Lecture 03

[Music] we'll learn about another key technology today which is arq and hybrid arq protocols okay so we'll see the need of it we'll understand today we are going to spend half an hour understanding this so what is motivation behind these protocol calls so we uh learned about this adaptive modation and coding yesterday and we saw that if like for a particular SNR you can get a particular rate okay and SNR is like typically a function of how far a user is from the base station okay now what happens if this SNR changes okay wherein like a user requested for an MCS corresponding to the high SNR okay and it moves to a low SNR region right it could happen when like you are on in a mall and then you suddenly go to the basement of a mall to a parking lot then the S drops because the signal considerably drops okay now you ask for this MCS 26 and suddenly you have gone to MCS 2 okay so what will happen we understood yesterday that receiver will not be able to uh decode this data without errors correct okay right so so ldbc will not work because it is receiving at a much higher rate than what can it can decode okay so this keeps happening in any Wireless Systems okay if the transmitter or receiver is mobile okay now have you ever wondered how your mobile is able to always provide you error-free data right okay if it is a simple thing if it is your email okay you always receive it without error then no errors in the data you have received how does that happen Okay so that's where these arq protocols come in okay and the first acronym I used was arq which was automatic repeat request and the second one is the some version of this called hybrid automatic repeat request okay is evolved version of this and this is the reference so we'll basically discuss these two protocols and how they ensure that you always receive the error free data that's an intention because you don't want to receive any data with error okay you don't know what was the actual data okay so this will first start with this arq protocol okay so remember the acronym automatic repeat request okay so this aru protocol uses an error detection code okay on top of this error correction code which is ldpc for us okay and that's what we are going to learn it is a fundamental block this error Det detection code okay how does this work that we'll see later but what it does is it can tell you whether your decoded data has an error or not okay how does it tell okay so there is multiple kinds of error detection codes you're going to the standard uses the cyclic redundancy check okay so it can only detect errors it cannot correct errors okay unlike this ldpc which can correct errors okay so let's see the flow and how this the CRC comes into picture at what stage the CRC comes into picture so you see this we are building upon this transceiver chain which we like learned in the last class so what did we see we started from here we added this module correct and now because there is a need that like there can be errors in data even though like you you know the exact capacity okay so what are this so so this is the data this module was not there in the last class CRC calculation and CRC validation these two modules were not there okay so what are we doing now okay so this is the data which we want to transmit okay and this is the data you want to receive at the receiver okay your mobile transmitted in the up link base station wants to receive or in the down link base station transmitted some data you want to receive okay so it should be error free now so this data okay is the intended data so there is this CRC calculation module so what happens okay so this module will calculate the CRC based on this data message okay how does it calculate what protocol is there that will come later okay let's say some uh CRC is calculated this module is able to do that okay and that is of course based on the data so it'll calculate some let's say uh some 24 bit of CRC okay it adds to this data fine okay so your actual data becomes data plus CRC at this stage instead of only encoding data what will this ldpc encoder do it will encode data plus CRC okay keep this in mind CRC is generated Based on data okay now we understood this yesterday right the whole of it right okay so what happens here and this is point number one the output of this data plus CRC the output of this what will happen you'll have data plus CRC plus parity bits which this ldpc encoder is going to add then you whatever is the rate okay right this you'll encode according to that rate we saw that yesterday then modulation your Channel demodulation ldpc decoder correct okay so here what happens at Point number three output of this what is the output you are going to get you'll get decoded data plus CRC at this stage is this clear that we at the receiver what is going to happen Okay that opposite of whatever happened in transmitter demodulation ldpc decoding will remove the par bits and give you the message bits which it tooken correct and then you have this decoded data plus CRC right now this CRC validation module will look at this CRC and will tell you whether this decoded data has errors or not do we get the flow okay so at Point number three you have decoded data plus CRC this CRC validation module we'll look at this both of them okay it can validate whether this decoded data has errors or not how does it happen what is the protocol will come later when we start understanding the standard okay so at the end of it what we know here is that the decoded data is error free erroneous fine okay so if no error is detected in the received data block here receiver will indicate to the transmitter that the receiver data is error free and how does it do that it sends a one bit message back to the transmitter and that's called an acknowledgement okay it's an act is a one bit act okay because error free or noner fre that's all okay that's the signal which needs to be sent to the transmitter okay so if you implement in a very simple way so transmitter will wait for this signal to come then only it will transmit the next block a very simple way of implementing this protocol okay so unless until the receiver tells the transmitter that I have successfully acknowledged I have successfully decoded this data transmitter is