Why Special Subframe is required in LTE?

In LTE Frame Type 2 (TD-LTE) there is a special sub-frame when switching from DL to UL but there is no special sub-frame or gap when switching from UL to DL. 

Different TDD modes

To understand this, it is important to know why a transmission gap is required when switching from DL to UL. The special sub-frame is made up of DwPTS, GP and UpPTS and all of these have configurable lengths while the sum of the lengths has to be 1 ms i.e. the length of the sub-frame. Now consider, the format 1, where the GP (guard period or TTG in WiMAX) is 4 symbols long which equates to 285 us approx. Consider a UE-A at a distance of 10 km from the eNB and UE-B at 50 km from the eNB. The time it takes the RF signals to reach the UE-A and UE-B will be

Time for UE-A = distance/velocity of light = 10000/3x10^8 = 33.3 us

Time for UE-B = distance/velocity of light = 45000/3x10^8 = 150 us

This means that after the eNB has transmitted the last symbol of DL data and it starts the GP, the last symbol will be received at UE-A after 33.3 us and at UE-B after 150 us. Now, every UE takes a small amount of time to switch from Rx to Tx mode and lets assume this switching period to be 50 us (it should be lesser for LTE UEs but this is just an assumption). So, the UE-A will start its switching period and will start transmitting after 33.5 + 50 = 83.5 us and it will take another 33.5 us to reach the eNB. This makes the total Round Trip Time for UE-A to be equal to 33.5 +50 + 33.5 = 117 us. Now we know that the GP at eNB is set 285 us so that means that the UE-A will be able to transmit the UL data within the GP. In actual practice, all the UEs know their Timing Advance from the eNB so the UE-A would wait that much amount before transmitting so that the UL data reaches the eNB at exactly the end of GP.

However, lets do the same analysis for UE-B. The total round trip time for UE-B would be 150 + 50 + 150 = 350 us which is greater than the GP (285 us) so, the UE-B would not be able to reach the first uplink symbol. Because of this, the GP is supposed to determine the maximum cell radius for a TDD system.

If there hadn't been a GAP or TTG (as in WiMAX) between the DL and UL transmission, these Over-The-Air timing delays and the switching period could not be compensated so we need to add a transmission gap when switching from DL to UL.

Now, lets consider the UL to DL switching. We will only consider UE-A for this example as UE-B has been proven beyond the cell range. The UE-A will transmit the last UL symbol and then start switching from Tx to Rx mode. The last UL symbol will reach the eNB after 33.5 us and the eNB would switch to Tx after receiving the last UL symbol. It will transmit the next DL symbol which will reach the UE-A after another 33.5 us and thus the UE-A will have a total of 33.5 + 33.5 = 67 us of time to switch from Tx to Rx mode without any presence of any RTG. So, while switching from UL to DL, a RTG or GP is not really required as the system already gets a virtual GP due to OTA delays.

There can be a query about the UEs that are very close to the eNB as they would have a very small Over-The-Air delay so it might not get enough time to switch to Rx mode. There can be two possible solutions for that

- In LTE, there is a 1 ms TTI so if the UE is too close to the eNB that it would not be able to switch to Rx mode in time, the eNB can allocate the DL resources in the next DL sub-frame so the UE will have 1 ms to make the switch. In WiMAX, this would not have been possible as it has 5 ms TTI and in case of No RTG, the UEs closer to WiMAX BTS would have to be scehduled in the next frame adding another 5 ms to the latency plane.

- Secondly, the UE switching from Rx to Tx and vice versa should now be extensively reduced. The 50 us limitation was there in beceem chipsets around 4 years back while I think that beceem and intel had made chipsets with switching periods of less than 20 us last year. SO, LTE UEs should have a much lower switching times.