Tom,
Please see my answers inline, below.
Tom DeReggi wrote:
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?
If a paging transmitter is transmitting spurious signals down below 928
MHz, a bandpass filter won't help but if the paging system is NOT
transmitting off-frequency but is overloading your receiver then the
bandpass filter WILL help.
(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?
That's a good idea. The installer should check throughput from a distant
client with and without the filter.
Basically meaning, who cares who the culprit is, if their is a way to
just cure the problem.
Knowing the "culprit" is important in order to troubleshoot the problem
cost effectively; per my previous (and above) explanations. Again, if
the paging system is transmitting outside it's authorized frequency
band, the bandpass filter will NOT help.
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.
Using the correct type of bandpass filter is also important. There are
narrow-band (single channel) filters and whole-band (902-928 MHz)
filters. I don't know what type of filters the "many Trango installers"
tried so I'm unable to provide further insight into why they got the
results they did.
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.
Maybe the internal filters weren't good enough to reduce the overloading.
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.
Unfortunately, there's no easy way to predict if a filter will help -
other than trying it. It would help to do an interference survey before
selecting a tower site. If there is a 929 MHz paging transmitter on a
tower (an the interference survey) would reveal this, then it is wise to:
1. Stay as far away as possible on that tower, or
2. Don't go on that tower, or
3. Plan to use a good bandpass or cavity filter.
This is just applying basic engineering skills and/or good common sense.
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?
I doubt that the $125 filters that I used are still available at that
price; remember that was 14 years ago. Here is a much better filter that
is available today and it even has an outdoor weatherproof case.
http://www.rflinx.com/products/filters/900/bpfx/
Bandpass filters are passive. They do not require electical power. Also,
it's likely a waste of money (as I mentioned earlier) to use these on
the CPE end unless the CPE end is close to a cell tower or paging tower.
These bandpass filters are most needed on the AP - if the AP is near (or
on) a cell/paging tower.
jack
Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband
----- Original Message ----- From: "Jack Unger" <[EMAIL PROTECTED]>
To: "WISPA General List" <[email protected]>
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" <[email protected]>
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" <[email protected]>
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
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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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