Tom,
Yes, their gear (the paging stuff) not only costs more but their
transmitters spurious emissions have to remain low or the paging company
risks being fined by the FCC. Sure, a transmitter can malfunction once
in a while and cause interference to the ISM band but this is not a
common occurance. Our gear has receivers where the manufacturing cost is
quite low. There may be $50 worth of parts in the receiver section of an
AP. The vendors typically do not spend a lot of money on components that
would raise the cost of their equipment and make it non-competitive such
as adding expensive filters to reduce the overloading problems that only
a minority of WISPs may ever experience. Similarly, the new cars that
people buy don't come with the most expensive tires as standard
equipment because most people would never notice a difference or be
willing to pay more for the premium tires.
I started deploying 900 MHz bridges in 1993 and 900 MHz APs (yes, for
WISP service) in 1995. I used Lucent "Wavelan" cards in those systems.
Whenever I was located within about 1/3 of a mile from a cell site (with
colocated 929 MHz and 930 MHz paging) I had to add an external bandpass
filter between the antenna and the antenna connector on the Wavelan
card. Until I did this, I could not get full throughput (which was about
1.3 Mbps in those days) through the card. The bandpass filter would
clear up the problem every time. Those filters weren't even that strong
- only about 6 dB of attenuation at 900 MHz and at 930 MHz (even less -
maybe 5 dB at 929 MHz) but it was enough to protect the Wavelan card's
receiver from being overloaded. These bandpass filters were made by a
3rd-party source and custom tuned by me in a calibration lab. My filter
cost was $125 each and they were not weatherproof so I mounted them
indoors. The inband attenuation was aboat 1 or 1.5 dB which was
insignificant in light of the fact that the filters worked to eliminate
the overloading and allow the AP to receive client signals up to 10 or
12 miles away.
Regarding Trango - I have not verified the accuracy of their spectrum
analysis tool but what you're seeing can be explained by one observation
and one guestimation. The -20 dBm to -30 dBm signal indications above
929 MHz are likely fairly accurate. Nearby paging transmitters could
easily be that loud. The fact that you're seeing signals down to 924 MHz
or so could be explained by the Trango receiver "front-end" (the first
stage connected to the antenna) being overloaded by one or more nearby
paging transmitters. When a receiver is overloaded, it generates
"spurious" signals that are not really being transmitted on the
frequency where they show up. The "spurs" are being generated inside the
receiver itself as a consequence of the overloading. It's fairly easy to
test to see if this is the case. Just insert a bandpass filter between
the antenna and the antenna connector (assuming a connectorized AP). If
the AP receiving distance and/or the throughput increases, you have just
proved that overloading was a problem. You can also re-run the
spectrum analysis tool and see if it no longer reports signals down to
924 MHz. It should now report that the non-WISP signals start around 929
MHz.
I hope this explanation helps.
jack
Tom DeReggi wrote:
Jack,
That all sounds good, and it brings up a good point, that we are just as
probable to be the culprit, not just the other guy.
Besides, their gear costs more, right :-)
However, what specific gear do you have experience with, on this issue,
to support your comment?
I'm not sure that I am knowledgable enough on the topic, to know for
sure which side is the flaw, how would we tell?
I use Trango 900. Trango's have a built-in specrum site survey tool,
that also scans a bit lower and higher than the ISM edge. My comment
was based on the fact that, when I do the site survey, I see signals in
the neg 20-30 range, spanning from significantly above 930 down to mid
portion of ISM channel 4 (924 or so).
Have you verified the accuracy of the Trango tool, and how it reacts to
this situation?
Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband
----- Original Message ----- From: "Jack Unger" <[EMAIL PROTECTED]>
To: "WISPA General List" <wireless@wispa.org>
Sent: Friday, October 27, 2006 1:07 PM
Subject: Re: [WISPA] Sprint / Nextel to use 900mz for iDen
"Bleed over" implies that the paging system is transmitting a signal
that is too wide. This is typically NOT the case. Our rather
inexpensive WISP AP receivers do not have adequate selectivity to
reject strong nearby signals. In other words, it's our equipment
problem not their equipment problem.
Also, WISP subscriber sites, unless located right under a
paging/cellular tower aren't close enough to be overloaded by
paging/cellular so they would not need the bandpass filter. Only our
APs which are located near paging/cellular towers should need the
bandpass filters.
jack
Larry Yunker wrote:
While filters can help, the problem that I see is that filters are:
1) expensive and 2) bulky. Last time I checked, a cavity filter for
the 902-928 range was roughly $300-$400. I don't see it being
practical to install one of these at every customer site!
Cavity filters are fine for your broadcast sites, but that is of
little help when the 900Mhz paging systems bleed over so much that
they "deafen" the subscriber radios.
- Larry
----- Original Message ----- From: "Mike Cowan"
<[EMAIL PROTECTED]>
To: "WISPA General List" <wireless@wispa.org>
Sent: Thursday, October 26, 2006 7:32 PM
Subject: Re: [WISPA] Sprint / Nextel to use 900mz for iDen
Filters fix this problem quite handily. We recommend one on every
system needed or not. I don't see an issue here.
Mike
At 07:07 PM 10/26/2006, you wrote:
ISM 902-928.
Exact band and Power limit is relevant. Currently, the top 25% of
ISM 900 bandwidth (channel 4) is unusable, in MANY areas, due to
blead over from 930 Licensed high power gear (500W). If the same
thing were to occur at the lower portion of 900 ISM bandwdith, it
could kill Channel 1 also, horribly effecting WISPs using
unlicenced. They also may be requesting to use higher power on the
actual ISM bands, argueing Public Safety is more important than
unlicensed use. Iftheir request is granted, specifics should be
lsited on how they are going to prevent interference with existing
unlicensed band users. Remember that the goal may not only be to
use the spectrum. They have benefit in killing off all the 900Mhz
WISPs, that could compete with Sprint/Nextel Next generation WiMax
type Licensed 700M-900M solutions.
Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband
Mike Cowan
Wireless Connections
A Division of ACC
166 Milan Ave
Norwalk, OH 44857
419-660-6100
[EMAIL PROTECTED]
www.wirelessconnections.net
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Jack Unger ([EMAIL PROTECTED]) - President, Ask-Wi.Com, Inc.
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Jack Unger ([EMAIL PROTECTED]) - President, Ask-Wi.Com, Inc.
Serving the License-Free Wireless Industry Since 1993
Author of the WISP Handbook - "Deploying License-Free Wireless WANs"
True Vendor-Neutral WISP Consulting-Training-Troubleshooting
Newsletters Downloadable from http://ask-wi.com/newsletters.html
Phone (VoIP Over Broadband Wireless) 818-227-4220 www.ask-wi.com
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