GSM: Global System for Mobile Communication Explained

GSM
Global System for Mobile
Communication
By: Mohammed Hassan
E-mail:

 Introduction to GSM
 Wireless Concepts
 Channel Concepts
 Switching System
 Base Station System
 Mobile Station
 Traffic Cases
 Cell Planning
Content

Introduction to
GSM
History of GSM system
1985 Standardization for second generation digital cellular systems is
initialized.
1992 GSM900 in World Wide.
1993 GSM1800 ( DCS ) in Europe.
1994 GSM1900 ( PCS ) was firstly commercial.
Global System for Mobile (GSM) is a 2G digital cellular telephone
system.
GSM became the world's leading and fastest growing mobile standard,
spanning over 174 countries .
Introduction to mobile systems (2G)

Introduction to
GSM
Difference between 1G and 2G mobile
networks
The main difference between 1G networks and 2G networks is
1 G systems was analog but 2 G systems was digital
The analog mobile systems have main restrictions of:
• the limited capacity,
• voice-only services
• high operational cost.
• Health Issues
• different systems are incompatible in terms of equipment and
operation, e.g NMT and TACS.

Introduction to
GSM
 Capacity
 While with digital systems such as GSM,
• the available frequency spectrum is used more efficiently,
leading to increased capacity
• reductions in associated costs for network operators, equipment
suppliers and subscribers.
 Services
 Analog mobile systems were originally designed for voice
 digital mobile systems can support voice, data and a range of
additional services such as :
• a short message service
• call forwarding
• ISDN compatible.
Difference between 1G and 2G mobile
networks

Introduction to
GSM
Introduction to mobile systems (2.5G)
 GSM offers circuit-switched with good voice quality, but it is providing
data rates of 9.6 kbps which is too slow.
 In 1999 General Packet Radio Service (GPRS) reuses the existing
GSM infrastructure to provide higher data rate
 It was lunched to increase the data rate to 115 kbps by:
– using the packet-switched in data transmission
– Defining new coding scheme.
 In 2001 Evolved Data rate for GSM Evolution (EDGE)
offers data rate of 384 kbps by using new modulation scheme ( 8 psk )

Introduction to
GSM
The main cellular standards

Introduction to
GSM
What is the GSM ?
 GSM is the Global System for Mobile telecommunications.
 It is the European standard for the Mobile telecommunications and it is
considered as one of the most popular standard worldwide.
 It is known as the second generation mobile telecommunications
system “2G system”.
 It is used in Egypt by the three existing operators; Vodafone , Orange ,
and Etisalat.

Introduction to
GSM
Evolution of GSM
 1982, Conference is held to specify a common European mobile
telecommunications in the band of 900 MHz.
 1987, GSM specs drafted “ TDMA+FDMA , Time multiplexing of order
8 with slow frequency hopping ”.
 1992, first GSM network was commercial in Finland .
 Global System for Mobile (GSM) is a second-generation digital cellular
telephone system.

Introduction to
GSM
 1996, first GSM operator in Egypt owned by the government.
 1998, Egypt privatizes its GSM operator.
 1998, Click GSM commercial launch.
 GSM became the world's leading and fastest growing mobile
standard, spanning over 174 countries, serving more than one in ten
of the world's population.
 2000, First GPRS roll out.
Evolution of GSM

Introduction to
GSM
Functionality
& capabilities
Speech
Circuit data
≤ 9.6 kbps
UMTS
capable systems
EDGE
384 kbps
WCDMA
2 Mbps
1998 1999 2000 2001 20021997 Time
HSCSD
57.6 kbps
Circuit
Switched
GPRS
115 kbps
Packet
Switched
Evolution Vs. Time

Introduction to
GSM
Transmission Medium
Basic Communication System Elements
 Transmitter
 Receiver
 Transmission Medium

Introduction to
GSM
Transmission Medium
 Transmission Medium Types

Guided or Wired transmission
 Twisted-pair :
It has very low bandwidth and it is easily tapped either physically or
by monitoring its electromagnetic radiation
 Coaxial cable :
It has greater bandwidth than twisted-pair but it is very
expensive.
 optical fibers :
It is very high bandwidth , very high bit rate and inherently
transmission medium.
Introduction to
GSM
Transmission Medium

