Re: M20 unmarked cars 'blotting out' mobile phones.

On Wed, 3 Dec 2008 18:29:52 -0000, "Jeff" <Jeff@xxxxxxxxxx> wrote:

That is not correct, GSM phones transmit on one set of frequencies and
receive on another. Although GSM phones are time division multiplexed they
do not use the sane frequency for both Tx and Rx. The reason for this is at
the base station rather than the phone. The base has 8 or 16 time slots to
transmit to, so if they are all occupied it must transmit continuously,
therefore it would be impossible to receive on the same frequency .

Actually it is *not* impossible for a station to transmit and receive
on the same frequency at the same time. The transmitted signal can be
cancelled out at the receiver in a similar way that transmitted audio
(microphone) on a two-wire telephone line can be cancelled out so that
it does not appear at the receiver (headphone) even though both
signals are on the same two-wire line.

There are a few ways to achieve it. One is to place two or more
transmit aerials with a receive aerial in between them. The phase
that the transmit aerials are fed in combination with the distances
chosen mean that the transmitted signals are in exact anti-phase and
so cancel out at the exact position where the receive aerial is
located.

Another way is to use an RF transformer arrangement (typically made
from lengths of co-ax) so that it acts in a similar way to a telephone
"hybrid", nulling out the transmit signal from the receiver.

It is only feasible on fixed base stations, because any variable
relections from nearby moving objects would upset the precise
phase-balance and allow the local transmitted signal to overpower the
distant signal.

Microwave links can receive and transmit on the same frequency
*through the same dish aerial*! Moreover, several different links,
all on the same frequency can operate from a single location.
Separation relies on the extreme directionality of the signal.

On shortwave HF systems where wavelengths are too large to be able to
effectively null out signals in the same way, long-distance duplex
links on the same frequency were effected by situating the transmitter
several miles away from the receiver. The receiver cannot pick up any
of the local transmitter's signal because it is within its "skip"
zone. Too far away to receive the signal by line-of-sight, but too
close to receive its "sky wave" from ionospheric refraction. Any
radio amateur will be familiar with the effect whereby they cannot
hear any signal at all from a powerful ham station in the next town,
but a signal from a country thousands of miles away is coming through
loud and clear.

The voice link between Rhodesia (now Zimbabwe) and England used to be
done in that way when I worked in the telephone company there. Voice
was transmitted and received on one sideband of the same frequency
signal, while data (telex) signals were sent and received on the other
sideband. Frequency division multiplexing was used on both signals
and allowed (IIRC) 4 voice channels and 16 telex channels to be
transmitted and received full duplex on a single HF frequency. In
addition, each voice channel was split into several frequency bands
which were each frequency-inverted before transmission as a form of
"scrambling" so that radio hobbyists were unable to listen in to
private phone conversations using ordinary SSB receivers. The telex
(sent in 7-bit baudot code rather than the normal 5-bit code) was
fully error-corrected so that the received data was always 100%
correct.

The voice channels could also be combined into a single "high
fidelity" audio channel when using the link for relaying commercial
radio content.

--
Cynic

.

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