Imam Ja’afar Al-Sadiq University

Technical College

Chapter Two
Stage Four

MOBILE
COMMUNICATION
Cellular Concept-System Design Fundamentals

Assist.Lect.Maryam Abdulhakeem
 Assist.Lect.Maryam Abdulhakeem
Cellular Concept-System Design Fundamentals
2.1 Cellular Mobile Systems

• GSM is a Public Land Mobile Network (PLMN)

• Components
• MS (mobile station)
• BS (base station)
• MSC (mobile switching center)
• LR (location register)
• Subsystems

• RSS (radio subsystem): covers all radio aspects

• NSS (network and switching subsystem): call forwarding, handover, switching
• OSS (operation subsystem): management of the network

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 Assist.Lect.Maryam Abdulhakeem
2.1.1 Network and Switching Subsystem

NSS is the main component of the public mobile network GSM

I. switching, mobility management, interconnection to other networks, system control

II. Components:
a. Mobile Services Switching Center (MSC) – high-performance digital ISDN
switches. controls all connections via a separated network to/from a mobile
terminal within the domain of the MSC - several BSCs can belong to an MSC
b. Databases (important: scalability, high capacity, low delay)
i. Home Location Register (HLR) central master database containing
user data, permanent and semipermanent data of all subscribers
assigned to the HLR (one provider can have several HLRs)
ii. Visitor Location Register (VLR) local database for a subset of user
data, including data about all users currently in the domain of the VLR

2.1.3 The MSC (mobile switching center)

In GSM, the MSC (mobile switching centre) is important.

• Switching functions
• additional functions for mobility support
• management of network resources
• interworking functions via Gateway MSC (GMSC)
• integration of several databases

2.1.3 Functions of a MSC

1- Assign the voice channel to each call

2- Performs handoffs.
3- Monitors the call for billing information

Standard Common Air Interface (CAI) is Communication between the base station and the
mobiles.

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 Assist.Lect.Maryam Abdulhakeem
2.1.3 GSM Channels Type

1) Voice channels (Traffic channels)

a. forward voice channels (FVC): The channels used for voice transmission from
the base station to mobiles
b. reverse voice channels (RVC): channels used for voice transmission from mobiles
to the base station
2) Control channels (setup channels)
a. initiating mobile calls, Control channels transmit and receive data messages that
carry call initiation and service requests, and are monitored by mobiles when they
do not have a call in progress
b. forward control channels (FCC)
c. reverse control channels (RCC).

2.2Cellular System Operation

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 Assist.Lect.Maryam Abdulhakeem
The diagram shows that
• The mobile device is connected to BTS (Antenna).
• BTS is connected to the Switching system called BSC.
• BSC is connected to the main switching system called MSC.
• MSC contains its own VLR (VLR: is a temporary database that stores the information
of the visitors under its coverage area.
• VLR stands for Visitor Location Register. When you roam in a different place VLR
stores your user information.).
• MSC's are connected to GMSC which is connected to HLR. (HLR stands for Home
Location Register, it is the main database where the documents or information of a
user is stored. all the documents that you give during the purchase of a SIM card are
stored in this HLR
• VLR Takes your information from HLR when you Roam in another state or region.).
• HLR also provides authentication by AuC(Authentication Center). AuC is connected
with HLR. If you initiate a call HLR and AuC will see if you are a genuine Mobile
user with valid IMEI number and Plan and then the call is set up from source to the
destination device.

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 Assist.Lect.Maryam Abdulhakeem
2.2.1 Mobile Terminated Call

1: calling a GSM subscriber

2: forwarding call to GMSC

3: signal call setup to HLR

4, 5: request MSRN from VLR

6: forward responsible MSC to GMSC

7: forward call to current MSC

8, 9: get the current status of MS

10, 11: paging of MS

12, 13: MS answers

14, 15: security checks

16, 17: set up connection

Functions performed by the system include the following:

1) Handoff (Handover): If a mobile unit moves out of the range of one cell and into the
range of another during a connection, the traffic channel has to change to one assigned to
the BS in the new cell. The system makes this change without either interrupting the call
or alerting the user.

2) Call blocking: During the mobile-initiated call stage, if all the traffic channels assigned
to the nearest BS are busy, then the mobile unit makes a preconfigured number of

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 Assist.Lect.Maryam Abdulhakeem
repeated attempts. After a certain number of failed tries, a busy tone is returned to the
user.
3) Call termination: When one of the two users hangs up, the MTSO is informed and the
traffic channels at the two BSs are released.
4) Call drop: During a connection, because of interference or weak signal spots in certain
areas, if the BS cannot maintain the minimum required signal strength for a certain period
of time, the traffic channel to the user is dropped and the MTSO is informed.
5) Calls to/from fixed and remote mobile subscriber: The MTSO connects to the public
switched telephone network (PSTN). Thus, the MTSO can set up a connection between a
mobile user in its area and a fixed subscriber via the telephone network.

