ECE - 1051: Basic Electronics
Chapter-9: Introduction to Mobile Communication
Mobile radio communication generally refers to any radio communication link between two
terminals. One or both are in motion or stationary in an unspecified location. The system can
establish communication even when the terminal is in transit or in a new area. Electronic
communication aims to enable communication with a person at any time, place, and in any
form. In general, Mobile communication meets this objective and enhances personal
communication. Mobile communication allows users to communicate in transit by providing
mobility to the terminal or device. Terminal or device mobility is enabled by wireless access.
Personal mobility can be supported by giving a unique number to the user and creating a
dynamic connection with the terminal. An additional facility such as service portability can
also be provided using intelligent network capabilities. In short, Mobile communication
provides unlimited reachability, accessibility, and rich services to the user.
With the increased number of users, accommodating them within the limited available
frequency spectrum became a significant problem. In cellular communication, a geographical
area is divided into a smaller size in the hexagon form called a cell. The cell consists of a base
station located at the center, acting as a transceiver.
In recent years, mobile communication has revolutionized communication by opening
up innovative applications not limited to one's imagination.
Learning Outcomes:
At the end of this module, students will be able to:
1. Explain the concept of basic cellular mobile communication.
2. Explain the multiple access techniques for cellular system.
3. Draw and explain the architecture of GSM system.
4. Explain the role of base station subsystem (BSS) and mobile switching center (MSC).
9.1.1 Principle of Cellular communication:
Cellular systems accommodate many mobile units over a large area within a limited frequency
spectrum. Figure 9.1.1 shows a basic cellular structure used in the mobile communication
system. Cell number 1 in the Figure 9.1.1 (shaded) has six neighboring cells surrounding it.
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ECE - 1051: Basic Electronics
Figure 9.1.1: An illustration of a cellular system
The geographic area is divided into smaller cells with a base station. Each mobile
communicates via radio with one or more base stations. An illustration of mobile-to-mobile
communication is shown in Figure 9.1.2.
Figure 9.1.2: Mobile to Mobile Communication
The base stations consist of transmitters and receivers, simultaneously handling full duplex
communication. The base station connects mobile calls via telephone lines, microwave links,
or fiber-optic cables to the switching center. The switching center coordinates the base stations'
activity and connects the entire cellular system to the public telephone network.
The channels used for transmission from the base station to the mobiles are called forward or
downlink channels and the channels used for transmission from the mobiles to the base station
are called reverse or uplink channels. The forward control and reverse control channels are the
two channels responsible for call initiation and service request.
Once a call is in progress, the switching center adjusts the transmitted power of the mobile (this
process is called power control). It changes the channel of the mobile and base station (handoff)
to maintain call quality as the mobile moves in and out of the range of a given base station. A
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ECE - 1051: Basic Electronics
call from a user can be transferred from one base station to another during the call. Handoff is
the process of transferring a call from one base station to another..
9.1.2. Multiple Access Technology:
A fixed amount of frequency spectrum is allocated to a cellular system by the telecom regulator
authority of India(TRAI). Multiple access techniques are then deployed so that many users can
efficiently share the available spectrum.
In 2010, 3G and 4G telecom spectrum were auctioned in a highly competitive bidding. The winners were
awarded spectrum in September, and Tata Docomo was the first private operator to launch 3G services
in India. The Government earned 677 billion from the 3G spectrum auction. While the broad band
wireless spectrum auction generated a revenue of 385 billion. The Government earned total revenue of
over 1062 billion from both auctions. The auction took place over 34 days and consisted of 183 rounds
of bidding. The five most expensive circles were Delhi, Mumbai, Karnataka, Tamil Nadu and Andhra
Pradesh. They accounted for 65.56% of the total bids. So the spectrum is very scarce and valuable
resource.
The three basic multiple access methods are :
• Frequency division multiple access (FDMA)
• Time division multiple access (TDMA)
• Code division multiple access (CDMA)
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ECE - 1051: Basic Electronics
Figure 9.1.3: Basic multiple access techniques
Figure 9.1.3 illustrates the three basic multiple access methods used in mobile communication.
