5G英文视频#8: 5G NR下行传输过程

原视频来自：https://www./watch?v=aW5nT_xW7qw&list=PLn8PRpmsu08rCL-Ejn25HMX6M6o7QjJoe&index=5。视频英文字幕根据声音自动产生，有拼写错误情况，翻译过程中做了纠正。

文中顺序依次是视频、中文译文和英文原文。

这是我们5G系列的新作，本视频中我们将讨论5G NR的下行传输过程， 包括下行共享信道（其中包括LDPC编码）、物理下行共享信道、PDSCH资源分配、不同类型的PDSCH映射，以及传输块的各种尺寸。

下行共享信道DL-SCH是承载用户数据的信道，它还承载其他信息，例如不同类型的系统信息块或SIB，对应的处理过程包括通用的步骤，例如CRC校验、码块分段 、速率匹配和级联，所有步骤都与LTE相似。与LTE的主要区别是5G使用LDPC编码，编码链路的输出对应一个码字。5G支持单个用户最多使用8层在下行链路上传输数据，这意味着它最多可以使用8流做并行传输，流数据来自一个码字或两个代码字，如果少于4层则对应一个码字 ，如果多于4层，则使用2个代码字。编码数据映射到物理下行链路共享信道（PDSCH）。

在这里，您可以看到下行链路共享信道处理的每个阶段如何映射到MathWorks 5G工具箱中的功能，例如CRC编码、码块分段、LDPC编码和速率匹配。

物理下行链路共享信道是高度可配置的，比LTE中的要多得多，我们将在下一张幻灯片中看到一些细节。eNB可以通过DCI和RRC消息配置PDSCH，其中DCI方式可以实现不同的时隙使用不同配置。我们看到这里不同的软件模块分别对应加扰，调制，层映射，用于MIMO处理的预编码和资源映射。这些都是已知的模块，几乎没有什么特殊之处，需要指出的是3GPP标准并未明确规定预编码步骤，尽管预编码处理一定会存在，预编码的细节将在本视频系列的另一集介绍。这里，我们在MathWorks22框中可以看到用于完成下行链路数据处理的代码。

这里突出显示了PDSCH处理阶段，但您可以看到在DL PDSCH阶段插入了调制参考信号（DMRS）。

LTE调制包括从QPSK到256 QAM，5GNR在下行链路上使用的调制方式与LTE完全相同，5G模块在一个功能调用中就能实现5G NR的调制。

层映射是将一个或两个码字映射到最多8层的操作，与LTE相比，该操作有所简化。如前所述。5G NR的一个码字对应最多4层，当层数大于4时才会使用第二个码字。 使用一层和两层传输时，数据的映射比较直观。

使用3层和4层传输时，每3个或4个比特数据做为一组映射成3层或4层。

对于5~8层，两个码字对应的数据在不同层之间的拆分如上图所示。

在将一个或两个码字映射到1~8层之后，需要对这些层进行预编码，有趣的是3GPP标准对下行链路没有规定做预编码。预编码是使用矩阵乘法将层映射到尽可能多的天线端口的操作。一种预编码的特殊情况是将一层映射到支持波束赋形的多个天线。对于视距传播，这可能意味着波束要指向特定的方向。

预编码的另一种情况是将多个层映射到多个天线，这种更普遍的情况有时称为空间复用。5G预编码的一个关键是与PDSCH相关联的解调参考信号(DMRS)必须采用相同的预编码，由于信道估计中包含了预编码器的影响，因此无需使UE知道预编码器，这就是为什么标准中未指定gNB使用的预编码器的原因。

如我们在上面幻灯片中所见，预编码器的输出直接或间接地映射到物理资源块。下行链路信道和信号（包括PDSCH和相关的DRMS）共享OFDM网格资源。

eNB首先将PDSCH符号映射到虚拟资源块，在把PDSCH符号映射到资源格的时候，应该避开那些保留给其他用途的资源格。

这包括所有物理信号，DRMS，信道状态信息参考信号或CSI-RS，以及相位跟踪参考信号或PTRS，其中还包括同步信号块（SSB），SSB将在本视频系列的另一集中做详细介绍。

虚拟块到物理资源块的映射可以采用交织方式或非交织方式，非交织方式映射是将每个虚拟块直接映射到物理资源网格中相同的位置。

交织映射通过在整个带宽部分上分配虚拟块来提供频率分集，交织粒度为2或4个资源块，该方案将连续的虚拟资源块分配给PDSCH，这种模式使用起始资源块的编号和需要的资源块的数量这两个参数，不但配置简单，同时还能获得频率分集带来的增益。

这里我们看到两个在时域分配PDSCH资源的示例，PDSCH可能会占用整个时隙，如网格底部所示。它也可能使用一部分时隙，这有时被称为部分时隙分配，这是5GNR的新功能。LTE总是为PDSCH分配一毫秒的完整子帧。

