Physics

UNIT 11:MOBILE PHONE AND RADIO COMMUNICATION

Topic Area: DIGITAL TECHNOLOGY
Sub-Topic Area: Analog and Digital Signals

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 learners will be able to;

◊ explain the concept and principles of cellular radio network.

◊ explain the need for cellular system in modern mobile communication.

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 Biology and 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 of interconnection 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, to which 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 thatcan 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.

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Operation can be programmable to update with newly upcoming technologies.
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Privacy is preserved by using data encryption.
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Greater dynamic range is possible.
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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.
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In long distance communication system, noise does not accumulate from repeater to repeater.
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Error detection and correction schemes can be employed by using coding techniques.

Limitations of Digital communication system
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Generally, more bandwidth is required than that for analog system.
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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 line based 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 according to 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 to hand-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 not interfere.

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 full systems 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 from radio 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 release the 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 possible ways 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)

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 the audio frequency but the amplitude remains constant.

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 the audio 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 low attenuation.

The modulated signal contains other frequencies called side frequencies which are created on either sides of the carrier. If the carrier frequency is f_c and modulated frequency is f_m, two new frequencies are f_c – f_m and f_c + f_m.

Phase modulation (PM)

Phase modulation is a form of modulation that encodes information as variations 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 often used 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.

                            END OF UNIT QUESTIONS

1. What is amplitude modulation?

2. What is modulation?

3. What are the different types of analog modulation?

4. What is the need for modulation?

5. What are the objectives met by modulation?

6. What is 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).