I
n the past, messages have been carried by runners, carrier pigeons, drum
beats, and torches. These schemes were adequate for the distance and “data
rates” of the age. In most parts of the world, these modes of communication
have been superseded by electrical communication systems, when can
transmit signals over much longer distances (even to distant planets and
galaxies) and at the speed of light.
Electrical communication is reliable and economical; communication
technology is alleviating the energy crisis by trading information processing
for a more rational use of energy resources. Some example: Important
discussions now mostly communicated face to face in meeting or
conferences, often requiring travel, are increasingly using “teleconferring.”
Similarly, teleshopping and telebanking will provide services by electronic
communication, and newspaper may be replaced by electronic services.
 The components of a communication system are as follows:
 The source originates a message, such as human voice, television
picture, a teletype message, or data.
 Input transducer which converts the original message into an
electrical waveform referred to as the baseband signal or message
signal.
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Chapter 2
 The transmitter: It modifies the baseband signal for efficient
transmission.
 The channel is a medium such as wire, coaxial cable, a waveguide, an
optical fiber, or radio link through which the transmitter output is sent.
 The receiver reprocesses the signal received from the channel by
undoing the signal modifications made at the transmitter and the
channel.
 The output transducer which converts the electrical signal to its
original form.
 The destination is the unit to which the message is communicated.
A channel acts partly as a filter to attenuate the signal and distort its
waveform. The signal attenuation increase with the length of the channel.
The waveform is distorted because of different amounts of attenuation and
phase shift suffered by different frequency components of the signal.
The signal is not only distorted by the channel, but it is also contaminated
along the path by undesirable signals lumped under the broad term noise,
which are random and unpredictable signals from causes external and
internal.
External noise include interference from signals transmitted on nearby
channels, human-made noise generated by faulty contact switches for
electrical equipment, automobile ignition radiation, fluorescent lights or
natural noise from lightning. With proper care, external noise can be
minimized or even eliminated.
Internal noise results from thermal motion of electrons in conductors,
random emission, and diffusion or recombination of charged carriers in
2
Chapter 2
electronic devices. With proper care, external noise can be minimized but
can never eliminate it.
The signal-to-noise ratio (SNR) is defined as the ratio of signal power to
noise power. The SNR is continuously decreasing along the length of the
channel.
Communication model
Communications can divide into two categories:
 Wired communication.
 Wireless communication.
Wired communication:
It is point-to-point communication.
Disadvantage:
1- Difficult to conducting the cable under the ground.
2- Big loss in long distance.
3- Low bandwidth.
Wireless communication:
Advantage:
1- User convenience.
2- Increased mobility of the workforce and society more generally.
3- High bandwidth.
3
Chapter 2
4- Roaming capabilities.
5- Very low installation fees.
6- Lower costs for certain fixed
applications.
7- More fungible investment.
Analog and digital systems
Analog system:
Analog messages are characterized by
data whose values vary over a
continuous range.
For example, the temperature or the
atmospheric pressure of a certain
location can vary over a continuous
range and can assume an infinite
number of possible values.
It is very hard to separate noise from an analog signal than from a digital
signal.
Digital system:
Digital messages are constructed with a
finite number of symbols. For example,
printed language consists of 26 letters, 10
numbers, a space, and several punctuation
marks. Thus, a text is a digital message
constructed from about 50 symbols.
Human speech is also a digital message, because it is made up from a
finite vocabulary in language.
Advantage:
1- Despite noise in this digital signal, you can still discern a high voltage
from low voltage.
4
Chapter 2
2- Easy standardization, flexibility in transmitter and receiver.
3- Viability of regenerative repeaters in digital communication.
4- Easy to insert control bits to control information.
5- Storage and retrieval fast.
6- Encryption is easy.
7- Better switching interface.
8- Digital communication system is more rugged than an analog
communication system in the sense that it can better withstand noise and
distortion.
9-Digital circuit is more simple than analog.
Disadvantage:
1- Overall system is more complex because need analog to digital and
digital to analog conversion.
2- Transmission bandwidth increase due to Nyquist rate (sampling
theory).
Pulse code modulation (PCM)
PCM is the most useful and widely used of all the pulse modulations
mentioned. Basically, PCM is a method of converting an analog signal into a
digital signal (A/D conversion).An analog signal is characterized by the fact
that its amplitude can take on any value over a continuous range. This means
that it can take on an infinite number of values. On the other hand, digital
signal amplitude can take on only a finite number of vales. An analog signal
can be converted into a digital signal by means of sampling and quantizing,
that is, rounding off its value to one of the closest permissible number (or
quantized levels). Finally, we say that PCM conversion used to convert
speech (human) signal to digital form.
5
Chapter 2
PCM process has four steps:
1- Filter:
Frequencies below 300 HZ and above 3400 HZ are filtered from the analog
signal. Filter for below 300 HZ because it is the range that contain noise
while the noise appear in low frequencies and filter above 3400 HZ because
greater than this will increase the number of bits that due to high cost.
So the actual bandwidth of the filtered signal is ( 3400-300 = 3100 HZ ), It is
often referred to as 4 KHZ, also this range have most power concentrated
,articulation (understanding) and the pitch freq located in this range.
2- Sampling:
If the highest frequency in the signal spectrum is B (in hertz), the signal can
be reconstructed from its samples, taken at a rate not less than 2B samples
per second. This means that in order to transmit the information in a
continuous-time signal, we need only transmit its samples.
Unfortunately, the sample values are still not digital because they lie in a
continuous range and can take on any one of the infinite values in the range
3-Quantization:
It defines the amplitude of each sample to decimal values; we quantize the
amplitude of the incoming samples to one of 255 amplitudes.
