IEEE C802.16j-07/524r1
Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
Corrections to HARQ ACK/NAK Scheduling
Date
Submitted
2007-09-19
Source(s)
Rakesh Taori
Samsung Advanced Institute of Technology
Voice: +82 31 280 9635
E-mail: rakesh.taori@samsung.com
E-mail: yb.chang@samsung.com
Youngbin Chang, Jung Je Son
Samsung Electronics
*<http://standards.ieee.org/faqs/affiliationFAQ.
html>
Eugene Visotsky
Motorola
Shashikant Maheshwari, Adrian Boariu, Yousuf
Saifullah
Voice : +1 972 839 1878
E-Mail : Shashi.maheshwari@nsn.com
Nokia Siemens Network
E-Mail : Liu.yang8@zte.com.cn
E-Mail : mchion@zteusa.com
Liu Yang, Mary Chion
ZTE
Re:
In response to the call for comments on the IEEE 802.16j D1
Abstract
In this document we propose some corrections to the HARQ ACK/NAK scheduling mechanism.
We propose a mechanism that is simpler, is based on more reasonable assumptions and fits the
design philosophy of centralized scheduling
Purpose
Discussion and Adoption in the IEEE 802.16j Draft
Notice
Release
Patent
Policy
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Corrections to HARQ ACK/NAK scheduling
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IEEE C802.16j-07/524r1
Rakesh Taori, Youngbin Chang, Jung Je Son
Problem
The current TGj draft (P802.16j D1) specifies an Equation which is used to compute “i + n”, where n is the
number of frames after which an RS sends encoded ACK/NAK for an HARQ burst received in frame i:
n = (H-1)*p + H*j
….Equation (1)
where H is equal to “hop_depth” that the represents number of hops that MR-BS/RS is away from the MS,
while p represents the DL burst processing delay at the RS and j the "HARQ_ACK_Delay for DL Burst" for RS
and MS RS and MS.
There are some problems with the application of this Equation:
(1) The Use of j in the above equation is basedon one of the following assumptions:
(A) The assumption that the delay incurred in forwarding an ACK/NAK (let us call it the ACK forwarding
delay) is the same as the delay incurred for an MS to process the DL burst and generate an ACK/NAK.
OR
(B) ACK/NAK forwarding delay in RS is the same as RS’s processing capability p in the above equation.
Either of these assumptions are not reasonable, however. Arguably, the ACK/NAK can even be forwarded in the
same frame that it was received. In any case, forwarding delay is likely to be faster than DL data burst
processing delay of RS and MS, respectively, p and j, because RS does not need to perform data burst decoding
or CRC verification.
(2) ACK/NAK forwarding delay is also dependent on the frame structure. Contrast, for instance, the cases when
UL frame has one access zone and one relay zone with the case that UL frame has one access zone and two relay
zones. In such cases, ACK/NAK can be forwarded within one frame or be forwarded in the next frame.
Furthermore, if we consider odd-hop and even-hop frames in a multi-hop situation, then it is quite different as to
how the delay should be calculated.
In short the above equation is applicable under specific assumptions that may not always be the case.
(3) Further more it introduces complexity at the RS that can be avoided. Centralized scheduling is mainly in the
specification for facilitating a simple design of the RS
Additionally, DL HARQ procedure for Non-transparent RS in centralized scheduling is defined in section
6.3.17.4.1 of IEEE 802.16j/D1. However, it is not clear how the HARQ ACK/NAK of retransmitted HARQ
bursts are relayed by the intermediate RSs. This contribution provides the clarification and fix for the issue.
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IEEE C802.16j-07/524r1
Suggested Remedy
First, let us clarify some definitions. If h represents the number of hops that an RS is away from the MS, we
denote the delay incurred at the hth RS in processing a DL burst and preparing the data for forwarding as ph,
while we denote the delay incurred in forwarding the ACK/NAK information as qh. The HARQ ACK delay
incurred at the MS is denoted by j (same as the current 802.16e specification).
BS
Frame i
RS 2
RS 1
p2
Data
DL burst
processing delay
Data
Frame i+1
p1
Data
Frame i+2
Frame i+3
q1
(H)ARQ_ACK
Delay
j
(N)ACK
(N)ACK
Frame i+4
Frame i+5
MS
q2
(N)ACK
ACK Forwarding Delay
Figure 1: The delay incurred in processing a DL burst is (p). The delay incurred in forwarding the
ACK/NACK information (q) can be different from the HARQ ACK delay at the MS (j) which is already defined
in the 802.16e specification.
