chapter iii wireless communication: a brief...
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CHAPTER III
WIRELESS COMMUNICATION: A BRIEF NOTE
3.1 INTRODUCTION
Wireless Communication is a broad and dynamic field that has spurred
tremendous excitement and technological advance over the last few decades.
Wireless Communication is, by any measure, the fastest growing segment of the
communications industry. As such as it has captured the attention of the media
and the imagination of the public. Cellular systems have experienced exponential
growth over the last decade and there are currently about two billion users
worldwide. These cellular phones have become a critical business tool and part of
everyday life in most developed countries. In addition wireless local area networks
currently supplement or replace wired networks in many homes, businesses and
campuses. This chapter presents briefly the history of wireless communication
from the smoke signals of the pre-industrial age to the cellular, satellite, and other
wireless networks of today. Further discusses the wireless vision in more detail,
including the technical challenges and current wireless systems along with
emerging systems and standards.
The first wireless networks were developed in the pre-industrial age. These
systems transmitted information over line of sight distances it will later extended
by telescopes and using smoke signals, torch signaling, flashing mirrors, signal
flares, or semaphore flags. These early communication networks were replaced
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first by the telegraph network (invented by Samuel Morse in 1838) and later by the
telephone. In 1895, a few decades after the telephone were invented. Marconi
demonstrated the first radio transmission from the Isle of Wight to a tugboat 18
miles away, and the radio communication was born1.
3.2 EVOLUTION OF WIRELESS COMMUNICATION
There are several smaller steps that take place in leading up to the
development of a new technology2. Tracing the development of these earlier
discoveries in brief can help us better understand how this technology actually
functions and contributes towards what could be the next development. The
ability to communicate with people on the move has evolved remarkably since
Guglielms Marconi first demonstrated radio ability to provide continues contact
with ships sailing the English Channel. That was in 1897, and since then new
wireless communications methods and services have been enthusiastically adopted
by people throughout the world. A brief review of the history of wireless
communications covering radio, television, radar, satellite, wireless and mobile
cellular and other wireless networks are presented in the following paragraph.
3.2.1 Radio and Television Communications
In 1874, Marconi performed simple experiments to send signals using
electromagnetic waves at short distances of only about 100 meters. At that time
scientists and experts believed that electromagnetic waves could only be
transmitted in a straight line, and the main obstacle to radio transmission was the
curvature of the earth’s surface. Finally Marconi successfully experimented to
prove that electromagnetic wave transmission was possible between two distant
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points even through obstacles in between. This proved the way for wireless
telegraphy, also known as radio communications. The word radio comes from the
term “radiated energy”3.
Marconi also studied microwaves and early television technology. In 1927,
Farnsworth gave the first public demonstration of the television system, and
developed several of the basic concepts of an electronic television system. North
America’s first television station, W3XK in Wheaton, Maryland, was started in
1930’s. By 1939, widespread commercial electronic television broadcasting
started in the United States. In 1941, the American Federal Communications
Authority set the standards for broadcast television. By 1970, television had
become the primary information and entertainment medium in the world.
3.2.2 Radar Communication
Radar has been recognized as one of the greatest scientific developments of
the first half of the 20th century. The first practical radar system was produced in
1935 by the British physicist Robert Watson-Watt.
Radar is an active remote-sensing system that operates on the principle of
echoes. A Radar display shows a map like picture of the area being scanned. The
centre of the picture corresponds to the radar antenna and the radar echoes are
shown as bright spots on the screen. Although radar is usually associated with
detecting airplanes in the sky or ships on the ocean, it is actually used in a variety
of different ways such as to forecast the weather, to scan entire regions for possible
archaeological sites from space satellites and airplanes, to study potential hidden
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dangers in highway tunnels, to locate stagnant pools of water in areas of dense
foliage on the earth, and to provide information about the universe4.
3.2.3 Satellite Communication
A satellite is an object that orbits or revolves around another object.
Satellites can be sent into space through a variety of launch vehicles. Sir Isaac
Newton in the 1720s was probably the first person to conceive the idea of a
satellite. In 1945, Arthur C Clarke a science fiction envisioned a network of a
communication satellites. Three satellites would be able to transmit signals around
the world by transmitting in a line-of-sight direction with other orbiting satellites.
In 1957, the Soviet Union launched the Sputnik 1 satellite, followed by Sputnik 2
and its passenger Laika, a dog who has the distinction of being the first living
creature to enter the earth’s orbit. In 1961, Yuri Gagarin became the first human in
orbit. In 1964, an international organization known as Intelsat was formed which
launched a series of satellites with the goal of providing total earth coverage by
satellite transmission5.
Today, Intelsat has 19 satellites in orbit that are open to use by all nations.
The INTELSAT (International Telecommunications Satellite) consortium owns the
satellites, but each country owns their earth-receiving stations. The explosive
popularity of cellular telephones advanced the idea of always being connected
everywhere on the earth. Several companies committed themselves to providing a
solution by using satellites in Low Earth Orbit (LEO) at a height of about 650 kilo
meters. Iridium, sponsored by Motorola, planned to launch 66 satellites into the
polar orbit to provide communications services to hand-held phones around the
world in 19986.
