Lecture 41 : 5G demodulation reference signal (DM-RS) design – part I

Okay, welcome to the class and you see this topic pi g demodulation reference signals. We discussed how they are used in the last class okay. Now we will see how they are defined in the standard okay. There is lot of flexibility which the standard provides while defining them and we will see why such flexibility is there and how to use that okay. That's the key idea and the main motivation behind this class and the next class okay. So just to recapture what are three different kind of reference signals which are defined in the standard sounding reference signal okay. Ruppling channel estimation okay. This is uplink focused correct and what is its downlink counterpart that is CSIRS okay for downlink okay. Then DMRS okay before we talk about DMRS we discussed the use of CSIRS for MCS calculation okay. Please remember that this is not the only use okay. The use for multiple other tasks. I have not told that to you till now okay. So if I get an opportunity we will discuss other uses of CSIRS also okay but do not get like tunneled by that effect that this is only used for MCS calculation. It has wide variety of uses okay. So this is CSIRS we have seen that also and finally what we saw was DMRS okay for channel estimation for demodulating data and this DMRS they are transmitted in both downlink and uplink okay. Key difference between this and other two is they are transmitted before data transmission happens this is transmitted along with data. So data can be PDSCH or PUSCH okay even if it is PUSCH transmission DMRS will be transmitted because the use of DMRS is to give instantaneous channel information to the receiver. So the receiver can be user or the base station does not matter okay right. So this is used for both downlink and uplink and similar use okay like the idea remains the same because the end objective is the same okay right. So now we are going to focus how this these DMRS are defined in the standard okay. That is what we will look at all the gory details of how they are defined, why they are defined like that okay right. So let us get going with that and these are just few general facts about DMRS okay we know how DMRS are used right okay whether it is like single layer transmission or multiple layer transmission. We discussed they are used for nl is equal to 1 and nl is equal to 2 but it does not matter once you have learnt it for nl is equal to 2 you can scale it up for any nl idea remains the same okay right. Now first key aspect which I did not tell you till now and it was not even required for our previous discussion that it is they are located early in the transmission and what do we mean by that okay that like even before the data comes DMRS will come okay. I will show you the pictures okay how they are how they come okay and why so they come early in the transmission and they help in achieving low latency this I will show you how we will discuss this aspect okay. So they are coupled they are coming they come early in the transmission it helps in achieving low latency okay right so sometimes they are known as front loaded reference signals okay loaded in front of data okay correct and so that is how they are one configuration I am showing you okay what is this type A I will come to it but what is this this is 1 PRB right okay 1 PRB 1 PRB over 14 symbols okay so this is the frequency axis and this is the symbol axis time axis correct 14 symbols look at this 0 to 13 index is running right time index and this is the RE index and the subcarrier index okay right. So you see this what is happening here the DMRS is coming in the third symbol okay correct and let us see this so why it is not coming in the first 2 symbols any guesses on that why because typically the first 2 symbols are used for PDCCH okay right and this aspect is mentioned later. So this is basically used for PDCCH and they have their own reference signals we discussed that core set remember that okay right the way this core set was defined it had its own reference signals right and UV uses those reference signals to decode its DCI right. So this is independent okay there is nothing to do with the design of this is separate of course it has everything to do with what comes later okay but the design of this is completely independent from this that is the aspect one has to keep in mind okay. Now so what is this front loaded you see this is basically the data is coming okay I will explain all these terms this term mapping type A I will explain everything in detail but look at point I am trying to make here is that this is where the data is coming okay right and you see where DMRS is coming correct okay. So of course by now UV knows where its data is located because it is decoded its DCI it has figured out its data is coming here correct okay. Now as soon as it starts receiving this data in the downlink what will it do it will basically use this DMRS to start estimating its channel does not wait for the rest of the transmission to happen to it does not wait for this reception okay correct right. So it gets this DMRS starts estimating the channel right so and what is happening with this you are basically cutting down on the processing delay correct if this DMRS instead of this front loading if you make it loaded at the back you will have to wait for the complete 14 symbols to come and then only you can start estimating the channel why right okay because you cannot do anything without estimating the channel right why make the receiver wait for this DMRS to come. So typically they come in the beginning of the of this PDSCH transmission front loaded low latency all these objectives are achieved okay because okay this is not an objective front loaded and loaded in