Department of Information

1
Department of Information Communication
Technology (ICT / TY)

• HDTN (45304)
• TN3431 Mobile Networks

2
Introduction

• Historical Background of Mobile Communication

The first mobile telephone service was introduced
in the United States in 1946 by ATT. This system
was based on FM transmission with a single
transmitter to provide coverage of up to 50 miles
or more. Within a year, mobile telephone systems
were offered in more than 25 American cities.
Demand of service grew rapidly and stayed ahead
of the available capacity.   In 1960s, the Bell
System introduced the Improved Mobile Telephone
Service (IMTS) with enhanced features to improve
the service.   In the late 1960S and early 1970s
work began on the first cellular telephone
systems.   In the early 1970s, the invention of
microprocessor and the advance of digital
techniques allowed more complex control
algorithms to be implemented. The quality and the
service of cellular systems were more enhanced. 
3
In the late 1980s interest emerged in a digital
cellular system, where both the voice and the
control were digital. The use of digital
technology for reproduction of music with CD
popularized the quality of digital audio. The
area was attractive to engineers to pay effort
for development. In the late 1990, digital
cellular service began to emerge to reduce the
cost of wireless communications and improve the
call-handling capacity of an analog cellular
system. The success of the first generation
analog cellular communications systems and the
enormous investment in development second
generation digital technologies have testified to
the world. Wireless communications continue to
experience rapid growth, and new applications and
approaches have spawned at an unprecedented rate.
The number of subscribers for cellular mobile
telephones, cordless phones, and radio pagers
have been increasing manifold. Within these 50
years, the third generation mobile system is
developed from a simple FM broadcasting network.
It is the power of human being. We should be
proud of being a human being.
4
Figure 1. Evolution of Wireless Technologies
5
Mobile Cellular Era and Hong Kong situation

• Overview of Mobile communications

Wireless communications Overview of terms,
Technologies and Services Wireless is an umbrella
term covering many kinds of communications. In
reality, each terms, such as cordless, cellular
and personal has a different connotation.
a.) Cordless It usually refers to devices used
in homes or offices that require no cord to
connect the handset to a base unit, which itself
is connected to the landline- based local
exchange network.
b.) Cellular It is a technology term,
designating the kind of wireless radio
communication that requires a cellular structure
of base stations. The transmission medium is
radio waves, and the underlying technology can be
analogue or digital.
6

• c.) Mobile
• Mobile communication enables connections during
  motion. Although mobility is usually easier to
  achieve with wireless than wirebased devices, it
  is not tied to wireless. Moreover, mobility is
  not tied to cellular. For example, telephones
  with long cords provide limited mobility.

d.) Personal Personal is a function term, can
describe a collection of functions of features of
communication services, such as small,
lightweight terminals, with the ability to
communicate from a variety of places and
reachable at all times and locations.
Limitation of conventional mobile systems   In
conventional mobile system, one or several
channels are selected to serve particular zones
as shown in figure 2.
7
Figure 2. Conventional Mobile Telephone System.
8
Disadvantages

• No hand off
• When an user starts a call in one zone has to
  reinitiate the call when moving into a new zone
  because the call will be dropped. This is an
  undesirable and unconventional radio telephone
  system since the call is incompleted when the
  user is moving from one zone to the other. This
  system is called no hand off which is a process
  of automatic changing frequencies as the mobile
  unit moves into a different frequency zone so as
  to keep the conversation connected without
  interrupting the users.

• Limited coverage
• Owing to the FCC maximum power regulation and
  handset transmitter power limitation, the maximum
  coverage of the communication is limited.

9

• Limited active users
• The number of active users is limited to the
  number of channels assigned to a particular
  frequency zone.

• Poor service performance
• The large number of subscribers created a high
  blocking probability during busy hour.

• Inefficient frequency spectrum utilization
• Each channel can only serve one customer at a
  time in a whole area.

