WHAT ARE THE LTE TDD – SPECIFIC LAYER 1 CHARACTERISTICS?

The main differences of TDD at L1 compared to FDD are summarized below.

Cell search. The P-SS is transmitted in the first symbol of DwPTS, and the S-SS is transmitted in the last symbol of sub-frame #0 in each 5 ms radio half frame. Currently, after the second step of the cell search, the UE does not know whether it is FDD or TDD and hence does not decode the frame timing. However, since the relative position of the PSS/SS is different for TDD and FDD, the UE may in a third step use reference signals to determine whether it is FDD or TDD. Another alternative is to do blind decoding of the physical broadcast channel in case of TDD.

Random access  For TDD, multiple frequency bands can be allocated to random access in the same UL sub-frame in order to increase the number of random access opportunities. Furthermore, there is a fifth RACH preamble format, a short RACH of duration 133.33us which, if configured, can be received in part of UpPTS.

HARQ processes  For TDD, the number of processes varies with the allocation of sub-frames to uplink and downlink.

DwPTS content  DwPTS can be used for control, data, and reference signals. Since the DwPTS contains the P-SS in the first position, reference signals and DL control signalling can not be transmitted in the same way as in other DL sub-frames. The minimum length of DwPTS is one symbol.

UpPTS content  The UpPTS may be used for short RACH reception, data transmission, as well as transmission of reference signals for sounding and demodulation. The minimum length of UpPTS is zero, at least for large Guard Period. No control signalling is transmitted in UpPTS.