I have a feeling this has more to do with building penetration than it does with altitude. Most towers are omnidirectional, thus the signal would be equally distributed in all planes. Since most towers are higher than most buildings (perhaps not the one you're in) the signal would pass through more building materials on the first floor than the 18th. I'm no expert on this, it's just a guess based on what I do understand about how these things work. In any event, the thing Sprint and others should (and in other cases ARE) doing is deploy microcells throughout metro areas. They're like little cell phone towers that work on a much more local level. This would improve signal in a near area.