WIRELESS & MOBILE COMMUNICATION

(Unit-1)
Introduction to Wireless Communication:

Before talking about wireless medium, we need to talk about the wired medium. Wired
network is a bounded medium where data travels over a path that a wire or cable takes. In
modern era of advanced and enormous no of devices, wired medium of communication
imposes a restriction on fluent communication. There are various problems with wired
networks. Consider a situation, you want to connect to 10 or more devices around you. You
need exact same no of ports to be able to connect to devices, but large no of ports seems to
be impractical but with wireless network, it can be easily done.

As the name suggests, wireless network eliminates needs to be tethered with wire or cable.
Convenience and Mobility becomes its main characteristics. Many different wireless devices
can connect to network easily and seamlessly. As wireless data travel through air, there must
be some constraints within which wireless communication takes place.

These are:

• All wireless devices must follow a common standard i.e., IEEE 802.11
• Wireless coverage must be there where devices are expected to use.
• Wired network comes under IEEE standard 802.3
• Wireless network comes under IEEE standard 802.11

IEEE stands for “Institute of Electrical and Electronics Engineers”, is an organization

composed of engineers that issues and manages standards for electrical and electronic
devices.

Basics of Wireless Communication:

Wireless communication takes places over free space through RF (radio frequency), one
device, a Transmitter, sends signal to another device, a Receiver. Two devices (transmitter
and receiver) must use same frequency (or channel) to be able to communicate with each
other. If a large number of wireless devices communicate at same time, radio frequency can
cause interference with each other. Interference increases as no of devices increases.

Wireless devices share airtime just like wired devices connect to shared media and share
common bandwidth. For effective use of media, all wireless devices operate in half duplex
mode to avoid collision or interference. Before the transmission begins, devices following
IEEE 802.11 standard must check whether channel is available and clear. Wireless
communication is always half duplex as transmission uses same frequency or channel. To
achieve full duplex mode, devices use different frequency or channel of transmission and
receiving of signals. You can say that wireless communication is Full duplex but technically
it is not.
Radio Frequency:

In free space, the sender (transmitter) sends an alternating current into a section of wire (an
antenna). This sets up a moving electric and magnetic fields that travel as travelling waves.
The electric and magnetic field moves along each other at a right angle to each other as
shown. The signal must keep changing or alternating by cycle up and down to keep electric
and magnetic field cyclic and pushing forward. The no of cycles a wave taking in a second
is called Frequency of the wave. So, frequency = no of cycles per second
Antennas in our daily lives send out Electromagnetic waves in all directions, like the waves
travelling in water when a stone is dropped in a water body.

Frequency Unit Names:

Unit Abbreviation Meaning

Hertz Hz Cycles per second

Kilohertz kHz 1000 Hz

Megahertz MHz 1, 000, 000 Hz

Gigahertz GHz 1, 000, 000, 000 Hz

Features of Wireless Communication:

The evolution of wireless technology has brought many advancements with its effective
features.

• The transmitted distance can be anywhere between a few meters (for example,
a television's remote control) and thousands of kilometres (for example, radio
communication).
• Wireless communication can be used for cellular telephony, wireless access to
the internet, wireless home networking, and so on.
• Other examples of applications of radio wireless technology include GPS units,
garage door openers, wireless computer mice, keyboards and headsets,
headphones, radio receivers, satellite television, broadcast television and
cordless telephones.

Wireless communication involves transfer of information without any physical connection

between two or more points. Because of this absence of any 'physical infrastructure', wireless
communication has certain advantages. This would often include collapsing distance or space.
Wireless communication has several advantages; the most important ones are discussed below.

Cost effectiveness

Wired communication entails the use of connection wires. In wireless networks,

communication does not require elaborate physical infrastructure or maintenance practices.
Hence the cost is reduced.

Example − Any company providing wireless communication services does not incur a lot of
costs, and as a result, it is able to charge cheaply with regard to its customer fees.

Flexibility

Wireless communication enables people to communicate regardless of their location. It is not

necessary to be in an office or some telephone booth in order to pass and receive messages.

Miners in the outback can rely on satellite phones to call their loved ones, and thus, help
improve their general welfare by keeping them in touch with the people who mean the most to
them.

Convenience

Wireless communication devices like mobile phones are quite simple and therefore allow
anyone to use them, wherever they may be. There is no need to physically connect anything in
order to receive or pass messages.

Example − Wireless communications services can also be seen in Internet technologies such
as Wi-Fi. With no network cables hampering movement, we can now connect with almost
anyone, anywhere, anytime.

Speed

Improvements can also be seen in speed. The network connectivity or the accessibility were
much improved in accuracy and speed.

Example − A wireless remote can operate a system faster than a wired one. The wireless control
of a machine can easily stop its working if something goes wrong, whereas direct operation
can’t act so fast.

Accessibility

The wireless technology helps easy accessibility as the remote areas where ground lines can’t
be properly laid, are being easily connected to the network.

Example − In rural regions, online education is now possible. Educators no longer need to
travel to far-flung areas to teach their lessons. Thanks to live streaming of their educational
modules.
Constant connectivity

Constant connectivity also ensures that people can respond to emergencies relatively quickly.

Example − A wireless mobile can ensure you a constant connectivity though you move from
place to place or while you travel, whereas a wired land line can’t.

