O.A. Nokhrin, V.V. Pechurin, I.E. Kalmychkov

HF data modems provide data interchange in standard bandwidth 4000 Hz by FSK2, 4, 8 with data rates 50 - 1200 bps, PSK2, 4, 8 and QAM16, 32, 64 with data rates up to 12800 bps. To receive such signals it is necessary to know the following parameters: center frequency, quantity of modulated parameter levels 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. An original algorithm of selection (detection) and estimation of FSK signals parameters is presented in the article. It is based on the use of flexible detection threshold calculated as a smoothed function of input power spectrum. Spectrum smoothing can be realized by median filtration or it can be calculated as a mean of m spectral coefficients of the initial spectrum in the running window. If quantity of smoothed function extremums exceeds 1, then a hypothesis on FSK signal presence is brought forth. In this case the number of extremums is taken as a quantity of M frequencies, and the extremum distance as Fd frequency deviation. Otherwise the hypothesis is rejected. The hypothesis is checked by the cross-correlation coefficient of the detected signal components.