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原视频来自：https://www./videos/5g-explained-demodulation-reference-signals-in-5g-nr-1566973057735.html。公众号作者进行了整理和翻译，欢迎指正和讨论。 文中顺序依次是视频、中文译文和英文原文。 中文： 这是“ 5G的解释” 系列的新内容。在本视频中，我们将讨论5G无线中的调制参考信号(DM-RS)。
DM-RS并不是为5G NR定义的唯一的物理信号。这里列举了下行链路的物理信号，其中一些在“5G解释”视频系列的其他章节中做过介绍。DM-RS用于信道估计和相关物理信道的解调。它们也可以用于估计接收信号功率，就像LTE中一样。 DM-RS伴随5G NR中的每条信息，因为3GPP标准假定传输中会使用预编码。由于数据和DM-RS都经过相同的预编码，因此接收机的信道估计包括了传播信道和预编码的影响。因此，接收机通过DM-RS就得到了预编码方案。 我们在有关下行链路数据的章节中提到，PDSCH映射有两种类型：类型A和类型B。类型A表示DM-RS位于时隙的符号2或3中。与LTE的PDSCH从OFDM符号3 开始分配资源相比，5G NR的PDSCH分配从OFDM符号0开始分配更有意义。 请注意，在两种配置中，DM-RS都接近于所分配资源的起始位置。 对于映射类型A，DM-RS是在符号2还是在符号3，是由5G工具箱中映射类型A的高层参数DL_DMRS_typeA_pos指定的。左侧图片的参数设置为2，右侧图片的参数设置为3。请记住，OFDM符号编号从0开始，而不是1。 DM-RS可以灵活地分配在1个OFDM符号或是2个OFDM个符号上。当分配使用2个OFDM符号时，DM-RS存在于两个连续的OFDM符号上，从而使DM-RS的数量加倍。使用1个OFDM符号分配还是2个OFDM符号分配，由5G工具箱中的参数DL_DMRS_max_len表示。左边的图片表示参数设置为1个符号，而右边图片表示参数设置为2个符号。 接下来，我们要查看每个时隙中包含的DM-RS的符号数量。到目前为止的讨论假定只有一个。实际上，3GPP标准允许每个时隙最多使用3个附加的OFDM符号。DM-RS的附加符号数是一个高层参数，由5G工具箱中的DL_DMRS_add_pos表示。左侧的图片将此参数设置为0，接下来的三张图片分别表示使用1、2和3个附加OFDM符号。
正如我们已经看到的，每个时隙中DM-RS最多可以使用4个OFDM符号。更加频繁的DM-RS类型有助于更好地跟踪快速变化的信道，高速移动场景是主要应用之一。当子载波间隔等于15 kHz时，一个时隙为1毫秒，每个时隙4个OFDM符号,这样的DRMS配置密度与LTE相同。当子载波间隔增大时，随着时隙的缩短，在时域中DM-RS密度会更高。
现在我们知道在使用DM-RS时，每个时隙最多可以有三个附加的OFDM符号，我不得不提到，只有在一个时隙中分配给PDSCH或PUSCH的符号足够多时才有意义。在这里，我们可以看到使用映射类型A的PUSCH分配的DM-RS符号的位置。您可以看到，第一个DM-RS符号始终是时隙的第2个或第3个符号。
对于PUSCH映射类型B，这里的主要区别是，第一个DM-RS符号始终是PUSCH分配的第1个符号。当PUSCH长度大于10个符号时，则最多可以配置4个DM-RS符号。
当我即将介绍DM-RS配置的最后一个参数DM-RS类型时，我想把这个概念与PDSCH或PUSCH映射类型进行对比。这两种类型是完全无关的。提醒一下，PDSCH或PUSCH映射可以是A或B类型，对于映射类型A，第1个DM-RS符号是在一个时隙的OFDM符号2的位置，或在OFDM符号3的位置。对于映射类型B，第1个DM-RS符号分配在第1个OFDM上。
该幻灯片显示了类型1和类型2的DM-RS模式和频率。左侧的类型1对应于频域上被DM-RS符号占用的所有资源单元。右侧的类型2显示了在每6个连续的资源单元上，DM-RS符号占用2个连续的资源单元。 类型2支持更多数量的正交信号，这适合于多用户MIMO。这两种类型考虑了密度、频率以及所支持的正交DM-RS序列数量之间的权衡。使用类型1还是类型2由5G工具箱中的DL_DMRS_config_type表示。左边的图片将此参数设置为1，而右边的图片将其设置为2。
每种类型支持多少个正交序列？要回答这个问题，我们首先必须认识到可以通过三种方式实现正交性：时间、频率和正交码。每个资源块可以使用两个不同的正交码。
对于类型2，当使用1个OFDM符号时可以使用6个正交信号，当使用2个OFDM符号时可以使用12个正交信号。
此幻灯片显示了1个符号DM-RS类型1的所有4个正交可能性。左边的前两个明显地与右边的两个正交，因为它们被分配给不同的资源单元。它们在频率轴上偏移1。 图片上看不到左侧的两个是正交的。正交的方法是将这些信号分配给的不同的天线端口，端口1000和端口1001，如5G工具箱中通过PortSet参数所示，该PortSet参数分别设置为0和1,这两个端口承载彼此正交的DM-RS值。右侧的端口1002和1003同样适用。
上面是完全相同的视图，是从空间的角度观察。由于每个栅格对应一个天线端口，因此该视图很明显地证明了这些栅格在时间和频率上是并发的。请注意，这些栅格在预编码后会映射到天线，因此在这里我们不看每个天线的视图。而是从天线端口的角度来看。
这是DM-RS类型2的视图。您可以看到3种集合，每个集合在频域中具有不同的DM-RS偏移。该图显示了对6个天线端口的支持，但请记住，使用2个符号的DM-RS最多支持12个天线端口。
在这里，我们研究具有30 kHz子载波间隔的下行5G波形的资源网格。资源网格在x轴上显示了一个子帧的数据。对于频域部分，y轴上有15个资源块或180个子载波。
我们可以看到全频带上PDSCH(用浅蓝色表示)的位置占据了所有15个资源块，并且仅使用每个时隙的前11个OFDM符号。
PDSCH映射类型为类型A，这意味着DM-RS从时隙的OFDM符号2的位置或OFDM符号3的位置开始。
绿色矩形显示为CORESet保留的空间，该空间携带带有下行链路控制信息的PDCCH。由于CORESet的长度为三个OFDM符号，因此可以从OFDM符号3开始配置DMRS, 而不是从OFDM符号2开始。
如果我们将PDSCH资源分配更改为类型B，则DM-RS始终从分配给PDSCH的资源的第一个OFDM符号开始。对于部分时隙分配，这尤其有意义，尤其是当分配不是从时隙的开头开始时。
让我们更改分配以占用OFDM符号5~10。现在，我们可以在PDSCH分配的第一个符号中看到DM-RS。
现在让我们看看其他的DM-RS位置。让我们回到使用OFDM符号0~10，PDSCH分配类型A。
现在，我们可以为每个时隙设置1个附加的DM-RS。
每个时隙设置2个附加的DM-RS。
每个时隙设置3个附加的DM-RS，但是第3个附加DM-RS没有配置成功。正如我们在上一张幻灯片中看到的那样，仅当PDSCH分配覆盖12个或更多符号时，才支持每个时隙配置3个附加的DM-RS。
让我们将分配更改为OFDM符号0~12。现在，每个时隙中都有一个完整的DM-RS符号。
最后，让我们将DM-RS类型从1切换到2。我们可以看到两个连续的资源单元位于两种频域分配之间的4个资源单元之间。
在类型1中，每2个资源单元就有1个分配给DM-RS，使得DMRS密度更高，但频域上可选的配置变少。关于DM-RS的讨论到此结束 英文： This is a new episode of our series, '5G Explained.' In this video, we discuss the modulation reference signals, or DM-RS, in 5G New Radio. We will look at what DM-RS are used for and detailed parameters that configure their number and positions, including PDSCH mapping type, single versus double symbols, additional DM-RS, as well as types 1 and 2.We will discuss the application of these configurations to different beamforming scenarios. DM-RS are not the only physical signals defined for 5G NR. This slide shows a list of physical signals for downlink and some of them are addressed in other