382 CHAPTER 8. MULTICHANNEL SYSTEMS
Figure 8.29: Schematic illustration of subcarrier multiplexing. Multiple microwave subcarriers
(SC) are modulated, and the composite electrical signal is used to modulate an optical carrier at
the transmitter (Tx).
been used commercially by the CATV industry since 1992 and can be combined with
TDM or WDM. A combination of SCM and WDM can realize bandwidths in excess
of 1 THz.
Figure 8.29 shows schematically a SCM lightwave system designed with a single
optical carrier. The main advantage of SCM is the flexibility and the upgradability
offered by it in the design of broadband networks. One can use analog or digital mod-
ulation, or a combination of the two, to transmit multiple voice, data, and video signals
to a large number of users. Each user can be served by a single subcarrier, or the multi-
channel signal can be distributed to all users as done commonly by the CATV industry.
The SCM technique has been studied extensively because of its wide-ranging practical
applications [231]–[234]. In this section we describe both the analog and digital SCM
systems with emphasis on their design and performance.
8.5.1 Analog SCM Systems
This book focuses mostly on digital modulation techniques as they are employed al-
most universally for lightwave systems. An exception occurs in the case of SCM sys-
tems designed for video distribution. Most CATV networks distribute television chan-
nels by using analog techniques based on frequency modulation (FM) or amplitude
modulation with vestigial sideband (AM-VSB) formats [232]. As the wave form of an
analog signal must be preserved during transmission, analog SCM systems require a
high SNR at the receiver and impose strict linearity requirements on the optical source
and the communication channel.
In analog SCM lightwave systems, each microwave subcarrier is modulated using
an analog format, and the output of all subcarriers is summed using a microwave power
combiner (see Fig. 8.29). The composite signal is used to modulate the intensity of a
semiconductor laser directly by adding it to the bias current. The transmitted power