True, using 10 MHz-wide channels instead of 22 MHz-wide channels should
help protect the adjacent-channel receivers from overloading and the
resulting throughput reduction.
The downside is that a narrower 10 MHz-wide channel won't deliver as
much throughput as a wider 22 MHz channel - everything else
(over-the-air protocol, interference levels, etc.) being equal. Because
the aggregate throughput of the 10 MHz-wide channel site will be lower
than a 22 MHz-wide site, the site won't be able to serve as many
Rather than build a lower throughput system, I'd rather build a higher
throughput system and just design in enough receiver overload protection
by using the methods that we've already discussed.
George Rogato wrote:
If one was to use 3 10MHz channels with 10MHz seperation between
channels, they would see the same or better performance than 3 full size
channels side by side.
So thats my advice, but it won't be wifi. Would be nice if there was a 5
gig option per sector so that you could do 2 2 gig channels at 1 and 11
and a 5 gig sector.
Jack Unger wrote:
Here's what I see that's good about the Pac Wireless antenna:
1. The 3-in-1 design may be the only solution where only one
antenna-mounting space exists on a tower.
2. The inclusion of electrical downtilt will minimize overshooting
3. The inclusion of null fill will minimize connectivity problems with
customers located very close to the tower.
4. The 15 dBi gain is in the appropriate and usable range.
5. The weight (14 lbs) will make the antenna easy to raise and mount.
6. The price range ($700) is moderate considering that you're really
getting three sector antennas in one.
Here's what I see that will need special consideration and planning.
1. The front-to-back ratio is only 15 dB. This is low compared to the
30 dB that I usually recommend for high-quality antennas.
2. The sector-to-sector isolation is 35 dB. When compared to the 83 dB
of isolation that 10 feet of vertical separation would provide, 35 dB
is a lot less.
In summary, this appears to be a good antenna system with the one
limitation that the somewhat low f/b ratio and sector-to-sector
isolation will combine to place the burden for good AP-to-AP isolation
on the quality of the co-located access point receivers and the
quality of the overall AP/site design. Receivers with poor or moderate
selectivity (in other words, receivers on Wi-Fi cards) will be
overloaded by the other AP transmitters with the result being a
throughput reduction that begins when traffic levels increase. The
more traffic, the more missed incoming packets, the more
retransmissions and the more throughput reduction. The result will be
that the site reaches saturation sooner and won't handle as much
traffic as a site where the AP receivers are not being overloaded.
The solution (other than to use really expensive equipment which has
good receiver filtering and selectivity built in) is to again use
single-channel bandpass filters on each AP. These will reduce the
level of signal from the co-located adjacent-channel AP transmitters
and permit the site to handle more traffic (reach saturation later)
than a site where the receivers ARE being overloaded.
Finally, in addition to the above, it's important not to "defeat" the
antenna isolation by letting RF energy "leak" directly from AP to AP
either on the ground or from PC-card to PC card. Proper shielding and
grounding will help to maintain that hard-won receiver-to-transmitter
isolation resulting in a high-performance, high-traffic handling,
reliable, profitable site.
Rick Harnish wrote:
While we are on this topic, look at the .pdf file and give me your
on this all in one sector from Pac Wireless?
OnlyInternet Broadband & Wireless, Inc.
Founding Member of WISPA
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On
Behalf Of Jack Unger
Sent: Friday, May 18, 2007 4:18 PM
To: WISPA General List
Subject: Re: [WISPA] 180* sector choices
Thanks for replying to my concerns and suggestions.
Have you tried using 5 to 10 feet of vertical separation between the
antennas rather than mounting them all at the same height? 10 feet of
vertical separation should be enough to prevent one AP transmitter
from overloading another AP receiver.
Secondly, single-channel bandpass filters are available which will
provide additional receiver protection from nearby transmitters
operating on non-overlapping frequencies.
Finally, higher-priced antennas normally have a higher front-to-back
ratio which increases isolation (minimizes interaction) between the
antennas. A minimum f/b ratio of 30 dBm is recommended.
Keep in mind that without dealing with these antenna isolation
issues, you will likely have the same problems using two 180* sector
antennas as you have had using three 120* sector antennas.
Mac Dearman wrote:
See in line please
Behalf of Jack Unger
Before you discard the idea of using three 120* sectors are you
absolutely certain that the "separation" issue is really due to the
antennas not being far enough apart? Have you thoroughly looked at
reasons why there could be interference between three APs like:
Well Jack - I'd like to think that I have exhausted the other
:-) but I am always willing to listen.
1. Using three cards on a single motherboard in one enclosure without
sufficient shielding between them.
All of our sector arrays utilize 3 different enclosures with one
the enclosure and then I try to separate the enclosures (and
far as possible.
2. Using three APs that are configured on frequencies that are too
together and are interfering with each other?
Unless we deploy 4 90* sectors we use non overlapping channels (1, 6 &
I guess what I'm asking is (before you go spend money on antennas that
may or may not solve the problem) what equipment are you currently
using, how is it configured, and how far apart can you actually get
three sector antennas on the 25G tower?
I have come to the conclusion that it is interference from the
the antennas due to not having adequate separation from each other.
small towers (although 180') like Rohn 25G aren't but 12" across -
wind up with the 3 sectors only 1' apart at their bases.
I wish you had some more ideas :-), but thanks for the thought and the
Jack Unger ([EMAIL PROTECTED]) - President, Ask-Wi.Com, Inc.
FCC License # PG-12-25133
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Author of the WISP Handbook - "Deploying License-Free Wireless WANs"
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