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3.2.4 Wireless and Mobile Communications
Based on the nature of wireless transmission, wireless communication
systems may be classified as simplex, half-duplex or full-duplex. In simplex
wireless systems, separate transmitters and receivers operate at the same frequency
and communication is possible in only one direction from the transmitter to the
receiver at any time. For example, paging and messaging systems are simplex
wireless communication systems in which short text or alphanumeric messages are
transmitted by fixed paging transmitters and received pagers but the received
messages are not acknowledged. Half-duplex wireless systems allow two-way
communication but a subscriber can only transmit or receive voice information at
any given time. The same frequency is used for both transmission and reception,
with a push-to-talk feature for enabling transmission only at a time. Walkie-talkie
wireless communication seats used by the police and paramilitary forces are the
examples of half-duplex wireless systems
Full-duplex wireless communication systems allow simultaneous radio
transmission and reception between the calling and called subscribers of the
system, either directly or via a base station. Full-Duplex mobile communication
systems provide many of the capabilities of the standard telephone for voice
communication, with the added convenience of communication on the move.
In Time Division Duplexing (TDD), a portion of the time is used to transfer
information data from base station to the mobile subscriber, and the remaining time
is used to transfer information data from the mobile subscriber to the base station
on the same frequency channel. Digital transmission formats and digital
modulation schemes are used in Time Division Duplixing (TDD). It is very
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sensitive to timing accuracies and needs synchronization between transmissions
and reception of the data at the transmitter and receiver ends respectively.
Therefore, Time Division Duplixing (TDD) has limited applications such as indoor
or small-area wireless applications where the physical coverage distances are much
smaller than those encountered in conventional cellular telephone systems so as to
keep the radio propagation delay within acceptable limits7.
3.2.5 Cellular Communication
In 1946, American Telephone & Telegraph (AT&T) introduced the first
American commercial mobile radio telephone service to private customers. It
consisted of a central transmitter with one antenna which could serve a wide area.
However, this system could not be used with mobile because of their limited
transmitter power. To overcome this limitation, smaller receivers with antennas
were placed on top of buildings and on poles around city, creating smaller cells.
When a person used his mobile, the conversation that he heard was transmitted on
one frequency by the central transmitter to the moving vehicle. In 1969, the Bell
system developed a commercial cellular radio operation using frequency reuse.
The first modern cellular telephone systems in the early 1980s used 666
channels. Advanced Mobile phone Service (AMPS) began setting up analog
cellular telephone operations in many parts of the world. Roaming form one city or
state in the United States was easy because the US system was based on an analog
cellular system. In Contrast, it was almost impossible to roam in Europe. During
the 1980s, a plan was launched to create a single pan European digital mobile
service with advanced features and easy roaming. This network stated operating in
19918.
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Cellular mobile communication systems provide full-duplex
communication, in which a pair of simplex RF channels with a fixed and known
frequency separation (Called duplex spacing) is used to define a specific radio
channel in the system.
3.2.6 Transition from Analog to Digital Systems
In the 1980s, most mobile cellular system was based on analog design. The
GSM system can be considered as the first digital cellular system. The different
reasons that explain this transition from analog to digital technology are the
following:
System Capacity: Cellular systems experienced a very significant growth in the
1980s. Analog systems were not able to cope with this increasing demand. In
order to overcome this problem, new frequency bands were allocated for the
development of mobile cellular radio and new modulation and coding technologies
were introduced. The digital radio was, therefore, the best option to handle the
capacity needs in a cost-efficient way.
Quality Aspects: The quality of the service can be considerably improved using a
digital technology rather than an analog one. In fact, analog systems carry the
physical disturbances in radio transmission such as fades, multipath reception,
spurious signals or interferences to the receiver. These disturbances reduce the
quality of the communication because they produce effects such as fadeouts,
crosstalk’s, hisses, etc.
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On the other hand, digital systems avoid these effects, transforming the
signal into bits. This transformation combined with other techniques, such as
digital coding, improve the quality of the transmissions. The improvement of
digital systems compared to analog systems is more noticeable under difficult
reception conditions than under good reception conditions.
Compatibility with other Systems such as Integrated Services for Digital Network
(ISDN): The decision of adopting a digital technology for GSM was made in the
course of developing the standard. During the development of GSM, the
telecommunications industry converted to digital methods. The ISDN network is
an example of this evolution. In order to make GSM compatible with the services
offered by ISDN, it was decided that the digital technology was the best option.
Additionally, a digital system allows, easily than an analog one, the
implementation of future improvement and the change of its own characteristics9.
