Why TD-LTE ?

Operators are spending millions of dollars just on the license only. It is expected that TDD spectrum will be sold at lower price as compared to FDD spectrum and secondly the TDD spectrum is also available in many countries. So certainly there is emerging interest in TD-LTE. Some of the main advantages of TD-LTE over FDD-LTE are as follows

In FDD LTE (Frequency division duplex)

• duplexing scheme requires paired spectrum which in other words means that the downlink and uplink transmission should be on different frequency spectrum
• There is wastage of frequency resources in up-link if most UEs in the cells are using downlink spectrum most the time or vice versa (For example if most users in he cells are watching YouTube or downloading some files)
• Since different range of frequencies are used in uplink and downlink, therefore channel characteristics are different in both directions. UE needs to report the downlink channel characteristics to the eNB on its uplink channels
• Additional hardware required on UE and eNB side to separate uplink and downlink frequencies which adds extra cost to the terminals and base stations

Now in TD-LTE (Time division duplex) ? 

• Paired spectrum is not required and both downlink / uplink communication occurs on single frequency channel
• Channel characteristics are same for both uplink and downlink channels
• Hardware cost is low, no need of diplexer in UE and eNB
• Spectrum is efficiently utilized since UEs can use all the slots for downlink traffic if there is no uplink traffic

TD-LTE frame structure is shown in the figure below

Subframe 0 and 5 carries synchronization signals and system information blocks

Subframe 1 is special subframe that is used to carry information for switching between uplink and downlink for 10ms switching period

For 5 ms switching period both subframe 1 and 6 are used