Free-space optical links for latency-tolerant traffic

Peer-to-peer (P2P) traffic and other latency-tolerant data sources form a large portion of present-day backbone traffic. Traffic shaping has been introduced to reduce the amount of P2P traffic at the expense of the end-user experience. In this work, free-space optical (FSO) links are proposed as a means to connect internet service providers to the backbone to carry latency-tolerant traffic. Although FSO links are less reliable than fibre, they are inexpensive and able to provide high throughput for traffic, which is able to tolerate delay. Single and adaptive modulation schemes are designed with the goal of maximising the throughput of the FSO link over a period of time rather than maximising the instantaneous reliability. Real world weather and visibility data from a major Canadian city are used to evaluate various techniques in terms of throughput, availability and outage. For a fixed modulation FSO system, four-pulse amplitude modulation (PAM) supports a yearly average rate of near 1.5 Gbps for a 1 km range, whereas conventional on off-keying (OOK) achieves 880 Mbps. With additional channel feedback data, adaptive modulation has over double the average throughput of OOK using four different PAM modulation schemes.