Wireless Communication is the fastest growing and most vibrant technological areas in the
communication field. Wireless Communication is a method of transmitting information from
one point to other, without using any connection like wires, cables or any physical medium.
Generally, in a communication system, information is transmitted from transmitter to receiver
that are placed over a limited distance. With the help of Wireless Communication, the
transmitter and receiver can be placed anywhere between few meters (like a T.V. Remote
Control) to few thousand kilometres (Satellite Communication).

We live in a World of communication and Wireless Communication, in particular, is a key part

of our lives. Some of the commonly used Wireless Communication Systems in our day to day
life are: Mobile Phones, GPS Receivers, Remote Controls, Bluetooth Audio and Wi-Fi etc.

What is Wireless Communication?

Communication Systems can be Wired or Wireless and the medium used for communication
can be Guided or Unguided. In Wired Communication, the medium is a physical path like Co-
axial Cables, Twisted Pair Cables and Optical Fibre Links etc. which guides the signal to
propagate from one point to other. Such type of medium is called Guided Medium. On the
other hand, Wireless Communication doesn’t require any physical medium but propagates the
signal through space. Since, space only allows for signal transmission without any guidance,
the medium used in Wireless Communication is called Unguided Medium.

If there is no physical medium, then how does wireless communication transmit signals? Even
though there are no cables used in wireless communication, the transmission and reception of
signals is accomplished with Antennas. Antennas are electrical devices that transform the
electrical signals to radio signals in the form of Electromagnetic (EM) Waves and vice versa.
These Electromagnetic Waves propagates through space. Hence, both transmitter and receiver
consists of an antenna.

What is Electromagnetic Wave?

Electromagnetic Waves carry the electromagnetic energy of electromagnetic field through

space. Electromagnetic Waves include Gamma Rays (γ – Rays), X – Rays, Ultraviolet Rays,
Visible Light, Infrared Rays, Microwave Rays and Radio Waves. Electromagnetic Waves
(usually Radio Waves) are used in wireless communication to carry the signals.

An Electromagnetic Wave consists of both electric and magnetic fields in the form of time
varying sinusoidal waves. Both these fields are oscillating perpendicular to each other and the
direction of propagation of the Electromagnetic Wave is again perpendicular to both these
fields. Mathematically, an Electromagnetic Wave can be described using Maxwell’s equations.
Pictorial representation of an Electromagnetic Wave is shown below, where the Electric
Field is acting in the Y – axis, magnetic field is acting in the Z – axis and the Electromagnetic
Wave propagates in X – axis.

A Brief History of Wireless Communication:

Since the use of smoke signals, flags and flashing mirrors in the pre – historic period, Wireless
communication has been a part of human life and it is continuously evolving. Modern Wireless
Communication i.e., using electrical signals and radio waves for communication has been
around us for more than 100 years. In the year 1897, Guglielmo Marconi successfully
demonstrated the Wireless Telegraphy by sending EM Waves for a short distance of 100
meters. This demonstration paved way for Radio Communication and the term Radio is derived
from Radiant Energy. By early 1900’s, Trans – Atlantic radio transmission had been
established, where Marconi successfully transmitted messages in the form of Morse code.
Since then, the technology related to wireless communication and wireless system has
advanced rapidly and thus enabling transmissions over longer distances at low cost with
cheaper devices.

Throughout the development of wireless communication, there are many wireless systems and
methods that flourished and many got disappeared. The best example for this is Telephone
Communication and Television Transmission. Initially, all telephone related communication
was carried out (and still is) using wired network, which we call it as Landline Telephone. But
the rapid growth of Mobile Communication started to replace the complex wired telephone
system. In this scenario, the wired technology became outdated and got replaced by wireless
communication. Another scenario where wireless communication got replaced by wired
communication is Television broadcasting. In the early days, television signals were
broadcasted using wireless radio transmitters. But this setup got replaced by cable television.
These two examples point out that with the development of technology, we always have to
choose what’s best for the situation i.e., in some areas we have to use wired communication
whereas in the other, going for wireless may be a better option.

Why Wireless Communication?

When wired communication can do most of the tasks that a wireless communication can, why
do we need Wireless Communication? The primary and important benefit of wireless
communication is mobility. Apart from mobility, wireless communication also offers
flexibility and ease of use, which makes it increasingly popular day – by – day. Wireless
Communication like mobile telephony can be made anywhere and anytime with a considerably
high throughput performance. Another important point is infrastructure. The setup and
installation of infrastructure for wired communication systems is an expensive and time
consuming job. The infrastructure for wireless communication can be installed easily and low
cost. In emergency situations and remote locations, where the setup of wired communication
is difficult, wireless communication is a viable option.

Advantages of Wireless Communication:

There are numerous advantage of Wireless Communication Technology, Wireless Networking

and Wireless Systems over Wired Communication like Cost, Mobility, Ease of Installation,
and Reliability etc.
Cost

The cost of installing wires, cables and other infrastructure is eliminated in wireless
communication and hence lowering the overall cost of the system compared to wired
communication system. Installing wired network in building, digging up the Earth to lay the
cables and running those wires across the streets is extremely difficult, costly and time
consuming job.

In historical buildings, drilling holes for cables is not a best idea as it destroys the integrity and
importance of the building. Also, in older buildings with no dedicated lines for communication,
wireless communication like Wi-Fi or Wireless LAN is the only option.

Mobility

As mentioned earlier, mobility is the main advantage of wireless communication system. It

offers the freedom to move around while still connected to network.