the front this low latency objective is achieved right okay correct. So it allows the receiver to obtain a channel estimate early and it what happens it processes receive signal on the fly without buffering and complete slot prior to data processing right. So let us look at this reduced memory requirement it can start estimating the channel estimating the channel then it can process the data of each symbol does not have to wait for this complete slot to arrive and then process its data is it cuts down on the memory requirement for the people who are working on implementing these modules you should ask them okay then how much of memory it saves okay right. So and this is how you do your cheaper receiver design okay you have cut down on the memory correct you have cut down on the delay okay right. So all these are this is basic common sense nothing more than that there is no like what fine engineering involved in that okay good fine okay I mean these are the key aspects okay. So you see the standards are designed like that most of the time it is common sense okay and these things only work. But of course it does not mean that then like you should not know about the fancy things because you saw what MIMO transmission you saw all this how all these like control coding interleaving they are fine nice modules okay you should know communication theory to understand that okay these ideas common sense okay. Now there are two different time domain DMRS structures are defined okay and they are type A and type B. So you see this type A I will explain what this type A and type B are okay each one of them has its own use. So you see this we are getting into the details of how DMRS are defined okay we just know as of now how to use them correct okay how they are defined why they are defined we are starting to see that okay and see another thing is that there is only one DMRS coming over here okay you estimate the channel and then you use it for the complete PRB. What is the basic assumption you are making here that the channel does not channel does not change for all these symbols correct okay and so if channel changes it will change okay if UV is moving and at the speed of 500 kilometer per hour or any like high speed okay so it will change okay even at like 50 kilometer per hour channel changes okay. So how DMRS allocation is done in that case that is what all these type A and type B are okay so basic idea will always be like it will be used for demodulating the data okay how they are configured and how what frequency they are going to come that is what these type A and type B are okay fine. So let us see that okay so what is we are going to focus first on mapping type A okay there are two types type A and type B okay we will look at type A mapping okay and what am I saying here the type A mapping is intended when PDSCH occupy most of a slot okay you have like the data is such that the PDSCH occupies complete slot or like most of it okay what could happen where this okay wherein there are other kind of transmission just it will make sense when I briefly tell you about type B okay that is used for URLC transmission okay URLC transmission can come anyway in a slot okay and it will occupy a small part of the slot because some very like low latency data but not high bandwidth data or not high amount of data okay right. So that is where type B mapping is used we will see all that also but keep I have to tell about type A so I am telling you about type B also okay. Type A is normal transmission okay wherein like high bandwidth transmission but not emergency transmission okay and it occupies most of the slot okay so data in 5G can occupy lesser than a slot that is there okay but still it will occupy most of the slot and depending on how many slot PDSCH is occupying you will have different configurations of type A okay we will see that. So this is the figure I showed you on the last slide okay so this is basically type A you see this in this figure data is basically occupied till the 11th symbol okay correct so most of it 3 symbols are left so they are used for something and maybe they are used for uplink okay or some other user fine. So this is where type A mapping is applicable okay and so you see this first DMRS is located in symbol 2 of the slot okay I jumped again and told you why it is not located here right okay so we know that why it is not located here because it is PDSCH okay. Now in this mapping type okay which is type A DMRS location this is called L bar okay location L bar is mapped relative to the start of the slot boundary start of the slot boundary is this this is a boundary okay right so if I say that the DMRS is beginning at second symbol so of course it is from the beginning okay you understand what I am saying here that L this DMRS location is told okay whenever I tell that DMRS is located at second symbol it is basically from the beginning of the slot okay it will change when the time mapping type is B okay right. So see this okay so and what is this L and then so L bar is the starting location okay and LD you see this let us read this is the duration between the first OFDM symbol of the slot and the last OFDM symbol of the schedule PDSCH resources in the slot this is LD what is L bar okay it tells the starting location relative to the beginning of the slot okay and LD is basically the duration between the first OFDM symbol of the slot and the last OFDM symbol of the schedule PDSCH resources in the slot. So LD is this not this that is how it is defined okay it will change for type B so that is why I am crucially pointing it out to you fine okay so let us see that so DMRS is located in symbol 2 in the downlink because of the facts we know now okay PDSCH which is located at the beginning of slot which