21CSE458T

WIRELESS MOBILE
COMMUNICATION
UNIT – 1
Department of CSE

1
Unit-1 - Introduction to Wireless Communication
Elements of wireless communication system- Frequencies for radio communication-
Signals, Noise – Types of Noise- Introduction to modulation and demodulation- Signals in
the modulation- Introduction to Analog modulation schemes- Amplitude Modulation
Frequency modulation- Phase Modulation- Introduction to Analog modulation schemes-
Amplitude Shift Keying Frequency Shift Keying Phase Shift Keying- BPSK, QPSK-
Multiplexing and multiple access techniques- Frequency-division multiplexing- Time-
division multiplexing- Code-division multiplexing- Spread spectrum modulation- frequency
hopping Spread spectrum- Direct Sequence Spread spectrum
 WHAT IS WIRELESS ?

The word wireless is dictionary defined “having no wires ” In networking

terminology , wireless is the term used to describe any computer network where
there is no physical wired connection between sender and receiver, but rather the
network is connected by radio waves and or microwaves to maintain
communications.
Wireless networking utilizes specific equipment such as NICs (Network
Interface Controllers) and Routers in place of wires (copper or optical fiber).
 Introduction to wireless Communication
⮚Wireless Communication is the process of transmitting voice and
data using Electromagnetic waves in open space (atmosphere)
⮚Wireless means Transmitting signals over invisible radio waves
instead of wires.
❖ WC – Limitations
⮚Bandwidth
⮚Frequency Spectrum
⮚Power
❖ Advantages and Disadvantages
❖ Technologies in Digital WC
❖ Wireless modulation schemes
❖ Infrared modulation Schemes
Wireless Communication Channel Specifications
▪ Two steps
⮚Duplexing method
⮚Multiple access method

Important terms used to describe elements of wireless

communication systems
Base station
Mobile Station
Mobile switching centre
 Basics of Wireless Networks

⮚ Network – a collection of terminals, computers, servers and

components which allows for the easy flow of data and use of
resources among them.
⮚ Wireless network
⮚ Wireless network Architecture
Elements of a Wireless Communication system
The essential components of a communication system are :
• Information Source
• Input Transducer
• Transmitter
• Communication Channel
• Receiver
• Destination.
Information Source
• A communication system serves to communicate a message or information. This
information originates in the information source.
Input Transducer
• The message from the information source may or may not be electrical in nature. In a
case when the message produced by the information source is not electrical in nature,
an input transducer is used to convert it into a time-varying electrical signal.
Transmitter
• In long-distance radio communication, signal amplification is
necessary before modulation.
• Modulation is the main function of the transmitter. In modulation, the
message signal is superimposed upon the high-frequency carrier signal.
• All these processing of the message signal are done just to ease the
transmission of the signal through the channel.
The Channel and The Noise
• The term channel means the medium through which the message travels
from the transmitter to the receiver.
• During the process of transmission and reception the signal gets distorted
due to noise introduced in the system.
• Noise is an unwanted signal which tend to interfere with the required signal.
Noise signal is always random in character.
Receiver
• The main function of the receiver is to reproduce the message signal in
electrical form from the distorted received signal. This reproduction of the
original signal is accomplished by a process known as the demodulation or
detection. Demodulation is the reverse process of modulation carried out in
transmitter.
Destination
• Destination is the final stage which is used to convert an electrical
message signal into its original form.
• For example in radio broadcasting, the destination is a loudspeaker
which works as a transducer i.e. converts the electrical signal in the
form of original sound signal.
 Continue....
Transmitter
Encodes and modulates voice, data, or video signals for sending.

Channel
The medium like air or space where signals travel amid noise.

Receiver
Decodes and demodulates signals back to original information.

Example
Cell towers send signals to smartphones through wireless channels.
Basic Components of a Wireless System
Transmitter Channel
Processes signals through modulation and amplification before The medium used for signal propagation, influenced by interference,

broadcasting. It converts input data into radio waves for transmission. noise, and other environmental factors affecting signal quality.

Receiver Examples
Demodulates and filters the incoming signal to recover original Cell towers, satellite links, and various wireless devices that implement

information accurately amidst noise and distortions. these components for communication.
Signals and Noise
Signal
The desired information like voice or data transmitted.

Noise
Undesired interference that degrades signal quality.

Signal-to-Noise Ratio
Higher SNR means clearer, better signal reception.

Types of Noise
• Thermal noise from electron motion
• Interference from other signals
• Atmospheric noise like lightning
 Noise & its Types
Noise
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.
 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.
Types of Noise
 External Noise

• 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
• Atmospheric noise (due to irregularities in the atmosphere).
• Extra-terrestrial noise, such as solar noise and cosmic noise.
• Industrial noise.
 Atmospheric Noise

• Static Noise
• Mainly due to lightning
• Results in transient electrical signal that generates harmonic
energy that can travel long distances
 Extraterrestrial Noise

• Solar noise
• Radiates broad spectrum of noise which includes frequencies used
for communication.
• These noise radiations vary with time
• Sun cycle repeats with 11 years
• Cosmic Noise
• Generated by stars
• Impact on frequency range 15-150MHz
Industrial noise
• Created by ignition systems, electric motors, fluorescent light. These
produce transients that create noise
 Internal Noise

• 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
• 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.
 Continue.....
• Shot Noise : These Noise are generally arises in the active devices
due to the random behaviour of Charge particles or carrierss. In case
of electron tube, shot Noise is produces due to the random emission
of electron form cathodes.
• Partition Noise : When a circuit is to divide in between two or
more paths then the noise generated is known as Partition noise.
The reason for the generation is random fluctuation in the division.
 Continue.....
• High- Frequency Noise : These noises are also known TRANSIT-
TIME Noise. They are observed in the semi-conductor devices
when the transit time of a charge carrier while crossing a junction
is compared with the time period of that signal.

• Thermal Noise : Thermal Noise are random and often referred as

White Noise or Johnson Noise. Thermal noise are generally
observed in the resistor or the sensitive resistive components of a
complex impedance due to the random and rapid movement of
molecules or atoms or electrons.
 Continue.....

• Low- Frequency Noise : They are also known as FLICKER

NOISE. These type of noise are generally observed at a frequency
range below few kHz. Power spectral density of these noise
increases with the decrease in frequency. That why the name is
given Low- Frequency Noise.
 Continue.....

Thermal noise power

PN = kTB
where
• PN = noise power in watts
• k = Boltzmann’s constant, 1.38 × 10−23 joules/kelvin (J/K)
• T = temperature in kelvins
• B = noise power bandwidth in hertz
 Problem
• A resistor at a temperature of 25 °C is connected across the input
of an amplifier with a bandwidth of 50 kHz. How much noise
does the resistor supply to the input of the amplifier?
 Continue.....

