WCDMA Frequency Division Duplex - FDD

Frequency division duplex (FDD) is a communications channel that allows the transmission of information in both directions via separate bands (frequency division). The WCDMA frequency division duplex (FDD) system pairs two 5 MHz radio channels to provide for simultaneous duplex communication.

FDD operation normally assigns the transmitter and receiver to different communication channels. One frequency is used to communicate in one direction and the other frequency is required to communicate in the opposite direction. The amount of frequency separation between the transmitter and receiver frequency bands varies based on the range of frequencies. Generally, the higher the frequency used, the larger the amount of duplex frequency spacing. The lower frequency band of the pair is generally designated for the mobile because it is easier (and often cheaper) to produce lower frequency devices.

Each radio channel is divided into 10 msec frames and each frame is further divided into 15 time slots (666 usec each). During a typical FDD voice conversation, all the time slots are dedicated for transmitting and for receiving.

Because there are times that a mobile device must perform other radio signal processing functions on other frequencies during conversation (during voice transmission), the mobile device may enter into a compressed mode of transmission. Compressed mode allows the mobile device to compress a 10 msec frame of data into only some of the 15 time slots within the 10 msec data frame (sending a higher data rate in some of the other slots). This allows the mobile device to use the time during the unused time slots to perform other functions such as retuning and measuring the signal strength and quality of other radio channels.

This figure shows the basic parts and operation of the WCDMA FDD system. This diagram shows that the WCDMA FDD system uses two 5 MHz radio channels that are nominally separated by 190 MHz (85 MHz separation in USA) to provide for full duplex two-way communication. Each radio channel is further divided into 10 msec frames and 15 time slots (666 usec each) that contain control and user data.

WCDMA Frequency Division Duplex - WCDMA FDD Diagram

WCDMA Frequency Division Duplex System- WCDMA FDD Diagram

UMTS WCDMA Books

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Introduction to WCDMA

This book explains the basic components, technologies used, and operation of UMTS WCDMA systems. You will discover why mobile telephone service providers have upgraded from 2nd generation digital systems to more efficient and feature rich 3rd generation system. You will also discover how 3rd generation systems are gradually evolving into 4th generation ultra broadband multimedia systems.

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