Amplitudes of the signal m(t) lie in the range (-mp,mp), which is partitioned
into L intervals, each of amplitude ∆v = 2mp / L .Each sample amplitude is
approximated to the midpoint of the interval in which the sample value falls.
Each sample is now approximated to one of the L numbers. The information
6
Chapter 2
is thus digitized. We can improve the accuracy of the quantized signal to
any desired degree by increasing the number of levels L.
3-Encoding:
It converts the decimal value (the output of quantization stage) to binary
form.
Represented each sample in 8 bits
So, Bit rate = 8000 sample/sec * 8 bits/word = 64 kb/sec
So, PCM process require 64 kb/sec channel to encode a 4 kHz audio input
signal.
Advantage of PCM:
 Can be modulated.
 High security.
 No distortion.
 No interference.
 No cross talk.
Important definitions
 Bandwidth:
The bandwidth of a channel is the range of frequencies that it can
transmit with reasonable fidelity. For example, if a channel can
transmit with reasonable fidelity a signal whose frequency
components occupy a range from 0 (dc) up to a maximum of 5000 HZ
(5 kHz), the channel bandwidth B is 5 kHz.
7
Chapter 2
 Bit rate:
-It is number of bits that pass a given point in telecommunication
network in a given amount of time, usually a second.
-Bit rate expression used only in digital telecommunication.
 Relation between bandwidth and bit rate:
B.W for baseband signal:
Bt base = (Bit rate / 2) (kHz)
for ideal Nyquist rate
Bt base = (Bit rate / 2) (1+α)
practical
Where α is Roll off factor (0< α <1)
For α =1
Bt base = Bit rate
B.W for passband signal = B.W for baseband signal *2
 Attenuation: Reduction in signal amplitude due to loss of energy.
 Distortion: Changing on the shape of the channel.
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Chapter 2
 Interference: Existence between two equal signal has equal
frequencies
 Noise: It is random signal.
 Cross talk: It is electrical coupling between paths.
Connections types
There are two principles for radio connection:
Simplex connection:
It is “one way street”
Example: Broadcast
television.
radio
and
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Chapter 2
Duplex connection:
Half duplex
Full duplex
Example: walkie talkie.
Duplex transmission
Frequency division duplex
(FDD)
Time division duplex
(TDD)
Separate from uplink frequency and Separate from uplink frequency and
downlink frequency by frequency. downlink frequency by time.
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Chapter 2
Multiplexing techniques
Frequency division multiplexing
(FDM)
One frequency for one user all time.
It is used for satellite.
Time division multiplexing
(TDM)
On one frequency, one time duration
for one user all time.
Access techniques
FDMA
TDMA
CDMA
If user need frequency If user need time slot in The difference between
take it, then release it.
frequency take it, then users by code.
release it.
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Chapter 2
Single cell system
- It is the first mobile telephone service.
- It supported “Duplex connection”.
- Analog system.
Disadvantage of early mobile (car)
telephone services:
 The equipment required was heavy,
bulky and expensive.
 Limited capacity.
 Low quality.
 No handover.
 Manual exchange.
 No switching between regions.
 Limited HF-freq because it had distributed among competitors such
as “The military, radio and television”.
Digital single cell
Because of new technology such as semiconductor, diodes, transistors,
integrated circuit … etc, we see digital single cell with new feature.
Advantage:





High capacity.
Lower system design cost.
Lower cost per subscriber.
Automatic switch
Increased high market penetration.
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Chapter 2
Cellular mobile radio systems
The breakthrough in capacity, which resulted in a significant increase in
the number of subscribers, was achieved with the introduction of the
Cellular Radio System in the late 1970s/early 1980s. The coverage of the
supply area of a mobile communication operator involves many radio
cells with cellular radio systems, in which the aforementioned limitation
of the available HF frequency range is neatly circumvented through the
repeated use of the HF channels.
Cell shape:
We put one frequency in one cell
or more than this by making
sectors.
35 km is the radius of one cell.
Cluster area:
When we reuse the frequency, then we
called this area (cluster).
The distance between original frequency
and the reuse of this frequency is reuse
distance.
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Chapter 2
Mobile system’s generations
First generation (1G):
Takes place via analog radio interface.
These systems were tested in many countries in the end of the 70s.
FDMA transmission technology.
FM modulation.
Networks that uses these systems:
In 1979:
In the USA  AMPS (Advanced Mobile Phone Service).
In Japan  NTT-MTS (Nippon Telegraph & Telephone co).
In the early 80s:
In Scandinavia  NMT (Nordic Mobile Telephone).
In England  TACS (Total Access Communication
System).
In Germany  C450 system.
Disadvantage of first generation:
1. Low capacity.
2. Low quality.
3. Incompatibility (Not one specification).
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Chapter 2
Second generation (2G):
A further and very significant innovation in mobile radio
communications took place with the introduction of the second
generation cellular mobile radio system (e.g. GSM).
Networks that uses these systems:
European  GSM  9.6 kbps.
American  CDMA IS – 95A  9.6 kbps.
Advantage of second generation:
1. Transmission via radio interface is now digital.
2. The increase in capacity.
3. The increase in subscribers.
4. Lighter.
5. Less expensive equipment with a wide range of possibilities for
use.
6. One specification.
7. Low power consumption.
8. TDMA transmission technology.
Second and half generation (2.5G):
It applied circuit switching and packet switching in the same time.
Networks that uses these systems:
European  GPRS {General Packed Radio Services}  171 kbps
American  CDMA IS-95B  144 kbps
Third generation (3G):
European  UMTS {Universal Mobile Telecommunication system}.
 2Mbps
American  CDMA 1X-EVDO  2Mbps
CDMA 1X-EVDV (4-5) Mbps
CDMA 2000 153.6 Kbps extended to 307.2 kbps
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Chapter 2
Third and half generation (3.5G):
HSDPA  {High Speed Data Packet Access}  14 Mbps
Forth generation (4G):
It will be used for voice over internet protocol.
