TELECOM Digest OnLine - Sorted: Re: GSM-900

Re: GSM-900

Michael D. Sullivan (userid@camsul.example.invalid)
Sat, 02 Apr 2005 05:10:32 GMT

Jason wrote:

> But since we transmit in x freq, then the receiver must tune to x feq
> in order to receive the signal right? Why transmit at x freq and
> receive at y freq?

> Or I have misunderstood. Kindly enlighthen.

Many (but not all) two-way radio transmission systems that operate in
full duplex (which means there are two full-time transmission paths, one
in each direction) use different frequencies for each of the two paths.

One path, known as the downlink, uses frequency X to transmit from the
base station to the mobile, and the other path, known as the uplink,
uses frequency Y to transmit from the mobile to the base station at the
same time. Under this scenario, the handset transmits on Y to a base
station receiver tuned to Y, while the base station transmits on X to
the handset, which is tuned to receive X. Operating in this manner is
known as frequency division duplex, or FDD.

Using paired frequencies that are sufficiently far apart allows the
receiver at each end to be able to operate without getting overloaded
and desensitized by the transmitter at the same end. If the receiver
were tuned to the same frequency being used to transmit at the same
time, it would pick up its own transmitter's strong signal and
wouldn't be able to pick up the much weaker signal coming from the
other end.

Some communications networks use a single frequency for both
transmissions, but alternate the use of that frequency in time so that
neither end is actually trying to receive when it is transmitting,
known as time division duplex or TDD. One way of operating in this
manner is to use "simplex" transmissions, such as on ham radio bands
or old-fashioned taxi dispatching systems, where you say "over" when
you are finished and then the other party keys its transmitter on to
respond. Another way to accomplish it is to alternate between
transmitting and receiving at a constant, high rate, with both units
carefully synchronized.

Keeping the units synchronized at a high enough rate for high-quality
speech is complex, and even more so when the distance between the two
units can vary considerably, since for every mile of distance, there
is a delay between transmission and reception of 1/18,600 second.
Thus, if the system is designed for a maximum transmission distance of
20 miles and a minimum of 0 miles, there must be at least 1/9300
second of dead air at the beginning and end of each time slice to keep
the two transmissions from overlapping, wasting at least 1/2325 second
for each pair of time slices (1 in each direction). If the time
slices themselves are short, as they must be for conversational speech
that isn't going to tolerate significant delay due to the time
compression and decompression involved, a significant amount of
transmission time is wasted.

And that's for just a single two-way voice transmission. GSM networks
combine many conversations into a single paired radio channel, which
is itself time-sliced, utilizing time division multiple access (TDMA);
but GSM separates the TDMA uplink and downlink transmissions by
frequency, using FDD. So GSM is an FDD/TDMA system. In applications
where an appreciable time delay is acceptable, TDD/TDMA can be used,
in which transmission time on a single frequency is sliced up between
up- and downlinks, each of which is further time-sliced into multiple
communications channels.

Michael D. Sullivan
Bethesda, MD (USA)
(Replace "example.invalid" with "com" in my address.)

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