CELLULAR COMMUNICATION SYSTEM

CHAPTER 4
CELLULAR COMMUNICATION SYSTEM

CHAPTER OUTLINE
CELLULAR COMMUNICATION COMPONENT
ANTENNA IN CELLULAR COMMUNICATION
CELLULAR COMMUNICATION CONCEPT

HAND OVER PROCESS IN CELLULAR
COMMUNICATION SYSTEM
RADIO CHANNEL & MODULATION TECHNIQUE
IN CELLULAR COMMUNICATION

PART 1
MAIN COMPONENT IN CELLULAR

AT THE END OF THIS TOPIC
STUDENT SHOULD BE ABLE
TO:
4.1 Understand the main component in cellular communication system.
4.1.1 Describe the Mobile Switching Centre (MSC).
4.1.2 Describe the elements connected to the MSC.
a. Home Location Register.
b. Visitor Location Register.
c. Equipment Identity Register (EIR).
d. Authentication Centre (AuC)
e. Gateway Mobile Switching Centre (GMSC).
f. SMS Gateway (SMS-G).

TO:
4.1.3 Describe the Base Station Subsystem (BSS).
4.1.4 Describe the types of Base Transceiver.
a. Base Transceiver Station (BTS).
b. Base Station Controller (BSC).
4.1.5 Describe the Mobile Unit that functions as a
transceiver.
4.1.6 Explain the function of sim card used in mobile units.
4.1.7 Relate between MSC, BSS, BTS, BSC and mobile unit
in the form of diagrams.

Wireless Communications

• Multimedia wireless Communications at any Time and Anywhere
• Brief history
•
•
•

Ancient Systems: Smoke Signals, Carrier Pigeons

•

Cellular has enjoyed exponential growth since 1988, with more than 2 billion
users worldwide today

•
•
•

Ignited the recent wireless revolution, 1980-2003

Radio invented in the 1880s by Marconi
Many sophisticated military radio systems were developed during and after
WW2

Growth rate tapering off
Is there a future for wireless?

Current Wireless Systems
•
•
•
•
•
•
•

Cellular systems
Wireless LANs
Satellite Systems
Paging Systems
Bluetooth
Ultrawideband Radios
Zigbee Radios

What is Cellular Communication
System
• Cellular communication is designed to provide
communications between two moving units,
or between one mobile unit and one stationary
phone or land unit (PSTN).

• A service provider must be able to locate and track a
caller, assign a channel to the call, and transfer the
channel from base station to base station as the caller
moves out of range (handover/handoff).

To make this tracking possible…..
• Each cellular service area is divided into small regions

called cells.
• Each cell contains an antenna and is controlled by
powered network station, called the base station (BS).
• Each base station is controlled by a switching office,
called a mobile switching center (MSC).
• The MSC coordinates communication between all the
base stations and the telephone central office (exchange).
It is a computerized center that is responsible for
connecting calls, recording call information, and billing.

COMPONENTS IN CELLULAR
3 main components:
• Mobile Station (MS) – UE, SIM
• Base Station Subsystem (BSS) – BTS,
RBS, BSC

• Network and Switching Subsystem
(NSS) – MSC, VLR, HLR,

Network & Switching Subsystem
(NSS)
•
•
•
•

Mobile Switching Center (MSC)

•

Authentication Centre (AuC)

•

Gateway Mobile Switching Center (GMSC)

•

SMS Gateway (SMS-G)

Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identify Register (EIR)

• The MSC is the heart of the GSM network.
• From technical perspective MSC is just an ordinary
Integrated Services Digital Network (ISDN) exchange

• One MSC can handles multiple BSCs and also
interfaces with other MSC's (Using E-Interface).

• It also handles inter-BSC handoffs as well as
coordinates
handoffs.

with

other

MSC's

for

inter-MSC

• MSC performs the telephony switching functions of
the system.

• Controls calls to and from other telephony and data
systems, such as the Public Switched Telephone
Network (PSTN) and Public Land Mobile Network
(PLMN).

• Difference between a MSC and an exchange in

a fixed network, MSC has to take into account
the impact of the allocation of radio resources
and the mobile nature of the subscribers and
has to perform in addition, at least the
following procedures:
• required for location registration
• procedures required for handover

• MSC can be connected to only one VLR or more
VLR. Therefore, all mobile stations that move
around under base stations connected to the MSC
are always managed by the same VLR.

• MSC would communicate typically with one EIR.

While it is possible for an MSC to communicate to
multiple EIRs, this is highly unlikely since the EIR
provides a centralized and geographic independent
function.

Ericsson Mobile Switching Center
Server (MSC-S)

The Mobile Switching Center Server (MSC-S) provides control
of high-capacity switching in mobile circuit core networks

The Elements Connected to The
MSC
The MSC connects to the following elements:
a.

The Home Location Register (HLR)

b.

The Visitor Location Register (VLR)

c.

Equipment Identify Register (EIR)

d.


e.

Gateway Mobile Switching Center (GMSC)

f.

SMS Gateway (SMS-G)

Home location register (HLR)
• HLR is a central database that contains details of
each mobile phone subscriber that is authorized to
use the GSM core network.

• The HLRs store details of every SIM card issued by
the mobile phone operator.

• Each SIM has a unique identifier called an IMSI
which is the primary key to each HLR record.

• When the mobile user visits a PCS network other than
the home system, a temporary record for the mobile
user is created in the visitor location register (VLR) of
the visited system.

• The VLR temporarily stores subscription information
for the visiting subscribers so that the corresponding
MSC can provide service.

