HF Email, Winmor, and AMPS, TDMA, CDMA, etc.

A small thread on the Winmor beta-test group on Yahoo groups arose around the subject of needing more Pactor (and eventually Winmor) stations because accessing email over HF is single threaded. Only one user can access a station at a time, and because the baud rate is so low for HF a station can be tied up for an extended period of time.

This kind of problem has been solved many times before. In the first generation of cellular phones (AMPS), an analog phone connection would tie up a channel (pair of frequencies) at the cell node for the duration of the call, much like the current HF approach does.

This first generation approach offers us some ideas for ham radio. The first is to use multiple channels to talk to multiple stations. In AMPS the channels were full duplex (using a pair of frequencies), but the same approach could be done with half-duplex.

This would probably require some custom, and very non-standard, radio equipment at the server end to handle multiple channels at once. It would be an interesting SDR project. Or one could just install multiple radios. In AMPS, there were two different bands for transmitting and receiving at the cell node, so duplexers are used to share antennas. This works well for a full duplex connection.

Even for a single radio, the Winmor channels work in 500hz bandwidths (and 1600 hz as well, but let's ignore that). You could easily put three of these in a standard SSB signal with spacing and have three "channels" for a single, normal radio. This gives a lot more capacity, albeit with more complexity. Issues around synchronizing the transmit and receive cycles on each channel arise, but that can be solved...

2G cellular moved to CDMA and TDMA digital, allowing sharing of channels. For HF email, this moves us into the realm of things like Aloha and Slotted Aloha, which are known, although not terrible efficient, means of coordinating multiple stations on a single frequency. But if the goal is to provide service to all and not allow a single user to "hog" a relay station, that would work.

Clearly, using an off-the-shelf radio is limiting (but not totally limiting), but custom hardware would really open this up to some sophisticated usage!