Good info Jack. In a past life I was a headend tech in the cable
television industry. and I also performed signal egress and ingress
troubleshooting using a Hewlett Packard 8591B analyzer. I spent a few
thousand hours on this tool and learned much about spectrum analysis at
that time. Here is some info for some of those out there who may be new
to spectrum analysis:
There is something that can make these paging transmitters appear to be
bleeding over into the ISM bands when in fact they may not be. The
setting on the analyzer is called "resolution bandwidth". This setting
takes all power within a given bandspace and averages it together as it
sweeps across the screen. The wider this setting is the fewer bumps you
see on the screen. The trace will smooth out as you increase this
setting because it is averaging power within a wider space of spectrum.
This has the added effect of making a "loud" carrier appear to cover a
wider space than it actually does and can cause you to believe that a
paging or other carrier is bleeding over into the ISM band. On the
contrary, narrowing the resolution bandwidth will show more accurate
representation of actual power in a given bandspace but is slower to
scan on most analyzers and produces a very sporadic display. If you are
looking for narrowband or adjacent channel interference into your band
then a narrow resolution bandwidth will be required. If you are wanting
to take a RSSI reading of your own carrier then a wider resolution
bandwidth will be required.
Resolution bandwidth is something you should learn to use and understand
if you want to get more from your work. It is an important part of
spectrum analysis. If you want to see how good an analyzer is then look
at how low the resolution bandwidth setting will allow. For our work a
minimum resolution bandwidth of about 100kHz is probably all you will
ever need. Also run it at its lowest resolution bandwidth and see how
long it takes to scan across the screen. If you are comparing multiple
analyzers make sure you always use the same span setting (difference
between upper and lower frequency on display). A narrower span will
display a narrow resolution bandwidth much faster. Better analyzers will
have a wide range of resolution bandwidth settings and will show a
sharp, clean display in any setting.
Learning to use a spectrum analyzer can seem daunting at first glance.
Do not let this intimidate you. You can learn to use this and get
meaningful information from it if you give it a try. You will not break
the analyzer by experimenting with it. If the unit you are using has
knobs and you had it set by someone previously then just take notes of
where they are set and then experiment with the unit. The most important
things to master are start frequency, stop frequency, span, center
frequency, reference level, attenuation, resolution bandwidth. Anything
else you learn is good to know but not as much as what I just outlined
here.
If anyone here is working with an analyzer and does not know what any of
those things mean then feel free to ask here onlist (or offlist if you
would prefer to not tell others you do not know) :-)
Scriv
Jack Unger wrote:
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
--
WISPA Wireless List: [email protected]
Subscribe/Unsubscribe:
http://lists.wispa.org/mailman/listinfo/wireless
Archives: http://lists.wispa.org/pipermail/wireless/
--
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
--
WISPA Wireless List: [email protected]
Subscribe/Unsubscribe:
http://lists.wispa.org/mailman/listinfo/wireless
Archives: http://lists.wispa.org/pipermail/wireless/
--
WISPA Wireless List: [email protected]
Subscribe/Unsubscribe:
http://lists.wispa.org/mailman/listinfo/wireless
Archives: http://lists.wispa.org/pipermail/wireless/