• In other words, the VLR is the "other" location register
used to retrieve information for handling calls to or
from a visiting mobile user.

Home Location Register (HLR)
• Examples of other data stored in the :
• GSM services that the subscriber has requested or been
given.

• GPRS settings to allow the subscriber to access packet
services.

• Current location of subscriber (VLR and serving GPRS
support node/SGSN).

• Call divert settings applicable for each associated
MSISDN.

Responsibilities of the HLR
include:
• management of service profiles
• mapping of subscriber identities (MISDN, IMSI)
• supplementary service control and profile updates
• execution of supplementary service logic e.g. incoming calls
barred.

• passing subscription records to VLR
• directly receives and processes MAP transactions and messages
from elements in the GSM network, for example, the location
update messages received as mobile phones roam around.

• VLR is a database as same as HLR that contains all subscriber
information data for call handling and mobility management

• VLR provide dynamic data management (HLR static data
management)

• The VLR keeps track of all subscribers roaming in the VLR
service area.

• In GSM system the VLR is integrated with the MSC

• VLR contains:
• Selective information function from the HLR

• IMSI (the subscriber's identity number).
• Authentication data.
• MSISDN (the subscriber's phone number).
• GSM services that the subscriber is allowed to access.
• access point (GPRS) subscribed.
• The HLR address of the subscriber.

Function of the VLR include:
• Executing supplementary service programs (outgoing calls
barred)

• Initiating authentication and ciphering
• Initiating paging

• Mapping of various identities (MSISDN, IMSI, TMSI,
MSRN)

• Passing location information to HLR

Function of the VLR include:
• To inform the HLR when subscriber has arrived in the area
covered by the VLR.

• To track where the subscriber when idle mode.
• To allow or disallow which services the subscriber may use.
• To allocate roaming numbers during the processing of
incoming calls.

• To purge the subscriber record if becomes inactive whilst in the
area and deletes the subscriber's data after some period and
informs the HLR

• To delete the subscriber record when a subscriber explicitly
moves to another, as instructed by the HLR.

Equipment Identity Register
(EIR)
• The EIR is a database that keeps tracks of handsets on the
network using the IMEI.

• The EIR was introduced to identify, track and bar such
equipment from being used in the network

• There is only one EIR per network.
• Composed of three lists.
• The White List
• The Gray List

• The Black List

Equipment Identity Register
(EIR)

• AUC is always integrated with HLR for the purpose of

the authentication.
• The Subscriber Authentication Key (Ki) is allocated to the
subscriber, together with the IMSI. The Ki is stored in the
AUC and used to provide the triplets, same Ki is also
stored in the SIM.
• AUC stores the following information for each subscriber
• The IMSI number,
• The individual authentication key Ki
• A version of A3 and A8 algorithm.

In AUC following steps are used to produce one triplet:

1.
2.

3.

A non- predictable random number, RAND, is produced
RAND & Ki are used to calculate the Signed Response
(SRES) and the Ciphering Key (Kc)
RAND, SRES and Kc are delivered together to HLR as one
triplet.

HLR delivers these triplets to MSC/VLR on request in such a way
that VLR always has at least one triplet.

Gateway Mobile Switching Center
(GMSC)
• There is another important type of MSC, called a
Gateway Mobile Switching Center (GMSC).

• The GMSC functions as a gateway between two
networks.

• If a mobile subscriber wants to place a call to a

regular land line, then the call would have to go
through a GMSC in order to switch to the Public
Switched
Telephone
Network
(PSTN).

Gateway Mobile Switching Center
(GMSC)

SMS Gateway (SMS-G)
• The SMS GMSC (SMS gateway MSC) is a gateway
MSC that can also receive short messages.

• The gateway MSC is a mobile network‟s point of
contact with other networks.

Base Station Subsystem (BSS)
• BSS is the section of a traditional cellular telephone
network which is responsible for handling traffic and signaling
between a mobile phone and the NSS.

• The BSS performs all the radio-related functions.
• The BSS is comprised of the following functional units:
• Base Station Controller (BSC)
• Base Transceiver Station (BTS)
• BSS communicate to Mobile Station (MS) using Air Interface

Base Station Subsystem (BSS)
• Function of BSS
• transcoding of speech channels,
• allocation of radio channels to mobile phones,
• paging,
• transmission and reception over the air interface
• and many other tasks related to the radio network.
• BTS + BSC = BSS.

Base Transceiver Station (BTS)
• BTS provides physical connection between MS to network
using Air Interface (Um)

• Main function of BTS is for maintaining the Um interface and
minimizing the transmission problem (Um very sensitive for
disturbance)

• Using Abis interface for connection between BTS and BSC

• A BTS usually placed on center of the cell
• Its transmitting power defines size of the cell
• Each BTS has between 1 to 16 transceiver
depending on density of the user in the cell
• Each BTS serve in a single cell










The site controller stations that function depends on the
directions from the MSC.
Using voice channels - VC (or trafic channel TC) and
control channels – CC as radio channels communications in
each cell
Supervise the call, monitoring the quality of the speech and
also the measurement of the strength of the voice signal.
Send and receive voice signals and data signals to/from users.
Interface between users equipment UE and switching systems
MCS

• The BTS is the Mobile Station's access point to the network.
• It is responsible for carrying out radio communications between
the network and the Mobile Station‟s.

• It handles speech encoding, encryption, multiplexing (TDMA),
and modulation/demodulation of the radio signals.

