Using Walsh Functions for Increase the Stealth Communication in a Digital Radio Channel

Abstract

Introduction. The use of wideband signals in telecommunication systems provides the desired speed of digital data transmission on radio channels of ultrahigh frequencies range with the required quality. In this case, in the range of ultrahigh frequencies, it is possible to provide a sufficiently low probability of detecting wideband signals, which corresponds to the needs of cybersecurity and data confidentiality.

Due to the optimal processing in wideband receivers, the power of the useful signal can be increased by of the signal base. A decrease in the spectral power density of a wideband signal reduces the detection range of the radiation of a broadband transmitter, as compared to a narrowband one with the same transmitter power. Further improvement of the stealth communication is possible with an increase in the base of the signal with a simultaneous decrease in the transmitter power.

Purpose of work. Evaluation of the possibility of using wideband signals in the shot wave range for communication digital data and determination the probability of the detection the transmitter radiation with lowing power by means of frequency monitoring.

Presentation of research material. Let the transmission of a voice signal with the upper frequency of the spectrum of 3,9 kHz used the analog-digital signal conversion with 256 quantization levels and a sampling period of 128 µs. Each 8 bits match a specific Walsh function in the encoder. With an elementary symbol duration of 0,5 µs, the signal spectrum is expanded to 2 MHz, and the signal base is 256. To organize multichannel transmission is supposed to use frequency separation of the channels with a phasemanipulation of the carrier frequency. This is not need account the mutual correlation the signals of other wideband channels, which use identical Walsh functions. Qualitative transmission of information, with binary eight-bit pulse-code modulation and the simultaneous use of Walsh orthogonal functions is provided by optimal signal / noise ratio at the input of the demodulator of the pulse-code modulation 15 dB. Consequently, an assessment of the possibility of transmitting information with specified quality indicators using a wideband signal is reduced to the calculation of the signal / noise ratio for a certain range of communication. The calculation results show that at the input of the demodulator of the pulse-code modulation of the wideband radio communication tool provides the necessary signal / noise ratio at a distance from the transmitter, which slightly exceeds 6 km. In addition, with a decrease the power transmitter’s in ten times, the range of the wideband radio communication decreases to 3 km. Verification of the possibility of detecting a radiation the means of frequency monitoring is to determine the signal / noise ratio that will be created by a wideband radio transmitter at the input of the frequency monitoring receiver. The wideband radio transmitter with a power of 5W creates at the entrance of the receiver of frequency monitoring at a distance of 1 km signal ratio / noise 16,3 dB. In this case, the conventional probability of the correct detection of the signal with an unknown initial phase and the amplitude is at least 0,8. Note, that a narrowband transmitter with the same power, at a distance of 1 km creates a signal / noise ratio of 36,3 dB at the input of the frequency monitoring receiver. When the transmitter power is reduced to 0,1 W, the transmission of information with the specified quality indicators is ensured over a distance of up to 2 km. In this case, the conditional probability of correct detection of the signal by the frequency monitoring receiver reaches a value of less than 0,5 already at a distance of 0,5 km. Despite the possibility of detecting a radio broadband signal, any other receiver that does not have a priori information about the information signal encoding parameters will not be able to restore the information contained in the signal.

Conclusions. The use of wideband signals with a large base degrades the conditions for detecting the emission of the transmitter compared to narrowband means of communication. A further decrease in the probability of detecting the emission of a wideband signal is possible by decreasing the transmitter's power.