Wednesday, September 10, 2008

Protecting receivers vs. authorizing transmitters

When governments hand out permissions to operate radios – licenses, for example – they think in terms of transmitters: within the licensed frequency range you can broadcast at such-and-such a power, and outside those frequencies you can transmit only at some other, much lower, power. [1] This distribution of broadcast power is often called a “transmission mask”.

In thinking about new ways of regulating radio, I’ve come to believe that a transmission mask is not sufficient; it helps to include receiver parameters. [2] But that’s not the point of this post; if you’re interested, read my paper at SSRN.

Today’s question is: if transmitter parameters are not sufficient, could one do without them completely? Can you go the whole hog, and only specify a receiver mask? I think you can, and I’m encouraged that Dale Hatfield tells me Robert Matheson concluded this some time ago, though I haven’t found a reference yet.

Receiver and transmitter parameters are figure and ground. Assume a steady state where all systems operate without interfering with each other. Transmissions will by definition have been chosen to prevent interference with receivers in adjacent frequency bands. The result of all the transmissions is a level of electromagnetic energy at every receiver which is low enough that no harmful interference results. (I’m ignoring receiver specifications [2] and the geographical distribution of transmitters in this discussion.) Each transmission propagates through space to a receiver, resulting in the allowed receiver mask:
{all transmission masks} -> {propagation} -> {resulting receiver mask}
One can also invert the calculation: given a receiver mask and propagation model, one can determine what the allowed transmissions should be.

A license defined in terms of receiver masks would allowed the licensee to transmit any amount of energy as long as it does not exceed the mask of anyone else. It would guarantee that nobody else is allowed to radiate energy which leads to the allowed levels being exceeded at the licensee’s receiver.

One can compare reception-based vs. transmission-based licenses by thinking about property rights. The two important attributes here are exclusivity (if I’m a licensee, I can prevent anyone from transmitting in my frequency “band”), and autonomy (within my “band”, I can do what I like, notably acting in a way that makes money). [3], [4]

A transmitter-based license focuses on autonomy by defining transmission parameters. It specifies what a licensee is allowed to do, but it doesn’t provide any guarantee of exclusivity. A receiver-only right reverses this emphasis: by specifying what would amount to harm to a receiver, it provides a way to make exclusivity real in practice. The constraints on autonomy are implicit in the receiver-rights of others: as long as a licensee doesn’t interfere with other licensees’ exclusivity, it can do what it likes.

Anything transmitter-only rights can do, receiver-only rights can also do. They are mirror images of each other. The information burdens are also mirror images. Receiver rights place a burden on all the other rights holders to figure out if their transmissions will transgress a receiver spec. While transmission rights don’t impose an information overhead upfront, the rights holder bears the burden of uncertainty: they may be blind sided at some future date by a new transmission right that reduces the value of the system they’ve deployed. The fight between M2Z and T-Mobile is a good example: T-Mobile claims that the proposed terms of a proposed new cellular license M2Z seeks (AWS-3) will cause harmful interference to their operations under a current license (AWS-1).

I like receiver-only rights because they put the focus on the ability of a wireless system to operate in the presence of noise, which one can only do by taking receiver parameters into account. However, enforcement proceedings may appear more complicated in this case, since it isn’t obvious which of many transmissions is responsible for a receiver mask being exceeded: a spectrum analyzer at a receiver can only measure the sum of all radiated power. Today the regulator has what looks like a big stick: it can objectively check whether a licensee’s equipment meets or violates its approved transmission mask. The stick doesn’t actually help solve the most difficult interference problem, the case where all transmissions meet their masks, but it gives the regulator power that it will be loath to give up.


There a choice in creating radio rights between protecting receivers and authorizing transmitters – or some mixture of the two. The current approach limits the discussion simply to ways of authorizing transmitters. A more nuanced analysis of the trade-offs is required, and was begun here.


[1] Since this is going to get pretty geeky, so I’m going to leave out a lot of important other stuff, including geography (radio licenses are typically restricted to a certain area) and duty cycles (how often transmitters operate).

[2] Receiver parameters. I distinguish between a receiver mask, by which I mean a distribution of RF energy (i.e., a spectrum) which represents the worst-case noise environment in which a receiver needs to be able to operate, and a receiver specification, by which I mean the ability of the receiver to detect and decode wanted signals in the presence of unwanted ones.

[3] According to Gary Libecap in Contracting for property rights (1989), p. 2 “. Private ownership . . . may involve a variety of rights, including the right to exclude nonowners from access, the right to appropriate the stream of rents from use of and investments in the resource, and the right to sell or otherwise transfer the resource to others.”

[4] “Band” here means the collection of constraints on operation. In the conventional approach, it’s usually taken to mean a frequency band and geographical region.

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