Ease of Installation

The setup and installation of wireless communication network’s equipment and infrastructure
is very easy as we need not worry about the hassle of cables. Also, the time required to setup a
wireless system like a Wi-Fi network for example, is very less when compared to setting up a
full cabled network.

Reliability

Since there are no cables and wires involved in wireless communication, there is no chance of
communication failure due to damage of these cables, which may be caused by environmental
conditions, cable splice and natural diminution of metallic conductors.

Disaster Recovery

In case of accidents due to fire, floods or other disasters, the loss of communication
infrastructure in wireless communication system can be minimal.

Disadvantages of Wireless Communication:

Even though wireless communication has a number of advantages over wired communication,
there are a few disadvantages as well. The most concerning disadvantages are Interference,
Security and Health.

Interference

Wireless Communication systems use open space as the medium for transmitting signals. As a
result, there is a huge chance that radio signals from one wireless communication system or
network might interfere with other signals.

The best example is Bluetooth and Wi-Fi (WLAN). Both these technologies use the 2.4GHz
frequency for communication and when both of these devices are active at the same time, there
is a chance of interference.
Security

One of the main concerns of wireless communication is Security of the data. Since the signals
are transmitted in open space, it is possible that an intruder can intercept the signals and copy
sensitive information.

Health Concerns

Continuous exposure to any type of radiation can be hazardous. Even though the levels of RF
energy that can cause the damage are not accurately established, it is advised to avoid RF
radiation to the maximum.

Basic Elements of a Wireless Communication System:

A typical Wireless Communication System can be divided into three elements: the Transmitter,
the Channel and the Receiver. The following image shows the block diagram of wireless
communication system.

The Transmission Path

A typical transmission path of a Wireless Communication System consists of Encoder,

Encryption, Modulation and Multiplexing. The signal from the source is passed through a
Source Encoder, which converts the signal in to a suitable form for applying signal processing
techniques.

The redundant information from signal is removed in this process in order to maximize the
utilization of resources. This signal is then encrypted using an Encryption Standard so that the
signal and the information is secured and doesn’t allow any unauthorized access.

Channel Encoding is a technique that is applied to the signal to reduce the impairments like
noise, interference, etc. During this process, a small amount of redundancy is introduced to the
signal so that it becomes robust against noise. Then the signal is modulated using a suitable
Modulation Technique (like PSK, FSK and QPSK etc.) , so that the signal can be easily
transmitted using antenna.

The modulated signal is then multiplexed with other signals using different Multiplexing
Techniques like Time Division Multiplexing (TDM) or Frequency Division Multiplexing
(FDM) to share the valuable bandwidth.

The Channel

The channel in Wireless Communication indicates the medium of transmission of the signal
i.e. open space. A wireless channel is unpredictable and also highly variable and random in
nature. A channel maybe subject to interference, distortion, noise, scattering etc. and the result
is that the received signal may be filled with errors.

The Reception Path

The job of the Receiver is to collect the signal from the channel and reproduce it as the source
signal. The reception path of a Wireless Communication System comprises of Demultiplexing,
Demodulation, Channel Decoding, Decryption and Source Decoding. From the components of
the reception path it is clear that the task of the receiver is just the inverse to that of transmitter.

The signal from the channel is received by the Demultiplexer and is separated from other
signals. The individual signals are demodulated using appropriate Demodulation Techniques
and the original message signal is recovered. The redundant bits from the message are removed
using the Channel Decoder.

Since the message is encrypted, Decryption of the signal removes the security and turns it into
simple sequence of bits. Finally, this signal is given to the Source Decoder to get back the
original transmitted message or signal.

Types of Wireless Communication Systems

Today, people need Mobile Phones for many things like talking, internet, multimedia etc. All
these services must be made available to the user on the go i.e. while the user is mobile. With
the help of these wireless communication services, we can transfer voice, data, videos, images
etc.

Wireless Communication Systems also provide different services like video conferencing,
cellular telephone, paging, TV, Radio etc. Due to the need for variety of communication
services, different types of Wireless Communication Systems are developed. Some of the
important Wireless Communication Systems available today are:

• Television and Radio Broadcasting

• Satellite Communication
• Radar
• Mobile Telephone System (Cellular Communication)
• Global Positioning System (GPS)
• Infrared Communication
 • WLAN (Wi-Fi)
• Bluetooth
• ZigBee
• Paging
• Cordless Phones
• Radio Frequency Identification (RFID)

There are many other system with each being useful for different applications. Wireless
Communication systems can be again classified as Simplex, Half Duplex and Full Duplex.
Simplex communication is one way communication. An example is Radio broadcast system.

Half Duplex is two way communication but not simultaneous one. An example is walkie –
talkie (civilian band radio). Full Duplex is also two way communication and it is a simultaneous
one. Best example for full duplex is mobile phones. The devices used for Wireless
Communication may vary from one service to other and they may have different size, shape,
data throughput and cost. The area covered by a Wireless Communication system is also an
important factor. The wireless networks may be limited to a building, an office campus, a city,
a small regional area (greater than a city) or might have global coverage.

Television and Radio Broadcasting

Radio is considered to be the first wireless service to be broadcast. It is an example of a Simplex

Communication System where the information is transmitted only in one direction and all the
users receiving the same data.

Satellite Communication

Satellite Communication System is an important type of Wireless Communication. Satellite

Communication Networks provide worldwide coverage independent to population density.

