8.5. SUBCARRIER MULTIPLEXING 385
travel at slightly different speeds. As a result, frequency fluctuations are converted into
intensity fluctuations during signal transmission. The dispersion-induced RIN depends
on laser linewidth and increases quadratically with fiber length. Fiber dispersion also
enhances CSO and CTB distortion for long link lengths [232]. It becomes necessary to
use dispersion-management techniques for such SCM systems. In a 1996 experiment,
the use of a chirped fiber grating for dispersion compensation reduced the RIN by
more than 30 dB for fiber spans of 30 and 60 km [236]. Of course, other compensation
techniques such as optical phase conjugation can also be used [237].
The CNR requirement can be relaxed by changing the modulation format from AM
to FM. The bandwidth of a FM subcarrier is considerably larger (30 MHz in place of
4 MHz). However, the required CNR at the receiver is much lower (about 16 dB in
place of 50 dB) because of the so-called FM advantage that yields a studio-quality
video signal (>50-dB SNR) with only 16-dB CNR. As a result, the optical power
needed at the receiver can be as small as 10μW. The RIN is not much of a problem for
such systems as long as the RIN value is below−135 dB/Hz. In fact, the receiver noise
of FM systems is generally dominated by the thermal noise. Both the AM and FM
techniques have been used successfully for analog SCM lightwave systems [232]. The
number of channels for AM systems is often limited by the clipping noise occurring
when the modulated signal drops below the laser threshold [238].
8.5.2 Digital SCM Systems
During the 1990s, the emphasis of SCM systems shifted from analog to digital modu-
lation. The frequency-shift keying (FSK) format was used for modulating microwave
subcarriers [231] as early as 1990 but its use requires coherent detection techniques
(see Chapter 10). Moreover, a single digital video channel requires a bit rate of more
100 Mb/s or more in contrast with the analog channel that occupies a bandwidth of
only about 6 MHz. For this reason, other modulation formats such as quadrature AM
(called QAM), carrierless AM/PM, and quadrature PSK have been explored. A com-
mon technique uses a multilevel QAM format. IfMrepresents the number of discrete
levels used, the resulting nonbinary digital signal is called M-ary because each bit can
haveMpossible amplitudes (typicallyM=64). Such a signal can be recovered at the
receiver without using coherent detection and requires a lower CNR compared with
that needed for analog AM-VSB systems. The capacity of an SCM system can be
increased considerably by employing hybrid techniques that mix analog and digital
formats [233].
To produce the QAM format from a binary bit stream, two or more neighboring bits
are combined together to form a multilevel signal at a reduced bit rate. For example,
if 2 bits are combined in pairs, one obtains a bit stream at the half bit rate but each
symbol represents four possible combinations 00, 01, 10, 11. To distinguish between
01 and 10, the signal phase should be modified. This forces one to consider both
quadratures of the microwave subcarrier (hence the name QAM). More specifically,
thejth combination is represented as
sj(t)=cjcos(ωct+θj)≡ajcos(ωct)+bjsin(ωct), (8.5.5)