10
First Generation Cellular System (Analog System)
System TACS ETACS AMPS
Frequency band (MHz) 890 915 935 960 872 915 917 960 824 849 869 894
Duplex spacing (MHz) 45 45 45
Number of Carriers 1000 1320 (some freq. band were used by other) 832
Carrier Spacing (KHz) 25 25 30
11
First Generation Mobile Service Cellular
services introduced in Hong Kong in
mid-eighties Analogue systems One AMPS, two
TACS and one E- TACS systems operated by three
companies Operating in 800 / 900 MHz bands
Spectrum capacity approaching saturation in
early nineties Eavesdropping problems Fraud
( cloning) problems
12
Second Generation Cellular System (Digital System)
System GSM900 DAMPS / USDC CDMA DCS1800
Frequency band (MHz) 890.0 915.0 935.0 960.0 835.0 842.5 880.0 887.5 826.59 834.09 871.59 879.09 1710 1785 1805 1880
Duplex spacing (MHz) 45 45 45 95
Carriers 124 250 6 374
Channels/Carrier 8 / 16 3 Dynamic 8 / 16
Spacing (KHz) 200 30 1250 200
13
Second Generation Mobile Service Introduced in
the 800 MHz/ 900 MHz bands in the early
nineties Digital systems ( analogue systems
fully phased out in 1998) Three GSM 900, one
CDMA and one TDMA systems operated by four
companies Personal Communications Services
(PCS) using GSM 1800 operating in 1.7 - 1.9 GHz
bands operational in 1997 Six PCS operated by
six companies (three are also operators in the
800 MHz/ 900 MHz bands)
14
Third Generation Cellular System (Digital System)
System IMT-2000 / WCDMA Unpaired IMT-2000 / WCDMA Paired
Frequency band (MHz) 1904.9 1919.9 2019.7 2024.7 1920.3 197.7 2110.3 2169.7
Duplex spacing (MHz) NA 190
Carriers 4 8
Channels/Carrier Dynamic Dynamic
Spacing (MHz) 5 15
15
Third Generation Mobile Service Introduced in
the 2 GHz bands in the last nineties Digital
systems Wideband CDMA systems operated by
four companies Auction in September 2001
Services operational in 2002/ 03
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• Basic Concept of User Capacity Calculation

• Traffic Model
• Mr. Anger K.Erlang, an Engineer at the Danish
  telephone administration, analysed traffic
  problem in 1919-2
• A n x T / 3600 Erlang
• where n number of calls per hour and subscriber
• T average conversation time (sec)
• A offered traffic per subscriber (Erlang)

17
Example 1 Q.1 In the past, a conventional mobile
telephone system was deployed in New York City.
It is called Improved Mobile Telephone Services
(IMTS) MK systems. At that moment, there were 6
channels serving 225 customers, with another 1300
customers on a waiting list. The large number of
subscribers created a high blocking probability
during busy hours. On average, the call holding
time is about 1.76minutes. Applying Erlang B
model, calculate the blocking probability of the
system and comment the performance when the
average holding time is increased.
Ans. The offered traffic is
From the Erlang B table, we can get the blocking
probability is 30 which means 100 calls
initiated only have 70 calls getting through. If
the actual average calling time is greater than
1.76min, the blocking probability can be even
higher.
18
Tutorial 1 Q1. In New York City, a conventional
mobile telephone system was called Improved
Mobile Telephone Services (IMTS) MJ systems. In
the system, the customers to channel ratio was 53
and there were 6 channels with another 2400
customers on a waiting list. On average, the call
holding time is about 105.6 seconds. By making
use of Erlang B table, calculate the blocking
probability of the system. (Ans 9.33 Erlangs
50 Blocking probability) Q2. Why are the taxi
drivers still using the conventional mobile
system to communicate between taxi station?
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• Hong Kong Mobile Market

Development of mobile telecommunication
systems Public Mobile Radiotelephone Service
(PMRS) Personal Communications Services
(PCS) 3G Wireless Communication Services (3G)
(Year 2002/03)
Public mobile radiotelephone service (PMRS)
Traffic is mainly between mobiles and Public
Switched Telephone Network (PSTN) users
Full Duplex Operation (Tx and Rx simultaneously)
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Public mobile radiotelephone service (PMRS)
(contd)
21
Personal communications services (PCS) PCS
is the system by which every user can exchange
information with anyone, at anytime, in any
place, through any type of devices, using a
single personal telecommunication number.
This implies low-cost ubiquitous services
22
Personal communications services (PCS) (contd)
23
3G Wireless Communication Services (3G) 3G
is the new generation of mobile
telecommunications, mobile access to personalised
multi-media services - anywhere, anytime .
Consumers will enjoy mobile-internet and
mobile-commerce services e.g. e-shopping, video
services .
24
3G Wireless Communication Services (3G) (contd)
25