Systems in between systems:
HSCSD  {High Speed Circuit Switching Data}.
Bit rate:  57.6 kbps.
It considers evolution in circuit switching, it increase the bit rate for
users by increasing a number of time slots for one user and it also
working by circuit switching.
It had invented between GSM GPRS.
It used for continuous data stream.
It is dedicated for one user (point to point).
Time slot up to 8  practical 4 time slot.
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Chapter 2
EDGE  {Enhanced Digital GPRS Evolution}
Bit rate for one time slot is 69.2 Kbps.
Theoretically up to 553.6 kbps
It is using 8PSK (Phase shift key) Modulation.
The difference between circuit switching and packet switching







circuit switching
Channel for one user
One path from transmitter and
receiver.
Fixed bit rate.
Cost per time.
Smallest error.
Cannot open many
applications in the same time.
Used for voice transmission
(Real time application)







17
packet switching
Channel for many user.
Multiple paths from
transmitter and receiver.
Variable bit rate.
Cost per volume.
Larger error
Can open many applications in
the same time.
Used for data transmission.
Chapter 2
Global System for Mobile Communication
History of GSM:
During the early 1980s, analog cellular telephone systems were experiencing
rapid growth in Europe, particularly in Scandinavia and the United
Kingdom, but also in France and Germany. Each country developed its own
system, which was incompatible with everyone else's in equipment and
operation. This was an undesirable situation, because not only was the
mobile equipment limited to operation within national boundaries, which in
a unified Europe were increasingly unimportant, but there was also a very
limited market for each type of equipment, so economies of scale and the
subsequent savings could not be realized.
The Europeans realized this early on, and in 1982 the Conference of
European Posts and Telegraphs (CEPT) formed a study group called the
Group Special Mobile (GSM) to study and develop a pan-European public
land mobile system. The proposed system had to meet certain criteria:







Good subjective speech quality
Low terminal and service cost
Support for international roaming
Ability to support handheld terminals
Support for range of new services and facilities
Spectral efficiency
ISDN compatibility
In 1989, GSM responsibility was transferred to the European
Telecommunication Standards Institute (ETSI), and phase I of the GSM
specifications were published in 1990. Commercial service was started in
mid-1991, and by 1993 there were 36 GSM networks in 22 countries.
Although standardized in Europe, GSM is not only a European standard.
Over 200 GSM networks (including DCS1800 and PCS1900) are
operational in 110 countries around the world. In the beginning of 1994,
there were 1.3 million subscribers worldwide, which had grown to more than
55 million by October 1997. With North America making a delayed entry
into the GSM field with a derivative of GSM called PCS1900, GSM systems
18
Chapter 2
exist on every continent, and the acronym GSM now aptly stands for Global
System for Mobile communications.
The developers of GSM chose an unproven (at the time) digital system, as
opposed to the then-standard analog cellular systems like AMPS in the
United States and TACS in the United Kingdom. They had faith that
advancements in compression algorithms and digital signal processors
would allow the fulfillment of the original criteria and the continual
improvement of the system in terms of quality and cost. The over 8000
pages of GSM recommendations try to allow flexibility and competitive
innovation among suppliers, but provide enough standardization to
guarantee proper interworking between the components of the system. This
is done by providing functional and interface descriptions for each of the
functional entities defined in the system.
Milestone of GSM standard:
 GSM 900 uses the circa 900 MHz band
 The frequency band used is 890-915MHz (mobile transmit) and 935960MHz (base transmit).
 The frequency band from 915 MHz to 935 MHz is called guard band
that uses to separate between uplink frequency and downlink
frequency.
 To allow maximum number of users access, each band is subdivided
into 124 carrier frequencies spaced 200 kHz apart, using FDMA
techniques.
 Each of these carrier frequencies is further subdivided into time slots
using TDMA.
 TDMA (Time Division Multiple Access) has 8 time slots (i.e.
transmitting for one eighth of the time).
 TDMA provides each user with the carrier frequency for
approximately 0.577ms.
 Frequency hopping May be optionally employed in order to avoid
'dead spots' and to minimize interference from other signals. The
hopping rate is one hop per TDMA frame (4.6µs), or 217 hops per
second.
 The method of modulation used is Gaussian Minimum Shift Keying
(GMSK).
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Chapter 2
 Phase and frequency synchronization must allow for Doppler shift for
vehicle speeds up to 250km/h as well as for frequency standard drift,
and timing advance to compensate for propagation delay due to round
trips for paths, in cells up to 35km radius.
The GSM phases:
In The Late 1980s, The Groups Involved in Developing the GSM
standard realized that within the given time frame they could not
complete the specification for the entire range of GSM services and
features as originally planned. Because of this, it was decided that GSM
would be realized in Phases, with phase 1 Consisting of a limited set of
services. Each New phase builds on the services offered by exiting
phases.
Phase 1:






Voice telephone.
Roaming.
Basic fax/data service up to 9.6 kbps.
Call forwarding.
Call barring.
Short message service.
Phase 2:







Advice of charge.
Calling line identification.
Call waiting.
Call hold.
Conference calling.
Post talk.
Closed user group.
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Chapter 2
Phase 2+:
 Multiple service profile.
 Private numbering plans.
 Inter working with GSM 1800, GSM 1900 and Digital Enhanced
Cordless Telecommunication (DECT) Standard.
GSM network architecture
Geographic network areas:
The GSM service area is the collection of PLMNs.
Public LAND Mobile Network (PLMN):
 A public land Mobile Network is a group of MSC/VLR areas.
 The largest geographical service area served by one network
operator.