3.3 EVOLUTION FROM 2G TO 3G CELLULAR NETWORKS
There are two steps of 3G evolution paths from 2G technologies based on
GSM and IS-95 CDMA respectively. An Evolution path from second generation
digital cellular GSM network to third generation network is depicted in the
following figure:
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Figure 3.1 An evolution path from GSM to 3G network
(Source: Wireless Communication, Singal (2010))
GSM is an Open, digital cellular technology which support voice calls and
data transfer speeds of up to 9.6 kbps, together with the transmission of SMS
(Short Message Service). GSM operates in the 900 MHz and 1.8 GHz bands in
Europe and the 850 MHz and 1.9 GHz bands in the US. GPRS offers throughput
rates of up to 53.6 kbps, so that users have a similar access speed to a dial-up
modern, but with the convenience of being able to connect from almost anywhere.
EDGE allows it to be overlaid directly onto an existing GSM network with simple
software –upgrade. WCDMA is the air interface for third-Generation mobile
communications Systems. It enables the continued support of voice, text and MMs
services in addition to richer mobile multimedia services.
Besides GSM, CDMA is the most popular mobile communication standard.
The initial evolution of CDMA started in 1991 as IS-95A CDMA one 2G digital
cellular technology for voice communication as well as data and multimedia
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services because it could allow multiple users to communicate within the spectrum,
avoiding interference or jamming among users. Code division ensures that each
user’s signal remains separate in the spectrum. An evolution path from second
generation digital cellular CDMA networks to third generation networks is
depicted in the following figure:
Figure 3.2 An Evolution path from CDMA to 3G Network
(Source: Wireless Communication, Singal (2010))
IS-95A describes the structure of the wideband 1.25 MHz CDMA channels,
power control, call processing, hand-offs, and registration techniques for system
operation. In addition to voice services, many IS-95A Operators provide circuit-
Switched data connections at 14.4 kbps. The IS-95B or CDMA one, categorized as
a 2.5G technology, defines a compatibility standard for 1.8 to 2.0 GHZCDMA
2000 multi-Carrier (MC) delivers improved systems capacity and spectrum
efficiency over 2G systems and its supports data services at minimum transmission
rates of 144 kbps in mobile (outdoor) and 2 Mbps in fixed (indoor) environments.
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3.4 GENERATIONS OF WIRELESS COMMUNICATION SYSTEMS
Wireless Communication system evolution has been categorized in to
generations, as follows:
3.4.1 First Generation Analog Cellular Systems
The first-generation cellular systems are based on analog transmission
technology. The most popular first generation cellular systems are AMPS (widely
deployed in most parts of US, South America, Australia, China), and ETACS
deployed throughout Europe10. The systems transmit speech signals employing
FM, and important control information is transmitted in digital form using FSK.
The entire service area is divided into logical cells, and each cell is allocated one
specific band in the frequency spectrum. To explore a frequency reuse pattern, the
frequency spectrum is divided among seven cells, improving the voice quality as
each subscribe is given a larger bandwidth.
AMPS and ETACS cellular radio systems deploy cell-sites with tall towers
that support several receiving antennas and have transmitting antennas that
typically radiate a few hundred watts of effective radiated power.
All these systems use two separate frequency bands for forward (from cell-
site mobile) and reverse (from mobile to cell-site) links. The typical allocated
overall band in each direction, for example, for AMPS, and NMT-900 is 25MHz in
each direction. The dominant spectra of operation for these systems are the 800-
and 900-MHz bands. In an ideal situation, all countries should use the same
standard and the same frequency bands.
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All the IG Cellular systems use analog frequency modulation (FM) for
which the transmission power requirement depends on the transmission bandwidth.
On the other hand, power is also related to the coverage and size of the cells.
Reduction in size of the cell increase the number of cells and the cost of
installation of the infrastructure. The channel spacing, or bandwidth, allocated to
each subscriber is either 30kHz or 25kHz or a fraction of either of them.
3.4.2 Second –Generation Digital Cellular Systems
The second generation (2G) Cellular systems represents the set of wireless
air interface standards that rely on digital modulation and sophisticated digital
signal processing in the handset and the base station. Digital cellular technologies
support a much larger number of mobile subscribers within a given frequency
allocation, thereby offering higher user capacity, providing superior security and
voice quality, and lay the foundation for value-added services that will continue to
be developed and enhanced in future.
There are four major standards in this category: the North American
Interim Standard (IS-54) that later on improved into IS-136; GSM, the pan-
European digital cellular; and personal digital cellular (PDC) –all of them using
TDMA technology; and IS -95 in North America, which uses CDMA technology.
The carrier spacing of IS-54/136 and PDC is the same as the carrier spacing of 1G
analog cellular system in their respective regions, but GSM and IS-95 use multiple
analog channels to form one digital carrier.
The most popular 2G cellular standards include three TDMA standards and
one CDMA standard. Interim Standard 54 or 136 (IS-54 or IS-136) also known as
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US Digital Cellular (USDC), which supports three time slotted mobile subscribers
for each 30-kHz radio channel in both the cellular 800MHz and PCS 1900 MHz
bands. Based on the analog AMPS cellular systems, the TDMA system IS-
54/136was developed in the US that adds digital traffic channels. The Is-136
includes digital control channels which enable to provide several additional
services such as identification, voice mail. SMS, call waiting group calling, etc11.
