Optical code division
multiplexing-OCDM
Whilst
WDM and OTDM techniques partition the available spectrum and time to
different users, respectively, OCDMA techniques multiplex users
simultaneously and asynchronously (or synchronously) across the same
spectrum and timeslot through a unique code.
An
alternative to providing additional multiplexing at a single
wavelength is through CDMA techniques. Direct-sequence (DS) CDMA is
a technique originating from spread spectrum (SS) communications.
DS-CDMA techniques play an important role in current wireless
communication systems such as cellular, microcellular, indoor and
satellite communications. Efficient multiple access capacity,
effective reuse of the frequency resource, asynchronous access
capacity and immunity to the multipath fading characteristic of
wireless transmission are important characteristics driving the use
of this approach. DS-CDMA is also the candidate for the next
generation of broad-band mobile communications.
CDMA
techniques are implemented directly in the optical domain based on
continuous-wave (CW) or ultra-short optical pulse laser sources.
Coherent OCDM techniques governed by similar principals to radio DS-CDMA,
can multiplex multiple users onto the huge optical spectrum via
spread spectrum techniques, offering the same transmission capacity
as OTDM systems (via interference cancellation in an asynchronous
transmission mode). The techniques can be used as an additional
optical layer of a multiplexing hierarchy based on WDM and allow
electrical user signals access to the optical network in a more
natural way. The asynchronous operation of OCDM systems is a very
desirable characteristic of any multiplexing technique especially at
high data rates, and the accurate time of arrival measurement
capability relaxes the problem of synchronization inherent in OTDM
implementations. OTDM and OCDM systems both utilize narrow pulsed
laser sources, but the latter has the advantage of de/multiplexing
and detection relying on commercial components rather than new
generation devices required by OTDM. Hence, the attraction of the
approach is compelling despite the added implementation complexity
brought about at the receiver due to need for carrier phase locking
and the phase stability requirements (the linewidth) of the laser
sources.
A
network employing OCDM/WDM is capable of establishing more traffic
connections than a network using WDMA only. These results also
suggest that OCDM/WDM reduces connection setup failure in any
network condition and hence, reduces connection setup time as well
as node buffering requirements. In the longer term, OCDM in tandem
with WDM is thus an important consideration in the goal of realising
an extensive, high capacity and easily managed optical
telecommunication infrastructure.
Since this
technology is still at an initial stage, we estimate that it will
not be a potential player in the commercial arena in the next 5
years. For this reason we shall ignore it in the rest of the
discussion.
|