Each system is different as to the number of carriers and a different number of modulation levels on the carriers.
Basically it is Shannon's law which defines the maximum rate of data that can be transmitted over a channel. But no real system can hit the Shannon limit. The imperfections in the demodulators vary from manufacturer. The simple answer is the more you try to pump through, the more signal you need. Shannon's law contains a signal to noise ratio as one of the components. If you want to have more throughput, you have to have more signal or less noise. The short answer is, read the manual. I would hope they are all conservative on the published signal levels needed for the different bit rates. As far as DSSS goes, I really don't know if the Shannon formula applies or not. Hmmm... ----- Original Message ----- From: "Tom DeReggi" <[EMAIL PROTECTED]> To: "WISPA General List" <wireless@wispa.org> Sent: Tuesday, June 17, 2008 6:24 PM Subject: Re: [WISPA] modulation question > Chuck, > > Nice posts! Thanks for the clarification. (PS. I clearly had some > misunderstanding/FUD in my last post, regarding QAM.) > > While you are at it.... > > Can you comment a bit on OFDM, regarding the math, of what occurs to power > levels as it divides into subcarriers and recombines on the receive end. > > There was a good post on the STAROS forums about it a year or so ago, but > I > couldn't find it again to repost to this list. > It brought forth some relevent info on how to correctly do link budget > calculations and what RSSI should be expected considering OFDM versus DSSS > style gear. > > Tom DeReggi > RapidDSL & Wireless, Inc > IntAirNet- Fixed Wireless Broadband > > > ----- Original Message ----- > From: "Chuck McCown - 2" <[EMAIL PROTECTED]> > To: "WISPA General List" <wireless@wispa.org> > Sent: Tuesday, June 17, 2008 9:04 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/ > -------------------------------------------------------------------------------- WISPA Wants You! 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