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Chapter 2
MSC/VLR Area:
 A Mobile Switching center/ vistor location register (MSC/VLR)
service area is made up of several LA’s and is covered by on MSC.
Location Area:
 A location Area is a group of cells.
 The largest area in which a mobile station may roam without
updating location.
 Served by one or more BSCs, but only one MSC.
 The area within which paging messages are sent out to all mobile
subscribers.
Cell Area:
 A cell is the smallest radio coverage area in the network.
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Chapter 2
Types of cell:
 According to cell coverage:
Omni cells that cover 360° and Sector cells that cover 120°.
 According to cell classification:
 Macro cell:
- It is a normal cell.
- Its ranges from 1 Km to several.
 Micro cell:
- Cells for the support of restricted areas with very high
mobile user density, (e.g.): shopping malls, railway,
subway station and airport terminal.
- Its ranges from 100 meters to approximately 1km.
 Pico cell:
- Cells for the support of indoor application (e.g.): offices.
- Its ranges should be several 10m.
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Chapter 2
 Umbrella cell:
- A cell that can give coverage to areas that don’t have
radio coverage.
- Usually the antenna is as high as possible that cell can fill
in the gaps left by the other cells.
The GSM Network Structure
Networks provisioned by an operator on a national level for public
mobile communication are called Public Land Mobile Networks PLMN.
A Public Land Mobile Network is divided into mobile and fixed
network components. They are connected via air interfaces.
24
Chapter 2
Mobile Station (MS):
The Mobile Station MS consist of:
 ME: Mobile Equipment.
 SIM: Subscriber Identification Module.
o The data in SIM card:
 Subscriber license.
 Subscriber key (ki, kc) 
Physical implemented.
 Personal identity.
 Personal phone book.
 Algorithm (A3, A8).
 SIM tool kits.
Base Station Subsystem (BSS):
Do all radio functions in the system such as coverage and responsible
about some switching functions.
It consists of four parts:




Base Transceiver Station (BTS).
Base Station Controller (BSC).
Transcoding and Rate Adoptions Unit (TRAU).
Local Maintenance Terminal (LMT).
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Chapter 2
 Base Transceiver Station (BTS):
BTS do a lot of functions such as:
o Do all radio coverage of the cell.
o Modulation and demodulation
functions.
o Transmit and receive.
o Ciphering and deciphering.
o Channel coding.
o Burst block formation.
o Power control.
o Timing advance.
o Frequency hopping.
o Synchronization.
Function of TRX:
1- Demodulation from microwave to base band signal.
2- Ciphering.
3- Control synchronization and time.
Function of HF-TX:
It converts base band signal to 900 MHz.
Function of HF-RX:
It converts 900 MHz to base band signal.
Function of O&M:
26
Chapter 2
Operation & Maintenance used for trying to repair error.
 Base Station Controller (BSC):
BSC do the following functions:
o Allocation of traffic channel.
o Release of traffic channel.
o Handover control.
o Supervision and control all BTSs that connected to it.
BSC’s block diagram
The terminal control element between BTS and Switch Matrix represent a
Microwave dishes.
27
Chapter 2
One terminal control element between MSC and Switch Matrix represent an
interface for one MSC and the other used for redundancy and traffic sharing.
The block of switch matrix used for switch function, it is connected to all
BTS that connected with internal BSC.
The DB block represents a data base for all BTS’s frequencies and power
and have information about all BTS.
The Central Module block uses to try to repair error, if not, it send a report
for “Network Management System”
Network Switching Subsystem (NSS):
The NSS forms the interface between the radio subsystem and the
public fixed networks (PSTN, ISDN, and PDN). It executes all
signaling functions for setting up connections from and to mobile
subscribers.
 Mobile Switching center (MSC):
Functions:
 Switching between users.
 Responsible for pilling.
 Responsible for originating call from mobile to mobile,
mobile to fixed phone and fixed phone to mobile.
 Control and supervision to all elements connected through
MSC.
28
Chapter 2
 Legal interception.
 Copying with abnormal signaling conditions (e.g.) loss of
signaling information.
 Routing functionality (Path selection).
 Evolution of available signaling information for destination
routing.
 Control of priority calls.
 Traffic measurement and traffic observation.
 Home Location Register (HLR):

The Home Location Register HLR is the main data base of the
mobile subscriber.
 The HLR is always associated with an Authentication Center
AC.
Functions:
 It sends all necessary data to VLR.
 Routing information to the GMSC.
 Transmit the triples from authentication center to VLR.
 Visitor Location Register (VLR):
The Visitor Location Register VLR is responsible to aid the MSC
with information on the subscriber, which are temporarily in the
MSC service area.
o Examples of subscriber data in the VLR:
 MSISDN: Mobile Subscriber ISDN number.
 IMSI: International Mobile Subscriber Identity.
 TMSI: Temporary Mobile Subscriber Identity.
 HON: Handover Number.
 MSRN: Mobile Station Roaming Number.
 Triples (Authorization parameters).
29
Chapter 2
VLR and HLR have a subscription information that mentioned earlier and
location information.
Location information:
If any user wants to call another one, to make this, it takes the following path
MS send to BTS, BTS send to BSC, BSC send to MSC, MSC send to VLR
and ask it, do you have this user? If not, send to HLR and ask it about the
MSC/VLR area.
 Authentication Center (AUC):
An Authentication Center AC contains all necessary means, keys
and algorithms for the creation of security related authorization
parameters, the so-called Triples. The Triples are created on VLR
request and delivered via HLR to the VLR. An AC is always
associated with an HLR.
 Central information contained in the AC are:
o IMSI: International Mobile Subscriber Identity
o Ki: Individual Key (top secret mobile subscriber identity)
o Algorithms for authentication and encryption: A3, A8.
