arq and harq

8/11/2019 Arq and Harq

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It's very important to use solutions that improve the efficiency of the adopted model in anydata communication system. If the transmission is 'Wireless', this need is even greater.

In this scenario we have techniques that basically checks, or verify if the information sentby the transmitter correctly arrived in the receiver. In the following example, we have apacket being sent from the transmitter to the receiver.

If the information arrived properly (complete , the receiver is ready to receive (and processnew data. If the information arrived with some problem, corrupted, the receiver mustrequest that the transmitter sent the packet again (retransmission .

!et's understand a little more about these concepts increasingly used (and required in thecurrent systems"


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Error Checking and Correction

We start talking about errors. #rrors are possible, and mainly due to the transmission link.In fact, we can even 'expect' errors when it comes to Wireless $ata %ransmission.

If we have errors, we need to take some action. In our case, we can divide it into two steps&error checking and error correction.

#rror checking is required to allow the receiver to verify that the information that arrived iscorrect or not.

ne of the most common methods of error checking is the ) , or ' yclic )edundancyheck', where bits ( ) are added to a group of information bits. %he ) bits are

generated based on the contents of the information bits. If an error happens with theinformation bits, the ) bits are used to verify and help recover the degraded information.

%he level of protection provided is determined by the ratio& number of ) bits by thenumber of information bits. *bove a certain error level, the process is eliminated. )protection is used practically in all existing +oice and $ata applications.

%he following diagram shows a simplified demonstration of how the ) is used.

*nd the ) is directly connected to the #rror orrection methods. %here are various waysof oward #rror orrection ( # , but the main idea is, given a level of quality in the link,try to get the lowest number of required retransmissions.
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-inimi ing the number of retransmissions we ended up having a more efficient data flowresult, including / mainly / the '%hroughput'.

In simplified way& the ) lets you know if a package arrived ' 0' or '1 % 0'. #verypacket that is sent has a ) , or a '2ignature'. *s an analogy, it's like when we send a letterto someone, and in the end we sign& '-y ull 1ame'. When the other person receives this

letter (information , he checks the signature& '-y Wrong'. In this case, he tells the-essenger& 'I don't know '-y Wrong', this information has some problems. 3lease asksender to send it again4'.

I.e. I do ) checks. If the ) is 'wrong', the information is 'wrong'. If the ) is 'correct',probably the information is 'correct'.

Retransmissions

)etransmissions are then& send information again (repeat to the receiver, after it makesuch a request. %he receiver requests that the information be retransmitted whenever itcannot decode the packet, or the result of decoding has been an error. %hat is, afterchecking that the information reached the receiver is not ' 0', we should request it to beretransmitted.

f course, when we have a good link (21) , without interference or problems that mayaffect data integrity, we have virtually no need for retransmissions.

In practice, in real World, this is very difficult to happen, because the links can face themost different adversities. %hus, an efficient mechanism to enable and manage theretransmission is essential.

We consider such a mechanism as efficient when it allow data communication in a link meetquality requirements that the service demands (5o2 .

+oice for example, is a service where retransmission does not apply. If a piece of information is lost, and is retransmitted, the conversation becomes intelligible.

n the other hand, data services practically rely on retransmission, since most have / orallows / a certain tolerance to delays 6 some more, some less. With the exception only for')eal %ime' services.


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7ut it is also important to take into account that the greater the number of neededretransmissions, lower the data transmission rate that is effectively reached& If theinformation have to be retransmitted several times, it will take long for the receiver toobtain the complete / final / information.

ARQ

%ill now we talked in a generic way about data retransmissions, error checking andcorrection. !et's now see some real and practical schemes.

%he simplest way (or more common control using what we described above is known as*)5, or '*utomatic )epeat )equest'.

In *)5, when we have a 'bad' package, the system simply discards it, and asks for aretransmission (of the same package . *nd for this, it sends a feedback message to thetransmitter.

%hese feedback messages are messages that the receiver uses to inform whether thetransmission was successful or not& '* 0nowledgement' (* 0 and '1on/* 0nowledgement'(1* 0 . %hese messages are transmitted from the receiver to the transmitter, andrespectively informs a good (* 0 or bad (1* 0 reception of the previous packages.

If in the new retransmission the packet keep arriving with errors, the system requests anew retransmission (still for this same package . %hat is, sends another '1* 0' message.


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%he data packets that are not properly decoded are discarded. %he data packets orretransmissions are separately decoded. %hat is, every time a packet that arrives is bad, itis discarded, and it is requested that this same package be retransmitted.

7ut see that if there were no retransmissions, the performance of the data flow would bemuch better. In the example below, compared with the previous, we transmit more

information / 8 times in the same time interval.

9nfortunately we don't have much to do about the link conditions. r better, we are able toimprove the links performance, for example with configuration parameters optimi ation, butwe'll always be sub:ect to face adverse conditions. In this case, our only way out is to try tominimi e retransmissions.

