Recently there has been a lot of articles about the 700 MHz FCC auction. With Google and a bunch of other big heavy hitters going after the auction, it is an exciting time for wireless. It got my mind spinning back to the year 1994. I think I can hear Stone Temple Pilots if I listen hard enough...
The company that I was working for at the time had partnered up with one of the IVDS license holders to create an asymetrical link demo for their newly minted 218-219 MHz license. The ink was barely dry on the paperwork before we were commissioned to have a system up and running. Older Ham radio folks get all worked up on this subject, as the lower end of the 1.25 meter band was "stolen" from them for the use of United Parcel Sercvice on an unbuilt truck tracking data network. When the UPS license expired, this prime 1 MHz chunk of bandwidth was auctioned off in metropolitan regions, with an A and B license holder scheme, similar to the old AMPS cellular network in the US. I can't remember the specifics on license costs, and I'm really too lazy to look it up. 500 KHz per license holder is a pretty huge chunk of spectrum. While small compared to the 700 MHz band, around 200 MHz you have some strange propogation characteristics. While it is VHF and works in a fairly line of sight manner, you also get some interesting multipath and beyond the horizon propogation without a huge amount of power.
If you think back to 1994, this is a time when 14.4kbps modems were a BIG deal and it cost and arm and a leg to get 128 kbps access via ISDN, so you can imagine that trying to figure out how to use 500 KHz of bandwidth to push data down to end users was very exciting. The first demo used a ton of ham radio equipment purchased from Ham Radio Outlet and modified for use on this project. The head end used a computer to take serial input from up to four sources, channelize it, then push it to a SSE satellite modem that used QAM modulation, but I can't remember exactly how many symbols. The IF coming out of the modem was 70 MHz, so a custom mixer was used to take the output up to the RF frequency. On the receive end a Standard Electronics ham radio HT (in layman's terms a walkie talkie), was used to receive the signal. The IF of the radio was tapped, and run into an SSE modem that demodulated the signal, sent it via synchronous serial to a computer, and in turn the computer output a regular RS-232 signal. The cool thing about this, is that what this ended up doing is to create an end to end one way RS-232 path that was transparent to the other equipment attached.
(images courtesy of DTN via archive.org)
The first application of this was the use of a DTN networks box. People on both coasts of the US probably have no idea what I am talking about, but DTN pretty much has owned the technology market for farmers from the beginning. Basically at the time they transmitted their data to a Ku band satellite transponder, which you pick up at your location with a 36" dish and run it to the DTN box, which is a very simple teletext device connected to a TV monitor. Crop reports, futures reports, etc. Very cool and very simple.
To test reliability we installed the head end unit in San Francisco on the top of the Hilton. In the pre-9/11 days nobody even asked questions when you were hefting 1/3 rack of equipment through the kitchen and on to the roof. Our equipment was installed along side the pager transmitters and other radio gear in the shack on the roof. We tuned up a ham radio Ringo Ranger for the 1.25 meter band and started transmitting. I think we had 50 watts of power or somewhere near it.
In the days after the installation we spent the evenings driving up and down the peninsula to see what type of coverage and interference we were getting. With three in the mini-van, one drove, one watched the HP spectrum analyzer and one manned the rack. We had incredible coverage all over the bay with the exception of Daly City. It is amazing what you can do with a meager signal when it is transmitted from high enough and transmitted with forward error correction.
In the end, the whole system wasn't developed, the IVDS license holders lapsed and just became an unhappy memory for company management. I imagine there is some ham club in the south bay that has been active on 220 MHz in a big way over the years due to all the equipment we sold off for surplus over Usenet, back in the days when it wasn't just porn.
When I think back to this whole experience it makes me smile. This is the first time that I really got to work hands on with RF equipment other than just talking with friends on the CB. The engineers that I worked with spent the time to explain things to me, even though I didn't have the math and physics background at the time to truly understand all of it. I hope that every kid that has a fondness for technology has a chance like this to learn on the job. It didn't pay a lot of money, but it was better than bagging groceries or flipping burgers.
Oh, and BTW, whenever I think about RF spectrum these days, I think about this XKCD comic.