12/11/12

Cours 7: Protocoles ARQ/HARQ
Communications sans fil, M2 ISIM 2012-2012
Iryna Andriyanova
Sunday, November 11, 12
Adaptation du lien sans fil
• Canal sans fil : variation rapides de qualité
• Adaptation du lien
• avant la transmission :
•
couche physique et MAC - type de modulation, codage de
canal, puissance émise
• après la transmission :
•
Sunday, November 11, 12
protocoles de retransmission de type Hybrid ARQ (HARQ)
Exemple de HSDPA (3G)
• QPSK pour les canaux bruités et 16-QAM
pour les canaux de bonne qualité
• 14 Mbps pour les canaux de bonne qualité (16QAM + code de rendement proche de 1)
• 2.4 Mbps pour les canaux bruités (QPSK +
code de rendement 1/3)
• le contrôle du lien s’effectue jusqu’a 500 fois
par sec
Sunday, November 11, 12
Hybrid ARQ
ARQ - mécanisme de contrôle ou on utilise les aquittements
(ACK/NACK) et les timeouts pour assurer une transmission
fiable lors d’une transmission peu fiable. Implementation
usuelle : bit(s) CRC
HARQ = ARQ + FEC. C’est une variation d’ARQ où :
✷ le code utilisé est un code plus puissant que CRC
(code de canal = FEC), par exemple un turbo code ou
un code convolutif, d’un certain rendement Rc
✷ les paquets reçus très bruités ne sont pas “oubliés”,
mais gardés et utilisés lors du décodage - on effectue
le “soft combining”
Sunday, November 11, 12
The standard measure of ARQ protocol efficiency is throughput, defined as the average number of user
level of redundancy of the error correcting code employed in an HARQ scheme has two opposing ef
data bits accepted at the receiving end in the time required for transmission of a single bit. Therefore the
HARQ de type CC (Chase combining)
on the scheme
efficiency, namely, with increased redundancy the probability of successful transmis
level of redundancy of the error correcting code employed in an HARQ scheme has two opposing effects
increases but
the scheme
percentage
of user
datawith
in the
frameredundancy
decreases.theUsually,
a fixed
rate code
which is
on the
efficiency,
namely,
increased
probability
of successful
transmission
increases
but characteristics
the percentage ofand
user throughput
data in the frame
decreases. Usually,
a fixed rate code which is well
suited to the
channel
requirements
is selected.
• suited
Retransmission
du
même
paquet
(codage
àofrépétition)
to with
the channel
characteristics
andconditions
throughput
requirements
is selected.
In applications
fluctuating
channel
within
a range
signal-to-noise ratios (SN
In applications with fluctuating channel conditions within a range of signal-to-noise ratios (SNRs),
such as•mobile
and satellite packet
data transmission,
the reçus
so called(diversité
incremental redundancy
Combinaison
de tous
les paquets
et SNR (IR) HA
such as mobile and satellite packet data transmission, the so called incremental redundancy (IR) HARQ
schemes exhibit
higher
throughput
efficiency by adapting their error correcting code redundanc
cumulé
plus
grand)
schemes exhibit higher throughput efficiency by adapting their error correcting code redundancy to
different channel
An IR-HARQ
protocol
operates
bythetheexample
example
in Figure
differentconditions.
channel conditions.
