Duplexing

The Problem

When you make a phone call, you need to talk and listen at the same time.

But radio transmission is like a walkie-talkie by default. You can either transmit OR receive, not both.

Duplexing solves this problem by enabling two-way communication.

Two Solutions

There are two ways to achieve full-duplex communication:

1. FDD (Frequency Division Duplex): Use two different frequencies
2. TDD (Time Division Duplex): Use one frequency, but take turns

FDD: Frequency Division Duplex

Use separate frequencies for each direction:

• Uplink: phone → tower (lower frequency band)
• Downlink: tower → phone (higher frequency band)

Both directions work simultaneously because they don’t interfere with each other.

A guard band sits between the two frequencies to prevent overlap.

Analogy: Two separate roads. One for northbound traffic, one for southbound. Both flow at the same time.

TDD: Time Division Duplex

Use one frequency, but alternate between sending and receiving:

• Transmit for a few milliseconds
• Switch to receive for a few milliseconds
• Repeat

The switching happens so fast (milliseconds) that you don’t notice any gaps.

Analogy: A single-lane bridge with a traffic light. Cars alternate directions, but it’s so fast it feels continuous.

Comparison

When To Use Which?

FDD is better when:

• Traffic is symmetric (equal upload and download)
• Low latency is critical
• Paired spectrum is available

TDD is better when:

• Traffic is asymmetric (more download than upload)
• Spectrum is limited or unpaired
• You want flexibility to adjust capacity dynamically

Real World Examples

LTE Band Numbers

In LTE, band numbers tell you the duplexing mode:

• Bands 1-31: FDD
• Bands 33-46: TDD

Most countries use FDD for wide-area coverage and TDD for capacity hotspots.