However, that’s not the only way to look at it. Non-spatial models such “Wireless as Trademark” work just as well (see my 2008 paper De-Situating Spectrum: Rethinking Radio Policy Using Non-Spatial Metaphors): by analogy, a trademark stands for both a part of the wireless resource (customarily, frequency band x geographic region x time slot), and signals. The wireless resource is all possible radio operations. In such an approach, one is much less likely to ignore the importance of receivers in this approach than spectrum-as-space, where only transmitters can “fill the space” with signals. Any radio operation includes the use of a receiver, and that receiver-transmitter pair influences what transmissions are possible by third parties.
Curiously, I found a relevant perspective on this problem in a book on ethics – Stephen Batchelor’s Living with the Devil. He writes:
One tends to think of space in terms of physical extension and location. A body “occupies” or “fills” a space. For there to be “no more space” means that nothing more can be fitted into a room or a vehicle or a document. Outer space is that virtually infinite expanse speckled with galaxies and stars separated by inconceivable distances. “Inner space” suggests a formless expanse of mind in which thoughts, mental images, memories, and fantasies rise and pass away. Space seems to be the relatively permanent place where temporal events happen.
Buddhist philosophers see space differently. They define it as the “absence of resistance.” The space in a room is under stood as the absence of anything that would prevent one moving around in it. To cross from one side of the room to the other is possible because nothing gets in your way. Rather than being the place where things happen, space is the absence of what prevents things from happening. The space in the room is nothing in itself; it is just the absence of chairs or tables, glass walls or hidden tripwires that would obstruct movement within it. In encountering no such resistance, we are able to move about freely. [In the footnotes, Batchelor ascribes this approach to the Geluk school of Tibetan Buddhism.]
The customary view that Batchelor outlines is “space as a set of dimensions” that informs the Spectrum as Space metaphor. One can transpose his summary to spectrum as “the relatively permanent place where [radio operations] happen.” The “Buddhist” view, on the other hand, would see spectrum as the absence of factors that would obstruct radio operations. Existing radio operations, including receivers, would provide resistance to new operations, even in quite distant frequency bands. And there is an interaction between the agent that wants to move about and the nature of obstructions: neither a mouse nor a monkey would have no trouble scurrying around in a restaurant, while a person would be obstructed by all the tables and chairs. Likewise, one has to first define the new operation one has in mind before deciding that spectrum is “occupied”; calculating utilization is not a straightforward matter of marking spectrum as “empty” or “full.”