An IR-HARQ
protocol
operatesasasillustrated
illustrated by
in Figure
1. At 1
• Décodage du paquet
at the
at
thetransmitter
transmitter
1 1 1 1 1
1 1 1 1 1
1
1
1 1
1 1
1
2 2
2
2 2
3
3
3
4
4
transmission # 1
transmission # 2
3
3
3
4
4
1 1 1 1 1 4 1 3 1 1 4 3 2 2 3 4 4 2 3 1 2 4 2 3 1
1 1 1 1 1 4 1 3 1 1 4 3 2 2 3 4 4 2 3 1 2 4 2 3 1
Sunday, November 11, 12
transmission # 1
transmission # 2
2
4 4
4 4
1
2
3
4
4
2
3
3
1
2
2
3
1
transmission # 3
transmission # 3
transmission # 4
transmission
#4
at the receiver
at the receiver
data bits accepted at the receiving end in the time required for transmission of a single bit. Therefore the
HARQ
de
type
IR
(Incremental
on the scheme efficiency, namely, with increased redundancy the probability of successful transmission
increases but the percentage of user
data in the frame decreases. Usually, a fixed rate code which is well
Redundancy)
level of redundancy of the error correcting code employed in an HARQ scheme has two opposing effects
suited to the channel characteristics and throughput requirements is selected.
In applications with fluctuating channel conditions within a range of signal-to-noise ratios (SNRs),
• Transmission du paquet fraction par fraction, en
such as mobile and satellite packet data transmission, the so called incremental redundancy (IR) HARQ
commençant par la partie “données”
schemes exhibit higher throughput efficiency by adapting their error correcting code redundancy to
different
channel conditions.àAnlaIR-HARQ
protocol
as illustrated
by the example in Figure 1. At
• Assemblage
réception
etoperates
décodage
commun
at the transmitter
1 1 1 1 1
1
1 1
1
2 2
3
4
3
4
2
3
1
2
2
3
4 4
transmission # 2
3
4
1 1 1 1 1 4 1 3 1 1 4 3 2 2 3 4 4 2 3 1 2 4 2 3 1
Sunday, November 11, 12
transmission # 1
transmission # 3
transmission # 4
at the receiver
power ratio that
terminals.
urce that is addressthe two-dimensiond a resource block.
ssembles 12 subcarof 180 kHz. In the
ock has a subframe
a short subframe
annel variations by
g on their current
time, a short hybrid
me of only 8 ms can
S1-U
S1-U
Exemple de LTE (Long Term
Evolution)
X2-U
E-UTRAN
eNB
X2-C
eNB
Stack de protocole :
OCOL
UE
ARQ
(incl. Seg/Conc.)
Re-ordering
HARQ
eNB
PDCP
PDCP
RLC
RLC
MAC
MAC
PHY
PHY
STACK
tially provides a bit
oding and a cyclic
e link-layer protoo upper layers by
y, and integrity. In
esponsible for the
d scheduling.
s for the LTE linkrequired reliability
Protocol (IP) data
f Sunday,
different
services
November 11, 12
Header
compression
! Figure 2. User plane protocol stack.
vides integrity protection to higher layer-control
protocols. The radio link control (RLC) sublayer
[5] comprises mainly ARQ functionality and supports data segmentation and concatenation. The
latter two minimize the protocol overhead independent of the data rate, as is explained in more
detail below. Finally, the medium access control
(MAC) sublayer [6] provides HARQ and is
Ciphering
Scheduling
umber of resource
electing a modulameet the current
scalable transport
ing in a wide range
on, it is possible to
by utilizing multiIMO) transmissions
ven further under
! Figure 1. Overview of the EPC/LTE architecture.
power ratio that
S1-U
terminals.
X2-U
urce that is addressE-UTRAN
the two-dimensiond a resource block.
ssembles 12 subcarX2-C
of 180 kHz. In the
eNB
ock
has a subframe
MEYER LAYOUT 3/25/09 2:18 PM Page 54
a short subframe
annel variations by
! Figure 1. Overview of the EPC/LTE architecture.
g on their current
time, a short hybrid
me of only 8 ms can
S1-U
Exemple de LTE (Long Term
Evolution)
eNB
Stack de protocole :
OCOL
corrected by the
lightweight HARQ
STACKprotocol. Only
PHY
RLC
RLC
Segmentation
concatenation
RLC SDU
Concatenation
Header
compression
IPPDCP
TCP
Ciphering Payload
RLC
Scheduling
umber of resource
UE
eNB
The two-layer ARQ
IP
electing a modulaPDCP IP UDP Payload
design achieves low
via S1 or from
meet the current
ARQ UE’s stack
latency and low
scalable transport
(incl. Seg/Conc.)
ing in a wideoverhead
range without
RLC
Re-ordering
on, it is possible
to reliability.
