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We propose atmospheric (free-space) optical code-division multiple-access (CDMA) systems. We analyze the bit error rate (BER) of the proposed system using pulse position modulation (PPM) taking into consideration the effects of scintillation, avalanche photodiode (APD) noise, thermal noise, and multiuser interference. We show that atmospheric optical CDMA systems can realize communications at a bit rate of 156 Mbps when the logarithm variance of the scintillation σs2 is smaller than, or equal to, 0.1. When σs2 is large, we need to use forward error correction codes (FECs).