Quasi-orthogonal subcarrier multiplexing for high-capacity optical data links

Short-reach interconnect requirements in digital computers and telephone switches may require throughputs of 10 Gb/s (50 parallel lines at 200 Mb/s each) in the near future. Time-division multiplexing (TDM) baseband transmission methods require high-speed multiplexing and precise clock recovery, and/or multiple fibres for such applications. As an alternative, the authors propose and analyze a quasi-orthogonal subcarrier multiplexing technique (QO-SCM) for short-reach digital fibre-optic interconnect applications. QO-SCM signal processing exploits the symbol-time synchronism which is inherent among the parallel lines of a typical data bus, to permit simplified filtering and higher spectral efficiency than conventional SCM. One can achieve bandwidth efficiencies of up to 2 b/s/Hz while eliminating RF channel filters and guard bands, and requiring only simple baseband pulse shaping. Link performance is studied through simulation to derive worst-case noise margins for a 50-channel QO-SCM pay-load signal with 10 Gb/s total capacity using a numerical model of the laser rate equations. Results predict that 10 Gb/s could be transmitted over 100 m at a bit error rate (BER) ≤10^-12 on a single fibre-optic link, given a laser RIN of -155 dB/Hz.