• Current core products and value-added products
  introduction

Curent core product in the mobile market
Mobile phone Services Value-added products in
the mobile market Short Message Services
(SMS) Wireless Application Protocol (WAP)
General Packet Radio Services (GPRS)
26
Frequency Spectrum Management

• Multiple Access

Spectrum Efficiency Considerations The greatest
problem in radio communication industry is the
limitation of available radio frequency spectrum.
In setting allocation policy, the FCC (Federal
Communications Commission) seeks systems which
need minimal bandwidth but provide high usage and
consumer satisfaction. The possible approaches
have a. Cellular concept reuses the allocated
frequency band in different geographic
locations b. Spread spectrum method allows
different users to occupy a wide frequency
bandwidth at a same time.
27
One of the most important concepts to any
cellular telephone system is that of "multiple
access", meaning that multiple, simultaneous
users can be supported. In other words, a large
number of users share a common pool of radio
channels and any user can gain access to any
channel (each user is not always assigned to the
same channel). A channel can be thought of as
merely a portion of the limited radio resource
which is temporary allocated for a specific
purpose, such as someone's phone call. A
multiple access method is a definition of how the
radio spectrum is divided into channels and how
channels are allocated to the many users of the
system.
Current Cellular Standards Different types of
cellular systems employ various methods of
multiple access. The traditional analog cellular
systems, such as those based on the Advanced
Mobile Phone Service (AMPS) and Total Access
Communications System (TACS) standards, use
Frequency Division Multiple Access (FDMA). FDMA
channels are defined by a range of radio
frequencies, usually expressed in a number of
kilohertz (kHz), out of the radio spectrum.
28
FDMA For example, AMPS systems use 30 kHz
"slices" of spectrum for each channel. Narrowband
AMPS (NAMPS) requires only 10 kHz per channel.
TACS channels are 25 kHz wide. With FDMA, only
one subscriber at a time is assigned to a
channel. No other conversations can access this
channel until the subscriber's call is inished,
or until that original call is handed off to a
different channel by the system.
TDMA A common multiple access method employed in
new digital cellular systems is Time Division
Multiple Access (TDMA). TDMA digital standards
include North American Digital Cellular (known by
its standard number IS-54), Global System for
Mobile Communications (GSM), and Personal
Di gital Cellular (PDC). TDMA systems commonly
start with a slice of spectrum, referred to as
one "carrier". Each carrier is then divided into
time slots. Only one subscriber at a time is
assigned to each time slot, or channel. No other
conversations can access this channel until the
subscriber's call is finished, or until that
original call is handed off to a different
channel by the system. For example, IS-54
systems, designed to coexist with AMPS systems,
divide 30 kHz of spectrum into three channels.
PDC divides 25 kHz slices of spectrum into three
channels. GSM systems create 8 time-division
channels in 200 kHz wide carriers.
29
CDMA With CDMA, unique digital codes, rather than
separate RF frequencies or channels, are used to
differentiate subscribers. The codes are shared
by both the mobile station (cellular phone) and
the base station, and are called "pseudo-Random
Code Sequences. All users share the same range
of radio spectrum at a same time. For cellular
telephony, CDMA is a digital multiple access
technique specified by the Telecommunications
Industry Association (TIA) as "IS-95". IS-95
systems divide the radio spectrum into carriers
which are 1,250 kHz (1.25 MHz) wide. One of the
unique aspects of CDMA is that while there are
certainly limits to the number of phone calls
that can be handled by a carrier, this is not a
fixed number. Rather, the capacity of the system
will be dependent on a number of different
factors. In September, 1995, Hutchison Telephone
of Hong Kong, a Motorola Cellular Infrastructure
Group customer, became the world's first operator
to initiate commercial CDMA service.
30
Multiple Access Comparison It is easier to
understand CDMA if it is compared with other
multiple access technologies. The following
sections describe the fundamental differences
between i. Frequency Division Multiple Access
Analog technology (FDMA) ii. Time Division
Multiple Access Digital technology (TDMA) iii.
Code Division Multiple Access Digital technology
(CDMA)
31
FDMA - Frequency Division Multiple Access FDMA
is used for standard analog cellular. Each user
is assigned a discrete slice of the RF spectrum.
FDMA permits only one user per channel since it
allows the user to use the channel 100 of the
time. Therefore, only the frequency "dimension"
is used to define channels. Figure 1
FDMA
Channel Width X 30 KHz (AMPS) X 25 KHz (TACS)
Time
Frequency
1 User per Narrowband Channel
32
TDMA - Time Division Multiple Access The key
point to make about TDMA is that users are still
assigned a discrete slice of RF spectrum, but
multiple users now share that RF carrier on a
time slot basis. Each of the users alternate
their use of the RF carrier. Frequency division
is still employed, but these carriers are now
further sub-divided into some number of time
slots per carrier. A user is assigned a
particular time slot in a carrier and can only
send or receive information at those times. This
is true whether or not the other time slots are
being used. Information flow is not continuous
for any user, but rather is sent and received in
"bursts." The bursts are re-assembled at the
receiving end, and appear to provide continuous
sound because the process is very
fast. Figure 2 TDMA
200 KHz Channels for GSM

