We are building a wearable device for low resource settings and are trying to figure out a way to be able to send the data collected over a long range say 5-6 miles at a base station/hospital. What would be the best protocol to use in this case? Also, since this is a wearable device we will have to take into account any possible effects on the body on using a particular kind of communication protocol. Apologies if this is vague. We are still fleshing out the final details and data transmission is a very critical piece which we are grappling with.
Standard solutions are just few: the Nordic packet-data transmitter, it cost about 1 USD and is capable UNDER 1 km;
the LORA(tm) product, packet data also, capable for 3 km yet costs 10x more;
the ordinary most prost transmitter, yet for 5-6 km You will need at least 5...15 Watts;
The WiFi, yet such is the maximum maximorum ever gained between two very high cost machines with large sharp-directed antennas;
and cellular phone networks, where Arduino phone sub-module costs few USD.
I would vote for the last solution, and if dislike, then for 15W prost radio.
You have a collection of options:
Mobile phone networks: any of the options, GPRS upto 4G depending on cost your willing to pay and data you need to transfer. Probably the easiest and most reliable, assuming that the mobile networks are available in the operational area.
Some analogue radio communication. I'm pretty sure that all of the frequencies that will give you the range are license controlled (in Europe, Asia and N America), but it might be worth investigating depending on how much you can work with the countries you plan to operate in. High power consumption, license fees and large antenna are all issues with that.
Satellite phone: lots of low level data communication takes place over satellite phones already, so I'm sure you could add something to that. Advantage is they cover all the world (depending on you provider) as long as you're outside (some work inside, sometimes). Power consumption is high, as is the required equipment cost.
Mesh network: if you have enough of these sensors spread from where you need the data to the furthest node, you could use a mesh network of nodes, one every 20 metres or so, and link one to another to get to the data centre. Clearly this is lots of nodes, lots of cost, and each node needs it's own power supply.
No doubt there are other methods, but looking at miles, you are best to use existing networks; mobile phones or satellite phones.