectrum.
V In communications technology, the signal spectrum is often deliberately limited. This is due to the fact that the equipment and the communication line have a limited bandwidth. The spectrum limitation is carried out based on the allowable signal distortion. For example, during telephone communication, two conditions must be met: so that the speech is legible and the correspondents can recognize each other by their voice. To meet these conditions, the speech signal spectrum can be limited to a band from 300 to 3400 Hz. The transmission of a wider range of speech in this case is impractical, as it leads to technical complications and increased costs.
A more general physical characteristic of a signal is the signal volume:
If ν ≤ 1, then the signals are called narrowband (simple). If ν >> 1, then - broadband (complex).
In natural conditions, signals created and received by living things propagate in their habitat. This environment can be called message transmission channel. We note right away that even
v In such a simple transmission system, the presence of interference in the channel is typical, i.e. signals generated by extraneous sources. With the emergence of the need for fast transmission of messages over long distances, a person has a need for the use of various devices ("technical means"). In modern transmission systems
v quality physical media information is used by electric currents or voltages, as well as electromagnetic oscillations.
When transmitting messages, it becomes necessary to use such technical means as sensors - converters of various
physical processes in low-frequency electric currents, called primary signals(for example, microphone, vidicon); devices for encoding discrete messages used both to match the power of the alphabet of the source M and the number of code symbols used in the transmission channel, and in order to ensure high reliability of transmission; devices for modulating high-frequency signal carriers with primary signals. Since the receiver perceives the message, as a rule, in the form that is presented at the output of the original source, the transmission system requires such technical means as a demodulator, a decoder, which carry out the inverse transformation of high-frequency signals into analogs of primary, low-frequency signals into analogs of the original messages ( for example, using a speaker, picture tube, etc.).
2.2. Communication systems
The set of hardware (hardware and software) and distribution media required to transfer a message from source to destination is called a communication system. In functional diagrams and their implementations, nodes such as an encoder and a modulator are combined in a transmitting device; similarly, the demodulator and the decoder are combined into a single device- the receiver. A typical functional diagram, including the main nodes of the communication system, is shown in Fig. 1.2. The communication line indicated here, in many cases identified with the transmission channel, is designed to transmit signals with the minimum possible loss of their intensity from the transmitter to the receiver. In electrical communication systems, a communication line, in particular, is a pair of wires, a cable or a waveguide, in radio communication systems, a region of space in which electromagnetic waves propagate from a transmitter to a receiver.
In the communication line, interference w (t) inevitably present in the communication system is localized, leading to random unpredictable distortion of the transmitted signal shape.
Rice. 2.1. Generalized structural scheme telecommunication systems
The receiver processes the received signal x (t), distorted by interference, and recovers the transmitted message u (t) from it. Typically, the receiver performs the opposite of the transmitter.
It is customary to call a communication channel a set of technical means serving to transfer a message from a source to a consumer. These means are a transmitter, a communication line and a receiver.
The communication channel together with the source and the consumer form information transmission and processing system... Distinguish discrete messaging systems(for example, a telegraph communication system) and continuous messaging systems(radio broadcasting, television, telephony systems, etc.). There are also mixed communication systems in which continuous messages are transmitted by discrete signals. Such systems include, for example, pulse code modulation systems.
When sending messages in one direction from the sender to the recipient, or from "point to point", a point-to-point one-way communication channel is used. If the source and the receiver alternately swap places, then for signal exchange it is necessary to use an alternate two-way communication channel that allows transmission both in one and in the opposite direction (half-duplex mode). Greater opportunities for exchange are provided by a simultaneous two-way communication channel, which provides simultaneous transmission of signals in opposite directions (full duplex mode).
A communication system is called multichannel if it provides the mutually independent transmission of several messages over one common communication channel.
If it is necessary to exchange messages between many senders and recipients, in this case called users or subscribers, it is required to create messaging systems (MTS) with a large number of communication channels. This leads to the concept of a message transmission and distribution system (MRS), i.e. communication systems in a broad sense. Such a system is commonly referred to as a communication (telecommunication) network, an information transmission network, or a messaging network. An example of an SRS is a fully connected network (Fig. 1.1), where end points (EP) are connected to each other on the principle "each with each".
Figure 2.2. Fully connected information transmission network
This network is nonswitched, and communication between subscribers is carried out through permanently fixed (nonswitched) channels. The distribution of information in such networks is provided by special access methods or information transfer control procedures that serve to notify which subscribers will exchange messages. With an increase in the number of subscribers in a multipoint network, delays in information transmission significantly increase, and in fully connected networks, the number of communication lines and the amount of equipment significantly increase. The solution to these problems is associated with the use of switched networks SPRS, where subscribers communicate with each other not directly, but through one or more switching nodes (CC).