Global system for mobile (GSM), which supports eight time slotted mobile
subscribers for each 200-kHz radio channel in both the cellular and PCS bands;
and Pacific Digital Cellular (PDC) a japans TDMA standard that is similar to IS-
136, are the other two most popular TDMA based digital cellular standards.
GSM supports eight users in a 200-kHz band; IS-54 and JDC support three
users in 30 and 25-kHz bands, respectively. In other words, GSM uses 25 kHz for
each user, IS-54 uses 10 kHz per user, and JDC uses 8.33 kHz per user. The
number of users for CDMZ depends on the acceptable quality of service; therefore,
the number of users in the 1250 kHz CDMA channels cannot be theoretically
fixed. But this number is large enough to convince the standards organization to
adopt CDMA technology for next-generation 3G systems12.
3.4.3 Third-Generation Digital Cellular Systems
The fundamentals purpose of the 3G mobile communications system is to
provide a globally integrated wireless communication system combining different
incompatible network technologies already deployed across the world. The modes
differ in how duplexing is accomplished and how many carriers are used. All
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variations operate in a 5-MHz channel, as compared to 1.25 MHz for CDMA one
systems.
The need for a capacity increase necessitates a greater spectrum allocation
(1885 MHz-2025 MHz and 2110 MHz-2200 Mhz) for 3G systems. The key
features of the IMT-2000 system defining the ITU’s (International
Telecommunication Union) view of 3G cellular network capabilities are as
follows:
• High degree of worldwide commonality of design
• Compatibility of services with fixed networks and within IMT-2000
• More efficient use of the available spectrum
• Voice quality comparable to the Public Switched Telephone
Network (PSTN)
Figure 3.3 Evolution of IMT-2000 Standards
(Source: Wireless Communication, Singal (2010))
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• 144-kbps data rate available to users in high-speed vehicles over large
areas
• 384 kbps available to pedestrians standing or moving slowly over
small area
• Support for 2-Mbps data rate for office use.
• Symmetrical and asymmetrical data- transmission rates
• Support for both circuit-switched and packet-switched data services
• Support for wide variety of mobile phones for worldwide use
including pico, micro, macro, and global cellular/satellite cells
• Worldwide roaming capability
• Capability for multimedia applications and a wide range of services
• Flexibility to allow the introduction of new services and technologies
The Third Generation aims to combine telephony, internet, and multimedia
into a single device. This entails an additional requirement that it supports the
internet protocols and be based on a packet-switched network backbone.
3.4.4 Fourth Generation Digital Cellular System
The emergence of new technologies in the mobile communication systems
and also the ever increasing growth of user demand have triggered researchers and
industries to Come up with a comprehensive manifestation of the up-coming fourth
generation (4G) mobile communication system. In contrast to 3G, the new 4G
framework to be established will try to accomplish new levels of user experience
and multi-service capacity by also integrating all the mobile technologies that exist
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(e.g. GSM - Global System for Mobile Communications, GPRS - General Packet
Radio Service, IMT-2000 - International Mobile Communications, Wi-Fi -
Wireless Fidelity, Bluetooth).
The fundamental reason for the transition to the All-IP is to have a common
platform for all the technologies that have been developed so far, and to harmonize
with user expectations of the many services to be provided. The fundamental
difference between the GSM/3G and All-IP is that the functionality of the RNC
and BSC is now distributed to the BTS and a set of servers and gateways. This
means that this network will be less expensive and data transfer will be much
faster. 4G will make sure - “The user has freedom and flexibility to select any
desired service with reasonable QoS and affordable price, anytime, anywhere.” 4G
mobile communication services started in 201013 but will become mass market in
about 2014-15
3.5 ADVANTAGES AND DISADVANTAGES OF WIRELESS
COMMUNICATIONS
3.5.1 Advantages
There are many advantages of wireless communications, using wireless
communications technology and wireless networking, as compared to wired
communications and networks. Some of the major advantages include mobility,
increased reliability, and ease of installation, rapid recovery and above all lower
cost.
Mobility: The primary advantage of wireless communications is to offer the user
freedom to move about while remaining connected with the network within its
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coverage area. Many business categories, such as the police department, require its
workforce to be mobile instead of fixed at one location. Wireless technology
enables a many industries to shift toward an increasingly mobile workforce,
whether they are in meetings or working on a factory floor or conducting research.
Increased reliability: The most common source of network problems is the failure
or damage of network cables due to environment conditions or erosion of metallic
conductors. A cable splice that is done incorrectly can cause unexplainable errors
and is very difficult to identify. Using wireless technology not only eliminates
these types of cable failures, but also increases the overall reliability of the
network.
Ease of Installation: Wireless communications and networks make it easier for
any office to be modified with new cubicles or furniture, without worrying about
providing network connectivity through cables. There is no need to first consider
the location of the computer jack in the wall when relocating furniture. Instead,
the focus can be on creating the most effective work environment without any
delay and hassles. The time required to install network cabling may take days or
even weeks to complete, thereby disrupting the whole work. Using a wireless
LAN eliminates such disruption.
