How to harness the unused radiowaves coming from cell sites?

By "harnessing unused radio waves" and the question being categorized in "energy" I guess you are referring to harnessing the effective radiated power from cell site antennas. The power from a cell site isn't really worth trying to harness to do anything other than communication. It's not like listening to an AM station on a crystal radio. Cell sites actually tend to use the least amount of power necessary for the network to work. That is one of the reasons cell sites tend to be clustered in more populated areas and the cell sites are close enough to overlap in range so that a cellphone can switch from using one tower to another seamlessly.