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TDD vs FDD-Difference between TDD and FDD

This page compares TDD vs FDD and describes difference between TDD and FDD topologies.

TDD means Time Division Duplex and FDD means Frequency Division Duplex. These topologies are widely used in advanced wireless communication systems such as WLAN, WiMAX(fixed/mobile), LTE and so on.

difference between TDD and FDD

As shown in the figure, in TDD system same frequency band FC is used by both Transmit and receive path at different time instants. In FDD system different frequency bands Fc1 and Fc2 are used by transmit and receive paths at same time instant.

We know that transmissions from base station to subscriber stations is referred as downlink and transmissions from subscriber stations to base station is referred as uplink in cellular/wireless communication systems. In TDD, both uplink and downlink transmissions are arranged one after the other on time scale i.e. uplink is transmitted at say 't1' instance and downlink is transmitted at 't2' instance. Here t2 is considered to be t1 plus some duration. Both uplink and downlink transmissions will take place at same RF carrier frequency(Fc).

In FDD, both uplink and downlink transmissions will be assigned two separate frequencies Fc1 and Fc2 respectively. Both utilizes same time slot('t1') to transmit.

TDD systems are cheap compare to FDD systems due to requirement of less RF modules such as Synthesizer, Local Oscillators, filters etc.

MAC layer in TDD system is complex compare to FDD system, as it has to deal with accurate time synchronization between the systems.

FDD system requires more power compare to TDD system due to more hardware RF modules.

TDD is favourable compare to FDD for advanced antenna technques such as beamforming and AAS(Adaptive Antenna System). This is due to channel reciprocity in TDD between the uplink and downlink paths.

LTE system utilizes both TDD as well as FDD frame structures to gain benefits of both. Refer LTE FDD vs TDD LTE which describes difference between FDD and TDD LTE topologies including frame structures of both.

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