| The communication channel established with HF waves reflected by the ionosphere is investigated. The background ionosphere is modeled by a horizontally stratified medium. This model is superimposed with random irregularities. The resulting channel is characterized by a random transfer function. Each frequency component of this transfer function is determined from the received wave field, knowing the transmitted wave field. The computation of the random fluctuations of the received wave field is performed by using the phase screen-diffraction layer method. This scheme simulates the propagation of an HF wave in the turbulent ionosphere. Based on the forward-scatter approximation, the scheme computes sequentially the effects of phase fluctuations due to the irregularities and the effects of diffraction due to phase mixing. Stepping along the ray path, phase fluctuations are imbedded into a number of phase screens. Diffraction effects are then computed between phase screens using FFT techniques. Special attention is given to the reflection region where the classical WKB approximations are invalid. The simulated received wave field is processed to determine some of the skywave channel characteristics. The computed statistics of the received signal include probability distribution, power spectrum, correlation function and scintillation index. For communication purposes, the pulse distortion is of major interest. The two-frequency mutual coherence function is computed to determine the statistical behavior of the received signal. The coherence bandwidth resulting from this computation is used to assess the ionospheric effects on HF spread spectrum systems. |
