Communication Systems — Modulation & Demodulation — Basic Electronics & Communication Engineering Notes

Last reviewed 16 Sept 2026 · 8 min read

Elements of a communication system

Information source → transmitter (modulator, amplifier) → channel (wire, cable, optical fibre, free space) → receiver (amplifier, demodulator) → destination. Noise enters mainly in the channel and receiver.

Baseband signal — original message (e.g. voice 300–3400 Hz, audio up to about 20 kHz, video several MHz).
Bandwidth — range of frequencies occupied by a signal or passed by a channel.
Analog and digital communication.
Transmission modes: simplex (one way — broadcasting), half-duplex (both ways, one at a time — walkie-talkie), full-duplex (simultaneous — telephone).

Noise

Type	Description
External	Atmospheric (lightning), extraterrestrial (solar, cosmic), man-made (motors, ignition)
Internal	Thermal (Johnson) noise from random electron motion in resistors; shot noise in active devices; flicker noise

FormulaNoise relations

Thermal noise power: $P_{n} = k T B$ ( $k = 1.38 \times 1 0^{- 23}$ J/K, T in K, B in Hz) Noise voltage: $V_{n} = 4 k T B R$ Signal-to-noise ratio: $S N R = \frac{P _{s}}{P _{n}}$ ; in dB: $10 lo g_{10} (P_{s} / P_{n})$ Noise figure: $F = \frac{S N R _{in}}{S N R _{o u t}}$ (≥ 1; NF in dB)

Need for modulation

Modulation — varying a property (amplitude, frequency or phase) of a high-frequency carrier in accordance with the message signal.

Practical antenna size — antenna length should be comparable to wavelength (commonly λ/4); low audio frequencies would need impractically long antennas.
Multiplexing — many signals share a channel using different carrier frequencies.
Reduced noise and interference (especially FM and digital).
Efficient radiation and longer range.
Narrowbanding — makes relative bandwidth small, simplifying circuits.

Amplitude modulation (AM)

Carrier amplitude varies with the message: $s (t) = A_{c} [1 + m cos ω_{m} t] cos ω_{c} t$ .

FormulaAM relations

Modulation index $m = \frac{A _{m}}{A _{c}} = \frac{V _{ma x} - V _{min}}{V _{ma x} + V _{min}}$ (0 ≤ m ≤ 1; m > 1 → over-modulation, distortion)

Spectrum: carrier $f_{c}$ and two sidebands $f_{c} \pm f_{m}$ Bandwidth $= 2 f_{m}$

Total power: $P_{t} = P_{c} (1 + \frac{m ^{2}}{2})$ ; total sideband power $= P_{c} \frac{m ^{2}}{2}$ Transmission efficiency $η = \frac{m ^{2}}{2 + m ^{2}}$ — maximum 33.3% at m = 1

Current relation: $I_{t} = I_{c} 1 + m^{2} /2$

Most transmitted power is in the carrier, which carries no information → power inefficient.
DSB-SC (double sideband suppressed carrier) — saves carrier power; bandwidth $2 f_{m}$ .
SSB (single sideband) — only one sideband: bandwidth $f_{m}$ and saves power; more complex receivers.
VSB (vestigial sideband) — used for analog TV video.

Uses of AM: medium-wave and short-wave broadcasting, aircraft communication.

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Elements of a communication system

Noise

Need for modulation

Amplitude modulation (AM)

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