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Chapter 2
 Equipment Identity Register (EIR):
The Equipment Identity Register EIR contains the Mobile
Equipment identity: the International Mobile Equipment Identity
IMEI. An IMEI clearly identifies a unique Mobile Equipment ME
and contains information about the place of manufacture, device
type and the serial number of the equipment.
- EIR contains 3 lists:
o White List (For Valid MS equipment).
o Black List (for denied service MS).
o Gray List (for mal-performance MS)
(e.g.) faulty software.
 Short Message Center (SMC):
It makes coding techniques for short message.
 Gateway Mobile Switching Center (GMSC):
It is the connection between mobile system and other networks.
 Interworking function (IWF):
It used for rate adaption and protocol conversion.
 Echo canceller (EC):
It converts 2-wire system to 4-wire system and otherwise.
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Chapter 2
Operation Subsystem (OSS):
The operation and maintenance center for radio part (BSS).
The operation and maintenance center for switching parts.
Radio Interface
The air or radio interface, i.e. the connection between the MS and fixed
network components.
There are advantage and disadvantage for radio transmission
 Advantage:
The main advantage of mobile communications is the unrestricted
mobility which can be achieved only via a radio interface.
 Disadvantage:
A mobile communication environment suffers from many problems
related to radio transmission, which may cause unacceptable degradation
of the service quality.
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Chapter 2
Transmission Problem:
o Path loss.
o Fading.
o Time delay.
o Interference.
o Time dispersion.
o Channel impairments.
o Bandwidth limitations.
 Path Loss Problem:
 Radio signal strength falls as some power  of the distance, called the
power-distance gradient or path-loss gradient.
 If the transmitted power is Pt, after a distance d, the signal strength
will be proportional to Ptd-.
Loss = 32.4 + 20 Log f (Mhz) + 20 Log d(Km)
Signal power (dbm)
BTS
Distance
MS
Solution:
Increase The Transmitted Power But By Limits To Compromise The
Frequency Reuse.
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Chapter 2
 Fading problem:
Fading means that the signal strength received fluctuates around a mean
value while changing the mobile position.
 Slow Fading = Log Normal Fading = Shadowing :The reason for shadowing is the presence of obstacles like large
buildings or hills in the path between the site and the mobile. The
distance between fading dips (minimum values of signal strength) is
from 10 to 20 meters.
 Solution:
To overcome the fading
problems, the fading
margin should be large
enough that the lowest
fading dip is still higher
than
the
receiver
sensitivity.
 Fast Fading = Rayleigh Fading = Multipath Fading:This occurs when a signal takes more than one path between the MS
and BTS antennas, The Signal is reflected off buildings, for
example, and is received from several different indirect paths.
Rayleigh fading occurs when the obstacles are close to the receiving
antenna. The received signal is the sum of many identical signals
that differ only in phase (and to some extent amplitude).
34
Chapter 2
 Solution:
1. Antenna Diversity:The cell transceiver will use two receiving antennas instead of
one. A distance of about 5 meters will separate them, and they
will receive radio signals independently, so they will be
affected differently by the fading dips and the better signal
received will be selected.
2. Frequency Hopping :The fading effect may not be the same for all of the
frequencies, so the time slot of the subscriber will jump or
“hop” between the frequencies of the cell when it is repeated in
each TDMA frame. If only one of the frequencies is affected
severely by fading, a small fraction of the signal will be lost.
The hopping sequence might be cyclic or random.
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Chapter 2
 Time delay problem:
Each MS on a call is allocated a time slot on a TDMA frame. This is an
amount of time during which the MS transmits information to the BTS.
The information must also arrive at the BTS within that time slot. The
time alignment problem occurs when part of the information transmitted
by an MS does not arrive within the allocated time slot. Instead, that part
may arrive during the next time slot, and may interfere with information
from another MS using that other time slot. A large distance between the
MS and the BTS causes time alignment. Effectively, the signal cannot
travel over the large distance within the given time.
 Solution:
Timing Advance:
If the Mobile Station is moving away from the base station during
a call, it will have to send the burst in advance, in relation to
synchronization time, in order for it to arrive in the given time slot
at the base station. The base station will therefore continuously
send a value between 0 and 63, telling the MS how many bit-times
(3.7ms) ahead of synchronization time it should transmit the burst.
This is one of the parameters, which puts a limit on the size of a
cell.
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Chapter 2
 Interference problem:
 Co-channel interference:
It is interference between carrier in
cluster and the same carrier in the
next cluster.
 Solution:
1-Sectorization of Cells, Then n Can be Reduced to be equal 1.
2-Increase carrier to interference ratio, it must greater than 9 db.
C/I = 10 log [(1/n). (D/R) ]
Where:
D: Reuse distance.
R: Radius of cell.
n: Number of interfering cells
D/R: Reuse ratio.
 : Propagation factor.
Carrier to interference ratio is influenced by the following factors:
1- Location of Mobile Station.
2- BTS antenna type.
3- Local geography and type of local scatters.
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Chapter 2
 Adjacent Channel Interference:
Adjacent frequencies are frequencies shifted 200 kHz from the carrier
frequency (C), must be avoided in the same cell and preferably in
neighboring cells also. Although adjacent frequencies are at different
frequencies to the carrier frequency they can still cause interference
and quality problems.
 Solution:
1- Sectorization.
2- Carrier to adjacent ratio must be greater than -9 db.
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Chapter 2
 Time dispersion:
Time dispersion is another problem relating to multiple paths to the Rx
antenna of either an MS or BTS.
Time dispersion causes Inter-Symbol Interference (ISI) where
consecutive symbols (bits) interfere with each other making it difficult
for the receiver to determine which symbol is the correct one.