Rapid Disaster Recovery: Accidents may happen due to fires, tornados, and floods
at any possible location, and that too without any prior warning. Any organization
that is not prepared to recover from such natural disasters will find itself quickly
out of business. Since the computer network is a vital part of the daily operations
of a business, the ability to have the network up and immediately working after a
disaster is critical. A documented disaster recovery plan is a must.
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Lower Cost: It will eliminating the need to install cabling and using wireless
communications results in significant cost savings. Installing network cabling in
older buildings can be an extremely difficult, slow and costly task.
3.5.2 Disadvantages
Radio Signal Interference: Signals from other wireless devices can disrupt its radio
transmission, or a wireless device may itself be a source of interference for other
wireless devices. For example, several commonly used office wireless devices
such as cordless telephones, microwave ovens, elevator motors, and other heavy
electrical manufacturing machines, transmit radio signals that may interfere with a
wireless LAN operation. These may cause errors to occur in the transmission
between a wireless device and an access Point. Similarly, Bluetooth and WLAN
devices both operate in the same radio frequency, potentially resulting in
interference between such devices.
Security: A Wireless communication device transmits radio signals over a wide
open area, and hence security becomes a major concern, It is possible for an
intruder with a notebook computer and wireless NIC to intercept the signals from
a nearby wireless network. Because much of business network traffic may contain
sensitive information, this becomes a serious concern for many users. Some
wireless technologies can provide added levels of security with authorization
features prior to gaining access to the network. Network administrators can also
limit access for approved wireless devices only. As further protection, data
transmitted between the wireless device and the access point can also be encrypted
in such a way that only the intended recipient can decode the message.
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Health Hazards: High Power levels of RF energy can produce biological damage.
However, it is not yet established accurately as to how much levels of RF can
cause adverse health effects. But continuous radiations even at lower levels can be
harmful to sensitive body organs. Radio transmitters in wireless communications
devices emit radio frequency (RF) energy. Typically, these wireless devices emit
low levels of RF while being used. Although some research has been done to
address these issues, no clear facts of the biological effects of this type of
radiations have emerged to date. The safety of cordless phone, which have a base
unit connected to the telephone wiring in a house and which operate at far lower
power levels and frequencies, has never been questioned. It is always wise to be
aware of the health concerns and to monitor ongoing scientific research, even
though the available science does not conclude either way about the safety of
wireless mobile devices14.
3.6 LANDMARK/S IN THE WIRELESS COMMUNICATION
TECHNOLOGY
In 1864 James C Maxwell laid down the theoretical foundation and
predicted the existence of electromagnetic (EM) waves. He explained that an
electromagnetic wave is formed of varying electric and magnetic fields, where an
accelerated charge produces a time varying magnetic field, which in turn produces
time varying electric field. Both the fields so produced are sources of each other
and are mutually perpendicular. In fact these varying magnetic and electric fields
produce an electromagnetic wave, which propagates in space in a direction
perpendicular to both of them. In his honor, the set of equations relating the
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existence and propagation of electromagnetic wave is known a Maxwell’s
equations.
In 1887 Heinrich Hertz was the first who carried out experiments in
laboratory to demonstrate the existence of electromagnetic waves (thus proved
Maxwell’s equations). He used spark transmitter and a resonant receiver to send
and receive electromagnetic waves of six meter wavelength. In his honor, the SI
unit of frequency is called Hertz (Hz).
In 1894 Jagadish Chandra Bose ignited gunpowder and rang a bell at a
distance using electromagnetic wave in a public demonstration in Kolkata, India.
He could successfully produce electromagnetic waves with wavelength in the
range of 5mm to 25mm. Based on the works of Maxwell and Hertz, Oliver Lodge
demonstrated wireless communication over a small distance.
In 1895 Guglielmo Marconi gave the first demonstration of wireless
communication using long wave transmissions with very high transmission power
(higher than 200kW). He discovered that if one of the spark gap terminals is
connected to an antenna and the other terminal is earthed, then an electromagnetic
wave was produced that could go up to several kilometers. In 1899 he was
established wireless communication across the English Channel. In 1901 he
transmitted the Morse message “S” across the Atlantic ocean, from Cornwall,
England to Signal Hill in Newfoundland, Canada over a distance of about 3200 km
(December 12). He’s success lead to wide spread use of electromagnetic waves for
ship to ship and ship to shore communications using Morse codes.
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In 1906 the first broadcast of voice and music was made by Reginald A
Fressenden using triode vacuum tube invented by Lee De Forest in the same year.
This technique is known as AM Radio. The triode vacuum tube allowed
modulation of continuous waves and made voice transmission possible.
In 1907 the first transatlantic commercial connection was established. In
1920 the first commercial radio station were established to broadcast signals
(began in United States and Canada). The broadcast was made to mobile receivers.
In fact the first mobile radio systems were one way with only the mobile receivers.
