Monday, July 02, 2012

Transmitter versus receiver specifications: measuring loudness versus determining understanding

In arguing that regulators should attend to receivers as well as transmitters, I’ve may have mistakenly left the impression that they’re symmetrical, e.g. that one is a reflection of the other. For example, in “Four Concerns about Interference Limits” I observed that a communication system can be engineered to operate successfully either by improving receivers, or by delivering more transmitted power.

However, this framing is potentially misleading. It could be taken to mean that transmission and reception are two sides of a coin, that they are similar in kind, but that one is the reflection or complement of the other. They’re actually more like the outside (transmission) and inside (reception) of a black box - a black box like the human head, say.

While it’s true that a transmitter and a receiver complement each other in completing a communication, transmission and reception differ in their nature. It’s like the difference between observing someone’s behavior (transmission) and observing their consciousness (reception). It’s pretty easy to observe whether a sentence has been uttered softly or loudly (transmitter characteristic), but much harder to check whether the sentence has been understood (receiver characteristic).

If one thinks of receivers as “just” the inverse of transmitters, it’s tempting to infer that since regulators have successfully managed transmission by specifying the operating parameters of transmitters, they could likewise easily specify the operating parameters of receivers.
However, the fact that regulating receivers has proved to be so difficult is a bit of a give-away… there are no simple receiver characteristics that can characterize reception without looking inside the box, in the way that one can enforce transmitter performance by simply looking at the power coming off an antenna.

Attributes that “look inside the box” entail making a host of assumptions about a receiver and its role in a communications system; these assumptions, encoded into rules, freeze the state of the art in receiver technology and applications. For example, transmitted power in an adjacent channel can be easily measured without saying or knowing anything about the transmitter’s design or use, whereas measuring a receiver’s adjacent channel rejection performance requires specifying acceptable receiver performance (which is different for, say, analog and digital systems) and the minimum desired signal type and level (again, a function of design and deployment choices).

Consequently, one shouldn’t expect to be able to regulate receivers in the same way we regulate transmission, i.e. one should expect that specifying device parameters will be as straightforward for receivers as for transmitters. That leads to my preference for interference limits (summarized in don’t try to specify the details of receiver performance, but rather specify the interference it has to tolerate before the operator can complain of harmful interference.

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