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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.


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