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Introduction to WCDMA: Physical Channels, Logical Channels, Network, and Operation

Time Division Duplex (TDD) WCDMA

Time division duplex (TDD) is a process of allowing two way communications between two devices by time sharing. When using TDD, one device transmits (device 1), the other device listens (device 2) for a short period of time. After the transmission is complete, the devices reverse their role so device 1 becomes a receiver and device 2 becomes a transmitter. The process continually repeats itself so data appears to flow in both directions simultaneously.

The WCDMA system can use time division duplex (TDD) format technology when paired frequencies are not available. The WCDMA TDD system uses the same CDMA channel coding process as the WCDMA FDD system. The WCDMA TDD system is well suited for indoor areas and wide area data systems.

During TDD operation, the mobile device receives a burst of data, waits until its assigned time slot, and transmits a burst of data on the same frequency. This process is continually repeated providing a steady flow of data in both directions.

A key challenge of TDD operation is the potential that the transmission time from mobile devices varies with the distance that the mobile device operates from the cell site and this can cause overlapping with transmission bursts received from other mobile devices. To overcome this challenge, a guard time period is added to each transmission burst to ensure it does not overlap with transmit bursts from other mobiles.

Figure 1.6 shows the basic parts and operation of the WCDMA TDD system. This...

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