Physical signals are used by the physical layer, but do not carry information originating from higher layers.
The two uplink physical signals are demodulation reference signals and sounding reference signals.
The demodulation reference signal is associated with the transmission of a PUSCH or PUCCH channel and is used for UE synchronization and for uplink channel estimation.
The sounding reference signal is not associated with the transmission of a PUSCH or PUCCH channel.
The same set of base sequences is used for the demodulation and sounding reference signals.
There are two types of downlink physical signals: reference signals and synchronization signals.
Downlink reference signals can be classified into three types:
Cell-specific reference signals, not associated with a multicast\broadcast single frequency network (MBSFN)
MBSFN reference signals
UE-specific reference signals
One reference signal per antenna port is transmitted.
Primary and secondary synchronization signals are transmitted in the downlink to enable the UE to acquire time and frequency synchronization with a cell.
The primary synchronization signal (P-SS) identifies the symbol timing and the cell ID within a cell ID group.
The secondary synchronization signal (S-SS) identifies the cell ID group.
For more information about physical signals refer to the 3GPP LTE Technical Specifications documents at http://www.3gpp.org.