COMMUNICATION SYSTEM – COMPLETE NOTES

Communication Modes

Full Duplex Communication

In Full Duplex Mode, communication is bidirectional, meaning data transmission takes place in both directions simultaneously.This mode allows two devices to transmit and receive information at the same time. Full duplex communication is commonly used in:

• Telephone networks
• Internet communication systems
• Modern digital communication systems

Full duplex differs from Half Duplex Communication, where data can travel in both directions but not simultaneously.

Other Communication Modes

Unidirectional Communication

Communication takes place only in one direction.Example:

• Traditional radio broadcasting

Multidirectional Communication

This term is not generally used for duplex communication systems.

Tridirectional Communication

This is not a standard communication system term.

Modulation in Communication Systems

Role of Modulator

The primary role of a Modulator is to:

Superimpose the Message Signal onto the Carrier Wave

Modulation combines:

• Low-frequency message signal
• High-frequency carrier signal

The process modifies one characteristic of the carrier wave according to the message signal.

Purpose of Modulation

• Efficient long-distance transmission
• Multiplexing of signals
• Improved noise immunity
• Better antenna size compatibility

Amplitude Modulation (AM)

Modulation Index

The Modulation Index (m) determines the extent of variation of carrier amplitude by the message signal.

Conditions of Modulation Index

As the modulation index increases:

• Amplitude variation increases
• Peak-to-peak amplitude increases

Overmodulation

When:

m > 1

the AM signal becomes distorted because the envelope no longer accurately represents the message signal.

Double Sideband Suppressed Carrier (DSB-SC)

Definition

In DSB-SC, the carrier is suppressed and only the sidebands carry information.

Generation of DSB-SC

DSB-SC is generated using:

• Balanced Modulator
• Ring Modulator

These modulators multiply the message signal with the carrier signal while suppressing the carrier component.

Output of Balanced/Ring Modulator

The output contains:

• Upper Sideband (USB)
• Lower Sideband (LSB)

Carrier frequency is suppressed.

Additional Processing Requirement

Vestigial Sideband (VSB) Modulation

Definition

Vestigial Sideband Modulation is a form of amplitude modulation in which:

• One sideband is transmitted completely
• A small portion of the other sideband is also transmitted

VSB reduces bandwidth while preserving important signal information.

Application

VSB is widely used in:

Television Broadcasting

VSB Demodulation

Correct Demodulation Method

Product Modulator followed by Low-Pass Filter

Working Principle

Step 1: Product Modulation

The received VSB signal is multiplied with a locally generated carrier signal.This shifts the signal back to the baseband frequency range.

Step 2: Low-Pass Filtering

After multiplication:

• Desired baseband signal
• High-frequency components

are produced.The low-pass filter removes unwanted high-frequency components and recovers the original modulating signal.

Advantages of VSB

• Reduced bandwidth requirement
• Efficient signal recovery
• Suitable for television transmission

Synchronous Detection in DSB-SC

Principle

Synchronous detection uses a locally generated carrier at the receiver for demodulation.For accurate demodulation:

• Frequency must match
• Phase must match

with the transmitted carrier.

Phase Error in Synchronous Detection

Cause

A phase difference between:

• Transmitted carrier
• Local carrier

creates phase error.

Mathematical Expression

Transmitted Signal

s(t)=Acos⁡(ωct) 

Local Carrier

cos⁡(ωct+θ)

Demodulated Output

After multiplication and filtering: y(t)=0.5Am(t)cos⁡(θ)

Effects of Phase Error

Phase error causes:

• Reduction in output amplitude
• Phase distortion

The amplitude gets multiplied by:

cos(θ)

Thus output decreases when phase mismatch increases.

Frequency Modulation (FM)

Wide Band Frequency Modulation (WBFM)

Wideband FM is used in:

FM Broadcasting

In WBFM, the carrier frequency deviation is large to achieve high-quality audio transmission.

Maximum Frequency Deviation

75 kHz

This is the standard deviation used in FM broadcasting systems.

Audio Baseband Bandwidth

15 kHz

Carson’s Rule

The bandwidth of an FM signal is estimated using Carson’s Rule.

B=2(Δf+fm)

Where:

• B = Total Bandwidth
• Δf = Maximum Frequency Deviation
• fm = Maximum Modulating Frequency

FM Broadcast Bandwidth Calculation

Using:

• Δf = 75 kHz
• fm = 15 kHz

B=2(75 kHz+15 kHz)=180 kHz 

Thus FM broadcasting occupies approximately:

180 kHz bandwidth

FM Broadcast Frequency Band

FM broadcasting operates within:

88 MHz – 108 MHz

Each FM station is allocated:

200 kHz bandwidth

Advantages of FM over AM

Noise Immunity

FM provides better noise immunity because information is carried through frequency variation rather than amplitude variation.

