GSM Interface

GSM Interface

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.

FDMA/TDMA Scheme

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.

GSM Hierarchy

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

Traffic Channel Multiframe

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.

Paging Channel

• PCH is used to alert the mobile station of

an incoming call.

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.

• Training Sequence is used to

synchronize the receiver w/ incoming

information and avoid the effect of

multipath.

• Guard Period is used to avoid

overlapping of two mobiles during ramping

time.