Re: Mobile networks have too much spectrum already
Some airport radars interfere with 5GHz (11a) WiFi so the network drivers look for radar chirps and avoid using this area of the spectrum if they detect them. Radar will usually be detected at the bottom of the band as badly formed packets with zero data length.
The same technique could be used to share other parts of the spectrum. These frequencies at the power that WiFi networks use are unlikely to go any distance so the radios could be required to 'see and avoid' any other users, something they could even do dynamically if necessary. The objection to this would be merely business -- business by its nature wants it all so any unlicensed use of spectrum is seen as lost revenue potential.
I should also mention that the entire reason why the ISM bands -- 2.4 and 5GHz -- are open and unlicensed is that they were regarded as useless for communication because these frequencies are absorbed by water molecules. (This is why microwave ovens work.) Its is nothing short of miraculous how radios were developed to make use of this spectrum despite this and it points the way to how spectrum should be used in general. Most people are still wedded to the notion of selecting traffic by frequency -- you 'tune' a radio -- but this is a grossly inefficient way to use spectrum, it just happened that it was easy to do so suited early radio technology. We're stuck with this legacy technology for the time being but for newer uses and bands there's no reason to keep thinking in terms of exclusive spectrum.