To demodulate different downlink physical
channels coherently, the UE requires complex valued channel estimates for each
subcarrier. Known cell-specific reference symbols are inserted into the
resource grid. The cell-specific reference signal is mapped to REs spread
evenly in the resource grid, in an identical pattern in every RB.
When transmitting with several antennas, each
antenna must transmit a unique reference signal. When one antenna transmits its
reference signal, the other antenna must be silent. The mapping of the
cell-specific reference signal on the resource grid therefore depends on the
antenna configuration, see Figure. The pattern of cell-specific reference
signals can be shifted in frequency compared to figure below. Which one of the
six possible frequency shifts to use depends on the Physical Cell Identity
(PCI) sent on Primary Synchronization Signal (PSS) and Secondary
Synchronization Signal (SSS).
Holes are REs that must be silent because the
cell-specific reference signal is transmitted on another antenna port. With one
antenna port, the number of REs in one scheduling block occupied by the
cell-specific reference signal and holes is 8. With two antenna ports the
number is 16. The following table shows the total number of REs occupied by the
cell-specific reference signal and holes, for the bandwidths available:
