Ⅰ、ANTENNA PORTS Antenna ports as defined in the 4G (LTE) standard do not (necessarily) correspond to physical antennas, but are logical entities distinguished by their reference signal sequence. Multiple antenna port signals may be transmitted on a single transmitter antenna (e.g., C-RS port 0 and UE-RS port 5); similarly a single antenna port may be distributed over multiple transmitter antennas (e.g., UE-RS port 5).
Ⅱ、PDSCH transmission in 4G (LTE) As an example of antenna ports used for PDSCH distribution, they may have the most variations. Initially the demodulator only supports transmission on pairs of antenna ports 0, (0 and 1), (0, 1, 2), or (0, 1, 2, 3); these ports are considered as C-RS antenna ports, each of which has a different arrangement of C-RS resource elements. Various configurations using these C-RS antenna ports are thus defined, including 2- or 4-port Tx diversity and 2-, 3- or 4-port spatial multiplexing.
Ⅲ、Beam Assignment The single layer PDSCH assignment that can be transmitted on port 5 after the introduction of beam assignment support. Since then LTE demodulators have been enhanced to support LTE Release9 This release adds transmission Mode8 - two-layer beam fouling (i.e. beamforming + spatial multiplexing) - where PDSCH is transmitted on antenna ports 7 and 8 (please note that single-layer beamforming in Rel9 can use either port 7 or port 8 in addition to port 5). The new transmission mode in the standard Rel10 - TM9 adds up to 8 layers of transmission using ports 7-14 (LTE-Advanced demodulators support TM9).
Ⅳ、Since ports 0-3 are indicated by the presence of C-RS, ports 5 and 7-14 are indicated by UE-specific reference signals (UE-RS); the following table summarizes the various PDSCH mappings that can be used with the corresponding reference signals and antenna ports.
V、 MIMO and Tx Diversity In a MIMO or Tx Diversity configuration each C-RS antenna port must transmit on a separate physical antenna creating spatial diversity between paths. On the other hand single layer beamforming is achieved by sending the same signal to each antenna but changing the phase of each antenna signal with respect to the other antennas. Since each antenna sends the same UE-RS sequence, the received UE-RS sequence can be compared with a reference sequence and the weights applied to the antennas to accomplish beamforming can be calculated.
VI、MULTILAYER BEAMFORMING The complexity of beamforming is increased by transmitting as many UE-RS columns as the number of layers to allow demodulation of the PDSCH data for each layer. The UE-RS sequence at each antenna port is orthogonal to the other sequences, both in the time/frequency domain and in the code domain. This can be thought of as independent beamforming for each layer. n Layer beamforming is an extension of two-layer beamforming that supports up to eight data layers being able to beamform each layer separately. For reference, the following table lists the different LTE downlink reference signals and the antenna ports used.
VII.Transmit-Receive Paths For single-layer, single-antenna LTE signals (using C-RS only) there is only one antenna port signal that can be received wirelessly, but in general the reception of LTE signals will contain a combination of multiple transmit antennas, each of which may be transmitting a combination of multiple antenna ports.LTE standards do not specify any particular transmit antenna setting, but since the C-RS antenna ports are are used for most control channels and PDSCHs, the LTE demodulator uses cell-specific RS antenna ports rather than transmit antennas when indicating the transmit path between the transmitter and receiver.