*nd that's where arise other techniques or more 'enhanced' schemes for retransmission.%he main one is ;*)5.

Hybrid ARQ (HARQ)

%he ;*)5 is the use of conventional *)5 along with an #rror orrection technique called'2oft ombining', which no longer discards the received bad data (with error .

With the '2oft ombining' data packets that are not properly decoded are not discardedanymore. %he received signal is stored in a 'buffer', and will be combined with nextretransmission.

%hat is, two or more packets received, each one with insufficient 21) to allow individualdecoding can be combined in such a way that the total signal can be decoded4

%he following image explains this procedure. %he transmitter sends a package . %hepackage arrives, and is ' 0'. If the package is ' 0' then the receiver sends an '* 0'.


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%he transmission continues, and is sent a package . %he package arrives, but let'sconsider now that it arrives with errors. If the package arrives with errors, the receiversends a '1* 0'.

nly now this package (bad is not thrown away, as it is done in conventional *)5. 1owit is stored in a 'buffer'.

ontinuing, the transmitter send another package


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$oes by adding (combining these two packages ( @


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;ere we can see the following& along with the received package , the receiver also haspackages and


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r retransmit that same information againD r retransmit only the redundancy.

*nd then, if we retransmit less information (only redundancy , we spend less energy, andthat will run much faster. With this we have a gain4

%hat is, we work with different 'versions of redundancy', that allows us to have a gain in theretransmission. %his is called ')edundancy +ersion', or what version of redundancy.

%he redundancy version, or ;*)5 scheme with '2oft ombining' can be ' hase ombination'or 'Incremental )edundancy'.

HARQ Chase Combination

EChase Combination F& when we combine the same information (the retransmission is anidentical copy of the original packet .

We transmit an information, which arrived wrong, and we need to do a retransmission. Weretransmit the same information / and there we don't have much gain.

HARQ Incremental Redundancy

EIncremental Redundancy F& where we retransmit only the portion that we didn'ttransmitted before. %hus we retransmit less information. !ess information means fewer bits,less energy. *nd this gives a gain4

)edundancy bits are retransmitted gradually to the receiver, until an * 0 is received.

With this, we adapt to changes in the condition of the link. %he first retransmission can, for

example, contain or not bits of redundancy. If necessary, a small number of these bits isretransmitted. *nd so on.

inishing for today! "hat are the # ste$s of HARQ% &hy it gi'es me aain% irst because from wrong packets = and ? we can get a correct one, since we do not discard

erroneous packets anymore. 2econd because we can / also in retransmission / send less information, and streamline the process.

%he use of ;*)5 with '2oft ombining' increases the received #bGIo effective value for eachretransmission, and therefore also increases the likelihood of correct retransmissionsdecoding, in comparison to conventional *)5.

We send a package, and it arrives with errors& we keep this package. )eceive theretransmission and then we add or combine both.

HARQ rocesses (Case Study)

What we have seen so far clarifies the concepts involved. In practice, in retransmission, thistype of 3rotocol is called '2top *nd Wait' (there are other kinds of similar protocols .


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What would be& send the information and stop. Wait for the response to send otherinformation. 2end, wait for response. 2end, wait for response ...

1o4 1ot so in practice. In practice, we work with a number of 'processes', which may varyfor example from B, H or . %he following image illustrates this more clearly.


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*ther ty$es of HARQ

1ew schemes are constantly being developed and used, as the type III ;*)5, which usesself/decodable packages.

7ut enter these variations, terminology and considerations, is not the scope of our tutorial,which was simply to introduce the concept of )etransmission, *)5 and ;*)5.

7ased on the key concepts illustrated here today, you can extend your studies the way youwant, however we believe that the most important thing was achieved 6 understand how itworks and what are all the cited concepts.

+A,A A$$let

7elow, you can see how some retransmission schemes work. %here are several *ppletsavailable, for the many possibilities (*)5, ;*)5, With 2liding Windows, 2elective, etc .

%he next is a link for a J*+* *pplet that simulates a '2elective )epeat 3rotocol transmission'.

http&GGmedia.pearsoncmg.comGawGawKkuroseKnetworkKBGappletsG2)Gindex.html

Conclusion

%his was another tutorial on important issues for those who work with I% and %elecom& data%ransmission and )etransmission techniques, *)5 and ;*)5.

*)5 is used for applications that allow a certain delay, as Web 7rowsing and 2treaming*udioGvideo. It is used widely in Wimax and Wi i communication systems. ;owever, itcannot be used in +oice transmission, as for example in C2-.

;*)5 for example is used in ;23* and !%#, and therefore must be a well/understoodconcept for those who work or want to work with these technologies.
http://media.pearsoncmg.com/aw/aw_kurose_network_4/applets/SR/index.htmlhttp://media.pearsoncmg.com/aw/aw_kurose_network_4/applets/SR/index.html


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We hope you en:oyed it. *nd until our next tutorial.

arq and harq

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