UNIT 11: MOBILE PHONE AND RADIO COMMUNICATION
Key unit competence: By the end of the unit I should be able to
distinguish mobile phone system from radio system of communication.
Unit Objectives:
By the end of this unit I will be able to;
◊ explain the concept and principles of cellular radio network.
◊ explain the need for cellular system in modern mobilecommunication.
Introductory Activity
The figure below shows how network for a certain telecommunications
company in Rwanda. Study it carefully and answer the followingquestions.
Network transmission
a. How many cells can you see in the figure above?
b. Identify different masts shown on the figure.
c. In regard to the figure, what is the importance of masts in
those different cells?
d. Why do you think in transmission of network, the targeted
area is divided into small portions?
e. Compare the number of cells that should be allocated for urbanareas to those for rural areas.
11.0 INTRODUCTION
The communication is the way of expressing our thoughts. In other words,
communication means sending or receiving message from one end to
other. We can express our feelings to others by speaking, writing or silent
indications. All living beings communicate to each other in different ways.
They have different types of voices and they understand meaning of voice
of their species. Human has also developed his dialect to communicate with
others. We learn different languages to understand meaning of other’s
dialects.
Devices used to talk, or to send message one end to other, or from one person
to other are called means of communication. Means of Communication are
the most necessary part of modern lifestyle. In modern age, there are many
types of means of communications like newspaper, Telephone, Mobile, TV,
Internet etc. They play very important role in our daily life activities.
This concept is closely related to the concepts of blood circulation (in Biologyand Medicine), transport networks, transmission of information etc.
11.1 CONCEPTS OF TRANSMISSION SYSTEM
In telecommunication, a communication system is a collection of individual
communication networks, transmission systems, relay stations tributary
stations and Data Terminal Equipment (DTE) usually capable ofinterconnection and interoperation to form an integrated whole.
In the transmission section, first of all, the source generated information is
fed to the input transducer, which converts energy of one form to another
form, usually in electrical form. This electrical signal or base band signal is
sent to the transmitter.
Transmitter:
Transmitter modifies the information signal for efficient transmission.
It modulates the information signal with a high frequency carrier. After
processing the signal transmitter transmits the signal, through channel to
the receiver.
Channel:
Channel, media or path implies the medium through which the message
travels from the transmitter to the receiver. A channel acts partly as a filter
to attenuate the signal and distorts its waveform. The signal attenuation
increases with the length of the channel. There are different types of
channels for different communication systems, such as wire, coaxial cable,
wave-guide, optical fiber or radio link through which transmitter output is
sent.
Receiver:
Receiver reprocesses the signal received from the channel by undoing the
signal modifications made at the transmitter and the channel. The receiver
output is fed to the output transducer, which converts the electrical signal
to its original form. By this way, the signal reached to its destination, towhich the message is communicated.
Digital communication:
Digital communication system exchange (both transmit and receive)
information to /from digital sources.
A digital (information) source produces a finite set of possible messages.
Typewriter is a good example of a digital source. There is a finite no. of
characters that can be emitted by this source.
Analog communication:
Analog communication system exchange (both transmit and receive)
information to /from analog sources. A microphone is a good example of an
analog source. An analog information source produces messages that are
defined on a continuum.
Why do we use digital not analog?
Digital communication has a number of advantages:
• Relatively inexpensive digital circuits may be used.
• Digital systems are relatively easy to design and can be fabricated on
IC chips.
• Information storage is easy.
• Operation can be programmable to update with newly upcoming
technologies.
• Privacy is preserved by using data encryption.
• Greater dynamic range is possible.
• Data from voice, video and data sources may be merged and transmitted
over a common digital transmission system. i.e. it is easy to multiplex
several digital signals.
• In long distance communication system, noise does not accumulate
from repeater to repeater.
• Error detection and correction schemes can be employed by using
coding techniques.
Limitations of Digital communication system
• Generally, more bandwidth is required than that for analog system.
• Synchronization is required, which calls for more sophisticated device
and costs more.
A/D converter
We use analog to digital converter, to convert analog signals to digital
signals.
A/D conversion has three steps:
(a) Sampling
In this process, Continuous-time signal is converted to Discrete-time signal
obtained by taking samples of the continuous-time signal at discrete-time
instants.
(b) Quantization
In this process, a Discrete-time Continuous- valued signal is converted
into a Discrete-time Discrete-valued (digital) signal. The sampled signal is
rounding off to the fourth nearest value which is permitted for transmission
by the system. The process of rounding off is called Quantization, while the
possible levels permitted for transmission are called Quantizing levels.
(c) Coding
In the coding process, each discrete value is represented by 8-bit binary
sequence e.g. 10010101. It consists of combinations of 0 and 1.
