---
title: "Importance and types of diversity"
subject: "Wireless Communication"
chapter: 5
key: "semester_7/wireless_communication:diversity-importance-types"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/diversity-importance-types
site: BCT Engineering
---

# Importance and types of diversity

Wireless Communication · Chapter 5: Equalization and Diversity

- Asked in: 2071 Bhadra, 2+6 marks
- Video: [Diversity Techniques in Antennas / Wireless Communication | Antenna and Wave Propagation Module - 6](https://www.youtube.com/watch?v=jJhxMI3jvi0) — THE BACKBENCH ENGINEERING COMMUNITY
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/diversity-importance-types
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-5

## Question

Why is diversity important in wireless communication system? Explain different types of diversity techniques.

## Short answer

Diversity reduces deep fade probability by combining multiple independently faded copies, lowering BER without extra power. Types: space diversity separates antennas; polarization diversity uses orthogonal polarizations; frequency diversity transmits on carriers spaced beyond coherence bandwidth; time diversity repeats data after coherence time; angle diversity uses directional beams.

## Answer

#### Importance of diversity

The mobile channel causes **multipath (Rayleigh) fading**: the received signal can drop by 20–40 dB in a fraction of a wavelength of movement. Diversity is important because:

- It gives several independently faded copies, so the chance of all being in a deep fade together (≈ *p*ᴹ) is very small.
- It greatly reduces BER and call drops without extra transmit power.
- Many forms (space, polarization) need no extra bandwidth.
- It needs no training overhead and is cheap compared with the improvement it gives (often 10–20 dB gain at low BER).

#### Types of diversity techniques

**1. Space (antenna) diversity**
- Two or more antennas separated so that fading is uncorrelated (≈ λ/2 at mobile, ≥ 10λ at base station).
- Combining: selection, feedback, maximal ratio and equal gain combining.

**2. Polarization diversity**
- Horizontally and vertically (or ±45°) polarized antennas at one location. Scattering de-correlates the two polarizations.
- Compact; widely used at base stations; gain is a little less than space diversity.

**3. Frequency diversity**
- The same information is sent on carriers separated by more than the **coherence bandwidth** (B_c ≈ 1/(5σ_τ)).
- Disadvantage: needs extra spectrum and a receiver per frequency. Frequency hopping and OFDM use this idea.

**4. Time diversity**
- Repeats the information at intervals greater than the **coherence time** (T_c ≈ 0.423/f_d).
- Implemented with channel coding plus interleaving, or with a RAKE receiver in CDMA.

**5. Angle (pattern) diversity**
- Directional antennas pointing in different directions pick up differently faded multipath components.

| Type | Separation needed | Extra cost |
|---|---|---|
| Space | Distance > coherence distance | More antennas |
| Polarization | Orthogonal polarization | Dual-pol antenna |
| Frequency | Δf > coherence bandwidth | More spectrum |
| Time | Δt > coherence time | Delay, coding |

## Check yourself

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

1. Why is diversity important?
   A. Several independently faded copies make simultaneous deep fade unlikely
   B. It increases transmit power
   C. It removes ISI
   D. It increases spectrum use always
   Answer: A. Probability of all fading is about p^M. (Key point: Importance)

2. Disadvantage of frequency diversity?
   A. Needs extra antennas
   B. Needs delay
   C. Needs extra spectrum and a receiver per frequency
   D. Needs training
   Answer: C. Frequency hopping and OFDM use this idea. (Key point: Frequency diversity cost)

3. Time diversity is implemented with...
   A. Cross-pol antennas
   B. Spaced antennas
   C. Directional beams
   D. Coding plus interleaving, or RAKE in CDMA
   Answer: D. Repeats info at intervals above coherence time. (Key point: Time diversity implementation)

## Related in Wireless Communication

- [ISI mitigation and space diversity techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aisi-space-diversity) — Wireless Communication, chapter 5
- [Need for diversity; combining techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-combining-techniques) — Wireless Communication, chapter 5
- [Diversity techniques and space diversity combining](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-space-combining) — Wireless Communication, chapter 5
- [Space diversity techniques with block diagrams](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspace-diversity-block-diagrams) — Wireless Communication, chapter 5
- [Benefits of wireless communication and 4G](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Awireless-benefits-4g) — Wireless Communication, chapter 1
- [Regulatory issues in wireless communication](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aregulatory-issues) — Wireless Communication, chapter 1, asked 6×

## Related in other subjects

- [Retrofitting techniques; why it is important](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aretrofit-techniques-importance) — Building Technology, chapter 11, asked 3×
- [Wired and wireless communication systems](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Awired-wireless-systems) — Basic Electronics Engineering, chapter 5, asked 2×
- [Optical fibre system vs wireless](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aofc-working-vs-wireless) — Propagation and Antenna, chapter 6
- [Switching techniques in data communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-techniques-data-comm) — Telecommunications, chapter 8, asked 3×