Chapter 7
The 2nd Generation Cellular Systems
GSM: Pan-European Digital Cellular System
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Background and Goals
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GSM (Global System for Mobile
Communications)
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Beginning from 1982
European standard
Full roaming in Europe
A purely digital system
Goals (principal/ original) -> Phase 1, 2, 2+:
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full international roaming
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Background and Goals
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provision for national variations in charging and
rates
efficient interoperation with ISDN systems
signal quality better than or equal to that of
existing mobile systems
traffic capacity higher than or equal to that of
present systems
lower cost than existing systems
accommodation of non-voice services and
portable terminals
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Architecture
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Network elements
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Mobile stations, base stations, and mobile
switching center
Three databases
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Home location registers (HLR)
Visitor location registers (VLR)
Equipment identity registers (EIR)
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Architecture
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In contrast to the original cellular, micro cells
are used in GSM
A BS separates into two parts: BTS (base
transceiver station) and BSC (base station
controller)
Typically, a BSC controls several BTS
To reduce the cost with the greatest possible
service extent
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Architecture
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Subscriber identity module (SIM)
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Two types: one like credit card and the one
smaller
An important GSM innovation
A removable card that stores information,
including ID number, abbreviated dialing code,
and subscriber’s service plan
Easy to change telephones
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Architecture
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As in the other systems, GSM uses a variety of
ID codes
GSM Identifiers
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International Mobile Subscriber Identity (15 digits)
Temporary Mobile Subscriber Identity (32 bits)
 Advantages: Privacy and save BW
International Mobile Equipment Identifier (15 digits)
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Architecture
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Authentication Key (max = 128 bits)
Cipher key (64 bits):
 Terminal and network use authentication key to
compute the cipher key
Mobile station classmark including:
 Version of the GSM standard
 RF power capability (power levels available)
 Encryption method
 Other properties of terminal
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Architecture
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Training Sequence (26 bits)
 help a terminal verify that it receives
information from the correct BS rather than
another BS using the same physical channel
BS Identity Code (6 bits)
Location Area Identity (40 bits) including:
 A mobile country code, network code, and area
code
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Radio Transmission
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GSM Spectrum
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There are two 25 MHz bands separated by 45 MHz
Initial GSM systems operate in the upper 10 MHz
Physical Channels
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GSM is a Hybrid FDMA/TDMA system
Each GSM band has carriers spaced at 200 kHz
The frame duration is 120/26 = 4.62 ms
Each frame contains 8 time slots
There are 25 MHz/200 k Hz = 125 carriers in per
direction
GSM specifies only 124 carriers (one is used as guard
band)
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Radio Transmission
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GSM time interval
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A hyperframe = 2048 superframe = 3 h 28 m
53.76 s
A superframe = 51 traffic multiframes = 26
control multiframes = 6.12 s
A traffic multiframe = 26 frames = 120 ms
A control multiframe = 51 frames = 235.4 ms
A frame = 8 time slots = 4.615 ms
A slot = 156.25 bits = 577 µs
A bit = 3.69 µs
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Physical Channels
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Traffic Channels
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A full-rate traffic channel (TCH/F) occupies one
time slot in 24 of 26 frames in every
multiframe
Traffic channel information travels in frames 011 and 13-24
Control information travels in frames 12 and 25
The SACCH occupies one frame in every traffic
multiframe
A SACCH associated with a full-rate traffic
channel alternatively occupies one slot in frame
12 and one slot in frame 25
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Physical Channels
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A half-rate traffic channel (TCH/H) occupies a
specific time slot in 12 of 26 frames in every
multiframe
Each carrier can carry up to 16 half-rate traffic
channels
Eight of these traffic channels have a SACCH in
frame 12 and the other eight half-rate channel
have a SACCH in frame 25
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GSM Bit Stream
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The contents of a GSM time slot is shown in
Fig. 7.8
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26 bits of training sequence serves as a purpose
similar to that of the SYNC field in NA-TDMA
GSM specifies 8 different training sequences with
low mutual cross-correlation
Network operators assign different training
sequences to nearby cells that use the same
carrier
The two DATA fields carry either user information
or network control information
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Radio Transmission
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The FLAG indicates whether the DATA field contains
user information or control one
The TAIL bits all set to 0
There is also a guard time 0f 30.5 µs
The GSM transmission rate is 270.833 kb/s
The modulation scheme in GSM is GMSK a form of
frequency shift keying
The modulation efficiency of GSM is 1.35 b/s/Hz
