General characteristics of noise immunity. Noise immunity and secrecy

Noise immunity of KPI transmission systems

The functioning of the SP KPI occurs in the conditions of interference. In the general case, it is necessary to assess the performance of the SP KPI when the enemy conducts electronic warfare (EW). At the same time, the most important indicator of the quality of functioning of the SP KPI is noise immunity.

The noise immunity of the RES is its property to remain operational in the conditions of the enemy’s electronic warfare.

In the general case, electronic warfare includes two successive stages – radio reconnaissance and radio countermeasures. The purpose of radio reconnaissance is to establish the fact of operation of the RES for radiation and to determine the parameters of the RES necessary for organizing radio countermeasures. The purpose of radio countermeasures is to create conditions that would complicate the operation of the RES or even lead to the disruption of the task. The main method of radio countermeasures is jamming. Jamming will be the more effective, the more information about the suppressed RES will be revealed at the stage of radio reconnaissance and used in the organization of radio countermeasures.

It follows that noise immunity as a qualitative indicator of the functioning of the SP KPI implies both the conduct of radio reconnaissance by the enemy (i.e., it takes into account the secrecy of the work of the SP KPI), and maintaining at an acceptable level the quality of the work of the SP KPI under the influence of interference (i.e., noise immunity).

The noise immunity of the RES depends on the technical characteristics of the RES, on the relative position of the RES and the equipment for reconnaissance and suppression, on the tactics of using the RES, on the operating time, etc. The combination of these characteristics and conditions is random, therefore, the noise immunity should be evaluated as the probability P pmz of the implementation of the RES tasks in electronic warfare conditions, determined by the ratio

Р pmz = 1 – Р р Р n

where P p is the probability of reconnaissance of the RES parameters necessary for organizing radio countermeasures;

P n – the probability of disruption of the RES as a result of radio countermeasures.

The probability Р р quantitatively reflects the secrecy of the RES – the ability of the RES to resist the measures of electronic intelligence aimed at detecting the fact of the operation of the RES and determining the signal parameters necessary for radio countermeasures. Accordingly, the value of P cr = 1 – P p can be taken as a criterion of secrecy.

The probability P n depends on the ability of the RES to perform the task under the influence of interference. Therefore, the value Р pmu = 1 – Р n can be taken as a criterion of noise immunity. This criterion determines the probability of the system performing a task in radio jamming conditions.

If the enemy does not reconnoiter the parameters of the radio link, then the setting of only noise barrage is obvious. If the parameters of the radio link in the process of reconnaissance by the enemy are determined, then most likely the setting of aimed interference. Thus, the noise immunity of the RES is determined by its secrecy and noise immunity. Let’s consider separate indicators of noise immunity.

Stealth . Radio reconnaissance, as a rule, involves the sequential execution of three main tasks: detecting the fact of the operation of the REM (signal detection), determining the structure of the detected signal (based on determining a number of its parameters) and disclosing the information contained (transmitted) in the signal. The last task sometimes has an independent value (it is one of the final goals). In the general case, the disclosure of the meaning of the transmitted information makes it possible to organize more effective radio suppression.

The listed tasks of radio reconnaissance can be opposed by three types of secrecy of RES: energy, structural and information.

Energy stealth characterizes the ability to resist measures aimed at detecting a signal by a reconnaissance receiver. To ensure energy secrecy, it is necessary to choose such a radiation power of the transmitter and such a radiation spectrum at which the signal power at the input of the reconnaissance receiver would be less than its real sensitivity. To ensure energy secrecy, it is possible to use broadband signals, since at a low spectral density and under the condition of a relatively narrow bandwidth of the reconnaissance receiver, the energy of the received reconnaissance signal will be small. Detection of a signal by a reconnaissance receiver occurs under the influence of interference (noise), and can be accompanied by two types of errors: signal omission when it is present at the input and false detection (false alarm) when there is no signal. These errors are probabilistic in nature. A quantitative measure of energy secrecy can be the probability of correct detection P n (for a given false alarm probability R lt ) , which in turn depend on the signal-to-interference ratio in the radio link and the decision rule for signal detection.

Structural stealth characterizes the ability to resist radio intelligence measures aimed at signal disclosure. This means recognizing the shape of the signal, determined by the methods of its coding and modulation, i.e., identifying the detected signal with one of the many a priori known signals. Structural stealth is provided by the use of signals, the complex structure of which makes it difficult for the enemy to reconnoiter them. Signals based on pseudo-random sequences of long duration, signals with complex modulation, etc. can be used as such signals. The use of complex signals imposes special requirements on the system in terms of synchronization accuracy of the receiving and transmitting sides. To increase structural secrecy, it is necessary to have as large an ensemble of used signals as possible and change the shape of signals quite often. The task of determining the structure of the signal is also a statistical one, and the probability of revealing the structure of the signal Р pp , provided that the signal is detected, can serve as a quantitative measure of structural secrecy. Thus, P str is a conditional probability.

Information secrecy is determined by the ability to resist measures aimed at revealing the meaning of information transmitted using signals. Disclosing the meaning of the transmitted information means identifying each received signal or their combination with the message that is being transmitted. This problem is solved by elucidating a number of signs of a signal, for example, the place of a given signal in the set of received ones, the frequency of its occurrence, the relationship between the factors in the appearance of a particular signal and a change in the state of the controlled object, etc. The presence of a priori and a posteriori uncertainties makes this problem probabilistic, and in as a quantitative measure of information secrecy, the probability of revealing the meaning of the transmitted information P inf is taken, provided that the signal is detected and isolated (i.e., its structure is revealed). Therefore, P inf is also a conditional probability.

Stealth is determined by the probability of reconnaissance of the RES signal . Often the task of disclosing the meaning of the transmitted information is not set, and then we can take P inf = 1 and P p = P obn P str . In some cases, to organize radio countermeasures, it is enough to detect the signal of a suppressed RES. In this case, R p is identified with R o n . Energy and structural secrecy are the most important characteristics of RES, which are faced by both design engineers of radio equipment and engineers operating it.

Thus, the secrecy of the SP KPI is ensured by taking into account the real conditions of operation, a combination of technical and organizational measures.

The criterion for assessing the noise immunity of the SP KPI is the probability of error Р osh when decoding a code combination, which is an encoded RK or a separate word of the EP. The value of this probability P osh , in turn, depends on the probability of distortion of the elementary symbol (digit) of the code combination p e and with non-redundant coding

Р osh u003d 1 – (1 – r e ) n

where n is the number of digits of the code combination.

Usually for systems of near space it is required to ensure the value of the probability of distortion of the message (commands or words of the control program) no more than 10 -8 – 10 -10 . The probability of distortion of an elementary symbol (element) of a message for systems of near space usually lies within 10 -3 – 10 -6 . Thus, the probability of distortion of the message represented in the SP CPI in the form of a code combination should be several orders of magnitude less than the probability of distortion of the symbols of this message. This determines the fundamental need for the application of special measures in the SP KPI to improve the reliability of transmitted messages.

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