Covert communication on optical fibres
Two researchers from Princeton University have proposed an implementation of a transmission process known from radio technology in a new medium. Optical Code Division Multiple Access (OCDMA) creates a covert communication channel in optical fibres transmitting WDM signals. Bernard Wu and Evgenii Narimanov pass the signal to be transmitted through a special optical system (grid and individual phase masks), which spread the signal spectrum in a manner similar to DSSS. Rather than being in a narrow-band light pulse with a high amplitude, the information is concealed in many weaker components which are on the same wavelength as the more powerful WDM signals. An attacker is unable to separate this mixture without the phase mask which complements the transmitter.
In order to prevent interference between the WDM transmission and the covert signal, the latter has to operate considerably more slowly. Wu and Narimanov determined in tests that the bit error rate (BER) of the WDM channel increased the stronger or quicker the overlaid OCDMA signal became. With a performance ratio of 3 (OCDMA to WDM) and a speed ratio of 1/256, the BER of the WDM signal was 10-6, whilst the OCDMA signal achieved a BER of about 10-4. If the OCDMA speed is now doubled, the BER of the WDM signal increases to 10-4. OCDMA should also work through a series of analogue amplifiers in a long optical fibre, but as soon as a digital regenerator is encountered, the signal is lost.