Privatizing Spectrum

According to Zimran Ahmed, the problem with spectrum is not that private ownership is too strong but that it is too weak.

All the considerable waste in spectrum exists because the spectrum users don't own it in perpetuity or it has encumbrances on its use, which means it can't be freely sold. Strong property rights would solve both problems.

Elsewhere, he writes

To date, spectrum licensing has been the single most damaging government intrusion into the technology world...

...there is nothing more logical than a spectrum auction. Those most able to profitably use the spectrum will bid the highest for it, and if they get it wrong they can then sell those rights to other people. The market will decide which uses it most values, and those holding spectrum inefficiently will bear the opportunity cost and so sell the spectrum to those who can use it better. Spectrum is a rival good, so strong property rights and a free market will allocate it best.

Discussion Question. Why is a spectrum auction, which allocates spectrum to private owners, so much more efficient that spectrum licensing, which allocates spectrum to specific uses?