Bandwidth

AM requires lower bandwidth compared to FM.

Power Efficiency

FM utilizes transmitted power more efficiently because all transmitted power carries information.In AM:

• Significant power is wasted in the carrier.

Demodulation

Definition

Demodulation is the process of recovering the original information signal from the modulated carrier wave.

Characteristics of Demodulation

Performed at Receiving End

Demodulation occurs after the signal reaches the receiver.

Recovery of Original Signal

It extracts the original low-frequency message signal from the carrier wave.

Digital Modulation

Definition

Digital modulation modifies a carrier signal to represent digital information.Carrier properties modified include:

• Amplitude
• Frequency
• Phase

Phase Shift Keying (PSK)

Definition

PSK is a digital modulation technique in which the phase of the carrier signal is varied according to digital data.

Types of PSK

Binary Phase Shift Keying (BPSK)

Uses two phase states:

• 0°
• 180°

to represent binary digits.

Quadrature Phase Shift Keying (QPSK)

Uses four phase states:

• 0°
• 90°
• 180°
• 270°

Each symbol represents two bits.

Higher-Order PSK

Examples:

• 8-PSK
• 16-PSK

These increase data transmission rate by transmitting more bits per symbol.

Advantages of PSK

• High spectral efficiency
• Better noise performance
• Suitable for modern communication systems

Applications of PSK

PSK is used in:

• Wi-Fi systems
• Cellular communication
• Satellite communication
• Optical fiber communication

Analog Modulation Techniques

Raised Cosine Filter

Definition

The Raised Cosine Filter is used in digital communication systems to minimize:

Inter Symbol Interference (ISI)

ISI occurs when adjacent pulses overlap and create errors during detection.

Working Principle

The raised cosine filter shapes the transmitted signal such that:

Zero ISI occurs at sampling instants

Roll-Off Factor (α)

The filter is characterized by the roll-off factor:

0 ≤ α ≤ 1

Conditions

Importance of Raised Cosine Filter

• Reduces ISI
• Improves symbol detection
• Maintains signal integrity
• Essential in digital communication systems

QUICK REVISION

Full Duplex

• Simultaneous bidirectional communication

DSB-SC

• Carrier suppressed
• Sidebands carry information
• Generated using balanced/ring modulator

VSB Demodulation

• Product modulator + low-pass filter

FM Broadcasting

• Maximum deviation = 75 kHz
• Frequency band = 88–108 MHz

Demodulation

• Performed at receiver
• Recovers original signal

PSK

• Digital modulation technique
• Information represented by phase changes

Raised Cosine Filter

• Minimizes intersymbol interference
• Provides zero ISI at sampling instants

Phase-Locked Loop (PLL) and FSK Demodulation

A Phase-Locked Loop (PLL) continuously tracks the frequency variations of an incoming signal. In FSK demodulation, whenever the input frequency changes, the PLL automatically adjusts the frequency of its Voltage-Controlled Oscillator (VCO) to lock onto the incoming signal.

Important Points

• PLL tracks changes in the frequency of the modulated signal.
• VCO frequency continuously adjusts to match the input frequency.
• PLL is widely used for FSK demodulation.
• The demodulated output is obtained from the PLL control voltage.

PLL-Based FM Demodulator

In a PLL FM demodulator, the Phase Detector compares:

• Incoming FM signal
• VCO output signal

The phase detector produces an output voltage proportional to the phase difference between these signals.

Working

1. FM signal is applied to the phase detector.
2. Phase detector compares it with VCO output.
3. Output voltage varies according to phase difference.
4. Frequency variations of FM appear as phase variations.
5. After low-pass filtering, the original message signal is recovered.

Important Point

The phase detector primarily detects phase difference, not amplitude.

Phase Modulation (PM)

Observation

The frequency deviation in PM is proportional to:

• Modulating signal amplitude
• Modulating signal frequency

Frequency Modulation (FM)

In Frequency Modulation, the carrier frequency varies according to the message signal.

Applications

• FM radio broadcasting
• Two-way radio communication
• Television audio transmission
• Data communication systems

Major Advantage

FM provides better noise immunity compared to amplitude modulation.

Carson's Rule

Carson's Rule estimates the bandwidth required for an FM signal.[

BT=2(Δf+fm)

Where:

• Δf = Peak frequency deviation
• fm = Maximum modulating frequency

Example

Given:

• Δf = 5 kHz
• fm = 3 kHz

BT=2(5+3)=16kHz

Result

Required FM bandwidth = 16 kHz.

