BROADBAND SIGNAL DISTORTION IN THE IONOSPHERE, CAUSED BY NONLINEAR FREQUENCY DISPERSION
At present for modern high-frequency communication (HF) systems and over-the-horizon radiolocation, the problem of radio signal bandwidth expansion is urgent, that is directly connected with another scientific problem of dispersive distortion which was not studied to the end. The purpose of the work is the research on factors of nonlinearity and stochasticity of the dispersive characteristic and their influence on the distortion of broadband signals with different modulation formats considering their compression during the processing. The methods of the mathematical analysis and natural experiments are used. In theory the frequency dependence of the group delay and in the experiment the delay time of the maximum value of the narrow-band pulse envelope are taken for the dispersive characteristic. It is shown that the problem of the propagation of an arbitrary spread-spectrum signal in the medium considering its compression in the receiver is reduced to the problem of propagation only in the medium of some equivalent signal. For broadband signals with LFM (SSB-C), pseudorandom operating frequency tuning (POFT) (SSB-FH), phase-coded modulation (PCM) (SSB-DS) the equivalent signal will have the common-mode phase-frequency characteristic and either homogeneous (in case of LFM and POFT) or Gaussian (in case of PCM) amplitude-frequency characteristic (AFC). In this connection the equivalent signal spectrum of the second type can be considered as the spectrum of the first type multiplied by Gaussian function. For the equivalent signal of the first type the spectrum of the received signal is proportional to the frequency response of a broadband radio signal. Therefore in this case the received equivalent signal will be proportional to radio channel pulse response. In the second case the equivalent signal is determined by the convolution integral of Gaussian function with radio channel impulse response. Thus, broadband signal distortion considering matched processing is determined by the distortion of channel impulse response. The models of the components of frequency dispersion: linear and nonlinear regular, nonregular and stochastic are suggested. The stochastic model is a Gaussian process with the zero first moment, the nonregular model is a periodic function and the regular model is a second-degree polynomial. The general problem solution of impulse response distortion (of broadband signals) for the channels with the nonlinear dispersion, described by incomplete Airy functions that with some values of distance characteristic parameters go to known solutions (without the dispersion and with the linear dispersion) is received. The well-known problem of the group delay for mediums with the dispersion is considered. The reasons for communication uncertainty of a phase-frequency characteristic derivative by frequency with the distorted by the dispersion signal are mathematically defined, that coincides with the envelope group delay only in the medium with the weak frequency dispersion and its meaning becomes undefined in dispersion mediums.