Blind Channel Equalization for Shortwave Digital Radio Communications

Sammanfattning: In this thesis the concept of Blind Channel Equalization has been examined and algorithms suitable to blindly equalize the channel are presented and evaluated on simulated data. The concept of blind equalization is to equalize a communica- tions channel without relying on a training sequence or pilot tone, which may be either unknown to the receiver or not exist at all. In total, seven blind equaliza- tion algorithms have been implemented, these are the: lms-cma (Constant Modu- lus Algorithm), rls-cma, mma (Multi Modulus Algorithm), rca (Reduced Constel- lation Algorithm), cna-6 (Constant Norm Algorithm), lms-dfe (Decision Feedback Equalization) and rls-dfe. The equalizers are designed as adaptive fir-filters that are recursively updated by either an lms- or rls-algorithm, according to a cost function specified by the chosen algorithm with the aim to appoximate the inverse h−1 of the communications channel h. Thanks to the recursive update the algorithms can easily be implemented either in offline or online systems. The results show that the rls-algorithms offer shorter convergence times and over all better performance than its lms counterparts. If the signal constellation is known by the receiver in advance the rls-dfe offers the best channel tracking ability, resulting in the lowest symbol error rate.The rls-cma offers the roughly the same mseR -performance (mean square error from the equalizer output to the closest radius of the constellation points) but it lacks the ability to handle the doppler shift as well as the rls-dfe does. The results also show that the mma, cna-6 and rca-algorithms do not offer any better performance than the more commonly used and studied lms-cma algorithm. When the receiver incorrectly assumes the signal constellation, it can identify the correct constellation. Test results show that the rls-cma is especially good at amplitde recovery, while the rls-dfe is suitable to recover the phase of the signal. Lastly the rca is useful to recover psk-4 modulated signals as its cost function match the psk-4 constellation.