Study On Equalization In Shortwave Communication Systems

Shortwave communication has the benefits of long transmission distance and its medium can't be destroyed, so it is extensively applied in military and emergency communication. In recent years, many adaptive techniques are used in digital shortwave radio and make its great development. The time-varying character of ionosphere bring the reflection and refraction components of shortwave signal, and lead to serious multi-path and fading effect, including serious inter-symbol interference (ISI). In order to overcome them, some effective methods must be used to mitigate the inter-symbol interference and ensure the signal transmission quality.After analyzing character of shortwave channel, the advantages and problems of several equalizer algorithms and structures are researched, and then an equalizing method adapt to shortwave communication is proposed. The main content of this paper is:①Establish the shortwave channel model combined with the propagation character and math model of shortwave channel referring to ITU-R, and then simulate the effect of multi-path fading, Doppler shift, Doppler spread and noise interference in this model.②Discuss and analyze several kinds of equalizer structures and algorithms with the 3G-ALE shortwave radio standard on burst wave transmitting in shortwave channel, then simulate and analyze the performances them based on the built shortwave channel model above.③In order to meet the demand of digital shortwave radio, an equalizer method using DFE structure and improved QR-RLS algorithm combining LMS algorithm is proposed to reduce the complexity and hardware resources. Because of the time-varying character, feed-forward filter of equalizer should have strong channel tracking ability, and improved QR-RLS algorithm can meet the demand and be used to mitigate the inter-symbol interference. As for the feedback filter, it's not necessary for it to have good channel tracking capability, so good performance can be acquired using the LMS algorithm. LMS algorithm can further eliminate the residual inter-symbol interference. The results of simulation and analysis showed that it can meet the system demand. Compared with traditional equalizer, it reduces the complexity of realization and hardware resources, meantime good stability. As mentioned above, the equalizer techniques are designed and simulated in shortwave communication system, but the related methods and results can be further extended and applied in the design and research of other communication systems.