• Noise figure describes the way in which a device adds noise to a

signal and thereby degrades the signal-to-noise ratio. It is defined as
follows:
NF=(S/N)i / (S/N)o
where
• (S/N)i = signal-to-noise ratio at the input
• (S/N)o = signal-to-noise ratio at the output
 Continue.....
• Converting noise figure to noise temperature is quite easy:
• Teq = 290(NF − 1) where
• Teq = equivalent noise temperature in kelvins
• NF = noise figure as a ratio (not in dB)
• The noise temperature due to the equipment must be added to the
noise
• Temperature contributed by the antenna and its transmission line
to find the total system noise temperature
 Radio Frequency Spectrum

Radio waves are a form of electromagnetic radiation, as are infrared, visible

light, ultraviolet light, and gamma rays. The major difference is in the
frequency of the waves. The portion of the frequency spectrum that is useful
for radio communication at present extends from roughly 100 kHz to about
50 GHz.
Radio Frequency Spectrum
Continue....
Key Considerations for Frequency Selection
Propagation Characteristics Environmental & Regulatory Factors

Factors like path loss, fading, and diffraction influence signal Weather conditions cause absorption effects; regulatory rules

strength and reliability over distance. govern license acquisition and spectrum usage rights to prevent
• Impact of terrain and obstacles
interference.
• Signal weakening over longer distances • Bandwidth availability based on frequency

• Compliance with legal limits for transmit power

Advanced Modulation Techniques

Analog Modulation
• Amplitude Modulation (AM)
• Frequency Modulation (FM)
• Phase Modulation (PM)

Digital Modulation
• Quadrature Amplitude Modulation (QAM)
• Orthogonal Frequency Division Multiplexing (OFDM)

Spectral Efficiency
Modern techniques like 8-PSK and 16-QAM optimize bandwidth use

and data rates in wireless networks.

Conclusion and Future Trends
Summary Emerging Technologies
Reviewed key frequency bands and their Introducing mmWave and Terahertz

practical applications in wireless

1 2 communication for ultra-high-speed

communication systems. data transmission.

Call to Action Future Outlook

Encourage ongoing research and Wireless communications are evolving
4 3
exploration to stay ahead in this rapidly, promising new capabilities and

dynamic field. efficiencies.

Introduction to Modulation and Demodulation
Modulation Demodulation Carrier Parameters Purpose

Encoding information by Recovering the original data • Amplitude Enables efficient transmission

varying a carrier wave's from the modulated carrier • Frequency and reduces noise effects.
• Phase
properties. wave.
Analog Modulation Schemes

Amplitude Modulation Frequency Modulation

(AM) (FM)
Changes carrier amplitude, used Varies carrier frequency, used

in AM radio (530-1700 kHz). in FM radio (88-108 MHz).

Simple but sensitive to noise.

More resistant to noise than

AM.

Phase Modulation (PM)

Alters carrier phase, related to FM, used in some digital systems.
Digital Modulation Techniques
Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK)

Bits represented by different amplitude Bits encoded by changes in frequency. Bits mapped onto different phase shifts.

levels.
• BPSK: Two phases (0°,180°)
• QPSK: Four phases (0°,90°,180°,270°)

Common in Wi-Fi and Bluetooth.

Shot Noise
Charge Carrier Randomness Semiconductor Origin Poisson Process
It is predominantly observed in Shot noise can be accurately modeled
Shot noise arises from the discrete and
semiconductor devices like diodes, as a Poisson process, reflecting the
random arrival times of individual
transistors, and vacuum tubes, where statistical independence of each charge
charge carriers, such as electrons and
charge carriers cross junctions or carrier's arrival. This type of noise is
holes, at a potential barrier within a
potential barriers. The magnitude of particularly significant in the receivers
device. This discontinuous flow, rather
shot noise is directly dependent on the of optical communication systems,
than a smooth, continuous current,
average DC current flowing through where light is converted into electrical
generates fluctuations.
the device. signals by photodetectors.
Understanding Noise
Definition of Noise Common Noise Types Signal-to-Noise Ratio (SNR)
Noise refers to unwanted signals that interfere • Thermal Noise: Random electron motion, SNR = Psignal / Pnoise. Higher SNR means

with the transmission of the desired signal, increases with temperature clearer, more reliable signals.
• Shot Noise: Fluctuations in current due to Nyquist formula (N = kTB) quantifies thermal
degrading communication quality.
discrete charge carriers noise power, where noise power is proportional to
• Flicker Noise: Low-frequency fluctuations
temperature and bandwidth.
in electronic devices
• Environmental Noise: Interference from

external sources
Why Modulation?
Efficient Transmission Frequency Allocation Hardware Limitations
Antennas operate effectively when their Modulation enables multiple signals to Modulation helps overcome physical

size is about a quarter of the wavelength share the same medium at different limitations of transmitting equipment,

(λ/4). Lower frequencies require frequencies, preventing interference and allowing better control of signal

impractically large antennas; modulation optimizing bandwidth use. propagation and reception.

shifts signals to higher frequencies for

manageable antenna sizes.

Modulation Techniques
Amplitude Modulation Frequency Modulation Phase Modulation Digital Modulation
(AM) (FM) (PM) Techniques like ASK, FSK,
The carrier's amplitude is The carrier frequency is altered Carrier phase changes
PSK, and QAM encode digital
varied in step with the message by the baseband signal’s according to the message
data via changes in amplitude,
signal amplitude. instantaneous amplitude. signal, affecting signal position
frequency, or phase.
in time.
Amplitude Modulation (AM)

AM Equation
s(t) = Ac[1 + m(t)] cos(2πfct)

Modulation Index
μ = Am/Ac; indicates depth of modulation

Overmodulation
Occurs when μ > 1, causing distortion and signal inefficiency.
Frequency Modulation (FM)

Frequency Deviation (Δf)

Δf = kf Am; the maximum frequency shift
2
FM Signal Equation
s(t) = Ac cos[2πfct + 2πkf∫m(τ)dτ] 1

Bandwidth (Carson's Rule)

3 BW = 2(Δf + fm); includes frequency deviation

and message bandwidth

Signals in Modulation:
Baseband Signal
Definition Examples
The baseband signal contains Audio signals, such as speech

the original information before and music; video signals for

modulation—commonly voice, broadcasting; digital data for

video, or data streams. communication systems.

Bandwidth
Determines the frequency range occupied by the signal. For voice, typically

300 Hz to 3.4 kHz, exposing the need for modulation to higher frequencies.
Signals in Modulation: Carrier Signal

Definition Carrier Equation

The carrier is a high-frequency c(t) = Ac cos(2πfct) where Ac is

sinusoidal wave modulated to carry amplitude and fc is frequency.

the baseband signal across

communication channels.

Stability Importance
A stable carrier frequency is critical to prevent distortion and ensure accurate

demodulation at the receiver side.

Demodulation Techniques
AM Demodulation FM Demodulation Digital Demodulation

Uses envelope detectors to extract amplitude Employs Foster-Seeley discriminator or Coherent detection requires phase

variations corresponding to the original phase-locked loops (PLL) to detect synchronization; non-coherent methods rely

message signal. frequency changes and recover the on energy detection and thresholding.

baseband.
Amplitude Modulation (AM) and Frequency
Modulation (FM)
Principles of Amplitude Modulation (AM)
In Amplitude Modulation (AM), the information signal, often referred to as the modulating signal, directly varies the amplitude of a high-

frequency carrier wave. This carrier wave typically operates in the kilohertz range, such as 530 kHz to 1700 kHz for standard AM radio

broadcasting.
A critical parameter is the Modulation Index (µ), calculated as the ratio of the amplitude of the modulating signal (Am) to the amplitude of the

carrier signal (Ac). If µ exceeds 1, a phenomenon known as overmodulation occurs, leading to significant signal distortion and loss of information.

The AM signal can be mathematically represented by the equation: s(t) = Ac[1 + µ*m(t)]cos(2πfct).

