---
title: "Noise, thermal noise, delay distortion"
subject: "Data Communication"
chapter: 1
key: "semester_5/data_communication:noise-types-thermal"
source: https://www.bctengineer.com/q/semester_5/data_communication/noise-types-thermal
site: BCT Engineering
---

# Noise, thermal noise, delay distortion

Data Communication · Chapter 1: Introduction

- Asked in: 2071 Chaitra, 4+2+2 marks
- Video: [Transmission Impairments – Attenuation, Distortion, Noise (Thermal, Induced, Impulse, Cross Talk)](https://www.youtube.com/watch?v=81-H2TEhlGk) — Dr. Ankit Verma
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/noise-types-thermal
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-1

## Question

Define noise. Briefly discuss the types of noise. Define thermal noise power density; calculate the thermal noise power density in Watts/Hz at a temperature of 17°C, the Boltzmann's constant is 1.38×10⁻²³ J/K. What is delay distortion and how can it be corrected? Why is digital transmission preferred over analog transmission?

## Short answer

Noise is any unwanted signal. Types include thermal, crosstalk, impulse. Thermal noise power density N0 = kT = 1.38×10^-23 × 290 K ≈ 4.0×10^-21 W/Hz. Delay distortion is corrected by equalizers. Digital transmission is preferred because repeaters regenerate signals and errors can be corrected.

## Answer

#### Noise
Noise is any unwanted signal that gets added to the useful signal between transmitter and receiver. It is the main factor limiting the performance of a channel and the reason for errors.

#### Types of noise
- **Thermal noise**: caused by random motion of electrons in conductors. It is present in all electronic devices, uniform over all frequencies (white noise), and N = kTB.
- **Intermodulation noise**: produced when signals of different frequencies share a medium with non-linear behaviour; new frequencies f₁ ± f₂ appear.
- **Crosstalk**: unwanted coupling between nearby wires, for example hearing another call.
- **Impulse noise**: short bursts of high amplitude from lightning, power-line surges, switching. It produces burst errors in digital data.
- **Shot noise and atmospheric noise** also occur in practice.

#### Thermal noise power density
N₀ = kT watts/Hz, with k = 1.38 × 10⁻²³ J/K.

T = 17 + 273 = 290 K

N₀ = 1.38 × 10⁻²³ × 290 = 4.0 × 10⁻²¹ W/Hz

**Answer: N₀ ≈ 4.0 × 10⁻²¹ W/Hz** (about 4.004 × 10⁻²¹ if 273.15 is used).

#### Delay distortion
In a guided medium, the velocity of propagation varies with frequency, so the frequency components of a signal arrive at different times and the pulse shape is distorted (spreading and overlapping of neighbouring pulses). It is critical for digital data. It is corrected by **equalisation**, using equalisers (delay-equalising filters) that give the faster components more delay, so all components arrive together.

#### Why digital transmission is preferred
- Repeaters regenerate the signal, so noise and distortion do not accumulate.
- Error detection/correction codes can be used.
- Easy multiplexing, encryption, compression and processing.
- Voice, data and video can share one integrated network, with cheap digital hardware.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Thermal noise power density is
   A. N0 = kTB
   B. N0 = k/T
   C. N0 = T/k
   D. N0 = kT W/Hz
   Answer: D. Multiplying by B gives total power N = kTB. (Key point: Thermal noise density)

2. 17 deg C in kelvin is
   A. 290 K
   B. 17 K
   C. 256 K
   D. 300 K
   Answer: A. 17 + 273 = 290 K. (Key point: Temperature)

3. N0 at 290 K is about
   A. 4.0 x 10^-23 W/Hz
   B. 4.0 x 10^-21 W/Hz
   C. 1.38 x 10^-21 W/Hz
   D. 4.0 x 10^-19 W/Hz
   Answer: B. 1.38e-23 x 290 = 4.0e-21. (Key point: Value)

4. Delay distortion is corrected by
   A. amplifiers
   B. repeaters
   C. equalisers
   D. error codes
   Answer: C. Equalisers delay faster components so all arrive together. (Key point: Delay distortion)

## Related in Data Communication

- [Advantages of digital over analog systems](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adigital-vs-analog-advantages) — Data Communication, chapter 1, asked 5×
- [Compare analog and digital communication](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adigital-vs-analog-comparison) — Data Communication, chapter 1, asked 4×
- [PCM and analog-to-digital conversion](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Apcm-working) — Data Communication, chapter 5, asked 4×
- [Define analog and digital signals](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aanalog-digital-signal-define) — Data Communication, chapter 1
- [Short note: Analog vs digital mux hierarchy](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aanalog-vs-digital-mux-hierarchy) — Data Communication, chapter 6
- [Digital vs analog spread spectrum](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-digital-vs-analog) — Data Communication, chapter 6

## Related in other subjects

- [Advantages of digital over analog transmission](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adigital-vs-analog-transmission-adv) — Instrumentation II, chapter 5
- [Noise, interference, distortion; thermal and high frequency noise](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-effects-thermal-hf) — Communication Systems, chapter 1
- [Analog and digital system block diagrams; linear and non-linear distortion](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aanalog-digital-block-linear-nonlinear-distortion) — Communication Systems, chapter 1
- [Analog vs digital source; atmospheric noise](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aanalog-vs-digital-source-atmospheric-noise) — Communication Systems, chapter 1