In general, the delays ph and qh will be different and p and q for different RSs will be different depending on the
capability of each RS. Therefore, according to the Figure, it is clear that the ACK/NAK scheduling delay at any
RS can be given by the following Equation:
H
n    ph  qh   j
h 1
…. Equation (2)
The next thing to consider is whether we need to transmit the hop depth. Even if the hop depth is transmitted, an
RS (e.g. RS2) needs to know the values p and q for all subordinate RSs (e.g. p1 and q1). This means that an RS
will not be able to calculate n based on hop depth alone.
We argue in this contribution that instead of transmitting the hop depth, it is beneficial for the BS to transmit the
frame number (offset) in which the RS should transmit the encoded ACK/NAK. This offset only need to be
transmitted once per HARQ DL MAP because one HARQ DL MAP shall consist of bursts that are same
number of hops away (See page 127, line 16/17).
Moreover, if the BS transmits the frame number at which the encoded ACK/NAK needs to be transmitted, the
RS need not calculate this by itself. This simplifies the RS implementation which is in line with the idea of
centralized scheduling.
The BS shall calculate n according to Equation 2 using p, q and j. The values for p and q may be exchanged
during capability negotiation.
.
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IEEE C802.16j-07/524r1
Remedy for DL HARQ Retransmission
DL HARQ procedure (Transmission and Retransmission) for non-transparent RS in centralized scheduling is
described using example network topology where we consider the complex topology and lots of packet loss and
on different links to show the overall operation and proposed specification changes are specified in specs
changes section.
Consider the network topology as shown in figure 1 where MR-BS has one sub-ordinate RS (RS1). RS1 has two
sub-ordinate RSs (RS2 and RS3) and assume multiple MSs are attached to RS2 and RS3. As mentioned in the
section 6.4.17.4.1 (page 127, line 16), MR-BS combines the HARQ bursts to MSs who are at the same hopdepth away. In this example, we consider that all HARQ burst are transmitted to 3 hop away MSs (MSs attached
to RS2 and RS3). Similar procedure is applicable for MSs who are attached to different hop-depth.
Figure 1, describes the first transmission of HARQ burst and figure 2 describes the retransmission of HARQ
burst along with additional new HARQ transmission.
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IEEE C802.16j-07/524r1
Explanation of figure 2:
MR-BS schedules 5 HARQ bursts for 3 hop away MSs. One of the burst (burst # 2) is assumed to be failed on
1st relay link. It is shown on the arrow as x(2). Similarly other HARQ bursts are failed on other links (access
/relay links) and corresponding failed HARQ burst is shown on the relevant arrow. RS1 failed to receive
HARQ burst #2 therefore it will replace the CID in the corresponding IE with its own Basic CID and at the time
of ACK reporting, it suppose to send code C1 for that burst. Same procedure is applied at RS2 and RS3. While
forwarding the ACK/NAK to super-ordinate RS/MR-BS, RS will perform Cx operation and send encoded
ACK/NAK.
BS
3
4
C2
5
C1
C3
C3
Frame
N+1
RS1
Frame
N+4
RS1
X(5)
3
Frame
N+2
MS
MS
X
x
C2
5
Frame
N+3
RS2
C1
X
5
x
NAK
C2
RS3
ACKCH = 3
HARQ MAP IE = 2
X(3)
x
4
RS3
HARQ MAP IE = 3
X
C1
ACKCH = 2
X(5)
RS2
1)
X(
X(3)
3
X(4)
x
X(
1)
ACKCH = 5
HARQ MAP IE = 5
1
C3
X(2)
2
BS
ACKCH = 5
X(2)
1
Frame
N+5
Frame N
X(4)
HARQ MAP IE = 5
MS
MS
X
NAK
Uplink ACK/NAK Transmission
Downlink HARQ burst Transmission
Figure 2: Initial transmission of HARQ burst
Explanation of Figure 3:
This figure describes the HARQ retransmission along with new transmission. Retransmission is only performed
on the failed link and onwards while the ACK/NAK reporting for the retransmission burst is sent back to MRBS.
MR-BS schedule transmission of new burst (6, 7 and 8) and also retransmit burst #2 which was failed on 1st hop
(see frame N in figure 2).
