On Tue, 20 Sep 2011 20:11:24 -0700, Jeff Liebermann wrote:
On Tue, 20 Sep 2011 07:16:40 -0500, dave wrote:
Theory and practice are quite different.
One day, you're going to eat those words, when you have to decide
whether to follow theory or practice. When I find that they're
different, it's usually because I'm doing something wrong. Also, if you
understand the theory, you can probably figure out the practice (what to
do). However, if you know the practice (i.e. seat of the pants
engineering), you're highly likely to fumble somewhere.
The tower owner should have an inventory of every transmit and every
receive frequency, plus all the standard I.F., plus nearby external high
powered sources. The owner should have cleared each frequency before it
went on the air, and should not add a tenant if doing so would create a
harmful spur to existing users. This is site management 101.
You almost made me spill my hot chocolate. You're correct. Site
managers should do all that. The problem is that all but one of the
site managers that I know of are business types, not engineers. They
hire engineers, tower jockeys, construction crews, and generally run the
business. It's not unusual for me to get a call or email with "I just
signed on to have [insert name] company put their radios in the
building. I'll let you know if anyone complains". This translates to
"Don't burn any billable hours doing calculations until AFTER someone
experiences interference. In short, I get paid to clean up the mess,
not to do the planning. If I want to enforce any engineering standards,
it's also done post mortem. At best, I would get an email asking where
in the building and tower I would guess the new radios should be
installed, usually without telling me the frequencies or equipment.
Interrogating the prospective new customer is something I try to do, but
often they contract out the repeater service to a comm shop, which
claims that they don't know anything because they're afraid I might
steal the customer. I don't wanna talk about licensing, HAAT calcs, and
coordination. Hopefully, your operation is a bit closer to theory than
practice.
I don't care how the WL people run their data streams. Cellular folks
don't like high mountains (except for backhaul).
Generally true. The CDMA crowd doesn't like high mountains for the same
reason they don't like CDMA operation in airplanes. The noise floor is
much higher up high and there are not enough channels available to
handle all the potential users if in a metro area. However, they do like
medium high mountain tops with fairly well controlled coverage areas.
They also like to share site ownership and management with public
agencies to reduce costs.
I know they use very
advanced techniques to hear signals below the noise floor; keeping that
noise floor as low as possible is of paramount importance when you are
looking at 100 mW devices in people's pockets 5 miles away.
100mw is about the maximum that a cell phone can belch. Power control
will usually keep that down to about 30-50mw.
FWIW, Tek has a real nice analyzer that will reverse engineer TDMA
spurs. make time-lapse spectrum analysis, and can even write on a map
for you.
Well, the 20+ year old P25 radios are finally being forced into service
by FCC edict, along with various incompatible TDMA implementations.
Meanwhile, cellular is heading towards various CDMA spread spectrum
technologies (CDMA200, WCDMA, LTE, etc), which makes TDMA look kinda
dated. Anyway, I can't afford much in the way of expensive test
equipment and usually borrow or rent what I need. I haven't actually
seen a spur, mix, intermod, or noise on a spectrum analyzer for many
years as the receiver sensitivities are well below the analyzer noise
floor. Same problem with PIM (passive intermod). It takes quite a bit
of power to produce PIM making it almost impossible to measure PIM while
the xmitters are in operation. Trying to see PIM on a spectrum analyzer
is futile.
I communicated sloppily; I mean the "theory and practice" of multiplexed
digital wireless is different than the "theory and practice" of SCPC
analog, e.g. your ability to see something meaningful on a scope.
http://www.tek.com/products/spectrum-analyzer/sa2600/
The scope is running Windows and can constantly compute intermod products
(aka predict spurs) based on the real time environment. It has an option
for advanced wireless signals. I could never get my boss to buy me one;
they are enamored of Rohde Fish 313s.