Where the sequences bits sent from BTS (1-2-3…), It reaches to the MS
by different paths, while MS processing bit 1 from path 1, it receive also
bit 1 from path 2, so time dispersion occurs.
One bit is transmitted every 3.7 μs. Radio waves travel at 3x10 8 m/s.
Therefore, one bit travels approximately 1 km within one bit period.
Thus, if the direct path is 1km and the indirect path is 3 km long, the first
bit transmitted will interfere with the 3rd bit transmitted.
 Solution:
1- Increase carrier to reflection ratio, it mean that increasing the
power of direct path .So MS choose the higher power.
2- Viterbi equalizer.
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Chapter 2
Viterbi equalizer:
Time dispersion is when transmitted signals being reflected by objects
far away from the RX antenna arrive at the antenna later than the
original signal. This can result in causing Inter-Symbol Interference,
ISI. The signals became spread out in time and adjacent symbols
interfere with each other. The receiver is unsure of the value of the
original signal transmitted.
Equalization is the process used to overcome time dispersion. The
equalizer can handle a limited number of reflections. The equalizer can
handle reflections delayed by about 15 ms, which is a signal path of
about 4.5 km.
The equalizer creates a model of the transmission channel and
calculates the most probable transmitted sequence. Data is transmitted
in bursts, which are placed within time slots. In the middle of the burst
a training sequence of a known pattern with good auto-correlation
properties. This training sequence is used by the equalizer to create the
channel model. This model changes all the time, but during one burst it
is considered to be constant. Rayleigh fading due to near region
reflections influences the reflected signal. This signal has an
independent fading pattern compared to the direct one and can be used
by the equalizer to enhance performance.
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Chapter 2
 Channel impairments problem:
 Channel Coding:
Channel Coding is used to detect and correct errors that have been
identified during the signal transmission process. These signal errors
are measured as a percentage of the total bits transmitted. The quality
of the transmitted signal is expressed in terms of Bit Error Ratio
(BER). BER is the percentage of faulty bits out of the total number of
bits transmitted.
Channel coding is performed inside the TRU for the downlink signal
and in the Mobile for the uplink signal. There are two types of channel
coders used: Block Coders and Convolutional Coders.
The speech coder delivers 20 ms slices of speech containing 260 bits
of speech information. The 260 bits of information is divided into:



50 very important bits
132 important bits
78 not so important bits
Three parity bits are added to the 50 bits (block coding). These 53 bits
together with the 132 important bits and 4 tail bits are convolutional
encoded to 378 bits (rate 1:2). The remaining bits are not coded.
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Chapter 2
 Interleaving:
Bit errors often occur in bursts of information sent over the Air
Interface. This is caused by long fading dips that affect several
consecutive bits. The Channel Coding process is only effective in
detecting and correcting single errors. A process called interleaving
solves this problem by separating the consecutive bits of information
in a message and transmitting them non-consecutively.
This helps overcome problems caused by bursts of noise. Through
interleaving single frames of information are spread out over several
bursts. In GSM two levels of interleaving are used.
 1st Level of Interleaving:The channel coder provides 456 bits for every 20 ms of speech. These are
interleaved, forming eight blocks of 57 bits each.
If one burst transmission is lost, there is a 25% BER for the entire 20 ms
of speech (2/8 = 25%).
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Chapter 2
 Second Level of Interleaving :In a normal burst there is space for 2 of the 57 bits blocks of information.
The second level of interleaving takes eight of the 57 bit blocks to be
transmitted and puts them in 8 different information bursts. Each
information burst will contain two 57 bit blocks of information from
different speech samples.
If only one level of interleaving is used, a loss of this burst results in a
total loss of 25%. This is too much for the channel decoder to correct. A
second level of interleaving can be introduced to further reduce the
possible BER to 12.5%.
Instead of sending two blocks of 57 bits from the same 20 ms of speech
within one burst, a block from one 20 ms and a block from next sample
of 20 ms are sent together
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Chapter 2
 Bandwidth limitation problem:
Because of The Limit B.W, Then In GSM we have different Radio
Transmission Process, Converting speech into the final bit stream that
will be transmitted over the air interface in GSM system goes through
more than one stage.
Because the bandwidth for GSM is 200 kbps, but we have 64kbps for
each user, 512kbps for 8 time slot.
So we need vocoding technique that take only parameter about a voice
and transmit it, but this make a decrease in a quality.
So we should increase bit rate by using analog to digital converter to
increase the quality.
o Analog to digital conversion:
1- Sampling:
Fs = 2B = 2*(3400-300)  8000 samples/sec.
2- Quantization:
We will use 8192 quantization level.
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Chapter 2
3- Coding:
L = 2n  so, 8192 = 2n  so n=13
104 kbps is the output bit rate for one subscriber per second on GSM
from A/D conversion.
o Segmentation:
Given that the speech organs are relatively slow in adapting to
changes, the filter parameters representing the speech organs are
approximately constant during 20 ms. For this reason, when coding
speech in GSM, a block of 20 ms is coded into one set of bits.
o Speech Coding:
The aim of speech coding in GSM is to send only information about
the original tone itself and about the filter.
Instead of using 13 bits per sample as in A/D conversion GSM speech
coding uses 260 bits.
Types of speech coder on GSM:
1- Hybrid coder.
2- Vocoder.
The output from speech coding is 13 kbps. This provides a speech
quality, which is acceptable for mobile telephony and comparable
with wire line PSTN phones.
