For channel estimation different types of reference signals are in use:
– Cell specific reference signals.
– MBSFN reference signals.
– UE specific reference signals
CELL-SPECIFIC REFERENCE SIGNALS
These signals are transmitted for all non-MBSFN subframes only where subcarrier spacing is 15 kHz. In MBSFN subframes only the first 2 OFDM symbols can be used. The 10 ms long pseudo-random sequence is cell-id specific and additionally 6 different cell-specific frequency shifts are defined.
MBSFN REFERENCE SIGNALS
Only in subframes allocated for MBSFN these signals are present and are used with extended CP only. A pseudo random sequence depending on MBSFN id will be transmitted on antenna port 4.
UE SPECIFIC REFERENCE SIGNALS
In case of single antenna port (port 5) transmission the reference signals are UE specific. UE takes these signals for channel demodulation of the PDSCH. The signals are only transmitted on RBs assigned to this specific UE.