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13 Modulation Spectrum Switched Mode
(Spectrum due to Modulation)
13.1 Description
In the GSM system the mobile phone (MS) transmitter (TX) output RF- signal is time-shared,
according to the principle of TDMA.
This implies the transmitter to be STARTED exactly at a controlled point of time to reach a
specific RF power-level.
Within a very short and clearly defined period of time < 28 µS.
This RF power-level shift, called upramp, must be as smooth and linear as possible during the
transition low to high, in order to limit the spectral propagation.
The control function for this level shift is accordingly pre-programmed in three steps, each of
which is smoothed in a digital function of a besselfilter.
When the transmitter is up at the decided RF power-level, it is ready to send the message in a
digital burstformat.
The message must be fully completed during a clearly defined period on the timeaxis.
Two different time interval exist.
It is during this interval, the real significant message is going to be sent.
Some of the messages are system information only and some are digital speech frames, but
mostly both types in the same datapacket.
The method chosen for the transmission via the transmitter carrier is decided to be GMSK
digital modulation.
Also during this time interval, the spectral propagation must be limited within a specified
bandwidth for the system.
Because of that limitation, there will be a compromise between readability / penetration and
the available frequency band.
After hopefully a completed message time-lapse, the transmitter shall be STOPPED exactly at
a controlled point of time, to return to RF off-level again, within a very short and clearly
defined period of time <28µS.
The transmitter RF power-level downshift is called downramp and also this must be as
smooth and linear as possible during the transition from high to low, in order to limit the
spectral propagation. Same control functionality, as in the previous upramp, over three steps
and a besselfilter, is also utilised for the downramp.
4/00021-3/FEA 209 544/25 C
©
Ericsson Mobile Communications AB
Trouble Shooting Guide, Advanced
42(78)
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