我们使用Mathworks 5G工具箱的用户界面交互地探索其中的一些分配选项，这里我们看的是数十个子载波间隔等于30 KHz的无线帧，总共包括20个时隙，PDSCH以蓝绿色显示在蓝线上，我们将在本系列5G的另一集中详细介绍资源网格的重置。资源块分配不必是连续的，尽管通过信令很容易分配不连续的资源块。

假设我们使用从0到20的连续的资源快，可以看到前10个时隙中有PDSCH传输，然后是5个空时隙，这是因为我们分配的时隙范围是0~9，周期是15个时隙。

让我们更改一下配置，去掉时隙6/7/9，请注意，在每个时隙中，PDSCH仅使用符号2到10， 这称为部分时隙分配。您也可以选择分配全部时隙，在这种情况下，PDSCH传输是连续的。PDSCH的参考信号以黄色显示，那些位置不可用于PDSCH映射，参考信号将在本系列5G的另一集中详细介绍。

这里我们可以看到MathWorks 5G工具箱中用于指定一个或多个PDSCH的一些参数，在实际示例中，我们已经看到了这些参数如何影响PDSCH链路。

正如我们已经看到的那样，PDSCH时隙分配可能从一个时隙的起始位置或从时隙的中间开始，这对应为两种不同的映射类型，即映射类型A和B。严格地说，PDSCH映射类型仅影响解调参考的位置信号，映射类型A 的DMRS信号是在一个时隙的第2个符号或第3个符号，映射类型B的DRMS信号是在PDSCH分配的第一个符号。映射类型A和B都支持全部和部分时隙分配，但实践中类型 B是配置部分时隙分配的首选选项，这些数据传输不是从时隙的起始位置开始，这些数据传输先分配资源给DMRS，这样可以减少处理延迟，这对于低延迟通信至关重要。

我们将在5G解释系列的另一集中详细介绍DMRS信号分配。作为本节的总结，我们想转向接收器，并探讨如何将传输块的大小通知给接收器，接收器需要知道传输块的大小，以便执行与LDPC解码相匹配的反向读取的操作，这一概念 与LTE类似，在此5G解释的视频系列的另一集中，我们将说明如何传输下行链路控制信息。但重要的是要知道5G没有用信令指示发送传输块大小，而是在信令中发送了一些其他的信息，包括调制编码方案MCS以及资源分配，哪些资源块分配给PDSCH以及OFDM符号的分配持续时间。

5G NR使用基于公式的方法来计算传输块大小，而LTE使用许多表格计算传输快大小。由于公式的定义方式包括量化，所有这些参数都有几种略有不同的配置，它们导致相同的传输块大小，但这不是问题，这使操作员可以更灵活地选择不同的参数做数据重传。有关下行数据传输的5G解释视频到此结束。

 英文：根据视频整理

This is a new episode in our series 5G explained, in this video we discussed downlink data transmission in 5G New Radio. 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.

The Downlink Shared Channel（DL-SCH） is the channel that carries user data, it also carries other pieces of information, such as the different types of system information blocks or SIB, the according chain includes the usual steps, such as CRC, Code block segmentation, rate matching and concatenation, all steps were familiar with from LTE. The main difference with LTE is the use of LDPC coding, the output of a coding chain is a code word. 5G supports transmission of up to 8 layers to single user on the downlink, this means that they can be up to 8 streams transmitted in parallel, those streams are coming from one or two code words, one code word if there are fewer than 4 layers, and two code words if there are more. The coded data is then mapped to the physical downlink share channel or PDSCH. 

Here you can seehow every stage of the downlink shared channel processing is mapped tofunctions in MathWorks 5G toolbox, you can recognize CRC encoding, code block segmentation, LDPC coding and rate matching. The physical downlink shared channel is highly configurable, much more so than in LTE, and we will see some of the details on the next slide. It is configured both by downlink control information, which can change from slot to slot, and radio resource control, which can setup some parameters as well. There isn’t much of a surprise here of virtuality, we find scrambling, modulation, layer mapping, precoding for MIMO processing and resource mapping. While those are all known blocks, there are few differences worth pointing out, chiefly the precoding step is not specified explicitly in the standard, although it is fully expected to be present, the detail of precoding will be addressed in another episode of this video series. Here we can see code for complete downlink data processing in MathWorks 22 box.

The PDSCH processing stage is highlighted, but you can also see the DL PDSCH stage as well as the insertion of the modulation reference signals or DMRS. 5G NR uses the exact same list of modulations on the downlink as LTE from QPSK through 256QAM, the NR symbol modulate function implements modulation for 5G NR in in onesimple call. Layer mapping is the operation that maps one or two code words to up to 8 layers, this operation is somewhat simplified compared to LTE, where you could see one or two code words for a given number of layers in 5G NR as mentioned earlier, anything up to 4 layers uses a single code word, anything beyond 4 layers uses a second code word.

