Communication Systems Summary

Atmosphere, Radio Wave Propagation, Modulation, and Optical Communication

Earth's Atmosphere
Wave Propagation
Modulation
Communication Channels
Optical Communication

1. Earth's Atmosphere

The earth's atmosphere mainly consists of nitrogen (78%) and oxygen (21%) along with argon, CO₂, water vapor, and other trace elements.

1.1 Atmospheric Layers

Troposphere

Height: 0-12 km from Earth's surface

Temperature: 298K to 220K

Characteristics: All climatic changes occur here

Stratosphere

Height: 12-50 km

Temperature: 220K to 280K (increases with height)

Mesosphere

Height: 50-80 km

Temperature: 280K to 180K (decreases with height)

Ionosphere

Height: 80-400 km

Temperature: 180K to 700K (increases with height)

Importance: Reflects radio waves for long-distance communication

1.2 Greenhouse Effect

Key Points:

  • Atmosphere is transparent to visible radiation but blocks most infrared radiation
  • Earth emits infrared radiation that gets trapped by the atmosphere
  • Low-lying clouds reflect back infrared radiation
  • This phenomenon keeps Earth warm at night

2. Radio Wave Propagation

2.1 Low Frequency Waves (AM Band)

Radio waves with frequencies less than 30 MHz (wavelengths more than 10 meters) form the AM band.

Propagation Characteristics:

  • Lower atmosphere is transparent to these waves
  • Ionosphere reflects them back to Earth
  • Two propagation paths: ground wave and sky wave
  • Frequencies around 1500 kHz are transmitted primarily through ground waves
  • Higher frequencies are mainly transmitted through sky waves

2.2 High Frequency Waves (TV Transmission)

Characteristics:

  • Above approximately 40 MHz, ionosphere does not reflect waves back to Earth
  • TV signals (100-200 MHz) cannot use sky wave propagation
  • Only direct reception through ground waves is possible
  • Requires very tall antennas for larger coverage

Transmitting Antenna Height

\[ h = \frac{d^2}{2R_e} \]

Where:
h = height of transmitting antenna
d = radius of area to be covered
Rₑ = radius of Earth

3. Types of Modulation

Modulation is the process of superimposing information signals on a high-frequency carrier wave for long-distance transmission.

3.1 Continuous Wave Modulation

Amplitude Modulation (AM)

Amplitude of carrier wave is changed in accordance with the intensity of the modulating signal.

Modulation factor (m) = Amplitude change of carrier wave / Amplitude of normal carrier wave
Frequency Modulation (FM)

Frequency of carrier wave is changed in accordance with the instantaneous value of the modulating signal.

\[ m_f = \frac{\text{maximum frequency deviation}}{\text{modulating frequency}} \]
Phase Modulation (PM)

Phase angle of the carrier signal varies in accordance with the modulating voltage.

3.2 Pulse Modulation

Type Description Application
Pulse Amplitude Modulation (PAM) Amplitude of pulses varies with modulating signal Analog systems
Pulse Width Modulation (PWM) Width/duration of pulses varies with modulating signal Analog systems
Pulse Position Modulation (PPM) Position of pulses varies with modulating signal Analog systems
Pulse Code Modulation (PCM) Digital representation of analog signals Digital communication (preferred)

4. Communication Channels

4.1 Types of Communication

Main Categories:

  • Line Communication: Through physical connections (wires, cables)
  • Space Communication: Through wireless transmission
  • Satellite Communication: Recent addition to space communication

4.2 Line Communication Channels

Type Description Applications
Two-wire Transmission Lines Parallel wire lines used where balanced properties are required Traditional telephone lines
Twisted Pair Wires Twisting helps minimize electrical interference Limited distance transmission
Coaxial Cables Used when unbalanced properties are needed Microwaves, UHF waves (up to 1 GHz)

Note:

Systems of conductors radiate RF energy if conductor separation approaches half the operating wavelength. This occurs more in parallel wires than in coaxial lines.

5. Optical Communication

5.1 Advantages of Optical Communication

Key Benefits:

  • Wide channel bandwidth: Uses frequencies around 10¹⁴ Hz compared to 10⁶-10⁸ Hz for radio
  • Large channel capacity: Can carry more information
  • Low transmission losses: Minimal signal degradation over distance
  • Signal security: Optical signals confined inside fibers, difficult to intercept
  • Immune to interference: Not affected by electromagnetic interference

5.2 Components of Optical Communication System

Component Function Examples
Optical Source and Modulator Generates and modulates light with information Lasers, LEDs
Optical Signal Detector Converts light back to electrical signal Semiconductor photodetectors
Optical Fiber Cable Transmits light signals over distance Glass/polymer fibers

5.3 Optical Fiber Structure

Core (n₁) → Cladding (n₂) → Protective Jacket

An optical fiber consists of a transparent core with refractive index n₁, surrounded by cladding with slightly lower refractive index n₂, enclosed in a protective jacket.

Principle: Total Internal Reflection

Light is transmitted through the fiber with minimal loss due to total internal reflection at the core-cladding interface.

Entrance Angle Formula

\[ \sin \theta_e = \frac{\sqrt{n_1^2 - n_2^2}}{n_0} \]

For air as external medium (n₀ = 1):

\[ \sin \theta_e = \sqrt{n_1^2 - n_2^2} \]

Where:
θₑ = entrance angle on the axis of the core
n₁ = refractive index of core
n₂ = refractive index of cladding
n₀ = refractive index of external medium