I've been following the topic of indoor coverage for mobile for almost 10 years now. The general perceived wisdom is that most voice and data traffic on cellular devices involves communication from inside buildings (50%, 60%, 70% - pick your own number). This assumption has been the primary driver behind the growth of femtocells, and various solutions looking at WiFi offload.
I regularly see forecasts predicting that this proportion is going to grow, and certainly with the current usage patterns of 3G laptops and smartphones, that seems reasonable, at first sight.
But.... how solid is that assumption? I'm starting to wonder if the ground might be less certain than everyone thinks. Because fundamentally, the indoor/outdoor split comes down to use cases and applications. The problem is that most radio-network folk in the industry are often a long way from those thinking about next-generation applications, devices and services, and what impacts these might have on the network and traffic patterns.
I can see a few early signs of a new generation of "outdoor-centric" applications, that might reverse the trend. Most obvious is mapping/navigation in various guises. But there's also streaming audio/radio - it will be interesting to see how Spotify grows on mobile for example. Then there's the whole area of "augmented reality", cloud computing and quite a few other examples I can think of. Several of these might be more symmetric (or upstream-heavy) than current applications, too.
On balance, I suspect that any shifts will happen slowly, but it's quite possible that we'll suddenly all be surprised by a new app coming out of nowhere and getting rampant adoption, thanks to appstores and widgets and other mechanisms that make viral uptake simpler. I can think of a few hypothetical examples that might emerge, although I'm not going to mention them here. (Let me know if you're a VC, I might do one of them myself....).
It's not just applications either - factors around device and OS architecture will also make a difference. If iPhones supported background applications, cellular networks might be suffering even more pain than they are currently, for example.
If you're working in the radio part of the industry - on either the vendor or operator side - and you'd like to explore the risks of "outdoor applications" more thoroughly, please get in touch with me.