Introduction to
GSM
 It is greatly depending on the particular frequency of the
electromagnetic wave
 Some of their advantages :
They are very flexible and suitable for all terrain.
Portable system can be installed very quickly
There are often the most cost-effective solution
As a summary, the radio transmission is used in case of mobile
telecommunications. Also, the using of Radio transmission is
combined with many problems that may cause the degradation of
the service quality.
Unguided or Wireless
transmission
Transmission Medium

FDMA
TDMA
CDMA
Introduction to
GSM
Multiple Access Techniques
 Multiple access schemes are used to allow
many users to share simultaneously a finite
amount of radio spectrum
 Sharing of spectrum is required to
increase capacity
 For high quality communication this sharing
of spectrum should not degrade
performance of the system
 high performance
 duplexing generally required
 frequency domain
 time domain

Introduction to
GSM
Duplexing
 What is Duplexing?
 to talk and listen simultaneously is called duplexing
 Classification of communication systems according to
their connectivity
 Simplex
 Half-duplex
 Duplex
A B
A B
A B

 Frequency division duplexing (FDD)
 two bands of frequencies for every user
 forward band (Uplink)
 reverse band (Downlink)
 duplexer needed
 frequency seperation between forward band and reverse band is
constant
frequency seperation
reverse channel forward channel
f
Introduction to
GSM

 Time division duplexing (TDD)
 uses time for forward and reverse link
 multiple users share a single radio channel
 forward time slot
 reverse time slot
 no duplexer is required
time seperation
t
forward channelreverse channel
Introduction to
GSM

 Frequency division multiple access (FDMA)
 Time division multiple access (TDMA)
 Code division multiple access (CDMA)
 Space division multiple access (SDMA)
 grouped as:
 narrowband systems
 wideband systems
Introduction to
GSM

Narrowband systems
 large number of narrowband channels
 usually FDD
 Narrowband FDMA
 Narrowband TDMA
 FDMA/FDD
 FDMA/TDD
 TDMA/FDD
 TDMA/TDD
Introduction to
GSM

Logical separation FDMA/FDD
f
t
user 1
user n
forward channel
reverse channel
forward channel
reverse channel
...
Introduction to
GSM

Logical separation FDMA/TDD
f
t
user 1
user n
forward channel reverse channel
...
Introduction to
GSM

Logical separation TDMA/FDD
f
t
user 1 user n
forward
channel
reverse
channel
forward
channel
reverse
channel
...
Introduction to
GSM

Logical separation TDMA/TDD
f
t
user 1 user n
forward
channel
reverse
channel
forward
channel
reverse
channel
...
Introduction to
GSM

Wideband systems
 large number of transmitters on one channel
 TDMA techniques
 CDMA techniques
 FDD or TDD multiplexing techniques
 TDMA/FDD
 TDMA/TDD
 CDMA/FDD
 CDMA/TDD
Introduction to
GSM

Logical separation CDMA/FDD
code
f
user 1
user n
...
Introduction to
GSM

Logical separation CDMA/TDD
code
t
user 1
user n
...
Introduction to
GSM

Multiple Access Techniques in use
Advanced Mobile Phone System (AMPS)
FDMA/FDD
Global System for Mobile (GSM) TDMA/FDD
US Digital Cellular (USDC)
TDMA/FDD
Digital European Cordless Telephone (DECT)
FDMA/TDD
US Narrowband Spread Spectrum (IS-95)
CDMA/FDD
Cellular System Multiple
Access Technique
Introduction to
GSM

Frequency division multiple access (
FDMA )
 one phone circuit per channel
 idle time causes wasting of resources
 simultaneously and continuously transmitting
 usually implemented in narrowband systems
 for example: in AMPS is a FDMA bandwidth of 30 kHz
implemented
Introduction to
GSM
FDMA
power

FDMA compared to TDMA
 fewer bits for synchronization
 fewer bits for framing
 higher cell site system costs
 higher costs for duplexer used in base station and subscriber
units
 FDMA requires RF filtering to minimize adjacent channel
interference
Introduction to
GSM