½ 200KHz ½
Time
Frequency
8 Users per Channel
33
CDMA - Code Division Multiple Access IS-95 uses
a multiple access spectrum spreading technique
called Direct Sequence (DS) CDMA. Each user is
assigned a binary, Direct Sequence code during a
call. The DS code is a signal generated by linear
modulation with wideband Pseudorandom Noise (PN)
sequences. As a result, DS CDMA uses much wider
signals than those used in other technologies.
Wideband signals reduce interference and allow
one-cell frequency reuse. There is no time
division, and all users use the entire carrier,
all of the time. Figure 3 DS-CDMA
Time
Frequency
N Users per Narrowband Channel
34
The International Cocktail Party To illustrate
the conceptual differences among the multiple
access technologies, the "International Cocktail
Party" analogy will be applied. Picture a large
room and a number of people, in pairs, who would
like to hold conversations. The people in each
pair only want to talk and listen to each other,
and have no interest in what is being said by the
other pairs. In order for these conversations to
take place, however, it is necessary to define
the environment for each conversation. First,
let us apply this analogy to an FDMA system. An
FDMA environment would be simulated by building
walls in the single large room, creating a larger
number of small rooms. A single pair of people
would enter each small room and hold their
conversation. When that conversation is complete,
the pair of people would leave and another pair
would be able to enter that small room.
35
In a TDMA environment, each of these small rooms
would be able to accommodate multiple
conversations "simultaneously." For example, with
an 8 slot TDMA system such as GSM, each "room"
would contain up to 8 pairs of people, with each
pair taking turns talking. Think of each pair
having the right to speak for 7.5 seconds during
each minute, with pair A able to use 00100
second through 00750 second, pair B using
007.60 second through 015.00 second, pair C
using 015.10 second through 022.50 second, pair
D using 022.60 second through 030.00
second, pair E using 030.10 second through
037.50 second, pair F using 037.60 second
through 045.00 second, pair G using 045.10
second through 052.50 second and pair H using
052.60 second through 060.00 second. Even if
there are fewer than three pairs in the small
room, each pair is still limited to its 7.5
seconds per minute.
36
Now, for CDMA, get rid of all of the little
rooms. Pairs of people will enter the single
large room. However, if every pair uses a
different language, they can all use the air in
the room as a carrier for their voices and
experience little interference from the other
pairs. The analogy here is that the air in the
room is a wideband "carrier" and the languages
are represented by the "codes" assigned by the
CDMA system. In addition, language "filters" are
incorporated, people speaking Chinese will hear
virtually nothing from those speaking English,
etc. We can continue to add pairs, each speaking
a unique language (as defined by the unique code)
until the overall "background noise"
(interference from other users) makes it too
difficult for some of the people to understand
the other in their pair (frame erasure rates get
too high). By controlling the voice volume
(signal strength) of all users to no more than
necessary, we maximize the number of
conversations which can take place in the room
(maximize the number of users per carrier). Of
course, if there is a person speaking too loud,
it will interfere to others significantly. So in
CDMA system, power control is a critical
issue. Therefore, the maximum number of users,
or effective traffic channels, per carrier
depends on the amount of activity that is going
on in each channel, and is therefore not precise.
It is a "soft overload" concept where an
additional user (or conversation, in our analogy)
can usually be accommodated if necessary, at the
"cost" of a bit more interference to the other
users.
37

