Hybrid Optical-Microwave Communications

A few weeks ago I visited the ITR at the University of South Australia, where I studied some time ago. My former PhD superviser, Bill Cowley is part of a team working on a interesting new hybrid optical/microwave communications scheme.

Their reasoning is that spectrum is becoming more and more congested. Free Space Optical (FSO) communications offers a great way to increase bandwidth. Free space means sending light signals through the air rather than through a fibre, for example over a distance of 10km. Like Wifi, but at higher (optical) frequencies, and with potentially enormous bandwidth for data.

The have a really neat trick – combining the FSO link with a regular microwave link, with Forward Error Correction (FEC) applied across the combined channels. This makes a system that is more robust than either link alone. The two channels have complimentary properties. For example if cloud affect the FSO link, the microwave link will be less affected.

I think this is a really novel idea. Normally FEC bits are sent through the same channel, e.g. on a CD extra bits on the disk are used to protect the music recording on the same disk from errors (like a scratch). On your cell phone both the speech data and the error protection bits pass through the same radio channel. With this scheme it’s like a DVD is combined with a CD to protect against bit errors.

Here is a picture of the optical part of the link, in a small room on the top of the ITR building:

The ‘receiver’ on the left hand side is a home-brew design based on 150mm Fresnel lens; the telescope of the right is a 125mm Meade which can either be used in visible wavelengths (via the web cam directly behind the scope) or at 1550 nm (detector and IR camera attached to the T splitter). The link extends over a 12km path to a site in the Adelaide Hills.

More information on the ITR’s FSO page. Thanks Bill for showing me the system and providing information for this post.