---
title: "Duplexing and its types"
subject: "Wireless Communication"
chapter: 1
key: "semester_7/wireless_communication:duplexing-types"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/duplexing-types
site: BCT Engineering
---

# Duplexing and its types

Wireless Communication · Chapter 1: Introduction

- Asked in: 2081 Baisakh, 2+4 marks
- Video: [Duplexing techniques | Wireless Communication | Lec-6](https://www.youtube.com/watch?v=BcdHIJ2Kr3g) — Quantum Study Circle - Clear your Concepts
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/duplexing-types
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-1

## Question

What do you mean by duplexing? Explain its different types.

## Short answer

Duplexing enables simultaneous two-way communication by separating forward and reverse channels. In Frequency Division Duplex (FDD), distinct frequency bands with a duplexer handle each direction; Time Division Duplex (TDD) uses a single frequency with alternating time slots.

## Answer

**Duplexing** is the technique that allows a communication link to send and receive at the same time (two-way communication), separating the forward (base to mobile) and reverse (mobile to base) channels either in frequency or in time.

A **simplex** system is one-way (paging), **half duplex** allows two-way but one direction at a time (walkie-talkie), and **full duplex** allows both directions at once (telephone). Duplexing methods give full or near full duplex operation.

#### 1. Frequency Division Duplexing (FDD)
- Two separate frequency bands are used: one for the forward channel and one for the reverse channel, separated by a fixed **duplex spacing**.
- A **duplexer** filter in the handset and base station lets one antenna transmit and receive together.
- Example: GSM-900 uplink 890–915 MHz, downlink 935–960 MHz, spacing 45 MHz.

```text
 Freq ↑  Downlink (forward) ███████████
       |      45 MHz spacing
       |  Uplink (reverse)   ███████████
       +------------------------------ time
```
- Advantages: true simultaneous transmission, no timing guard needed, suited to symmetric voice traffic.
- Disadvantages: needs paired spectrum and a duplexer; fixed split cannot adapt to asymmetric data.

#### 2. Time Division Duplexing (TDD)
- A single frequency is used; time is divided into slots for forward and reverse transmission, switched fast enough that the user sees it as full duplex.
- Guard time is needed between transmit and receive.
- Example: DECT cordless phones, TD-SCDMA, LTE-TDD, WiMAX, 5G NR TDD bands.

```text
 Freq ↑  [DL][UL][DL][UL][DL][UL]...  one carrier
       +------------------------------ time
```
- Advantages: unpaired spectrum, flexible DL/UL ratio for asymmetric data traffic, same channel in both directions helps beamforming.
- Disadvantages: needs strict time synchronisation, guard time reduces efficiency, limited range due to propagation delay.

| Point | FDD | TDD |
|---|---|---|
| Separation | Frequency | Time |
| Spectrum | Paired | Unpaired |
| Duplexer | Needed | Not needed (switch) |
| Asymmetric traffic | Poor | Good |
| Example | GSM, WCDMA, LTE-FDD | DECT, TD-LTE |

## Check yourself

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

1. What does duplexing achieve?
   A. Two-way communication by separating forward and reverse channels in frequency or time
   B. Splitting one signal into many subcarriers
   C. Spreading signals with codes
   D. Reusing frequencies in far cells
   Answer: A. Duplexing separates uplink and downlink, by frequency (FDD) or time (TDD). (Key point: Definition)

2. What does FDD need that TDD does not?
   A. Paired spectrum and a duplexer
   B. Strict time synchronisation
   C. Guard time between slots
   D. A single frequency only
   Answer: A. FDD uses two bands separated by duplex spacing and a duplexer; TDD needs sync and guard time. (Key point: FDD)

3. Why is TDD good for asymmetric data traffic?
   A. The DL/UL time ratio can be adjusted flexibly
   B. It has a fixed 45 MHz split
   C. It uses two paired bands
   D. It needs no unpaired spectrum
   Answer: A. Slot allocation between DL and UL can be changed, unlike FDD's fixed split. (Key point: TDD advantage)

4. In GSM-900 FDD, what is the duplex spacing?
   A. 25 MHz
   B. 45 MHz
   C. 200 kHz
   D. 935 MHz
   Answer: B. Uplink 890-915 MHz and downlink 935-960 MHz are 45 MHz apart. (Key point: Duplex spacing)