IEEE C802.16m-08/322
Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
HARQ Operational Mode for 802.16m
Date
Submitted
2008-05-05
Source(s)
Yih-Shen Chen, Kelvin Chou, I-Kang Fu and
Paul Cheng
Yihshen.Chen@mediatek.com
Kelvin.Chou@mediatek.com
IK.Fu@mediatek.com
MediaTek Inc.
No.1, Dusing Rd. 1, HsinChu Science-Based
Industrial Park, HsinChu, Taiwan 300, R.O.C.
Re:
IEEE 802.16m-08/016, “Call for Contributions on Project 802.16m System Description
Document (SDD)”. In response to the following topics:
• Hybrid ARQ
Abstract
This contribution proposes HARQ operational modes for 802.16m.
Purpose
To be discussed and adopted by TGm for the 802.16m SDD.
Notice
Release
Patent
Policy
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HARQ Operational Modes for 802.16m
Yih-Shen Chen, Kelvin Chou, I-Kang Fu and Paul Cheng
MediaTek Inc.
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IEEE C802.16m-08/322
1. Introduction
This contribution discusses both downlink and uplink HARQ operational modes for 802.16m, including
asynchronous adaptive HARQ, synchronous adaptive HARQ and synchronous non-adaptive HARQ. Comparing
the HARQ modes, we propose that all the three HARQ modes are applicable for DL HARQ transmissions while
synchronous HARQ modes are for UL HARQ transmissions. And also, we discuss the lost-of-map problem in
asynchronous adaptive HARQ and the signaling overhead reduction for synchronous HARQ. Additionally, the
acknowledgement and data multiplexing issues are addressed.
2. DL and UL HARQ modes
2.1. Comparing of HARQ operational modes
Based on the characteristics of retransmission timing and resource allocation, the HARQ operational modes can
be generally categorized into asynchronous adaptive, asynchronous non-adaptive, synchronous adaptive and
synchronous non-adaptive mode, separately. Here, the synchronous operation means that a retransmission
occurs at a fixed delay after an initial transmission if no ACK is received by the transmitter. The adaptive
operation means that the transmission attributes (e.g.: MCS, packet length, location and etc) may be changed
during a retransmission. Since the asynchronous non-adaptive is non-realistic, only the other three modes are
discussed in the contribution.
Table 1 shows the comparisons between the three operational modes. The asynchronous operation provides
transmission prioritization and can take advantage of scheduling over multiple connections; therefore, it has
multi-user diversity gain. The adaptive mode can dynamically change the transmission attribute; therefore, it has
link adaptation gain. Obviously, the synchronous adaptive HARQ is the most flexible in bandwidth management.
However, the cost for the high flexibility is high signaling overhead. On the other hand, the most prominent
benefit of synchronous non-adaptive mode is simplicity, which requires less signaling overhead. But, it imposes
restrictions on the bandwidth management and can not exploit the frequency diversity gain for retransmission.
The synchronous adaptive HARQ can provide a moderate flexibility for bandwidth management, with restricted
scheduling. And, the signaling overhead is an issue to be addressed.
Advantage
Disadvantage
Asynchronous adaptive
Synchronous adaptive Synchronous non-adaptive
HARQ
HARQ
HARQ
- Multi-user diversity gain - Link adaptation gain -Less signaling overhead
- Link-adaptation gain
- Frequency diversity - Easy scheduling
- Frequency diversity gain gain
- Priority-based
- Easy scheduling
scheduling
- Moderate flexibility
- High flexibility on
on bandwidth
bandwidth management
management
- Large signaling overhead - Large signaling
- No multi-user diversity gain
- Lost-of-MAP issue
overhead
- No link adaptation gain
- Restricted scheduling
- No frequency diversity
gain
- Less flexibility on
bandwidth management
Table 1. Comparison of three HARQ operation modes
2.2. Choices of DL HARQ operation
For DL transmission, all the three modes are possible candidates for DL HARQ operations and each has its own
suitable applications. BE and nrtPS services have higher tolerance in transmission delay so they can be
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transmitted over asynchronous adaptive HARQ channels. As to UGS services, they are suitable to be transmitted
over synchronous HARQ channels due to their transmission periodicity. Since BS has to schedule the
transmission of multiple services/connections over DL frame/subframe, we propose asynchronous adaptive
HARQ as the basic operational mode and synchronous HARQ as supplementary mode to effectively reduce the
signaling overheads.
ACID=0, AISN=0
NACK
ReTx = 0
ACID=0, AISN=0
NACK
Soft buffer
size
...
ReTx = 1
ACID=0, AISN=0
NACK
ReTx = 2
ACID=0, AISN=0
...
Discard!
