Demodulation Reference Signals (DMRS) in 5G NR

This is a new episode of our series, "5G Explained." In this video, we discuss the modulation reference signals, or DM-RS, in 5G in your 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 5GNR. 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 pause 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 pause 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 three 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 web DM 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 type 1 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 the location 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 core set, which carries the PDCCH with downlink control information. As the core set is 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.