• It is also capable of frequency hopping (changing carrier
frequency while communicating)

• One BTS usually covers a single 120 degree sector of an area.

• Usually a tower with 3 BTSs will accommodate all 360 degrees
around the tower.

Base Station Controller (BSC)
• The BSC controls multiple BTS.
• It handles allocation of
- radio channels,
- frequency administration,
- power and signal measurements from the MS,
- Handovers from one BTS to another (if both BTSs are
controlled by the same
BSC).

Base Station Controller (BSC)
• A BSC my be collocated with a BTS
or it may be geographically separate.
• It may even be collocated with the
Mobile Switching Center (MSC).

Mobile Station (MS)
• MS is the physical equipment used by a GSM subscriber
• It comprises two parts:
• Subscriber Identity Module (SIM)
• The Mobile Equipment (ME)

Mobile Equipment (ME)
• ME provides the radio and processing needed to access the
GSM network, plus a man machine interface MMI to enable the
user to access services.

ME Function
• Radio transceiver and signal processing
• Radio related operations: power control; timing advance;
discontinuous transmission (DTX); slow frequency
hopping (SFH).

• Call handling
• man-machine interface, display, keypad, speech
transducers.

• interfaces to external equipment e.g. laptops / palmtops

Subscriber Identity Module (SIM)
• Provides personal mobility - user can have access to subscribed
services irrespective of a specific terminal.

• Contains the International Mobile Subscriber Identity (IMSI)
used to identify the subscriber to the system, a secret key for
authentication, and other information.

• The SIM card may be protected against unauthorized use by a
password or personal identity number.

SIM

Function of SIM card
• SIM is a smart card which plugs into the mobile equipment and
contains information about the mobile subscriber.

• Carries all the subscriber specific information used by an MS.
• Major functions are to identify the current user of an MS and
to take part in security and confidentiality procedures.

• Stores recent location data and may also store personal

information for the user such as abbreviated dialing codes
(telephone directory).

Specific functions include:
• Permanent storage of a subscriber‟s International Mobile
Subscriber Identity (IMSI) and Authentication key (Ki)

• Semi permanent storage of system information e.g. current
Location Area Identity (LAI), encryption key Kc and lists of
preferred / forbidden GSM networks

• Semi permanent storage of user data, „telephone directory‟,
short messages

• Participation in mobility procedures e.g. user authentication,
generation of ciphering key, instigation of location updates.

• Protected by PIN

The SIM contains several pieces of
information:




International Module Subscriber Identity (IMSI) – This
number identifies the mobile subscriber. It is only transmitted
over the air during initialisation.
Temporary Mobile Subscriber Identity (TMSI) – This
number identifies the subscriber, it is periodically changed by
the system management to protect the subscriber from being
identified by someone attempting to monitor the radio
interface.

The SIM contains several pieces of
information:






Location Area Identity (LAI) – Identifies the current location
of the subscriber .
Subscriber Authentication Key (Ki) – This is used to
authenticate the SIM card.
Mobile Station International Services Digital Network
(MSISDN) – This is the telephone number of the mobile. It is
comprised of a country code, a national a subscriber number.

PART 2
ANTENNA USE IN CELLULAR

TO:
4.2 Know the types of antenna used in cellular communication
systems.
4.2.1 Identify the types of antenna.
a. Omnidirentional antenna.
b. Sectorized antenna.
4.2.2 State the characteristics of the antenna in

Types of Antenna Used in Cellular
Communication System
a. Omnidirectional Antenna
b. Sectorized Antenna

Omni Directional
• The omnidirectional antenna radiates or receives equally well in
all directions.

• It is also called the "non-directional" antenna because it does
not favor any particular direction.

• This antenna is built using a metallic bar

Omni Directional
• The highest radiated power is in the direction perpendicular to

the antenna and reradiated power drops as the angle increases
above or below the horizon.
• Such an antenna is suitable for cell phone towers because most
of the radiated power travels parallel to the horizon.
• In an omnidirectional cell, the BTS (centre-excited) is equipped
with an antenna system that transmits and receives equally well
in all directions. A theoretical circular shape of coverage will be
achieved with this antenna

Sectorized Antenna
• By focusing the beam in a more focused area, offers
greater range and throughput with less energy.

• Many operators will use sector antennas to cover a
360-degree service area rather than use an Omni
directional antenna due to the superior
performance of sector antennas over an Omni
directional antenna.

Sectorized Antenna
• In sectorized cell, it can be either edge excited or centre excited.
In a centre excited cell, it can be a 3x120o and 6x60osectors
while in the edge excited, the BTS is located at the edge of the
cell and provides coverage to multiple(3) cells.

Sectorized Antenna
• Advantages:
• More power utilisation
• More user can access the channel

• Disadvantages
• Need more antenna to cover 1 cell
• High cost

PART 3
CONCEPTS OF CELLULAR

4.3 Understand the concepts of
cellular communication system.
• 4.3.1 Define Cell and Cluster.
• 4.3.2 Describe techniques to improve coverage and capacity in
cellular systems.
a.
Cell splitting
b.
Sectoring
c.
Microcell zone concept
• 4.3.3 Describe the term
communication systems.

frequency

reuse

in

cellular

4.3 Understand the concepts of
• 4.3.4 Describe the relationship between frequency reuse and cell
splitting as techniques to maximize the traffic capacity of
cellular systems.
• 4.3.5 Describe how to determine total number of channels in a
given bandwidth.
• 4.3.6 Describe the Signal to Noise ratio (S/N ratio).

Cell and Cluster
CELL

• A cell is formally defined as an area where in the use of radio
communication resources by the MS is controlled by a BTS.

• The size and shape of the cell and the amount of resources
allocated to each cell dictate the performance of the system to a
large extent, given the number of users, average frequency of
calls being made, average duration of call time, and so on.

Cell and Cluster
• Cell Area

Cell and Cluster
• In cellular communication, we used hexagon to represent the
cell area

• Rural Area population Density is low; cell area is kept large
• Built up area population density is large cell area is small

Cell and Cluster
• The middle circles represent cell sites.
• This is where the base station radio equipment and their
antennas are located.

• A cell site gives radio coverage to a cell.
• Most cells have been split into sectors or individual areas to

make them more efficient and to let them to carry more calls.

• Antennas transmit inward to each cell
• It cover a portion or a sector of each cell, not the whole thing

Cell Types
• The density of population in a country is so varied
that different types of cells are used:
• Macrocells
• Microcells
• Selective cells

Cell and Cluster
• Cluster:
• A set of hexagons (cells) can be packed in clusters such that
no two similar cell are adjacent.

• Possible cluster sizes are 1,3,4,7,9,12, etc.
• Frequency can only be reused outside and not within the
same cluster.

4.3.2 Describe techniques to improve
coverage and capacity in cellular
systems.
a. Cell splitting
b. Sectoring
c. Microcell zone concept

System Expansion Techniques
• As demand for wireless services increases, the number
of channels assigned to a cell eventually becomes
insufficient to support the required number of users.
More channels must therefore be made available per
unit area.
• This can be accomplished by dividing each initial cell area
into a number of smaller cells, a technique known as cellsplitting.

• It can also be accomplished by having more channels per cell,
i.e. by having a smaller reuse factor. However, to have a
smaller reuse factor, the co-channel interference must be
reduced. This can be done by using antenna sectorization.

System Expansion Techniques -Sectorization
• Keep the cell radius but decrease the D/R ratio. In
order to do this, we must reduce the relative
interference without increasing the transmit power.

• Sectorization relies on antenna placement and
directivity to reduce co-channel interference. Beams
are kept within either a 60° or a 120° sector.

• If we partition a cell into three 120° sectors, the

number of co-channel cells are reduced from 6 to 2
in the first tier.
• Using six sectors of 60°, we have only one cochannel cell in the first tier.
• Each sector is limited to only using 1/3 or 1/6 of the
available channels. We therefore have a decrease in
trunking efficiency and an increase in the number of
required antennas.
• But how can the increase in system capacity be
achieved?

System Expansion Techniques -Micro cells
• Micro cells can be introduced to alleviate capacity
problems caused by “hotspots”.

• By clever channel assignment, the reuse factor is
unchanged. As for cell splitting, there will occur
interference problems when macro and micro cells
must co-exist.

System Expansion Techniques -Micro cells

Cell Splitting
 The process of subdividing a congested
cell into smaller cells each with its own
base station.

 Lowering antenna height, antenna
down tilting and reducing transmitter
power.

Cell Splitting
• Increasing capacity by increasing the
number of times that a channels reused

Cell splitting
• During splitting, the designer must

minimize changes in the system
• The value of the reuse factor must be
relatively prime with respect to the value
of the type of split.
• Hence the voice channel frequency
assignments at the old cell remain the
same when new cell are added.

Cell splitting
• Cell splitting increases the number of BSs in order to
increase capacity. There will be a corresponding
reduction in antenna height and transmitter power.

• Cell splitting accommodates a modular growth

capability. This in turn leads to capacity increase
essentially via a system re-scaling of the cellular
geometry without any changes in frequency planning.

• Small cells lead to more cells/area which in turn leads
to increased traffic capacity.

Cell splitting
• For new cells to be smaller in size, the transmit power must be

reduced. If n=4, then with a reduction of cell radius by a factor
of 2, the transmit power should be reduced by a factor of 24
(why?)
• In theory, cell splitting could be repeated indefinitely.
• In practice it is limited
• By the cost of base stations
• Handover (fast and low speed traffic)
• Not all cells are split at the same time : practical problems of BS sites,

such as co-channel interference exist
• Innovative channel assignment schemes must be developed to address
this problem for practical systems.

Sectoring
• Sectorization
• increased C/I (carrier-to-interference
ratio, C/I) by eliminating some cochannel cell at the

• expense of reducing trunking efficiency
• cell size can be reduced

Sectoring
• Cell sectoring will improved the value of

C/I by reducing the number of interferers.
• However, trunking effiency (the number
of users which can be offered a particular
GOS) will be reduced
• Trunking efficiency can also be measured
in term of number of channel per sector
(cell)

Microcell Zone Concept
• The increased number of handoffs required when
sectoring is employed results in an increased

• load on the switching and control link elements of
the mobile system.

• In this scheme, each of the three (or possibly more) zone sites

(represented as Tx/Rx in Figure 4.1) are connected to a single
base station and share the same radio equipment.
• The zones are connected by coaxial cable, fibre optic cable, or
microwave link to the base station.
• Multiple zones and a single base station make up a cell. As a
mobile travels within the cell, it is served by the zone with the
strongest signal.
• This approach is superior to sectoring since antennas are placed
at the outer edges of the cell, and any base station channel may
be assigned to any zone by the base station.

Figure 4.1 The microcell concept

• As a mobile travels from one zone to another within
the cell, it retains the same channel.

• Thus, unlike in sectoring, a handoff is not required at
the MSC when the mobile travels between zones
within the cell.