2.3 RF Planning

RF Planning is the process of assigning frequencies, transmitter locations, and parameters of

a wireless communications system to provide sufficient coverage and capacity for the
services required.

The RF plan of a cellular communication system has two objectives: coverage and capacity.

• Coverage relates to the geographical footprint within the system that has sufficient
RF signal strength to provide for a call/data session.
• Capacity relates to the capability of the system to sustain a given number of
subscribers. Capacity and coverage are interrelated.

To improve coverage, capacity has to be sacrificed, while to improve capacity, coverage will
have to be sacrificed. It is necessary to restructure the radiotelephone system to achieve high
capacity with limited spectrum.

a. Increase the capacity of the system: by using lower-power systems with shorter
radius and using numerous transmitters/receivers (Base stations). Thereby
providing additional radio capacity with no additional increase in radio spectrum.
b. Distributing the available channels throughout geographic regions: by
systematically spacing base stations and their channel groups. The available
channels can be reused as long as the interference between co-channel stations is
kept below acceptable level.

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 Assist.Lect.Maryam Abdulhakeem
2.4 Cell types

• Macro cell: their coverage is large (aprox. 6 miles in diameter);

• used in remote areas,
• high-power transmitters and receivers are used
• Micro cell: their coverage is small (half a mile in diameter) and are used in urban zones.
• low-powered transmitters and receivers are used to avoid interference with cells in
another cluster
• Pico cell: is a small cellular system typically covering
• a small area, such as building (offices, shopping malls, train stations).
• In cellular networks, picocells are typically used to extend coverage to indoor areas
where outdoor signals do not reach well.
• Selective cells: located at the entrances of tunnels where a
• coverage of 360 degrees is not needed in this case,
• a selective cell with a coverage of 120 degrees is used

2.4.1 Decreasing the cell size gives

• Increased user capacity

• Increased number of handovers per call
• Increased complexity in locating the subscriber
• Lower power consumption in mobile terminal: so it gives longer talk time, safer
operation

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 Assist.Lect.Maryam Abdulhakeem
2.5 Cellular Network Coverage

The essence of a cellular network is the use of multiple low-power transmitters, on the order
of 100 W or less. Because the range of such a transmitter is small, an area can be divided into
cells, each one served by its own antenna.

a. Each cell is allocated a band of frequencies and is served by a base station (consisting of
transmitter, receiver, and control unit).
b. Adjacent cells are assigned different frequencies to avoid interference or crosstalk.
However, cells sufficiently distant from each other can use the same frequency band.

While it might seem natural to choose a circle to represent the coverage area of a base station,
adjacent circles cannot be overlaid upon a map without leaving gaps or creating overlapping
regions.

2.5.1 The hexagon

• No gaps or overlapping
• The largest area compared with square and triangle.
• Fewest number of cells can cover a geographic region,
• Closely approximates a circular radiation pattern which would occur for an
omnidirectional base station antenna and free space propagation.
• A hexagonal pattern provides for equidistant antennas.
• When using hexagons to model coverage areas, base station transmitters are depicted
as either:

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 Assist.Lect.Maryam Abdulhakeem
- In the center of the cell (center-excited cells): omnidirectional antennas are
used in center-excited cells.
- On three of the six cell vertices (edge-excited cells): sectored directional
antennas are used in corner-excited cells

The radius of a hexagon is defined to be the radius of the circle that circumscribes it
(equivalently, the distance from the center to each vertex; also equal to the length of a side of
a hexagon).

cell radius(R),is the distance(d) between the cell center and each adjacent cell center

𝒅 = √𝟑𝑹

area of the hexagon

𝟑 √𝟑 𝟐
𝑨𝒓𝒆𝒂 = 𝑹
𝟐
In practice, a precise hexagonal pattern is not used. Variations from the ideal are due to:

- Topographical limitations.

- Local signal propagation conditions.

- Practical limitation on siting antennas.

2.6 Frequency Reuse

Frequency reuse (frequency planning): is the design process of selecting and allocating
channel groups for all of the cellular base stations within a system.

