388 CHAPTER 8. MULTICHANNEL SYSTEMS
without requiring synchronization, and microwave subcarriers can be processed using
commercial electronic components. Each user is assigned a unique wavelength for
transmitting multiple SCM messages but can receive multiple wavelengths. The main
advantage of multiwavelength SCM is that the network can serveNMusers, whereN
is the number of optical wavelengths andMis the number of microwave carriers by us-
ing onlyNdistinct transmitter wavelengths. The optical wavelengths can be relatively
far apart (coarse WDM) for reducing the cost of the terminal equipment. In another
approach, the hybrid fiber/coaxial (HFC) technology is used to provide broadband in-
tegrated services to the subscriber. Digital video transport systems operating at 10 Gb/s
by combining the WDM and SCM techniques are available commercially since 1996.
The use of WDM and SCM for personal communication networks is quite appealing.
The SCM technique is also being explored for network management and performance
monitoring [247].
8.6 Code-Division Multiplexing
The multiplexing techniques discussed so far in this chapter can be classified as sched-
uled multiple-access techniques in which different users use the network according to a
fixed assignment. Their major advantage is the simplicity of data routing among users.
This simplicity, however, is achieved at the expense of an inefficient utilization of the
channel bandwidth. This drawback can be overcome by using a random multiple-
access technique that allows users to access any channel randomly at an arbitrary time.
A multiplexing scheme well known in the domain of wireless communications makes
use of thespread-spectrum technique[248]. It is referred to ascode-division multiplex-
ing(CDM) because each channel is coded in such a way that its spectrum spreads over
a much wider region than occupied by the original signal.
Although spectrum spreading may appear counterintuitive from a spectral point of
view, this is not the case because all users share the same spectrum, In fact, CDM is
used extensively in microwave communications (e.g., cell phones) as it provides the
most flexibility in a multiuser environment. The termcode-division multiple access
(CDMA) is often employed in place of CDM to emphasize the asynchronous and ran-
dom nature of multiuser connections. Even though the use of CDMA for fiber-optic
communications has been studied since 1986, it was only after 1995 that the technique
of optical CDM was pursued seriously as an alternative to OTDM [249]–[271]. It can
be easily combined with the WDM technique. Conceptually, the difference between
the WDM, TDM, and CDM can be understood as follows. The WDM and TDM tech-
niques partition the channel bandwidth or the time slots among users. In contrast, all
users share the entire bandwidth and all time slots in a random fashion in the case of
CDM.
8.6.1 Direct-Sequence Encoding
The new components needed for CDM systems are the encoders and decoders located
at the transmitter and receiver ends, respectively. The encoder spreads the signal spec-
trum over a much wider region than the minimum bandwidth necessary for transmis-