You probably remember mobile operators raving about the promise of 5G several years ago. Now, they're getting excited about a new upgrade: 5G Advanced.Angel Garcia | Bloomberg | Getty ImagesBARCELONA, Spain — Telecom operators haven't yet finished rolling out 5G wireless mobile networks. And yet bosses of major carriers are already talking about building something called "5.5G," or "5G Advanced."There was a lot of chatter about 5.5G at the Mobile World Congress tech trade show in Barcelona, Spai...