episodes of these '5G Explained' video series. DM-RS are used for channel estimation and demodulation of associated physical channels. They can also be used to estimate receive signal power, as is already the case in LTE. CSI-RS, or channel state information reference signals, help the receiver provide estimate of the channel that can be exploited for resource allocation, beamforming, and beam management. PT-RS, or phase tracking reference signals, are used for phase tracking, which is of particular importance in millimeter wave transmission where phase noise is more prevalent. Finally PSS and SSS, the primary and secondary synchronization signals, play a key role in synchronization and cell search procedures, as explained in detail in another episode of this series. DM-RS accompany every piece of information in 5G NR because the standard assumes precoding is used.As both data and DM-RS go through the same precoding, channel estimation at the receiver includes the effect of both the propagation channel and precoding. Therefore, DM-RS made precoding transparent to the receiver. This is very similar to DM-RS in LTE for transmission mode 9, corresponding to ports 7 through 14 transmission. When sent along PDSCH, DM-RS only occupy resource blocks in that part of PDSCH allocation. The number, position, and density of DM-RS are highly configurable, contrary to what was the case with LTE, and the next slide explain in detail those configurations. You may recall from the episode of this '5G Explained' series about downlink data that there are two types of PDSCH mapping: type A and type B. Type A implies that DM-RS are located in symbol 2 or 3 of a slot. Naturally, this makes more sense that PDSCH allocation starts at symbol 0, as opposed to past symbol 3. Type B implies that DM-RS are located in the first symbol of a PDSCH allocation. This makes sense in all cases, but particularly so when the allocation starts midway through a slot. This case is likely to require short latency, and having the DM-RS right away makes for the fastest possible demodulation. This slide summarizes those statements. Note that in both configurations, DM-RS are toward the beginning of the allocation. For mapping type A,whether DM-RS are to be found in symbol 2 or symbol 3 is specified by a higher layer parameter represented by DL-DM-RS type A position in 5G Toolbox. The picture on the left has this parameter set to 2, where the picture on the right has it set to 3. Remember that symbol numbering starts at 0 and not 1. Another degree of flexibility in allocating DM-RS is whether to allocate single symbols or double symbols. When allocating double symbols, DM-RS are present in two consecutive symbols, thereby doubling the number of DM-RS. Whether single or double symbol allocation is used, it's a higher layer parameter represented by DL-DM-RS max length in 5G Toolbox. The picture on the left has this parameter set to 1, or single symbol, while the picture on the right has it set to 2, or double symbols. Next, we want to look at the number of symbols with DM-RS per slot. The discussion so far assumed there was only one. In fact, the standard allows for up to three additional symbols per slot. The number of additional symbols with DM-RS is a higher layer parameter, represented by DL-DM-RS at pos in 5G Toolbox. The picture on the left has this parameter set to 0 and the next three pictures show the layout with 1, 2, and 3 additional symbols respectively. As we have just seen, we can have up to four symbols with DM-RS per slot. More frequent DM-RS in type helps track a fast-varying channel better, hence one of the main applications for th ree additional DM-RS symbols is high-speed scenarios. Also note that at the 15 kilohertz sub-carrier spacing, once slot is 1 millisecond and 4 