• Carrier Wave: High-frequency signal (e.g., 530 kHz - 1700 kHz)

• Modulating Signal: Information signal (audio, data)
• Modulation Index (µ): µ = Am/Ac (Amplitude of Modulating Signal / Amplitude of Carrier Signal)
• Overmodulation: µ > 1, causes distortion
• AM signal equation: s(t) = Ac[1 + µ*m(t)]cos(2πfct)
Principles of Frequency Modulation (FM)
Frequency Modulation (FM) operates by varying the frequency of the carrier wave in proportion to the instantaneous amplitude of the modulating

signal. Unlike AM, the amplitude of the FM carrier wave remains constant, which significantly contributes to its robust performance against noise.

Key parameters in FM include Frequency Deviation (Δf), which indicates the maximum shift in carrier frequency from its center frequency,

calculated as Kf * Am (where Kf is the frequency sensitivity). The Modulation Index (β) is the ratio of frequency deviation to the modulating

signal's frequency (fm), and the signal's bandwidth is approximated by Carson's Rule: BW ≈ 2(Δf + fm). The FM signal is defined by: s(t) = Ac

cos[2πfct + βsin(2πfmt)].
Carrier Wave Frequency Deviation (Δf)
Higher frequency signal (e.g., 88 MHz - 108 MHz) Δf = Kf * Am (Kf is the frequency sensitivity)

Modulation Index (β) Bandwidth (BW)

β = Δf / fm (fm is the frequency of the modulating signal) BW ≈ 2(Δf + fm) (Carson's Rule)
Key Differences: AM vs FM
The fundamental distinction between AM and FM lies in how they encode information onto the carrier wave. AM varies the amplitude, while FM

varies the frequency. This difference leads to several practical implications in terms of performance and application. FM generally requires a wider

bandwidth, allowing it to carry more information and provide superior audio quality, but also making it more complex to implement compared to AM.

Crucially, FM's constant amplitude makes it far more immune to noise, which often manifests as amplitude variations. AM, with its varying amplitude,

is more susceptible to these interferences. This noise immunity contributes to FM's higher power efficiency as the transmission power remains

constant, unlike AM where power fluctuates with the signal.

AM FM
• Parameter Variation: Varies amplitude • Parameter Variation: Varies frequency
• Bandwidth: Narrower • Bandwidth: Wider
• Noise Immunity: Lower • Noise Immunity: Higher
• Power Efficiency: Less efficient • Power Efficiency: Constant power
• Complexity: Simpler • Complexity: More complex
Advantages and Disadvantages of AM
Amplitude Modulation (AM) holds a significant place in the history of radio communication due to its relative simplicity. Its straightforward

implementation makes AM receivers less expensive to produce, contributing to its widespread adoption, especially in early broadcasting. This low

cost and ease of setup are significant advantages for many basic communication needs.

However, AM's major drawback is its susceptibility to noise. Electrical disturbances, atmospheric interference, and even static from everyday

appliances can easily corrupt an AM signal, leading to poorer audio quality. This inherent vulnerability limits its use in applications where high

fidelity or critical reliability is paramount.

Advantages Disadvantages
• Simple to implement • Susceptible to noise
• Low cost receivers • Lower audio quality

Applications include traditional AM radio broadcasting, which benefits from its long-range propagation, and aviation communication, where its

simplicity allows for reliable voice transmission.

Advantages and Disadvantages of FM
Frequency Modulation (FM) stands out for its superior audio quality and remarkable immunity to noise. Because the information is carried in

frequency variations rather than amplitude, FM signals are far less affected by electrical interference, leading to a much clearer and more stable

listening experience. This makes FM ideal for applications where high-fidelity sound is essential.

Despite these advantages, FM comes with its own set of challenges. Its implementation is more complex, requiring more sophisticated circuitry for

both modulation and demodulation. Furthermore, FM signals demand a wider bandwidth for transmission compared to AM, which can limit the

number of available channels in the radio spectrum.

High Audio Noise Immunity Complex Wider Bandwidth
Quality Resistant to electrical Implementation
Clearer sound due to Requires advanced Occupies more spectrum
interference.
noise immunity. circuitry. space.

FM's applications are prevalent in areas requiring high-quality transmission, such as FM radio broadcasting and various wireless communication

systems.
Applications of AM
Amplitude Modulation (AM) has found enduring applications across various sectors due to its inherent characteristics. Its ability to propagate over long distances, particularly at lower frequencies, makes

it a prime choice for AM radio broadcasting, reaching wide audiences with relatively simple receiver technology. This long-range capability is also crucial for aviation communication, enabling pilots to

communicate reliably with air traffic control over vast areas.

Beyond broadcasting and aviation, AM's simplicity makes it suitable for emergency broadcast systems, ensuring alerts can be widely disseminated even in adverse conditions where more complex

systems might fail. It also forms the backbone of many two-way radio systems, like walkie-talkies and CB radios, providing straightforward and cost-effective communication for short to medium

ranges.

Aviation Communication
AM Radio Broadcasting
Enables air-to-ground communication, providing reliable voice
Used for long-range communication, reaching wide audiences
transmission for pilots and air traffic controllers over extensive
due to its ability to propagate over vast distances and its relatively
areas, crucial for flight safety.
simple receiver technology.

Two-way Radios Emergency Broadcast Systems

Commonly employed in devices like walkie-talkies and CB Utilized for alert systems due to its simplicity and resilience,

radios, offering straightforward and cost-effective ensuring critical information can be disseminated effectively even

communication for personal and professional use over short to in challenging conditions.

medium ranges.
Applications of FM
Frequency Modulation (FM) is the cornerstone of many high-quality communication systems due to its superior noise immunity and ability to carry high-fidelity audio. Its most recognizable application is in FM radio broadcasting, delivering

crystal-clear sound that is largely unaffected by static and interference, making it the preferred choice for music and talk radio.

FM's robustness against noise makes it ideal for wireless communication systems such as cordless phones and wireless microphones, ensuring reliable and clear transmission in environments prone to interference. Its inherent noise immunity is

also a significant advantage in satellite communication, where signals travel long distances and are susceptible to various forms of distortion. Furthermore, FM is extensively used in telemetry for transmitting data from remote sensors, where data

integrity is paramount.

FM Radio Broadcasting
• Provides high-fidelity audio transmission.

• Less susceptible to static and interference.

• Preferred for music and high-quality speech.

Wireless Communication
• Used in cordless phones and wireless microphones.

• Ensures clear and reliable transmission in various environments.

Satellite Communication
• Valued for its noise immunity over long distances.

• Crucial for maintaining signal integrity in space communication.

Telemetry
• Transmits data from remote sensors.

• Ensures high data integrity and reliability for critical measurements.

Recent Advancements and Research
The landscape of AM and FM is continuously evolving with significant advancements driven by digital technology. Digital AM and FM leverage digital signal processing (DSP) to

achieve improved efficiency, enhanced audio quality, and greater spectral efficiency compared to traditional analog methods. This allows for clearer signals and better utilization of

the available frequency spectrum.

Software-Defined Radio (SDR) represents a revolutionary shift, offering unprecedented flexibility in modulation and demodulation. SDR systems can be reconfigured via software,

allowing them to adapt to different modulation schemes and communication standards without hardware changes. Research is also exploring hybrid AM/FM systems, aiming to

combine the benefits of both techniques, such as AM's long-range capabilities with FM's noise immunity. Furthermore, ongoing research focuses on advanced modulation

techniques to maximize bandwidth usage, a critical need in an increasingly crowded wireless environment.