In FDMA, users share the available spectrum allocated to each user on demand.
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ECE - 1051: Basic Electronics
In TDMA available spectrum is are divided into a number of time slots. An individual user is
assigned a time slot that permits access to the frequency channel for the duration of the time
slot.
The CDMA system utilizes the spread spectrum technique, whereby a spreading code (called
a Pseudo-random Noise or PN code) allows multiple users to share a block of the frequency
spectrum.
Self- test:
1. How call set up takes place between two mobile subscribers?
2. Explain the need for cellular concept in mobile communication.
3. Explain frequency reuse concept in cellular systems.
9.1.3. Global System for Mobile Communications (GSM):
Global System for Mobile Communications (GSM) is a standard developed by the European
Telecommunications Standards Institute (ETSI) to describe protocols for second-generation
digital cellular networks used by mobile phones. The GSM architecture, as shown in figure
10.1.4
Features:
• Higher digital voice quality.
• Low cost
• Ability to deploy equipment from different vendors
• GSM allows network operators to offer roaming services
Mobile Station (MS): The Mobile Station is made up of two entities:
1. Mobile Equipment (ME):
▪ The Mobile Equipment is the hardware used by the subscriber to access the
network.
▪ Uniquely identified by an IMEI (International Mobile Equipment Identity)
2. Subscriber Identity Module (SIM):
▪ Smart card containing the International Mobile Subscriber Identity (IMSI)
▪ Allows the user to send and receive calls and receive other subscribed services
▪ Protected by a password or personal identification number (PIN)
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ECE - 1051: Basic Electronics
Figure 9.1.4:GSM architecture
Base Station Subsystem (BSS): Base Station Subsystem is composed of two parts:
• Base Transceiver Station (BTS)
• Base Station Controller (BSC)
Base Transceiver Station (BTS): Houses the radio transceivers that define a cell and handle
radio-link protocols with the Mobile Station
Base Station Controller (BSC): The tasks performed by BSC are
• Manages Resources for BTS
• Set up a call
• Location update
• Handover for each MS
Network subsystem (NSS): The NSS provides the link between the cellular network and the
public switched telephone network (PSTN). The NSS controls handoffs between cells in
different BSSs, authenticates users and validates their accounts, and includes functions
enabling the worldwide roaming of mobile users.
Mobile Switching Center (MSC):
• The central component of the Network Subsystem
• Handles billing activities
• Handover management
• Communication with HLR,VLR, MSC’S
• Controlling of connected BSC’S
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ECE - 1051: Basic Electronics
Home Location Registers (HLR):
It stores the permanent and temporary information about each of the subscribers that “belongs”
to it.
Visitor Location Registers (VLR): The VLR maintains information about subscribers currently
physically in the region covered by the switching center.
Authentication Center (AuC): This database is used for authentication activities of the system;
for example, it holds the authentication and encryption keys for all the subscribers in both the
home and visitor location registers.
Equipment Identity Register (EIR): The EIR keeps track of the type of equipment that exits
the mobile station. It also plays a role in security, e.g., blocking calls from stolen mobile
stations and preventing networks used by stations that have not been approved.
Summary
1. A cellular radio system provides a wireless connection to the public telephone network
for any user location within the radio range of the system.
2. Mobile phone networks are divided into thousands of overlapping, individual
geographic areas or cells each with a base station.
3. The channels used for transmission from the base station to the mobiles are called
forward or downlink channels, and the channels used for transmission from the mobiles
to the base station are called reverse or uplink channels.
4. A call from a user can be transferred from one base station to another during the call.
The process of transferring is called handoff.
5. The three basic multiple access methods are: Frequency division multiple access
(FDMA), Time division multiple access (TDMA), Code division multiple access
(CDMA).
6. Global System for Mobile Communications (GSM), is a standard developed by the
European Telecommunications Standards Institute (ETSI) to describe protocols for
second generation digital cellular networks used by mobile phones.
Exercise:
1. Compare different generations of cellular mobile communication in terms of
underlying modulation schemes, data rate, and applications offered etc.
2. Compare CDMA with that of GSM system.
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