<<

. 13
( 132 .)



>>

890.0 MHz, 890.2 MHz, 890.4 MHz etc. up to 896.0 MHz in the uplink and 935.0 MHz,
935.2 MHz, 935.4 MHz etc. up to 941.0 MHz in the downlink, as shown in Figure 3.8.
TDM further splits each of these frequency channels into eight separate time slots, each
of which may be allocated to a user or used for control purposes. These time slots are
individually referred to as slot 0 through to slot 7, and form a TDM frame. A single time
slot in GSM is also referred to as a burst; however, this should not be confused with the
term ˜error burst™. If a cell is allocated a single frequency (one TRX) then slot 0 on this
frequency is reserved as a control channel. If two or more frequencies are employed within
the cell then it may require additional control channels to increase the overall ef¬ciency.
The slot 0 control channel always includes the broadcast and control channel (BCCH),
which is broadcast from the base station in the downlink to provide information to the
mobile devices registered in the cell, such as the cell identi¬er, network operator etc.,
54 GSM FUNDAMENTALS


200kHz
spacing




channel 10
channel 11
channel 12
channel 13
channel 14
channel 15
channel 16
channel 17
channel 18
channel 19
channel 20
channel 21
channel 22
channel 23
channel 24
channel 25
channel 26
channel 27
channel 28
channel 29
channel 30
channel 31
channel 1
channel 2
channel 3
channel 4
channel 5
channel 6
channel 7
channel 8
channel 9




zzz z frequency
HHH H
MMM M
.0 .2 .4 .0
90 890 890 6
89
8

Figure 3.8 200 kHz GSM uplink channels


thus the maximum number of users for the ¬rst TRX is actually only seven. If more than
one frequency is used within the cell these additional frequencies can share the control
channel of the ¬rst TRX and therefore have eight time slots available for subscriber
traf¬c channels, allowing slot 0 on these additional frequencies to be used as a normal
traf¬c channel. A single time slot allocated to control and signalling may not possess a
high enough bit rate, giving rise to congestion on the control channel. This may occur,
for example, in a cell which contains a large number of short message service (SMS)
users, since SMS in GSM is sent over the control channels, or in an airport where many
users may try to access the network at the same time. In this case, the operator may set
aside another time slot to alleviate this congestion. The actual con¬guration is speci¬c to
the unique requirements of a particular cell. The rapid rise globally of SMS traf¬c has
presented somewhat of a dimensioning dilemma to operators for this reason.
Figure 3.9 shows an active TRX, where the shaded boxes indicate a particular time
slot currently in use. The top frame indicates the uplink transmission from the mobile
station and the bottom frame indicates the downlink transmission from the base station.
It can also be seen from the diagram that the transmission by the base station is actually
retarded by the duration of three time slots so that the TRXs do not have to listen while
transmitting.2 Note that the bursts are still paired, e.g. if the mobile device transmits on
time slot 1 then it will receive on time slot 1 because the whole frame has been retarded
by three time slots.
Figure 3.10 shows an example of how the eight time slots can be used. It can be seen
that over time the ¬rst time slot (time slot 0) starts off broadcasting system information
across the cell. This same time slot is then used to transfer an SMS message to a single
subscriber and is then used for a location update for a new mobile device, which has
just entered this cell. The other time slots are used by subscribers as and when they wish

2
The mobile station may be transmitting a 0.3 W signal; it is expected to be able to receive a very
weak signal of about 1 millionth of a watt from the base station on the same antenna. Clearly if it
is transmitting and receiving at the same time this is particularly dif¬cult to do.
3.3 GSM AIR INTERFACE 55


TDM frame

01234567012345670123456701234567
890.2Mhz
Mobile Station Transmission
TDM frame

56701234567012345670123456701234
935.2Mhz
Base Station Transmission

Figure 3.9 TDM channels in GSM

T/slot 0 T/slot 1 T/slot 2 T/slot 3 T/slot 4 T/slot 5 T/slot 6 T/slot 7
Broadcast Voice Call Free Voice Call Free Voice Call Voice Call GPRS Call

