Basics of Communication System — Block Diagram, Types & Working
A communication system enables the transfer of information from a source to a destination through a channel. From early telegraph systems to modern 5G networks, the fundamental architecture remains the same — a transmitter, channel, and receiver working together to deliver messages reliably.
What is Communication?
Communication is the process of transferring information (message) from one point (source) to another point (destination). In electrical engineering, we specifically study electronic communication — where electrical signals carry information over a distance using wired or wireless media.
A few centuries ago, humans relied entirely on physical modes — messengers carrying letters. The speed was low, cost was high, and reliability depended on geography and weather. Modern electronic communication has revolutionised this — whether you're watching a live cricket match or making a video call across continents, you're using an electronic communication system.
Block Diagram of Electronic Communication System
Every communication system, regardless of complexity, follows this fundamental block diagram:
Block Diagram of Electronic Communication System
Elements of a Communication System
1. Input Transducer
The input transducer converts the original message (sound, image, data) into an equivalent electrical signal. This conversion is essential because electrical signals travel at the speed of light, enabling high-speed transmission.
- Microphone — converts sound waves into electrical voltage variations
- Camera sensor — converts light intensity into electrical signals
- Keyboard/Sensor — converts physical input into digital data
2. Transmitter
The transmitter processes the electrical signal to make it suitable for transmission through the channel. Key operations performed by the transmitter include:
- Modulation — impresses the message signal onto a high-frequency carrier
- Amplification — boosts signal power for long-distance transmission
- Encoding — adds error-correction codes for reliability
- Filtering — limits bandwidth to allocated frequency band
3. Channel
The channel is the physical medium connecting transmitter to receiver. It introduces attenuation, distortion, and noise.
4. Noise and Distortion
Noise is any unwanted random signal that corrupts the transmitted information. Sources include thermal noise in resistors, atmospheric interference (lightning), and man-made interference (electrical switching, motors).
Distortion is the systematic alteration of the signal waveform caused by non-ideal channel characteristics — such as frequency-dependent attenuation or phase shifts.
5. Receiver
The receiver extracts the original message from the received signal. It performs the inverse operations of the transmitter — demodulation, decoding, amplification, and filtering to remove noise.
6. Output Transducer
Converts the recovered electrical signal back into a form usable by the destination — a loudspeaker for audio, a display for video, or a printer for text.
Types of Communication Systems
Need for Modulation
Baseband signals (audio: 20 Hz–20 kHz) cannot be transmitted directly over long distances because:
- Antenna size — efficient radiation requires antenna length ≈ λ/4. For a 5 kHz audio signal, this would be 15 km — impractical.
- Multiplexing — multiple users can share the same channel by using different carrier frequencies.
- Noise immunity — higher frequency signals are less susceptible to low-frequency noise.
- Bandwidth utilisation — modulation shifts signals to frequency bands where the channel has better characteristics.
For f = 5 kHz: λ/4 = 3×10⁸ / (4 × 5000) = 15,000 m = 15 km
Bandwidth and Channel Capacity
Bandwidth is the range of frequencies a channel can pass without significant attenuation. Channel capacity defines the maximum data rate achievable with negligible errors.
Where: C = capacity (bits/s), B = bandwidth (Hz), SNR = signal-to-noise ratio
For example, a telephone channel with B = 3.4 kHz and SNR = 30 dB (1000) has a theoretical capacity of approximately 34 kbps.
Noise and Signal-to-Noise Ratio
The quality of a communication system is measured by its Signal-to-Noise Ratio (SNR):
Applications of Communication Systems
- Telecommunications — mobile networks (4G/5G), landline telephony
- Broadcasting — AM/FM radio, digital television (DTH)
- Internet — fibre optic backbone, Wi-Fi, satellite internet
- Navigation — GPS, IRNSS (NavIC) for India
- Defence — radar, encrypted military communication
- Medical — telemedicine, remote patient monitoring
Frequently Asked Questions
Q1. What is the basic block diagram of a communication system?
A communication system consists of five blocks: Input Transducer → Transmitter → Channel → Receiver → Output Transducer. The input transducer converts the message to an electrical signal, the transmitter modulates it for transmission, the channel carries it, the receiver demodulates it, and the output transducer converts it back to the original form.
Q2. Why is modulation necessary in communication?
Modulation is necessary because baseband signals have very low frequencies requiring impractically large antennas. Modulation shifts the signal to higher frequencies, enables multiplexing of multiple signals, improves noise immunity, and allows efficient use of the electromagnetic spectrum.
Q3. What is the difference between noise and distortion?
Noise is random and unpredictable interference added to the signal from external or internal sources. Distortion is a systematic, repeatable alteration of the signal waveform caused by non-ideal channel characteristics. Noise cannot be completely eliminated, while distortion can be compensated using equalisation techniques.
Q4. What is Shannon's channel capacity theorem?
Shannon's theorem states that the maximum error-free data rate (C) of a channel equals B × log₂(1 + SNR), where B is bandwidth in Hz and SNR is the signal-to-noise ratio. This sets the theoretical upper limit — no real system can exceed this rate without errors.
Q5. What are the advantages of digital communication over analog?
Digital communication offers better noise immunity (regeneration at repeaters), error detection and correction capability, encryption for security, easier multiplexing, and compatibility with modern computing systems. However, it requires more bandwidth than analog for the same signal.