Number of channels in a FDMA system
• N … number of channels
• Bt … total spectrum allocation
• Bguard … guard band
• Bc … channel bandwidth
N=
Bt –
2*BguardBc
Introduction to
GSM

Example: Advanced Mobile Phone System ( AMPS )
• AMPS
• FDMA/FDD
• analog cellular system
• 12.5 MHz per simplex band - Bt
• Bguard = 10 kHz ; Bc = 30 kHz
N=
12.5E6 - 2*(10E3)
30E3
= 416 channels
Introduction to
GSM

Time Division Multiple Access ( TDMA )
 time slots
 one user per slot
 buffer and burst method
 noncontinuous transmission
 digital data
 digital modulation
Introduction to
GSM
TDMA
power

Slot 1 Slot 2 Slot 3 … Slot N
Repeating Frame Structure
Preamble Information Message Trail Bits
One TDMA Frame
Trail Bits Sync. Bits Information Data Guard Bits
The frame is cyclically repeated over time.
Introduction to
GSM

Features of TDMA
 a single carrier frequency for several users
 transmission in bursts
 low battery consumption
 handoff process much simpler
 FDD : switch instead of duplexer
 very high transmission rate
 high synchronization overhead
 guard slots necessary
Introduction to
GSM

Number of channels in a TDMA system
• N … number of channels
• m … number of TDMA users per radio channel
• Btot … total spectrum allocation
• Bguard … Guard Band
• Bc … channel bandwidth
N=
m*(Btot - 2*Bguard)
Bc
Introduction to
GSM

Example: Global System for Mobile (GSM)
• TDMA/FDD
• forward link at Btot = 25 MHz
• radio channels of Bc = 200 kHz
• if m = 8 speech channels supported, and
• if no guard band is assumed :
N=
8*25E6
200E3
= 1000 simultaneous
users
Introduction to
GSM

Efficiency of TDMA
 percentage of transmitted data that contain information
 frame efficiency f
 usually end user efficiency < f ,
 because of source and channel coding
 How get f ?
Introduction to
GSM

Slot 1 Slot 2 Slot 3 … Slot N
Repeating Frame
Structure
Preamble Information Message Trail Bits
One TDMA Frame
Trail Bits Sync. Bits Information Data Guard Bits
The frame is cyclically repeated over time.
Introduction to
GSM

Introduction to
GSM
 Transmitting voice and data using electromagnetic waves in
open space (atmosphere)
 Electromagnetic waves
 Travel at speed of light (c = 3x108 m/s)
 Has a frequency (f) and wavelength (λ)
c = f *λ
 Higher frequency means higher energy photons
 The higher the energy photon the more penetrating is the
radiation
What is Wireless Communication ?

Modulation
Introduction to
GSM
You may be ask, when the baseband signal can be transmitted directly
why to use the modulation ?
The answer is that the baseband transmission has many limitations which
can be overcome using modulation .
What is Modulation ?
Modulation is a process through which audio, video, image or text
information is added to an electrical or optical carrier signal to be transmitted
over a telecommunication or electronic medium.

Modulation
Introduction to
GSM
 Reduction in the height of antenna
 Avoids mixing of signals
 Increases the range of communication
 Multiplexing is possible
 Improves quality of reception
Advantages of Modulation ?

Introduction to
GSM
For the transmission of radio signals, the antenna height must be multiple of λ
/4 ,
where λ is the wavelength λ=c/f
The minimum antenna height required to transmit a baseband signal of f = 10
kHz
is calculated as follows :
So ,The antenna of this height is practically impossible to install .
Now, let us consider a modulated signal at f = 1 MHz .
The minimum antenna height is given by,
This antenna can be easily installed practically .
Thus, modulation reduces the height of the antenna .
 Reduction in the height of antenna
Modulation

