I think some disambiguation may be in order. QAM is a vector modulation method:. http://en.wikipedia.org/wiki/Constellation_diagram It shares almost nothing with OFDM methods.
Irrespective, all receivers (CW, AM, FM, SSB, VSB, angle modulation, OFDM, QAM, TCM, etc) can have an RSSI output from the AGC, limiter or demodulator. Strictly speaking it only means Received Signal Strength Indicator. It is modulation agnostic. It is not related to the modulation. ----- Original Message ----- From: "Tom DeReggi" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]>; "WISPA General List" <wireless@wispa.org> Sent: Monday, June 16, 2008 10:10 PM Subject: Re: [WISPA] modulation question > I'm not sure exactly your question? > I'm also not sure there is a "purpose", as much as reporting what occurs. > I'm also not sure if you are looking for an answer at the "waveform" level > versus the "Link budget" level? > > RSSI is a factor related to Modulation. More specifically with OFDM. > > Often in manufacturer spec sheets, it will list the minimum rssi level in > order to use a specific modulation. This is not just a random number > picked. > Someone else will probably explain it better, and I welcome them to, but > for > now I'll try :-) > It has something to do with how OFDM takes the signal and breaks it up > into > lower powered sub carriers. > As modulations are higher, they get broken down into more sub carriers, > hence QAM 32,64,256, etc. > I believe it has something to do with how the math works with Watts versus > DB, when the signal gets split and added back togeather again at a later > process. > There are two side effects that come... As higher modulations are used, it > more work for the card and Transmit power becomes less, and receive > sensitivity becomes higher (worse). > > I guess what I'm saying is that its possible to use a specific modulation > at > many different power levels/ receive levels (RSSI). It depends on the > power > rating of the card. However, the point I'm making is, at a given set power > level or sensitivity rating of a card running a low modulation, if the > modulation type is raised, it will have a calculatable/predictable effect > on > the signal strength received and sent. > > Tom DeReggi > RapidDSL & Wireless, Inc > IntAirNet- Fixed Wireless Broadband > > > ----- Original Message ----- > From: "Rogelio" <[EMAIL PROTECTED]> > To: "WISPA General List" <wireless@wispa.org> > Sent: Monday, June 16, 2008 10:06 PM > Subject: [WISPA] modulation question > > >> I've got a question, which I'm afraid might be a little stupid to some, >> particularly those with RF backgrounds... >> >> I've always thought that modulation rate was directly tied to RSSI (for >> some law of physics reason or something), but someone else told me that >> it's not like that (in theory) and what I'm seeing is just certain >> vendors do that for a particular purpose. >> >> What is this purpose? >> >> >> -------------------------------------------------------------------------------- >> WISPA Wants You! Join today! >> http://signup.wispa.org/ >> -------------------------------------------------------------------------------- >> >> WISPA Wireless List: wireless@wispa.org >> >> Subscribe/Unsubscribe: >> http://lists.wispa.org/mailman/listinfo/wireless >> >> Archives: http://lists.wispa.org/pipermail/wireless/ > > > > -------------------------------------------------------------------------------- > WISPA Wants You! Join today! > http://signup.wispa.org/ > -------------------------------------------------------------------------------- > > WISPA Wireless List: wireless@wispa.org > > Subscribe/Unsubscribe: > http://lists.wispa.org/mailman/listinfo/wireless > > Archives: http://lists.wispa.org/pipermail/wireless/ > -------------------------------------------------------------------------------- WISPA Wants You! Join today! http://signup.wispa.org/ -------------------------------------------------------------------------------- WISPA Wireless List: wireless@wispa.org Subscribe/Unsubscribe: http://lists.wispa.org/mailman/listinfo/wireless Archives: http://lists.wispa.org/pipermail/wireless/
