Direct-detection APD-based optical CDMA using orthogonal signalings

Direct-detection optical, code-division multiple-access (CDMA) communication systems with avalanche photodiode (APD) photodetectors are investigated. Chernoff upper bound, modified Chernoff upper bound, and Gaussian approximation of the probability of bit error (pbe) are presented for general APDs and {0, 1}-valued optical signature sequences. Multiple-user interference, shot-noise, and receiver thermal noise effects on the pbe are studied in detail. Equal-weight orthogonal signaling formats that do not require dynamic estimation of the receiver threshold are studied. The results suggest that orthogonal signaling schemes are preferable to the on-off orthogonal signaling schemes. It is also shown that prime codes are much better than the optical orthogonal codes for optical code-division multiplexing