Aerial platform using beamed energy

Would anyone care to comment on the feasibility or lack thereof associated
with the following idea?

In this concept, there would be a large stationary aerial platform located
30,000 to 40,000 meters above a major metropolitan area. The platform
would be supported primarily by an Ultra Long Duration Balloon (ULDB, c.f.,
http://www.space.com/news/spaceagencies/nasa_balloon_010226_wg.html).
Beamed microwave energy from the ground directly below would be used to
keep the ballon stationary over a fixed point on the ground. Solar cells
might also supplement power needs on the platform.

The platform would serve as a cellular/mobile telephone transmission tower
(special care would, I guess, have to be taken to ensure that the frequency
of the beamed energy did not interfere with the frequency of
cellular/mobile transmission). Such a platform might replace many ground
towers. Also on the platform might be cameras and/or SAR that could
provide near-constant traffic monitoring, reducing the need for
helicopters.

Alternative uses of such a platform might be to lift a telescope above the
Earth's atmosphere for lower cost than launching one into orbit. The
platform and its telescope could be retrieved to the ground for maintenance
at regular intervals, too.

Ron Menich