The Mitigation of MAI for OOK-CDMA Systems with Optical Hard-Limiters by Transmitting Optical Pulses with Two-Level Intensities

In this paper, we propose OOK-CDMA system with optical hard-limiters (OHLs) using schemes of transmitting optical pulses with two-level intensities. The proposed system can be applied to both cases with single OHL and double OHLs. In the proposed system, a frame corresponding to a bit time is divided into two frames, where the low intensity pulses are transmitted in the weighted positions at the former frame, and the high ones are transmitted in the weighted positions at the latter frame. At the receiver side, in each weighted position of low intensity level, the ability of Multiple Access Interferences (MAIs) cancellation by OHL is equal to that of the conventional systems. However, in each weighted position of high intensity level, the OHL with the higher-level threshold value can easily eliminate MAIs with low intensity, while the MAIs are not eliminated in the conventional system. As a result, the ability of interference cancellation by OHL is significantly increased. When using single OHL in the proposed system, the optimum combinations of the values of low-level weights and low-level intensity ratio depend on the received laser power. This means that deploying dynamic control of the values of low-level weights and low-level intensity ratio is an imperative requirement in the proposed system with single OHL. On the other hand, when using double OHLs, the optimum combinations of the values of low-level weights and low-level intensity ratio are always constant. We analyze the performance of the proposed system and show its advantage in terms of bit error probability compared to the conventional system in both cases with single OHL and double OHLs. We also show that the proposed system with double OHLs is more effective and practical than the one with single OHL depending on the received laser power.