Abstract
The purpose of this thesis is to study and improve echo cancellation (EC) techniques for discrete multitone (DMT) based asymmetric digital subscriber line (ADSL) transceivers. In this thesis, a multi-carrier EC technique with cyclic echo synthesis (CES) and two 2-stage approaches are investigated and compared. The CES-based approach is to eliminate the cross echo produced by each subcarrier in the time domain first, and then the echo can be canceled by using one-complex tap filter on each subcarrier independently in the frequency domain. The two-stage structures we considered were originally proposed for the integrated service digital network (ISDN). Here we apply them in ADSL systems. They have the same main echo cancellers in the first stage, but different echo tail cancellers in the second stage. Some parameters must be determined off line for these 2-stage structures. For the first echo tail canceller using orthonormal functions, the pole location has to be chosen in advance and we use an easy way to find it during the training procedure. The other echo tail canceller is based on adaptive interpolated finite impulse response (AIFIR) filtering. For the original AIFIR approach, the coefficients of the interpolator are fixed. In this thesis, we propose an adaptive interpolator to make this kind of structure more attractive. Both computer simulations and complexity analyses have been performed for these approaches. The results show that all of them can meet the performance requirements for ADSL transmission with similar computational complexity.