Jack,
That is helpful information, and helps explain the situation, thank you.
However, I'm not sure that I understand how the response answers my primary
confusion, "how to tell whether the interference is caused by me or them".
Inserting a filter inline, will of course tell me if my interference can or
can not be cured with a bandpass filter. Whether it was the other party
bleeding outside of its channels, or power overloading my receiver, the
bandpass filter would help reduce either problem, wouldn't it?
(reduction of power and reduction of bandwidth bleedover). So what it sounds
like is, an installer should always have a filter on-hand to insert and test
with at time of installation, to see if it helps? Basically meaning, who
cares who the culprit is, if their is a way to just cure the problem.
Where my question specifically related to Trango was.... Many Trango users
had installed filters to try and stop the interference from paging
companies, and it did not help. Normally, this makes no sense, because in
theory the filters would always help. One of Trango's benefits were that it
in fact had quality 900 filters installed already to help reduce
interference. It is one of the features that it had above Canopy, Waverider,
and OEM wifi products.
And its not always cheap, to find out wether the filter would help. It
sometimes means making a second climb to 500 feet, or bringing power up
500feet for the test, that did not yet have Coax or a second DC power feed.
Not difficult to do, but clearly an added cost for something that may or may
not improve above what Trango already has built-in, based on other's
experience.
What would be interesting is learning more about the filter that you
previously procured and/or how to make them. At $125 each, I'd have a slew
of them laying around for using on the fly. At $125, it would be cheap
enough to istall on every CPE radio as well, if needed. But I haven't found
them for less than $450, and they typically had closer to 3 db power
reduction on their spec sheets. And is it possible to build a passive
Filter that does not require additional electric power?
Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband
----- Original Message -----
From: "Jack Unger" <[EMAIL PROTECTED]>
To: "WISPA General List" <wireless@wispa.org>
Sent: Saturday, October 28, 2006 1:03 AM
Subject: Re: [WISPA] Sprint / Nextel to use 900mz for iDen
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.
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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