Modern communications demand higher data rates and performance.  Multiple-input multiple-output (MIMO) antenna systems, also known as multiple-element antenna (MEA) systems, achieve greater spectral efficiency for allocated radio frequency (RF) channel bandwidths by utilizing space or antenna diversity at both the transmitter and the receiver, or in other cases, the transceiver. 
In MIMO systems, each of a plurality of data streams is individually mapped and modulated before being precoded and transmitted by different physical antennas or effective antennas.  The combined data streams are then received at multiple antennas of a receiver.  At the receiver, each data stream is separated and extracted from the combined signal.  This process is generally performed using a minimum mean squared error (MMSE) or MMSE-successive interference cancellation (SIC) algorithm. 
Additionally, a downlink physical signal corresponds to a set of resource elements used by the physical layer but does not carry information originating from higher layers.  The following downlink physical signals are defined: Synchronization signal and Reference signal. 
The reference signal consists of known symbols transmitted at a well defined OFDM symbol position in the slot.  This assists the receiver at the user terminal in estimating the channel impulse response to compensate for channel distortion in the received signal.  There is one reference signal transmitted per downlink antenna port and an exclusive symbol position is assigned for an antenna port (when one antenna port transmits a reference signal other ports are silent).  Reference signals (RS) are used to determine the impulse response of the underlying physical channels.