RS1 has to forward all four burst along with 2 retransmitted burst (#1 and #4) witch were failed at 2nd hop (see
frame N+1 in figure 2), therefore RS1 transmit 6 burst in total.
Assume out of 6, 3 bursts are meant for RS2 and other 3 bursts are meant for RS3. RS2 transmit total 4 HARQ
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IEEE C802.16j-07/524r1
bursts, 3 burst received from RS1 and one retransmitted burst (#3) similarly RS3 transmit 4 HARQ bursts, 3
received from RS1 and one retransmitted burst (#5) (see frame N+2 in figure 1).
In frame N+3, MS sends the ACK/NAK according to 802.16e HARQ procedure.
In frame N+4, RSs will transmit the ACK/NAK for the burst received from super-ordinate RS in the order it has
received. However, RSs will transmit the UL ACK/NAK for the retransmitted burst in the separate region
indicated by “Retx_ACKCH_offset” in the HARQ_ACKCH_region_allocation_for_Relay_Data IE.
For RS2, one HARQ burst was retransmitted (#3) therefore ACK/NAK for it will be reported in the separate
region indicated by “Retx_ACKCH_offset” in the HARQ_ACKCH_region_allocation_for_Relay_Data IE.
For RS3, one HARQ burst was retransmitted (#5) therefore ACK/NAK for it will be reported in the separate
region indicated by “Retx_ACKCH_offset” in the HARQ_ACKCH_region_allocation_for_Relay_Data IE.
RS1 will receive ACK/NAK for the burst it has transmitted along with the ACK/NAK for the retransmitted
burst from RS2 and RS3. RS1 shall peform the Cx operation and relay the ACK/NAK of retransmitted burst to
super-ordinate station. It shall also append its own ACK/NAK of retransmitted burst.
BS
HARQ MAP IE = 3 + 1 (R)
2(R)
6
7
MS1
ACKCH = 4 + 2 (RS1) +
1 (RS2) + 1 (RS3)
8
C0
Frame
N+5
MS2
C0
4(R)
2
3(R)
7
C0
C0
C0
Retx_ACKCH_offset
for RS1
RS1
6
4
5(R)
C0
C0
C0
C0
C0
C0
C0
7
ACK/NAK related to IEs
Frame
N+4
RS2
Retx_ACKCH_offset
for RS2
for RS3
RS3
ACKCH = 4
HARQ MAP IE = 4+1
MS1
C0
8
Frame
N+2
RS3
HARQ MAP IE = 3+1
1
C0
Frame
N+1
6
RS2
C0
ACK/NAK related to IEs
RS1
2
C0
ACKCH = 6 + 1( RS2)
+ 1( RS3)
HARQ MAP IE = 4 + 2 (R)
1(R)
BS
Frame N
C0
8
Frame
N+3
MS2
Downlink HARQ burst Transmission and
retransmission
C0
C0
ACKCH = 4
C0
C0
MS1
MS2
Uplink ACK/NAK Transmission
Figure 3: retransmission and new transmission of HARQ bursts
6
C0
C0
C0
IEEE C802.16j-07/524r1
Proposed Text
Change lines 42-45 on Page 126 as follows:
The number of prescheduled retransmissions for a HARQ flow may be provided to the access RS from the MRBS in the “hop_depth” field of the RS_HARQ_DL_MAP_IE. The UL ACK channel from the access RS to the
MR-BS shall be allocated after the prescheduled retransmission attempts have been acknowledged by the MS.
This ACK/NACK delay shall be specified in the ACK_frame_delay field of the RS_HARQ_DL_MAP_IE.
Change lines 16-23 on Page 127 as follows:
MR-BS shall allocate the HARQ region which contain bursts destined to MSs which have same number of hops
away in common the same ACK_frame_delay by using RS HARQ DL MAP IE. Similarly MR-BS shall allocate
ACKCH by using HARQ_ACKCH region allocation for Relay Data IE.
MR-BS shall indicate the hop_depth ACK_frame_delay in RS HARQ DL MAP IE as well as in
HARQ_ACKCH region allocation for Relay data IE so that RS can map the HARQ burst and corresponding
HARQ ACK/NAK accordingly to indicate to the RS the offset from the current frame for transmitting the
encoded ACK/NAK.
