Lastly, depending on the amount of time, the signal spends at a particular frequency channel;
there could be a coherent-detection (i.e., carrier phase recovery) acquisition time issue.
343
Let??™s discuss the techniques in more detail. We will begin with the FH method (see Fig. 7.2). This
spread spectrum technique uses N frequency channels to transmit the data signal. The N channels
are ???hopped??? in a predetermined, but appearing in a random manner. Assume we have N channels
with an information bandwidth of Rb 10 kHz. The transmit bandwidth now becomes spread to
N 10 kHz.
The information-bearing signal is denoted as b(t), which is then spectrally up-converted by
the carrier signal c(t). The PN sequence will select one of the N channels in a random fashion.
There are two classifications that depend on the hopping rate fh. The first is fast hopping, where
we hop more than one channel for the duration of a symbol (fh Rb). The second is slow hopping,
where we hop on a channel for the duration of many symbols (fh Rb). Since the WCDMA
standard is based on the DS technique, we will end our discussion on FH and emphasize the DS
method next.
The DS technique encodes each bit of information with many chips.
Pages:
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570