The GSM radio interface uses FDMA to divide the frequency band into channels and TDMA to divide each frequency channel into time slots to allow multiple users, with each user assigned a single time slot. The normal GSM burst carries digitized voice data or other information in a 57-bit data field, and includes guard periods and training sequences to help with timing synchronization and equalization between the mobile station and base transceiver station. GSM networks operate at different frequencies around the world, with GSM-900 being most common in Europe and other parts of the world.

1. GSM Radio Interface
Asst.Prof. Rupali Lohar
Dept. of Computer Science & Engineering
B. R. Harne College Of Engineering & Technology, Karav, Post Vangani (W Tal Ambernath,
Mumbai, Maharashtra 421503

2. Radio Interface
Another MSC
HLR/AUC
SMC
PSTN
ISDN
OMC
MS
Um
MS
A-bis
BSC
A
BTS
MSC/VLR

3. Radio Access Technology
Power
Power
Power
FDMA
TDMA
CDMA
Time
FDMA
Frequency
Time TDMA
Frequency
CDMA
Frequency
Time
Code

4. 7 0 1 2 3
Physical Channel of Logical Channel
0
different
message
types
Logical channel
Logical channel
Logical channel
Logical channel
Logical channel
TDMA FRAME
1 2 3 4 5 6
200K
577 s
Physical Channel
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5. Uplink
• the uplink (radio link from the mobile to the network-that is, mobile
transmit, base receive)

6. Downlink
• downlink (from the network to the mobile-that is, base transmit,
mobile receive) are transmitted on different frequency bands.

8. General Characteristics
• Currently, there are several types of networks in the world using the GSM standard, but at
different frequencies.
• The GSM-900 is the most common in Europe and the rest of the world. Its extension is E-GSM.
• The DCS-1800 operates in the 1,800-MHz band and is used mainly in Europe, usually to cover
urban areas. It was also introduced to avoid saturation problems with the GSM-900.
• The PCS-1900 is used primarily in North America.
• The GSM-850 is under development in America.
• The GSM-400 is intended for deployment in Scandinavian countries in the band previously used
for the analog Nordic Mobile Telephony (NMT) system.

11. • FDMA consists in dividing the frequency band of the system into several
channels. In GSM, each RF channel has a bandwidth of 200 kHz, which is
used to convey radio modulated signals, or carriers. Each pair of uplink/
downlink channels is called an absolute radio frequency channel (ARFC)
and is assigned an ARFC number (ARFCN
• TDMA is the division of the time into intervals: within a frequency channel,
the time is divided into time slots. This division allows several users (eight)
to be multiplexed on the same carrier frequency, each user being assigned
a single time slot. A packet of data information, called a burst, is
transmitted during a time slot. The succession of eight time slots is called a
TDMA frame, and each time slot belonging to a TDMA frame is identified
by a time slot number (TN), from 0 to 7.

12. GSM 900
• 124 channels
• 200 KHz USED FOR FDMA
• Channels 1 and 124nare not used for transmission
• 32 channels are reserved for organizational data
• 90 are used for customer
• 248 channels separated in time via GSM TDMA frame
• Duration frame 4.615 ms
• Each frame again subdivided into 8 GSM time slot
• Data is transmitted small portion , called bursts

13. 8bit 41 synchronous
bits
36 encrypted
bits
3bit 68.25bit
Guard interval
• Access burst (AB): Used in MS initial connection sretup
Tail bit Data Tail bit
3bit 142bit 3bit 8.25bit
• Frequency correction burst (FB): Used in frequency synchronization
between MS and BTS, avoid interference.
Tail bit Data Tail bit Guard interval
3bit 39 encrypted
bits
64 synchronous bits
39 encrypted
bits
3bit 8.25bit
• Synchronous burst (SB): Used in timing synchronization between
MS and BTS
Tail bit Data Data Tail bit Guardinterval
Burst
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14. 3bit 142 modulation bits 3bit 8.25bit
Tail bit Tail bit Guardinterval
Burst
3bit 57 encrypted bits 1 26bit 1 57 encrypted bits 3bit 8.25bit
Tail bit Data Training sequence Data Tail bit Guard interval
Frame stealing flag
 Normal burst (NB)
 Used to carry the information of the traffic channel and the control channel
except for RACH
 Dummy burst (DB)
 Used if no data is available for a slot
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16. GSM normal burst
• This GSM burst is used for the standard communications between the
base station and the mobile, and typically transfers the digitised voice
data.
• Data is transmitted small portion , called bursts.
• 1 bit flag: This bit within the GSM burst indicates the type of data in
the previous field.

17. • Guard space- To avoid overlapping with other bursts due to different
path delays.
• 3 tail bits: These tail bits at the start of the GSM burst give time for
the transmitter ,are all set to 0 and can be used to enhance the
receiver performance.
• 57 data bits: This block of data is used to carry information, and
most often contains the digitized voice data

18. • 26 bits training sequence: -
• This training sequence is used as a timing reference and for
equalization.
• Used to adapt the parameter of receiver to the current path
propagation .
• Select strongest signal in case of multipath propagation
• 1 bit flag S- indicates the type of data in the data field.