DESIGN OF MULTILEVEL DECISION-FEEDBACK EQUALIZERS

Multilevel decision feedback equalization scheme (MDFE) is an efficien t and simple realization of the fixed-delay tree search with decision feedback (FDTS/DF) for channels using RLL(1, k) codes, In MDFE, the en tire tree-search is replaced with a 2-tap transversal filter and a bin ary comparator with negligible loss in performance. This 2-tap filter can be combined with the forward and feedback equalizers resulting in a structure that is physically identical to DFE but requires very diff erent equalizer settings, This paper focuses on equalizer design for M DFE. It is first shown that the MDFE scheme can also be derived withou t using the principle of tree-search by exploiting the run-length cons traints imposed by the RLL(1, k) code in conjunction with the maximiza tion of an appropriately defined signal-to-noise ratio (SNR), Recogniz ing that a multilevel eye is formed at the comparator, we define this SNR as the eye-opening divided by noise plus intersymbol interference, This formulation directly leads to a novel adaptive scheme based on t he well known LMS algorithm, The relationship between this work and th e earlier derivation of MDFE is then clarified, We also develop a noni terative analytical approach for the optimum equalizer design. Because of the economy of implementation, there is particular interest in the design of continuous-time forward equalizers, A noniterative analytic design approach,,which does not suffer from local minima problems, is developed for such equalizers, Computer simulation results are presen ted for comparing the different design approaches.