I'm taking an online course called Networks: Friends, Money and Bytes from a Princeton lecturer Mung Chiang. As an evolution of universities putting pre-lecture notes on an intranet, then videos of slide presentations, they have started to open them up to the general public. What a fantastic idea! I can't afford (or am too scared to jump) to study full-time, plus the geographical constraint is removed.
Anyway, Q1 of the twenty questions the course is structured around is discussing cellular networks. As signals attenuate over distance you will eventually be too far away to hear it. This means that the signal frequency can be re-used on another of your network's aerials. So, divide your spectrum allocation into seven parts and you can have a hexagonal design where each neighbour uses a different frequency. This reminded me of the map-making question about the minimum number of colours you need to colour the countries on a map of the world.