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*
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> > > > > > >
> > > > > > > --
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> > > > >
> > > >
> > > > --
> > > > "Speed, it seems to me, provides the one genuinely modern pleasure."
> > > > -- Aldous Huxley (1894 - 1963)
> > > >
> > > > --
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> > >
> > >
> > >
> >
> > --
> > "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)
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