Reciprocity-enhanced time-division multiplexed optical switching with spatial diversity for free-space optical communication links

Abstract

This thesis investigates a performance analysis and experimental measurements of a free-space optical communications system that exploits optical reciprocity to switch between multiple (N) spatially-diverse apertures to increase data transmission through a scintillating channel. Electromagnetic signals, specifically optical, propagating along either direction of the same path experience the same changes in signal strength (reciprocity). A duplex link on a fading channel can use knowledge of the channel condition to decide to switch between multiple spatially diverse apertures, functionally improving the quality of the link. This concept will be referred to as "time division multiplexed (TDM) reciprocity." Higher throughput, higher received power, and lower latency can be achieved by exploiting reciprocity to choose a higher quality channel. The experimental work in this thesis includes systems measurements and assessment of components and sub-systems that enable TDM reciprocity.

This thesis outlines system requirements and trades for the N-aperture switching system. Important engineering decisions are discussed: sizing N, clock recovery, real-time atmosphere assessment, and direct versus coherent detection. The efficacy of increasing data rate with TDM reciprocity is verified by conducting an experimental BER (bit error rate) measurement campaign on a physical DPSK (differential phase shift keying) modem. The BER experiment uses scintillation data to simulate the atmosphere. The thesis concludes with results of testing of the optical switch and and a description of the driving logic for the optical switch. This switch architecture was designed with the capability to be integrated with a variable-rate, burst-mode, DPSK modem that can achieve data rates from 72 Mbps to 2.880 Gbps.

The switch is fast (<200 ns) to reduce impact on higher data rates produced by the modem, and needs to handle high power to reduce the need for multiple fiber amplifiers in the transmitter. An initial synchronization test of the switch with signal generators indicates switching without impacting error rate is possible within the DPSK modulation dead time. Aggregate gain includes a higher average received power, better receiver performance, and fixed switch insertion loss. For a link with a error-free-communication BER threshold of 1% and a scintillation index of 0.28, total gain is estimated to be 1.7 dB for N 4 and 2.7 dB for N = 10 apertures. For a link with a error-free-communication BER threshold of 1% and a scintillation index of 1.0, total gain is estimated to be 5.1 dB for N = 4 and 6.5 dB for N = 10 apertures.

Future work on the reciprocity concept could incorporate a fast switch into a dedicated fiber or free space optical experiment, rather than as an addition to an engineering modem. Future experiments should also investigate the potential of other mechanisms (e.g. high speed buffering or variable channel rates) to use knowledge of a fading channel.