If you have been keeping track of EN events, you may remember that Dr. Robert McGwier N4HY and Dr. Tom visited CTVR in September and where Dr. Bob gave a superb two-part talk on software-defined radio and amateur satellites.

I posted the slides from the first part of Bob’s presentation here and have received the slides from the second half of his talk (well, we all had a hectic few weeks since then). This part of his talk focused on his long and exciting involvement with amateur radio and amateur satellites through his leadership in the Radio Amateur Satellite Corporation AMSAT.

Well, now you can download these slides and learn some more about his involvement as co-founder of the open source DttSP project, which is used with Flex Radio, and on co-designing AMSAT’s new satellite.

Download the slides as a zip archive [3.9MB]

A further presentation from Tom Clark K3IO regarding this satellite, the Geostationary Eagle, is also included in the zip archive. Tom prepared these slides for the 2008 AMSAT Space Symposium in Atlanta, GA, which was held during the last week of October. This is a goldmine of information for anyone interested in satellites.

At the recent IEEE DySPAN demonstrations in Chicago, one of the impromptu demonstrations set up by Tom and I showed what happened when a TV whitespace user was faced with interference from a narrowband FM transmission. This demonstration was set up on the fly; the TV whitespace user in this case happened to be Shared Spectrum Company’s nodes. A signal generator was used to generate a 25kHz FM signal. The range of operation was restricted to the 482MHz-500MHz frequency range; the frequency range specified in our FCC special temporary authority (STA) licence issued to CTVR and the demonstrators by the FCC.

Spectrogram showing Shared Spectrum Company's transmissions and a frequency-agile narrowband FM interference source.

A spectrogram displays frequency usage over time. In this case, the colours range from blue to red; blue indicates little or no activity towards red, which would indicate a very strong received signal.

This spectrogram, captured using an Anritsu MS2721B analyser, shows Shared Spectrum Company’s nodes operating in a ‘detect and avoid’ mode and sucessfully avoiding channels occupied by the narrowband source. When the frequency-agile narrowband interferer, controlled by me, moved onto the channel used by SSC, SSC’s OFDM transmitting node switched to an unoccupied channel.

Karl and Filip from Shared Spectrum Company were initially unaware of this demonstration so there was no preparation beforehand on their part other than running their demonstration as normal. SSC’s application in this case involved video streaming. This continued to operate just fine even though the nodes were changing channels to avoid the intentional interference.

To make things more interesting, the TX power of the narrowband interferer was reduced until it just about reached the detection threshold used by SSC’s nodes. As people moved between the interferer and SSC’s location, the interferer alternated between the detection and non-detection region over at SSC’s area. In this case, the TX power of the interferer was -18dBm using a suboptimal antenna, ~15 feet between the two locations, and obstructed by a pillar and members of the public in the room.

This was a 25kHz bandwidth example. Wireless microphones typically use approximately 200kHz of bandwidth. It is important to note also that wireless microphones certainly would not be roaming around in the spectrum segment in this manner either. The combination of wider bandwidths and static frequency usage for long periods of time makes them even easier to detect and avoid.

This was a dynamic spectrum access system operating under the shadow of several high power TV transmitters located a couple of blocks away, and no, the sky didn’t fall in.