Exit Chart Analysis Applied to Adaptive Turbo Equalization

In this paper we consider iterative channel estimation, equalization, and decoding, or adaptive Turbo equalization, as a receiver technology for digital communication systems where the channel imposes time-varying intersymbol interference. We show how a semianalytical technique called EXIT charts, originally developed by S. ten Brink for analysis of Turbo codes, can be used to predict the performance of the iterative receiver for such systems. To demonstrate the usefulness of the technique, we use EXIT charts to address interesting questions about adaptive Turbo equalization for time-varying channels: Which pattern of training sequences should be used, what is the performance difference between optimal MAP equalization and suboptimal linear equalization, and between the case of a known channel and the case of an estimated channel, and what can be gained by using a recursive precoder in conjunction with the symbol mapper.