not going to transmit the next packet of data okay so this modules guarantees that the received data is error free so if error is detected in this arq protocol what happens is the receiver discards this data okay and then notifies the transmitter by sending a negative acknowledgement it's called an act so it transmitter knows that receiver has not been able to successfully decode the data for whatsoever reasons okay I gave one simple reason that you move to the basement or that is one you move from this room to the outside or come from outside the room to inside okay signal strength changes because inside the room the signal will get reflected by multiple walls it has to like cross these walls so there will be a lot of attenuation okay so the SNR has reduced so multiple reasons wherein like this decoded data can fail okay so in response to the snack transmitter R transmit the same information fine okay this is basically arq protocol you see this it'll keep happening in Loop and it'll guarantee that you will receive your data correctly the receiver is able to receive the data correctly okay so this is in a very simple form the arq protocol okay so we'll understand what hybrid arq protocol is okay it builds on top of it and there is only one simple difference and what is that that in arq if the error is detected receiver throws away that data in hybrid arq it doesn't throw away it keeps that data it uses that okay and I'll show you how it improves the performance okay there should be some reason to retain the data correct and it retains the data and requests for retransmission of data because the receive data is erroneous right so it is the data is retained okay and this is called hybrid arq okay it we'll see what are different forms of this hybrid arq and how does it help that is the intention okay because if you keep doing it you are bound to receive your data without any error because negative acknowledgement keep will keep going and at some stage there is hope that like you will be able to recover your data it might happen that you in the basement of a building for let's say some time you be able to receive the data transmitter will give up after making maximum number of attempts okay it give up then I'm not able to reach you that's all stops okay but so what happens there are two flavors of hybrid arq first one is called Chase combining and another one is called incremental redundancy we'll see both these flavors how do they help okay so I've shown you this block let's see what is happening we'll understand everything information bits right okay so CRC is inserted then you encode it okay this is rate 3x4 so you there rate 3x4 you have used for encoding as an example okay right so you take this rate 3x4 encoded data okay send it don't look at anything else here okay look only here okay so you sent this this was the first transmission these bits were input to the decoder okay and what happens is the CRC validation module says that these bits are in error okay there are not an error you done you want to understand what happens if there is an error so Knack goes to the transmitter transmitter will do the first ransmission this transmission this is first ransmission okay you get this the same data is transmitted again exactly the same okay so this is called Chase combining Chase transmission and now you see the same data is transmitting what will this do what will the receiver do because it has received a fresh copy of the transmit data there second copy right okay correct so R Transmissions consist of same set of coded coded bit as the original transmission just showed it to you okay so after each R transmission receiver combines each received bit with this previous transmission okay so what am I doing I receive some let's say 100 bits I have another set of 100 bits I simply add them okay signals I I'm adding them okay they're fine okay so this combined signal is fed to the FC decoder this added signal is fed to the FC decoder okay FC decoder will decode it again will CRC validation module again check the validity of it if the CRC passes it's fine right if CRC is not able to pass then again It'll ask for R transmission same set of data is sent again you again add that okay how will this addition help I'll tell you in the next slide okay but that's the process that same data is transmitted you keep adding that data at the receiver okay and feed this combined data to the FC decoder okay now FC decoder decodes CRC gets validated or not okay based on that another set of ransmission another set of retransmission okay if it how is this helping that let's understand that okay first thing you will note here is when I retransmit this old data okay the code rate is not changing you transmitted with 3x4 okay what's happening here you got the if it is a rate 3x4 okay I'm explaining this part okay so what what is happening if it is rate 3x4 code three message bit and one par bit when you are retransmitting you retransmitting so same three message bits and one par bit you add that what's happening at the end of it right same three message bit and one pirate bit rate remains 3x 4 okay I'm stressing upon this because the second flavor that IR I'm talking about will change the rate okay that's why I'm alert in it to you that this is not happening here okay rate remains 3x4 after each combining I gave you the reason all of us understood that okay because message bits and the same message bits and the same parity bit is being retransmitted okay now my claim is that if we are doing it it should improve the performance right it should basically like help you in decoding the data because for the first transmission we could not do it how is it helping the second transmission okay what is happening and one reason you remember why did we even face the situation because the SNR has reduced correct okay you ask for a higher MCS but you are in a Zone where in you cannot decode that okay so the SNR has reduced if it is other way around the SNR increases you will peacefully be able to decode that data right because you asked for a lower MCS and you have gone to a higher MCS Zone you'll be able to decode that right okay but other way around it'll not be able to decode because of the obvious reasons okay because of what Shannon told