• Classification description of CDMA DSSS FHSS

CDMA Technology Though CDMA's application in
cellular telephony is relatively new, it is not a
new technology. CDMA has been used in many
military applications, such as anti-jamming
(because of the spread signal, it is difficult to
jam or interfere with a CDMA signal), ranging
(measuring the distance of the transmission to
know when it will be received), and secure
communications (the spread spectrum signal is
very hard to detect). Spread Spectrum CDMA is a
"spread spectrum" technology, which means that it
spreads the information contained in a particular
signal of interest over a much greater bandwidth
than the original signal. The standard data rate
of a CDMA call is 9600 bits per second (9.6
kilobits per second). This initial data is
"spread," including the application of digital
codes to the data bits, up to the transmitted
rate of about 1.23 megabits per second. The data
bits of each call are then transmitted in
combination with the data bits of all of the
calls in the cell. At the receiving end, the
digital codes are separated out, leaving only the
original information which was to be
communicated. At that point, each call is once
again a unique data stream with a rate of 9600
bits per second.
38
Spread-Spectrum Modulation Spread Spectrum is
defined as a communication technique in which the
intended signal is spread over a bandwidth in
excess of the minimum bandwidth required to
transmit the signal. The advantage of a
spread-spectrum communication system is it can
reject interference. Pseudo-noise
Sequences Spreading is accomplished by using high
rate code known as pseudo-noise code (PN). It is
defined as coded sequence which has long period
to repeat. An example is maximum length
sequence which has a period of N 2m - 1 m is
the length of the shift register. N is the PN
sequence length.
39
There are several ways to classify CDMA schemes.
The most common is the division based on the
modulation method used to obtain the wideband
signal, Direct Sequence (DS) and Frequency
Hopping (FH). Direct-Sequence Spread-Spectrum
(DSSS) In DSSS, a data sequence b(t) is modulated
by a wide-band PN sequence c(t) by applying
multiplication. Figure 4 shows the basic model of
baseband DSSS system. transmit
receive Figure 4 Idealised model of
baseband spread spectrum system By
multiplication, each information data bit is
divided up into a number of small time segments,
as shown in Figure 5. These small time segments
are commonly called as chips.
40
(a) Data b(t)
(b) Spreading code c(t)
(c) Product signal m(t)
Figure 5 Illustrating the waveforms in the
transmitter For baseband transmission, the
product signal m(t) represents the transmitted
signal. The expression is as the
following m(t) c(t)b(t)
41
The received signal r(t) consists of the
transmitted signal and the channel noise
or interference denoted by i(t). Hence r(t)
m(t) i(t) c(t)b(t) i(t) To recover the
data sequence b(t), the received signal will be
multiplied with the locally generated PN sequence
which is exact replica of that used in the
transmitter. The result of demodulation is
therefore given by z(t) c(t)r(t) c(t) 2
b(t) c(t)i(t) The spreading code c(t)
alternates between the levels -1 and 1, and the
alternation is destroyed when it is squared,
therefore, c(t) 2 1 for all t From above
equation, z(t) c(t) 2 b(t) c(t)i(t) can be
simplified as z(t) b(t) c(t) i(t)
42
So we can pass the processed signal into the low
pass filter which bandwidth just large enough to
accommodate the recovery of the information
sequence b(t) (narrow band). It removes most of
the power of the interferener (wide band). The
effect of the interference i(t) is thus
significantly reduced at the receiver output. The
direct sequence is not only suitable for baseband
transmission. It also can incorporate with
coherent binary phase-shift keying (PSK) to
transmit over a bandpass channel.
43
Frequency-Hopping Spread-Spectrum (FHSS) In
Frequency hopping, the carrier frequency of the
modulated information signal is not constant but
changes periodically. During time intervals T,
the carrier frequency remains the same, but after
each time interval the carrier hops to another
(or possibly the same) frequency. The hopping
pattern is decided by the spreading code. The set
of available frequencies the carrier can attain
is called the hop-set. Figure 6 show how the
difference between FHSS and DSSS of frequency
usage in the whole frequency band.


