Research in Spectral
Hybrid AM/FM Systems
Software-Defined Radio Efficiency
Research into combining advantages of Focuses on advanced modulation
Digital AM and FM
(SDR) both modulation techniques, leveraging techniques to maximize bandwidth usage,
Improved efficiency and quality using Provides flexible modulation and
the strengths of each for specific optimizing the utilization of limited radio
digital signal processing (DSP) for clearer demodulation, allowing systems to adapt
applications. spectrum.
signals and better spectral utilization. to various standards through software

updates.
Binary Phase Shift Keying
(BPSK)
BPSK is a straightforward digital modulation technique that represents binary data by

shifting the phase of a carrier signal. It uses two distinct phases, typically 0° and 180°,

to represent the two binary bits: 0 and 1. This means that for each symbol transmitted,

only one bit of information is conveyed, making it a relatively low data rate modulation

scheme but robust in noisy environments.

Two Phases One Bit per Symbol
Represents data using a 0° or Each symbol carries a single bit

180° phase shift. of information.

Lower Data Rate

Simpler but transmits less data per unit time.
BPSK Modulation Process
The BPSK modulation process involves transforming digital data into analog signals. A unipolar Non-Return-to-Zero (NRZ) signal typically

serves as the modulating input, where a positive voltage represents a binary '1' and a zero voltage represents a binary '0'. This modulating signal

then controls the phase of a continuous carrier wave. For a bit '0', the carrier phase remains at 0°, while for a bit '1', the phase shifts by 180° (π

radians). The resulting modulated signal s(t) is expressed as Acos(2πfct + θ(t)), where θ(t) is either 0 or π.

Phase Shift
Carrier Signal
θ(t) = 0 for bit 0, θ(t) = π for bit 1.
Modulating Signal
A sinusoidal wave: Acos(2πfct).
Unipolar NRZ signal (0 or 1).
BPSK Constellation Diagram
A constellation diagram visually represents the set of all possible symbols that a modulation scheme can transmit. For BPSK, the diagram consists

of only two points, positioned 180° apart on a circle centered at the origin. These points correspond to the two possible phase shifts (0° and 180°)

of the carrier signal. While its simplicity makes it robust against noise, the limited number of points signifies its low data rate capability, as each

point represents only one bit of information.

Two Points Robust Low Data Rate

Represents 0° and 180° phase shifts. Less susceptible to noise due to distinct Transmits only one bit per symbol.

phase separation.
Quadrature Phase Shift Keying
(QPSK)
QPSK is an advancement over BPSK, employing four distinct phase shifts to encode digital

data. Unlike BPSK's two phases, QPSK uses 0°, 90°, 180°, and 270° (or π/2, π, 3π/2 radians)

to represent combinations of two bits. This means that each symbol transmitted carries two

bits of information (dibit), effectively doubling the data rate compared to BPSK for the same

bandwidth. This makes QPSK a more spectrally efficient modulation technique, crucial for

Four Phases
applications requiring higher throughput.
Two Bits per Symbol
0°, 90°, 180°, 270°. Each symbol encodes a "dibit" (00, 01,

10, 11).

Higher Data Rate

Doubles throughput compared to BPSK.
QPSK Modulation Process
The QPSK modulation process leverages two orthogonal carrier signals — a cosine wave and a sine wave — to simultaneously transmit two

independent bit streams. The incoming data stream is first split into two parallel sequences: the in-phase (I) component and the quadrature (Q)

component. Each of these components then modulates its respective carrier. The modulated signals are then summed to produce the final QPSK signal.

The phase θ(t) of the resulting signal can be 0, π/2, π, or 3π/2, depending on the combination of the input dibits.

Split Data Stream

Into In-phase (I) and Quadrature (Q) components.

Modulate Carriers
I with cosine, Q with sine carrier signals.

Combine Signals
Sum modulated I and Q components.
QPSK Constellation Diagram
The constellation diagram for QPSK displays four points, each representing a unique combination of two bits (a dibit) and its corresponding phase shift. These

points are typically located on a circle at 0°, 90°, 180°, and 270°. This arrangement clearly illustrates how QPSK achieves its higher data rate: by encoding two

bits per symbol, it effectively doubles the spectral efficiency compared to BPSK. While it packs more information, the points are closer together, making it

slightly more susceptible to noise than BPSK, a trade-off for increased throughput.

Four Points Doubled Data Rate

Representing 00, 01, 10, 11 dibits. Achieved by encoding two bits per symbol.
1 2

Increased Spectral Efficiency

Phase Shifts
0°, 90°, 180°, 270°. 4 3 More data transmitted in the same bandwidth.
BPSK vs QPSK: Performance
Comparison
When comparing BPSK and QPSK, several performance aspects stand out. BPSK offers a

simpler implementation due to its fewer phase states, which also makes it more robust against

noise and channel impairments. However, this robustness comes at the cost of a lower data rate.

QPSK, on the other hand, provides a higher data rate and significantly better spectral efficiency

by transmitting two bits per symbol. Both modulation techniques are inherently susceptible to

noise and interference, as highlighted by Singal TL, requiring robust error correction mechanisms

in practical wireless systems.

Implementation Simpler More Complex

Data Rate Low High

Spectral Efficiency Lower Higher

Robustness to Noise Higher Lower

Real-World Applications
BPSK and QPSK are widely utilized across various wireless communication systems due to their distinct characteristics. BPSK, with its

robustness and simplicity, is often preferred for low data rate applications where reliability is paramount, such as in satellite communication

systems, as discussed by Blake. QPSK's ability to transmit data at a higher rate makes it ideal for more demanding applications. It is commonly

found in wireless LANs (Wi-Fi), modern mobile communication standards (3G, 4G, 5G), and digital TV broadcasting, as noted by Agarwal &

Zeng, where spectral efficiency is a key requirement for delivering rich multimedia content and high-speed internet access.

BPSK: Satellite QPSK: Wireless QPSK: Mobile QPSK: Digital TV

Communication LANs Communication
Enables faster data Efficiently broadcasts
Supports high-speed data
transfer for network high-quality video
Reliable for long-
for smartphones and
access. content.
distance, low-data links.
devices.
Introduction to Multiplexing
Efficiency Through Sharing Optimizing Resources

Multiplexing is a foundational concept in telecommunications, By efficiently combining diverse data streams, multiplexing

allowing multiple independent signals to be combined into a single significantly reduces the need for multiple physical channels, thereby

composite signal for transmission over a shared communication saving costs and improving network infrastructure. It’s an

channel. This process is vital for maximizing the utilization of indispensable technology for everything from telephone networks to

available bandwidth, transforming a single pathway into a conduit for high-speed internet, ensuring that communication resources are used

vast amounts of information. to their fullest potential.

Frequency-Division Multiplexing (FDM)
Bandwidth Segmentation Simultaneous Transmission Analog Applications
Beyond TV, FDM is historically
FDM works by dividing the total This technique ensures that signals
prevalent in analog radio broadcasting
available bandwidth of a travel in parallel, each within its
and early telephone systems, where it
communication channel into several dedicated frequency range. A classic
facilitated the concurrent transmission
non-overlapping frequency sub- application is analog television
of multiple voice calls over a single
channels. Each sub-channel is then broadcasting, where each channel
cable by assigning each call a unique
allocated to carry a distinct signal, occupies a specific 6 MHz bandwidth,
frequency band.
allowing multiple signals to be enabling viewers to tune into different

transmitted simultaneously without programs transmitted concurrently.

interference.
Time-Division Multiplexing (TDM)
Sequential Access
TDM enables multiple users to share a single channel by dividing the transmission time into discrete, short intervals or "time slots."

