OK, I fill in one hole and another appeared. (Thanks Brian) I honestly don't know if Orthogon puts the QAM directly on each discrete RF carrier of the OFDM scheme or if those carriers are FMed with a subcarrier containing QAM. So, let me retreat to a previous statement that I still believe to be correct: OFDM does not have any relationship with QAM.
(I have CODFM on the brain as that was a superior method of doing OTA HDTV but the ATSC selected 8VSB. But that was in another life...) OFDM strictly speaking only means taking a channel, transmitting multiple RF carriers within that channel. The frequencies are calculated such that there is a minimum of interference between them. Those individual RF carriers are (I discover) commonly refered to by some(most) as subcarriers. Strictly speaking, they are not subcarriers in the classical sense but that is a whole other discussion. I will concede that using the term subcarrier is in common use but is not accurate. (It really depends on what they are doing at the baseband level, so I might be wrong about that opinion as well.) The second channel (R-L) of an FM broadcast signal is on a subcarrier. Color of NTSC video is a subcarrier. Musak is on subcarriers of FM broadcast channels. OFDM bearer channels (howz that for mixing technologies) are discrete RF carriers. But since my opinion of the use of the term subcarrier is in an antiquated minority, I will yield that the term is the commonly accepted method of referring to those carriers. Gheeze it is hard to change. Glad I learned something today... ----- Original Message ----- From: "Chuck McCown - 2" <[EMAIL PROTECTED]> To: "WISPA General List" <wireless@wispa.org> Sent: Tuesday, June 17, 2008 8:04 AM Subject: Re: [WISPA] modulation question > >what is a sub carrier? > > For IP guys, think of VLANs. You can cram a bunch of VLANs on an Ethernet > link. Each VLAN appears to be its own Ethernet link. But to the trunk, > all > the VLANs appear just to be payload data. > > Same thing with RF. The on-the-air signal is modulated. OFDM or FM (or > morse code or AM or whatever) method. That is the Ethernet. Inside that > modulation, if you pick it apart, you may find the raw data (like Canopy > does) or you may find other modulated signals (like the VLAN) that have to > be further demodulated (QAM on OFDM like Orthogon). > > QAM methods are used to cram a ton of information on a link. Whether it > is > on the raw RF signal on in a subcarrier. But they are not as robust as > simple direct modulation. But there is always a tradeoff. > > The following is not really accurate but it may give a data guy another > way > of thinking about it. > Level 1 2.4 GHz RF using antennas > Level 2 OFDM modulation on the RF > Level 2 VLANs QAM Sub Carriers > Level 3 Ethernet data > > ----- Original Message ----- > From: "Chuck McCown - 2" <[EMAIL PROTECTED]> > To: "WISPA General List" <wireless@wispa.org> > Sent: Tuesday, June 17, 2008 7:47 AM > Subject: Re: [WISPA] modulation question > > >> Bryan Scott brought up an important point: >> You can put QAM subcarriers on OFDM. >> >> But you can put almost any other type of subcarrier on OFDM too. >> That may be a confusing thing. OFDM is method of putting multiple FM >> modulated carriers on the air. Almost like sending multiple channels >> containing their own info and then combining all the channels at the far >> end. Those channels can have QAM subcarriers, but the RF is still FM >> modulated, not QAM modulated. That may be confusing to some. The FM >> modulation of OFDM gives it the inherent advantage of angle only >> modulation >> methods. >> >> Then there are systems that use QAM to modulate the RF. Those systems >> are >> less resistant to link problems but are one of the best ways to cram a >> bunch >> of data on the link. (V.90 dial up modems are a good example). When >> they >> added color to the B&W TV signal, they used a QAM method. The original >> stereo AM radio signal was QAM. Cable modems use QAM. >> >> >> ----- Original Message ----- >> From: "Chuck McCown - 2" <[EMAIL PROTECTED]> >> To: "WISPA General List" <wireless@wispa.org> >> Sent: Monday, June 16, 2008 11:15 PM >> Subject: Re: [WISPA] modulation question >> >> >>>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/ >>> >> >> >> >> -------------------------------------------------------------------------------- >> 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/
