That's a good idea. If the micro cells are on a different piece of spectrum, that would work really well.
The only thorn in this strategy is how well do "large cells" work in a city with lots of tall buildings? It's pretty easy to imagine how the every-street-corner micro cells work...there's always one within line-of-sight. Of course given the traffic in a big city, we need a definition of "fast travelling" user.