11.2 PRINCIPLE OF CELLULAR RADIO
The cellular concept was a major breakthrough in solving the problem of
spectral congestion and user capacity. It offered very high capacity output
in a limited spectrum allocation without any major technological changes.
The cellular concept is a system-level idea which calls for replacing a single,
high power transmitter (large cell) with many low power transmitters (small
cells), each providing coverage to only a small portion of the service area.
Each base station is allocated a portion of the total number of channels
available to the entire system, and nearby base stations are assigned
different groups of channels so that all the available channels are assigned
a relatively small number of neighbouring base stations. Neighbouring base
stations are assigned different groups of channels so that the interference
between base stations (and the mobile users under their control) is
minimized.
By systematically spacing base stations and their channel groups throughout
a market, the available channels are distributed throughout the geographic
region and may be reused as many times as necessary so long as the
interference between co-channel stations is kept below acceptable levels.
11.3 STRUCTURE OF CELLULAR NETWORK
An overall cellular network contains a number of different elements from
the base transceiver station (BTS) itself with its antenna back through
a base station controller (BSC), and a mobile switching centre (MSC)
to the location registers (HLR and VLR) and the link to the public switched
telephone network (PSTN).
Of the units within the cellular network, the BTS provides the direct
communication with the mobile phones. There may be a small number of
base stations linked to a base station controller. This unit acts as a small
centre to route calls to the required base station, and it also makes some
decisions about which base station is the best suited for a particular call.
The links between the BTS and the BSC may use either land lines of even
microwave links. Often the BTS antenna towers also support a small
microwave dish antenna used for the link to the BSC. The BSC is often
co-located with a BTS.
The BSC interfaces with the mobile switching centre. This makes more
widespread choices about the routing of calls and interfaces to the land linebased PSTN as well as the location registers.
11.4 PRINCIPLE OF CELLULAR NETWORK
Increase in demand and the poor quality of existing service led mobile
service providers to research ways to improve the quality of service and
to support more users in their systems. Because the amount of frequency
spectrum available for mobile cellular use was limited, efficient use of the
required frequencies was needed for mobile cellular coverage. In modern
cellular telephony, rural and urban regions are divided into areas accordingto specific provisioning guidelines.
Deployment parameters, such as amount of cell-splitting and cell sizes,
are determined by engineers experienced in cellular system architecture.
Provisioning for each region is planned according to an engineering plan
that includes cells, clusters, frequency reuse, and handovers.
Cells
A cell is the basic geographic unit of a cellular system. The term cellular
comes from the honeycomb shape of the areas into which a coverage region
is divided. Cells are base stations transmitting over small geographic areas
that are represented as hexagons. Each cell size varies depending on the
landscape. Because of constraints imposed by natural terrain and man
made structures, the true shape of cells is not a perfect hexagon
Clusters
A cluster is a group of cells. No channels are reused within a cluster.
Fig.11-2 illustrates a seven-cell cluster. In clustering, all the available
frequencies are used once and only once. As shown on Fig.11-3, each cell
has a base station and any mobile user moving remains connected due tohand-offs between the stations.
Frequency Reuse
Because only a small number of radio channel frequencies were available
for mobile systems, engineers had to find a way to reuse radio channels in
order to carry more than one conversation at a time. The solution was called
frequency planning or frequency reuse. Frequency reuse was implemented
by restructuring the mobile telephone system architecture into the cellular
concept.
The concept of frequency reuse is based on assigning to each cell a group of
radio channels used within a small geographic area. Cells are assigned a
group of channels that is completely different from neighbouring cells. The
coverage area of cells are called the footprint. This footprint is limited by a
boundary so that the same group of channels can be used in different cells
that are far enough away from each other so that their frequencies do notinterfere.
Cells with the same number have the same set of frequencies. Here, because
the number of available frequencies is 7, the frequency reuse factor is 1/7.
That is, each cell is using 1/7 of available cellular channels.
Cell Splitting
Unfortunately, economic considerations made the concept of creating fullsystems with many small areas impractical. To overcome this difficulty,
system operators developed the idea of cell splitting. As a service area
becomes full of users, this approach is used to split a single area into
smaller ones. In this way, urban centers can be split into as many areas
as necessary in order to provide acceptable service levels in heavy-traffic
regions, while larger, less expensive cells can be used to cover remote rural
regions.
Handoff
The final obstacle in the development of the cellular network involved the
problem created when a mobile subscriber travelled from one cell to another
during a call. As adjacent areas do not use the same radio channels, a call
must either be dropped or transferred from one radio channel to another
when a user crosses the line between adjacent cells. Because dropping the
call is unacceptable, the process of handoff was created. Handoff occurs
when the mobile telephone network automatically transfers a call fromradio channel to radio channel as a mobile crosses adjacent cells.