PDCP
H
sacrificing
Header compression
by utilizing multiMost errors are
MAC PDCP PDCP SDU
HARQand ciphering
IMO) transmissions
captured and
ven further under
H
MAC
PDCP
PHY
PDCP
PDU
Segmentation
RLC
residual
HARQ errors
! Figure 2. User plane protocol stack.
tially provides
a bit
oding and a are
cyclic
detected and
MAC
MAC SDU
MAC
e link-layerresolved
proto-by the more
Multiplexing
vides integrity protection to higher layer-control MAC
o upper layers by
expensive (in terms
protocols. The radio link control (RLC) sublayer
y, and integrity. In
latency and
[5] comprisesL1mainly ARQ functionality and supesponsible forof the
Coding,
ports data segmentation
and concatenation. The
d scheduling. overhead) ARQ
interleaving,
latter two minimize
the protocol overhead indes for the LTEretransmissions.
linkmodulation
pendent of the data rate, as is explained in more
required reliability
detail below. Finally, the medium access control
Protocol (IP) data
! Figure 3.
of data
flow through
(MAC) sublayer
[6]Illustration
provides
HARQ
and isL2 protocol stack.
f Sunday,
different
services
November 11, 12
RLC SDU
RLC
PDU
Multiplexing (padding)
MAC SDU
Transport block
MAC
PDU
CRC
Exemple de LTE : HARQ+ARQ
MEYER LAYOUT
3/25/09
2:18 PM
Page 55
RLC SDU
BLER ~10-6
UL ARQ
transmitter
RLC
Sliding window ARQ
RLC PDU
BLER ~10-4-10-3
RLC STATUS
BLER ~10-4-10-3
MAC
DL HARQ
receiver
UL ARQ
receiver
UL HARQ
transmitter
Stop and wait HARQ
HARQ ACK/NACK
error rate=10-4-10-3
RLC STATUS
as DL HARQ data
UL HARQ
receiver
DL HARQ
transmitter
Transport block
BLER e.g.10-1
Uplink L1
Downlink L1
! Figure 4 HARQ and ARQ retransmissions on MAC and RLC layer.
link
tion
ent)
4
tc.)
eNB
2
nt, etc.)
this message. Consequently, errors or loss of the
ARQ feedback can be detected and recovered
by sending another RLC status. Upon reception
of the RLC status message, the ARQ transmitter
triggers a retransmission of the corresponding
Sunday, November 11, 12
Exemple de LTE : perfrormances
MEYER LAYOUT
3/25/09
100
2:18 PM
Page 58
UM-3gpp-case3
AM-3gpp-case3
90
80
70
CDF
60
50
40
30
20
10
0
0
10
20
30
40
Object bit rate [Mbps]
50
60
70
! Figure 6. Performance samples of consecutive file downloads with RLC AM
and RLC UM.
Sunday, November 11, 12
to the 3GPP case 3 scena
2×2 MIMO scheme is
peak-bit rate for this ca
The HARQ block error
percent. HARQ failures
number of five transm
here) are exceeded or d
errors. The residual los
layer is on the order of
these errors propagate to
be handled by TCP; wh
they are recovered by the
Figure 6 shows the pe
bit rate (OBR), that is,
the file transfer time, of
AM and UM radio bear
tribution function (CDF
radio bearer using RLC
achieves better performa
bearer. Whereas the bes
transfers for RLC UM p
ty suffers significantly fro
ures that trigger the TCP
thereby increase the tr
50th percentile of the OB
cent to 15 Mb/s. The ac
file transfer over RLC U