Allpass forward equalizer for decision feedback equalization

Abstract

An allpass forward equalizer (APFE) for use with decision feedback equalization (DFE) is described. The APFE has poles, unlike the normally implemented FIR (finite impulse response) forward equalizer; equalizers with poles can provide superior performance or reduced hardware complexity when compared to all-zero equalizers, but can have local optima and are more dimcult to design. A theoretical basis fair the allpass is presented and a systematic design procedure is described. It is shown that there are no local optima in a sufficient region about the global optimum such that adaptation is feasible. A performance vs. complexity advantage is shown for the APFE relative to the FIR forward equalizer, and implementation as a continuous-time filter is proposed.

A Low-Voltage Low-Power CMOS Analog Adaptive Equalizer for UTP-5 Cables

“…This pole is designed such that it allows the zero to boost only a limited range of frequency that enables the recovery of the signal without excessive boosting of frequencies outside of the needed range. In a high-pass implementation, the FE has a zero and only parasitic poles that might appear at a higher than needed frequencies leading to much worse noise and reflection performance [9], [10]. The adaptation to the transmission channel is achieved through an automatic tuningloop that compares the edge rate of the output of the FE to a reference edge rate to generate an error signal.…”

Equalization in hard disk drive read channels

“…Conventional DFE uses a FIR filter as a forward equalizer to cancel the non-causal precursor ISI, however [13] suggests that an adaptive analog implementation can show equivalent performance to that of a long (24 taps) FIR filter, whilst reducing power consumption. In this work we propose and show simulation results for a 2nd order and for a 3rd order allpass continuous-time filter, both capable of implementing the adaptive forward equalizer.…”

“…Alternatively the capacitor array can be loaded in parallel and subsequently adapted to optimum values. Considering equal-valued transconductors with a nominal value of 60µS we used C 0 = 0.102pF, C 1 = 0.49pF, C 2 = 1.77pF nominal values for the integrating capacitors, matching the theoretical work [13] for the ideal pole locations of the forward equalizer on a third order filter. An identical procedure was followed to calculate the ideal integrating capacitors and corresponding pole location values for the second order allpass filter.…”

An Adaptive Constraint Method for Paraunitary Filter Banks with Applications to Spatiotemporal Subspace Tracking

“…Paraunitary filters and their one-dimensional cousins, allpass filters, are important for a number of useful signal processing tasks, including coding, deconvolution and equalization, beamforming, and subspace processing [1][2][3][4][5][6][7][8][9][10][11][12]. Paraunitary filters are lossless devices, such that no spectral energy is lost or gained in any targeted spatial dimension of the multichannel input signal being filtered.…”