WDMA SUBCARRIER-FDMA LIGHTWAVE NETWORKS - LIMITATIONS DUE TO OPTICAL BEAT INTERFERENCE

Microwave subcarrier frequency-division multiple access (SFDMA) provides concurrency in an optical wavelength band, and can be used to increase the capacity of a wavelength-division multiple access (WDMA) network. This paper considers a passive broadcast packet network architecture in which each address is specified by a single subcarrier frequency within a single optical wavelength band, with one laser per user (WD-SFDMA). For simultaneously accessible subcarrier channels, the capacity should ideally increase linearly with the number of subcarrier channels. However, with multiple optical carriers in the same optical channel, the beat notes produced by the mixing of different optical carriers at a photodetector may interfere with the microwave subcarriers (optical beat interference), and cause transmission failures. This paper presents a general model of optical beat interference, and its contribution to channel outage in a WD-SFDMA network. The probability of channel outage due to optical beat interference is determined from analysis and computer simulation for externally and directly modulated single-mode lasers. These results are used to calculate the ultimate limitation on the throughput and capacity of the network due to beat note interference, for a simple retransmission protocol. As more subcarrier channels are added, network capacity increases sublinearly and eventually saturates.