has its own DMRS right okay so we have discussed that. So DMRS can be located in symbol 3 if PDSCH occupies 3 symbols so PDSCH can occupy third symbol also depending on how many users DCI you are sending in the PDSCH okay right so and typically it occupies 2 to 3 symbols okay so if PDSCH is occupying 3 symbols it will come here the DMRS will come here okay in this picture it is only occupying 2 symbols that is what this bullet is telling okay right so these are see you will have to configure base station has to configure the DMRS okay how is it going to configure is going to configure like that okay and mapping type A mapping where is it scheduled okay this is you will see that this all this information has to go in the DCI it has to be told correct okay where this DMRS is scheduled right whether in this symbol or this symbol you will see that a lot of flexibility will come okay it has to be told okay right so this is you see this is over one symbol correct okay now I will show you another configuration with type A itself okay so we are only focusing on type A as of now the earlier DMRS was single symbol okay and what I am going to show you is double symbol okay so DMRS can span over 2 consecutive OVDM symbols also okay like this okay and earlier DMRS was single symbol this is double symbol it will be told okay you of course knows about all this that this is how its DMRS is coming and why 2 why is it spanning 2 symbols okay and the reason here is used for defining higher number of orthogonal DMRS ports okay right you see this if basically you need only if it is a 2 port DMRS you need at least 2 subcarriers if it is 8 port DMRS you need at least 8 subcarriers okay right so as the number of port increases the number of resources required for defining DMRS is going to increase so this accounts for that the second symbol accounts for that okay so that you instead of 12 resource element you have 24 resource element to define a DMRS which will contain larger number of ports okay right so and that is the idea is clear that as the number of port increases the number of resources required to define the port will increase and this accounts for that okay right just gave an example then from 2 to 4 you will need 4 at least 4 resource elements okay. Now what happens okay that although this front loaded reference signal reduce the latency okay that they have come in the beginning saw that whether it was single symbol or double symbol they were coming in the beginning of the PDSCH transmission right okay and we know the objective reduce the latency they may not be sufficiently dense in that time domain in the case of rapid channel variations what am I saying here will decode everything okay right the channel is varying fast correct okay user is moving at a speed of very high speed or moderate speed or low speed okay depending on that okay the channel will change within a slot okay right so and how fast does it change okay that you base station let us say has the complete information how does it figure out that is left for future discussion but it has this information that ue is moving at a slow speed or moderate speed or a high speed okay. Now depending on that it can send 3 additional DMRS time domain occasion in a slot okay that is how it is written in the standard okay what does that mean let me show you this okay correct. So what was this there was only one DMRS okay coming in the second symbol now let us say ue is moving at a moderate speed okay base station knows that what will it do by the time this part of the symbol comes okay ue has channel has changed okay right so it will send one additional DMRS towards the end of the slot okay correct to see this that is the configuration that is what we are talking about that how DMRS needs to be configured and this also standard only tells you that these DMRS are defined okay how to configure them it is entire the designers decision it is entirely the schedulers decision okay that is where scheduler will again come into picture okay that it needs this information that how fast the user is moving okay so it will get that information let us assume that okay then based on that it knows that it is moving at a moderate speed okay right typically 20 kilometer, 30, 40 kilometers okay right. So now this is one additional location this is called see what you should read this up to 3 additional DMRS time location time domain occasions in a slot are allowed okay. So this is one additional this is basic this will always be transmitted okay right so this is one additional DMRS location what happens here speed increases two additional locations correct okay and a highest speed that is what it is supported okay in release 15 okay I am not sure about other releases where the DMRS density is further increased okay but if you still design the standard for a higher speed then you will have to further increase the DMRS occasion but till release 15 there are 3 additional DMRS occasions which are defined okay right. Now you see this if you do a bad system design okay you do a if the scheduler does not take care of the fact that like you is moving or you is stationary or is highly mobile okay always let us say always transmits 3 additional occasion then you are using lot of bandwidth to transmit unnecessary information your users data rate is reducing right correct okay because this will all this will be accounted during rate matching correct right. So because rate matching will tell you how many resource elements are there in per PRP that is you remember that formula correct in that formula all these were accounted for additional overhead this is that additional overhead at that stage I could not tell you okay so this additional overhead is going to reduce the number of resource element