During a call, two parties are on one voice channel. When the mobile unit
moves out of the coverage area of a given cell site, the reception becomes
weak. At this point, the cell site in use requests a handoff. The system
switches the call to a stronger-frequency channel in a new site without
interrupting the call or alerting the user. The call continues as long as the
user is talking, and the user does not notice the handoff at all.
11.5 MOBILE COMMUNICATION SYSTEMS
Mobile communication systems have become one of the hottest areas in the
field of telecommunications and it is predicted that within the next decade,
a considerable number of connections will become partially or completely
wireless. Rapid development of the Internet with its new services and
applications has created fresh challenges for the further development of
mobile communication systems.
We can say that mobile communication system is a high capacity
communication system arranged to establish and maintain continuity of
communication paths to mobile stations passing from the coverage of one
radio transmitter into the coverage of another radio transmitter. A control
center determines mobile station locations and enables a switching center
to control dual access trunk circuitry to transfer an existing mobile station
communication path from a formerly occupied cell to a new cell location.
The switching center subsequently enables the dual access trunk to releasethe call connection to the formerly occupied cell.
ACTIVITY 11-1: The Concept of Communication
Aim: this activity aim at understanding the concept of
communication.
a) The figure below shows the Amahoro village. Explain all the possibleways of communication according to the infrastructure shown.
b) Use the equipment below and create 2 communication stories. You
must use at least 4 equipments.
11.6 RADIO TRANSMISSION (AM, FM, PM)
Application Activity
Radio receiver
While listening to radio on one of the evening, Mukagatsinzi heard
that the tuned channel was on FM at 100.7 MHz But her radio works
efficiently when she pulls up the antenna.
f. What do you think is the significance of the antenna on her
radio?
g. Hoping you have ever used/played a radio. Where do you think
the information/sound from the radio come from?
h. Explain the mode of transmission of information as suggested
in b) above to the receiving radio.
i. While going to sleep, her radio fell down and the speaker got
problems. Do you think she was able to listen to late night
programs on the same channel?
j. As indicated on the radio, what does FM, MW, and SW mean?
Modulation is a technique used for encoding information into a RF channel.
Typically the process of modulation combines an information signal with
a carrier signal to create a new composite signal that can be transmitted
over a wireless link. In theory, a message signal can be directly sent into
space to a receiver by simply powering an antenna with the message signal.
However, message signals typically don’t have a high enough bandwidth
to make efficient direct propagation. In order to efficiently transmit data,the lower frequency data must be modulated onto a higher frequency wave.
The high frequency wave acts as a carrier that transmits the data through
space to the receiver where the composite wave is demodulated and the
data is recovered. There are a few general types of modulation; Frequency
Modulation (FM), Phase Modulation (PM) and Amplitude modulation (AM).
Frequency modulation (FM)
This is a kind of modulation which is used in every high broadcasts. The
frequency of the carrier is altered at a rate equal to the frequency of theaudio frequency but the amplitude remains constant.
Frequency modulation is widely used for FM radio broadcasting. It is
also used in telemetry, radar, seismic prospecting monitoring newborns
(for seizures via Electroencephalography), two-way radio systems, music
synthesis, magnetic tape-recording systems and some video-transmission
systems. In radio transmission, an advantage of frequency modulation is
that it has a larger signal-to-noise ratio and therefore rejects radio frequency
interference better than an equal power amplitude modulation (AM) signal.For this reason, most music is broadcast over FM radio.
Amplitude modulation (AM)
In amplitude modulation, the information signal is used to vary the
amplitude of the carrier so that it follows the wave shape of information
signal. Here, before the information is transmitted, it is first mixed to a
carrier signal so that it can be transmitted over a long distance with lowattenuation.
The modulated signal contains other frequencies called side frequencies
which are created on either sides of the carrier. If the carrier frequency is
fc and modulated frequency is fm
, two new frequencies are fc – fm and fc + fm.
Phase modulation (PM)
Phase modulation is a form of modulation that encodes information asvariations in the instantaneous phase of the carrier wave. It is widely
used for transmitting radio waves and is an integral part of many digital
transmission coding schemes that underlie a wide range of technologies
like WiFi, GSM and satellite television. In this type of modulation, the
amplitude and frequency of the carrier signal remains unchanged after
PM. The modulating signal is mapped to the carrier signal in the form of
variations in the instantaneous phase of the carrier signal.
Phase modulation is closely related to frequency modulation and is oftenused as intermediate step to achieve FM.
