10 dB for spatial diversity. additional decibels for frequency diversity.
On Fri, 2003-08-01 at 08:44, Jim Thompson wrote: > > Sure, but someone said "14dB". > > Greg DesBrisay writes: > > > > > > Yup. So we agree! > > > > Greg > > > > > > On Mon, 2003-07-28 at 01:32, Jim Thompson wrote: > > > > > > 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) > > > > > > > > -- > "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
