3 The disadvantage of this technique is that
the sampling clocks need to be rather high and the memory storage requirements
can be excessive. An alternative approach is to sample the received
signal at a rate equal to the Nyquist rate and then perform the interpolation
operations as needed. This will result in the minimal storage of intermediate
values.
Hence the timing offset , is varied so as to maximize the above function over the L symbol observation
window. A block diagram showing these operations is shown in Fig. 6.27. Here a timing
instance, j, is chosen and its respective samples enter the decision device so that the output can be
used in the comparison. The likelihood value is computed and then the next timing instance, j1, is
chosen and a new likelihood value is computed. This procedure continues for as many times as the
symbol was oversampled. The best choice, that is, the sample timing with the largest likelihood value,
is chosen and used for the remainder of the signal processing functions downstream.
t^
322 CHAPTER SIX
Oversampled
Memory
r(k)
a(kT)
Decision
Device
Memory
Compute
Likelihood
Choose
Max
Sample Selection
Control, j
ADC
~
r(t)
Sampling
Clock, fs
tj
t??? FIGURE 6.
Pages:
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533