1. To allow communication within the cell using a given frequency

2. To limit the power at that frequency that escapes the cell into adjacent ones.

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 Assist.Lect.Maryam Abdulhakeem
The objective is to use the same frequency: to allow the frequency to be used for multiple
simultaneous conversations.
Note
• 10 to 50 frequencies are assigned to each cell
• determine how many cells must intervene between two cells using the same frequency
so that the two cells do not interfere with each other.

Various patterns of frequency reuse are possible.

• If the pattern consists of N cells

• each cell is assigned the same number of frequencies K(the total number of
frequencies allotted)
• each cell can have K/N frequencies
• For AMPS K = 395, and N = 7 This implies that there can be at most 57 frequencies
per cell on average.
• Is the smallest pattern that can provide sufficient isolation between two uses of the
same frequency.
The following parameters are commonly used in characterizing frequency reuse:
a) D = minimum distance between centers of cells that use the same frequency band
(called co-channels)
b) R = radius of a cell
c) d = distance between centers of adjacent cells
𝒅 = √𝟑𝑹
d) N = number of cells in a pattern (Cluster size)
Reuse Factor (Each cell in the pattern uses a unique set of frequency bands)

In a hexagonal cell pattern:(to connect without gaps between adjacent cells), only the
following values of N are possible:
𝑁 = 𝐼 2 + 𝐽2 + (𝐼 × 𝐽) 𝐼, 𝐽 = 0,1,2,3,4, …
possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21,…

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Choice of N (assuming constant cell size)

Small N Large N
• More cluster are required to cover the • Less cluster are required to cover the
service area service area
• More capacity • Less capacity
• Higher probability of co-channel • Less probability of co-channel interference
interference
𝑫
= 𝒒 = √𝟑𝑵
𝑹
Where q is the reuse ratio.
𝑫
= √𝑵
𝑹
Theory

Consider a cellular system that has a total of K duplex channels available for use. If each cell is
allocated a group of C channels (C < K), and if the K channels are divided among N cells into
channel groups which each have the same number of channels, the total number of available radio
channels can be expressed as:

𝐾 = 𝐶𝑁
𝑆𝑝𝑒𝑐𝑡𝑟𝑢𝑚𝑏𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ 𝑜𝑟 𝑇𝑜𝑡𝑎𝑙𝑏𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ
𝐾=
Channelbandwidth

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 Assist.Lect.Maryam Abdulhakeem
• Cluster is the number of cells that collectively use the complete set of available
frequencies, The cluster size (N) is typically equal to 4, 7, or 12.
• If a cluster is replicated M times within the system, the total number of duplex
channels can be used as a measure of capacity and is given:

𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦 = 𝑀𝐶𝑁 = 𝑀𝐾

The capacity of a cellular system is directly proportional to the number of times a cluster is
replicated in a fixed service area.

NOTE

If N is reduced while the cell size is kept constant, more clusters are required to cover a given
area and hence more capacity is achieved.
• A large cluster size indicates that the ratio between the cell radius and the distance
between co-channel cells is large.
• A small cluster size indicates that co-channel cells are located much closer together.

To find the nearest co-channel neighbors of a particular cell, one must do the following:

I. Move i cells along any chain of hexagons and then

II. Turn 60 degrees counter-clockwise and move j cells.

i = 3 and j = 2 (N = 19).

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 Assist.Lect.Maryam Abdulhakeem
Example 1: Assume a system of 32 cells with a cell radius of 1.6 km, a total of 32 cells, a
total frequency bandwidth that supports 336 traffic channels, and a reuse factor of N = 7.

a) If there are 32 total cells, what geographic area is covered, how many channels are
there per cell, and what is the total number of concurrent calls that can be handled?
b) Repeat for a cell radius of 0.8 km and 128 cells.

Solution:

a) The area of a hexagon of radius R is

3√3 2 3√3 2
Areaa = R = 1.6 = 6.65 km2
2 2

Total Area Covered = 6.65 × 32 = 213 𝑘𝑚2

336
Number Of Channels Per Cell = = 48
7

𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦 = 48 × 32 = 1536 𝑐ℎ𝑎𝑛𝑛𝑒𝑙𝑠

b) The area of a hexagon of radius R is

3√3 2 3√3 2
Areaa = R = 0.8 = 1.66 km2
2 2

Total Area Covered = 1.66 × 128 = 213 𝑘𝑚2

336
Number Of Channels Per Cell = = 48
7

𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦 = 48 × 128 = 6144 𝑐ℎ𝑎𝑛𝑛𝑒𝑙𝑠

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Example 2: Consider a cellular system in which total available voice channels to handle the
traffic are 960. The area of each cell is 6 𝑘𝑚2 and the total coverage area of the system is
2000 𝑘𝑚2 .
Calculate:
a. The system capacity if the cluster size N is 4
b. The system capacity if the cluster size is 7.

