One motivation for my recent Summer Ham Project is an idea that has been kicking around in my head for a few years. I figure that low bandwidth data over HF could be used for low cost communications in parts of the world where there is no infrastructure at all. For example chat, SMS, or email type services over distances of 1000’s of km.
Now HF email exists but a solar powered HF email station (HF radio, modem, solar power system) can cost $10,000 (this is a quote from some missionary friends of mine in New Guinea). The power levels are also quite high, for example a transmitted power of 100W. It’s probably possible to do it a little cheaper (say with Ham radio gear) for a minimum or perhaps several thousand dollars. Plus 1st-world service connection fees for the server side of the HF email service.
Now the trend in radio design is to move the signal processing from analog electronics to software. This tends to reduce the cost and complexity of the analog side. Software, as we know, can be free. So it should be possible to use a low cost radio (less than $100) coupled to some DSP software (say running on a laptop) to provide low cost data transmission over HF.
The problem with HF radio channels is that they are unreliable, they fade in and out over the course of the day and the solar sunspot cycle. Having a high power level helps punch that message through, but perhaps there are other ways. Some Hams have been experimenting with modes such as QRSS, which use low data rates and DSP techniques to send signals over 1000’s of km using just milliwatts of power. PSK31 is another interesting Ham mode for “HF chat” that uses sophisticated modulation and error correction coding techniques in just 31 Hz of bandwidth.
Over the past few months I have been working with Elektra on the Mesh Potato project. This has given me some exposure to mesh Wifi networks. In mesh networks client nodes forms ad-hoc networks by relaying messages. A central access point is not required. The network protocols automatically sense when a nearby node can be reached and adapt as nodes come and go. This sounds ideal for relaying HF radio messages over unreliable HF channels.
The figure below (thanks Elektra) shows the advantages of mesh networks over regular networks with a fixed central access point. Node C may not be able to see the access point, but can easily see nodes D & B. However a mesh network can use D & B to route traffic for C.
Now Mesh Wifi nodes might be a few 100m apart and operate at 11 Mbit/sec. Mesh HF networks might be 100-1000km apart and operate at 300-9600 bits/sec. Much slower but still fine for emails, SMS, and maybe even chat. Imagine a bunch of MF mesh nodes on a Pacific Island chain. They have no other form of communication. VSAT is far far too expensive due to the monthly fees and even a $8,000 HF email station is pushing it. A packet might bounce around several of the nodes before reaching the main island, and a shared Internet gateway to the rest of the world.
HF Mesh Nodes could be built very cheaply. For example a simple, low cost radio plus a laptop or even a small embedded DSP processor. As the data rates are low the signal processing could be done on a very low end chip, even a microcontroller. Low transmit powers are adequate as the mesh automatically senses a way to get the message through even under poor radio conditions. If the HF mesh nodes are cheap there can be many nodes, enhancing the performance of the network.
The HF mesh idea is just an idea at the moment that I am brainstorming with Elektra and Kim Hawtin. The potential is basic (but very useful) communications on a global scale at very low cost and with minimal infrastructure.