STRUGGLE AGAINST INTERSYMBOL INTERFERENCE BY USING EQUALIZERS AND ORTHOGONAL TIME-DIVISION MULTIPLEXING

Abstract

Multipath propagation of radio waves leads to the occurrence of intersymbol interference (MSI). Therefore, the implementation of one of the ways to combat ISI in mobile digital troposcatter stations is an urgent task. In this article explores two options for dealing with intersymbol interference in a multipath communication channel: the use of equalizers and orthogonal time division multiplexing (OTDM). One of the known ways to combat ISI is based on the use of equalizers that equalize the frequency response of the channel. It should be noted that the channel condition assessment is carried out only on the receiving side. If evaluate the channel on the transmitting and receiving sides, then this requires a feedback channel. In this case, it is necessary to have a channel precoder that predistorts the signal so that after passing through the communication channel the signal takes the desired shape and there is no MSI. The designs of adaptive equalizers and the application of channel precoder for implementing orthogonal time multiplexing are considered. With OTDM, the digital reception algorithm is reduced to character-by-character processing of received signals, which simplifies the implementation of demodulators. However, in this case, the OTDM system loses in energy efficiency to a system that implements reception “in general” according to the Viterbi algorithm, but is more preferable from this point of view in comparison with the use of linear transverse equalizers. It is shown in the work that when implementing radio systems operating on channels with ISI, the choice of the control method can be determined by the developer depending on the requirements for such a system. In channels with variable parameters, and such is the troposcatter communication channel, adaptive equalizers with feedback by decision may turn out to be preferable to the OTDM system in the fight against ISI. The results of this work can be used in the development of mobile digital troposcatter stations for special purposes and combined mobile digital troposcatter-radiorelay stations.