I've always accepted that technology advances. Being born in the 90s, everything just becomes faster, smaller, cheaper and generally better if you wait a few years. This was most obvious with consumer electronics such as TVs, PCs and cellphones.
However, it occurs to me now that I know what drives most of this changes, except for one. Computers and cellphones get better and faster mainly because we are able to build smaller and more efficient transistors (I hear about twice the transistor count per unit of silicon area every two years).
The Internet got faster first with DSL which pushed the bandwidth of landline copper twisted pair to its maximum. When we ran out of usable spectrum inside the copper wire we turned to optic fiber, and it was a whole new game.
TL;DR: But, what is it that makes it possible for cellular networks to keep getting faster? I've had 2G, 3G and now LTE cellphones and the speed differences are astronomical, akin to the differences observed in household internet in the last decade.
Yet, LTE channels don't necessarily have a bigger bandwidth (in fact, I believe LTE uses less: 3G uses 5 MHz channels, whereas LTE can have smaller channels, from 1.4 to 20 MHz). Moreover, I've heard many times that LTE is more efficient in terms of bps per channel Hz (I would add 'citation needed' here, I'll be the first to admit that it at least sounds dubious).
So what is it? Just more spectrum? Better and smaller electronics? Or are we getting better at this in other ways? How so?