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U. Glaeser

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Noise-Predictive Maximum Likelihood Detectors (NPLD)<br />

Maximum likelihood detection combined with PR equalization is a dominant type of detection electronics<br />

in today’s digital magnetic recording devices. As described earlier, in order to simplify hardware realization<br />

of the receiver, the degree of the target PR polynomial is chosen to be small with integer coefficients to<br />

restrict complexity of Viterbi detection trellis. On the other hand, if the recording density is increased,<br />

to produce longer ISI, equalization to the same PR target will result in substantial noise enhancement<br />

and detector performance degradation. Straightforward solution is to increase the duration of the target<br />

PR polynomial decreasing the mismatch between channel and equalization target. Note that this approach<br />

leads to undesirable increase in detector complexity fixing the detector structure in a sense that its target<br />

polynomial cannot be adapted to changing channel density.<br />

The (NPML) detector [20,32] is an alternative data detection method that improves reliability of the<br />

PRML detector. This is achieved by embedding a noise prediction/whitening process into the branch<br />

metric computation of a Viterbi detector. Using reduced-state sequence-estimation [43] (see also the<br />

description of the generalized VA in this chapter), which limits the number of states in the detector trellis,<br />

compensates for added detector complexity.<br />

A block diagram of a NPML system is shown in Fig. 34.61. The input to the channel is binary sequence,<br />

i, which is written on the disk at a rate of 1/T. In the readback process data are recovered via a lowpass<br />

filter as an analog signal y(t), which can be expressed as y(t) = Σni n h(t − nT) + u(t), where h(t) denotes<br />

the pulse response and u(t) is the additive white Gaussian noise. The signal y(t) is sampled periodically<br />

at times t = nT and shaped into the PR target response by the digital equalizer. The NPML detector then<br />

performs sequence detection on the PR equalized sequence y and provides an estimate of the binary<br />

information sequence i. Digital equalization is performed to fit the overall system transfer function to<br />

some PR target, e.g., the PR4 channel.<br />

M<br />

The output of the equalizer yn + in + Σi=1 fi xn−i + wn consists of the desired response and an additive<br />

total distortion component wn, i.e., the colored noise and residual interference. In conventional PRML<br />

detector, an estimate of the recorded sequence is done by the minimum-distance criteria as described<br />

for the Viterbi detector. If the mismatch between channel and PR target is significant, the power of<br />

distortion component wn can degrade the detector performance. The only additional component<br />

compared to the Viterbi detector, NPML noise-predictor, reduces the power of the total distortion by<br />

FIGURE 34.61 Block diagram of NPVA detector.<br />

FIGURE 34.62 NPML metric computation for PR4 trellis.<br />

© 2002 by CRC Press LLC<br />

j<br />

i<br />

Magnetic<br />

Recording<br />

Channel<br />

u(t)<br />

+<br />

AGC<br />

Whitened PR equalizer output :<br />

Lowpass<br />

Filter<br />

.....<br />

y(t)<br />

−<br />

t=nT<br />

N<br />

∑<br />

i=<br />

1<br />

PR digital<br />

equalizer<br />

y<br />

.....<br />

NPVA detector<br />

Viterbi<br />

detector<br />

Predictor<br />

P(D)<br />

= ( x n−1<br />

xn−<br />

2)<br />

Sk<br />

= ( xn<br />

xn−1)<br />

S ) − x ( S ) / x ( S )<br />

Channel<br />

memory state<br />

y<br />

n<br />

w<br />

n−i<br />

p<br />

xn( k n−2<br />

j n k<br />

i<br />

.....<br />

i

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