Design of Cost Efficient Optical Code Division Multiple Access (CDMA) Channels Overlay Wavelength Division Multiplexing (WDM) Grid

Abstract

This paper proposes a cost-efficient hybrid configuration based on optical code division multiple access (CDMA) system overlaid WDM grid. In the proposed hybrid system only 15 wavelengths with channel separations up to 25GHz have been used to carry out 120 users’ data with spectral efficiency of 1.6 b/s/Hz. In other words, a number of users can share one wavelength in an optical CDMA technique, at a 40Gbps of line rate for each wavelength. Hence, an improvement on the wavelength utilization efficiency can be achieved. Results show that the hybrid system supports 120 users over 90km of single mode fiber and 15.075km of dispersion compensating fiber. The results also show that optical CDMA, which is based on spread spectrum technology, has the ability to suppress the unavoidable nonlinear effect, i.e., four-wave mixing (FWM), where the optical energy of a bit is spread over signature sequence code. The effect of MAI in the proposed hybrid system is reduced due to the zero cross-correlation property of Multi-diagonal (MD) code; therefore the FWM light associated with MAI is reduced.   Moreover, the squeezing of the chips interval into a quarter of bit duration guarantees the avoidance of inter-symbol interference (ISI).
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