Increasing the Capacity of SAC-OCDMA: Forward Error Correction or Coherent Sources?

We consider three different strategies for maximizing the capacity while minimizing the cost of spectral amplitude coding optical code-division multiple access (SAC-OCDMA): incoherent sources, multilaser sources and forward error correction (FEC). Due to their low cost and wide optical bandwidth, incoherent sources are often considered for SAC-OCDMA. Such sources exhibit reduced spectral efficiency due to their intensity-noise- limited performance. For single user systems, coherent sources offer greater spectral efficiency and improved performance; this is not necessarily the case for OCDMA. Even coherent sources are ultimately intensity noise limited in SAC-OCDMA due to the beating of coherent signals from different users overlapping in bandwidth. The intensity noise in coherent systems can be eliminated by having the center frequencies of spectral bins be offset from nominal values by a unique differential amount for each user. This requirement, however, leads to exacting requirement for source quality control and stability, and thus greater cost. We examine via simulation how system performance is affected for coherent sources under various assumptions about the precision of frequency offsets during manufacture. Finally, we examine the effectiveness of FEC in combating intensity noise in a cost effective manner. We find that coherent sources must have precise frequency placement to outperform FEC combined with incoherent sources. FEC systems work best in networks with low statistical utilization, while multilaser systems work best under high load.