Communication Systems Fundamentals and Modulation Techniques

Posted on Feb 5, 2026 in Electronics

1. Communication System Definition and Block Diagram

A communication system is a setup used to transmit information from a source to a destination. The general block diagram includes the following functional elements:

Information Source: Produces the message to be transmitted (e.g., voice, text, video).
Input Transducer: Converts the message (if non-electrical) into a time-varying electrical signal.
Transmitter: Processes the electrical signal for transmission. Its main function is modulation, where the message signal is superimposed on a high-frequency carrier signal.
Channel: The medium through which the message travels from the transmitter to the receiver (e.g., wire, optical fiber, or free space).
Receiver: The main function is to reproduce the message signal in electrical form from the distorted received signal through a process called demodulation (the reverse of modulation).
Output Transducer: Converts the electrical message signal back into its original physical form (e.g., sound).

2. Communication Channel Types

A communication channel is the medium through which the message travels from the transmitter to the receiver. Channels are classified into two types based on the nature of the medium:

A. Guided Medium (Wired)

Signals are confined within a physical path (wire or cable).

Twisted Pair Cable: Two insulated copper conductors twisted together to decrease interference. Used in local telephone and short-distance data communication.
Coaxial Cable: Consists of a central copper wire surrounded by insulation and a copper mesh shield. It offers a higher frequency range and better noise immunity than twisted pairs. Used in Cable TV (CATV) and LANs.
Fiber Optic Cable: Consists of glass or plastic threads; information is transmitted as light. It offers high bandwidth, low attenuation, and immunity to electromagnetic interference.

B. Unguided Medium (Wireless)

Electromagnetic waves are transported without a physical conductor.

Radio Waves (3 kHz – 1 GHz): Omnidirectional signals used for AM/FM radio and TV.
Microwaves (1 GHz – 300 GHz): Highly directional signals used for satellite communication and radar.
Infrared (300 GHz – 400 THz): Short-range, line-of-sight signals used for remote controls.

3. Need for Modulation

Modulation is necessary for the following reasons:

Reduction in Antenna Height: To transmit a signal effectively, antenna height must be a multiple of $\lambda/4$. For a low-frequency baseband signal (e.g., 10 kHz), the required height is approximately 7.5 km, which is practically impossible. Modulation increases the frequency (e.g., to 1 MHz), reducing the required height to ~75 meters.
Avoid Mixing of Signals: Without modulation, all baseband sound signals would occupy the 0–20 kHz range and get mixed. Modulation shifts signals to different frequency slots (channels), allowing them to be separated.
Increase Range of Communication: Low-frequency baseband signals are heavily attenuated over long distances. Modulated high-frequency signals travel longer distances with less attenuation.
Multiplexing is Possible: Modulation allows multiple signals to be transmitted simultaneously over the same channel (e.g., different TV channels).
Improves Quality of Reception: Techniques like Frequency Modulation (FM) and digital schemes reduce the effect of noise.

4. Modulation Definition and Types

Modulation is the process where some parameter of a high-frequency carrier wave (amplitude, frequency, or phase) is varied in accordance with the modulating (message) signal.

Types of Analog Modulation:

Amplitude Modulation (AM): The amplitude of the carrier wave is varied in proportion to the message signal, while frequency and phase remain constant.
- Waveform: The “envelope” of the carrier wave follows the shape of the message signal.
Frequency Modulation (FM): The frequency of the carrier wave changes with respect to the instantaneous value of the message signal, keeping amplitude and phase constant.
- Waveform: The wave compresses (higher frequency) and expands (lower frequency) based on the message signal.
Phase Modulation (PM): The phase of the carrier signal varies according to the amplitude variations of the message signal.

5. Superheterodyne Receiver Operation

The superheterodyne receiver converts the incoming Radio Frequency (RF) signal to a lower, fixed Intermediate Frequency (IF) for efficient amplification and demodulation.