Thus, the switched SPRS is a set of OP, switching nodes and communication lines connecting them.
The main task of modern SPRS is to provide a wide range of users (people or organizations) with a variety of information services, which include, first of all, the efficient delivery of messages from one point to another, which meets the requirements for speed, fidelity, delay time, reliability and cost.
The statistical characteristics of the call flow are studied by methods of queuing theory, in particular teletraffic theory. This theory makes it possible to establish requirements for switching devices and the number of lines, at which satisfactory communication quality is guaranteed for a given percentage of failures or latency.
So, for example, the load of the telephone network depends on the number, time of occurrence and duration telephone conversations.
The load intensity is understood as the mathematical expectation of the incoming load, referred to a unit of time (in telephony - 1 hour).
Erlang (1 hour session) is taken as the unit of measurement of the intensity of the load. During the day, the load changes, the hour of the greatest load is called CHNN. Each subscriber, on average, gives a load in the range of 0.06 ...
0.15 Earl. These values are used to calculate the telephone network and its switching systems.
The source of information in the communication system (see Fig. 2.1) is the sender of the message, and the consumer is its recipient. In some systems of information transmission, the source and consumer of information can be a person, while in others - various kinds of automatic devices, computers, etc.
Converting a message to a signal involves three operations:
– conversion from non-electrical to electrical form;
– primary coding;
– transformation in order to match the characteristics of the signal with the characteristics of the communication channel.
These three operations can be independent or combined.
At the first stage, the message is converted with the help of sensors into an electrical quantity - the primary signal.
The main primary telecommunication signals are: telephone (voice), sound broadcasting, facsimile, television, telegraph, data transmission (for example, text input from the keyboard).
In order for the received message to correspond most closely to the transmitted one, it is advisable to carry out the transmission of signals in discrete form. Analog signals are converted to discrete in the quantization process, in which the continuous range of signal values is subdivided into discrete areas so that all signal values falling into one of these areas are replaced with one discrete value. In this case, quantization takes place not only in terms of some signal parameter, for example, in amplitude, but also in time.
The second stage of converting a message into a signal - encoding - consists in converting letters, numbers, signs into certain combinations of elementary discrete symbols, called code combinations or words. The rule for this transformation is called code. The purpose of coding, as a rule, is the matching of the message source with the communication channels, which ensures either the maximum possible information transfer rate or the specified noise immunity. The coordination is carried out taking into account the statistical properties of the message source and the nature of the effect of interference.
At the third stage, the primary signals u (t) are converted into signals convenient for transmission over the communication line (in shape, power, frequency, etc. These operations are performed in the transmitter. In the simplest case, the transmitter may contain an amplifier of primary signals or only a filter , limiting the bandwidth of transmitted frequencies. In most cases, the transmitter is a carrier generator (carrier) and a modulator. The modulation process consists in controlling the carrier parameters with the primary signal u (t). At the output of the transmitter, we obtain a modulated signal s (u, t).
An information transmission system is called multichannel if it provides the mutually independent transmission of several messages over one common communication channel.
The communication channel can be characterized in the same way as the signal, by three parameters: the time during which the channel is transmitting, the dynamic range and the channel bandwidth. For undistorted signal transmission, the channel capacity V k must not be less than the signal volume.
The common features of the various channels are as follows. First, most channels can be considered linear. In such channels, the output signal is simply the sum of the input signals (superposition principle). Secondly, at the channel output, even in the absence of a useful signal, there is always interference. Third, the signal, when transmitted over the channel, undergoes a time delay and attenuation in level. And, finally, in real channels there are always signal distortions caused by channel imperfections.
The signal at the channel output can be written as follows:
x (t) = µ s (t - τ) + w (t),
where s (t) is the signal at the channel input; w (t) - interference; µ and τ are the quantities characterizing the attenuation and delay time of the signal.
2.3. The main indicators of the quality of the communication system
Based on the purpose of any telecommunication system - the transmission of information from source to consumer - it is possible to evaluate the operation of the system by two indicators: the quality and quantity of transmitted information. These indicators are inextricably linked.
The quality of transmitted information is usually assessed by the reliability (fidelity) of message transmission. Quantitatively, reliability is characterized by the degree of correspondence between the received message and the transmitted one. Decrease in reliability in the communication channel occurs due to the action of interference and distortion. But since the distortion in the channel, in principle, can be compensated for and in properly designed channels they are small enough, then the main reason decreases in confidence are interference. Thus, the fidelity of message transmission is closely related to noise immunity systems, i.e. its ability to withstand the interfering effects of extraneous signals. The system is the more noise-resistant, the higher the transmission fidelity it provides for given characteristics of interfering influences and a certain power of transmitted signals reflecting the state of the source. The quantitative measure of reliability is chosen differently depending on the nature of the message.