Each user is allocated a specific time slot during which they can transmit their data.

Time-Shared Channel
Although users share the same frequency band, they transmit at different moments, creating a seamless flow of data. This sequential

allocation ensures that signals from different sources don't interfere with each other, as they never occupy the channel simultaneously.

Telephone Networks
A prime example of TDM's application is in digital telephone networks, specifically T1 lines. These lines aggregate 24 individual

voice channels, each transmitting data at 64 kbps, into a single 1.544 Mbps stream, making efficient use of network infrastructure.
Code-Division Multiplexing (CDM)
Unique Code Assignment Signal Separation CDMA Cellular Systems
CDM stands out by allowing multiple At the receiver, the same unique code is CDM is most famously employed in

signals to occupy the same frequency used to despread and recover the desired Code-Division Multiple Access (CDMA)

band and time slot simultaneously. This signal, while signals encoded with cellular systems, such as IS-95. Its spread

is achieved by assigning each user a different codes appear as random noise spectrum technology provides enhanced

unique orthogonal code (spreading code). and are rejected. This inherent ability to security, increased capacity, and

The user's data is then modulated with separate signals, even when they overlap, improved voice quality, revolutionizing

this distinct code, effectively spreading makes CDM incredibly robust against mobile communication by allowing more

the signal across a wider spectrum. interference. users to share the same airwaves.
Wavelength-Division Multiplexing (WDM)
Light as a Carrier
WDM is an optical networking technology that significantly boosts the capacity of fiber optic cables. It functions similarly to FDM, but

instead of radio frequencies, it uses different wavelengths (colors) of light to carry distinct optical signals simultaneously over a single optical

fiber.

Spectral Diversity
Each unique wavelength acts as an independent channel, allowing vast amounts of data to be transmitted in parallel. This approach

dramatically increases the bandwidth available over existing fiber infrastructure, postponing the need for costly new cable deployments.

Fiber Optic Backbones

WDM, particularly Dense Wavelength-Division Multiplexing (DWDM), forms the backbone of modern high-speed internet and

telecommunication networks. It’s essential for long-haul and metropolitan area networks, facilitating the global exchange of digital

information at unprecedented speeds.

Introduction to Multiple Access Techniques

Multiple access techniques are foundational protocols that govern how multiple users can simultaneously share a

common communication medium or channel, ensuring orderly and efficient resource allocation. These techniques

are distinct from multiplexing, which focuses on combining signals, by addressing the challenge of managing

diverse user requests for channel access. They are indispensable for preventing interference and ensuring fair usage

in shared environments.

100% 99%
Channel Sharing Interference Management
Enables optimal utilization of limited radio frequency Minimizes signal collision and ensures clear

spectrum or cable bandwidth. communication for all active users.

50%
Resource Allocation
Dynamically assigns channel capacity to individual

users based on their needs.

Frequency-Division Multiple Access (FDMA)
Exclusive Channels
FDMA operates by dividing the total available frequency band into narrower, distinct channels,

1 each assigned exclusively to a single user for the duration of their transmission.

No Overlap
Once a channel is allocated, no other user can transmit on that frequency until it
2
becomes free, effectively eliminating interference between users sharing the same

medium.

Analog Systems
A prime example is the Advanced Mobile Phone System (AMPS), an
3
early analog cellular technology, where each phone call occupied a

unique frequency channel.

Time-Division Multiple Access (TDMA)
Shared Time Slots

1 TDMA allows multiple users to share a single frequency channel by dividing it into repeating time slots. Each user is assigned

specific, non-overlapping time slots for their transmissions.

Sequential Transmission
Users take turns transmitting in their allocated slots, ensuring that only one user occupies the channel
2
at any given moment. This prevents collisions and allows for efficient sharing of the medium.

GSM Technology
A widely recognized application of TDMA is in the Global System for

3 Mobile Communications (GSM) cellular networks. GSM systems typically

divide a single radio frequency channel into 8 time slots, allowing 8

different users to share the same frequency resource sequentially.

Frequency-Division
Multiplexing (FDM): A Core
Wireless Technology
Frequency-Division Multiplexing (FDM) is a foundational technique in wireless

communication systems and a critical multiplexing method in telecommunication. It

operates by dividing the total available bandwidth into non-overlapping frequency

bands, allowing multiple signals to be transmitted simultaneously over a single

medium. This concept is extensively covered in core textbooks by prominent authors

like Roy Blake and Jochen Schiller, highlighting its importance in understanding

wireless communication.

preencoded.png
Basic Principles of FDM

Bandwidth Division Independent Signals Simultaneous Guard Bands

Total bandwidth is segmented Each dedicated channel carries an Transmission Small frequency ranges inserted
Enables concurrent transmission
into separate, non-overlapping independent signal without between channels to prevent
of multiple signals over a single
channels. mutual interference. inter-channel interference.
physical medium.

The fundamental principle of FDM involves subdividing the entire communication bandwidth into multiple distinct frequency channels. Each of

these channels is then used to carry a separate signal independently. A crucial aspect of FDM is the inclusion of "guard bands," which are narrow

unused frequency ranges strategically placed between adjacent channels. These guard bands are essential for maintaining orthogonality and

preventing signals from one channel from bleeding into another, thereby minimizing inter-channel interference and ensuring signal integrity.
FDM System Architecture
Transmitter Components

• Modulators: Convert baseband signals to higher frequencies.

• Carrier Generators: Produce high-frequency carrier waves.
• Linear Mixer: Combines modulated signals for transmission.

Various modulation techniques such as Amplitude Modulation (AM),

Frequency Modulation (FM), and Single Sideband (SSB) are employed to

shift the baseband signals to their respective carrier frequencies.

Receiver Components

• Band-Pass Filters: Isolate specific frequency channels.

• Demodulators: Convert modulated signals back to baseband.

The modulated signals are then sent over the transmission medium, where the

receiver uses filters to select the desired channel and demodulators to recover

the original information.

Mathematical Foundation of FDM

Δf fg
Channel Bandwidth Guard Band Width
Determined by signal type and modulation scheme. Calculated to prevent adjacent channel interference.

SNR η
Signal-to-Noise Ratio Spectral Efficiency
Critical for signal quality and error rates. Maximizing data rate per unit of bandwidth.

The mathematical underpinning of FDM involves precise calculations to ensure optimal system performance. This includes determining the appropriate channel

bandwidth allocations, which are crucial for accommodating the signal while minimizing spectral waste. Guard band calculations are vital to effectively isolate

channels and prevent inter-channel interference. Furthermore, considerations of Signal-to-Noise Ratio (SNR) are paramount for maintaining signal integrity and

minimizing bit error rates. Finally, analyses of inter-modulation distortion and spectral efficiency are conducted to optimize overall system capacity and
Advantages and Limitations of FDM
No Time Synchronization Continuous Transmission
Unlike time-division methods, FDM does not require precise All signals can be transmitted continuously and

time synchronization between transmitters and receivers. simultaneously, which is beneficial for real-time applications.