Satellite Communication Systems offer telecommunication (Satellite Phones), positioning and

navigation (GPS), broadcasting, internet, etc. Other wireless services like mobile, television
broadcasting and other radio systems are dependent of Satellite Communication Systems.

Mobile Telephone Communication System

Perhaps, the most commonly used wireless communication system is the Mobile Phone
Technology. The development of mobile cellular device changed the World like no other
technology. Today’s mobile phones are not limited to just making calls but are integrated with
numerous other features like Bluetooth, Wi-Fi, GPS, and FM Radio.

The latest generation of Mobile Communication Technology is 5G (which is indeed successor

to the widely adapted 4G). Apart from increased data transfer rates (technologists claim data
rates in the order of Gbps), 5G Networks are also aimed at Internet of Things (IoT) related
applications and future automobiles.
Global Positioning System (GPS)

GPS is solely a subcategory of satellite communication. GPS provides different wireless

services like navigation, positioning, location, speed etc. with the help of dedicated GPS
receivers and satellites.

Bluetooth

Bluetooth is another important low range wireless communication system. It provides data,
voice and audio transmission with a transmission range of 10 meters. Almost all mobile phones,
tablets and laptops are equipped with Bluetooth devices. They can be connected to wireless
Bluetooth receivers, audio equipment, cameras etc.

Paging

Although it is considered an obsolete technology, paging was a major success before the wide
spread use of mobile phones. Paging provides information in the form of messages and it is a
simplex system i.e. the user can only receive the messages.

Wireless Local Area Network (WLAN)

Wireless Local Area Network or WLAN (Wi-Fi) is an internet related wireless service. Using
WLAN, different devices like laptops and mobile phones can connect to an access point (like
a Wi-Fi Router) and access internet.

Wi-Fi is one of the widely used wireless network, usually for internet access (but sometimes
for data transfer within the Local Area Network). It is very difficult to imagine the modern
World without Wi-Fi.

Infrared Communication

Infrared Communication is another commonly used wireless communication in our daily lives.
It uses the infrared waves of the Electromagnetic (EM) spectrum. Infrared (IR) Communication
is used in remote controls of Televisions, cars, audio equipment etc.

Cellular System Infrastructure

Early wireless systems had a high-power transmitter, covering the entire service area. This
required a very huge amount of power and was not suitable for many practical reasons.

The cellular system replaced a large zone with a number of smaller hexagonal cells with a
single BS (base station) covering a fraction of the area. Evolution of such a cellular system is
shown in the given figures, with all wireless receivers located in a cell being served by a BS.
Wireless devices need to be supported for different types of services, the wireless device could
be a wireless telephone laptop with wireless card, personal digital assistant (PDA), or web
enabled phone. For simplicity, it could be called an MS.

In a cellular structure, a MS (mobile station) needs to communicate with the BS of the cell
where the MS is currently located and the BS acts as a gateway to the rest of the world.
Therefore, to provide a link, the MS needs to be in the area of one of the cells (and hence a BS)
so that mobility of the MS can be supported. Several base stations are connected through hard-
wires and are controlled by a BS controller (BSC), which in turn is connected to a mobile
switching center (MSC). Several mobile switching centers are interconnected to a PSTN
(public switched telephone network) and the ATM (asynchronous transfer mode) backbone.
To provide a better perspective of wireless communication technology, simplified system
infrastructure for cellular system is shown in the figure:
A cellular system requires a fairly complex infrastructure. A generic block diagram in shown
in the figure:

A BS consists of a base transceiver system (BTS) and a BSC. Both tower and antenna are a
part of the BTS, while all associated electronics are contained in the BSC. The HLR (home
location register) and VLR (visitor location register) are two sets of pointers that support
mobility and enable the use of the same telephone numbers worldwide.
The AUC (authentication center) unit provides authentication and encryption parameters that
verify the user's identity and ensure the confidentiality of each cell. The EIR (equipment
identity register) is a database that information about identity of mobile equipment. Both AUC
and EIR can be implemented as individual stand-alone units or as a combined AUC/EIR unit.
The HLR is located at the MSC where MS is initially registered and is the initial home location
for billing and access information.

In simple words, any incoming call, based on the calling number, is directed to the HLR of the
home MS where the MS is registered. The HLR then points to the VLR of the MSC where the
MS is currently located. Bidirectional HLR-VLR pointers help in carrying out various
functionalities, as illustrated in the figure:

The VLR contains information about all MS visiting that particular MSC and hence points to
the HLR of the visiting MSs for exchanging related information about the MS. Such a pointer
allows calls to be routed or rerouted to the MS, wherever it is located. In cellular systems, a
reverse direction pointer is needed that allows traversal of many control signals back and forth
between the HLR and VLR such bidirectional HLR-VLR pointers help in carrying out various
functionalities.

Frequencies for Radio Communication:

Radio frequency (RF) is a measurement representing the oscillation rate of electromagnetic

radiation spectrum, or electromagnetic radio waves, from frequencies ranging from 300
gigahertz (GHz) to as low as 9 kilohertz (kHz). With the use of antennas and transmitters, an
RF field can be used for various types of wireless broadcasting and communications.

How radio frequency works?