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Chapter 2
o Ciphering:
Ciphering is a technique used in both GSM to minimize
eavesdropping. Ciphering involves taking a portion of the digitized
signal stream of l’s and 0’s and combing it with a random stream of
l’s and 0’s. This random stream is known only by the mobile station
and BTS. The result is a signal that is unintelligible to all mobile
stations, or anyone else, and can only be deciphered by the station
with the matching code. The A5/1 and A5/2 encryption algorithms are
available for use in GSM.
o Burst formatting:
Every transmission from an MS/BTS must include some extra
information such as the training sequence. The process of burst
formatting is to add these bits (along with some others such as tail bits) to
the basic speech/data being sent. This increases the overall bit-rate, but is
necessary to counteract problems encountered on the radio path.
o Modulation & Transmission:
The bits must then be sent over the air using a carrier frequency. GSM
uses the Gaussian minimum shift key (GMSK) modulation technique; it
is digital modulation to sent bits over air.
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Chapter 2
Types of channels
Physical channel
Logical channel
Physical channel:
A physical channel in GSM is defined by a frequency pair for UL/DL
and a Time Slot TS of the TDMA frame. The frequency bandwidth in
GSM is 200 kHz. A Time Slot TS has duration of 0.577 ms. 8 TS
form a TDMA frame; the duration of a TDMA frame is 4.615 ms.
Logical channel:
Many types of logical channels exists, each designed to carry a
different message to or from an MS. All information to and from an
MS must be formatted correctly.
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Chapter 2
o Logical channel:
The Logical Channels are divided into two categories, Control Channels
and Traffic Channels. The Control and Traffic Channels are further
subdivided; there are two types of Traffic Channels and three categories
of Control Channels with a total of nine different types.
 Traffic Channels:
Traffic Channels (TCH) carry either speech or data. There are two
types of traffic channels: Full-rate and half-rate. The TCH can be
located in any time slot on any frequency defined in the cell, except
for the first time slot (TS0) on the first carrier (C0). )
Full-Rate: The full-rate traffic channel (TCH) handles encoded speech
or data, So Full rate (TCH) Transmits full rate speech (13 Kbits/s).
Half-Rate: With a half-rate channel, a mobile station will only use
every second time slot (every other one is idle). As a result, two
mobile stations will be able to use the same physical channel for calls
leading to a doubling of the capacity on the channel; So Half rate
(TCH/2) transmits half rate speech (6.5 Kbits/s).
 Control channels:
These are used to carry signaling or synchronization data. They are
divided into three types:
A. Broadcast Channels (BCH)
B. Common Control Channels (CCCH)
C. Dedicated Control Channels (DCCH)
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Chapter 2
 Broadcast Channels (BCH):
All Broadcast Channels are transmitted point to multi-point over the
downlink.
Frequency Correction Channel (FCH) - Provides the frequency correction
information used by the mobile station.
Synchronization Channel (SCH) - Contains the Base Station Identity
Code (BSIC) and the TDMA frame number used for synchronization of
the mobile station to the frame structure of a new BTS.
Broadcast Control Channel (BCCH) Used to broadcast general
information to all mobile stations
 Common Control Channels (CCCH):
All Common Control Channels are transmitted point to point.
Paging Channel (PCH) - Used to page the mobile station. PCH
information is transmitted over the downlink.
Random Access Channel (RACH) - Used by a mobile station to request
access to the system such as paging response, request the call, request
message and reserved “SDDCH”. RACH information is transmitted over
the uplink
Access Grant Channel (AGCH) - Used to assign a SDCCH. AGCH
information is transmitted over the downlink.
 Dedicated Control Channels (DCCH):
All Dedicated Control Channels are transmitted point to point over both
the downlink and the uplink.
Stand alone Dedicated Control Channel (SDCCH) - Carries signaling
information during call setup.
Uplink: Allocation of free traffic channel.
Downlink: The MS receives TCH assignment information (carrier and
time slot).
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Chapter 2
Slow Associated Control Channel (SACCH) – Transmits call control data
and measurement reports.
Uplink: -Mobile station output power.
-Measurement of adjacent BCCH carrier.
Downlink: - Time advance information.
- Set output power at MS.
Fast Associated Control Channel (FACCH) - Carries urgent signaling
information such as handover information decision.
Types of frames
TDMA Frame
In GSM 8 TS form one TDMA frame (4.615ms), i.e. 8 physical channels are
using the same frequency band being cyclically (Every 4.615 ms) allocated
to a certain user / application.
Multiframes
 Multiframes of physical channels allocated for user traffic (Traffic
Channels TCH) are repetition cycles of 26 TDMA frames.
 Multiframes of physical channels allocated for signaling data (mostly on
one / several of the TS0 of the carrier of one cell) are repetition cycles of
51 TDMA frames.
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Chapter 2
Superframe
 51 Traffic Multiframes.
 26 Control Multiframes.
Time of Superframe = 51*26*40615 = 6.12 Sec.
Hyperframe
 2048 Superframe.
Time of Hyperframe = 2048*6.12 = 3.5 hours.
Time slot staggering
It used for solving complex electronic system and simply the manufacturing
receiving.
For example, if we send a data in time slot 2 for uplink, we receive data in
time slot 5 for downlink.
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Chapter 2
The Burst
The information contained in one time slot in a TDMA frame over the air
interface is called a burst. The TRU, inside the BTS, and the Mobile
Station perform the function putting the information into the proper burst
format.
The 156.25 bit are used / defined as follows:
142 bit for the transmission of “Information” (not only user’s data /
signaling
3 bit as Tail Bits TB for edge limitations of the TS.”
8.25 bit as Guard Period GP.
Types of bursts
Normal Burst
Dummy Burs
Frequency Correction Burst
Access Burst
Synchronization Burst
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Chapter 2
Normal Burst :
`The Normal Burst is used to carry Information on Traffic Channels
and Control Channels; BCCH, PCH, AGCH, SDCCH, SACCH and
FACCH.A Normal Burst consists of two 57-bit packets of encrypted
data or speech, 2 flag bits, a 26-bit Training Sequence, and two 3-bit
packets called tail bits. The flag bits (stealing flags) indicate FACCH
signaling is in progress. The Training Sequence is a known bit pattern
used by the equalizer to create a channel model. The tail bits (TB) are
always 0, 0, and 0 and they are used to help the equalizer indicate the
start and stop points. A time slot has room for 156.25 bits, but the
burst contains only 148 bits. The remaining 8.25 is empty, and is
called the Guard Period. The Guard Period is used to protect bursts
from overlapping on each other.
