TDD : Why a Guard Period only in DL-to-UL Switching ?

One of my friend, Sripathi has been asking this question about Time Division Duplexing (TDD) for over a week. I thought to visualize the answer for him this time.

What was his question ?

In TDD, why special sub-frame is used for switching Downlink (DL) to Uplink (UL) . Why not for UL to DL ? What’s the reason Please clarify.

There are 2 versions of the answer , some people will get satisfied with version 1, others with version 2. Both are correct, but that’s how we all learn , what convinces us.

Version 1:

For TDD, in the Downlink , due to path distance DL signal can be delayed. Due to path distance , path delay is created. Delay can cause collision between UL and DL signals. The guard period provides enough time for DL delayed signal to arrive due to path distance, and also gives enough opportunity for (User Equipment)UE to receive UL timing advance command from the Base station.

Follow up Question. Why special sub-frame only in DL to UL, why not from UL to DL ?

UE always transmit in UL after receiving grant from the BS. BS can advance or retard the UL timing command as needed. Once DL signal is received completely then UE can send command /signal in return with respective timing ( UE cannot transmit on its own before receiving a complete response from the DL) . So we need a guard period for DL to UL switching, to avoid collisions. Whereas in case of UL to DL switching there is no need of Guard period, as eNodeB has timing advance feature plus there will be minimum chance of collision in UL to DL case.

In DL to UL swtiching , a Guard Period is needed , ” To avoid the time advanced UL to collide with the delayed DL”

Version 2:

A simple answer is in TDD because there is no duplex Frequency and all management must be done on the same path, then UE should first receive info about up-link(most important one is Time scheduling ). Therefore, Downlink  should always be done before Uplink in TDD mode.

If you have anything else to add , feel free to write it in the comments below.

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