The mapping is pretty straightforward direct for one layer, alternatively for two. Similar for3 and 4 layers, each group of 3 or 4 input bits is mapped to a set of 3 or 4layers. For 5 through 8 layers the two code words are split as shown here between the different layers. After one or two code words got mapped to between 1 and 8 layers, the layers undergo precoding, which interestingly is not specified in the standard for downlink. Precoding is the operation that maps the layers to as many or more antenna ports using a matrix multiplication with the precoder. The special case of precoding is mapping one layer to multiple antennas, which enables beam forming. For Line-of-Sight transmission, this would likely mean targeting a particular direction.

 Another case of precoding is mapping several layers to multiple antennas, this more general case is sometimes referred to as spatial multiplexing. One key aspect of precoding in 5G is that the associated demodulation reference signals or DMRS must undergo the same precoding, as aresult the UE doesn’t need to be made aware of the precoder, as the effect of a precoder is included in channel estimation, this is why the exact precoder the gNB is to use is not specified in the standard. The precoder output is then mapped to physical resource blocks either directly or indirectly, as we will see on the next two slides. Downlink channels and signals including the PDSCHand associated DRMS share the OFDM grid. PDSCH symbols are first mapped to virtual resource blocks, when map to the grid, PDSCH symbols avoid locations reserved for other purposes.

This includes all physical signals, DRMS, channel state information reference signals or CSI-RS, and phase tracking reference signals or PTRS, this also includes any resource block that is fully or even partially used by synchronization signal blocks or SSB, SSB are explained in detail in another episode of this video series. Mapping of virtual blocks to physical resource blocks can be interleaved or non-interleave, non-interleave mapping consists in directly mapping each virtual block to the same position in the physical resource grid. Interleave mapping provides frequency diversity by distributing the virtual blocks over the whole bandwidth part, the interleave granularity is 2 or 4 resource blocks, this scheme that assign consecutive virtual resource blocks to a PDSCH, a pattern that is easy to signal simply with a starting resource block and numberof resource blocks, while still getting frequency diversity.

Here we see two examples of PDSCH resource allocation in time, the PDSCH may spend the whole slot as shown at the bottom of the grid, it may also use part of a slot, this is sometimes referred to as partial slot allocation, and it is a new capability in 5G New Radio compared to LTE. As you may remember LTE always allocate a full subframe of one millisecond for PDSCH. Let us explore some of those allocation options interactively with a user interface that uses Mathworks 5G toolbox, here we are looking at tens of frames with 30 KHz subcarrier spacing, which means a total of 20 slots, the PDSCH is shown in teal on line blue and this is what I want you to look at here, we will focus on the reset of a resource grid in detail in another episode of this series 5G explained.

Resource block allocation does not have to be contiguous, although it is easier to signal when it is, let’s make it contiguous from 0 to 20, we can see PDSCH transmission inthe first 10 slots, followed by 5 empty slots, this is because we are located slot 0 through 9 with a periodicity of 15 slots, let’s change the allocation to something different, now slot 6, 7 and 9 do not have PDSCH transmission, finally notice that within each slot, the PDSCH only uses symbols 2 through 10, this is called partial slot allocation, you can choose to allocate the full slots, in which case there is no break between PDSCH transmission. Reference signals for PDSCH are shown in yellow, those locations are not available for PDSCH mapping, reference signals will be addressed in detail in another episode f this series 5G explained.

Here we can see some of the parameters available in MathWorks 5G toolbox to specify one or multiple PDSCH, we have just seen how these parameters impact the PDSCH chainin the live example. As we have just seen a PDSCH slot allocation may start atthe beginning of a slot or midway through a slot, this corresponds to two different mapping types mappings type A and B. Strictly speaking the PDSCH mapping type only affects the location of the demodulation reference signals, for mapping type A DMRS in symbol 2 or 3 of a slot. While for mapping type B, DRMS are in the first symbols of the PDSCH allocation. Both mapping types A and B support full and partial slot allocation, in practice however mapping type B is the preferred option for partial slot allocation, and specially for transmission that do not start at the beginning of a slot having DMRS at the beginning of the allocation enables to reduce processing latency, which is critical for low latency communications.

We will coverDMRS allocation in more detail in another episode of this 5G explained series. To conclude this section, we want to turn to the receiver and explore how transport block sizes are communicated to the receiver, the receiver needs to be able to figure outthe transport block size in order to perform inverse read matching an LDPC decoding, the concept is similar to LTE, in another episode of this 5G explained video series we will explain how downlink control information is transmitted. But it is important to know that the transport block size itself is not signaled, instead a few different pieces of information are signaled, they include the modulation coding scheme MCS as well as the resource allocation, which resource blocks are assigned to the PDSCH and the duration of the assignment in OFDM symbols.

5G NR uses a formula based approach to compute the transport block size, where LTE uses a number of tables. Because of the way the formula is defined including quantization, there are several slightly different configurations of all those parameters, theyresult in the same transport block size, far from being an issue, this gives the operator more flexibility in selecting different parameters for the retransmission of a packet that didn’t go through the first time. This concludes this episode of the 5G explained video series on downlink data transmission.