Today, while reading IT mainstream magazine "c't", I stumbled across an article about GSM deployments (and popularity) all over Africa.
One of the interesting things in that article was that one Operator had modified their network in a way to only use four timeslots (out of the eight available timeslots) per frequency in order to extend the range of a single cell to something like 70 kilometers.
For those who are not as familiar with the GSM Um air interface: It uses TDMA (multiple devices each get one timeslot on a given frequency). So let's assume we have eight timeslots on one frequency, all the transmitters (handsets) need to be synchronized with regard to that timeslot. Radio travels at speed of light and not with infinite speed. Therefore, since the handsets can be at a lot of distance to the receiver (base station), they might send in the correct timeslot, but the signal arrives out of the timeslot. GSM uses what's called "timing advance" in order to compensate for that effect. The base station tells the handset how much time earlier than the actual timeslot it needs to transmit to ensure arrival within the timeslot.
Now in that African GSM network in question, it seems like even the maximum timing advance is not sufficient. The frame still arrives late, i.e. in the next timeslot. By allocating only every second timeslot, there cannot be any clash and thus the range of a single cell can be extended. This is actually a very cool idea, I would almost call it a "hack", and it is possible within the GSM spec without requiring any change to existing mobile phones!
I only wonder how much of such cool hacks we would see if GSM base stations were more open and available. If there was a full FOSS stack that many people could use on off-the-shelf hardware, it would lead to a lot more innovative experiments and thus innovation. There would suddenly be more than a handful of GSM experts with access to proprietary technology looking at what kind of good, useful, cool and/or creative things one can do...