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Frequency Allocation
• Uplink Direction(MS to BS)
– GSM 900 (890-915 MHz) “BW=2*25Mhz
– GSM 1800 (1710-1785 MHz) “BW=2*75Mhz
• Downlink Direction(BS to MS)
– GSM 900 (935-960 MHz)
– GSM 1800 (1805-1880 MHz)
Multiple Access Scheme
• Since the radio spectrum is a limited resource shared by
all users, a method must to be devised to divide up the
bandwidth among as many users as possible. The
multiple access scheme defines how different
simultaneous communications, between different mobile
stations situated in different cells, share the GSM radio
spectrum. A mix of Frequency Division Multiple
Access (FDMA) and Time Division Multiple Access
(TDMA), combined with frequency hopping, has been
adopted as the multiple access scheme for GSM.
TDMA frames are grouped into two types of
multiframes:
• 26-frame multiframe (4.615ms x 26 = 120
ms) comprising of 26 TDMA frames. This
multiframe is used to carry traffic channels
and their associated control channels.
• 51-frame multiframe (4.615ms x 51 @
235.4 ms) comprising 51 TDMA frames.
This multiframe is exclusively used for
control channels.
Two Types of Channel
• Traffic Channel• Used to transport speech and data information.
• Control Channel
– Used for network management messages and
some channel maintenance task.
Traffic Channel
• Defined by using 26-frame Multiframe.
• Specified with two general forms:
– Full rate traffic channel (TCH/F)=22.8kbps
– Half rate traffic channel (TCH/H)=11.4kbps
Control Channels
• Control channels carry system signalling and
synchronisation data for control procedures such
as location registration, mobile station
synchronisation, paging, random access etc.
between base station and mobile station. Three
categories of control channel are defined:
• Broadcast
• Common
• Dedicated
Broadcast Channel (BCH)
• It is used by the base station to provide
the MS with the sufficient information it
needs to synchronize with the network.
The three types of BCH:
• Broadcast Control Channel
• Synchronization Channel
• Frequency-Correction Channel
Broadcast Control Channel
• BCCH provides system parameters
needed to identify and access the cellular
network.
Frequency-Correction Channel
• FCCH supplies the MS with the frequency
reference of the system in order to
synchronize it with the network.
Synchronization Channel
• SCH gives the MS the training sequence
needed in order to demodulate the
information transmitted by the base
station.
Common Control Channel
• CCCH helps to establish the calls from the
mobile station or the network. There are 3
types of CCCH:
• Access Grant Channel
• Paging Channel
• Random Access Channel
• Cell Broadcast Channel
Access Grant Channel
• AGCH is the answer of BS to a RACH
from MS. It is used to inform the MS about
the channel selection provided by the BS.
Random Access Channel
• RACH is used by the MS to request an
access to the network or to establish a
connection.
Dedicated Control Channel
• Used in signaling message exchange
between the mobile station and the
network. There are 3 types of DCCH:
• Standalone Dedicated Control Channel
• Slow Associated Control Channel
• Fast Associated Control Channel
Standalone Dedicated Control
Channel
• SDCCH is used in order to exchange
signaling information in the downlink and
uplink direction.
Slow Associated Control
Channel
• SACCH is used for channel maintenance
and channel control. It carries the general
information between MS and BS.
Fast Associated Control
Channel
• FACCH carries the same information as
the SDCCH. It replaces the traffic channel
when there is an immediate transmission
of signaling information.
Burst Structure
• Burst is made up of a useful part and a
guard band. During the time slot period a
frequency carrier is modulated by a stream
of data bits organized into blocks.
Types of Burst
• Normal Burst is used to carry speech or
data information. It consist of 116 data
bits(2 groups of 58 bits), 8.25 bits guard
period, 26 bits training sequence and 6
bits for tail bits.
• Frequency-correction burst has the same
guard period and tail bits from normal
burst but the encrypted data and
sequence form a fixed series of zeros for
frequency synchronization.
• Synchronization burst is used for time
synchronization. It has the same start and
stop bits of FCB but has “extended training
sequence”.
• Access Burst is used by the MS for initial
access to the system. It has a shorter
burst because it doesn’t require the MS to
be fully synchronize.
• Tail bits are group of 3 bits placed at the
beginning and end of a burst to cover the
periods of ramping up and down of
mobile’s power.
• Stealing flags indicates to the receiver if
the information in the burst is a traffic or
signaling.