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4G LTE differences in FDD and TDD modes.
LTE technological differences in FDD and TDD modes of operation.

FDD and TDD are different only at physical layer which interacts with hardware. The upper layers of software don’t get impacted while switching in-between the two access modes. This saves lots of operational efficiency.

At physical layer, a different hardware architecture is involved for the two access modes.

Following is a list of most noticeable hardware architecture differences.
A dedicated frame structure is available for just TDD access mode.  This frame structure is discussed in Alt frame format section.

The Alt frame format allows TDD to coexist with other TDD systems.
Some other TDD differences are listed below.

Availability of short random access form at in TDD.
Multiple acknowledgements (bundling) on Uplink Control Channel.
Variable number of H-ARQ based on UL/DL allocation ratio.
FDD and TDD modes also differs in the synchronization signals placement in frame symbols.  

In FDD, primary and secondary synchronization signals are placed adjacent to each other in a single subframe.

In TDD,  primary and secondary synchronization signals are placed in different sub frames and are separated by two OFDM symbols.