To focus on only one domain seems to be an old fashion approach. Instead focus on the interception between different domain levels.
If you take a look at the wireless sensor networks you have to design new efficient com protocols and special analog circuits. In between there are hardware MAC implementations, low energy DSPs, clever RF architectures ... and so on.
All domains are interessting but most of the solutions space is in between the knowledge domains.
So take a broad approach to fill your interest.