Tom,
Good summary.
jack
Tom DeReggi wrote:
John,
Helpful post!
In summary of this thread, I guess the message I'm getting is, there is
no substitution for the right tool for the job.
And its clear that an analyzer is the correct tool, to truely learn the
characteristics of your RF colocated neighbor, without risk of false
assumptions.
Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband
----- Original Message ----- From: "John Scrivner" <[EMAIL PROTECTED]>
To: "WISPA General List" <wireless@wispa.org>
Sent: Saturday, October 28, 2006 12:12 PM
Subject: Re: [WISPA] Sprint / Nextel to use 900mz for iDen
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" <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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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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