S.V. Ivkov1, V.А. Nikitenko2, S.А. Undilashvili3

1−3Military University of Radioelectronics (Cherepovets, Russia)

On foreign military radio communication lines in the high frequency range, radio signals with M-ary phase manipulation (PM) are used to transmit discrete messages both at a fixed frequency and in systems with frequency hopping. Signals with frequency hopping include signals of frequency-adaptive radio lines and with pseudorandom adjustment of the operating frequency. Radio signals by pseudorandom tuning of the operating frequency are a set of individual elements of the radio signal, spaced by frequency and orthogonal in time. A separate element of the radio signal is a short-term signal, that is, radio signals with a duration much shorter than the duration of the input implementation, processed for the purpose of its detection, recognition and subsequent measurement of its technical parameters. Reception of radio signals from PM is performed by the method of coherent detection, using a local carrier generator controlled by a phase-locked loop (PLL).

The article discusses fading that occurs due to interference at the receiving point: general, when the amplitudes of the rays are approximately the same, and the mutual delay is insignificant, and frequency-selective, when the mutual delay of the rays exceeds the duration of the clock interval. Analysis of the operation of the PLL system under fading conditions allows us to conclude that the error of setting the local generator to the carrier frequency of the radio signal does not allow dividing the signal into common-mode and quadrature components, which leads to errors in the decision-making scheme. Under these conditions, the disadvantages of the method of coherent demodulation of radio signals with PM are due to the use of a PLL system and consist in the dependence of the error in determining the mismatch of the instantaneous frequency of the reference harmonic oscillation by a voltage-controlled generator and the carrier oscillation of the received radio signals on the number of elementary parcels for the duration of the short-term signal sampling, the SNR and the intensity of fading (general and frequency-selective) on the radio line, which can lead to demodulation errors. Frequency-selective fading is observed during multipath propagation of radio waves, when the difference in the course of the rays exceeds the duration of the elementary sending. Let's agree to call the ray with the shortest propagation path the first ray. Then the first parcel of the first beam is not subjected to the distorting effect of the radio signals of other rays. The amplitude of the radio signal arriving at the receiving point along the shortest trajectory significantly exceeds the amplitude of the radio signal of other beams. The sum of the energy of the products of the first (undistorted) parcel on the signal voltages of the remaining beams on the k-th clock interval will be less than the energy of the product of the first (undistorted) parcel on the i-th elementary parcel of the first beam. The probability of an erroneous decision during demodulation in comparison with single-beam reception of radio signals decreases in proportion to the ratio of the energy of the rays.

In addition, the PLL system used in quasi-coherent demodulation to restore the local carrier, under fading conditions, generates a control signal distorted by multiplicative interference. The influence of such interference leads to a false capture or disruption of the synchronization of the local carrier generator and, as a result, the appearance of long series of errors, i.e. a sharp increase in the probability of a demodulation error. It is proposed to abandon the use of a local generator with a PLL and detect PM signals relative to the first elementary parcel using its Hilbert copy in the quadrature channel as a support. To use the proposed solution, it was necessary to remove the uncertainty about the beginning of the first modulation symbol. During the analysis of the synchronization system of the radio signal demodulator with PM, a requirement was put forward for the magnitude of the error in determining the start of a short-term signal not exceeding 10% of the duration of the clock interval. In the conditions of fading, the correlation processing of the input implementation did not ensure the fulfillment of the stated requirement. In order to fulfill the stated requirement about the magnitude of the error in determining the beginning of a short-term signal under fading conditions, a method of time synchronization of a demodulator with a short-term signal of multilevel phase modulation based on Pearson linear correlation was proposed.

The use of the proposed method removed the uncertainty about the first modulation symbol. The changes made to the well-known method of incoherent reception made it possible to abandon the use of a local carrier during detection. This made it possible to eliminate errors caused by false frequency capture by the PLL system. In addition, the results of mathematical analysis allow us to conclude that the changes made reduce the sensitivity of the demodulator to the influence of interfering rays. The assumptions about the higher stability of the incoherent demodulation method in comparison with the coherent one under fading conditions are confirmed by the results of the model experiment.

Ivkov S.V., Nikitenko V.А., Undilashvili S.А. A method of relative demodulation for short-term signals with multilevel phase manipulation in the conditions of fading. Electromagnetic waves and electronic systems. 2022. V. 27. № 1. P. 19−27.

DOI: https://doi.org/10.18127/j15604128-202201-03 (in Russian)

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