How many times would a cluster of size 4 have to be replicated to cover the entire cellular
area? Does decreasing N increase the system capacity? Explain.

Solution:

a) N=4

Area of a cluster = 4 × 6 = 24 𝑘𝑚2

2000
Number of clusters for covering total area = = 83.33 ~83
24

960
Number of channels per cell = = 240
4

𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦 = 83 × 960 = 79, 680 𝑐ℎ𝑎𝑛𝑛𝑒𝑙𝑠

b) N=7

Area of a cluster = 7 × 6 = 42 𝑘𝑚2

2000
Number of clusters for covering total area = = 47.62 ~ 48
42

960
Number of channels per cell = = 137.15 ~ 137
7

𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦 = 48 × 960 = 46,080 channels

It is evident when we decrease the value of N from 7 to 4, we increase the system capacity
from 46,080 to 79,680 channels. Thus, decreasing N increases the system capacity.

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2.7 Channel Assignment Strategies

• The choice of channel assignment strategy impacts the performance of the system,
particularly as to how calls are managed when a mobile user is handed off from one
cell to another.
• Channel assignment strategies can be classified as either fixed or dynamic.
a. Fixed channel assignment strategy: each cell is allocated a predetermined set of
voice channels.
- Borrowing strategy: a cell is allowed to borrow channels from a neighbouring
cell if all of its own channels are already occupied the mobile switching center
(MSC) supervises.
b. Dynamic channel assignment strategy: voice channels are not allocated to different
cells permanently.
- Each time a call request is made, the serving base station requests a channel
from the MSC.
2.8 Co-channel Interference

The S/I ratio at the desired mobile receiver is given as:

𝑆 𝑆
= 𝑁𝐼
𝐼 ∑
𝑘=1 𝐼𝑘

Where 𝑁𝐼 =the number of interfering cells in the first tier

𝐼𝑘 =the interference due to the kth interferer

Note
In a fully equipped hexagonal-shaped cellular system, there are always six co-channel
interfering cells in the first tier (i.e., 𝑁𝐼 = 6).

In a small cell system, interference will be the dominating factor and thermal noise can be
neglected. Thus, the S/I ratio can be given as:

𝑆 1
=
𝐼 𝐷 −𝛾
∑6𝑘=1 ( 𝑘 )
𝑅

Where 2 ≤ γ ≤ 5: the propagation path loss, and γ depends upon the terrain environment.
𝐷𝑘 : the distance between mobile and kth interfering cell
R: the cell radius

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If we assume 𝐷𝑘 is the same for the six interfering cells for simplification, or D = 𝐷𝑘 , then
Equation above becomes:
𝑆 1 𝑞𝛾
= =
𝐼 6(𝑞)−𝛾 6

1
𝑆 𝛾
∴ 𝑞 = [6 ( )]
𝐼
Since 𝑞 = √3𝑁 therefore
2
1 𝑆 𝛾
𝑁 = [6 ( )]
3 𝐼

Example3: Consider the AMPS in which an S/I ratio of 18 dB is required for the accepted
voice quality. Assume γ = 4.
a. What should be the reuse factor for the system?
b. What will be the reuse factor of the GSM system in which an S/I of 12 dB is required?

Solution:

a.

2
1 𝑆 𝛾
𝑁 = [6 ( )]
3 𝐼
2
1 18 4
𝑁𝐴𝑀𝑃𝑆 = [6 (1010 )] = 6.486 ≈ 6
3
b.
2
1 12 4
𝑁𝐺𝑆𝑀 = [6 (1010 )] = 3.251 ≈ 3
3
Example 4: Consider a cellular system with 395 total allocated voice channel frequencies.
Calculate the mean S/I ratio for cell reuse factor equal to 4, 7, and 12. Assume
omnidirectional antennas with six interferers in the first tier and a slope for path loss of 40
dB/decade (γ = 4). Discuss the results.
Solution:

For a reuse factor N = 4, the number of voice channels per cell site = K/N = 395/4 = 99.
2
1 𝑆 𝛾
𝑁 = [6 ( )]
3 𝐼

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2
1 𝑆 4
4= [6 ( )]
3 𝐼
𝑆
= 24(13.8 𝑑𝐵)
𝐼

The results for N = 7 and N = 12 are given in Table below.

