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 optical code division multiple access(CDMA) system, we consider M-ary pulse-amplitude modulation(PAM) coded system. However, when the 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-tevel threshold detection instead of the correlator. The proposed system can obtain the synergistic effect between the PAM and the chip-level receiver, which the chip-level receiver reduces more MAIs compared with the correlator and the PAM has the larger number of transmitted bits per frame than on-off keying(OOK), and achieves higher tolerance to MAIs than the OOK-CDMA system with the 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.