Optical orthogonal code-division multiple-access system .II. Multibits/sequence-period OOCDMA

For part I see ibid. vol.42, no.7, p.2470 (1994). In a recently proposed optical orthogonal code division multiple-access (OOCDMA) system, one bit of user´s data is transmitted per sequence-period, and a threshold is employed for the final bit decision. In this paper, a system that can transmit multibits per sequence-period is introduced, and avalanche photodiode (APD) noise, thermal noise, and interference, are included. This system, derived by exploiting orthogonal properties of the OOCDMA code sequence and using a maximum search (instead of a threshold) in the final decision, is log₂ F times higher in throughput, where F is sequence-period. For example, four orders of magnitude are better in bit error probability at -56 dBW received laser power, with F=1000 chips, 10 “marks” in a sequence, and 10 users of 30 Mb/s data rate for one-bit/sequence-period and 270 Mb/s data rate for multibits/sequence-period system. Furthermore, an exact analysis is performed for the log₂F bits/sequence-period system with a hard-limiter placed before the receiver, and its performance is compared to the performance without hard-limiter, for the chip-synchronous case. The improvement from using a hard-limiter is significant in the log₂F bits/sequence-period OCCDMA system (while it is not in a one-bit/sequence-period OOCDMA system proposed in Part I)