Narrowband FM (NBFM)

Characteristics

• Small frequency deviation (typically ±5 kHz or less)
• Small bandwidth
• Suitable for voice communication

Applications

• Police communication
• Ambulance communication
• Taxi communication
• Aviation VHF radios
• Marine communication systems
• Amateur radio communication

Frequency Shift Keying (FSK)

FSK is a digital modulation technique in which the carrier frequency changes according to the binary data.

Operation

• Binary 1 → Higher frequency (Mark Frequency)
• Binary 0 → Lower frequency (Space Frequency)

Features

• Better noise immunity than AM.
• Simple implementation.
• Reliable data transmission.

Differential Phase Shift Keying (DPSK)

DPSK is a variation of PSK in which phase changes are measured relative to the previous symbol instead of a fixed reference.

Advantage

• Eliminates phase ambiguity problems.
• Does not require an absolute phase reference.

Amplitude Modulation Variants

Double Sideband Suppressed Carrier (DSB-SC)

Features

• Carrier is suppressed.
• Both sidebands are transmitted.
• Generated using a balanced modulator.

Output Contains

• Upper Sideband (USB)
• Lower Sideband (LSB)
• Modulation frequency components

Advantage

• More power efficient than conventional AM.

Single Sideband (SSB)

Features

• Only one sideband is transmitted.
• Carrier is suppressed.

Advantages

• Maximum bandwidth efficiency.
• Maximum power efficiency.

Applications

• HF communication
• Military communication
• Long-distance radio communication

Vestigial Sideband (VSB)

Features

• One sideband transmitted completely.
• Other sideband transmitted partially.

Advantages

• Reduced bandwidth.
• Simple envelope detection.

Application

• Television video transmission.

Pulse Modulation Techniques

Pulse Time Modulation (PTM) involves varying time-related pulse parameters according to the modulating signal.

Pulse Width Modulation (PWM)

Characteristics

• Pulse width varies.
• Amplitude remains constant.

Also Known As

• Pulse Duration Modulation (PDM)
• Pulse Length Modulation (PLM)

Applications

• Motor speed control
• LED dimming
• DC-DC converters

Pulse Position Modulation (PPM)

Characteristics

• Information is carried by pulse position.
• Pulse amplitude remains constant.
• Pulse width remains constant.

Advantages

• Good noise immunity.
• Less affected by amplitude variations.

Applications

• Optical communication systems
• Telemetry systems
• Fiber-optic communication

PPM Generation

1. Message signal and sawtooth signal are compared.
2. PAM is generated.
3. Monostable multivibrator converts PAM into PPM.
4. Pulse position varies according to message amplitude.

Eye Diagram

An Eye Diagram is a graphical tool used to evaluate digital communication system performance.

Applications

• Investigation of Inter-Symbol Interference (ISI).
• Determination of optimum sampling instant.
• Evaluation of signal quality.

Important Features

Eye Opening Width

Represents the time interval where the signal can be sampled without ISI.

Wide Eye Opening

• Less distortion.
• Lower error probability.

Narrow Eye Opening

• Higher ISI.
• Greater probability of errors.

Observation Equipment

Eye diagrams are typically observed using a Digital Storage Oscilloscope (DSO).

Guided Transmission Media

Guided media use a physical path for signal transmission.

Optical Fibre

Construction

• Core
• Cladding

Working

Light travels through the core and is continuously reflected by the cladding.

Advantages

• High bandwidth
• Low attenuation
• High efficiency
• Immunity to electromagnetic interference

Non-Guided Media

• Radio waves
• Satellite communication
• Infrared transmission

Superheterodyne Receiver

A superheterodyne receiver converts the incoming RF signal into a fixed Intermediate Frequency (IF).

Intermediate Frequency

Image Frequency

Image frequency is an unwanted frequency that also produces the same intermediate frequency and causes interference.

Formula

Phase Shift and Time Delay

For periodic signals:

360° Phase Shift

A phase shift of 360° corresponds to one complete cycle.

Relation

[

\Delta t = T

]Where:

• Δt = Time delay
• T = Signal period

Conclusion

A 360° phase shift produces a time delay equal to one period.

Noise Figure (NF)

Noise Figure measures degradation in the Signal-to-Noise Ratio (SNR) caused by system components.

Definition

Channel Capacity

Nyquist Formula

For a noiseless channel:

Time Division Multiplexing (TDM)

TDM combines multiple data streams into a single transmission channel by allocating different time slots.

Transmission Rate