If a message is a discrete sequence of elements from a certain finite set, the effect of interference is manifested in the fact that instead of the actually transmitted element, some other element can be received. This event is called an error. The probability of error p or any increasing function of this probability can be taken as a quantitative measure of confidence.
An indirect measure of quality can be an estimate of the degree of distortion of the form of the received standard signals (edge distortion, fragmentation, fluctuations of the fronts, etc.). These distortions are also normalized for discrete channels. There are simple relationships for converting waveform distortion into error probability.
When transmitting continuous messages, the degree of correspondence of the received message v (t) to the transmitted u (t) can be a certain value ε, which is the deviation of v from u. The criterion of square deviation is often adopted, expressed by the ratio:
ε 2 = 1 T ∫ [v (t) - u (t)] 2 dt. T 0
The root-mean-square deviation ε 2 takes into account the influence on the received message ν (t) of both interference and all kinds of distortions (linear, non-linear).
The fidelity of the transmission depends on the signal-to-interference power ratio. The larger this ratio, the less the probability of error (the greater the reliability).
For a given interference intensity, the error probability is the less, the more the signals corresponding to different elements of the message differ from each other. The challenge is to select signals with a large difference for transmission.
The reliability also depends on the method of reception. It is necessary to choose such a reception method that best realizes the distinction between signals for a given signal-to-interference ratio. A properly designed receiver can increase the signal-to-interference ratio, and quite dramatically.
An indirect assessment of the quality of transmission of continuous messages is given according to the characteristics of the channels (frequency, amplitude, phase, noise level, etc.), according to some parameters of signals and interference (distortion factor, signal-to-noise ratio, etc.), according to subjective perception messages. The quality of a telephone connection, for example, can be assessed by speech intelligibility.
There is a significant difference between discrete and continuous messaging systems. In analog systems, any, even arbitrarily small, interfering effect on the signal, causing distortion of the modulated parameter, always entails the introduction of the corresponding error into the message. In systems for transmitting discrete messages, an error occurs only when the signal is reproduced (recognized) incorrectly, and this occurs only with relatively large distortions.
In the theory of noise immunity developed by V.A. Kotelnikov, it is shown that for a given coding and modulation method, there is a limiting (potential) noise immunity, which in a real receiver can be achieved, but cannot be surpassed. A receiving device that implements potential noise immunity is called an optimal receiver.
Along with the reliability (noise immunity), the most important indicator of the operation of the communication system is transmission speed. In discrete messaging systems, the rate is measured by the number of transmitted binary symbols per second R. For one channel, the transmission rate is determined by the ratio
R = 1 log 2 m,
where T is the duration of the signal elementary message; m is the base of the code. For m = 2, we have R = 1 / T = v, Baud.
The maximum possible transmission rate R max is usually called
system bandwidth. The capacity of an analog message transmission system is estimated by the number of simultaneously transmitted telephone conversations, broadcasting or television programs, etc.
System capacity Rmax should not be confused with
bandwidth of the communication channel C (see chapter 4). The bandwidth of a communication system is a technical concept that characterizes the equipment used, while the bandwidth of a channel determines the potential of a channel to transmit information. In real systems, the transmission rate R always less bandwidth WITH. In information theory, it is proved that for R ≤ C it is possible to find such transmission methods and corresponding reception methods in which the transmission reliability can be made arbitrarily large.
From what has been considered it follows that the quantity and quality of the transmitted information in the communication channel is mainly determined by the interference in the channel. Therefore, when designing and operating communication systems, it is necessary to achieve not only small distortions of the received primary signal, but also a specified excess of the signal over the interference. Usually the signal-to-noise ratio for the received primary signals is normalized.
Latency is an important characteristic of a communication system. The delay is understood as the maximum time elapsed between the moment the message is sent from the source to the input of the transmitting device and the moment the restored message is issued by the receiving device. The delay depends, firstly, on the nature and length of the channel, and secondly, on the duration of processing in the transmitting and receiving devices.