Introduction to
GSM
 Avoids mixing of signals
If the baseband sound signals are transmitted without using the
modulation by more than one transmitter, then all the signals will be
in the same frequency range i.e. 0 to 20 kHz . Therefore, all the signals get
mixed together and a receiver can not separate them from each other .
 Increase the Range of Communication
The frequency of baseband signal is low, and the low frequency
signals can not travel long distance when they are transmitted . They
get attenuated .
The attenuation reduces with increase in frequency of the
transmitted signal, and they travel longer distance .
modulation process increases the frequency of the signal to be
transmitted .
Modulation

Introduction to
GSM
 Multiplexing is possible
Multiplexing is a process in which two or more signals can be
transmitted over the same channel simultaneously . This is possible only with
modulation.
The multiplexing allows the same channel to be used by many signals
. Hence, many TV channels can use the same frequency range,
without getting mixed with each other or different frequency signals can be
transmitted at the same time .
 Improves Quality of Reception
With frequency modulation (FM) and the digital communication
techniques such as PCM, the effect of noise is reduced to a great extent . This
improves quality of reception .
Modulation

Analog-to-analog modulation
Introduction to
GSM
Modulation

Amplitude modulation ( AM )
The total bandwidth required for AM can be determined from
the bandwidth of the audio signal: BWt = 2 x BWm.
Introduction to
GSM
Modulation

AM band allocation
 Bandwidth of an audio signal is 5KHz. An AM radio station needs at least a
minimum bandwidth of 10 KHz.
 AM stations are allowed carrier freq between 530 and 1700 KHz.
 If One station uses 1100 KHz the next one uses 1110 KHz
Introduction to
GSM
Modulation

Frequency modulation ( FM )
The bandwidth of a stereo audio signal is usually 15 KHz. Therefore, an FM station
needs at least a bandwidth of 150 KHz. The minimum bandwidth is at least 200 KHz
(0.2 MHz).
Introduction to
GSM
Modulation

FM band allocation
Introduction to
GSM
Modulation

Phase modulation ( PM )
Introduction to
GSM
Modulation

Introduction to
GSM
Modulation
Digatalization

Introduction to
GSM
Modulation
Digatalization
What sampling rate for a signal with a bandwidth of 10KHz (1000 to
11,000 Hz) ?
The sampling rate must be twice the highest frequency in the signal:
Sampling rate = 2 x (11,000) = 22,000 samples/s
2. We want to digitize the human voice. What is the bit rate, assuming 8
bits per sample?
The human voice normally contains frequencies from 0 to 4000 Hz
.
Sampling rate = 4000 x 2 = 8000 samples/s
Bit rate = sampling rate x number of bits per sample
= 8000 x 8 = 64,000 bps = 64 Kbps

Digital-to-analog modulation
Types of digital-to-analog modulation
Introduction to
GSM
Modulation

Aspects to digital-to Analog conversion
 Bit Rate / Baud Rate
 Bit rate is the number of bits per second. Baud rate is the number
of signal units per second. Baud rate is less than or equal to the bit
rate.
 Bit rate is important in computer efficiency
 Baud rate is important in data transmission.
 Baud rate determines the bandwidth required to send signal
 Baud rate = ( bit rate / # bits per signal unit )
 An analog signal carries 4 bits in each signal unit. If 1000 signal
units are sent per second, find the baud rate and the bit rate
 Baud rate = 1000 bauds per second (baud/s) Bit rate = 1000 x 4 = 4000
bps
 The bit rate of a signal is 3000. If each signal unit carries 6 bits,
what is the baud rate?
 Baud rate = 3000/6 = 500 bauds/sec
Introduction to
GSM
Modulation

Amplitude Shift Keying (ASK)
 The strength of the carrier signal is varied to represent binary 1 and 0.
 Frequency and phase remains the same.
 Highly susceptible to noise interference.
 Used up to 1200 bps on voice grade lines, and on optical fiber.


