The major differences between FDD and TDD mode of operation are found in the following areas:
• eNodeB synchronization and coordination
To avoid interference between UL and DL on a TDD carrier, especially eNodeB-to-eNodeB interference, the eNodeBs are preferably synchronized in time and use the same allocation of sub-frames to UL and DL with large enough guard periods.
• Efficient co-existence between TDD systems on adjacent carriers
To avoid interference between UL and DL TDD systems operating on adjacent carriers, an efficient solution is to synchronize the two systems to a common time reference and use allocations of sub-frames to UL and DL on each carrier so there is no overlap between them in time.
• L1/L2 control signaling and HARQ operation
In the general case, there is no one-to-one association between DL and UL sub-frames, and this has a major impact on the control signalling design as well as on latency and HARQ operation.
• LTE TDD specific L1 characteristics
There are some differences between LTE TDD and FDD when it comes to the physical layer: related to the synchronization signals, random access preambles, use of the special fields DwPTS and UpPTS as well as guard periods.
• UE transmit power
A TDD UE could take advantage of the fact that a duplex filter, with an insertion loss of 1.5-3.5dB, could be avoided to increase the UE peak power from the 24dBm currently agreed for TDD and FDD. This could then compensate for the lower user plane coverage due to the fact that the UL is not continuous.
• Channel reciprocity
Since UL and DL use the same frequency, the instantaneous UL and DL radio channels may be highly correlated, at least within the coherence time of the channel. So far, the main use of reciprocity in an implementation appears to be direction of arrival estimation for DL beamforming, with more than four transmit antennas using UE specific reference signals. However, a parameter like Difference of Arrival (DoA) is reciprocal also for FDD, and a UE specific reference signal is defined for both FDD and TDD.