Abstract
Discrete Multitone (DMT) modulation has been chosen to provide the high-speed link for Asymmetric Digital Subscriber Lines (ADSL) service. Hybrid circuits are commonly used to provide full-duplex transmission over subscriber loops. However, the imperfect characteristics in the hybrid circuits cause signal leakage and result in a serious echo problem. Therefore, a high performance echo canceller is necessary for a full-duplex ADSL transceiver. Cyclic echo synthesizer (CES)-based echo cancellation (EC) is an effective technique for DMT EC. This technique features the combination of echo reconstruction in time domain and echo cancellation in frequency domain. However, the CES approach normally requires enormous computational and hardware complexity as a consequence of the relatively high sampling rate in ADSL and the large amount of samples of echo channel impulse response. In this thesis, we propose a structure to shorten the length of echo channel impulse response, and thereby reducing the computational complexity. The proposed structure uses a shortened impulse response filter (SIRF) and cancels echo in time domain. We also discuss the mutual impact caused by SIRF and the time domain equalizer (TEQ) which is used for subscriber loop shortening to avoid intersymbol interference (ISI). The time domain EC can solve this problem effectively. Moreover, the shortening efficiency of TEQ could be improved by SIRF, which operates as a prefilter of TEQ. Computer simulations have shown that the proposed structure requires less computational and hardware complexity, however, with a slight performance loss as compared with the CES approach for echo cancellation.