The performance of optical code-division multiple-access (CDMA) communication systems with avalanche photodiodes (APD) is analyzed. The bit error rate (BER) can be accurately computed by using the saddlepoint method. The effects of the multiple-user interference (MUI), signal-dependent shot noise, and receiver thermal noise are investigated. Results of the numerical integration illustrate the non-Gaussian property of the receiver output distribution. Exact means and variances are derived for the Gaussian approximation. It is found that when the MUI increases, the saddlepoint approximation yields satisfactory results, but the Gaussian approximation yields higher BER floors. Prime codes and on-off keying (OOK) are considered. Examples illustrate the effects of the system parameters such as the APD gain, threshold, prime code length, and the number of simultaneous users.