Change the row regarding hop_ depth in Table 411, page 178, as follows:
hop_depth ACK_frame_delay
3 bits 4bits
Reserved
1 bit
B000 and b001 are invalid. When MR-BS/RS
transmits HARQ burst for the nth hop away
MSs, it shall set hop_depth = n. hop_depth
represents the value of H defined in subcluase
8.4.5.4.25. The RS shall transmit encoded
ACK/NAK after ACK_frame_delay frames
from receiving the DL HARQ MAP.
[Change Table 484a as follows]
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IEEE C802.16j-07/524r1
Change Table 484a on Page 186 as follows:
Table 484a—HARQ_ACKCH region allocation for UL Relay Data IE
Syntax
Size
Notes
Extended 2 UIUC
Length
4 bits
8 bits
HARQ ACKCH Region for Relay Data IE = 0x09
Length in bytes
Direction 1 bit
1 bit
0 = IE is related to UL HARQ Data IE
1 = IE is related to DL HARQ Data IE
HARQ_ACKCH_Region_for Relay Data IE()
{
If (direction == 1) {
N_hop
4bits
for (i = 0; i < N_hop; i++) {
hop_depth
3 bits
Reserved
1 bit
ACKCH_offset
8 bits
B000 and b001 are invalid. When MR-BS/RS
transmits HARQ burst for the nth hop away
MSs, it shall set hop_depth =n.
ACKCH_offset indicates the starting
point in the ACKCH region for sending
HARQ ACK/NAK for corresponding
hop_depth.
}
retx_region_present
1 bit
If (retx_region_present) {
N_RS
3 bits
Number_of_RS
RCID_IE()
Variable
RS basic CID
Retx_ACKCH_offset
8 bits
For(j=0; j< N_RS; j++){
}
}
} else {
Reserved
3 bits
}
OFDMA Symbol
8 bits
Subchannel offset
7 bits
No. OFDMA symbols
5 bits
No. subchannels
4 bits
}
Retx_ACHCH_offset
Retx_ACKCH_offset indicates the starting point in the ACKCH region for sending ACK/NAK signals for the retransmitted burst.
RS uses this region to forward the ACK/NAK signals of retransmitted burst and along with relaying the ACK/NAK signals of
retransmitted bursts received from its sub-ordinate RSs. The RS shall transmit encoded ACK/NAK after ACK_frame_delay frames
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IEEE C802.16j-07/524r1
from receiving the DL HARQ MAP.
Change lines 47 (Page 186) – Line 4( page 187) as follows:
When RS receives HARQ DL sub-burst for relaying to MS at frame i, it shall transmit the encoded ACK/ NAK
signal through ACK Channel in the ACKCH region at frame (i + n) where n is given by the ACK_frame_delay
in HARQ DL MAP IE which is transmitted by the MR-BS to the RS. calculated at each RS according to the
following equation.
n = (H-1)*p + H*j
H is equal to “hop_depth” transmitted in RS HARQ DL MAP IE and HARQ_ACKCH region allocation for
relay Data IE. It represents number of hops MR-BS/RS is away from the MS. p is defined by the
"HARQ_burst_Delay for DL Burst" field in the DCD messages.
j is defined by the "HARQ_ACK_Delay for DL Burst" field in the DCD messages. It is applicable to both RS
and MS.
In 2-hop case, there is only one RS and n=p + 2*j.
ACK/NAKs in response to sub-bursts belonging to HARQ DL MAP IEs received in different frames, shall be
transmitted in the order in which the MAPs were received.
Delete lines 6 -16 on page 187:
If pre-scheduling of retransmissions on the access link on the DL is enabled, only HARQ flows with the same
number of pre-scheduled retransmission attempts shall be scheduled into the same HARQ region. An UL
HARQ feedback region is allocated in the frame i + n at an RS for a HARQ region received in frame i, where
n= (H˜ – 1)× p +H˜×j
and and k denotes the number of pre-scheduled attempts. For pre-scheduled bursts, H˜ is specified in the
“hop_depth” field of the RS_HARQ_DL_MAP_IE. At an access RS, H = 1, and hence the number of prescheduled attempts for a HARQ burst can be computed at the access RS as . k=H˜ – 1.
