Methodology for assessing the effect of ionospheric diffusivity and the choice of operating frequency on the noise immunity of shortwave communications

It is known that the noise immunity of shortwave communication channels with one discrete beam (mode) depends on the fading depth of the received signals. The latter, in turn, depends on the ionosphere diffuseness and the choice of the operating frequency of the wave. In a single-mode shortwave communication channel, signal fading is usually described by the Rician distribution. A technique is known for estimating the Rician fading parameter in a single-mode shortwave channel from the choice of the operating frequency. However, it is advisable to develop it in the direction of determining the dependence of the Rician parameter on the ratio of the operating frequency to the maximum usable frequency and the level of ionospheric diffuseness. The objective of the paper is to develop a technique for estimating the probability of erroneous reception of signals with binary orthogonal frequency modulation in a single-mode shortwave communication channel depending on the level of ionospheric diffuseness and the choice of the ratio of the operating frequency of the wave to the maximum usable frequency. This technique is developed in 2 stages: 1) determining the dependence of the Rician parameter on the ratio of the operating frequency to the maximum usable frequency and the level of ionospheric diffuseness; 2) determining the dependence of the probability of erroneous signal reception on the ratio of the operating frequency to the maximum usable frequency and the ionospheric diffuseness level. It was found that with a strong ionospheric diffuseness, fading in a single-mode shortwave communication channel in the range of operating frequency to maximum usable frequency ratios from 1 to 0.4 will be close to the Rayleigh fading, and with a normal diffuseness level, the Rice parameter value can vary within a wide range: from 0.2 to 100. Analytical expressions were obtained for determining the dependence of the probability of erroneous signal reception with binary orthogonal frequency modulation on the ratio of the operating frequency to the maximum usable frequency and the ionospheric diffuseness level. The effect of an increase in the ionospheric diffuseness during its disturbances on the value of the signal-to-noise ratio at the receiver input permissible for ensuring the required error probability was analyzed for different operating frequency to maximum usable frequency ratios. On this basis, recommendations have been developed for selecting the ratio of the operating frequency to the maximum usable frequency, which ensure a reduction in the permissible signal-to-noise ratio at the receiver input at different levels of ionospheric diffuseness.