In 1927 John Baird made demonstration of first television broadcast in
United Kingdom. In the meantime the mobile receivers were developed with
reasonable success. The Detroit Police Department in United States commenced
first regular one-way radio communication at 2MHz radio frequency band. In
1930 the mobile transmitters were developed and the first two way mobile radio
systems were placed in operation for police radio. In 1933 the frequency
modulation was invented by Edwin H Armstrong, which resulted in improved
quality in communication.
In 1939 – 1945 the World War II fueled the development of mobile
communication systems. In 1946 after the World War II, vast availability of
mobile communication systems was extended for commercial use. The first Public
Mobile Telephone System (PMTS) was established in 25 cities in United States.
In 1947 Citizen Band (CB) radio services were set up in United States. It enabled
easy to set up, and cheap solution for short distance communication. However CB
radio offers no privacy and no support to PSTN connectivity.
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In 1957 a new era in wireless communication was initiated with the launch
of satellite Sputnik I by Soviet Union. In 1970 the cellular concept came into
existence for mobile communication, using the idea of frequency reuse at Bell
laboratories.
In 1971 the first wireless network based on packet radio, ALOHANET was
developed in the University of Hawaii. It enabled seven computers spread over 4
islands to communicate using star topology. ALOHANET incorporated the first
set of protocols for channel access and routing in packet radio systems.
In 1979 Neppon Telephone and Telegraph (NTT) introduced the first
analog cellular mobile communication service in Japan.
In 1981 another analog mobile cellular communication system, called NMT
(Nordic Mobile Telephone) was launched in Scandinavia.
In 1983 one more analog mobile cellular communication system, called
AMPS (Advanced Mobile Phone Systems) was launched in Chicago.
In 1991 the first digital cellular service began in United States. This
marked the beginning of second generation (2G) mobile communication systems,
enabling high power efficiency, higher capacity and lower cost. The Global
system for Mobile Communication (GSM) was standardized. In 1992 the GSM
was launched.
In 1993 the Code Division Multiple Access (CDMA) based mobile
communication system came in to existence.
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In 1997 the WCDMA (Wideband Code Division Multiple Access) was
considered as one of the technologies for UMTS (Universal Mobile Telephone
System) development. WLAN (Wireless Local Area Network) was standardized
in United States. It supported wireless networking within a small region with
reasonably good data rate with limited mobility. HIPERLAN (High Performance
Radio Local Area Network) was also standardized in Mobile Communication
System was proposed for the third generation mobile communication system,
defined as IMT-2000, aiming to offer a common worldwide framework for mobile
communication.
In 2001 the 3G (Third Generation) mobile communication services using
WCDMA (Wideband Code Division Multiple Access) was started in Japan, named
as FOMA (Freedom of Mobile Multimedia Access) service15.
3.7 EMERGING WIRELESS NETWORK TECHNOLOGY
One of the most striking changes in the use of technology in the recent
years has been the explosive growth in the use of wireless networks for Internet
and local network access. The promise of ubiquitous wireless networks
dramatically enhances the usefulness of small Internet-capable devices such as:
• Infrared
• Bluetooth
• Wi-Fi
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3.7.1 Infrared
Infrared (IR) ports have been standard on most laptops and Personal Digital
Assistants (PDA) for quite some time. Some printers and cell phones come
equipped with infrared ports as well. The principal use has been to provide a
communication channel between devices for synchronization, backup, or file
transfer. The transfer rate is not as fast (4 Mbps or megabits per second) as wired
connections (such as USB), although now some IR ports can transfer at a zippier
16 Mbps. IR ports are also used to transfer contact information or calendar entries
between hand-held devices. This use is quite popular in Japan and Europe,
particularly for exchanging business cards and downloading short messages.
Utilities are available which allow for IR interoperability among Palms, Windows
CE/ Pocket PC devices, and even older Newton Messagepads (JetSend,
Peacemaker, BackTalk). While IR is the granddaddy of wireless protocols, new
applications continue to be developed for the its use, including InfoPort, a product
for beaming large documents to Palm devices from kiosks or other public terminals
(being used at the University of South Dakota for transferring documents to
students), and Infrared Financial Messaging (IrFM), a new "point and pay"
wireless payment standard. Financial transactions, in fact, are seen as a major
future use of IR, as it is a more secure means of communication than other wireless
protocols, since devices have to be lined up in close proximity to one another.
In a rare example of not re-inventing the wheel with each new
technological advance, OBEX has been selected as the standard for file exchange
on the new Bluetooth wireless protocol. IrDA capability is built into mainstream
operating systems including MS Windows, Linux, and MacOS. But IrDA
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compatible ports are also being added to devices such as cameras (the Casio
WQV3 cameras) and scanners (Hewlett-Packard CapShare and the QuickLink Pen
from WizCom). Scanning text or images into a hand-held scanner, which can then
be beamed and stored on a hand-held computer offers interesting possibilities for
collecting such materials as newspaper clippings or regalia for language learning
purposes.
