O.A. Nokhrin, V.V. Pechurin

Transfer of discrete messages in the HF band is carried out on one or several channels by frequency, phase or quadrature amplitude modulation methods with various parameters. To receive such signals it is necessary to know the following parameters: carrier frequency, type of modulated and modulation rate. These parameters are approved beforehand or included into a service data packet. Modulation parameters transmission leads to the reduction of the channel use efficiency. To avoid it modulation type recognition units should be included into data modems. Signal recognition based on trial demodulation or use of reference patterns demands the difficult calculations, large quantity of reference patterns and necessity of their aprioristic set. It conditions necessity of working out heuristic recognition methods using a generality of radio signals properties. In this methods recognition is performed under the multilevel scheme, where each level takes part in formation of the final decision about a signal class. It demands on the top levels use of the recognition features are immunity of various harmful influences. Distant radio propagation in HF band is characterized by multiplicative distortions of signals. This distortions leads to errors of the recognition. In this connection in article the analysis of an envelope of the discrete signals has been performed. Which allowed to reveal single and multitone signals feature - is the various number of predominating harmonic components in an envelope spectrum. Researches of influence of additive noise and multiplicative distortions at power of this component showed that predominance of the harmonic component is watched at the signal to noise ratio more than 3 dB. It allows to use the given feature for the separation of single and multitone HF signals into recognition multilevel systems.