P.S. For more detail on OFDM and field tests of (non-802.11) OFDM systems, please take a look at my presentation on the "Basics of Orthogonal Frequency Division Multiplexing (OFDM)" at http://www.ewh.ieee.org/r6/scv/comsoc/0101.PDF or http://www.wca.org/Year2000/gregdesbrisay.pdf .
Regards, Greg On Thu, 2003-07-24 at 23:00, Greg DesBrisay wrote: > > > Ladjicke, > > OFDM is essentially a multi-tone modulation, with some tricks thrown in > that allows the tones to be closer together than traditional multi-tone > systems. Imagine a 256-tone OFDM signal; each tone is modulated > independently; since there are 256 tones the symbol rate is 1/256 times > the symbol rate of a single-tone system; therefore the 256-tone OFDM > signal can tolerate 256 times more multipath than a single-tone modem > without equalization; or alternatively, the 256-tone system can send > data 256 times faster in a multipath environment than a simple > single-tone modem without equalizers can. And the OFDM system doesn't > require expensive processor-intensive equalizers to do so. > > So how does this apply to non-line-of-sight (NLOS) communications? > In NLOS situations the direct signal, if it exists, is greatly > attenuated, so multipath becomes far more significant than in LOS > situations. OFDM is excellent at handling multipath, so it is very well > suited to NLOS communications. > > I've set up several OFDM 45-Mbps links in the 5.8-GHz U-NII band over > distances of about 10 miles with absolutely no LOS--diffracting over > roof tops, trees, and hills. These links ran for weeks with no packet > errors. Note: you still have to obey the laws of physics so you have to > have enough signal at the receiver to detect! For these links I used > 4-ft dishes on each end of the link. > > Different systems have different power output levels, have different > data rates, have different modulation on the tones, have different > error-correction schemes, have different adjacent-channel sensitivity, > and are designed to operate in different frequency bands which have > different regulations on allowable power output and antenna gain, but > your question asked about OFDM in general, so this is my general answer. > > Criticisms from others about 802.11a on this list don't seem to be > related to OFDM, they seem to be related to other issues of receiver and > transmitter design. > > I hope this helps! > > Greg DesBrisay > [EMAIL PROTECTED] > > > > > > > On Thu, 2003-07-24 at 14:26, Ladjicke Diouf wrote: > > Can somebody shed some light on how OFDM helps NLOS, I thought it was just a > > coding scheme like DSSS for 802.11b > > > > Ladjicke > > > > > > >From: Patrick Leary <[EMAIL PROTECTED]> > > >To: 'George' <[EMAIL PROTECTED]>, "802.11B NEWS GROUP" > > ><[EMAIL PROTECTED]> > > >Subject: RE: [BAWUG] 802.11b Long Range non line of sight > > >Date: Thu, 24 Jul 2003 11:20:35 -0700 > > > > > >No such animal exists. 802.11b is by definition of the technology on which > > >it is based a LOS only solution (this includes our own 802.11b). For NLOS > > >you will need an OFDM based bridge, such as those offered by us or a few > > >others. > > > > > >Patrick Leary > > >Alvarion > > > > > >-----Original Message----- > > >From: George [mailto:[EMAIL PROTECTED] > > >Sent: Thursday, July 24, 2003 2:16 PM > > >To: 802.11B NEWS GROUP > > >Subject: [BAWUG] 802.11b Long Range non line of sight > > >Importance: High > > > > > > > > >Hi guys, > > > > > >I was wondering if anyone had came across with an 802.11b equipment > > >that doesnt need clear line of sight. > > >I am tasked of bridging a link for a10mile remote site. But Line Of > > >sight is not very clear. Can anyone please > > >suggest what can i possibly do to accomplish this. > > > > > >thanks in advance > > > > > >George > > > > > > > > >This mail passed through mail.alvarion.com > > > > > >**************************************************************************** > > >******** > > >This footnote confirms that this email message has been scanned by > > >PineApp Mail-SeCure for the presence of malicious code, vandals & computer > > >viruses. > > >**************************************************************************** > > >******** > > > > > > > > > > _________________________________________________________________ > > Help STOP SPAM with the new MSN 8 and get 2 months FREE* > > http://join.msn.com/?page=features/junkmail > > > > -- > > general wireless list, a bawug thing <http://www.bawug.org/> > > [un]subscribe: http://lists.bawug.org/mailman/listinfo/wireless > > > -- > general wireless list, a bawug thing <http://www.bawug.org/> > [un]subscribe: http://lists.bawug.org/mailman/listinfo/wireless -- general wireless list, a bawug thing <http://www.bawug.org/> [un]subscribe: http://lists.bawug.org/mailman/listinfo/wireless