Key Blocks and Operation:

RF Amplifier: Amplifies the weak signal received from the antenna and improves the signal-to-noise ratio before the noisy mixer stage.
Mixer / Local Oscillator: The oscillator generates a frequency that mixes with the incoming RF signal. The mixer translates the RF signal to a fixed Intermediate Frequency (IF).
IF Amplifier: Provides the majority of the gain (amplification) and selectivity in the receiver.
Detector: Performs demodulation to recover the original audio signal from the IF carrier.
AF Amplifier: Amplifies the audio signal to a level sufficient to drive a speaker.

6. Mobile Phone Principle of Operation

A mobile phone acts as a sophisticated two-way radio with various processing units.

Principal Components:

RF Part: Contains an Up-Converter (converts baseband to RF for transmission) and a Down-Converter (converts incoming RF to baseband).
Baseband Part (DSP): Processes voice/data. It uses a Codec to compress/decompress speech (converting 8 kHz audio to 13 kbps for traffic channels).
ADC and DAC: The ADC converts analog voice to digital signals for processing, and the DAC converts digital signals back to analog for the speaker.
CPU & Application Layer: Runs the operating system (e.g., Android) and applications (audio, video).
Input/Output: Includes the Microphone (converts voice to electric signal), Speaker (converts electric signal to sound), Display, Keypad, and Camera.
SIM: Stores the user’s identity (IMSI).

7. GSM Architecture

The GSM network is divided into four main subsystems:

Mobile Station (MS): The user equipment comprising the physical phone hardware (with IMEI) and the SIM card (with IMSI).
Base Station Subsystem (BSS): Handles radio communication.
- BTS (Base Transceiver Station): The radio towers/antennas that communicate directly with mobile phones.
- BSC (Base Station Controller): Controls a group of BTSs, managing radio resources and handovers.
Network Switching Subsystem (NSS): The “core network” managing switching and subscriber data.
- MSC (Mobile Switching Centre): Routes calls and handles mobility management.
- HLR (Home Location Register): Database of permanent subscriber info and location.
- VLR (Visitor Location Register): Temporary database for subscribers currently in the area.
- AuC (Authentication Centre): Stores security keys for authentication.
- EIR (Equipment Identity Register): Checks IMEI to identify stolen phones.
Operation and Support Subsystem (OSS): Used to monitor and control the overall network traffic and maintenance.

8. AM, FM, and PM Comparison

Here are the differences between Amplitude Modulation (AM), Frequency Modulation (FM), and Phase Modulation (PM) in point format:

Carrier Variation

AM: The amplitude of the carrier wave is varied in accordance with the message signal.
FM: The frequency of the carrier wave is varied in accordance with the message signal.
PM: The phase of the carrier wave is varied in accordance with the message signal.

Noise Immunity

AM: This technique is much affected by noise.
FM: It is more immune to noise compared to AM.
PM: The noise voltage is constant, offering better immunity than AM.

System Fidelity

AM: The system fidelity is generally poor.
FM: It offers improved system fidelity.
PM: It also offers improved system fidelity.

Linearity

AM: This is a type of linear modulation.
FM: This is a type of non-linear modulation.
PM: This is also a type of non-linear modulation.

Bandwidth

AM: The bandwidth is equal to $2f_m$, which is much less than FM.
FM: The bandwidth is equal to $2[\Delta f+f_m]$, which is significantly larger and depends on the modulation index.

9. AM Transmitter Block Diagram

The AM transmitter generates a high-power amplitude-modulated signal for broadcast.

Block Diagram Explanation:

Audio Block: Processes the voice signal from the microphone using amplifiers and processors.
RF Oscillator: Generates the specific carrier frequency (Radio Frequency) assigned to the transmitter to avoid interference.
AM Modulator: This is the mixing stage where the audio signal modulates the amplitude of the RF carrier generated by the oscillator.
RF Power: Amplifies the modulated signal to the required power level before it is sent to the antenna for transmission.