correct okay. Now this is other extreme and you is moving at a very high speed and you do not allocate additional DMRS so channel has changed you is using the H channel to decode its data again it will create errors right okay. So it is going to like lead to a knack so then that is a system design okay correct and so typically what people do is initially design the system which does not support mobility so that okay you show that system works then you say okay I want to like my system will support mobility so you will have to configure all that okay so okay supports pedestrian mobility or like low vehicular mobility will configure then you will have to decide correct whether this or this what people typically do is it is difficult to get this information the speed of the user you configure this okay live with that that you always send this okay it is a bulletproof way okay and it will support at least pedestrian mobility or low vehicular mobility. So there will be some degradation is of course in the throughput because you are always transmitting this correct but you do not need that the scheduler does not need that speed information we have a dumb way of doing it but then you progressively evolve and you get that information scheduler gets that information somehow and then either this or that okay then there are other options we see the flexibility you see the importance of scheduler design okay. So channel estimator in the receiver can use this additional DMRS for better channel estimation correct okay right for example to interpolate between the DMRS within a slot what will happen okay so you will estimate a channel here you will estimate a channel here what will it do between for this intermediate this one symbols will interpolate the channel correct okay. Now how can what kind of interpolator you design again the UE designs it is again the like is the function of the processing power UE has okay if it is an like 5000 rupees phone it will simply copy these estimates here will not interpolate okay copy to a see these three estimates here this estimate for three symbols copy estimate for three symbol and somewhere do averaging in between okay right. So these are the kind of games which are played while designing a system okay right. So this is where you see this is additional occurrence are being allowed okay right one additional two and three additional occurrences are being allowed okay correct. Now what happens there is a certain cases where in this additional occurrences are not allowed okay we will see that that is what I am going to show discuss the case when no additional single symbol DMRS can be transmitted okay there are two different kind of transmission right I showed you here additional occurrence for only for single symbol okay even the two symbol can be there can be additional occurrence for two symbol transmission also I am not showing you here that is also possible okay right. So is this clear what are we showing here that in certain scenarios these additional occurrences are allowed okay in certain scenarios the additional occurrences are not allowed okay looking at the single symbol DMRS where additional occurrences are not allowed okay. So what are what is happening okay if the PDSES duration is between 3 to 7 symbols okay right like this then because you see that data is not being transmitted in the rest of the symbols there is no need to configure additional DMRS occurrence okay right. So standard prohibits the transmission of additional transmission additional DMRS occurrence because data is not scheduled in the most of the slot or in this part so what is it saying if the PDSES duration is between 3 to 7 symbols okay right. So this is ID is equal to 3 and this is ID is equal to 7 okay LD this LD is between 3 to 7 okay so if the PDSES duration LD is between 3 to 7 symbols okay right so DMRS location L is always L0 is equal to 2 this is L0 is the location of the first DMRS symbol okay right. So what are we saying here what did this slide tell you okay in the previous slide I told you that the additional DMRS occurrences are possible fine okay you see that okay right. Now in the next slide what am I telling you in which case these additional occurrences are not allowed and what am I showing you if the data is coming till here correct data is coming till here you know that channel will not age okay most likely because even if you move at a high speed channel ages you see its effect coming only after 7th symbol or something okay you see that whereas where are these additional occurrences coming okay here not 7th symbol okay here again after right okay so this is the second occurrence you can say that here it is coming okay right but before that you have already defined these occurrences which essentially means that data is spanning the whole slot correct okay because how do you define first you define this which the basic assumption is that data is spanning the whole slot data is not spanning the whole slot you do not care right then this is first this is defined this is additional occurrence okay correct okay so if data is coming till here then what standards here is forget you do not assign any additional symbol DMRS symbol correct okay right. So and in this case so this DMRS location L bar is L bar is the DMRS location L0 is the standard uses for the first DMRS location so L0 is what L0 is 2 this is L0 right location of the first DMRS symbol which is 2 second symbol okay and what are we saying Ld okay which is the duration is between 3 to 7 symbols no additional DMRS is allowed okay right so we will continue discussing this in the next class okay what are the additional flexibilities which are provided for DMRS okay I will stop here we will take it up in the next class thank you.