Dumb question about OFDM

/usr

The CWNA book states that OFDM is not considered a spread spectrum technology, yet the requirements given for a technology to be classified as such are somewhat vague, stating only that the system must use a broader frequency range than necessary to transmit the data.

My question is, why exactly is OFDM not considered spread spectrum?

I'll admit that I haven't looked anywhere but the CWNA book. Maybe I'm just overlooking some information within the book, or maybe it's just that vague.

JDMurray

I don't have the latest CWNA Study Guide in front to me to read the exact wording, but OFDM is a spread spectrum frequency modulation technique that is an alternative to Direct Sequence Spread Spectrum's phase modulation technique. OFDM "spreads" a transmission over many different frequencies, which is what DSSS also does. The way the transmission signals are modulated are different.

If the CWNA Study Guide claims that OFDM isn't a spread spectrum technology, I'd have to ask, "what is their definition of spread spectrum technology?"

http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing

/usr

OFDM "spreads" a transmission over many different frequencies, which is what DSSS also does.

That's the reason why I was curious as to why they didn't consider it a spread spectrum technology.

On page 197 of the Third Edition it says...

Although it [OFDM] is not considered a spread spectrum technique by the FCC, OFDM shares many qualities with spread spectrum communicators, including using a low transmit power and wider-than-necessary bandwidth.

There is no additional reasoning as to why it isn't.

JDMurray

OK, there's the reason. The FCC does not consider OFDM to be a "spread spectrum technology." Now you need to consult the FCC's web site (www.fcc.gov) as to the reason. It would probably be quicker and easier to simply search for this question on the CWNA forums at www.cwne.com/phpBB2/index.php, and post the question if you do not find an explanation already there.