ЦИФРОВОЙ SDR-ЭКВАЛАЙЗЕР ДЛЯ РАБОТЫ БЫСТРОЙ ПРОГРАММНОЙ ПЕРЕСТРОЙКИ РАБОЧЕЙ ЧАСТОТЫ В ДИСПЕРГИРУЮЩИХ ИОНОСФЕРНЫХ КАНАЛАХ ШИРОКОПОЛОСНОЙ КВ-СВЯЗИ
Authors: Овчинников В.В. 
Published: February 2025

Решалась задача создания цифрового адаптивного эквалайзера в полосе частот широкополосной КВ-связи для преодоления негативного эффекта нелинейной фазовой дисперсии среды. Методики, алгоритмы и программное обеспечение (ПО) разработаны применительно к технологии SDR и реализованы с использованием универсальной платформы USRP N210. Применялись современные подходы к цифровой обработке сигналов. Верификация созданных алгоритмов и программного обеспечения осуществлялась в натурных экспериментах на радиотрассе о. Кипр – г. Йошкар-Ола, протяжённостью 2 600 км. В качестве физической модели программной перестройки рабочей частоты (ППРЧ) использовалась модуляция типа LFMCW. Экспериментально исследовалась эффективность разработанных алгоритмов преодоления частотной дисперсии в каналах с полосой 1 МГц.

Currently, HF communication systems are rapidly developing due to the application of digital methods of signal synthesis and processing. This has triggered the research into the application of wideband signals with bandwidth significantly greater the channel coherence bandwidth. Spreading signal bandwidth rises the link energy budget and fundamentally solves the problem of covertness (or, both, covertness and noise immunity) of HF communication. However, propagating in the ionosphere, that exhibits dispersive features, wideband signal suffers shape distortions. Overcoming frequency dispersion poses a great challenge due to intra-mode (magnetoionic) interference, narrowband and fluctuating additive interferences and time variability of the channel. These negative factors should be taken into account to design adaptive equalizer. In the framework of the research there is considered the application of breakthrough SDR technology to ensure the integration of the equalizer into the state-of-the-art HF communication systems. The aim of the research was the development and full-scale experiment verification of adaptive SDR-equalizer to correct for frequency dispersion in ionospheric HF radio channels to support wideband FFH systems. Methods, algorithms and software were developed according to the SDR technology and implemented with the use of USRP N210 hardware platform. Their verification was carried out in the full-scale experiments on the mid-latitude radio path Cyprus-to-Yoshkar-Ola of length of 2600 km. LFMCW test signal was used as a physical model of the FFH signal. Megahertz bandwidth channels were selected from the single mode propagation band. The efficiency of the SDR equalization was estimated according to the depth of compression of the test signal of 1 MHz bandwidth from the single mode propagation band. The compressed LFMCW signal represents the channel impulse response. Thus, there were compared the impulse responses of 1 MHz bandwidth channels before and after SDR equalization. Impulse responses were calculated from the complex samples of the channel frequency response. The durations of the impulse responses were observed up to 20 times greater than 1 μs. This spread was caused by the frequency dispersion, that results in the delay spread over the wideband channel. The IR durations measured after the equalization of the ionospheric channel with 1 MHz bandwidth were always compressed to 1 μs. Research findings proved that the developed algorithm of adaptive equalization allows to correct for phase dispersion and amplitude variations caused by the intra-mode interference.