The purpose of this topic is to give an overview and explanation of the relationships between antenna ports, physical transmit antennas, and receive channels. Included in this discussion is information about reference signals, PDSCH, usage of antenna ports, and beamforming. Also, this discussion is not an exhaustive listing of all the variations supported by the LTE standard, but is focused around the analysis capabilities of the 89600 VSA.
For exact information about what is possible in LTE, see 3GPP TS 36.211 and 36.213. Also, see App Note 5990-9997: Verify and Visualize Your TD-LTE Beamforming Signals.
The LTE standard defines what are known as antenna ports. These antenna ports do not correspond to physical antennas, but rather are logical entities distinguished by their reference signal sequences. Multiple antenna port signals can be transmitted on a single transmit antenna (C-RS port 0 and UE-RS port 5, for example). Correspondingly, a single antenna port can be spread across multiple transmit antennas (UE-RS port 5, for example).
Let us consider antenna ports used for PDSCH allocations since they probably have the most variations. Initially, the 89600 VSA’s LTE demodulator supported only analysis of PDSCH transmitted on Antenna Ports 0, (0 and 1), (0, 1, 2), or (0, 1, 2, 3). These ports are considered C-RS antenna ports, and each port has a different arrangement of C-RS resource elements. Various configurations are defined that use these C-RS antenna ports, including 2- or 4-port Tx Diversity and 2-, 3-, or 4-port Spatial Multiplexing.
Then beamforming support was added and single-layer PDSCH allocations transmitted on Port 5 could be analyzed. The LTE demodulator has since been enhanced to support the LTE Release 9 which added Transmission Mode 8–Dual-Layer Beamforming (i.e. beamforming + spatial multiplexing)–where PDSCH is transmitted on Antenna Ports 7 and 8 (note that single-layer beamforming in Rel 9 can also use port 7 or port 8 in addition to port 5). In Rel 10 of the standard, the new transmission mode 9 (TM9) added up to 8-layer transmissions using Ports 7-14. TM9 is supported by the LTE-Advanced demodulator.
As Ports 0-3 are indicated by the existence of C-RS, so Ports 5 and 7-14 are indicated by the UE-specific Reference Signal (UE-RS). The following is a table that summarizes the various PDSCH mappings that can be used along with the corresponding reference signal and antenna ports.
|Reference Signal||PDSCH Mapping||# layers||# physical antennas||Antenna Ports||LTE Release|
|Tx Diversity||2 or 4||2 or 4||0-1 or 0-3||8|
|Sp Multiplexing||2, 3, or 4||2, 3, or 4||0-1, 0-2, or 0-3||8|
|5, 7, 8||9|
|N-layer, N <=8||N||>= N||7-(6+N)||10|
In a MIMO or Tx Diversity configuration, each C-RS antenna port must be transmitted on a separate physical antenna to create spatial diversity between the paths. Single-layer beamforming, on the other hand, is accomplished by sending the same signal to each antenna but changing the phase of the each antenna’s signal relative to the others. Since the same UE-RS sequence is sent from each antenna, the 89600 VSA can compare the received UE-RS sequence with the reference sequence and calculate the weights that were applied to the antennas to accomplish the beamforming.
Multi-layer beamforming adds some complexity to beamforming by transmitting as many UE-RS sequences as there are layers to allow demodulation of each layer’s PDSCH data. The UE-RS sequence for each antenna port is orthogonal to the others, either in time/frequency domain or in the code domain. This can be thought of as beamforming of each layer independently. N-layer beamforming is an extension of dual-layer beamforming and supports up to 8 data layers with the ability to beamform each layer separately.
For reference, the following table lists the different LTE downlink reference signals and the antenna ports they use.
|Reference Signal||Antenna Ports||LTE Release|
|5, 7, 8||9|
In the case of a single-layer, single-antenna LTE signal (using only C-RS), there would only be one antenna port signal that could be received over the air, but in general, the reception of an LTE signal will contain a combination of multiple transmit antennas, each of which may be transmitting a combination of multiple antenna ports. The LTE standard does not specify any particular setup for transmit antennas, but since the C-RS antenna ports are used for most control channels and PDSCH, the 89600 VSA LTE demodulator uses Cell-specific RS antenna ports instead of transmit antennas when indicating transmit paths between transmitter and receiver.
The VSA denotes C-RS antenna ports by the mnemonic C-RSn on the user interface and in the documentation, where n is the antenna port number. Correspondingly, the receive channel is denoted by Rxm, where m is the measurement channel number – 1.
Together, these two endpoints constitute an transmit-receive path from transmitter to the receiver (ultimately the 89600 VSA). A transmit-receive path is denoted by C-RSn/Rxm, so for example, C-RS2/Rx1 on the MIMO Info Table shows metrics calculated from the (C-RS) antenna port 2 signal received on VSA measurement channel 2.