11.7 POST, TELEGRAPH AND TELEPHONE (PTT)
A postal, telegraph and telephone service (or PTT) is a government
agency responsible for postal mail, telegraph and telephone services. Such
monopolies existed in many countries, though not in North America or
Japan. Many PTTs have been partially or completely privatized in recent
years. In some of those privatizations, the PTT was renamed completely,
whereas in others, the name of the privatized corporation has been only
slightly modified.
Postal services transport mail and small packages to destinations around
the world, and they are mostly public corporations. However, there has
been increased privatization of postal operators in the past 20 years, and
government restrictions on private postal services have eased. Postal
authorities are often also involved in telecommunications, logistics, financial
services and other business areas.
Rwanda is part of the Universal Postal Union, which recommends a
maximum of 9,000 people per one post office branch. The ‘iPosita Rwanda
is the company responsible for postal service in Rwanda.
A telegraph is a communication system in which information is transmitted
over a wire through a series of electrical current pulses, usually in the form
of Morse code. The basic components include a source of direct current, a
length of wire or cable, and a current-indicating device such as a relay,buzzer or light bulb.
Telephony is the technology associated with the electronic transmission
of voice, fax, or other information between distant parties using systems
historically associated with the telephone, a handheld device containing
both a speaker or transmitter and a receiver. With the arrival of computers
and the transmission of digital information over telephone systems and
the use of radio to transmit telephone signals, the distinction between
telephony and telecommunication has become difficult.
Aim: The purpose of this activity is to give the real structure ofcommunication network and the terms used.
Procedure: Use the following clues to fill the puzzle. The sentences to
help in filling the puzzle are also given below.
ANTENNA, CAMERA , CELLULAR, FAX, FILM, HEADPHONE,
KEYBOARD, LENS, MICROPHONE, PEN, PLUG, PRINTER, RADIO,
SATELLITE, SPEAKER, TELEPHONE, TELEVISION, TRIPOD,
TURNTABLE, VIDEO.
ACROSS:
4. I’m out of my office. I’m calling you on my cellular telephone.
8. The signal bounces off a satellite high up in outer space.
10. The …………… needs a new link cartridge.
13. The …………… makes his voice sound much louder.
16. The sound from the radio can out of a …………….
17. I have the car ………………. tuned to my favorite station.
18. I used a …….. to write a letter.
20. I type on my computer ………………
ACTIVITY 11-2: Structure of Communication Networks
DOWN:
1. You have to ……….. it in before it will work.
2. I bought a new …….. for my camera.
3. On the airplane everyone listened to the movie through …………
4. The ….. on my car helps distant radio stations come in more clearly.
5. My favorite ………. Channel is the one that carries Oprah.
6. What is your ………… number? I’ll call you tomorrow.
7. That ……… was directed by Steven Spielberg
8. You play vinyl records on a ………….
9. He took photographs of their vacation with his digital ………….
10. The band shot a ……….. of their latest song.
11 …………… is short for facsimile.
12. The camera was perched on a ……………
END OF UNIT ASSESSMENT
1. What do you understand by the term Modulation.
2. Explain the meaning of Amplitude Modulation.
3. Explain the different types of analog modulation.
4. In modern system, Modulation very important while transmitting signals.
Discuss why modulation should be done in transmission of signals and
information.
5. Discuss the objectives that are achieved when modulation is done.
6. Explain the meaning of frequency modulation.
UNIT SUMMARY
Concepts of transmission system
In telecommunication, a communication system is a collection of
individual communication networks, transmission systems, relay stations,
tributary stations, and data terminal equipment (DTE) usually capable of
interconnection and interoperation to form an integrated whole.
Principle of cellular radio
The cellular concept is a major breakthrough in solving the problem of
spectral congestion and user capacity. It involves dividing the area into
small parts called cells. The neighbouring base stations are assigned
different groups of channels so that the interference between base stations
(and the mobile users under their control) is minimized. It offers very high
capacity in a limited spectrum allocation without any major technological
changes.
Structure of cellular network
An overall cellular network contains a number of different elements from
the base transceiver station (BTS) itself with its antenna back through
a base station controller (BSC) and a mobile switching centre (MSC) to
the location registers (HLR and VLR) and the link to the public switched
telephone network (PSTN).
The BSC is often co-located with a BTS. The BSC interfaces with the mobile
switching centre. This makes more widespread choices about the routing
of calls and interfaces to the land line based PSTN as well as the HLR and
VLR.
Principle of cellular network
Because the amount of frequency spectrum available for mobile cellular use
was limited, efficient use of the required frequencies was needed for mobile
cellular coverage. In modern cellular telephony, rural and urban regions
are divided into areas according to specific provisioning guidelines.
Modulation techniques
Modulation is a technique used for encoding information into a RF channel.
There are a few general types of modulation; Frequency Modulation (FM),Phase Modulation (PM), and Amplitude modulation (AM).