4G LTE Frame Structure : TDD and FDD frame types

The LTE frame structure is fundamental to how data is transmitted in the 4G cellular network, with different configurations for Time Division Duplex (TDD) and Frequency Division Duplex (FDD) modes. FDD uses separate frequency bands for uplink and downlink, while TDD alternates between them within the same frequency band, making the frame structure crucial to determining how resources are allocated in both modes.

As mentioned, there are two topologies used in LTE system viz. FDD and TDD. In FDD Type 1 system, different carrier frequencies such as 'Fc1' and 'Fc2' are used by transmit and receive paths at the same time instant say 't1'. In TDD Type 2 system, same carrier frequency is used in both the transmit and receive paths at different time instants say 't1' and 't2'.

LTE FDD and LTE TDD

As shown in the figure, for LTE FDD mode, 'f1' and 'f2' are one pair of carrier frequencies allocated separately for both the uplink and downlink directions. The downlink refers to transmission from LTE eNodeB to UEs and uplink refers to transmission from UEs to eNodeB.

As shown in the figure, for LTE TDD mode, same carrier frequency 'f1' is allocated in both the uplink and downlink directions. This frequency is used by both uplink and downlink at different time slots for mapping their information data. Refer FDD vs TDD LTE >> which describes LTE FDD and LTE TDD modes with examples.

4G Frame structure : FDD Type 1, TDD Type 2

As mentioned as per topologies, TDD and FDD there are two frame structures viz. type-1 and type-2 LTE frame structure. Total Frame duration is about 10ms. There are total 10 subframes in a frame. Each subframe composed of 2 time slots.

Type 1 is used as LTE FDD frame structure. As shown in the figure below, an LTE TDD frame is made of total 20 slots, each of 0.5ms. Two consecutive time slots will form one subframe. 10 such subframes form one radio frame. One subframe duration is about 1 ms. Hence LTE radio frame will have duration of about 10ms. Each radio frame will have 307200 Ts. Where in one Ts equals 1/(15000 x 2048) seconds.

Type 2 is used as LTE TDD frame structure. As shown in the figure, here radio frame composed of two half frames, each of 5ms duration resulting in total frame duration of about 10ms. Each radio frame will have total 10 subframes,each subframe will have 2 time slots. subframe configuration is based on Uplink downlink configuration(0 to 6). Usually in all the cases, subframe #0 and subframe#5 is always used by downlink. The Special subframe carry DwPTS(Downlink Pilot Time Slot),GP(Guard Period) and UpPTS(Uplink Pilot Time Slot). For the 5ms DL to UL switch point periodicity case, SS(Special subframe ) exists in both the half frames. For the 10ms DL to UL switch point periodicity case, SS exists only in first half frame.

The table mentions DL and UL configuration which determines the contents of the subframes in LTE frame structure. In the table, 'U' stands for Uplink, 'D' stands for Downlink and 'S' stands for special subframe.

Summary

The 4G LTE frame structure varies significantly between TDD and FDD, with each mode optimized for different network requirements. FDD offers simultaneous uplink and downlink channels, while TDD efficiently shares a single frequency band, allowing flexibility in adapting to traffic patterns. Both frame structures play a vital role in ensuring high performance and reliability in LTE 4G networks.

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