Planes & Cell phones

"Greg Copeland" wrote in message
news
On Tue, 20 Apr 2004 16:35:03 -0500, FUji wrote:

Huh? Maximum output of most handheld cell phones is 0.6 watts with the
old
in-car and bag phones going up to 3 watts.
The radius of
interference from 0.6 watt phones transmitting from inside an aluminum
can
would be rather small.

I'm not sure how far you could even transmit on 0.6 of watt.

From London, on the 144MHz band, under the right conditions, I
once talked to a guy in Germany through a repeater in Belgium on
my 1W handheld. I then switched down to 150mW and could still
talk to him clearly. This was from the 12th floor of a building.

It's amazing how far 0.6W will go from a plane...if not for a good
quality phone call, at least to cause some interference.

People forget that cell switching is not magical. And it's certainly is
not zero cost. I must admit I do not fully understand everything that
goes on, but I am sure it's not as simple as you imply. Everytime a call
switches cells, it creates lots of work for the cell network to make sure
only a single tower handles that call. So, while it may take a fraction
of a second from a given phone and a given tower, there is lots going on
behind the scenes. Worse, instead of it going on with one, two or maybe
three towers, now it's causing a flurry of on twenty or more (highest
estimates I've read) towers. Let's also not forget that each tower can
only process and multiplex n-number of signals at a given time. DSPs,
just like your CPU, does have finite capacity. During cell switching, as
I understand it, this finite resource is being used on each tower in
contact with the phone. So, to say, "it causes all sorts of problems on
the cell network", does seem like a spot on statement to me.

Not only does it cause these problems with signalling in the network,
it also causes interference in other ways. With analogue, it will render
the frequency used unusable for the same frequency, repeated through
the 7 or 21 cell re-use pattern over the transmitted area. With TDMA
(D-AMPS or GSM), you'd wipe out that time slot on that frequency.
Greater than about 20 miles away, the time delay on the transmitted
signal would mean you'd also wipe out the adjacent time slot. You could
be decreasing capacity significantly over a wide area.

With CDMA I'm not sure about the interference potential, though it's
bound to increase the noise. I took our test gear up in the plane over
Newbury one day to see what I could receive. I got well over 20
cells and it was adding, deleting and replacing radio links like mad.
Absolute chaos on the network. Note that this network had not yet
been launched to the public (Vodafone UMTS - 3G).

It's not so much the "tower"s processing capacity, but that of the
BSC/MSC (or RNC on CDMA) controlling the base stations, and
also the capacity of the signalling links. All this is finite.

Someone said something (I've heard it said a few times) that the phone
"locks on" to the nearest cell. In laymans terms, yes. In actual terms,
you're transmitting, just like any other transmitter. The cell used for
communication will be the one receiving you the strongest. Note that
the neighbouring cells will be on different frequencies, so your phone
will tune to one of the frequencies used by the strongest cell. There is
then a re-use pattern, and the same frequencies will be re-used somewhere
a certain distance away, depending on the cell size. The cell size depends
on whether you're in an urban or rural environment...in an urban
environment they use a smaller cell size - lower power, more obstructions,
so the next cell using the same frequencies will be closer. They're banking
on the other cells not being able to pick you up due to your low power
and the buildings, hills,. trees and so on blocking the signal. (note that
the frequency thing doesn't apply to CDMA)

If conditions are right (wrong?) you may be picked up by other cells
on the same frequency...this is interference. If this is above a certain
level, this same frequency on the other cell will be rendered unusable.
(or that time slot on that frequency...and maybe the adjacent one, as
mentioned above).

If you use a phone in the air over an urban environment, you will cause
a large amount of interference. If you're down low over a rural area,
you may not cause any at all.

Paul

"Ash Wyllie" wrote in message
...


Cirrus 1234A: I need to phone home.

"SHUT UP AND FLY!!!" :

My instrument instructor is also a real estate agent. Her cell phone rang
during runup yesterday and she confirmed a closed sale, but not without
difficulty.

-c

gatt wrote:

My instrument instructor is also a real estate agent. Her cell phone rang
during runup yesterday and she confirmed a closed sale, but not without
difficulty.

My wife forwarded me an Andy Cap cartoon yesterday. Some dude is chatting on the
phone at the pub trying to set up a business deal. He asks Andy if he has the time.
Andy says "Yes, it's exactly two minutes before that phone goes through the window."
Love it.

George Patterson
None of us is as dumb as all of us.