The phone of a decade from now will not just be shaped by usage scenarios; the technical constraints also matter. Some are more stringent than others. Let's decompose a phone into its main hardware components: processing, storage, interface, connectivity and battery.
Storage is the least constrained component. Memory capacity is growing faster than Moore’s Law. The storage in the phone is essentially unlimited. The video comm-pod is just the start; we’ll be carrying around most of the data we need with us for the unlikely event that we’re not connected.
Compute power is also not constrained much. The key issue will be power consumption (see batteries, below). However, it’s reasonable to assume that Phone 2020 will offer all the fancy graphics and natural language computation that algorithms can offer (for example, speech synthesis that reads messages like Winston Churchill). Multiple radios and software defined radio (SDR) pose power consumption challenges, too.
The small size of the phone poses some user interface challenges. Audio and video input/output aren’t a problem. The standing challenge is quiet message input. Small keyboards are the current best bet, but they’re only marginally useful. Projected or roll-up keyboards don’t offer the tactile feedback required for efficient typing. Chorded input is the obvious solution – if it weren’t for the fact that chord keyboards have failed in the market time after time. Designers will try all sorts of other whacky options in coming years, from sub-vocalization to reading brainwaves.
Data connectivity is limited by the spectrum available, channel use efficiency, and electrical power. Ultra wideband will provide very fast short-range communication; system capacity for ranges beyond one’s personal space is more debatable. We are approaching the Shannon limit for single channel communication (cf. turbo and LDPC codes; MediaFLO uses turbo codes), but the wild card is multi-user channels, that is, mesh networks. It boils down to the question: is spectrum scarce? Since technology is driving up both data rates and user demand, the answer is up in the air. If spectrum capacity becomes a non-issue, we’ll see a lot of person-to-person live high definition video, and cellular companies as we know them today will disappear.
Battery technology is the biggest obstacle to size reduction and strange new form factors. Reducing battery size while maintaining power output increases power density; keeping size constant while increasing the charge (and thus battery life) does the same thing. Sony’s exploding batteries show how close to the hairy edge we are at the moment. If we can somehow break through the energy density barrier the world will look very different in ten years: tiny wireless earphones that go for weeks on a charge, self-winding phones, paper-thin devices, and even long-range wireless power supplies.