386 CHAPTER 8. MULTICHANNEL SYSTEMS
Figure 8.30: Frequency allocation in a multiwavelength SCM network.wherej=1 to 4 andωcis the frequency of the subcarrier. As an example,θjcan
be 0,π/ 2 ,π, and 3π/2 for the four combinations. The main point is that the four
combinations of the 2 bits are represented by four pairs of numbers of the form (aj,bj),
whereajandbjhave three possible values (−1, 0, 1). The same idea can be extended
to combine more than 2 bits. The combination ofmbits yields the QAM format with
M= 2 mlevels. It is common to refer such systems as M-QAM SCM systems.
The hybrid SCM systems that combine the analog AM-VSB format with the digital
M-QAM format have attracted considerable attention because they can transmit a large
number of video channels over the same fiber simultaneously [233]. The performance
of such systems is affected by the clipping noise, multiple optical reflections, and the
nonlinear mechanisms such as self-phase modulation (SPM) and SBS, all of which
limit the total power and the number of channels that can be multiplexed. Nevertheless,
hybrid SCM systems can transport up to 80 analog and 30 digital channels using a
single optical transmitter. If only QAM format is employed, the number of digital
channels is limited to about 80. In a 2000 experiment, 78 channels with the 64-QAM
format were transmitted over 740 km [234]. Each channel had a bit rate of 30 Mb/s,
resulting in a total capacity of 2.34 Gb/s. Such a SCM system can transport up to 500
compressed video channels. Further increase in the system capacity can be realized by
combining the SCM and WDM techniques, a topic discussed next.
8.5.3 Multiwavelength SCM Systems
The combination of WDM and SCM provides the potential of designing broadband
passive optical networks capable of providing integrated services (audio, video, data,
etc.) to a large number of subscribers [239]–[243]. In this scheme, shown schemat-
ically in Fig. 8.30, multiple optical carriers are launched into the same optical fiber
through the WDM technique. Each optical carrier carries multiple SCM channels using
several microwave subcarriers. One can mix analog and digital signals using different
subcarriers or different optical carriers. Such networks are extremely flexible and easy
to upgrade as the demand grows. As early as 1990, 16 DFB lasers with a wavelength
spacing of 2 nm in the 1.55-μm region were modulated with 100 analog video channels
and six 622-Mb/s digital channels [240]. Video channels were multiplexed using the
SCM technique such that one DFB laser carried 10 SCM channels over the bandwidth
300–700 MHz. The ultimate potential of such WDM systems was demonstrated in a