Jan W. M. Bergmans

Density improvements in digital magnetic recording by decision feedback equalization

The information density improvements are estimated that are feasible by applying decision feedback equalization instead of conventional linear equalization in digital magnetic recording systems, with differentiating playback heads. Significant improvements are shown to be within reach for an important class of transmission codes.

A class of data-aided timing-recovery schemes

This paper develops a class of data-aided timing-recovery schemes whose structure and operating principle are reminiscent of zero-forcing equalizer adaptation. The schemes exhibit no pattern-dependent jitter and are applicable to a variety of modulation codes and reception techniques. They come in both oversampled and baud-rate forms, and permit maximum-likelihood tracking performance whenever the oversampling factor satisfies the sampling theorem. Subject to this condition, even simple schemes can be close to optimum. It is shown that various existing; schemes belong to the class under study or are closely related to it. >

Efficiency of data-aided timing recovery techniques

This paper assesses the tracking capabilities of data-aided timing recovery schemes in terms of an efficiency that characterizes, coarsely speaking, the amount of timing information that the scheme manages to extract from the incoming signal per unit of time and signal-to-noise ratio. Efficiencies of maximum-likelihood and various baud-rate timing recovery schemes are determined as a function of the data and channel parameters, and are optimized with respect to design freedoms of the schemes. These results are illustrated for partial-response channels and idealized digital recording channels. >

Partial-response decoding of rate 1/2 modulation codes for digital storage

In digital storage systems, receivers for rate 1/2 modulation codes are usually oversampled by a factor of two with respect to the data stream that they attempt to reconstruct. It is shown that oversampling may be avoided by using partial-response techniques to detect, instead of the encoded binary signal, a decimated ternary one, from which the original data can be recovered by means of a simple decoder. A method is described to find all such decoders for a given rate 1/2 code. Examples treated are FM, MFM, Miller-squared,

Decision feedback equalization for digital magnetic recording systems

The merits of decision feedback equalization are investigated as an alternative to the conventional linear method of equalization for combining intersymbol interference and noise in digital magnetic recording systems. It is argued and exemplified that several benefits can be expected from its application. The benefits are derived by theoretical considerations and simulation results for idealized system models. >

SNR merits of binary modulation codes in equalized digital recording systems

For a digital recording system with a binary modulation encoder and a linearly dispersive channel with additive noise, the author determines optimum mean-square performance of the linear, partial-response, and decision-feedback equalizers. The analysis revolves around the power spectral density A( Omega ) of the code and the folded signal-to-noise ratio X( Omega ) of the channel. The latter function is analyzed for stylized optical and magnetic recording channels. For all equalizers it is shown that the effect of coding is similar to increasing X( Omega ) by an additive portion 1/A( Omega ). This favors depressions of A( Omega ) at frequencies where X( Omega ) is poor. However, for A( Omega ) to have any depressions, the signaling rate must be increased with respect to uncoded storage, and this inevitably degrades X( Omega ). At high information densities the degradation is often too large for coding to be rewarding. Examples serve to illustrate these results. >

On the performance of data receivers with a restricted detection delay

It is well known that the performance of a data receiver for an intersymbol interference (ISI) channel can depend strongly on the detection delay d. For a discrete-time communication system, this paper derives a lower bound on the bit-error probability as a function of d. This restricted delay bound is governed by a restricted-delay distance d(d). In many instances, it improves upon Forneys (1972) bound, which is governed by the minimum distance dmin. For instance, for partial-response channels, d(d) does not converge to dmin even as d?8. For channels without spectral zeros, a finite detection delay suffices for d(S) to coincide with dmin. For all finite d, d(d) is determined by a finite number of error patterns and may be computed in a straightforward manner. Unlike dmin, d(d) depends on the phase characteristics of the channel. Minimum phase is proved to maximize d(d). The lower bound is generalized to discrete-time channels with colored noise and to continuous-time channels. The effect of transforming a continuous-time channel into a discrete-time channel is discussed. Transformation via a matched filter, as in the ISI canceller and a Viterbi detector due to Ungerboeck and MacKechnie (1973), is shown to result in poor restricted-delay properties. Implications of these results are illustrated by means of examples

A technique to reduce error propagation in M-ary decision feedback equalization

A technique is described for reducing the error propagation resulting from the use of decision feedback equalization in an M-ary, varying system. The technique is applicable when the actual system response varies moderately about a nominal response that can be approximated by a partial response. The decision feedback equalizer (DFE) is supplemented with partial response precoding and detection such that the resulting error propagation is a function of only the difference between the actual system response and the partial response. Simulation results for a system with a nominal response equal to the duobinary partial response are given to illustrate the performance improvement.

Effect of binary modulation codes with rate R=1/N on equivalent discrete-time models for channels with intersymbol interference

For a data transmission system with intersymbol interference and noise in which signaling occurs by means of nonoverlapping rectangular pulses, a relation between the equivalent discrete-time models for uncoded and coded transmission is derived. It applies to binary modulation codes with rate R=1/N, where N is a positive integer. Examples suggest that these models are often affected by coding in a manner that is incompatible with a commonly adopted definition of coding gain. >

Transition detector for CD and DVD

This bit detector for d=2 modulation codes is remarkably simple, supports very high data rates, can be adaptive, and is capable of handling nonlinearities. The performance approaches that of a full-fledged maximum-likelihood sequence detector.