3.7.2 Bluetooth
A wireless protocol which has been highly touted in the last several years is
Bluetooth, developed originally by Ericsson in Sweden in 1994 and named for
Harald Bluetooth, the Viking king who united Denmark and Norway in the 10th
century. Bluetooth uses a short-wave, always-on radio signal that lets devices of all
kinds communicate with one another, including cell phones, printers, laptops, and
hand-held computers. Since it uses RF (radio frequency) waves, communication
does not require a line-of-sight connection between devices, as does IR. Like IR,
Bluetooth is short range (the normal limit is 10 meters) but is also Omni directional
and can travel through non-metal obstructions (clothes, furniture, walls). Longer
range transmitters, capable of sending signals up to 100 meters, are also being
developed. Bluetooth transmits at a maximum rate of 1 Mbps. There has been quite
a buzz about Bluetooth and the era of "personal area networks" (also being called
"piconets") or "information clouds" this wireless technology promises to create.
The idea is that once Bluetooth components become inexpensive enough, they will
become embedded in all kind of machines, including VCRs, washer-dryers, stoves,
microwaves, and CD-players, all of which could be monitored and controlled by
Bluetooth. Ericsson envisions a scenario in which mall shoppers would access
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sales information on their PDAs as they stroll, or cameras might send instantly
pictures to relatives as they are taken. For some, this kind of all-encompassing
network is more threatening than enticing. Of interest to language teachers is the
fact that Bluetooth supports voice as well as data. Others see a more modest role
for Bluetooth principally as a cable replacement technology, taking over the role of
wired serial or USB connections. In contrast to IR, Bluetooth allows point-to-
multi-point connections, thus creating an ad-hoc wireless connection of "master"
and (up to seven) "slaves." Also, as opposed to IR, Bluetooth communication can
be initiated by the devices themselves, allowing for self-monitoring and automated
interactions.
3.7.3 Wi-Fi (Wireless Fidelity)
If Bluetooth is being promoted as a cable replacement technology, Wi-Fi is
seen widely as a replacement of wired Ethernet. Actually wireless LAN (local area
network) technology has been around since the late 1980's. However, different
proprietary approaches were used, and the networks operated at low-speed
(1-2 Mbps). In 1997, the standards setting body, IEEE, released the 802.11
standard for wireless local area networking using the unlicensed 2.4 GHz
frequency band (as opposed to the 900 MHz band used previously). This standard
was later updated to 802.11b, which raised the transmission speed from 2 to 11
Mbps or approximately the same transmission speed as traditional wired Ethernet
connections. This is the standard generally referred to today as Wi-Fi ("wireless
fidelity") or wireless LAN. As opposed to Bluetooth Wi-Fi requires use of a "base
station" or "access point" for transmitting signals to clients, which generally use
Wi-Fi PC cards or desktop adapters to connect to the base station. Like Bluetooth
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(which also transmits at 2.4 GHz), Wi-Fi signals can travel through solid objects,
although they traverse better through wood or drywall than through stone and
brick. Transmission distances vary from 50 to 300 feet, depending on equipment
and configuration, and can be extended up to 20 miles through the use of high gain
antennas. Wi-Fi began to be widely used when in 1999 Apple introduced its
"Airport" wireless networking technology which uses the 802.11b standard. Today
many Wi-Fi base stations and adapters are available from a variety of vendors.
Wi-Fi connections are also showing up in public and commercial spaces such as
airports or coffee shops, where connections are available for a fee.
Despite Wi-Fi's popularity there are several concerns in its use voiced by
users and system administrators, namely limited bandwidth, radio interference
from other devices, and security. A revision of the specification called
802.11aaddresses these issues, at least in part. Although Wi-Fi runs at about the
same speed as 10 Mbps wired Ethernet, configuration and security concerns
usually reduce throughput to something more like 5-7 Mbps. 802.11a runs at the
higher speed of 54 Mbps, although real-world use will be lower. This is still a
significant increase for applications needing higher bandwidth such as streaming
media. Wi-Fi runs on a radio frequency (2.4 GHz) shared by microwave ovens,
most cordless phones, and Bluetooth devices, creating the potential for serious
interference problems. 802.11a runs at 5.4 GHz, thus avoiding that conflict. Both
wireless standards have a built-in security protocol called WEP ("Wireless
Encryption Protocol") which allows for encrypted transmissions. Often, however,
WEP has not been used on Wi-Fi networks out of concern that throughput will be
negatively affected. The higher bandwidth of 802.11a may encourage greater use
of WEP. Security experts, however, point out that WEP is not impenetrable and
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recommend use of VPN ("Virtual Private Network") software for secure network
access with wireless clients. A new security protocol, 802.1x, is just being added to
Wi-Fi setups16.
