Time-Frequency Jammer excision for Multi-carrier spread spectrum using adaptive filtering

by Coelho, Brenno Beserra

The development of new wireless technologies, the improvement of existing ones and the reduction on the wireless devices prices is increasing the number of users, the demand for bandwidth and the demand for higher data rates. The spread of the technology however bring some drawbacks. One is the increasing interference level that can degrades the wireless communications. Many different techniques are used to minimize the interference and the effect of the channel (multipath, Doppler etc) in a wireless channel. This thesis considers the frequency and time processing of a jammer affected multi-carrier spread spectrum (MC-SS) system. A linear chirp is used as a spreading sequence. Such a sequence not only provides a constant envelope, but also allows the estimation of the channel parameters using a linear time-invariant model. Hence time-delays and Doppler frequency shifts can be represented by effective time shifts. The discrete evolutionary transform (DET) time-frequency repre- sentation is used for estimating the channel characteristics and for detecting jammers. Once the jammers are detected, the original spreading function corresponding to the jammed fre- quency is adapted to minimize the jammer effects. The bit detection is then performed using a least mean square (LMS) adaptive filter and it is done in both time- and frequency-domain. To illustrate the performance of the method, simulations with different signal to noise ratios, different jammer to signal ratios and different Doppler shifts were performed. The results indicate that the method is capable of excising the jammers providing a good bit error rate in low Doppler situations.