Enhancement of tolerance to MAIs by the synergistic effect between M-ary PAM and the chip-level receiver for optical CDMA systems

First, in order to increase the number of transmitted bits per pulse in an optical CDMA system, we consider an M-ary PAM coded system. However, when a correlator is applied in the receiver, the multiple access interferences (MAIs) with high intensities deteriorate the system performance significantly, even if the number of MAIs is small. Consequently, we propose an optical CDMA system with M-ary PAM in conjunction with the chip-level receiver by M-level threshold detection instead of a correlator. The proposed system can obtain a synergistic effect between the PAM and the chip-level receiver, in which the chip-level receiver reduces MAIs more compared with the correlator and the PAM has a larger number of transmitted bits per frame than on-off keying (OOK), and achieves higher tolerance to MAIs than the OOK-CDMA system with a chip-level receiver at the same bit rate and number of subscribers. We show that the proposed PAM-CDMA system with the chip-level receiver achieves better bit error probability than the OOK-CDMA system with the chip-level receiver.