Radio frequency is measured in units called hertz (Hz), which represent the number of cycles
per second when a radio wave is transmitted. One hertz equals one cycle per second; radio
waves range from thousands (kilohertz) to millions (megahertz) to billions (gigahertz) of cycles
per second. In a radio wave, the wavelength is inversely proportional to the frequency. Radio
frequencies are not visible to the human eye. As the frequency is increased beyond that of the
RF spectrum, electromagnetic energy takes the form of microwaves, infrared radiation (IR),
visible, ultraviolet, X-rays and gamma rays.

RF technology

Many types of wireless devices make use of RF fields. Cordless and cell phones, radio and
television broadcast stations, Wi-Fi and Bluetooth, satellite communications systems and two-
way radios all operate in the RF spectrum. In addition, other appliances outside of
communications, including microwave ovens and garage door openers, operate at radio
frequencies. Some wireless devices, like TV remote controls, computer keyboards and
computer mice, operate at IR frequencies, which have shorter electromagnetic wavelengths.

How the radio frequency spectrum is used?

The radio frequency spectrum includes the set of frequencies of the electromagnetic framework
ranging from 30 Hz to 300 GHz. It is divided into several ranges, or bands, and given labels,
such as low frequency (LF), medium frequency (MF) and high frequency (HF), for easier
identification.

With the exception of the lowest-frequency segment, each band represents an increase of
frequency corresponding to an order of magnitude (power of 10). The following table depicts
the eight bands in the RF spectrum, showing frequency and bandwidth ranges. The super high
frequency (SHF) and extremely high frequency (EHF) bands are often referred to as
the microwave spectrum.

Signals and Noise in Communication System:

A signal is an electromagnetic or electrical current that carries data from one system or network
to another. In electronics, a signal is often a time-varying voltage that is also an electromagnetic
wave carrying information, though it can take on other forms, such as current. There are two
main types of signals used in electronics: analog and digital signals. This article discusses the
corresponding characteristics, uses, advantages and disadvantages, and typical applications of
analog vs. digital signals.
Signal Strength:

Signal strength in mobile communication refers to the measurement of the power level of the
radio signal received by a mobile device from a cellular network. It indicates the quality and
reliability of the wireless connection between the mobile device and the network. Signal
strength is typically measured in decibel milliwatts (dBm) or signal bars on the mobile device's
display. The dBm scale represents the power level of the received signal relative to a reference
level. A higher positive dBm value indicates a stronger signal, while a lower negative dBm
value indicates a weaker signal. In most cases, a signal strength of around -50 dBm to -70 dBm
is considered excellent, providing a strong and reliable connection. As the signal strength
decreases, the quality of the connection may deteriorate, leading to dropped calls, slower data
speeds, or difficulty establishing a connection. Signal strengths below -100 dBm may result in
a complete loss of connectivity.

Several factors can affect signal strength, including:

Distance from the cellular tower: The farther away you are from a cellular tower, the weaker
the signal is likely to be.

Obstructions: Buildings, trees, and other obstacles can attenuate or block the signal, reducing
its strength.

Interference: Other electronic devices, physical obstructions, or nearby networks operating on

the same frequency can cause signal interference.

Network congestion: High network usage or heavy traffic in a specific area can impact signal
strength.

Weather conditions: Extreme weather conditions like storms or heavy rainfall can temporarily
affect signal strength.

To improve signal strength, you can try the following:

Move closer to a window or go to an open area: This can reduce obstructions and improve the
signal reception.

Avoid physical obstructions: Position yourself away from objects that may block the signal,
such as large buildings or dense vegetation.

Restart your device: Sometimes, restarting your mobile device can help it reconnect to the
network and improve signal strength.

Use Wi-Fi calling: If available, using a Wi-Fi network for calling can provide a more stable
and reliable connection.

Consider using a signal booster: Signal boosters or repeaters can amplify the signal strength in
areas with weak reception.
It's important to note that signal strength can vary depending on the cellular network, the
technology used (2G, 3G, 4G, 5G), and the specific location you are in.

Analog Signal

An analog signal is time-varying and generally bound to a range (e.g. +12V to -12V), but there
is an infinite number of values within that continuous range. An analog signal uses a given
property of the medium to convey the signal’s information, such as electricity moving through
a wire. In an electrical signal, the voltage, current, or frequency of the signal may be varied to
represent the information. Analog signals are often calculated responses to changes in light,
sound, temperature, position, pressure, or other physical phenomena.

Digital Signal

A digital signal is a signal that represents data as a sequence of discrete values. A digital signal
can only take on one value from a finite set of possible values at a given time. With digital
signals, the physical quantity representing the information can be many things:

• Variable electric current or voltage

• Phase or polarization of an electromagnetic field
• Acoustic pressure
• The magnetization of a magnetic storage media

Digital signals are used in all digital electronics, including computing equipment and data
transmission devices. When plotted on a voltage vs. time graph, digital signals are one of two
values, and are usually between 0V and VCC (usually 1.8V, 3.3V, or 5V)

In any communication system, during the transmission of the signal, or while receiving the
signal, some unwanted signal gets introduced into the communication, making it unpleasant
for the receiver, questioning the quality of the communication. Such a disturbance is called
as Noise.

What is Noise?

Noise is an unwanted signal which interferes with the original message signal and corrupts the
parameters of the message signal. This alteration in the communication process, leads to the
message getting altered. It is most likely to be entered at the channel or the receiver.

The noise signal can be understood by taking a look at the following example.
Hence, it is understood that noise is some signal which has no pattern and no constant frequency
or amplitude. It is quite random and unpredictable. Measures are usually taken to reduce it,
though it can’t be completely eliminated.