Figure 4.2 Illustration of how a distributed antenna system (DAS) may be used
inside a building. Figure produced in Site Planner®. (Courtesy of Wireless
Valley Communications Inc.)

• This technique is particularly useful along highways
or along urban traffic corridors.

• The advantage of the zone cell technique is that
while the cell maintains a particular coverage radius,
the co-channel interference in the cellular system is
reduced since a large central base station is replaced
by several lower powered transmitters (zone
transmitters) on the edges of the cell.

4.3.3 Describe the term frequency reuse
in cellular communication systems.

Frequency Reuse
• Base stations in adjacent cells are assigned channel
groups which contain completely different channels
than neighbouring cells.

• The base station antennas are designed to achieve the
desired coverage within the particular cell.

Frequency Reuse
• By limiting the coverage area to within the
boundaries of a cell, the same group of channels
may be used to cover different cells that are separated
from one another by distances large enough to keep
interference levels within tolerable limits.

Frequency Reuse
• The design process of selecting and allocating
channel groups for all of the cellular base stations
within a system is called frequency reuse or frequency
planning.

Based on Figure 4.2:
• Illustration of the cellular frequency reuse concept.
• Cells with the same letter use the same set of frequencies.
• A cell cluster is outlined in bold and replicated over the
coverage area.

• In this example, the cluster size, N, is equal to seven, and
the frequency reuse factor is 1/7 since each cell contains
one-seventh of the total number of available channels.

Frequency Reuse
• Figure 4.2 is conceptual and is a simplistic model of
the radio coverage for each base station, but it has
been universally adopted since the hexagon permits
easy and manageable analysis of a cellular system.

• The actual radio coverage of a cell is known as the
footprint and is determined from field measurements or
propagation prediction models.

4.3.4 Describe the relationship between frequency
reuse and cell splitting as techniques to maximize the
traffic capacity of cellular systems.

The relationship between frequency
reuse and cell splitting
• To understand the frequency reuse concept, consider a cellular
system which has a total of S duplex channels available for use.

• If each cell is allocated a group of k channels (k < S), and if the S
channels are divided among N cells into unique and disjoint channel groups
which each have the same number of channels, the total number of
available radio channels can be expressed as:
S = kN

(4.1)

• The N cells which collectively use the complete set of available
frequencies is called a cluster.

• If a cluster is replicated M times within the system, the total
number of duplex channels, C, can be used as a measure of
capacity and is given by
C = MkN = MS

(4.2)

• As seen from Equation (4.2), the capacity of a
cellular system is directly proportional to the number
of times a cluster is replicated in a fixed service area.

• The factor N is called the cluster size and is typically equal
to 4, 7, or 12.

• If the cluster size N is reduced while the cell size is kept
constant, more clusters are required to cover a given
area, and hence more capacity (a larger value of C) is

achieved.

• A large cluster size indicates that the ratio between the cell
radius and the distance between co-channel cells is small.

• Conversely, a small cluster size indicates that cochannel cells are located much closer together.

• The value for N is a function of how much interference a
mobile or base station can tolerate while maintaining
a sufficient quality of communications.

• From a design viewpoint, the smallest possible value
of N is desirable in order to maximize capacity over a
given coverage area (i.e., to maximize C in Equation
(4.2)).

• The frequency reuse factor of a cellular system is given by
1/N, since each cell within a cluster is only assigned 1/N of
the total available channels in the system.

4.3.5 Describe how to determine
total number of channels in a given
bandwidth.
• Example 4.1
If a total of 33 MHz of bandwidth is allocated to a particular FDD cellular telephone
system which uses two 25 kHz simplex channels to provide full duplex voice and
control channels, compute the number of channels available per cell if a system uses
a.

four-cell reuse,

b.

seven-cell reuse, and

c.

12-cell reuse.

If 1 MHz of the allocated spectrum is dedicated to control channels, determine an
equitable distribution of control channels and voice channels in each cell for each of
the three systems.

Solution
Given:
Total bandwidth = 33 MHz
Channel bandwidth = 25 kHz × 2 simplex channels
= 50 kHz/duplex channel
Total available channels = 33,000/50 = 660
channels
(a) For N = 4, total number of channels available
per cell = 660/4 ≈ 165 channels.
(b) For N = 7,total number of channels available
(c) For N = 12,total number of channels available

A 1 MHz spectrum for control channels implies
that there are 1000/50 = 20 control channels
out of the 660 channels available. To evenly
distribute the control and voice channels,
simply allocate the same number of voice
channels in each cell wherever possible. Here,
the 660 channels must be evenly distributed
to each cell within the cluster. In practice, only
the 640 voice channels would be allocated,
since the control channels are allocated
separately as 1 per cell.

• (a) For N = 4, we can have five control
channels and 160 voice channels per cell.
• In practice, however, each cell only needs a
single control channel (the control channels
have a greater reuse distance than the voice
channels). Thus, one control channel and 160
voice channels would be assigned to each cell.

• (b) For N = 7, four cells with three control
channels and 92 voice channels, two cells with
three control channels and 90 voice channels,
and one cell with two control channels and 92
voice channels could be allocated.
• In practice, however, each cell would have one
control channel, four cells would have 91
voice channels, and three cells would have 92
voice channels.

• (c) For N = 12, we can have eight cells with
two control channels and 53 voice channels,
and four cells with one control channel and 54
voice channels each.
• In an actual system, each cell would have one
control channel, eight cells would have 53
voice channels, and four cells would have 54
voice channels.

