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Goal: To learn the name and understand the concept behind a certain RF theory. The name of the theory is what I am looking for.

History: A few months ago, I ran across a theory that explained why you did not need a 100mw client card to make use of the range of a 100mw access point (contrary to popular belief). I believe the link was even referenced off somewhere on the cwnp.com site, but for the life of me after searching here and on Google, I cant find it again.

I am hoping someone can inform me of the name of the theory so I can research it some more to better understand what I am talking about.

Here is a picture of the situation I am referencing:
www.toastresearch.com/unknown_theory.jpeg

The only way I could see this working is if the access point has a better receive sensitivity than the client card. On the other hand, as I think about a cellular network (WARNING: I'm not a RF engineer), it seems that with sector antenna's and very low mw phones, their has to be some explanation of how this works since it does not seem likely that the antenna's and the cell phone have the same power output (again, I could only explain this in my mind with receive sensitivity).

Hopefully that makes my question clear. If not, let me know what else I need to provide and I will do my best.

Thanks for the help, have a Happy New Year.

Josh

Josh,
I believe you are referring to Antenna Reciprocity.
The Rayleigh-Helmholtz reciprocity theorem is well explained on this page:
http://www.connect802.com/wcu/2005/newsletter_051001.htm

Happy New Year from Milano!

Sacim,

Brilliant. The Rayleigh-Helmholtz reciprocity theorem was exactly was I was looking for.

Thanks for the information, I really appreciate it! Have a good one.

Josh

Josh,
glad I could help. If you find other interesting info/links on the subject, let me know.

Josh,

What you are reffering to is called up link (UL) and down link (DL) link budgets.

The DL link budget, from the AP to the CPE (Customer Premise Equipment, which can be a PCMCIA card, or external modem, either could have an external antenna) consists of the transmit power(+), cable loss (-), transmit antenna gain (+), Path loss (-), receive (CPE) antenna gain (+) (again, could be a PCMCIA, external modem, and both could have external antennas) and then reference the sum against the receive noise floor.

Do the exact same process for the UL. Notice that the UL tx pwr is much lower. Also, in the UL you will get an extra 3 db of receive diversity gain because you have 2 receive antennas. Also, the receiver noise floor of the base station (AP may be much better than that of the CPE.

The Link Budget calculations determine the UL and DL throughput for any given set of equipment for any given distance.

Key word: Link Budgets- RF

Really interesting article. I read a Cisco Press book (Designing VoWLAN) and the author pushed for the idea of having the APs force the power settings of the clients if possible. Did anyone have to design a VoWLAN network and ran into that issue? I believe it's the UL that determins the signal coverage.
Doesn't it conflict with that Rayleigh-Helmholtz theorem?