Most common examples of noise are −

• Hiss sound in radio receivers

• Buzz sound amidst of telephone conversations
• Flicker in television receivers, etc.

Effects of Noise

Noise is an inconvenient feature which affects the system performance. Following are the
effects of noise.

Noise limits the operating range of the systems

Noise indirectly places a limit on the weakest signal that can be amplified by an amplifier. The
oscillator in the mixer circuit may limit its frequency because of noise. A system’s operation
depends on the operation of its circuits. Noise limits the smallest signal that a receiver is
capable of processing.

Noise affects the sensitivity of receivers

Sensitivity is the minimum amount of input signal necessary to obtain the specified quality
output. Noise affects the sensitivity of a receiver system, which eventually affects the output.
Types of Noise:

The classification of noise is done depending on the type of the source, the effect it shows or
the relation it has with the receiver, etc.

There are two main ways in which noise is produced. One is through some external
source while the other is created by an internal source, within the receiver section.

External Source

This noise is produced by the external sources which may occur in the medium or channel of
communication, usually. This noise cannot be completely eliminated. The best way is to avoid
the noise from affecting the signal.

Examples

Most common examples of this type of noise are −

• Atmospheric noise (due to irregularities in the atmosphere).

• Extra-terrestrial noise, such as solar noise and cosmic noise.
• Industrial noise.

Internal Source

This noise is produced by the receiver components while functioning. The components in the
circuits, due to continuous functioning, may produce few types of noise. This noise is
quantifiable. A proper receiver design may lower the effect of this internal noise.

Examples

Most common examples of this type of noise are –

• Thermal agitation noise (Johnson noise or Electrical noise).

• Shot noise (due to the random movement of electrons and holes).
• Transit-time noise (during transition).
• Miscellaneous noise is another type of noise which includes flicker, resistance effect
and mixer generated noise, etc.

Modulation and Demodulation:

Modulation is the process of encoding information in a transmitted signal, while demodulation

is the process of extracting information from the transmitted signal. Many factors influence
how faithfully the extracted information replicates the original input
information. Electromagnetic interference can degrade signals and make the original signal
impossible to extract. Demodulators typically include multiple stages of amplification and
filtering in order to eliminate interference. A device that performs both modulation and
demodulation is called a modem -- a name created by combining the first letters of Modulator
and Demodulator. A computer audio modem allows a computer to connect to another computer
or to a data network over a regular analog phone line by using the data signal to modulate an
analog audio tone. A modem at the far end demodulates the audio signal to recover the data
stream. A cable modem uses network data to modulate the cable service carrier signal.

Amplitude Modulation and Frequency Modulation:

Amplitude modulation and frequency modulation are used to transmit data using the method
of modifying a carrier signal. The main difference between both modulations is that in
frequency modulation, the carrier wave frequency is modified as per the transmit data. In
contrast, in amplitude modulation, the carrier wave is modified according to the data.

For instance, if several sets of data are required to be transmitted using the same medium, then
each set off is sent using different frequency waves. This is the process of how radio broadcasts
are done.

Amplitude Modulation

Amplitude modulation is a modulation technique where the amplitude of a carrier varies

depending on the information signal. AM radio broadcast signals use lower carrier frequencies,
which helps them travel long distances. Sometimes AM signals can be able to bounce off the
ionosphere. The distance travelled by the AM is much larger than the FM.

Frequency Modulation

In this module, the carrier wave frequency is modified according to the signal that carries
information. The radio signals have large bandwidth than AM radio signals, which helps to
offer much better sound quality. Frequency modulation also enables to transmit stereo signals.
 Difference Between AM and FM

Amplitude Modulation (AM) Frequency Modulation (FM)

The first successful audio transmission Developed in 1930 by Edwin Armstrong,

was carried out in the mid-1870s in the United States

The radio wave is called a carrier wave, The radio wave is called a carrier wave,
and the frequency and phase remain the but the amplitude and phase remain the
same same

Has poor sound quality, but can transmit Has higher bandwidth with better sound
longer distance quality

The frequency range of AM radio varies The frequency range of FM is 88 to 108

from 535 to 1705 kHz MHz in the higher spectrum

More susceptible to noise Less susceptible to noise

Digital Modulation Techniques:

Digital Modulation provides more information capacity, high data security, quicker system
availability with great quality communication. Hence, digital modulation techniques have a
greater demand, for their capacity to convey larger amounts of data than analog ones.

There are many types of digital modulation techniques and we can even use a combination of
these techniques as well. In this chapter, we will be discussing the most prominent digital
modulation techniques.

Amplitude Shift Keying:

The amplitude of the resultant output depends upon the input data whether it should be a zero
level or a variation of positive and negative, depending upon the carrier frequency.

Amplitude Shift Keying (ASK) is a type of Amplitude Modulation which represents the binary
data in the form of variations in the amplitude of a signal.

Any modulated signal has a high frequency carrier. The binary signal when ASK is modulated,
gives a zero value for LOW input and gives the carrier output for HIGH input.
Following is the diagram for ASK modulated waveform along with its input.

Frequency Shift Keying:

The frequency of the output signal will be either high or low, depending upon the input data
applied.

Frequency Shift Keying (FSK) is the digital modulation technique in which the frequency of
the carrier signal varies according to the discrete digital changes. FSK is a scheme of frequency
modulation.