Soft buffer
size
ACID=1, AISN=0
Figure 1: Data reception in asynchronous DL HARQ mode (soft buffer overflow)
2.2.1. Lost-of-MAP issue for asynchronous DL HARQ operation
The asynchronous DL HARQ mode is currently applied in the 802.16e specification. In the asynchronous
operation, MS has to wait for DL MAP to determine the reception timing of an HARQ burst. The retransmission
procedure is controlled by the maximum number of retransmission. However, MS would have no idea about
whether BS retransmits the HARQ burst or not if the DL MAP happens to loose for some time instances. A
problem of soft buffer overflow may occur. As illustrated in Fig. 1, BS sends an HARQ burst and MS cannot
correctly decode the burst. During the retransmission, MS misses some DL MAPs and the retransmission
exceeds a pre-defined threshold. BS then discards the burst, but MS does not notice that. When BS transmits a
new HARQ burst over another HARQ process, the soft buffer of MS may be overflowed because the discarded
copy still occupies the memory.
To cope with the problem, we propose to add a timer parameter, maximum scheduling delay, for the
asynchronous DL HARQ mode. Figure 2 shows the proposed operation. Every time MS sends a NACK in the
feedback channel, it starts a timer. If no DL_HARQ_IE is received before the timer expiry, it deems that BS had
already retransmitted the HARQ burst. With the timer checking and the maximal retransmission counting, MS
can determine the validity of the data that occupy the soft buffer volume.
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IEEE C802.16m-08/322
ACID=0, AISN=0
NACK
Soft buffer
size
...
ReTx = 0
ACID=0, AISN=0
NACK
ReTx = 1
ACID=0, AISN=0
NACK
ReTx = 2
ACID=0, AISN=0
Soft buffer
size
...
Discard!
ACID=1, AISN=0
Figure 2: Data reception in asynchronous DL HARQ mode with maximum scheduling delay
2.3. Choices of UL HARQ operation
For UL transmission, asynchronous HARQ is not considered because the other-cell emission interference varies
with time and there is some leakage on the UL scheduling gain [1]. We think both synchronous adaptive and
synchronous non-adaptive mode are suitable for UL HARQ operations. As stated in Table 1, there are pros and
cons for adaptive and non-adaptive operations. Although non-adaptive operation has the benefit of low signaling
overhead, adaptive operation can provide flexibility for bandwidth management, which has some compelling
advantages:
1.
Under some bandwidth-congested circumstances, the adaptive HARQ transmission may be temporarily
suspended or changed its MCS.
2.
The retransmitted HARQ burst may be allocated to another radio unit (RU) to exploit frequency diversity
gain.
3.
The retransmitted HARQ burst may be allocated to another carrier in the multi-carrier 802.16m system [2].
Here, we propose a synchronous hybrid mode for UL HARQ transmission, where the synchronous non-adaptive
mode is the baseline and the synchronous adaptive mode is the supplementary one. As shown in Fig. 3, MS
retransmits an HARQ burst with the same parameters if no further UL_HARQ_IE is received from BS.
Otherwise, it retransmits the HARQ burst according to the parameters extracted from the newly received
UL_HARQ_IE. For the adaptive operation, the parameters may be updated partially to effectively reduce the
signaling overhead. The detail updating procedure is TBD.
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IEEE C802.16m-08/322
UL_HARQ_IE
(parameter set #1)
UL_DATA (#1)
UL_HARQ_IE
(parameter set #2)
UL_DATA (#2)
UL_DATA (#2)
UL_HARQ_IE
(parameter set #3)
UL_DATA (#3)
Figure 3: Synchronous hybrid mode for UL HARQ transmission
4. Acknowledgement feedback for HARQ
In DL-HARQ, a common shared UL feedback channel is required for ACK/NACK in an explicit and
synchronous fashion whatever synchronous or asynchronous DL HARQ is applied. Synchronous ACK offset (N)
should be chosen to meet SRD requirement; that is, data transmission latency is within 10ms. The offset value is
also determined by DL/UL ratio and MS processing time. In UL-HARQ, BS may explicitly send ACK/NACK to
MS or implicitly toggle AI_SN for allocating a new initial transmission.
5. Data multiplexing over HARQ
During service negotiation, each HARQ-enabled connection is configured to be transported over multiple
HARQ processes. And, packets from multiple connections can be multiplexed onto a single HARQ process and
distinguished by the MAC headers. It is recommended that the data multiplexing is per-QoS basis, with respect
to delay latency requirement. The numbers of DL and UL HARQ processes are TBD, which is an attribute of
MS terminal capability.
The broadcast channel (BCH) messages and MAC management messages can be also considered to transport
over HARQ. Since the PER and transmission latency requirement for control signaling and data connections are
different, dedicate HARQ processes for BCH and MAC management messages are preferable. The detail data
multiplexing scheme is TBD.