4.3.6 Describe the Signal to
Noise ratio (S/N ratio).

• The presence of noise degrades the performance of analog and digital
communication.

• The extent to which noise affects the performance of communication systems is
measured by the output signal to noise power ratio or SNR.

• Signal-to-noise ratio is defined as the power ratio between a signal (meaningful
information) and the background noise (unwanted signal).

• Signal-to-noise ratio (SNR or S/N) is defined as the ratio of signal power to
the noise power. A ratio higher than 1:1 indicates more signal than noise.

• SNRs are often expressed using the logarithmic decibel scale (dB).

• Noise factor (F): The noise factor of a system is defined as;

F

Si N i
So N o

• where SNRin and SNRout are the input and output
power signal-to-noise ratios, respectively.

• Noise figure (NF): a measure of degradation of the signal-tonoise ratio (SNR), caused by components in a radio
frequency (RF) signal chain. It is the noise factor, given in dB.

Noise Calculation
• SNR is ratio of signal power, S to noise power, N.

• Noise Factor, F

• Noise Figure, NF

F

SNR 10 log

Si N i
So N o

NF 10 log F
10 log

Si N i
(dB)
So N o

S
dB
N

4.0
CELLULAR
COMMUNICATION
SYSTEM
4.5
Understand the process of hand-over in cellular
communication system.
139

4.5 Understand the process of handover in cellular communication system
4.4.1 Define the process of hand-over

4.4.2 Sketch the process of hand-over between cells.
4.4.3 Describe the relationship between Signal to Noise ratio (S/N
ratio) and the hand-over when the call is in progress.
4.4.4 Describe the types of hand-over.
a.

Hard hand-over.

b.

Soft hand-over.

140
4.4.5 Describe roaming and paging in cellular communication system.

4.4.1 Define the process of handover
• When a mobile moves into a different cell while a conversation

is in progress, the MSC automatically transfers the call to a new
channel belonging to the new base station.

• This handoff operation not only involves identifying a new base

station, but also requires that the voice and control signals be
allocated to channels associated with the new base station.

• Processing handoffs is an important task in any cellular radio
system.

• Many handoff strategies prioritize handoff requests over call
initiation requests when allocating unused channels in a cell site.

141

Handoff
• When a mobile user is engaged in conversation, the
MS is connected to a BS via a radio link.

• If the mobile user moves to the coverage area of
another BS, the radio link to the old BS is
eventually disconnected, and a radio link to the
new BS should be established to continue the
conversation.

• This process is variously referred to as automatic
link transfer, handover, or handoff.

Handover / Handoff
• Occurs as a mobile moves into a different cell during
an existing call, or when going from one cellular
system into another.
• It must be user transparent, successful and not too
frequent.

• Not only involves identifying a new BS, but also requires
that the voice and control signals be allocated to channels
associated with the new BS.

Handover / Handoff
• =handoff threshold Minimum acceptable
signal to maintain the call

•

too small:

• Insufficient time
to complete handoff
before call is lost

• More call losses
•

too large:

• Too many handoffs
• Burden for MSC

Hard handoff between the MS and
BSs.

145

Handoff
• Handoffs must be performed successfully and as
infrequently as possible, and be imperceptible to the
users.

• In order to meet these requirements, system
designers must specify an optimum signal level at
which to initiate a handoff.

146

Handoff acceptance signal level
• Once a particular signal level is specified as the minimum
usable signal for acceptable voice quality at the base station
receiver (normally taken as between –90 dBm and –100
dBm), a slightly stronger signal level is used as a threshold at
which a handoff is made.

• This margin, given by Δ = Pr handoff – Pr minimum usable,
cannot be too large or too small. If Δ is too large, unnecessary
handoffs which burden the MSC may occur, and if Δ is too
small, there may be insufficient time to complete a handoff
before a call is lost due to weak signal conditions.
147

Illustration of a handoff scenario at cell

148

Understand the process of hand-over in
4.4.4 Describe the types of hand-over.
a.

Hard hand-over.

b.

Soft hand-over.

c.

Softer hand-over

149

Hard handover
• The definition of a hard handover or handoff is one where an existing
connection must be broken before the new one is established.

• One example of hard handover is when frequencies are changed.
• As the mobile will normally only be able to transmit on one frequency at a
time, the connection must be broken before it can move to the new channel
where the connection is re-established. This is often termed and interfrequency hard handover. While this is the most common form of hard
handoff, it is not the only one.

• It is also possible to have intra-frequency hard handovers where the frequency
channel remains the same.

• Although there is generally a short break in transmission, this is normally
short enough not to be noticed by the user.
150

Soft handover
• The new 3G technologies use CDMA where it is possible
to have neighbouring cells on the same frequency and this
opens the possibility of having a form of handover or
handoff where it is not necessary to break the connection.

• This is called soft handover or soft handoff, and it is

defined as a handover where a new connection is
established before the old one is released.

• In UMTS most of the handovers that are performed are
intra-frequency soft handovers.

151

Softer handover
• The third type of hand over is termed a softer

handover, or handoff. In this instance a new signal is
either added to or deleted from the active set of
signals.

• It may also occur when a signal is replaced by a

stronger signal from a different sector under the
same base station.

• This type of handover or handoff is available within
UMTS as well as CDMA2000.

152

4.4.5 Describe roaming and paging in cellular
communication system.

153

Roaming
• In short, roaming is a term used to describe the ability of
phones to connect to the network of a different carrier,
abroad or at home in

• to offer users the same features they use while on their
“home” network – making and receiving calls and text
messages and surfing the web.

