Direct-detection optical asynchronous CDMA systems with double optical hard-limiters: APD noise and thermal noise

Performance of optical asynchronous code-division multiple-access (CDMA) systems with double optical hard-limiters is analyzed in the presence of avalanche photodiode (APD) noise and thermal noise. The following effects on the optimal thresholds of double optical hard-limiters and on-off keying (OOK) decoder are clarified: sequence codes, the number of users, the received laser power, and the modulation extinction ratio. The results show that the optical asynchronous CDMA systems with double optical hard-limiters have good performance in the presence of the APD noise and the thermal noise even when the number of simultaneous users is large. Moreover the optical asynchronous CDMA systems with double optical hard-limiters are shown to be practical in the respect of setting optimal thresholds.