Optical code division
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
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
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 years. For this reason we shall
ignore it in the rest of the discussion.
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