Following is the diagram for FSK modulated waveform along with its input.

The output of a FSK modulated wave is high in frequency for a binary HIGH input and is low
in frequency for a binary LOW input. The binary 1s and 0s are called Mark and Space
frequencies.
Phase Shift Keying:

The phase of the output signal gets shifted depending upon the input. These are mainly of two
types, namely BPSK and QPSK, according to the number of phase shifts. The other one is
DPSK which changes the phase according to the previous value.

Phase Shift Keying (PSK) is the digital modulation technique in which the phase of the carrier
signal is changed by varying the sine and cosine inputs at a particular time. PSK technique is
widely used for wireless LANs, bio-metric, contactless operations, along with RFID and
Bluetooth communications.

PSK is of two types, depending upon the phases the signal gets shifted. They are −

Binary Phase Shift Keying (BPSK)

This is also called as 2-phase PSK (or) Phase Reversal Keying. In this technique, the sine wave
carrier takes two phase reversals such as 0° and 180°.

BPSK is basically a DSB-SC (Double Sideband Suppressed Carrier) modulation scheme, for
message being the digital information.

Following is the image of BPSK Modulated output wave along with its input.

Quadrature Phase Shift Keying (QPSK)

This is the phase shift keying technique, in which the sine wave carrier takes four phase
reversals such as 0°, 90°, 180°, and 270°.

If this kind of techniques are further extended, PSK can be done by eight or sixteen values also,
depending upon the requirement. The following figure represents the QPSK waveform for two
bits input, which shows the modulated result for different instances of binary inputs.
QPSK is a variation of BPSK, and it is also a DSB-SC (Double Sideband Suppressed Carrier)
modulation scheme, which send two bits of digital information at a time, called as bigits.

Instead of the conversion of digital bits into a series of digital stream, it converts them into bit-
pairs. This decreases the data bit rate to half, which allows space for the other users.

Differential Phase Shift Keying (DPSK)

In DPSK (Differential Phase Shift Keying) the phase of the modulated signal is shifted relative
to the previous signal element. No reference signal is considered here. The signal phase follows
the high or low state of the previous element. This DPSK technique doesn’t need a reference
oscillator.

The following figure represents the model waveform of DPSK.

It is seen from the above figure that, if the data bit is LOW i.e., 0, then the phase of the signal
is not reversed, but is continued as it was. If the data is HIGH i.e., 1, then the phase of the
signal is reversed, as with NRZI, invert on 1 (a form of differential encoding).

If we observe the above waveform, we can say that the HIGH state represents an M in the
modulating signal and the LOW state represents a W in the modulating signal.
Multiple Access & It’s Techniques:

Sometimes a satellite’s service is present at a particular location on the earth station and
sometimes it is not present. That means, a satellite may have different service stations of its
own located at different places on the earth. They send carrier signal for the satellite.

In this situation, we do multiple access to enable satellite to take or give signals from different
stations at time without any interference between them. Following are the three types of
multiple access techniques.

• FDMA (Frequency Division Multiple Access)

• TDMA (Time Division Multiple Access)
• CDMA (Code Division Multiple Access)

Now, let us discuss each technique one by one.

FDMA

In this type of multiple access, we assign each signal a different type of frequency band (range).
So, any two signals should not have same type of frequency range. Hence, there won’t be any
interference between them, even if we send those signals in one channel.

One perfect example of this type of access is our radio channels. We can see that each station
has been given a different frequency band in order to operate.

Let’s take three stations A, B and C. We want to access them through FDMA technique. So we
assigned them different frequency bands.

As shown in the figure, satellite station A has been kept under the frequency range of 0 to 20
HZ. Similarly, stations B and C have been assigned the frequency range of 30-60 Hz and 70-
90 Hz respectively. There is no interference between them.
The main disadvantage of this type of system is that it is very burst. This type of multiple access
is not recommended for the channels, which are of dynamic and uneven. Because, it will make
their data as inflexible and inefficient.

TDMA

As the name suggests, TDMA is a time based access. Here, we give certain time frame to each
channel. Within that time frame, the channel can access the entire spectrum bandwidth

Each station got a fixed length or slot. The slots, which are unused will remain in idle stage.

Suppose, we want to send five packets of data to a particular channel in TDMA technique. So,
we should assign them certain time slots or time frame within which it can access the entire
bandwidth.

In above figure, packets 1, 3 and 4 are active, which transmits data. Whereas, packets 2 and 5
are idle because of their non-participation. This format gets repeated every time we assign
bandwidth to that particular channel.

Although, we have assigned certain time slots to a particular channel but it can also be changed
depending upon the load bearing capacity. That means, if a channel is transmitting heavier
loads, then it can be assigned a bigger time slot than the channel which is transmitting lighter
loads. This is the biggest advantage of TDMA over FDMA. Another advantage of TDMA is
that the power consumption will be very low.

Note − In some applications, we use the combination of both TDMA and FDMA techniques.
In this case, each channel will be operated in a particular frequency band for a particular time
frame. In this case, the frequency selection is more robust and it has greater capacity over time
compression.

CDMA

In CDMA technique, a unique code has been assigned to each channel to distinguish from each
other. A perfect example of this type of multiple access is our cellular system. We can see that
no two persons’ mobile number match with each other although they are same X or Y mobile
service providing company’s customers using the same bandwidth.
The basic advantage of this type of multiple access is that it allows all users to coexist and use
the entire bandwidth at the same time. Since each user has different code, there won’t be any
interference.