Cross-talk & Distortion Bandwidth Wastage

Susceptible to cross-talk between adjacent channels and Guard bands, while necessary, lead to a certain degree of

intermodulation distortion due to non-linearities. bandwidth inefficiency.

FDM offers significant advantages, such as its simplicity in implementation and the absence of a need for strict time synchronization, allowing

continuous transmission of multiple signals. However, it also has limitations, including susceptibility to cross-talk between channels and

intermodulation distortion. The requirement for guard bands, while preventing interference, inevitably leads to some bandwidth wastage, which

can impact overall channel capacity. These factors must be carefully balanced in system design.
FDM Applications in Wireless Systems

FDM has been widely adopted across various wireless communication systems. Historically, it was the backbone of first-generation cellular telephony through Frequency-

Division Multiple Access (FDMA). It remains prevalent in traditional radio broadcasting for both AM and FM signals, enabling multiple stations to transmit simultaneously

without interference. FDM is also a core technology in cable television systems, allowing numerous channels to be delivered over a single cable, and it plays a role in satellite

communications for multiplexing different data streams. Even in modern broadband, like certain DSL implementations, FDM principles are utilized.
Orthogonal Frequency-Division Multiplexing (OFDM)

Conventional FDM OFDM Evolution FFT/IFFT Cyclic Prefix

Separate channels, guard bands, Overlapping, orthogonal Efficient implementation using Added to combat inter-symbol

lower spectral efficiency. subcarriers, increased spectral Fast Fourier Transform. interference from multipath.

efficiency.

Orthogonal Frequency-Division Multiplexing (OFDM) represents a significant evolution from conventional FDM. Its key innovation lies in using

overlapping yet orthogonal subcarriers, which dramatically increases spectral efficiency by eliminating the need for large guard bands between

subcarriers. OFDM systems are efficiently implemented using Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) algorithms. A

crucial feature is the cyclic prefix, which is added to each OFDM symbol to mitigate the effects of multipath propagation, providing robust resistance to

frequency-selective fading that plagues traditional FDM in wireless channels.

OFDM in Modern Wireless Standards
Wi-Fi Standards Cellular Systems Digital Broadcasting
• IEEE 802.11a/g/n/ac/ax • 4G LTE and 5G NR • DVB (Digital Video Broadcasting)
• Enables high-speed wireless local • Supports high data rates and low
• DAB (Digital Audio Broadcasting)
area networks. latency for mobile users.

• Enhances quality and spectral

efficiency in media delivery.

OFDM is a cornerstone technology in nearly all modern wireless communication standards. It is integral to Wi-Fi networks (IEEE

802.11a/g/n/ac/ax), providing the robust performance needed for high-throughput local area communication. In cellular systems, OFDM is

fundamental to both 4G LTE and the latest 5G New Radio (NR) standards, enabling faster data speeds and improved reliability. It also underpins

digital broadcasting technologies like DVB and DAB, delivering superior audio and video quality. Features like guard intervals and advanced

channel coding, including interleaving, further enhance its performance in these diverse applications.
Frequency-Division Multiple Access (FDMA)

Channel Assignment Mobile Communication

Each user is assigned a unique frequency band for Widely used in first-generation cellular systems.

communication.

Comparison Spectral Efficiency

Contrasted with TDMA (Time-Division Multiple Impacted by guard bands and channel allocation

Access) and CDMA. strategies.

Frequency-Division Multiple Access (FDMA) extends the principles of FDM to allow multiple users to share a common communication medium. In FDMA, each user is allocated

a distinct frequency band for the duration of their call or data session. Channel assignment strategies are crucial to efficiently manage the available spectrum among many users.

While effective, the spectral efficiency of FDMA can be limited by the necessity of guard bands. It is often compared with Time-Division Multiple Access (TDMA) and Code-

Division Multiple Access (CDMA) as foundational multiple access schemes, with hybrid approaches often employed in modern systems.
Future Trends and Conclusion

Challenges & Research

Cognitive Radio
Addressing issues like interference
Advanced Techniques
Dynamic spectrum access and
OFDMA Dominance management, spectrum scarcity, and
Exploration of Filter Bank
utilization through intelligent radio
OFDMA continues to be a energy efficiency.
Multicarrier (FBMC) and Non-
technologies.
cornerstone for next-generation
Orthogonal Multiple Access
wireless systems, providing
(NOMA).
enhanced flexibility.
The evolution of FDM continues with advanced techniques like Orthogonal Frequency-Division Multiple Access (OFDMA), which is pivotal in next-generation

wireless systems. Research is actively exploring innovations such as Filter Bank Multicarrier (FBMC) and Non-Orthogonal Multiple Access (NOMA) to further

improve spectral efficiency and capacity. Cognitive radio applications are also gaining traction, enabling intelligent and dynamic use of the frequency spectrum.

The challenges for the future include managing increasing interference, overcoming spectrum scarcity, and enhancing energy efficiency, ensuring that the

fundamental principles of FDM continue to underpin robust and efficient wireless communication.
Time-Division Multiplexing
(TDM)

preencoded.png
What is Time-Division Multiplexing?
Definition How it Works

Time-Division Multiplexing (TDM) is a technique that enables TDM operates by allocating specific, distinct time slots to each

the transmission of multiple data streams or signals over a single individual signal. These signals are then interleaved sequentially

shared communication channel or physical medium. in time, sharing the same transmission medium. This method

ensures efficient utilization of the available bandwidth by

preventing simultaneous transmissions.

TDM in Wireless Communication
GSM (2G) Integration User Allocation
Time-Division Multiple Access In TDMA, each individual user is

(TDMA), a direct application of assigned a unique and dedicated

TDM, was a cornerstone time slot within the

technology in second-generation communication frame. This

(2G) cellular networks, most structured allocation ensures that

notably in Global System for multiple users can share the same

Mobile Communications (GSM). frequency channel without

Collision Prevention
interference.
The precise time slot assignment is crucial for preventing data collisions and

minimizing electromagnetic interference between different users. This systematic

approach contributes to the overall stability and reliability of the wireless

network.
TDMA Frame Structure (GSM Example)
Frame Division User Assignment
In the GSM standard, each TDMA frame is precisely divided A single user is assigned exclusively to one of these 8 time

into 8 distinct time slots. This segmentation is fundamental to slots within a given frame. This ensures an organized and

how multiple users can share the same radio frequency non-overlapping transmission for each active communication

channel. link.

Frame Duration Data Burst

The total duration of a complete GSM TDMA frame is a Each individual time slot within the frame carries a "burst" of

fixed 4.615 milliseconds. This standardized timing is critical data. This burst contains the modulated information that the

for network synchronization and efficient data flow. user is transmitting or receiving during their allocated time.
Types of TDM
Synchronous TDM
In Synchronous TDM, time slots are rigidly assigned to each channel, irrespective of whether that channel currently has data to transmit. This fixed allocation simplifies

control but can lead to unused capacity.

Asynchronous TDM (Statistical TDM)

Asynchronous TDM, often called Statistical TDM, dynamically allocates time slots only when a channel has data ready for transmission. This demand-driven approach

significantly enhances bandwidth utilization.

Efficiency for Variable Data Rates

Statistical TDM proves particularly more efficient in scenarios where data rates are highly variable or bursty. By avoiding fixed assignments for idle channels, it maximizes

throughput and minimizes wasted bandwidth.

