Let's say, for simplicity, that all of the traffic is compressed into 6 hours each day, so that you see a load of about 0.003 Erlang per subscriber during this peak traffic time. A minimum 3-sector GSM BTS site provides about 10.5 Erlangs at 2% blocking and thus serves 3,500 subscribers at your typical daily peak load. If your cost of operation is $6/sub/mo, that corresponds to a cost of about $252k/year per BTS site to run your network, with most of that cost in the BTS site itself: about $200k/year. (!) When we first estimated this, we though we'd misplaced a decimal point somewhere. Then we did we read this article in Balancing Act that put the cost of operating an off-grid BTS site in Africa at around $210k/year. Then we talked to some telecom people from Africa who said the cost was well over $150k/yr but they didn't know by how much. So it probably really is around $200k/yr. Why?
It's all about power. Suppose you have a BTS that draws 5 kW. And since it's in the tropics you have to cool it, which brings your power budget up to 7 kW. To supply that, you need a generator. And since a generator is a target for theft, you need security lighting and cameras, which drive up your power budget and add at least 1 Mb/s to your backhaul requirement, which requires yet more power. Before long, the site is drawing over 1o kW continuously and you are burning at least 25 gallons of diesel fuel every day. Now you need a crew with a truck to drive around fixing generators and fences and filling fuel tanks, which is complicated by the fact that most of these sites aren't even near roads. It starts looking like war logistics, where Sun Tzu tells us that every sack of rice at the front cost 10 more just to get there. By the time you have everything in place you're spending nearly $20k/mo to keep this beast running.
This matters a lot to the long term development of these countries, because most of the people who live out in the countryside cannot afford $6/mo for anything, meaning that they will never get telephone service, not even on a non-profit basis. To achieve universal service, someone will need to try something completely different.
So here's the good news: if you can keep site power consumption down to just a few hundred Watts, this all changes dramatically. Instead of a generator, you can run the whole site on solar panels or microturbines in many parts of the world. No more diesel fuel. No more crews in trucks. Every two years, you replace the batteries in the power system. That's all. That's why the design target for OpenBTS is 75 Watts per transceiver, a target that we are very near already just using off the shelf equipment.
Other other cost components in the subscriber rate are internetworking and capital amortization. Most connections between African carriers happen in Europe. That means that if you call from your MTN cell phone to a wired phone down the street that call may well get routed through France at French long distance rates. And the capital cost of rolling out a rural GSM network is at least $100/subscriber. But those are topics for other posts.