frequency ¾¾
frequency ¾¾
¾¾ time
¾¾ time
FH
DS
Figure 6 Time/frequency occupancy of FH and DS
signals
44
FH system use only a small part of the bandwidth
when it transmits, but the location of this part
differs in time. DS system occupies the whole
frequency band when it transmits. In power
consumption, DS system transmitting the signal
power over the whole frequency band is spread,
so the power is much less than FH system in the
same time period of that small part of frequency
band. But on average, DS system transmits the
spread signal power during all time periods while
FH system only uses this band part of the time,
so both systems will transmit the same power in
that small part of frequency band.
45
Figure 7 shows the basic model of baseband FHSS
system. transmit
receive
b(t)
m(t)

Frequency syntheziser
c(t)
z(t)
r(t)
output
BasebandLPF
Frequency syntheziser
c(t)
Figure 7 Idealised model of baseband frequency
hopping spread spectrum system
46
Using a fast frequency syntheziser that is
controlled by the code signal, the carrier
frequency is converted up to the transmission
frequency. To recover the data, using a locally
generated code sequence, the received signal is
converted down to the baseband just like DSSS.
Fast Hopping and Slow Hopping Within FHSS, a
distinction is made that is based on the hopping
rate of the carrier. If the hopping rate is
(much) greater than the symbol rate, the
modulation is considered to be fast frequency
hopping (F-FH). In this case the carrier
frequency changes a number of times during the
transmissioning of one symbol, so that one bit is
transmitted in different frequencies. If the
hopping rate is (much) smaller than the symbol
rate, one speaks of slow frequency hopping
(S-FH). In this case multiple symbols are
transmitted at the same frequency.
47

