Theoretical Performance of Multi-Weight Spreading Codes for Multimedia Optical Access Network

The Optical Code Division Multiple Access (OCDMA) technique is a potential solution to provide access on the optical channel to different users but also to different services applying performance differentiation. For this purpose, we focus here on an OCDMA system using 2 Dimensional (2D) multi-weight spreading codes. At the reception, a Hard Limiter (HL) device in front of the Conventional Correlation Receiver (CCR) is used to reduce the multiple access interference. To evaluate the performance of this scheme in a noisy optical channel, we develop an approximated calculation which provides a theoretical upper bound of the multi-weight code performances. In addition, for any noise amount, the optimal CCR threshold can be exactly determined. We use the theoretical results to design the multi-weight codes adapted to the transmission of three services with respectively low, medium and high Bit Error Rates (BER). Thanks to our theoretical approach, the noise impact is analyzed and shown being very significant for each service. It is possible to take the noise power into account in the code design, but at the cost of the increase of the spreading code length. To overcome these drawbacks, we investigate the BER gain provided by Low Density Parity Check (LDPC) codes. The results obtained show that Forward Error Correction (FEC) is a good way to improve the multimedia access network potentialities.