How is Basic Frame Structure in LTE ?
- In FDD, the DL and UL Radio Frames (RFs) are not on the same carrier.
- The RF frame is called Type 1 by the 3GPP.
- The RF length is 10 ms.
- The radio frame is made up of 10 sub-frames of 1 ms.
- Each sub-frame is made up of 2 slots of 0.5ms.
For FDD, 10 subframes are available for downlink transmission and 10 subframes are available for uplink transmissions in each 10 ms interval. Uplink and downlink transmissions are separated in the frequency domain.
A Frame structure type 2 is also defined and is applicable to TDD
Each slot is made up of:
- 7 symbols in case of normal CP (guard time between symbols)
Tu = Useful Symbol Duration
Tcp = Cyclic Prefix duration
Tecp = Extended Cyclic Prefix duration
Since OFDM offers a better flexibility in terms of sub-frame structure and pilot allocation, there is no reason to consider the same structure as for DFT-SOFDM.
- The frame structure for the type 2 frames used on LTE TDD is somewhat different.
- The 10 ms frame comprises two half frames, each 5 ms long.
- The LTE half-frames are further split into five sub frames, each 1ms long.
The subframes may be divided into standard subframes of special subframes.
The special subframes consist of three fields:
- DwPTS – Downlink Pilot Time Slot
- GP – Guard Period
- UpPTS – Uplink Pilot Time Stot.
These three fields are also used within TD-SCDMA and they have been carried over into LTE TDD (TD-LTE) and thereby help the upgrade path. The fields are individually configurable in terms of length, although the total length of all three together must be 1ms.