Gains from diversity are large in multipath fading channels, and low in LOS with no scatterers.
At a nominal 10e-3 BER, two-branch diversity yeilds approximately 10dB of gain, while a 6-way diversity scheme may generate 15dB of gain (both compared with a system with only one antenna at each end.) This figure assumes uncorrelated Rayleigh flat fading channels and MRC, and, say BPSK modulation. When signals are coherently combined at the receiver using techniques such as MRC, the average received SNR can be increased by 10*log10(M), where M is the number of (receive) antennas. Greg DesBrisay writes: > > Jim, > > Thanks for taking the time to look at the presentation! It nice to know > it gets looked at every once in a while. > > My memory is getting a little fuzzy about this particular system, but > here's what I recollect. > > In an extreme-scattering environment (e.g. a near-Rayleigh environment) > MRC (maximal ratio combining) with 2-antenna spatial diversity will give > you about 10 dB or so of gain [See books by Lee, Rappaport, and > others]. And with 256 tones spread over a 10-MHz bandwidth combined > with a bunch of FEC (forward error correction) you get several dB of > gain due to frequency diversity. Combine spatial diveristy and > frequency diversity and you get more diversity gain than if you just use > one type of diversity. > > > Greg > > P.S. Obviously I'm not hawking any proprietary wares because the system > I described in those slides was discontinued over two years ago! I was > also careful not to mention who made the particular OFDM system I > described. > > I posted those links because they're the best tutorial on OFDM that I > know of. I was very careful to make those presentations tutorials on > OFDM and _not_ sales presentations. Both the IEEE and the WCA don't > appreciate sales presentations (neither does the Naval Post Graduate > School where I was requested to make that presention as well). > > > > On Fri, 2003-07-25 at 01:08, Jim Thompson wrote: > > > > I was mostly OK with these (even though VOFDM is BS) until I got to > > slide 19, where you claim "3-14dB of Gain" from 2-antenna diversity. > > Not even MRC will generate *14dB* of gain over 2-antennas. > > > > What has BAWUG become, "hawk our proprietary warze?" Geez. > > > > Jim > > > > Greg DesBrisay writes: > > > > > > 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 > > > > > > > -- > > "Speed, it seems to me, provides the one genuinely modern pleasure." > > -- Aldous Huxley (1894 - 1963) > > > > -- > > general wireless list, a bawug thing <http://www.bawug.org/> > > [un]subscribe: http://lists.bawug.org/mailman/listinfo/wireless > > > -- "Speed, it seems to me, provides the one genuinely modern pleasure." -- Aldous Huxley (1894 - 1963) -- general wireless list, a bawug thing <http://www.bawug.org/> [un]subscribe: http://lists.bawug.org/mailman/listinfo/wireless
