---
title: "Small scale fading and influencing factors"
subject: "Wireless Communication"
chapter: 3
key: "semester_7/wireless_communication:small-scale-fading-factors"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/small-scale-fading-factors
site: BCT Engineering
---

# Small scale fading and influencing factors

Wireless Communication · Chapter 3: Radio Wave Propagation in Mobile Networks

- Asked in: 2072 Magh, 3+6 marks
- Video: [Factors Influencing Small Scale Fading - Mobile Radio Propagation - Mobile Communication System](https://www.youtube.com/watch?v=bvesKUOvNdA) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/small-scale-fading-factors
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-3

## Question

What is small scale fading? Describe the different factors that influence the small scale fading.

## Short answer

Small-scale fading is rapid variation in amplitude, phase and delay over a few wavelengths due to multipath. The influencing factors are multipath propagation, speed of the mobile, speed of surrounding objects, and transmission bandwidth. Delay spread and Doppler spread determine flat or frequency-selective and fast or slow fading.

## Answer

#### Small scale fading
**Small scale fading** is the rapid change in the amplitude, phase and multipath delay of a received radio signal over a short distance (a few wavelengths) or a short time (a few seconds). It happens because two or more copies of the transmitted signal (multipath waves) reach the receiver at slightly different times and with different phases, and add constructively or destructively. Large-scale path loss can be treated as constant over such a small distance.

Its three main effects are:
- Rapid changes in signal strength over a small travel distance or time interval.
- Random frequency modulation due to different Doppler shifts on different multipath signals.
- Time dispersion (echoes) caused by multipath propagation delays.

```text
 Power (dB)
   |   /\    /\  /\     /\
   |  /  \/\/  \/  \/\_/  \   <- deep fades
   | /                     \     every ~ λ/2
   +----------------------------> distance
```

#### Factors influencing small scale fading
1. **Multipath propagation** – Reflecting and scattering objects (buildings, vehicles, trees) create many waves with random amplitudes, phases and arrival times. Their vector sum fluctuates strongly, and the spread of arrival times causes time dispersion and ISI.

2. **Speed of the mobile** – Relative motion between the base station and the mobile gives each multipath component a different Doppler shift, f_d = (v/λ) cos θ. Higher speed means a larger Doppler spread and faster fading (smaller coherence time).

3. **Speed of surrounding objects** – Moving cars, people and other objects in the radio channel also create time-varying Doppler shifts. If they move faster than the mobile, their effect dominates; otherwise it can be ignored.

4. **Transmission bandwidth of the signal** – The channel has a coherence bandwidth B_c (inversely related to its delay spread).
   - If signal bandwidth B_s < B_c, all frequency components fade together: **flat fading**, little distortion.
   - If B_s > B_c, different frequency components fade differently: **frequency selective fading**, pulse spreading and ISI.

Together, the delay spread (from multipath) and the Doppler spread (from motion) decide whether the channel is flat or frequency selective, and fast or slow fading.

## Check yourself

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

1. Small scale fading is best defined as:
   A. Slow decrease of mean power with distance
   B. Rapid change of amplitude, phase and delay of the signal over a short distance or time
   C. Loss caused by walls
   D. Loss due to antenna mismatch
   Answer: B. It is the rapid change of the received signal over a few wavelengths due to multipath copies adding with different phases. (Key point: Definition)

2. What happens when the mobile speed increases?
   A. Doppler spread decreases
   B. Coherence time increases
   C. Fading becomes faster (smaller coherence time)
   D. Fading becomes flat
   Answer: C. Higher speed gives a larger Doppler spread and so faster fading with smaller coherence time. (Key point: Speed of mobile)

3. When does a signal experience flat fading?
   A. B_s > B_c
   B. B_s < B_c
   C. Only when the mobile is stationary
   D. When there is no multipath delay but high speed only
   Answer: B. If the signal bandwidth is less than the coherence bandwidth, all frequency components fade together. (Key point: Bandwidth and flat fading)

4. When can the motion of surrounding objects be ignored?
   A. When they move slower than the mobile
   B. When they move faster than the mobile
   C. Never
   D. When bandwidth is large
   Answer: A. If surrounding objects move faster than the mobile their effect dominates; otherwise it can be ignored. (Key point: Speed of surrounding objects)

## Related in Wireless Communication

- [Small scale fading types and factors](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Asmall-scale-fading-types-factors) — Wireless Communication, chapter 3, asked 2×
- [Large scale path loss and small scale fading](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Alarge-scale-loss-small-fading) — Wireless Communication, chapter 3
- [Fading factors and Doppler shift derivation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afading-factors-doppler-shift) — Wireless Communication, chapter 3
- [Large vs small scale models; propagation mechanisms](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Alarge-small-scale-propagation) — Wireless Communication, chapter 3

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