• Concept of frequencty reuse

Overall Cellular Mobile Network Configuration
In a cellular system (Fig. 8), the area to be
covered is divided into a number of small areas
called cells. A base station is used in the cell
to serve the mobiles within its coverage. Each
base station is connected by some fixed links
(depends on traffic requiremnt) to a MSC. The
MSCs are interconnected to each other and to the
public switched telephone network (PSTN).
Figure 8 Cellular network configuration   Abbrev
iation MS ----- Mobile station BS
----- Base station MSC -- Mobile switching
centre
48
Radio Cells   Hexagonal-shaped cell layouts are
used in initial design to partition coverage
areas. In flat terrain, circular cell layouts
can be used. In most urban area, irregular
layouts are used (Fig. 9). Overlapping areas
among cells for call handover.
Fictitious
Ideal
Real
Figure 9 Cell coverage
49
Cluster and Frequency Re-use   Each cellular
network has assigned two bands of radio spectrum
for duplex operation. The duplex bands are
divided into a number of channel pairs and are
then assigned to a defined number (N) of cells.
The N cells form a group known as the cluster.
The cluster repeats itself over the whole
coverage area and therefore frequencies are
re-used as many times as possible depending upon
the number of clusters. The pattern of the cells
within a cluster is fixed (Fig. 10) as it is
optimized for minimizing interference and
therefore N is also known as reuse pattern.
Figure 10 3-cell cluster reuse patterns
50
Frequency Channels   There are two groups of
channel operate in forward or down (BS to MS) and
reverse or up (MS to BS) direction.   ? Traffic
channels are for speech or data communication. ?
Control channels are for management
purposes.     Figure 11 Communication
between mobile and base station   Location
Registration   When a mobile is not engaged in a
call, it tunes to the control channel of the
situated cell and monitors the forward signalling
information. As the mobile moves across the
network, it will scan all control channels and
lock onto the strongest one which is usually the
situated cells control channel.
51
The mobile checks the location information being
broadcast and if this differs from the previous
cell, the mobile automatically updates the
network of its location. Therefore, the network
continues to maintain an updated location
database of all mobiles.   Handoff /
Handover   It is a process of transferring a call
to another base station. Handoff normally occurs
when the mobile is at the cell boundary or in
signal-strength holes within a cell.   Handoff
decision is based on one or more of the following
conditions          Received power is below
certain threshold (e.g. -95dBm).        Received
C/I is below certain threshold (e.g.
18dB).        A better channel is available from
adjacent cell.        The local cell is too
congested while the adjacent cell is not (forced
handoff).
52
Frequency Reuse Pattern Selection   Because of
the frequency re-use, nearby mobiles may use the
same frequency and cause co-channel interference.
The key objective of planning a cellular radio
system is to design the reuse pattern N and
frequency allocation in order to maximize the
capacity of the network whilst controlling
co-channel interference and other interference to
within acceptable limits.   Typical values of N
are 3, 4, 7 and 12 (Fig. 12). Reducing N will
increase the trunking efficiency. However, as N
decreases, the distance between cocells (cells
with same channels) also reduces, which increases
the co-channel interference.   Figure 12
Cell reuse patterns
53
The minimum distance which allows the same
frequency to be reused is called reuse distance
(D). D is related to the cell radius R as shown
in Fig. 13 and the ratio of D to R, called the
reuse ratio, is a function of N,
namely     Figure 13 D and R of a
cluster Co-channel Interference   During a call,
the mobile receives wanted carrier signal (C)
from the base station in which it is located, and
also interfering signal (I) from other cells.
The carrier to interference ratio (C/I) is
related to D/R.  
54
For the 3-cell, 4-cell and 7-cell cluster there
could be up to six immediate interferes, as shown
in Fig. 14.     Figure 14 The geometry
associated with interfering cells using 3-cell
4-cell and 7-cell clusters   Assuming the fourth
power propagation law, an approximate value of
C/I is   C/I  
55
Using the D/R ratio,   C/I   This shows
that the C/I is a function of the reuse pattern
N. For example, suppose N 7, then C/I ? 73, or
18 dB.   Cellular radio systems are designed to
tolerate a certain amount of C/I. Beyond this
level, speech quality will be severely degraded.
This lower limit on C/I effectively sets the
minimum D/R ratio that can be used.   The C/I
requirement has two other factors may need to be
taken into account                     
Adjacent channel interference from near channels
in neighbouring cells.                   
Multipath fading which may weaken C as compared
to I.   The C/I requirement for analogue cellular
systems varies from 18 to 21 dB. The C/I
requirement for GSM is 9 dB in theory and is 15
dB in practice to provide quality services.
56
Frequency Reuse Pattern of CDMA System   In CDMA
system, because of spread spectrum technique,
much high C/I is acceptable. Therefore, all
frequencies are re-use in every sector of
cell.     Figure 15 CDMA reuse pattern

• Power Control
•  
• Both the base station and the mobile transmission
  power needed to be controlled. This has multiple
  effects of
•  
• reducing power consumption of the mobile,
• reducing the co-channel and adjacent channel
  interference,
• reducing the generation of intermodulation
  product, and
• (iv)       conforming the coverage of cell.
•  