symbols per slot correspond to density of several reference symbols in LTE. At higher sub-carrier spacings, as the slot is shorter, the DM-RS density in the time domain is higher. Now that we know that we can have up to three additional OFDM symbols per slot with DM-RS, I have to mention that this is only the case for PDSCH or PUSCH allocations that occupy enough symbols in a slot, which should make a lot of sense. Here, we can see the position of DM-RS symbols for PUSCH allocations that use mapping type A. And as you can see, the first DM-RS symbol is always the second or third symbol of the slot. If one additional DM-RS symbol is configured, its position in time depends on whether the PUSCH is allocated 8 or 9, 10 to 12, or more than 13 symbols. If the allocation is shorter than eight symbols, no additional DM-RS can be allocated. Similarly, the second DM-RS can only be added if the allocation is at least 10 symbols long, and its position depends on whether the allocation is more or less that 13 symbols long. Finally at the bottom of the slide, you can see the positions of addition DM-RS when three additional DM-RS are requested. The only configurations where you can have four symbols with DM-RS are other ones with at least 12 symbols allocated to the PUSCH. Here we have the exact same view, but for PUSCH mapping type B. The main difference here is that the first DM-RS symbol is always the first symbol of the PUSCH allocation. You can have up to four DM-RS symbols as soon as the allocation is at least 10 symbols long. As I'm about to introduce the last parameter for DM-RS configuration, the DM-RS type, I want to contrast it right away with the PDSCH or PUSCH mapping type. Those two types are completely unrelated. As a reminder, PDSCH or PUSCH mapping can be type A or B, and it applies whether the first DM-RS symbol is at position 2 or 3 of the slot, or in the first symbol of the allocation, respectively. DM-RS type 1 and type 2 are something completely different. They specify the density of DM-RS in the frequency domain and they impact a number of possible orthogonal sequences. This slide shows the DM-RS pattern and frequency for type 1 and type 2. Type 1 on the left corresponds to every other resource element in frequency being occupied by a DM-RS symbol.Type 2 on the right shows two consecutive resource elements occupied by DM-RS symbols out of each group of six resource elements. Therefore, type 1 has a denser occupancy at 50% of the resource estimates, versus one-third of the resource elements for type 2. On the other hand, you can only have two such columns of type 1 DM-RS, whereas there can be three different sets of type 2 DM-RS as there are two more possible positions for a set of two DM-RS in each group of six resource elements. This means that type 2 supports a larger number of orthogonal signals, which is more suitable for multi-user MIMO. These two types correspond to a trade-off between density and frequency and the number of orthogonal DM-RS sequences supported. Whether type1 or 2 is used is a higher layer parameter, represented by DL-DM-RS config type in 5G Toolbox. Picture on the left has this parameter set to 1, while the picture on the right has it set to 2. How many orthogonal sequences are supported for each type? To answer this question, we first have to recognize that orthogonality can be achieved in three ways: via time,frequency, and code. The code component gives us a set of two DM-RS as the basic unit, meaning that we can have two different orthogonal DM-RS per resource block. The time component is single symbol versus double symbol, giving us an additional factor of 1 or 2. Finally, type 1 let us place those symbols at two possible locations in the frequency domain, giving another factor of 2. Therefore, type 