6. Text Proposal
----------------------- Text Start ----------------[Insert the following text in TGm SDD [5]]
10.x Hybrid-ARQ (HARQ)
10.x.1 Comparing of HARQ operational modes
Table 1 shows the comparisons between the three possible operational modes. The asynchronous operation
provides transmission prioritization and can take advantage of scheduling over multiple connections; therefore,
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IEEE C802.16m-08/322
it has multi-user diversity gain. The adaptive mode can dynamically change the transmission attribute; therefore,
it has link adaptation gain. From the table it can be found that the synchronous adaptive HARQ is the most
flexible in bandwidth management. However, the cost for this high flexibility is higher signaling overhead. On
the other hand, the most prominent benefit of synchronous non-adaptive mode is simplicity, which requires less
signaling overhead. But, it imposes restrictions on the bandwidth management and can not exploit the frequency
diversity gain for retransmission. The synchronous adaptive HARQ can provide a moderate flexibility for
bandwidth management, with restricted scheduling. But, the signaling overhead is an issue in synchronous
adaptive HARQ.
Advantage
Disadvantage
Asynchronous adaptive
Synchronous adaptive Synchronous non-adaptive
HARQ
HARQ
HARQ
- Multi-user diversity gain - Link adaptation gain -Less signaling overhead
- Link-adaptation gain
- Frequency diversity - Easy scheduling
- Frequency diversity gain gain
- Priority-based
- Easy scheduling
scheduling
- Moderate flexibility
- High flexibility on
on bandwidth
bandwidth management
management
- Large signaling overhead - Large signaling
- No multi-user diversity gain
- Lost-of-MAP issue
overhead
- No link adaptation gain
- Restricted scheduling
- No frequency diversity
gain
- Less flexibility on
bandwidth management
Table 1. Comparison of three HARQ operation modes
10.x.2 Choices of DL HARQ operation
For DL transmission, all the three modes are possible candidates for DL HARQ operations and each has its own
suitable applications. BE and nrtPS services have higher tolerance in transmission delay so they can be
transmitted over asynchronous adaptive HARQ channels. As to UGS services, they are suitable to be
transmitted over synchronous HARQ channels due to their transmission periodicity. Since BS has to schedule
the transmission of multiple services over DL frame/subframe, asynchronous adaptive HARQ is chosen as the
basic operational mode and synchronous HARQ as supplementary mode to effectively reduce the signaling
overheads.
10.x.3 Choices of UL HARQ operation
For UL transmission, asynchronous HARQ is not considered because the other-cell emission interference varies
with time and there is some leakage on the UL scheduling gain [1]. Both synchronous adaptive and synchronous
non-adaptive mode are suitable for UL HARQ operations. Therefore, a synchronous hybrid mode is preferable
for UL HARQ transmission, where the synchronous non-adaptive mode is the baseline and the synchronous
adaptive mode is the supplementary one. As shown in Fig. x, MS retransmits an HARQ burst with the same
parameters if no further UL_HARQ_IE is received from BS. Otherwise, it retransmits the HARQ burst
according to the parameters extracted from the newly received UL_HARQ_IE. For the adaptive operation, the
parameters may be updated partially to effectively reduce the signaling overhead. The detail updating procedure
is TBD.
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IEEE C802.16m-08/322
UL_HARQ_IE
(parameter set #1)
UL_DATA (#1)
UL_HARQ_IE
(parameter set #2)
UL_DATA (#2)
UL_DATA (#2)
UL_HARQ_IE
(parameter set #3)
UL_DATA (#3)
Figure x: Synchronous hybrid mode for UL HARQ transmission
10.x.4 Acknowledgement feedback for HARQ
In DL-HARQ, a common shared UL feedback channel is required for ACK/NACK in an explicit and
synchronous fashion no matter either synchronous or asynchronous DL HARQ is applied. Synchronous ACK
offset (N) should be chosen to meet SRD requirement; that is, data transmission latency is within 10ms. The
offset value is also determined by DL/UL ratio and MS processing time. In UL-HARQ, BS may explicitly send
ACK/NACK to MS or implicitly toggle AI_SN for allocating a new initial transmission.
10.x.5 Data multiplexing over HARQ
During service negotiation, each HARQ-enabled connection is configured to be transported over multiple
HARQ processes. And, packets from multiple connections can be multiplexed onto a single HARQ process and
distinguished by the MAC headers. It is recommended that the data multiplexing is per-QoS basis, with respect
to delay latency requirement. The numbers of DL and UL HARQ processes are TBD, which is an attribute of
MS terminal capability.
The broadcast channel (BCH) messages and MAC management messages can be also considered to transport
over HARQ. Since the PER and transmission latency requirement for control signaling and data connections are
different, dedicate HARQ processes for BCH and MAC management messages are preferable. The detail data
multiplexing scheme is TBD.
-----------------------Text End -------------------
References
[1] IEEE 802.16m-08/151, “Downlink Control Structure related to Hybrid-ARQ”
[2] IEEE 802.16m-08/178r1, “Proposal for IEEE 802.16m Resource Allocation and Control Structure for multiCarrier Operation”
7