On the decision-directed equalization of constant modulus signals

Decision-directed (DD) equalization is the most primitive blind equalization (BE) method for the cancelling of inter-symbol-interference (ISI) in data communication systems. Even through DD equalizers are believed to be unable to open the channel eye when it is initially closed, this does not seem to be true in the of constant-modulus (CM) constellations (pure phase modulation). We investigate the shape of the DD cost function in this case and obtain several interesting results that indicate that the DD algorithm should be capable of opening a closed channel eye in the CM case. Based on this fact, we propose a novel hybrid CMA-DD equalization scheme that offers an appealing alternative to the generalized Sato (GSATO) algorithm, for QAM constellations. Our theoretical claims about the performance of DD equalizers as well as the performance of our novel scheme are verified through computer simulations