GSM BS turn off its transmitter at the end of each time
slot. It resume transmitting after a pause of 30.5µs to
send to another terminal in the next time slot
The BS turn off its transmitter in unassigned time slots
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Slow Frequency Hopping
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The signal moves from one frequency to another in
every frame
The purpose of FH is to reduce the transmission
impairments
Without FH, the entire signal is subject to distortion
whenever the assigned carrier is impaired
Network operator assigns different hopping
patterns to different cells
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Radiated Power
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GSM specifies 5 classes of mobile stations
transmitting power, ranging from 20 W (43 dBm)
to 0.8 W (29 dBm)
Typically, vehicle-mounted terminal is 8 W and
portable terminals is 2 W
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Spectrum Efficiency
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The reuse factor of N = 3 or 4
The number of physical channel is 124 carriers x 8
channels/carriers = 992 physical channels
The efficiency of GSM is E = 992 channels/4
cells/cluster/50 MHz = 4.96 conversation/cell/MHz
(N= 4) or
The efficiency of GSM is E = 992 channels/3
cells/cluster/50 MHz = 6.61 conversation/cell/MHz
(N= 3)
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Logical Channels
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Traffic channels (two-way)
Broadcast channels (base-to-mobile)
Common control channels (base-to-mobile or
mobile-to-base)
Dedicated control channels (two-way)
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Broadcast channels and Common
control channels
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The broadcast channels always occupy time
slot 0
The common control channels can also
occupy time slots 0
Control Multiframe
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There are 5 groups of frames, each containing ten
frames beginning with a frequency-correction
frame and a synchronization frame
In the reverse direction, time slot 0 is assigned to
random access channels in all 51 frames
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Figure 7.11 shows the contents of time slot 0 in
each of the 51 frames
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Logical Channels
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Frequency Correction Channel (FCCH)
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The FCCH simply transmits 148 0s
The FCCH always occupies time slot 0 in a frame
of 8 time slots
A terminal without a call in progress searches for a
FCCH
Synchronization Channel (SCH)
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A BS transmits a SCH in time slot 0 of every frame
that follows a frame containing an FCCH
The SCH contains a TRAINING sequence
The DATA fields contain BS identity code (6 bits)
and the present frame number
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Logical Channels
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Broadcast Control Channel (BCCH)
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BS use the BCCH to transmit the information that
terminals need to set up a call, including the
control channel configuration and the access
protocol
The message length is 184 bits and the encoded
message is 456 bits occupying 4 time slots
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Logical Channels
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Paging Channel (PCH) and Access Grant
Channel (AGCH)
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The purpose of the AGCH is to direct a terminal to
a stand-alone dedicated control channel (SDCCH)
Both channels use the same coding scheme as the
BCCH
They occupy 36 frames of time slot 0 per
multiframe
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Logical Channels
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Random Access Channel (RACH)
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GSM terminals send messages on the RACH to
originate phone calls, initiate transmissions of
short messages, respond to paging messages, and
register their locations
Terminals with information to transmit use the
slotted ALOHA protocol to gain access to the time
slot
The Ack directs the terminal to a stand-alone
dedicated control channel (SDCCH) to be used for
further communications
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Logical Channels
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The RACH slot includes a 41-bit TRAIN and 36-bit
DATA
The 36-bit DATA field carries a simple 8-bit
message
Three of the 8 bits indicate the purpose of the
access attempt and the other 5 bits are produced
by a random number generator
The 5-bit random code is likely (with probability
31/32) to distinguish the successful terminal from
the other
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Logical Channels
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Stand-Alone Dedicated Control Channel
(SDCCH)
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SDCCH is a two-way channel assigned to a specific
terminal
The physical channel used by an SDCCH is a set of
four time slots in each 51-frame control multiframe
With 114 data bits per time slot, the data rate of the
SDCCH is 1937.25 b/s (see eq. 7.7)
Each SDCCH has a slow associated control channel
The SACCH occupies an average of two time slots
per control multiframe (969 b/s)
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Logical Channels
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Traffic Channels (TCH)
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GSM defines two traffic channels, a full-rate
channel occupies 24 time slots in every 26frame and a half-rate channel
The bit rate of a full-rate traffic channel is
22,800 b/s
SACCH occupies time slots in frames 12 or 25
of each 26-frame traffic multiframe
The transmission rate of a traffic SACCH is 950
b/s
With 456 bits transmitted per message, a
message spans four traffic multiframes, a time
interval of 480 ms
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Logical Channels
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Fast Associated Control Channel (FACCH)
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Use the traffic channel to transmit control
information, which is an in-band signaling channel
Each FACCH message is multiplexed with user
information and interleaved over 8 frames.