154

Roaming
• Roaming is possible thanks to the international agreements
carriers have with other carriers, in order to offer their
wireless services in other regions of a country or of the

world.

• There are various types of roaming agreements between
carriers, with some of them being free, but most of them
will bring extra charges to your monthly cell phone bill.

155

Roaming
• Also worth remembering is that roaming services have to be
activated with some carriers in order for your phone to work
abroad.

• So, if you plan to use the handset in other countries, you‟ll
have to enable the service with your mobile operator before
departing.

156

Why is it so important for them to
offer roaming features?
• First of all, it‟s all about marketing.
• Each carrier, especially major ones, want their
subscribers to know that they‟ll be able to use the
handset, which is purchased in most cases for a
subsidized two-year contract, abroad and enjoy the
same services.

157

Why is it so important for them to
offer roaming features?
• And second of all, roaming agreements between
carriers aren‟t exactly controlled by a regulator
(except in the EU), which means that mobile
operators can jack up prices and raise their profit
margins when it comes to charging for used voice
minutes, SMS and MMS messages, and especially data
used when roaming.

158

Roaming
• When a mobile user moves from one PCS system
(e.g., the system in New York City) to another (e.g.,
the system in Los Angeles), the system should be
informed of the current location of the user.
Otherwise, it would be impossible to deliver the
services to the mobile user.

• To support mobility management, protocols such
as EIA/TIA Interim Standard 41 (IS-41 or
ANSI-41) or Global System for Mobile
Commu-nications (GSM) Mobile Application
Part (MAP) have been defined for PCS networks.

Roaming
• A wireless roaming network has FIVE(5) components that make

it work:
1. A database for storing customer profile information such as
features, dialing capabilities, and the home serving area
identification. This is called the home location register
(HLR).
2. A database of mobile numbers used by each switch on the
network.
3. A signaling network for transmitting data messages between
switches.
4. Routing specifications that direct the data messages to the
appropriate destination.
5. Public long-distance connections for call delivery
160

Roaming
• A registration cycle keeps track of a phone as it travels around the network.

It begins when a wireless user powers on their phone. The general steps for
this process are:

• When the phone is powered on, it sends a data message to the cellsite. This

data message contains the Mobile Identification Number (MIN or phone
number) and the Electronic Serial Number (ESN). The cellsite forwards
this information to the switch.

• The switch compares the MIN with a table of all MINs in the network. It
will determine if the MIN belongs to a home customer, or to a visiting
customer. In either case, the switch will request the subscriber's feature
profile from the Home Location Register (HLR). The HLR for home
customers may be integrated into the same switch or stored on a separate
platform.
161

Roaming
• If the HLR is a separate platform, or if the
customer is visiting from another system, the
switch then sends a data message to the HLR
across the signaling network.

• Routing specifications stored at Signaling
Transfer Points (STPs) provide the necessary
information to direct the message to the home
location register.
162

Roaming

• When the Home Location Register (HLR)
receives the message, it checks the MIN & the
ESN.

• If the numbers are valid, the HLR records the
location of the phone and returns a message
containing the subscriber's feature list and
calling restrictions to the visited switch.
163

Roaming
• Once the visited switch receives the return
message, it creates a Visitor Location
Register (VLR) to store information about
the roamer, including the MIN, ESN,
features, etc... This register will be used by
the roamer as long as they are registered in
the visited system.
164

Paging Systems
• Broad coverage for short messaging

• Message broadcast from all base stations
• Simple terminals
• Optimized for 1-way transmission

• Answer-back hard
• Overtaken by cellular

Paging
• Practically every cellular system has some kind of

broadcast mechanism.
• This can be used directly for distributing information to
multiple mobiles, commonly, for example in mobile
telephony systems, the most important use of broadcast
information is to set up channels for one to one
communication between the mobile transceiver and the
base station.
• This is called paging. The three different paging
procedures generally adopted are sequential, parallel and
selective paging.
166

Paging
• The details of the process of paging vary somewhat
from network to network, but normally we know a
limited number of cells where the phone is located
(this group of cells is called a Location Area in the
GSM or UMTS system, or Routing Area if a data
packet session is involved; in LTE, cells are grouped
into Tracking Areas).

167

Paging
• Paging takes place by sending the broadcast message
to all of those cells.

• Paging messages can be used for information
transfer.

• This happens in pagers, in CDMA systems for
sending SMS messages, and in the UMTS system
where it allows for low downlink latency in packetbased connections.
168

4.5 UNDERSTAND RADIO
CHANNELS AND MODULATION
TECHNIQUES USED IN
CELLULAR COMMUNICATION
SYSTEM.

169

Subtopic
4.5.1 Describe the types of radio channel.
a. Control channel (CC).
i.
ii.

b.

Reverse Control Channel (RCC).
Forward Control Channel (FCC).

Voice channel (VC).

i.

Forward Voice Channel (FVC).

ii.

Reverse Voice Channel (RVC).

170

Subtopic
4.5.2 Describe the modulation technique used in radio
channel in 4.6.1.
4.5.3 Describe the cellular call procedures involved in
making different types of calls.
a.

Mobile to wireline

b.

Mobile to mobile

c.

Wireline to mobile

171

4.5.1 Describe the types of radio channel.
a. Control channel (CC).
i. Reverse Control Channel (RCC).
ii. Forward Control Channel (FCC).

b. Voice channel (VC).
i. Forward Voice Channel (FVC).
ii. Reverse Voice Channel (RVC).