[Insert following paragraph after 3rd paragraph of sub clause 6.3.17.4.1]
When a transmission of an HARQ burst fails, MR-BS shall schedule retransmission of HARQ burst only on the
failed links. The retransmission status (ACK/NAK) shall be conveyed back to the MR-BS. The MR-BS shall
schedule additional UL ACK channel along the path from the failed link to the MR-BS for the retransmission
status such that encoded ACK/NAK report is received by the MR-BS.
MR-BS shall provide the offset to each RS in the “HARQ ACKCH region allocation for data IE” for relaying
the ACK/NAK reports for the retransmitted HARQ burst. The RS has assigned a specific ACKCH sub-region
given by the “Retx_ACHCH_offset” field in the “HARQ ACKCH region allocation for relay data IE” where RS
reports the ACK/NAK signals of the corresponding retransmitted HARQ bursts.
When an RS is performing retransmission of HARQ burst, all the sub-ordinate RSs consider this retransmission
as new transmission and follow the procedure as described in section 6.3.17.4.1. However, the RS that
performed the retransmission of HARQ burst shall report the ACK/NAK signals to the super-ordinate RS in the
ACKCH sub-region indicated by the “Retx_ACKCH_offset” field. Before relaying the ACK/NAK signals of
the retransmitted HARQ bursts to the superordinate station, the RS shall append in the corresponding
retransmission ACKCH sub-region, its own ACK/NAK signals of the retransmitted bursts that failed at its own
link; AI_SN field in the (RS-)HARQ-MAP IE identifies the retransmission. The order in which the RS appends
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IEEE C802.16j-07/524r1
the ACK/NAK reports is the same as the order in which the CIDs of the retransmitted bursts have been appeared
in the (RS-)HARQ_DL-MAP IE. . The RS from where the retransmission was originated shall transmit encoded
ACK/NAK signals to super-ordinate RS/MR-BS after ACK_frame_delay frames from receiving the DL HARQ
MAP. Intermediate RS along the path to MR-BS when receive the encoded ACK/NAK signals from the
retransmission ACKCH sub-region in frame ‘n’ shall forward to superordinate RS in frame ‘n + j’, where j is
defined by the "HARQ_ACK_Delay for DL Burst" field in the DCD messages
Following example figures (xxa and xxb) shows the initial transmission and retransmission of HARQ burst
BS
HARQ MAP IE = 5
4
C2
5
C1
C3
C3
Frame
N+1
RS1
Frame
N+4
RS1
X(5)
3
Frame
N+2
Frame
N+3
MS
MS
X
X(
1
C2
RS2
C1
X
5
x
NAK
ACKCH = 2
MS
MS
Uplink ACK/NAK Transmission
Downlink HARQ burst Transmission
Figure xxa: example of Initial transmission of HARQ burst
1
0
C2
RS3
ACKCH = 3
HARQ MAP IE = 2
X(3)
x
x
5
RS3
HARQ MAP IE = 3
X
C1
4
X(5)
RS2
1)
X(
X(3)
3
X(4)
x
)
ACKCH = 5
HARQ MAP IE = 5
1
C3
X(2)
3
X(4)
2
BS
ACKCH = 5
X(2)
1
Frame
N+5
Frame N
X
NAK
IEEE C802.16j-07/524r1
HARQ MAP IE = 3 + 1 (R)
2(R)
6
7
MS1
ACKCH = 4 + 2 (RS1) +
1 (RS2) + 1 (RS3)
BS
8
C0
Frame
N+5
MS2
C0
4(R)
2
3(R)
7
C0
C0
RS1
6
4
5(R)
C0
Retx_ACKCH_offset
for RS1
C0
C0
C0
C0
C0
C0
C0
7
ACK/NAK related to IEs
Frame
N+4
RS2
Retx_ACKCH_offset
for RS2
for RS3
RS3
ACKCH = 4
HARQ MAP IE = 4+1
MS1
C0
8
Frame
N+2
RS3
HARQ MAP IE = 3+1
1
C0
Frame
N+1
6
RS2
C0
ACKCH = 6 + 1( RS2)
+ 1( RS3)
HARQ MAP IE = 4 + 2 (R)
2
C0
ACK/NAK related to IEs
RS1
1(R)
BS
Frame N
C0
8
Frame
N+3
MS2
Downlink HARQ burst Transmission and
retransmission
C0
C0
ACKCH = 4
C0
C0
MS1
MS2
Uplink ACK/NAK Transmission
Figure xxb: example of retransmission and new transmission of HARQ bursts
1
1
C0
C0
C0