Frequency Correction Burst:
The frequency correction burst is used to carry the data in the
Frequency Correction Channel (FCCH).
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Chapter 2
Synchronization Burst:
The TDMA frame number is sent on the SCH channel, which carries
also the Base station Identity code (BSIC).
Access Burst:
The access burst is used to carry data in the Random Access Channel
(RACH). It has a longer guard period to compensate for the fact that
the mobile station does not know the timing advance value for
transmission at first access. The mobile station may be far away from
the BTS, which indicates the initial burst will arrive late.
Dummy Burst:
The dummy burst is sent from the BTS when there is nothing else to
be sent.
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Chapter 2
Power measurements
Idle Mode
Active mode
Idle Mode:
Idle mode is when the mobile station is turned on and not in a callconnected mode (the mobile is not sending signals). When the mobile
station is turned on it measures all the radio frequencies in the system
and stores the signal strength for each one. The mobile station tunes to
the frequency with the strongest signal and determines if it is a
BCCH-carrier. The BCCH data describes if the cell is available for the
mobile station (cell belongs to the right PLMN). The mobile tunes to
the best cell to receive pages or to request a connection. The mobile
continues to monitor all neighboring cells, and if a better cell is
available, the mobile will then tune to that cell.
The mobile station is constantly updating a measurement report which
contains a mean signal strength for neighboring cells, and the signal
strength and BER of the serving BTS. The signal strength of the
serving BTS is measured each time a mobile station receives on its
assigned time slot.
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Chapter 2
The idle mode task can be subdivided into four processes:




PLMN selection
Cell selection
Cell reselection
Location updating
The relationship between these processes is illustrated in Figure
below:
Active mode:
During a call, the mobile makes power measurements on the BCCH
carriers received from the adjacent cells and the signal received from
its cell, to produce a list of the six strongest carriers, which will be
reported regularly to the BSC via the SACCH channel. The BSC will
use this report to determine whether it is required to make hand over
to a new cell or not.
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Chapter 2
Handover
The handover procedure for a mobile on Circuit Switched call connection is
performed in BSC. The decision for a handover is based on measurement
reports from the MS and cell data. The MS is using the SACCH to transmit
the measurement reports.
Handover can take place between traffic channels belonging to:
Different MSC/VLR (and different BSC’s)
Different BSC’s (Inter BSC Handover)
Different cells within a BSC (Intra BSC Handover)
The same cell (Intra cell Handover)
INTRA BSC HANDOVER:
The main steps involved in an intra BSC handover are shown in Figure here.
During the call, the MS measures the signal strength and quality on its own
TCH and the signal strength of the neighboring cells. The MS evaluates the
average value for each of these.
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Chapter 2
INTER BSC HANDOVER:
If the MS moves to an area covered by a cell belonging to another BSC and
handover is required, an inter BSC handover takes place.The serving BSC
decides from the measurement reports that the call must be handed over to a
cell belonging to the new BSC.
INTER MSC HANDOVER:
If, during a call, the serving BSC decides to perform a handover and the new
cell belongs to another MSC.
INTRA CELL HANDOVER:
The intra cell handover makes it possible to switch from one TCH to another
within the same cell during a call. An intra cell handover is initiated when
the channel quality is worse than expected from the signal strength level.
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Chapter 2
Numbering Planning
International Mobile Subscriber Identity [IMSI]:
o It is used for location update.
o Locate at SIM card.
o Locate at VLR&HLR in network.
o Each SIM card has different IMSI.
o It is used for first active with new MSC.
o It is very important, so it is not known for users.
o It is consist of 15 digits:
 MCC: Mobile Country Code.
 MNC: Mobile Network Code.
 MSIN: Mobile Station Identification Number.
Temporary Mobile Subscriber Identity Number [TMSI]:
o The TMSI can be allocated to the mobile subscriber in order to
be used instead of his IMSI during all radio communications.
The purpose is to keep subscriber information confidential on
the air interface.
o The TMSI is relevant on the local MSC/VLR level only and is
changed at certain events or time intervals. Each local operator
can define its own TMSI structure.
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Chapter 2
Mobile Station ISDN Number [MSISDN]:
o MSISDN length: 2 / 3 + 3 + max. 7 digits = max. 12 digits.
o MSISDN = CC + NDC + SN.
 CC: Country Code.
 NDC : Network Destination Code.
 SN : Subscriber Number.
o The MSISDN is "the user’s telephone number".
o A user has one IMSI (with one contract), but he can have different
MSISDN (e.g. for fax, phone...).
International Mobile Equipment Identity [IMEI]:
o It is a special number for mobile equipment.
o One IMEI uses for one IMSI.
o It consists of 14 digits.
 TAC: Type Approval Code (6 digits).
 FAC: Final Assembly Codes (2 digits).
 SN: Serial Number (6 digits).
Mobile Station Roaming Number [MSRN]:
o When a mobile terminating call is to be set-up, the HLR of the
called subscriber requests the MSC/VLR to allocate an MSRN to
the called subscriber.
o This MSRN is returned via the HLR to the GMSC.
o The GMSC routes the call to the MSC/VLR exchange where the
called subscriber is currently registered.
o The routing is done using the MSRN. When the routing is
completed, the MSRN is released.
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Chapter 2
Traffic Cases
61
Chapter 2
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Chapter 2
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Chapter 2