Control questions
1. What is meant by a message and a signal?
2. Draw a functional diagram of the information transmission system.
3. What is called a communication channel? What types of channels do you know?
4. How is a continuous message converted into a signal?
5. What is transmission fidelity and how is it quantified?
6. Give a definition of the main characteristics of the signal?
7. What is modulation?
8. How is the transmitted message restored at the receiver?
9. What parameters determine the quality of information transmission and the amount of transmitted information?
10. What is meant by the bandwidth of a communication system?
State exam
(State examination)
Question No. 3 “Communication channels. Classification of communication channels. Communication channel parameters. Condition for signal transmission over a communication channel ".
(Plyaskin)
Link. 3
Classification. 5
Characteristics (parameters) of communication channels. ten
Condition for signal transmission over communication channels. 13
Literature. fourteen
Link
Link- a system of technical means and a signal propagation medium for transmitting messages (not only data) from a source to a receiver (and vice versa). The communication channel, understood in the narrow sense ( communication path), represents only the physical medium of signal propagation, for example, a physical communication line.
The communication channel is designed to transmit signals between remote devices. Signals carry information intended for presentation to a user (person), or for use by computer applications.
The communication channel includes the following components:
1) transmitting device;
2) receiving device;
3) transmission medium of various physical nature (Fig. 1).
The information-carrying signal generated by the transmitter, after passing through the transmission medium, enters the input of the receiving device. Further, information is extracted from the signal and transmitted to the consumer. The physical nature of the signal is chosen so that it can propagate through the transmission medium with minimal attenuation and distortion. The signal is necessary as a carrier of information, it itself does not carry information.
Fig. 1. Communication channel (option number 1)
Fig. 2 Communication channel (option no. 2)
Those. this (channel) - technical device(technique + environment).
Classification
There will be exactly three types of classifications. Choose the taste and color:
Classification No. 1:
There are many types of communication channels, among which the most commonly distinguished channels wired communication ( aerial, cable, light-guide etc.) and radio communication channels (tropospheric, satellite and etc.). Such channels, in turn, are usually qualified based on the characteristics of the input and output signals, as well as on the change in the characteristics of the signals, depending on such phenomena occurring in the channel as fading and attenuation of signals.
By the type of distribution medium, communication channels are divided into:
Wired;
Acoustic;
Optical;
Infrared;
Radio channels.
Communication channels are also classified into:
Continuous (at the input and output of the channel - continuous signals),
Discrete or digital (at the input and output of the channel - discrete signals),
· Continuous-discrete (continuous signals at the channel input, and discrete signals at the output),
· Discrete-continuous (discrete signals at the channel input, and continuous signals at the output).
Channels can be like linear and nonlinear, temporary and spatio-temporal.
Possible classification communication channels by frequency range .
Information transmission systems are single-channel and multichannel... The type of system is determined by the communication channel. If a communication system is built on the same type of communication channels, then its name is determined by the typical name of the channels. Otherwise, the specification of the classification features is used.
Classification No. 2 (more detailed):
1. Classification by frequency range
Ø Kilometer (LW) 1-10 km, 30-300 kHz;
Ø Hectometric (SV) 100-1000 m, 300-3000 kHz;
Ø Decameter (HF) 10-100 m, 3-30 MHz;
Ø Meter (MV) 1-10 m, 30-300 MHz;
Ø Decimeter (UHF) 10-100 cm, 300-3000 MHz;
Ø Centimeter (CMB) 1-10 cm, 3-30 GHz;
Ø Millimeter (MMV) 1-10 mm, 30-300 GHz;
Ø Decimitre (DMMV) 0.1-1 mm, 300-3000 GHz.
2. By direction of communication lines
- directed ( different conductors are used):
Ø coaxial,
Ø twisted pairs based on copper conductors,
Ø fiber optic.
- non-directional (radio links);
Ø line of sight;
Ø tropospheric;
Ø ionospheric
Ø space;
Ø radio relay (retransmission on decimeter and shorter radio waves).
3. By the type of transmitted messages:
Ø telegraph;
Ø telephone;
Ø data transmission;
Ø facsimile.
4. By the type of signals:
Ø analog;
Ø digital;
Ø impulse.
5. By the type of modulation (manipulation)
- In analog communication systems:
Ø with amplitude modulation;
Ø with single sideband modulation;
Ø with frequency modulation.
- In digital communication systems:
Ø with amplitude shift keying;
Ø with frequency shift keying;
Ø with phase shift keying;
Ø with relative phase shift keying;
Ø with tone shift keying (single elements manipulate the subcarrier oscillation (tone), after which the keying is carried out at a higher frequency).
6. By the value of the base of the radio signal
Ø broadband (B >> 1);
Ø narrow-band (B "1).
7. By the number of simultaneously transmitted messages
Ø single-channel;
Ø multichannel (frequency, time, code division of channels);
8. In the direction of messaging
Ø one-sided;
Ø
bilateral.