1 can have 4 orthogonal signals for a single symbol and 8 for double symbols. Similarly for type 2, the numbers come out to be 6 and 12 respectively, because there are three possible positions in frequency for each group of two resource elements, as discussed earlier. This increased number is motivated by multi-user MIMO, where the total number of layers can be larger than in the single user case. This slide shows all four orthogonal possibilities for a single symbol DM-RS type 1. The first two on the left are clearly orthogonal to the two on the right because they are allocated to different resource elements. They're shifted by 1 on the frequency axis. The fact that the two on the left are orthogonal to each other is not visible on the picture. The only way to spot it is the fact that the antenna port those signals are allocated to are port 1000 and port 1001 respectively, as shown in 5G Toolbox by the port set parameter, which is set to 0 and 1 respectively. Those two ports carry DM-RS values that are orthogonal to each other. The same applies to ports 1002 and 1003 on the right. Here is the exact same view, but in space. As each grid corresponds to one antenna port, this view makes obvious the fact that those grids are concurrent in time and frequency. Note that those grids get mapped to antennas after precoding, so we are not looking here at the view for each antenna; rather, this is the view from an antenna port perspective. Here is the exact same view for DM-RS type 2. You can see the three sets of two grids. Each one of the sets has DM-RS offset by two resource elements in the frequency domain. This picture shows support for six antenna ports, but remember that up to 12 antenna ports are supported with double symbol DM-RS. Let's review some of those modes in an interactive example, written with MathWorks 5G Toolbox. Here, we are looking at the resource grid for downlink 5G waveform with a 30 kilohertz sub-carrier spacing.The resource grid shows one sub-frame worth of data on the x-axis. For bandwidth part, 15 resource blocks, or 180 sub-carriers on the y-axis. We can see full band allocation of PDSCH, occupying all 15 resource blocks while using only the first 11 symbols of each slot. The PDSCH mapping type is type A, which means that the DM-RS starts at position 2 or 3--3 here--of the slot. The green rectangles show space reserved for the CORESET, which carries the PDCCH with downlink control information. As the CORESET three symbols long, it makes sense to start the DM-RS on symbol 3 and not 2. If we change the PDSCH resource allocation to type B, DM-RS always starts with the first symbol of the PDSCH allocation. This makes most sense for partial slot allocation, and in particular when the allocation doesn't start at the beginning of the slot. Let's change the allocation to occupy symbol 5 through 10. We can now see DM-RS in the first symbol of the PDSCH allocation. Let's now look at additional DM-RS positions. Let's switch back to using symbols 0 through 10 and PDSCH allocation type A. We can now request one additional DM-RS per slot. Now two. Now three. What happened? As we saw in the previous slide, three additional DM-RS per slot is only supported when the PDSCH allocation covers 12 or more symbols. Let's change the allocation to cover symbols 0 to 12. Now we get a full DM-RS symbol in each slot. Finally, let's switch the DM-RS type from 1 to 2. We can see that two consecutive resource elements are located with a gap of four resource elements between two allocations in frequency. With type 1, every other resource element is allocated to a DM-RS, leading to a higher density but fewer possible configurations. This concludes this episode of the '5G Explained' series on DM-RS.
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