Multilevel and trellis coded modulation in asynchronous FOCDMA networks

An asynchronous fibre optic code division multiple access (FOCDMA) network employing multilevel modulation for data transmission is investigated. Symbol error probability (SEP) of the network is evaluated assuming the use of optical orthogonal code (OOC) and a double hard-limited, passive correlation PIN diode detector receiver. The SEP is evaluated in the presence of multiple access interference (MAI), detector dark current noise, photon detection quantum noise, and circuit thermal noise. Use of spectrally efficient trellis coded modulation (TCM) techniques based on set partitioning of the signal constellation to enhance the FOCDMA network SEP performance in the presence of these performance degradation sources is also discussed along with the transmitter and receiver architectures for realising multilevel modulation with four levels of modulation and four-state, eight-point TCM techniques in the network. Results show that use of multilevel and TCM techniques in asynchronous FOCDMA networks can result in significant improvement in performance and in bandwidth efficiency compared to uncoded conventional (two-level OOK modulated) asynchronous FOCDMA networks.