![]() ![]() The precoder output is then mapped to physical resource blocks, either directly or indirectly, as we will see on the next two slides. This is why the exact precoder the G node b is to use is not specified in the standard. As a result, the UE doesn't need to be made aware of the precoder, as the effect of a precoder is included in channel estimation. One key aspect of precoding in 5G is that the associated demodulation reference signals, or DM-RS, must undergo the same precoding. This more general case is sometimes referred to as spatial multiplexing. For line-of-sight transmission, this would likely mean targeting a particular direction.Īnother case of precoding is mapping several layers to multiple antennas. The special case of precoding is mapping one layer to multiple antennas, which enables beam forming. Precoding is the operation that maps the layers to as many or more antenna boards using a matrix multiplication with a precoder. For five through eight layers, the two codewords are split, as shown here, between the different layers.Īfter one or two codewords got mapped to between one and eight layers, the layers undergo precoding, which, interestingly, is not specified in the standard for downlink. Similarly, for three and four layers, each group of three or four input bits is mapped to a set of three or four layers. The mapping is pretty straightforward-direct for one layer, alternatively for two. Anything beyond four layers uses a second codeword. In 5G NR, as mentioned earlier, anything up to four layers uses a single codeword. This operation is somewhat simplified compared to LTE, where you could see one or two codewords for a given number of layers. Layer mapping is the operation that maps one or two codewords to up to eight layers. The NR symbol modulate function implements modulation for 5G NR in one simple call. 5G NR uses the exact same list of modulations on the downlink as LTE, from QPSK through 256QAM. The PDSCH processing stage is highlighted, but you can also see the DL-SCH stage, as well as the insertion of demodulation reference signals, or DM-RS. Here we can see code for complete downlink data processing in MathWorks 5G Toolbox. The detailed precoding will be addressed in another episode of this video series. Chiefly, the precoding step is not specified explicitly in the standard, although it is fully expected to be present. While those are all known blocks, there are a few differences worth pointing out. We find scrambling, modulation, layer mapping, precoding for MIMO processing, and resource mapping. There isn't much of a surprise here compared to LTE. It is configured both by downlink control information, which can change from slot to slot, and radio resource control, which can set up some parameters as well. And we will see some of the detail on the next slide. The physical downlink shared channel is highly configurable, much more so than in LTE. You can recognize CRC encoding, code block segmentation, LDPC coding, and rate matching. Here you can see how every stage of the downlink shared channel processing is mapped to functions in MathWorks 5G Toolbox. The coded data is then mapped to the physical downlink shared channel or PDSCH. ![]() Those streams are coming from one or two codewords-one codeword if there are fewer than four layers, and two codewords if there are more. This means that there can be up to eight streams transmitted in parallel. 5G supports transmission of up to 8 layers to a single user on the downlink. The output of a coding chain is a codeword. The main difference with LTE is the use of LDPC coding. The coding chain includes the usual steps, such as CRC, code block segmentation, rate matching, and concatenation-all steps we're familiar with from LTE. It also carries other pieces of information, such as the different types of system information blocks, or SIB. The downlink shared channel, or DL-SCH, is the channel that carries user data. We will look at the downlink shared channel chain, which includes LDPC coding, the physical downlink shared channel chain, our resource elements are allocated for PDSCH transmission, the different types of PDSCH mapping, and conclude with a quick word on transport block sizes. This is a new episode in our series, "5G Explained." In this video, we'll discuss downlink data transmission in 5G New Radio. ![]()
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