---
title: "Need for interleaving"
subject: "Wireless Communication"
chapter: 5
key: "semester_7/wireless_communication:interleaving-need"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/interleaving-need
site: BCT Engineering
---

# Need for interleaving

Wireless Communication · Chapter 5: Equalization and Diversity

- Asked in: 2080 Bhadra, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/interleaving-need
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-5

## Question

Why is interleaving needed in wireless communication? Explain its working in brief.

## Short answer

Interleaving is needed to convert burst errors from fading into random errors that error-correcting codes can handle. A block interleaver writes coded bits row by row into a matrix, reads them column by column for transmission, and the receiver reverses the process, so a deep fade damages at most one bit per code word.

## Answer

**Interleaving** is needed because mobile fading channels cause **burst errors**: when the mobile is in a deep fade, many consecutive bits are lost. Most error-correcting codes are designed for random, scattered errors and cannot correct long bursts. Interleaving spreads the bits of each code word over time so that one fade damages only a few bits of each code word. It provides **time diversity** without adding redundant bits.

#### Working (block interleaver)
```text
 Write row-wise:        Read column-wise:
  a1 a2 a3 a4           a1 b1 c1 d1 a2 b2 ...
  b1 b2 b3 b4
  c1 c2 c3 c4           Burst hits a2 b2 c2 d2
  d1 d2 d3 d4
```
1. Coded bits are written into an m × n array row by row (each row = one code word).
2. They are read out column by column and transmitted.
3. The receiver's de-interleaver writes column by column and reads row by row, restoring the order.
4. A burst of up to m bits in the channel becomes at most one error per code word, which the decoder corrects.

Trade-off: deeper interleaving handles longer fades but adds delay. Example: GSM interleaves each 456-bit speech block over 8 TDMA bursts.

## Check yourself

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

1. Why is interleaving needed in mobile wireless links?
   A. Fading causes only random single errors
   B. Interleaving increases the data rate
   C. It removes the need for any channel code
   D. Fading causes burst errors, which most error-correcting codes cannot correct
   Answer: D. Codes are designed for scattered errors; interleaving turns fade bursts into scattered errors. (Key point: Why interleaving is needed)

2. At the receiver's de-interleaver, how are bits handled?
   A. Written column by column, read row by row
   B. Written row by row, read column by column
   C. Written and read row by row
   D. Discarded and regenerated
   Answer: A. The de-interleaver reverses the transmitter's operation to restore the order. (Key point: Write/read order)

3. With an m x n block interleaver (each row one code word), a channel burst of up to m bits produces what after de-interleaving?
   A. m errors in a single code word
   B. At most one error per code word
   C. No errors in any code word at all, regardless of code
   D. n errors in each code word
   Answer: B. Consecutive channel bits come from different rows, so each code word gets at most one error. (Key point: Burst size limit)

## Related in Wireless Communication

- [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×
- [Trends in wireless communication beyond 3G](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Atrends-beyond-3g) — Wireless Communication, chapter 1
- [Interleaving and MLSE](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ainterleaving-mlse) — Wireless Communication, chapter 5
- [Interleaving and adaptive equalization of ISI](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ainterleaving-adaptive-equalizer) — Wireless Communication, chapter 5
- [Short note: Wireless LAN (WLAN)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Awlan-short-note) — Wireless Communication, chapter 8

## Related in other subjects

- [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×
- [Communication protocol and its need](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Acommunication-protocol-need) — Telecommunications, chapter 8, asked 3×
- [Communication system block diagram; need for modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acomm-block-diagram-needs-modulation) — Communication Systems, chapter 1
- [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