us now how does it help how does this combining help I just told you that SNR is reducing okay how will this addition help in increasing the SNR okay I'll show you that if I increase if I do this combining I'll be able to increase the SNR okay right if SNR gets increased right there so I can basically there is a hope that I'll be able to decode the MCS which I had earlier asked for which corresponded to the higher MCS or the higher SNR okay right so let's see that okay this combined receive signal the added signal which is being fed to the FC decoder this was the first let's say this is okay this is the first time you received your data okay I'm looking only at one bit this is if there are thousand bit same thing like it will be like X1 plus the same model is applicable okay now second time what happens you first time you not able to decode Knack second time data comes to you what is changing across this the noise okay I'm taking very simple example to elucidate the fact a lot of things other things also change but forget them for a moment so what am I doing here y1 + Y2 that's what I'm doing what happens what will be the resultant signal 2x + N1 + N2 okay now you look at the SNR of this when you transmitted the signal for the first time what is the SNR P by Sigma Square that's what the signal power is p noise power is Sigma Square that's the notation we used for the SNR so P by Sigma Square okay right now what happens so you were not able to decode the data you asked for the second the first retransmission okay you added that now look at the SNR of this signal okay we'll calculate the SNR of this signal what is the signal power if x has power P what will be the power of this 2x it will be 4 P 2x² right power of this a signal Square you get the power right so that the power is 4 P what is the power of noise you square this also okay so what happens so the power is N1 plus simple okay this is okay there some expectation sitting here but I'm not using that so what happens okay this is actually random signal okay what happens what will happen What N1 squ plus right so expectation of N1 Square we saw we have assumed it to be Sigma Square N2 is also same variance Sigma Square what happens to this expectation of this is what if you have not seen it this is zero because N1 and two are independent noise with zero mean okay so this goes to zero okay so power of this is 2 Sigma Square noise power is 2 Sigma Square signal power is 4 P what is the SNR right here here SNR was P by Sigma Square what have I done by simple combining I've doubled the SNR okay right so I'm able to by this using this combining I'm able to double the if I add it thce that's how this R transmission you add them the SNR will become thce four times that's what is happening so you increasing the SNR there should be some reason right that's why we are doing it okay so SNR dropped you're artificially boosting it by asking for the fresh packet of data okay right so this is simple Chase combining because like there's no other way this that you can increase the SNR in the simple setup okay right so that's Chase combining that is implemented is used in standard day in and day out okay right so this is first flavor second flavor is incremental Rundy okay so if you have any doubts retain them we I'll answer at the end of this recording okay so look at a similar kind of setup okay we are looking now what we are looking at is the incremental redundancy second flavor of hybrid arq okay right so what is happening look at the first difference I'm decoding it at a rate 1x4 I'm not decoding it at a rate 3x 4 okay right even though the user had asked for a rate of 3x4 okay this we are assuming that user had asked for rate 3x4 code okay I'm not doing that I'm decoding I'm encoding it at a rate of 1x4 okay fine now what happens here is that so this is basic code when by 4 so for three message bits how many output bits will be there 12 rate 1x4 right okay 3 input bits and 12 output bits so nine pity bits will be added if rate 3x 4 three input bit and four output but when pirate bit gets added extending that idea okay right because I'm using a basic 1x4 code okay for initial transmission user had asked for rate 3x4 okay right so what is happening so only every third coded bit is transmitted what does that mean how many bits I transmit I 12 bits okay so I'll see let's say this what am I doing here I'll take this this is rate 1x4 okay so three bits and nine message bits okay now user had asked for rate 3x four code what does that mean three message bit and one parity bit so what will I do I'll take this three message bit and take one p bit from here transmit it right so user had ask for 3x4 I'm giving 3x4 is this clear okay though I've generated rate 1x4 code okay I'm not sending these additional par bits for the first instance okay because user are not asked for it though I'm generating 1x4 okay this rate 1x4 is called the mother code okay right and you see this from this rate 1x4 I'm generating a higher rate code which is 3x4 3x 4 is higher than 1X 4 by doing what I'm dropping these parity bits what is it called it's called punching okay right so you drop these extra eight bits which were generated okay now if the user had asked for 3x4 and if the conditions are fine user will be able to decode this 3x4 fine right things don't change okay now what happens if if the user position changes if there is a knack okay so yeah so this is I just gave you an example this is written for you rate 3x 4 so effective number of transmit is four okay now the situation changes okay what what is happening you see this there is the first retransmission okay what am I doing I'm sending four additional par bits not the same message again understand this is a difference between the Chase and IR okay I'm not retransmitting this message which is 3x4 I'm sending some fresh par bits get it now what is the effective code rate now three earlier right what was four you send it four now what are you sending four fresh par bits correct so this part you are sending what is it this is rate 3x 8 fine digest this okay so what have I done another way of solving the same problem because you mooved from highest Sr to low SNR region right so your rate should reduce what is MCS lower MCS is basically