GSM tutorial radio /air interface
an overview or tutorial of the radio or air interface and channels for the
GSM system
There are a number of elements to the GSM radio or air
interface. There are the aspects of the physical power levels, channels and the
like. Additionally there are the different data channels that are employed to
carry the data and exchange the protocol messages that enable the radio
subsystem to operate correctly.
Basic signal characteristics
The GSM system uses digital TDMA technology combined with a
channel bandwidth of 200 kHz. Accordingly the system is able to offer a higher
level of spectrum efficiency that that which was achieved with the previous
generation of analogue systems. As there are many carrier frequencies that are
available, one or more can be allocated to each base station. The system also
operates using Frequency Division Duplex and as a result, paired bands are
needed for the up and downlink transmissions. The frequency separation is
dependent upon the band in use.
The carrier is modulated using Gaussian Minimum Shift Keying
(GMSK). GMSK was used for the GSM system because it is able to provide features
required for GSM. It is resilient to noise when compared to some other forms of
modulation, it occupies a relatively narrow bandwidth, and it has a constant
power level.
The data transported by the carrier serves up to eight
different users under the basic system. Even though the full data rate on the
carrier is approximately 270 kbps, some of this supports the management
overhead, and therefore the data rate allotted to each time slot is only 24.8
kbps. In addition to this error correction is required to overcome the problems
of interference, fading and the like. This means that the available data rate
for transporting the digitally encoded speech is 13 kbps for the basic vocoders.
Power levels
A variety of power levels are allowed by the GSM standard,
the lowest being only 800 mW (29 dBm). As mobiles may only transmit for one
eighth of the time, i.e. for their allocated slot which is one of eight, the
average power is an eighth of the maximum.
Additionally, to reduce the levels of transmitted power and
hence the levels of interference, mobiles are able to step the power down in
increments of 2 dB from the maximum to a minimum 13 dBm (20 milliwatts). The
mobile station measures the signal strength or signal quality (based on the Bit
Error Rate), and passes the information to the BTS and hence to the BSC, which
ultimately decides if and when the power level should be changed.
A further power saving and interference reducing facility is
the discontinuous transmission (DTx) capability that is incorporated within the
specification. It is particularly useful because there are long pauses in
speech, for example when the person using the mobile is listening, and during
these periods there is no need to transmit a signal. In fact it is found that a
person speaks for less than 40% of the time during normal telephone
conversations. The most important element of DTx is the Voice Activity Detector.
It must correctly distinguish between voice and noise inputs, a task that is not
trivial. If a voice signal is misinterpreted as noise, the transmitter is turned
off an effect known as clipping results and this is particularly annoying to the
person listening to the speech. However if noise is misinterpreted as a voice
signal too often, the efficiency of DTX is dramatically decreased.
It is also necessary for the system to add background or
comfort noise when the transmitter is turned off because complete silence can be
very disconcerting for the listener. Accordingly this is added as appropriate.
The noise is controlled by the SID (silence indication descriptor).
Multiple access and channel structure
GSM uses a combination of both TDMA and FDMA techniques. The
FDMA element involves the division by frequency of the (maximum) 25 MHz
bandwidth into 124 carrier frequencies spaced 200 kHz apart as already
described.
The carriers are then divided in time, using a TDMA scheme.
The fundamental unit of time is called a burst period and it lasts for
approximately 0.577 mS (15/26 mS). Eight of these burst periods are grouped into
what is known as a TDMA frame. This lasts for approximately 4.615 ms (i.e.120/26
ms) and it forms the basic unit for the definition of logical channels. One
physical channel is one burst period allocated in each TDMA frame.
There are different types of frame that are transmitted to
carry different data, and also the frames are organised into what are termed
multiframes and superframes to provide overall synchronisation.
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