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What Should be the HARQ Unit and Why?
Slide1
Date:2019-09-15
Authors:
Sep, 2019
Background
Hybrid-ARQ is a strong candidate feature forTGbeMany results (MPDU and CW) have shown the benefits of HARQHowever, the HARQ unit is not defined yetDefinition of HARQ unit is extremely important towards standardization of HARQTherefore, in this presentation we present the HARQ unit which is most practical and natural to HARQ
Slide2
Sep, 2019
HARQ Unit
Many contributions [2-3] have shown and presented the HARQ unit as an option between the following two:MPDUCodeword (CW)We want to highlight that considering MPDU as a HARQ unit candidate is not even an optionIn the layered architecture of 802.11 there is no 1-to-1 mapping between MPDUs and CWsHence, CW is the only and most natural candidate for HARQ unit
Sep, 2019
Slide3
Framing in 802.11
Sep, 2019
Slide4
MPDU-1
MPDU-2
MPDU-3
MPDU-N
MPDU-1
MPDU-2
MPDU-3
MPDU-N
A-MSDU
MPDU Header
Delimiters


EOF Padding
PSDU
Pre-FEC padding
Tail bits
Post-FEC padding
Scrambled DATA
Scrambling
Encoding
MAC
PHY
Service field
Codewords
There is no MPDU to CW mapping apparent in this layered architecture
MPDU Level Limitations - I
It was shown in [1] that HARQ cannot be supported at the MPDU level due to,Misalignment of CWs in retransmitted MPDUs (from [1]) makes combining impossibleArbitrary numbers of delimiters (DL) among the MPDUsEach MPDU Header’s flipped retry bit upon retransmissionDifferent Cipher Text on retransmissionsDifferent CRC bits on retransmissions
Slide5
MPDU-1
MPDU-2
MPDU-3
MPDU-4



MPDU Header
Ext-IV
Cipher Text
MIC
CRC
DL (Delimiter)
Pre-FEC Padding
Failed MPDU

CW-4
CW-5
Re-Tx MPDU

CW-1
CW-2
CW-3
At the Tx for Retransmission
At the RX
Sep, 2019
MPDU Level Limitations - II
Though MPDU is perceived to be desired and simple HARQ unit we want to highlight that it is not even an option for HARQ for following reasons,different payload (even few bits) in each MPDU in retransmissions,the contents of the CWs will change, thus no combining at the PHY levelthe misalignment in the MPDU to CW mapping between first transmission and the retransmissions,Due to this misalignment it is not possible to combine LLRs for retransmitted MPDU at the PHYno knowledge of MPDUs at the PHY,MPDU is a MAC construct and has no notion at the PHYTherefore, MPDU cannot be a unit for HARQ in 802.11be
Slide6
Sep, 2019
CW as HARQ Unit - I
Codeword level HARQ was presented in [1] for 802.11beHARQ combining is closely tied to the forward error correction (FEC) that happens at the PHY levelFEC unit is the codeword (CW)MDPUs are not FEC unitthere is no 1-1 mapping between MPDUs and FEC unitMPDU cannot be used as a unit for PHY layer since it is a MAC constructTherefore, CW is the natural unit for HARQ as it provides the basic unit for PHY layer where the combining is supposed to happen
Slide7
Sep, 2019
Efficiency of CW as HARQ Unit
Lets consider an example with CW as HARQ unit and show how it can be beneficialFor Example,Only three CWs are decoded wrong and due to this all of the MPDUs CRC failsNot only that the retransmission of these MPDUs is spectrally in-efficient but it is also not possible to combine the failed codewords at the receiver due to the reasons mentioned previously
Slide8
Sep, 2019
MPDU-1
MPDU-2
MPDU-3
MPDU-4



MAC
CRC Failed
CRC Failed
CRC Failed
CRC Failed
CW-1
CW-2
CW-3
CW-4
CW-5
CW-6
CW-7
CW-8
CW-9
CW-10
CW-11
Decoding Failed
Decoding Failed
Decoding Failed
PHY
Flow of CW Retransmissions
In this case,RX sends back the CW level feedbackTX receives this CW level feedback and it prepares for the transmission such that it informs the PHY to attach the failing CWs to the next transmissionRX will receive this retransmission and decodes the HARQ-SIG part to find out which CWs are to be combined
Slide9
Sep, 2019
RxPHY
RxMAC
HARQ Combining
TxPHY
TxMAC
SIG Decoding
Data Decoding
SIG Decoding
Data Decoding
. . .
Summary
MPDU is a MAC constructNot identifiable on PHY layerWhen handled by the MAC, retransmitted MPDUs:don’t have the same bit contentdon’t map to the same codewords as the original transmissionEven an entire A-MDPU retransmission does not result in the same bit stream on the PHY layer (PSDU)Combining (both CC and IR) is closely tied to error correction and happens at the PHY levelMDPUs do not have self-contained FEC unitNo consistent mapping between MPDUs and CWsMultiple CW per MPDUCW can overlap multiple MPDUs
Sep, 2019
Slide10
Conclusion: HARQ Unit = Codeword
Sep, 2019
Slide11
PHY provides natural unit for HARQ and that is CodewordMPDU as a HARQ unit is a practical impossibilityTherefore, we propose to use the codeword as a HARQ unit and perform all simulations and studies with CW level retransmissions
Straw-Poll
Do you agree that HARQ unit for 802.11be shall be based on codewords?YNA
Slide12
Sep, 2019
References
[1] 11-19/0873r1 – HARQ Framing – Imran Latif (Quantenna)[2] 11-19/1038r1 – HARQ with A-MPDU – Shimi Shilo (Huawei)[3] 11-19/1131r0 - Consideration on HARQ Unit -TaewonSong (LG Electronics)
Sep, 2019
Slide13
Sep, 2019
Slide14
Backup Slide
What about BCC ?
Sep, 2019
Slide15
Previous slides showed HARQ unit for LDPC encodersHowever, in BCC there is no notion of codewordsBCC has very limited support in 11ax (only 20 MHz)LDPC is the de-facto FEC encoder/decoder for 11ax onwardsTherefore, focus of HARQ forTGbeshould be on LDPCIt is not necessary to define HARQ for BCC

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