lower rate correct okay lower modation scheme also okay so at least I'm able to reduce the rate right so instead of boosting the SNR what am I doing I'm reducing the rate correct that should also help me in solving the problem because I move to a low SNR region and I should be able to work with the lower rate okay it works it's fine if it doesn't work what will happen Knack right okay what happens you send four fresh par bits so what will eight 1 by4 okay so you see this rate is changing further down because SNR had gone so much below or so it had reduced so much correct okay if you're not able to decode you ask for another retransmission at this stage you see this what are we doing I'm transmitting the old data right till here the rate was changing here what is happening I'm transmitting the old data right rate will not change here it's just that this part when you do the SNR will get boost okay because what is happening here you're sending the old message bit and old parity bit you're not changing the rate you're only boosting the SNR so the hope that combination of this will work okay so for third R transmission four old bits are transmitted code rate remains 1x4 at this stage right SNR of this part will increase and you hope that things work out for you right this is incremental redundency what is the name telling you why is this called incremental redundency because you are increasing redundency in steps what is redundancy redundancy is the par bits your original message bit is not redundant it's just the par bits which are redundant with respect to the message okay so that's what is happening so there's a hope that it'll work okay and things work as I said if it doesn't work Beyond a certain point of time transmitter will give up okay now why two different flavors in which Flav at what circumstances one is better than other let's understand that for a few minutes okay so Chase versus incremental redundency these are natural questions okay why two different flavors okay Chase combining is easier to implement than IR right okay you simply combine you just retain the packet if the user has asked for 3x4 Generate 3x4 retransmit but IR you have to generate at a lower rate then punching okay so slightly more sophisticated framework than the chase combining okay but uh 5G provides framework to implement both of them okay so now if all redundancy versions provide the same amount of information about the data packet which is the message there are different so okay I did not tell you this is called these versions they are given names okay redundency version zero redundency version one okay so we'll Define it formally so if what are we saying here if all redundency version provide the same amount of information about the data packet which is the message okay you can transmit these versions in any order okay why should I transmit the second one let's go back why should I transmit this after this I can transmit this also after this right these are also four parity bits right but if so all these parity bits give me the same information about the message bit you can transmit them in any order okay but for some code structure not all redundancy versions are of equal importance for example ldpc codes okay so ldpc codes where the systematic bits what are systematic bits this is a technical name given to the message bits okay so the systematic bits are of higher importance than the parp bits okay so what am I saying here what are the comment I'm making here that redundancy version zero where in we sent the message bits plus few par bits is more important than the other redundancy versions okay because the first rcy version contains the message bit okay and that is crucial for the ldpc decoder to decode the message okay so keeping this in mind initial trans should include all the systematic bits and some parity bits that's what we are doing now if in the in the R transmission par BS not in initial transmission can be included so if you have already you transmitted the Rundy version zero okay and somehow you figure out that at the receiver when you're making an error message bits are fine and the problem is with the apparative bits okay most of the message bits are fine okay so what will you do that you will ask for the receiver will ask for next redundancy version because somehow intelligently it has figured out that message bits most few message bits are in error okay and other message bits plus the DAT parity bits are in are in error few message bits and the par bits are not in error okay right so what will happen here is it will ask for the next redundency version okay now however if the initial transmission was received in poor quality on not at all right what happens look at these two bullets okay what I saying here here most of the message bits were find somewhere in error you ask for next redundency version okay because you need message bits for the decoder to work now what happens here the initial transmission was bad okay right then the figure out that the first packet redundancy version is completely an error right then what will happen here is that you will not ask for the next redundancy version you will ask for the same redundancy version because of the fact that ldpc needs these message bits to decode the data okay right so what am I saying here that if the initial transmission or the initial reception is so poor that all the message bits are in error you will ask for the retransmission of the same data which is Ry version zero what does that become that becomes jsing okay so R transmission with only only parity bit is not appropriate some of the systematic bits will provide better performance and what is that where are the systematic bits they are there in the first redundancy version rv0 correct okay and what does it become it is the CH combining so to summary of it is that if the initial transmission is bad you go for CH combining the initial transmission is okay but you are still not able to decode go for incremental redundancy okay because that's the code structure ldpc needs that there are multiple other ways of understanding it but for this class this is if you understand this this is enough okay so I'll stop here and we'll discuss uh in the next class the next technology [Music] [Music] [Music]