Synchronous TDM
Simplicity vs. Efficiency
Fixed Time Slots
While its implementation is straightforward
Synchronous TDM assigns predetermined,
due to predictable timing, this approach can
fixed time slots to each communication 1 2
be inefficient as bandwidth is wasted when
channel, regardless of whether that channel is
assigned channels are idle.
actively transmitting data.
Example Allocation Legacy System Use
Consider a system with 4 channels; in Synchronous TDM was widely employed in

Synchronous TDM, each channel would 4 3 older telecommunication systems, such as the

consistently receive 25% of the total T1 lines used in North America, for their

available transmission time, even if some robust and reliable fixed-rate data

channels have no data. transmission.

Asynchronous (Statistical) TDM
Dynamic Allocation
Unlike synchronous TDM, time slots in statistical TDM are allocated dynamically, exclusively

when a particular channel requires transmission capacity, responding to actual demand.

Bandwidth Optimization
This demand-driven allocation significantly enhances overall bandwidth utilization, as idle

channels do not consume precious time slots, leading to more efficient network performance.

Addressing Requirement
To facilitate dynamic allocation and ensure data reaches the correct destination, each

transmitted time slot must carry explicit addressing information, increasing frame overhead

slightly.

Modern System Adoption

Asynchronous TDM principles are extensively applied in contemporary communication

systems, particularly where data traffic is bursty or highly variable, such as in internet packet

networks.
Advantages of TDM
Simple Implementation
TDM systems, particularly synchronous ones, are relatively straightforward to

design and implement due to their clear time slot assignments.

Efficient Bandwidth Use

Especially with statistical TDM, bandwidth is used efficiently by allocating slots

only when data is present, maximizing throughput over the shared medium.

Digital Signal Suitability

TDM is inherently well-suited for transmitting digital signals, making it a natural

fit for modern digital communication networks.

Secure Communication
By utilizing encryption within each dedicated time slot, TDM can facilitate

secure communication, protecting data integrity and confidentiality.

Disadvantages of TDM
Synchronization Criticality
Accurate timing is paramount in TDM; any clock drift between transmitter and receiver can lead to misalignment and data loss.
1

Overhead Challenges

2 Both time slot allocation and the inclusion of addressing information (in asynchronous TDM) introduce overhead,

reducing the effective data rate.

Clock Drift & Jitter Susceptibility

3 TDM systems are vulnerable to clock drift, where timing sources diverge, and jitter, which is the

deviation of the actual timing from ideal periodic timing, both impacting data integrity.

Asynchronous Complexity

4 While more efficient, asynchronous TDM introduces increased system complexity due to

the need for dynamic slot management and addressing mechanisms.

Conclusion
Fundamental Multiplexing
Time-Division Multiplexing stands as a core technique in communication systems,

essential for managing multiple data streams over shared channels.

Crucial in Wireless
Its application, particularly as TDMA in GSM, highlights its critical role in the

evolution and functionality of wireless communication networks.

Essential for CSE Students
A deep understanding of TDM principles is fundamental for Computer Science

and Engineering students, providing a foundation for advanced networking

concepts.
Further Study Areas
We encourage you to delve deeper into specific TDM variations like TDMA

and Statistical TDM, and explore their modern applications in evolving

communication technologies.
What is CDMA?
CDMA is a form of spread-spectrum multiple access where multiple users can share the same frequency band simultaneously without interfering

with each other. Unlike Frequency-Division Multiple Access (FDMA) which assigns different frequencies, or Time-Division Multiple Access

(TDMA) which assigns different time slots, CDMA assigns a unique code to each user. This code is used to spread the user's signal across the

entire available bandwidth. Common codes include Walsh codes, Gold codes, and Kasami codes. The receiver then uses the same unique code to

despread and recover the desired signal, while other signals appear as low-level noise due to their different codes.

CDMA FDMA TDMA

Users share the same frequency and time, Users are assigned distinct frequency bands Users share the same frequency but transmit

differentiated by unique codes. for communication. in different time slots.

Spreading Codes
Spreading codes are at the heart of CDMA technology, enabling multiple users to coexist in the same bandwidth. These codes possess critical

properties such as orthogonality, meaning that the cross-correlation between different codes is nearly zero. This orthogonality allows the receiver

to isolate a specific user's signal while rejecting others. Autocorrelation, on the other hand, ensures that a code correlates strongly only with itself,

facilitating synchronization.
Walsh Codes Gold Codes Kasami Sequences
Ideal for synchronous CDMA systems Pseudo-random sequences with good Known for their excellent correlation

due to their perfect orthogonality. They cross-correlation properties, suitable for properties and large family size, offering

are commonly used in the forward link asynchronous systems like the reverse more unique codes for a given length.

(base station to mobile). link (mobile to base station).

CDMA Encoding and Decoding
The elegance of CDMA lies in its encoding and decoding processes. At the transmitter, each user's data bit is multiplied by their unique spreading

code, expanding the signal's bandwidth significantly – a process known as spreading. This results in a wideband, low-power signal that is then

transmitted.
At the receiver, the magic of despreading occurs. The received signal, which is a composite of all active users' spread signals plus noise, is

correlated with the receiver's specific spreading code. Due to the orthogonality of the codes, only the desired signal is despread back to its original

narrowband form, while other users' signals are effectively averaged out and filtered as noise.

Encoding (Transmitter)
Data bit x Spreading Code = Transmitted Signal

Decoding (Receiver)
Received Signal x User's Code = Original Data
Near-Far Problem
One significant challenge in CDMA systems is the "near-far problem." This occurs when a mobile user close to the base station transmits with

higher power than a distant user, causing the close user's signal to overpower and drown out the weaker signal from the far user. This can severely

degrade the performance of the entire system.

To combat this, precise power control is crucial. Power control techniques ensure that all signals arrive at the base station with approximately the

same power level, maintaining signal integrity and system capacity.

Open-Loop Power Control 1

Mobile adjusts its transmit power based on the received

signal strength from the base station. Simpler but less

2 Closed-Loop Power Control
accurate.
Base station measures received signal power and sends

commands to the mobile to increase or decrease its power.

More accurate and dynamic.

Advantages of CDMA
CDMA offers compelling advantages that have cemented its role in wireless communication. One of its primary benefits is increased capacity,

often significantly higher than FDMA and TDMA systems due to its spread spectrum nature and the ability to reuse frequencies across all cells.

This allows more users to share the same spectrum efficiently.

Furthermore, the inherent coding mechanism provides enhanced security, as it's difficult for unauthorized users to despread a signal without the

correct code. CDMA also excels in soft handover, allowing a mobile device to communicate with multiple base stations simultaneously, ensuring

seamless transitions and fewer dropped calls. Its resistance to multipath fading, where signals arrive at the receiver via multiple paths, further

enhances reliability.
Increased Capacity Enhanced Security
More users per bandwidth unit. Difficult to intercept or jam.

Soft Handover Multipath Resistance

Seamless transitions between cells. Robust against signal echoes.
Disadvantages of CDMA
Despite its numerous advantages, CDMA is not without its drawbacks. The primary challenge lies in its complexity of implementation. The intricate

design required for precise synchronization and power control demands sophisticated hardware and software, making system setup and maintenance more

demanding.
Maintaining perfect orthogonality between codes in real-world environments is also a significant hurdle. Imperfect orthogonality leads to inter-user

interference, which can reduce system capacity and call quality. Furthermore, the stringent requirement for precise power control, particularly the near-far

problem, means that even minor deviations can severely impact overall system performance. Code management, especially in large networks with many

users, presents a substantial logistical challenge.

