Why are Low Frequency Bands Configured in FDD Mode?

Mandatory Guard Period in TDD Bands makes them highly inefficient if used in lower frequency configuration

In this note, we will discuss why low bands are used in FDD (Frequency Duplex Mode) mode and why not in TDD (Time Duplex Mode). But before we dive into details let’s first discuss the basics.

FDD vs TDD

The following diagram explains the difference.

FDD (Frequency Division Duplex) is like a double road separated by a divider, whereas TDD (Time Division Duplex) is a single road where the traffic is allowed to move in only one direction at a time. In order words, for a TDD BTS to start receiving it has to first Stop Transmission and immediately switch ON its receiver to process incoming signal. Hence all elements of the TDD network need to be synchronised using a GPS clock for the network to function properly.

TDD Frame Structure

A typical LTE TDD Frame looks something like this.