simon]
On Friday, July 25, 2003, at 02:09 AM, Greg DesBrisay wrote:
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
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