One of the biggest challenges in any modern wireless cellular communications system is the performance at the cell edge. This is a major reason why beamforming technology has a key role to play in delivering LTE services.
188.8.131.52 LTE downlink transmission mode support for beamforming
LTE defines many downlink transmission modes, which are listed in Table 2.4-1. Of particular interest from a beamforming point of view are transmission modes 7, 8, and 9. Release 8 introduced TM7, which supports single layer beamforming on antenna port 5. Release 9 added TM8, which supports dual layer beamforming on antenna ports 7 and 8. Finally, Release 10 added TM9, which supports up to eight layer transmission on antenna ports 7 to 14.
It should be noted that the ports mentioned above are all virtual antenna ports representing particular configurations of reference signals. The physical geometry and number of antenna elements are not defined in the LTE specifications. In practice each virtual port’s physical realization may comprise four or more spatially separated physical antenna elements.
The following examples focus on TM7 and TM8, which are the focus of development for initial TD-LTE market deployments.
184.108.40.206 Signal processing for TM7 and TM8
A summary of the defined downlink signal processing flow for TM7 and TM8 is shown in Figure 6.9-6. As with other transmission modes, the PDSCH data transport block information is channel-encoded and has rate matching applied, producing either one or two code words, which are then mapped onto layers.
It's worth noting that for TM7 and TM8, the precoding block is not codebook based. It is left up to the eNB to determine the optimal beamforming precoding to apply. This coding can be derived by the eNB from direct measurement of the received uplink sounding reference signal, and can include the use of any configured UE channel feedback (CQI/PMI/RI) information. Also worth noting is that the beamforming precoding can be dynamic and vary on a per subframe and resource block basis to adapt to changing channel conditions.
For demodulation purposes, TM7 and TM8 include the mapping of UE-specific reference signals (UE-specific RS), also known as demodulation reference signals (DMRS) in each PDSCH resource block. The E-specific RS undergo the same beamforming precoding as the associated PDSCH. This concept is shown in Figure 6.9-6 where the UE-specific RS feed into the precoding block. The beamforming precoding is calculated primarily to maximize the SINR observed by the target UE, but the precoding will also attempt to minimize interference to other UE within the serving or adjacent cells.
Click image to enlarge
Figure 6.9-6. Signal processing flow for TM7 and TM8
In addition to producing user-specific beam patterns, the base station has the ability to choose a different sector-wide broadcast beam pattern for common control channel content, which is received by all UE within the cell. This beamforming of the control channels is possible when the number of beamforming antenna elements is greater than the number of configured cell RS ports, as shown in Figure 6.9-6.
220.127.116.11 LTE UE-specific reference signals structure
To support beamforming for TM7, TM8, and TM9, UE-specific RS are defined for port 5 and ports 7 through 14. The physical structure of the UE-specific RS is shown in Figure 6.9-7 for the TM7 and TM8 cases.
Figure 6.9-7. UE-specific RS structure for TM7 and TM8
Transmission mode 7 supports only single layer beamforming transmissions. For this purpose port 5 UE-specific RS resource element mappings are defined in time and frequency within each scheduled PDSCH resource block assignment as shown in Figure 6.9-7. Since the UE-specific RS undergo the same beamforming weight precoding as the associated PDSCH data, it is possible for the target UE to directly demodulate the precoded PDSCH using the similarly precoded UE-specific RS as the reference.
Transmission mode 8 extends beamforming to dual layer spatial multiplexing. For this purpose ports 7 and 8 UE-specific RS resource element mappings are defined. Each port corresponds to a different spatially multiplexed MIMO transmission layer. It is worth noting that the same physical resource elements are used by both port 7 and port 8. In order that the UE can correctly separate these simultaneously transmitted UE-specific RS, orthogonal UE-specific RS sequences are used.
The orthogonality of UE-specific RS resource mappings for ports 7 and 8 are further extended using a combination of frequency division multiplexing (FDM) and code division multiplexing (CDM) resources in order to support ports 9 through 14 as required for TM9. From a test point of view, it is essential that the UE-specific RS content for TM7, TM8, and TM9 be verified for baseband correctness as well as for relative magnitude and phase weighting accuracy observed at the calibrated RF output of the antenna element array.