9. By order of message exchange
Ø simplex communication- two-way radio communication, in which the transmission and reception of each radio station is carried out in turn;
Ø duplex communication- transmission and reception are carried out simultaneously (the most efficient);
Ø half-duplex communication- refers to the simplex, which provides for an automatic transition from transmission to reception and the possibility of re-asking the correspondent.
10. By methods of protection of transmitted information
Ø open communication;
Ø closed communication (classified).
11. By the degree of automation of information exchange
Ø non-automated - control of the radio station and exchange of messages is performed by the operator;
Ø automated - only information is entered manually;
Ø automatic - the process of messaging is performed between an automatic device and a computer without the participation of an operator.
Classification number 3 (something can be repeated):
1. By appointment
Telephone
Telegraph
Television
Broadcasting
2. By transfer direction
Simplex (transmission in one direction only)
Half duplex (alternate transmission in both directions)
Duplex (simultaneous transmission in both directions)
3. By the nature of the communication line
Mechanical
Hydraulic
Acoustic
Electrical (wired)
Radio (wireless)
Optical
4. By the nature of the signals at the input and output of the communication channel
Analog (continuous)
Discrete in time
Discrete by signal level
Digital (discrete and in time and in level)
5. By the number of channels per communication line
Single channel
Multichannel
And another drawing here:
Fig. 3. Classification of communication lines.
Characteristics (parameters) of communication channels
1. Channel transfer function: is presented in the form amplitude-frequency characteristic (AFC) and shows how the amplitude of the sinusoid at the output of the communication channel decays in comparison with the amplitude at its input for all possible frequencies of the transmitted signal. The normalized frequency response of the channel is shown in Fig. 4. Knowing the frequency response of a real channel allows you to determine the shape of the output signal for almost any input signal. To do this, it is necessary to find the spectrum of the input signal, transform the amplitude of its constituent harmonics in accordance with the amplitude-frequency characteristic, and then find the shape of the output signal by adding the transformed harmonics. For experimental verification of the amplitude-frequency characteristic, it is necessary to test the channel with reference (equal in amplitude) sinusoids over the entire frequency range from zero to some maximum value that can occur in the input signals. Moreover, it is necessary to change the frequency of the input sinusoids with a small step, which means that the number of experiments should be large.
- the ratio of the spectrum of the output signal to the input
- bandwidth
Fig. 4 Normalized frequency response of the channel
2. Bandwidth: is a derivative of the characteristic from the frequency response. It is a continuous range of frequencies for which the ratio of the amplitude of the output signal to the input signal exceeds a certain predetermined limit, that is, the bandwidth determines the range of signal frequencies at which this signal is transmitted through the communication channel without significant distortion. Typically, the bandwidth is measured at 0.7 times the maximum frequency response. The bandwidth has the greatest impact on the maximum possible data transfer rate over the communication channel.
3. Attenuation: is defined as the relative decrease in the amplitude or power of a signal when a signal of a certain frequency is transmitted over a channel. Often, during channel operation, the fundamental frequency of the transmitted signal is known in advance, that is, the frequency whose harmonic has the highest amplitude and power. Therefore, it is enough to know the attenuation at this frequency in order to approximately estimate the distortion of the signals transmitted over the channel. More accurate estimates are possible by knowing the attenuation at several frequencies corresponding to several fundamental harmonics of the transmitted signal.
Attenuation is usually measured in decibels (dB) and is calculated using the following formula: 
, where
Signal power at the channel output,
Signal strength at the channel input.
The attenuation is always calculated for a specific frequency and is related to the channel length. In practice, the concept of "linear attenuation" is always used, i.e. signal attenuation per unit of channel length, for example, attenuation 0.1 dB / meter.
4. Transmission speed: characterizes the number of bits transmitted over the channel per unit of time. It is measured in bits per second - bit / s, as well as derived units: Kbps, Mbps, Gbps... The transmission rate depends on the channel bandwidth, noise level, type of coding and modulation.
5. Channel immunity: characterizes its ability to provide signal transmission in the presence of interference. It is customary to divide the interference into internal(represents thermal noise of the apparatus) and external(they are diverse and depend on the transmission medium). Noise immunity of the channel depends on the hardware and algorithmic solutions for processing the received signal, which are embedded in the transceiver. Immunity transmission of signals through the channel can be increased at the expense of encoding and special processing signal.
6. Dynamic range : logarithm of the ratio maximum power signals transmitted by the channel to the minimum.
7. Interference immunity: this is noise immunity, i.e. noise immunity.