4.6 Understand radio channels and modulation
techniques used in cellular communication
system.
172

•

Operational Channels
In each cell, there are FOUR(4) types of channels that take
active part during a mobile call.

• These are:
a.

Control channel (CC).

i.
ii.

b.

Forward Control Channel (FCC).
Reverse Control Channel (RCC).

Voice channel (VC).

i.

Forward Voice Channel (FVC).

ii.

Reverse Voice Channel (RVC).

173

Forward Control Channel (FCC)
• Forward Control Channel (FCC): Control channels
are generally used for controlling the activity of the
call, i.e., they are used for setting up calls and to
divert the call to unused voice channels.

• Hence these are also called setup channels.
• These channels transmit and receive call initiation
and service request messages. The FCC is used for
control signalling purpose from the BS to MS.

174

Reverse Control Channel (RCC)
• Reverse Control Channel (RCC): This is used for the
call control purpose from the MS to the BS.

• Control channels are usually monitored by mobiles.

175

Forward Voice Channel (FVC)
• Forward Voice Channel (FVC): This channel is used
for the voice transmission from the BS to the MS.

176

Reverse Voice Channel (RVC)
• Reverse Voice Channel (RVC): This is used for the
voice transmission from the MS to the BS.

177

4.5.2 Describe the modulation technique used in
radio channel in 4.6.1.

178

Modulation Basics
• The primary difference between analog and digital
modulation is the source information.

• If the source is analog, the modulation is analog.
• If the source is digital, the modulation is digital.
• The carrier is always analog
• The modulated wave is always propagated through
the air as a series of sine or cosine waves - not
pulses!

180

Modulation and Demodulation

181

Amplitude Modulation (AM)

184

Frequency Modulation (FM)

185

Amplitude Shift Keying (ASK)
• Amplitude Shift Keying (ASK)
– The digital source bits are represented by different amplitude
levels

– Not commonly used alone in wireless
– The amplitude in a wireless environment is much more prone
to impairments than the frequency or phase of the signal

– ASK types of modulation are more commonly used in wire
line where amplitude doesn’t suffer so much degradation

187

Frequency Shift Keying (FSK)
• Frequency Shift Keying (FSK)
– The frequency of the carrier is switched depending
on whether the source data is a “one” or “zero”

– GSM uses a type of FSK called Gaussian Minimum
Shift Keying (GMSK)

188

Phase Shift Keying (PSK)
• Phase Shift Keying (PSK)
– The phase of the carrier is switched depending on
whether the source data is a “one” or “zero”

– Most current and future wireless systems use a
type of PSK

189

Quadrature Phase Shift
Keying (QPSK)
• Quadrature Phase Shift Keying (QPSK)
– The modulated wave shifts between four phases,
90° apart to create a “00”, “01” “10” or “11”

– CDMA uses QPSK on Forward Link

190

Quadrature Amplitude
Modulation (QAM)
• Quadrature Amplitude Modulation (QAM)
– ASK and PSK are combined
– More common on Microwave and wireline
technologies such as DSL

– 16-QAM shown in diagram. 256-QAM is also
possible

191

QPSK, BPSK and QAM

QPSK

BPSK

16-QAM
192

4.5.3 DESCRIBE THE
CELLULAR CALL
PROCEDURES INVOLVED IN
MAKING DIFFERENT TYPES
OF CALLS.
MOBILE TO WIRELINE
MOBILE TO MOBILE
WIRELINE TO MOBILE

193

Introduction
• Phone calls over the cellular network requires the use
of two full-duplex voice channels simultaneous user
channels and control channels.

• RBS transmit and receive signals using channels
called the forward control channel and forward voice
channel.

• While the cellular unit is transmitting and receiving
signals using the back channel and control channel
speakers.
194

Cellular users dial the number he/she want,
and click the 'SEND'. This will cause the
number dialled and cellular dialler will be
sent to the MTSO
If the number identification is valid , MTSO
will connect the call to the PSTN network.
Next to complete the guide called

Through RBS, MTSO will get the channel guide
that is free and direct the cellular unit to tune
it to the channel
After MTSO received confirmation that the
mobile unit is tuned, mobile users will receive
a progress tone from the MTSO.

How to call
from mobile
unit to the
PSTN (wireline)

MTSO will stop ringing when cellular users
began to answer the call and conversation
started.
195

Mobile users who want to make a call,
dialled the desired number and press
'SEND'.

How to call
from mobile
unit to the
other mobile
units.

MTSO will receive the caller's identification
number and the number is then checked its
authenticity and it determines whether the
mobile unit is in use(on-hook) or not (off-hook).

MTSO will send the command 'paging' to
all users, then called RBS and will receive
instruction

Positive signals from users who are called will
cause the MTSO try and get a free channel and
directing users ,manually tuned to their respective
channels. Then the cellular units called will ring.

MTSO will stop ringing when the called user
answered and the conversation began to be
instituted by both users.
196

MTSO received a call from the caller via
the PSTN.

MTSO will check digit dialling and
determine whether the cellular unit is
used or not.

How to call
from PSTN
(Wireline)
to
mobile units

If used (on-hook), the MTSO will switching
'paging' to mobile users and try to get the
free channels and direct him to tune to that
channel

Mobile unit will send confirmation to the
MTSO through the RBS channel and then send
a call progress tones which produce tones.

MTSO will stop ringing when it receives a
positive signal that the mobile user has
answered the call and the conversation can
begin.
197

CELLULAR COMMUNICATION SYSTEM

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