It is well known that the performance of a data receiver for an intersymbol interference (ISI) channel can depend strongly on the detection delay 6. For a discrete-time communication system, this paper derives a lower bound on the bit-error probability as a function of 6. This "restricted delay bound" is governed by a "restricted-delay distance" d(6). In many instances, it improves upon Forney's bound, which is governed by the minimum distance &in. For instance, for partial-response channels,

more »

... 6) does not converge to dmin even as 6-00. For channels without spectral zeros, a finite detection delay suffices for d(6) to coincide with &,in. For all finite 6, d(6) is determined by a finite number of error patterns and may be computed in a straightforward manner. Unlike &,in, d(6) depends on the phase characteristics of the channel. Minimum phase is proved to maximize d(6). 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 IS1 canceller and a Viterbi detector due to Ungerboeck and MacKechnie, is shown to result in poor restricted-delay properties. Implications of these results are illustrated by means of examples.