In this technique, a number of stations can have number of channels unlike FDMA and TDMA.
The best part of this technique is that each station can use the entire spectrum at all time.

1. Frequency Division Multiple Access (FDMA) : FDMA is a type of channelization

protocol. In this bandwidth is divided into various frequency bands. Each station is allocated
with band to send data and that band is reserved for particular station for all the time which
is as follows :

Figure – FDMA

The frequency bands of different stations are separated by small band of unused frequency
and that unused frequency bands are called as guard bands that prevents the interference of
stations. It is like access method in data link layer in which data link layer at each station
tells its physical layer to make a band pass signal from the data passed to it. The signal is
created in the allocated band and there is no physical multiplexer at the physical layer.

2. Time Division Multiple Access (TDMA) : TDMA is the channelization protocol in which
bandwidth of channel is divided into various stations on the time basis. There is a time slot
given to each station, the station can transmit data during that time slot only which is as
follows :
 Figure – TDMA

Each station must aware of its beginning of time slot and the location of the time slot. TDMA
requires synchronization between different stations. It is type of access method in the data
link layer. At each station data link layer tells the station to use the allocated time slot.

3. Code Division Multiple Access (CDMA) : In CDMA, all the stations can transmit data
simultaneously. It allows each station to transmit data over the entire frequency all the time.
Multiple simultaneous transmissions are separated by unique code sequence. Each user is
assigned with a unique code sequence.

Figure – CDMA
In the above figure, there are 4 stations marked as 1, 2, 3 and 4. Data assigned with respective
stations as d1, d2, d3 and d4 and the code assigned with respective stations as c1, c2, c3 and c4.

Spread Spectrum Modulation:

The increasing demand for wireless communications has problems due to limited spectrum
efficiency and multipath propagation. The use of spread spectrum communication has
simplified these problems. In the spread spectrum, signals from different sources are
combined to fit into larger bandwidth.

Most stations use air as the medium for communication, stations must be able to share the
medium without an interception and without being subject to jamming from a malicious
intruder. To achieve this, spread-spectrum techniques add redundancy means it uses extended
bandwidth to accommodate signals in a protective envelope so that more secure transmission
is possible. The spread code is a series of numbers that looks random but are actually a
pattern. The original bandwidth of the signal gets enlarged (spread) through the spread code
as shown in the figure.

Spread Spectrum

Principles of Spread Spectrum process:

1. To allow redundancy, it is necessary that the bandwidth allocated to each station

should be much larger than needed.
2. The spreading process occurs after the signal is created by the source.

Conditions of Spread Spectrum are:

1. The spread spectrum is a type of modulation where modulated signal BW is much

larger than the baseband signal BW i.e. spread spectrum is a wide band scheme.
2. A special code (pseudo noise) is used for spectrum spreading and the same code
is to be used to despread the signal at the receiver.

Characteristics of the Spread Spectrum are:

1. Higher channel capacity.

 2. Ability to resist multipath propagation.
3. They cannot easily intercept any unauthorized person.
4. They are resistant to jamming.
5. The spread spectrum provides immunity to distortion due to multipath
propagation.
6. The spread spectrum offers multiple access capabilities.

Two types of techniques for Spread Spectrum are:

1. Frequency Hopping Spread Spectrum (FHSS)

2. Direct Sequence Spread Spectrum (DSSS)

Frequency Hopping Spread Spectrum (FHSS):

In Frequency Hopping Spread Spectrum (FHSS), different carrier frequencies are modulated
by the source signal i.e. M carrier frequencies are modulated by the signal. At one moment
signal modulates one carrier frequency and at the subsequent moments, it modulates other
carrier frequencies. The general block diagram of FHSS is shown in the below figure.

Frequency Hopping Spread Spectrum

A pseudorandom code generator generates Pseudo-random Noise of some pattern for each
hopping period Th. The frequency corresponding to the pattern is used for the hopping period
and is passed to the frequency synthesizer. The synthesizer generates a carrier signal of that
frequency. The figure above shows the spread signal via FHSS.

Advantages of FHSS:

• Synchronization is not greatly dependent on distance.

• Processing Gain is higher than DSSS.
Disadvantages of FHSS:

• The bandwidth of the FHSS system is too large (in GHz).

• Complex and expensive Digital frequency synthesizers are required.

Direct Sequence Spread Spectrum (DSSS):

In DSSS, the bandwidth of the original signal is also expanded by a different technique. Here,
each data bit is replaced with n bits using a spreading code called chips, and the bit rate of
the chip is called as chip-rate. The chip rate is n times the bit rate of the original signal. The
below Figure shows the DSSS block diagram.

Direct Sequence Spread Spectrum

In wireless LAN, the sequence with n = 11 is used. The original data is multiplied
by chips (spreading code) to get the spread signal. The required bandwidth of the spread
signal is 11 times larger than the bandwidth of the original signal.

Advantages of DSSS:

• The DSSS System combats the jamming most effectively.

• The performance of DSSS in presence of noise is superior to FHSS.
• Interference is minimized against the signals.

Disadvantages of DSSS:

• Processing Gain is lower than DSSS.

• Channel Bandwidth is less than FHSS.
• Synchronization is affected by the variable distance between the transmitter and
receiver.

Prepared by Dr. Chiranjit Dutta