Dale Hatfield and Phil Weiser write,

For a band like that traditionally used for AM broadcasting, it seems impractical, if not impossible, to provide licenses with anything close to certainty in terms of interference protection…a station in an adjacent–or more remote–geographic area could seek damages or injunctive relief based on a series of natural conditions that happen only infrequently…the realities of radio wave propagation in this region of the spectrum simply do not lend themselves to clear and enforceable boundaries for the geographic are dimension of the spectrum resource.

The authors do point out that the properties of the PCS band (the frequency range used by cell phones) are less problematic for a property-rights regime. Moreover,

the commonality of interest among cellular and PCS providers reflects a shared understanding that there is a mutual threat of interference and a mutual benefit to cooperation…Consequently, even though the reality of the spectrum property right is “muddy,” the affected parties are still able to agree on mutually beneficial accomodations.