57
Cell Size   Cell size need not be the same. The
cells are subdivided into smaller cells
(microcells) towards the middle of the city to
permit management of a higher density of users.
The peripheral is served by large cells
(macrocells). The underlying principle is if
each cell (whether large or small) has the same
amount of frequency channels, a small cell in
size can carry more traffic per unit area than a
large cell.   Sectorization   Sectorization is a
standard practice in most cellular systems. In a
regular cellular layout, co-channel interference
will be received from six surrounding cells. One
way of cutting the interference is to use several
directional antennas at the base stations, with
each antenna illuminating a sector of the cell,
and with a separate channel set allocated to each
sector. One pair of control channel is assigned
in each sector.   60o and 120o cell sectorization
are commonly employed (Fig. 16). It reduces the
number of prime interference sources to one and
two respectively (Fig. 17)
58
Figure 16 60o and 120o sectors
Figure 17 Interference in 7-cell cluster with
sectors
59
From Fig. 16 and Eq. 17, worst case
C/I   (7-cell cluster with six
sectors/cell)   C/I
779 29 dB   (7-cell cluster with three
sectors/cell)   C/I
282 24.5 dB   A disadvantage
is that the channel sets are divided between
sectors and trunking efficiency is reduced.
However, improved C/I allows the system to use a
smaller reuse pattern N. The net effect of
sectorization is an increase in the total system
capacity (Table 1). After sectorization, the
original cell coverage is no more valid. Fig. 18
shows the radiation pattern and coverage of the
new 120o sectored cell. Fig. 19 shows the
3-site, 9-cell cluster and the 4-site, 12-cell
cluster which are evolved from the 3-cell and
4-cell clusters.
60
System (with 396 channels) N Channels per Sector Offered Load (E) per Cell Mean C/I (dB)
Omni 4 4 4 4
Omni 99 99 99 99
Omni 86.7 86.7 86.7 86.7
120o Sector 13.8 13.8 13.8 13.8
120o Sector 7 7 7 7
120o Sector 56/57 56/57 56/57 56/57
60o Sector 45.9 / 46.9 45.9 / 46.9 45.9 / 46.9 45.9 / 46.9
60o Sector 18.7 18.7 18.7 18.7
60o Sector 12 12 12 12
Table 1 Omni vs. Sectorized cellular system
performance
61
Figure 18 Cell and 120o sectored cell
62
Þ
3-cell cluster
3/9-cell cluster
Þ
4/12-cell cluster
4-cell cluster
Figure 19 Examples of reuse patterns Frequency
Channel Assignment   Cautions have been taken in
controlling co-channel interference (Fig 12).
Frequency assignment schemes are able to control
co-channel interference but mainly it is used to
control adjacent channel interference (ACI).
Main criterion is to maintain frequency
separation between channels in the same cell or
in an adjacent cell.
63
Consider the 4-cell cluster in Fig. 10 as an
example, a simple frequency assignment will
be   f1 ? cell A, f2 ? cell B, f3 ? cell C, f4 ?
cell D, f5 ? cell A, f6 ? cell B, f7 ? cell C, f8
? cell D, .   The main advantage of this
arrangement is that all frequencies within one
cell are widely separated (e.g., in B f2, f6,
f10, ). It will greatly reduce the ACI.   ACI
also appears when two adjacent frequencies (e.g.,
f3 and f4) are assigned to two adjacent cells.
It can be shown that a careful frequency
assignment provides smaller ACI but cannot
eliminate ACI. In general, ACI decreases with
the increase of cluster size N.   For sectored
cases, the cells are numbered as shown in Fig.
18. Consider the 4/12-cell cluster as an
example, a simple frequency assignment will
be   f1 ? cell A1, f2 ? cell B1, f3 ? cell
C1, f4 ? cell D1, f5 ? cell A2, f6 ? cell B2, f7
? cell C2, f8 ? cell D2, f9 ? cell A3, f10 ? cell
B3, f11 ? cell C3, f12 ? cell D3,
64
f13 ? cell A1, f14 ? cell B1, f15 ? cell C1, f16
? cell D1, f17 ? cell A2, f18 ? cell B2, f19 ?
cell C2, f20 ? cell D2, .   It can be shown that
there is no adjacent frequency in adjacent cells
when cluster is equal or larger than 4/12-cell
cluster.
A3
D2
A2
D1
C2
C3
B2
B1
C3
A1
C3
C2
B2
C1
A3
A2
B1
A1
B3
B2
D1
A1
A3
C1
A3
C1
C2
D3
B3
B2
A1
A2
B1
C2
B1
B1
D1
D2
C3
C2
C1
A3
C2
B2
A1
D3
C3
A1
B2
B3
D1
B1
A2
3/9-cell cluster
4/12-cell cluster
Figure 20 Frequency assignments of 3/9,
4/12-cell clusters
65
Reuse Pattern Selection   7/21 reuse pattern It
provides ideal isolation of co-channel and
adjacent channel interference.   4/12 reuse
pattern Most GSM systems are planned around this
reuse pattern over flat terrain.   3/9 reuse
pattern This pattern can be used by GSM in
theory, but it has the problem of the adjacent
channel interference.   Antenna
Arrangement   Fig. 21a is a typical antenna
configuration layout for a 3-sector cell site.
Two receive antennas provide space-diversity
against multi-path fading. Fig. 21b shows the
6-sector antenna array. Control channels are
transmitted and received through omni antennas or
they can be simulcasted with the traffic channels
through combiner and splitter.
66
Figure 21 Antenna array of cell site