Voice Call
SMS
Free
Free Voice Call
Location
Update Free

Broadcast Voice Call
Free
Voice Call


Figure 3.10 Example use of time slots

to make telephone calls (voice or data). The duration of these obviously depends on the
call itself.
The network selects which band to use, whether there is an option, for example
900 MHz or 1800 MHz, the frequency, if there is more than one available in this cell,
and the time slot the subscriber will use. This selection is completely transparent to the
subscriber; in fact, the frequency and time slot will normally change throughout the call,
with the user perhaps noticing a small amount of interference or possibly nothing at all.
The ability to change the frequency of a subscriber is required when a subscriber roams
from one cell into another since adjacent cells cannot use the same frequency.
Within a cell, most GSM systems implement frequency hopping, where the BTS and
mobile unit transmit consecutive frames on different carrier frequencies across the radio
channels. Frequency hopping is a key part of the GSM system and is used to alleviate
some of the inherent problems with radio links such as multipath fading by avoiding
prolonged use of one frequency. The BCCH channel is not part of the frequency-hopping
scheme since it needs to be located by mobile devices wishing to connect to the cell.
Figure 3.11 below shows a simpli¬ed example of the frequency hopping process. In a
practical situation each individual time slot can hop independently to the TDMA frame.


3.3.1 GSM multiframes
The above eight time slot framing structure as shown in Figure 3.9 is part of a much
larger multiframe. There are two types of multiframe: the traf¬c channel and control
56 GSM FUNDAMENTALS


Subscriber
timeslots

TRX3




BCCH remains
frequency




on t/slot 0
TRX2
BCCH




BCCH




BCCH




BCCH
TRX1




TDMA
TDMA TDMA time
block 3
block 1 block 2

Figure 3.11 Frequency hopping in GSM

channel multiframes. The traf¬c channel multiframe consists of 26 groups of 8 TDM
frames whereas the control multiframe consists of 51 groups of frames.



3.3.2 Traf¬c channel multiframe
Although this is a designated traf¬c channel consisting of 26 frames, at most only 24 of
these are used for TCH user data such as voice. The frame is also used to carry two logical
control channels, the slow associated control channel (SACCH) and the fast associated
control channel (FACCH). Time slots 12 and 25 are speci¬cally used for SACCH. In fact,
the SACCH may actually alternate between these on different multiframes and so there is
only one SACCH channel transmitted per multiframe. During the frame that the mobile
device is not transmitting or receiving on its dedicated TCH, it is constantly monitoring
the strength of the received signals from the cell it is attached to as well as other cells
in the area. The mobile station can actually monitor up to six surrounding cells. The
SACCH is then used for sending these measurement results to the network. A SACCH
message is 456 bits long (4 bursts — 114 bits per burst); it therefore has a time interval
of 480 ms before repeating itself. This information can be used to increase or decrease
the transmitted power levels of both the mobile device and the network every 480 ms, or
just a little over twice a second. The mobile device uses this information it receives but
actually alters its power in steps every 60 ms. Enhanced circuit switched data (ECSD)
can also use a fast power (FP) method which is sent within the data stream every 20 ms.
Figure 3.12 illustrates the SACCH power control mechanisms: both the mobile device
and the BTS send measurement reports to the BSC, which makes the decision to increase
or decrease power.
The SACCH may also be used for sending SMS messages to and from the mobile
device while a call is in progress. User data frames 0“11 and 13“24 also carry control
information in the form of the FACCH channel, as described below. A single user voice
3.3 GSM AIR INTERFACE 57


SACCH every 480ms Base Station Subsystem

Power Control and measurement reporting
Power Control Decision

Mobile
Abis
Station
SIM
Power Control and
measurement reporting
ME
Power Control
BTS Decision BSC



Figure 3.12 SACCH power control


call will be transmitted in frame 0 of the multiframe for a particular amount of time,
followed by a particular amount of time in frame 1 etc. Eight such calls can be made on
this carrier frequency. A traf¬c channel is only assigned when the mobile device is in
dedicated mode, whereas in idle mode the mobile device does not have a traf¬c channel
assigned to it. Figure 3.13 shows the relationship of the TDM frame within a traf¬c
channel multiframe.
Each of the eight time slots in the TDM frame consists of 148 bits lasting 547 µs and
has the structure shown in Figure 3.14.

Multi-frame 32,500 bits in120msec
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Control Control
Frame Frame
01234567
TDM frame
1250 bits in 4.615msec

Figure 3.13 GSM multiframe

<<

. 13
( 132 .)



>>