 

00
1)2cos(
)(
binary
binarytfA
ts c
On/Off keying
Introduction to
GSM
Modulation

Relationship between baud rate and bandwidth in ASK
 Find the minimum bandwidth for an ASK signal transmitting at
2000 bps. The transmission mode is half-duplex.
 In ASK the baud rate and bit rate are the same. The baud rate is therefore
2000. An ASK signal requires a minimum bandwidth equal to its baud rate.
Therefore, the minimum bandwidth is 2000 Hz.
BW .. is the bandwidth;
N .. baud is the baud rate
fc .. carrier frequency
d .. modulation Factor
BW = (1 + d) * Nd
Introduction to
GSM
Modulation

Frequency Shift Keying ( FSK )
 Frequency of the carrier is varied to represent digital data
(binary 0/1)
 Peak amplitude and phase remain constant.
 Avoid noise interference by looking at frequencies (change of a
signal) and ignoring amplitudes.
 Limitations of FSK is the physical capabilities of the carrier.
 f1 and f2 equally offset by equal opposite amounts to the carrier
freq.
 In MFSK more than 2 freq are used, each signal element
represents more than one bit






0)2cos(
1)2cos(
)(
2
1
binarytfA
binarytfA
ts
Introduction to
GSM
Modulation

Relationship between baud rate and bandwidth in FSK
 FSK shifts between two carrier frequencies
 FSK spectrum = combination of two ASK spectra centered on fc1
and fc0.
 BW = fc1-fc0 + Nbaud Nbaud >> Baud Rate
Introduction to
GSM
Modulation

FSK Examples (cont.)
 What is the maximum bit rates for an FSK signal if the bandwidth of the medium
is 12,000 Hz and the difference between the two carriers is 2000 Hz.
Transmission is in full-duplex mode.
 Because the transmission is full duplex, only 6000 Hz is allocated for each
direction.
BW = baud rate + fc1 - fc0
Baud rate = BW - (fc1 - fc0 ) = 6000 - 2000 = 4000
But because the baud rate is the same as the bit rate,
 What is the Find the minimum bandwidth for an FSK signal transmitting
at 2000 bps. Transmission is in half-duplex mode, and the carriers are
separated by 3000 Hz.
 Because For FSK
BW = baud rate + fc1 - fc0
BW = bit rate + fc1 - fc0 = 2000 + 3000 = 5000 Hz
Introduction to
GSM
Modulation

Phase Shift Keying
 Phase of the carrier is varied to
represent digital data (binary 0 or
1)
 Amplitude and frequency
remains constant.
 If phase 0 deg to represent 0,
180 deg to represent 1. (2-PSK)
 PSK is not susceptible to noise
degradation that affects ASK or
bandwidth limitations of FSK
Introduction to
GSM
Modulation

4-PSK (QPSK)
method
Introduction to
GSM
Modulation

8-PSK
 We can extend, by varying the the signal by shifts of 45 deg
 With 8 = 23 different phases, each phase can represents 3 bits (tribit).
Introduction to
GSM
Modulation

Relationship between baud rate and bandwidth in PSK
 Bandwith similar to ASK, but data rate can 2 or more times greater.
 What is the bandwidth for a 4-PSK signal transmitting at 2000 bps. Transmission
is in half-duplex mode.
 For PSK the baud rate is the same as the bandwidth, which means the baud rate is
5000. But in 8-PSK the bit rate is 3 times the baud rate, so the bit rate is 15,000 bps.
 Given a bandwidth of 5000 Hz for an 8-PSK signal, what are the baud rate and
bit rate?
 For PSK the baud rate is the same as the bandwidth, which means the baud rate is
5000. But in 8-PSK the bit rate is 3 times the baud rate, so the bit rate is 15,000 bps.
Introduction to
GSM
Modulation

Quadrature Amplitude Modulation
 PSK is limited by the ability of the equipment to distinguish between small
differences in phases.
 Limits the potential data rate.
 Quadrature amplitude modulation is a combination of ASK and PSK so that a
maximum contrast between each signal unit (bit, dibit, tribit, and so on) is
achieved.
 We can have x variations in phase and y variations of amplitude
 x • y possible variation (greater data rates)
 Numerous variations. (4-QAM, 8-QAM)
# of phase shifts > # of amplitude shifts
Introduction to
GSM
Modulation

8-QAM and 16-QAM
First example handles noise best
Because of ratio of phases to amplitudes
ITU-T recommendation.
Second example, recommendation of OSI.
not all possibilities are used, to increase
readability of signal, measurable differences
between shifts are increased
Introduction to
GSM
Modulation