3.8 ACCESS TECHNIQUES FOR WIRELESS COMMUNICATION
In wireless communication systems it is often desirable to allow the
subscriber to send simultaneously information to the base station while receiving
information from the base station. A cellular system divides any given area into
cells where a mobile unit in each cell communicates with a base station. The main
aim in the cellular system design is to be able to increase the capacity of the
channel i.e. to handle as many calls as possible in a given bandwidth with a
sufficient level of quality of service. There are several different ways to allow
access to the channel. These include mainly the following:
• Frequency Division Multiple Access (FDMA)
• Time Division Multiple Access (TDMA)
• Code Division Multiple Access (CDMA)
• Space Division Multiple Access (SDMA)
3.8.1 Frequency Division Multiple Access (FDMA)
This was the initial multiple-access technique for cellular systems in which
each individual user is assigned a pair of frequencies while making or receiving a
call. One frequency is used for downlink and one pair for uplink. This is called
frequency division duplexing (FDD). That allocated frequency pair is not used in
the same cell or adjacent cells during the call so as to reduce the co channel
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interference. Even though the user may not be talking, the spectrum cannot be
reassigned as long as a call is in place. Different users can use the same frequency
in the same cell except that they must transmit at different times. The features of
FDMA are as follows: The FDMA channel carries only one phone circuit at a time.
If an FDMA channel is not in use, then it sits idle and it cannot be used by other
users to increase share capacity. After the assignment of the voice channel the BS
and the MS transmit simultaneously and continuously. The bandwidths of FDMA
systems are generally narrow i.e. FDMA is usually implemented in a narrow band
system The symbol time is large compared to the average delay spread. The
complexity of the FDMA mobile systems is lower than that of TDMA mobile
systems. FDMA requires tight filtering to minimize the adjacent channel
interference17.
3.8.2 Time Division Multiple Access (TDMA)
In digital systems, continuous transmission is not required because users do
not use the allotted bandwidth all the time. In such cases, TDMA is a
complimentary access technique to FDMA. Global Systems for Mobile
communications (GSM) uses the TDMA technique. In TDMA, the entire
bandwidth is available to the user but only for a finite period of time. In most cases
the available bandwidth is divided into fewer channels compared to FDMA and the
users are allotted time slots during which they have the entire channel bandwidth at
their disposal, TDMA requires careful time synchronization since users share the
bandwidth in the frequency domain. The number of channels are less, inter channel
interference is almost negligible. TDMA uses different time slots for transmission
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and reception. This type of duplexing is referred to as Time division duplexing
(TDD).
The features of TDMA include the following: TDMA shares a single
carrier frequency with several users where each users makes use of non
overlapping time slots. The number of time slots per frame depends on several
factors such as modulation technique, available bandwidth etc. Data transmission
in TDMA is not continuous but occurs in bursts. This results in low battery
consumption since the subscriber transmitter can be turned OFF when not in use.
Because of a discontinuous transmission in TDMA the handoff process is much
simpler for a subscriber unit, since it is able to listen to other base stations during
idle time slots. TDMA uses different time slots for transmission and reception thus
duplexers are not required. TDMA has an advantage that is possible to allocate
different numbers of time slots per frame to different users. Thus bandwidth can be
supplied on demand to different users by concatenating or reassigning time slot
based on priority
3.8.3 Code Division Multiple Access (CDMA)
Spread spectrum multiple access (SSMA) uses signals which have a
transmission bandwidth whose magnitude is greater than the minimum required RF
bandwidth. A pseudo noise (PN) sequence converts a narrowband signal to a
wideband noise like signal before transmission. SSMA is not very bandwidth
efficient when used by a single user. However since many users can share the same
spread spectrum bandwidth without interfering with one another, spread spectrum
systems become bandwidth efficient in a multiple user environment.
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There are two main types of spread spectrum multiple access techniques:
Frequency hopped multiple access (FHMA) Direct sequence multiple access
(DSMA) or Code division multiple access (CDMA).18
3.8.4 Space Division Multiple Access (SDMA)
DMA utilizes the spatial separation of the users in order to optimize the use
of the frequency spectrum. A primitive form of SDMA is when the same frequency
is reused in different cells in a cellular wireless network. The radiated power of
each user is controlled by Space division multiple access. SDMA serves different
users by using spot beam antenna. These areas may be served by the same
frequency or different frequencies. However for limited co-channel interference it
is required that the cells are sufficiently separated. This limits the number of cells a
region can be divided into and hence limits the frequency re-use factor. A more
advanced approach can further increase the capacity of the network. This technique
would enable frequency re-use within the cell. In a practical cellular environment it
is improbable to have just one transmitter fall within the receiver beam width.
Therefore it becomes imperative to use other multiple access techniques in
conjunction with SDMA. When different areas are covered by the antenna beam,
frequency can be re-used, in which case TDMA or CDMA is employed, for
different frequencies FDMA can be used.
3.9 CONCLUSION
From the foregoing discussion, it is clear that, the field of Wireless
Communication through its developments has attained the status of an academic
discipline. It was also found that a large quantum of research output has been
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emerging as noticed from the SCOPUS database. Therefore, the present study has
concentrated on the Scientometric analysis of research productivity on this subject
which has been analysed in Chapter V through the Scientometric indicators and
statistical techniques as described in Chapter IV.
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