It is evident from the results that, by increasing the reuse factor from N = 4 to N = 12, the
mean S/I ratio is improved from 13.8 to 23.3 dB.
2.9 Co-channel Interference Reduction
• We should avoid increasing the number of cells N in a frequency reuse pattern.
• As N increases, the number of frequency channels assigned to a cell is reduced,
thereby decreasing the call capacity of the cell
• Instead of increasing N:
a) Perform cell splitting to subdivide a congested cell into smaller cells
b) Use a directional antenna arrangement (sectorization) to reduce co-channel
interference. In this case, each cell is divided into three or six sectors and uses three or
six directional antennas at the base station to reduce the number of co-channel
interferers.

• Co-channel interference with 120° sectorized cells.

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 Assist.Lect.Maryam Abdulhakeem

• Co-channel interference with 60° sectorized cells

• Each sector is assigned a set of channels (frequencies) (either 1/3 or 1/6 of the
frequencies of the omnidirectional cell).
2.10 Adjacent Channel Interference (ACI)
• Signals which are adjacent in frequency to the desired signal cause adjacent channel
interference.
• ACI is brought about primarily because of imperfect receiver filters which allow nearby
frequencies to move into the pass band, and nonlinearity of the amplifiers.
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 Assist.Lect.Maryam Abdulhakeem
• The ACI can be reduced by:
1. Using modulation schemes which have low out-of-band radiation.
2. Carefully designing the band-pass filter (BPF) at the receiver front end.
3. Assigning adjacent channels to different cells in order to keep the frequency
separation between each channel in a given cell as large as possible.
• The effects of ACI can also be reduced using advanced signal processing techniques
that employ equalizers.

2.11 Handoff (Handover) Strategies

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.
The Handoff decision is made depending on:
a. Power
b. Traffic
c. Channel quality
d. Distance
e. Administration
The handoff operation involves:
1. Identifying a new base station,
2. Allocate the voice and control signals to channels associated with the new base
station.

Note
acceptable voice quality at the base station receiver (normally taken as between -90 dBm
and -100 dBm)
This margin (cannot be too large or too small) is given by:
𝐴 = 𝑃𝑟𝐻𝑎𝑛𝑑𝑜𝑓𝑓 − 𝑃𝑟𝑀𝑖𝑛𝑖𝑚𝑢𝑚 𝑢𝑠𝑎𝑏𝑙𝑒
• If A is too large, unnecessary handoffs which burden the MSC may occur.
• If A is too small, there may be insufficient time to complete a handoff before a call is
lost due to weak signal conditions
A is chosen carefully to meet these conflicting requirements
Figure below demonstrates the case where a handoff is not made and the signal drops below
the minimum acceptable level to keep the channel active. This dropped call event can happen
when there is an excessive delay by the MSC in assigning a handoff, or when the threshold A
is set too small for the handoff time in the system. The length of time needed to decide if a
handoff is necessary depends on the speed at which the vehicle is moving

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 Assist.Lect.Maryam Abdulhakeem
Excessive delays may occur during high traffic conditions due to
- Either computational loading at the MSC
- Or no channels are available on any of the nearby base stations

• Value of delta is large enough. When the 𝑃𝑟𝐻𝑎𝑛𝑑𝑜𝑓𝑓 is reached, the MSC initiates the
handoff.

• The MSC was unable to perform the handoff before the signal level dropped below
the minimum usable level, and so the call was lost.

2.12 Handoff Types

Handoff can be categorized as hard handoff, soft handoff, and softer handoff.

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 Assist.Lect.Maryam Abdulhakeem
If the hand-off is needed between two cells (BTS) controlled by the same Base Station
Controller (BSC), the MSC is not needed as the BSC does it all.
Types of hand-off are:
1) INTRA-CELL, within a cell, narrow-band interferences could make transmission at a
certain frequency impossible. The BSC decides to change the carrier frequency.
2) INTRA BSS, between cells controlled by the same BSC. The BSC performs the
handover, assigns a new radio channel in the new cell and releases the old one
3) INTER BSS, between cells controlled by different BSCs, and the MSc is involved.
4) INTER MSC-from region to region where more than one MSC is involved. Between
two cells belonging to different MSCs. Both MSCs perform the handover together.

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2.13 Umbrella cell approach

Is technique using different antenna heights (often on the same building or tower) and
different power levels, it is possible to provide "large" and "small" cells which are co-located
at a single location.

• Used to provide large area coverage to high-speed users while providing small area
coverage to users traveling at low speeds
• When the speed of the mobile is too high, the mobile is handed off to the umbrella
cell. The mobile will then stay longer in the same cell (in this case the umbrella cell).
• large cell → high speed traffic → fewer handoffs
• small cell → low speed traffic

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