参考：5G NR The Next Generation Wireless Access Technology，2018
9.8.1 DOWNLINK PRECODING
In the downlink, the demodulation reference signal (DMRS) used for channel esti-
mation is subject to the same precoding as the PDSCH (see Fig. 9.9). Thus, the
precoding is not explicitly visible to the receiver but is seen as part of the overall channel.
...
11.2 DOWNLINK MULTI-ANTENNA PRECODING
All NR downlink physical channels rely on channel-specific DMRSs to support
coherent demodulation. Furthermore, a device can assume that the DMRSs are
jointly precoded with the data in line with Fig. 11.7. Consequently, any downlink
multi-antenna precoding is transparent to the device and the network can, in prin-
ciple, apply any transmitter-side precoding with no need to inform the device
what precoding is applied.2
2 The device must still know the number of transmission layers, that is, the number of columns in
the precoder matrix applied at the network side.