sing chain The following figure indicates the processing chain for the PDSCH. It is very similar to LTE. The PDSCH supports up to two codewords, which are scrambled, modulated, and then mapped onto layers. The layers are then mapped onto the Resource Elements (REs), together with the DMRS, and transmitted: Downlink physical channels and signals Modulation Resource Element OFDM Signal Scrambling Mapper Mapper Generation Layer Mapper Modulation Resource Element Scrambling OFDM Signal Mapper Mapper Generation Figure 6.4 - The PDSCH processing chain Notice that there is no precoder defined as in LTE transmission mode 9, for instance. Depending on the device feedback, the gNB employs a suitable precoder, which is applied to both data and DMRS. PDSCH eMBB and URLLC multiplexing So, we have covered the basic structure of the downlink data channel for NR. We have talked about multiplexing the eMBB traffic with the low-latency traffic. Both are carried onto the PDSCH. We already discussed that low-latency transmissions are typically done using mini slots, which allows the transmissions to start without having to wait for a slot boundary. In NR, it is possible to share resources between eMBB and low-latency traffic. This can happen in two ways. The first one is preemption. This allows low-latency traffic to be transmitted while eMBB traffic is on the air, sharing the same resources dynamically with the eMBB traffic. The second way is to assign the low-latency traffic to frequency resources that don't overlap with the eMBB traffic. Both options are optionally indicated to all devices receiving the original eMBB traffic. Downlink Pls In the following figure, we have an example of eMBB traffic being preempted by low-latency traffic. A device receives the eMBB slot and suddenly a mini slot is scheduled with some low-latency mission- critical traffic. The situation is flagged to the device using the PI in either the same or a later slot. Why is that useful? Well, if the device knows that an OFDM symbol was punctured by other tr