Therefore, the transmission time of an FACCH
message is approximately 40 ms
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Messages
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GSM Protocol Layers
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GSM provides a large number of open interfaces
Message Structure
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All of the signaling message length is 184 bits with
the exception of the FCCH, SCH, and RACH
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Network Operations
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Call to a GSM Terminal
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Terminal uses the frequency correction channel
(FCCH) to synchronize its local oscillator
It then gains timing information from the SCH
The terminal then obtains important information
from broadcast control channel (BCCH)
After the initialization procedure, the terminal
monitors a paging channel (PCH)
Eventually, it detects a paging request message
and this message cause the terminal to transmit a
channel request message on the random access
channel (RACH)
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Network Operations
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The network response this request by transmitting
an immediate assignment message on an access
grant channel (AGCH)
This message established a stand-alone dedicated
control channel (SDCCH) to be used for exchange
of mobility management messages and call
management messages
When terminal moves to SDCCH, it transmits a
paging response message to BS
The BS then initiates the GSM authentication
procedure
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Network Operations
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Authentication and Encryption Procedure
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The terminal received a 128-bit random number
(RAND) from BS
Then it applies a GSM encryption algorithm A3
to compute a 32-bit signed response, SRES
The inputs of A3 are RAND and secret key Ki
The secret key Ki is stored in the subscriber
information module (SIM)
The terminal applies another encryption
algorithm A8 to compute a 64-bit ciphering key
Kc from SRES and Ki
The network also uses A3 to compute SRES
from RAND and Ki
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Network Operations
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If the two values of SRES are identical, the
network accept the the user as an authorized
subscriber
To encrypt user information and network control
information, the BS and network derive a 114-bit
mask to be added (modulo 2) to the two DATA
fields
The inputs of A5 are the 64-bit ciphering key Kc
and the current 22-bit frame number
Because A5 uses the frame number to compute
the ciphering mask, the mask change from frame
to frame
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Network Operations
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To Setup a Call
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BS transmits a setup message to the terminal
The terminal Ack this message with a call
confirmed
The terminal then send a connect message to the
network
In response, the network moves the call to a
traffic channel by means of an assignment
command message
Note that, GSM assigns a traffic channel after the
mobile subscriber accepts the call
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Network Operations
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Location-Based Registration
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Terminal registers its location when it moves to
a new cell
Mobile-Assisted Handover
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When mobile terminal finds a channel quality is
better than present one the handover
procedures will be executed
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Status of GSM
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GSM operates in 900 MHz, 1800 MHz, and 1900
MHz bands
GPRS (generalized packet radio service) with 100+
kbits/s data rate
Enhanced Data Rate for GSM Evolution (EDGE) with
300+ kbits/s data rate
Universal Mobile Telecommunication Services
(UMTS): 3G telecommunication technology up to 2
Mbits/s data rate using WCDMA or TD/CDMA
(IMT2000) transceiver
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Review Exercises
1.
2.
What is the advantage of transmitting the
training sequence in the middle of a slot?
What is the benefit of using the frame
number to calculate encryption marks?
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References
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D. J. Goodman “Wireless Personal
Communications Systems,” Chapter 7.
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