Complexity
1 High implementation and maintenance costs.

Power Control
2 Requires extremely precise power management.

Code Management
3 Challenges in allocating and managing unique codes.
CDMA Standards
CDMA has evolved through various standards, each building upon its predecessor to offer improved performance and capabilities. IS-95, introduced by

Qualcomm, was the first commercial CDMA-based cellular standard, often referred to as "2G CDMA." It offered data rates of up to 14.4 kbps for voice

and basic data services.

An evolution of IS-95, cdma2000 brought higher data rates and more advanced features, marking the transition to 3G. It supported data rates up to 3.1

Mbps in its 1xEV-DO revision, providing mobile internet access. W-CDMA, or Wideband CDMA, became the foundation for 3G UMTS (Universal

Mobile Telecommunications System) networks, offering even higher data rates up to 42 Mbps (HSPA+), and was widely adopted globally, showcasing

CDMA's versatility in mobile communications.

IS-95 2G 14.4 kbps First commercial CDMA system

cdma2000 3G 3.1 Mbps Evolution of IS-95 with higher

speeds

W-CDMA 3G 42 Mbps Used in UMTS networks

globally
Applications of CDMA
CDMA's robust features have led to its widespread adoption across various wireless communication systems. It formed the backbone

of many 2G and 3G cellular networks, providing reliable voice and data services to millions of users. Its ability to support multiple

users simultaneously and its resistance to interference made it ideal for mobile communication.

Beyond traditional cellular, CDMA is integral to the Global Positioning System (GPS). Each GPS satellite transmits unique CDMA

signals, allowing receivers on Earth to determine their precise location by measuring the time difference of arrival of these signals. It

is also used in satellite communication due to its ability to handle multiple signals over long distances and its inherent security.

Emerging applications in IoT and military communication systems further highlight its versatility.

Cellular GPS Satellite IoT &

Systems Enables precise Comm. Military
Emerging uses in
Foundation for 2G and Robust for long-
location tracking. various secure
3G mobile networks. distance
applications.
transmissions.
What is Spread Spectrum?

Definition Textbook Insight Key Concept

Spread spectrum involves intentionally According to Roy Blake in "Wireless The primary advantage of spread spectrum

distributing signal energy over a much wider Communication Technology," spread is its enhanced resistance to interference and

bandwidth than the minimum required to spectrum refers to "Modulation techniques jamming. By spreading the signal, it

transmit the information. This intentional that distribute the signal over a bandwidth becomes less susceptible to concentrated

spreading makes the signal more resilient to much wider than the minimum required to noise, allowing for more robust

various forms of interference. transmit the information." This highlights communication even in challenging

the core principle of bandwidth expansion. environments.

Types of Spread Spectrum
Direct Sequence Spread Frequency Hopping Spread Other Techniques
Spectrum (DSSS) Spectrum (FHSS) Beyond DSSS and FHSS, other
DSSS spreads the signal by directly FHSS rapidly switches the carrier
spread spectrum methods exist,
multiplying it with a pseudo-random frequency of the signal according to a
including Time Hopping (TH), which
noise (PN) code, as noted by Dharma pseudo-random sequence, a concept
varies transmission times, and Chirp
Prakash Agarwal and Qing-An Zeng detailed by Jochen Schiller in "Mobile
Modulation, which sweeps
in "Introduction to Wireless and Communications." This hopping
frequencies. These offer alternative
Mobile Systems." This code's high pattern makes it difficult to jam or
ways to achieve spread spectrum
chip rate leads to bandwidth intercept.
benefits.
expansion.
Frequency Hopping Spread Spectrum (FHSS)
1 2 3 4

Process Hop Rate Example Dwell Time

FHSS operates by rapidly The rate at which the carrier Bluetooth technology is a "Dwell time" refers to the

switching the carrier frequency changes can be prime example of FHSS, duration a signal remains on

frequency of the signal either "slow" or "fast" utilizing 79 channels with 1 a specific frequency before

across a wide range of relative to the symbol rate. MHz spacing within the 2.4 hopping to the next one in

available frequencies. This Slow hopping means GHz industrial, scientific, the pseudo-random

hopping pattern is multiple symbols are sent on and medical (ISM) band. sequence. This time is a

determined by a pseudo- a single frequency, while This is highlighted by G.I. critical parameter in FHSS

random noise (PN) fast hopping means the Papadimitriou and his system design.

sequence. frequency changes multiple colleagues in "Wireless

times per symbol. Networks."

Direct Sequence Spread Spectrum (DSSS)
Process
In DSSS, each bit of the original data is replaced by a much longer sequence of bits, known as "chips." This chip sequence is what spreads

the signal across the wider bandwidth.

Spreading Code
The crucial element is the pseudo-random noise (PN) sequence, which has a significantly higher chip rate than the data bit rate. As Singal

TL points out in "Wireless Communication," this PN sequence dictates the spreading.

Example
Consider a 1 Mbps data rate. If we use an 11-chip PN sequence, the signal is spread to an effective rate of 11 Mbps. This illustrates the

significant bandwidth expansion achieved.

Despreading
At the receiver, the incoming spread signal is correlated with an identical PN sequence. This process effectively "despreads" the signal,

recovering the original data while pushing narrowband interference into the background.
DSSS vs. FHSS: Key Differences
Bandwidth Usage Utilizes a constant, wide Hops across multiple

bandwidth for narrow frequency bands.

transmission.
Implementation Generally more complex Simpler in some aspects,

Complexity due to precise chip-level particularly in frequency

synchronization synthesis, but requires

requirements. robust frequency hopping

mechanisms.
Jamming Resistance Spreads the jamming Avoids jammed

signal power over its frequencies by rapidly

entire wide bandwidth, hopping to a new, clear

reducing its impact. frequency, making

sustained jamming

difficult.
Advantages of Spread Spectrum
Interference Resistance Security
Spread spectrum significantly reduces the impact of narrowband The signal is inherently difficult to detect and intercept without

interference. By spreading the signal, the interference power is knowledge of the unique spreading code. This provides a layer of

diluted, making the original signal easier to recover. privacy and security against unauthorized access.

Multipath Mitigation Code Division Multiple Access (CDMA)

As noted by Roy Blake, the wide bandwidth of spread spectrum Spread spectrum enables multiple users to share the same

allows receivers to effectively resolve multipath components. This bandwidth simultaneously. Each user is assigned a different

reduces fading and improves signal quality in challenging spreading code, allowing their signals to be distinguished at the

environments. receiver without interference.

Disadvantages of Spread Spectrum

Increased Bandwidth
Spread spectrum inherently requires a significantly wider bandwidth for transmission compared to

traditional narrowband modulation techniques. This can be a limiting factor in spectrum-constrained

environments.

Complexity
The design of both the transmitter and receiver becomes more complex. Additional circuitry

is needed for spreading and despreading operations, which increases manufacturing costs

and power consumption.

Synchronization
As highlighted by Dharma Prakash Agarwal and Qing-An Zeng, precise

synchronization of the spreading code at the receiver is critical. Any

misalignment can severely degrade performance or prevent signal recovery

entirely.
THANK YOU