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 System- WCDMA FDD Diagram
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