A FAST TRACKING ADAPTIVE MLSE FOR TDMA DIGITAL CELLULAR-SYSTEMS

This paper presents an adaptive MLSE (Maximum Likelihood Sequence Esti mator) suitable for TDMA cellular systems. The proposed MLSE has two s pecial features such as handling wide dynamic range signals without an alogue gain controls and fast channel tracking capability. In order to handle wide dynamic range signals without conventional AGCs (Automati c Gain Controller), the proposed MLSE uses envelope components of rece ived signals obtained from a non-linear log-amplifier module which has wide log-linear gain characteristics. By using digital signal process ing technique, the log-converted envelope components are normalized an d converted to linear values which conventional adaptive MLSEs can han dle. As a channel tracking algorithm of the channel estimator, the pro posed MLSE adopts a QT-LMS (Quick-Tracking Least Mean Square) algorith m, which is obtained by modifying LMS algorithm to enable a faster tra cking capability. The algorithm has a fast tracking capability with lo w complexity and is suitable for implementation in a fixed-point digit al signal processor. The performances of the MLSE have been evaluated through experiments in TDMA cellular environments with pi/4-shifted QP SK, 24.3 k symbol/sec. It is shown that, under conditions of 65 dB amp litude variations and 80 Hz Doppler frequency, the MLSE successfully a chieves less than 3% B.E.R., which is required for digital cellular sy stems.