Novi pristup rešavanju problema frekvencijskog ofseta kod sistema sa ortogonalnim frekvencijskim multipleksom

Abstract

In this dissertation, principles of orthogonal frequency division multiplex (OFDM), a method of encoding digital data on multiple carrier frequencies, are given. After theoretical basics of wireless channel and discrete adaptive filters, advantages and disadvantages of this kind of transfer are presented. Also, basic blocks of classical OFDM system are described, with comments on how behavior and implementation of each of them can be improved. OFDM systems with MDPSK modulation and differential detection in receiver are considered. Performances for different values of modulation levels and different OFDM parameter values are analyzed, in accordance with existing standards and OFDM systems that do not belong to certain standard classification. For this purpose, a modular simulation environment is developed, in which a universal model of the OFDM system with possibility of all OFDM parameters adjustment and wireless channel parameters is possible. The main OFDM system disadvantage is frequency offset sensitivity, which destroys subcarrier orthogonality and produces intercarrier interference. Hence, frequency offset is the main factor which limits the subcarrier bandwidth and increase of system bitrate. In order to resolve frequency offset problem, we analyze OFDM receivers with different configuration and complexity. Proposed receivers are designed by modifying existing differential detection algorithms, such as double differential detection, multisymbol differential detection and decision differential detection algorithm. The new approach to the frequency offset problem solving in system with orthogonal frequency division multiplex is based on adaptive transversal filter application. We proposed optimal OFDM receivers with good performance in wide frequency offset range in the Rician and Rayleigh fading channel. Proposed receivers significantly improve the quality of the received signal and can be applied in modern wireless telecommunication systems.