Bit Baud
comparison
• Assuming a FSK signal over
voice-grade phone line can send
1200 bps, it requires 1200 signal
units to send 1200 bits (each
frequency shift represents one bit,
baud rate 1200)
• Assuming 8-QAM, baud rate is
only 400 to achieve same data
rate.
Modulation Units
Bits/Ba
ud
Baud
rate
Bit
Rate
ASK, FSK, 2-PSK Bit 1 N N
4-PSK, 4-QAM Dibit 2 N 2N
8-PSK, 8-QAM Tribit 3 N 3N
16-QAM Quadbit 4 N 4N
32-QAM Pentabit 5 N 5N
64-QAM Hexabit 6 N 6N
128-QAM Septabit 7 N 7N
256-QAM Octabit 8 N 8N
Introduction to
GSM
Modulation

 A constellation diagram consists of eight equally spaced points on a
circle. If the bit rate is 4800 bps, what is the baud rate?
 The constellation indicates 8-PSK with the points 45 degrees apart. Since
23 = 8, 3 bits are transmitted with each signal unit. Therefore, the
baud rate is 4800 / 3 = 1600 baud
 What is the bit rate for a 1000-baud 16-QAM signal.
 A 16-QAM signal has 4 bits per signal unit since log216 = 4. Thus, (1000)(4)
= 4000 bps
 Compute the baud rate for a 72,000-bps 64-QAM signal.
 A 64-QAM signal has 6 bits per signal unit since log2 64 = 6.
 Therefore, 72000 / 6 = 12,000 baud
Bit Baud comparison (examples)
Introduction to
GSM
Modulation

Wireless Challenges
 Radio Communication applied to mobile telephony has the
following limitations :
 Powerful transmitters & huge equipment are required.
 Capacity is limited to the frequency band allocated.
 So, in order to get rid of these shortages the cellular system is
being used in the mobile communications.
Introduction to
GSM
Wireless
Challenges

Wireless Challenges
 The Area to be covered is divided into
small cells.
So,
 Low Transmission power.
 Smaller equipment size.
 Capacity of the system can be
increased by reuse the frequency.
Introduction to
GSM
Wireless
Challenges

Cell Geometry
The simplest shape is the circular shape
So, why not using this shape ?
Dead Spots
Introduction to
GSM
Wireless
Challenges

Cell Geometry
So, what are the possible shape and what is the used shape ?
RR
R

Introduction to
GSM
Wireless
Challenges

Types of Antennas
 Two types of antennas are being commonly used; the Omni-directional
antenna and the directional antenna.
 A site is the place where the transmission equipment is placed; this
maybe on the top of the buildings or on the top of the advertising.
 In case of directional type antenna; one site is corresponding to 3 cells
and in case of Omni-directional antenna one site is corresponding to
one cell and one antenna.
Sectorial CellsOmni-Directional Cell
Introduction to
GSM
Wireless
Challenges

Frequency Reuse
 As the frequency resources that are given to an operator
”Vodafone or Orange ” are limited; the frequency used in a cell
should be reused again in a cell that is away from this cell in
order to increase the capacity of the system. Moreover, for a
certain frequency 8 time slots are used for more capacity of the
system.
 So, the access technique that is used is FDMA+TDMA with
frequency reused.
Introduction to
GSM
Wireless
Challenges

Cellular System Concepts
 Reuse Pattern(Cluster):
 Cells are grouped into Clusters
 Available Band is distributed among the
cells of the cluster
 “N” is the number of cells in a cluster .
 Each frequency is reused after the same
distance “D”
 Reuse Plan = (D/R)²= 3N
Where R is the cell radius
5
2
3
4
7
1
6
5
N=7 Cell ClusterN=7 Cell Cluster
7 Cell Reuse Plan7 Cell Reuse Plan
2
3
4
7
1
6
5
2
3
4
7
1
6
5
2
3
4
7
1
6
5
2
3
4
7
1
6
5
D
Introduction to
GSM
Wireless
Challenges

GSM: Global System for Mobile Communication Explained

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