IEEE C802.16m-09/0306r1
Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
HARQ buffer management for IEEE 802.16m
Date
Submitted
[2009-1-7]
Source(s)
Tom Harel, Yuval Lomnitz
E-mail:
tom.harel@intel.com
Intel Corporation
Re:
802.16m amendment working document
Abstract
Text proposal for FEC and HARQ section, on the subject of HARQ buffer management
Purpose
To be discussed and adopted by TGm for the 802.16m amendment.
Notice
Release
Patent
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HARQ buffer management for IEEE 802.16m
Tom Harel, Yuval Lomnitz
Intel Corporation
Introduction
802.16m AAI MS is required to support HARQ by combining the information in a re-transmission with
information from previous transmissions. The information from transmission that was not decoded correctly
might be saved in form of soft-bits or received bits’ metrics or received symbols.
In practical implementation, the MS saves information from bursts that were not decoded correctly in a buffer
called HARQ buffer. This buffer has a limited size, and additional bursts cannot be further saved when the buffer
is full. The size and management scheme of this buffer affects both the maximum throughput of the MS and the
performance of HARQ. Since large buffers are required in order to handle the traffic rates supported in 802.16m
the buffer management scheme should balance the buffer utilization with HARQ performance.
In [4] we had shown that gains of x2 in maximum throughput can be attained with negligible effect on spectral
efficiency, by over-utilization of the buffer. However the scheme presented in [4] is only one particular example
and to our opinion 802.16m should support a wider scope of buffer optimization schemes.
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IEEE C802.16m-09/0306r1
Discussion
Buffer overflow
In case of buffer overflow, which is defined as reception of more (erroneously decoded) data than the buffer
capability, the HARQ performance deteriorates. This problem becomes more severe for HARQ-IR and adaptive
HARQ since:
1. In HARQ-IR the re-transmission might include only parity bits. The CTC decoder performance in the
absence of some initial information on systematic bits is bad, and in some cases it is impossible to
decode a re-transmission without using the original transmission even without noise (SNR  ∞).
2. In adaptive HARQ the size of the re-transmission might be very small, assuming most of the information
needed for decoding was received in the previous transmissions.
From the reasons above, HARQ buffer overflow should be managed by the BS’s scheduler, which manages the
data transmission to the MS. The MS shall report its HARQ buffer size, and BS shall track the occupancy level
in MS’s buffer, and limit the amount of new information sent to the MS.
Maximal throughput and HARQ buffer occupancy
A conservative buffer management scheme avoiding overflow limits the maximal throughput that can be sent to
an MS with given HARQ buffer size. Supporting high throughput traffic, about 180 Mbits/sec for 2x2 MIMO
and 20 MHz BW, is one of the goals for 16m. Using conservative design as applied in 802.16e might require
large buffers in order to support such throughput, as depicted in the calculation:
The maximal supported throughput is given by:
thput 
BufferSize * Rmax
RTT
Rmax is the maximal code-rate (e.g. 5/6). RTT (Round Trip Time) is the time from a transmission to its
retransmission or new transmission (including receiver processing time, (N)ACK channel signaling and
scheduling of re-/new transmission) in 16m is at least 1 subframe (5 ms), and BufferSize is measured in
number of soft-bits or metrics in the buffer.
The reason for this throughput higher bound is that BS cannot send more coded bits than the MS buffer size
before receiving an acknowledgement that some bursts were received and decoded correctly, making room for
new information.
This worst case approach ensures that HARQ buffer is never overflowed (unless there are errors in control
signaling), but actually it makes poor usage of the buffering capability. Assuming the information in one frame
is divided into 16 bursts of equal size, each one on a different HARQ channel, and the probability of error is 0.3
for each burst and independent among them. Then the probability of having fully occupied buffer by the end of
the frame is as small as 4*10-9. This shows that more information could have been sent to the MS with
negligible error probability. The following figure shows that with probability 10-2 up to one half of the buffer (8
bursts) is occupied at the end of the frame.
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IEEE C802.16m-09/0306r1
Probability for x failures in 16 trials with fail probability 0.3
Binomial distribution
0
10
-2
10
Probability
-4
10
-6
10
-8
10
-10
10
0
2
4
6
8
10
12
Buffer occupancy at the frame end
14
16
HARQ buffer management and MS behavior
The conservative buffer management protocol of avoiding overflow wastes resources and limits the maximal
user’s throughput. Therefore BS might over-use the MS buffer by transmitting more information than the
declared MS buffer size, using statistical assumption on the maximal number of failures in frame (that won’t
cause buffer overflow). However, the statistical assumption for the buffer over-usage depends on MS buffer
management.
MS might effectively enlarge its buffer size, therefore allowing more buffer over-usage, for example:
1. Evacuate correctly decoded FEC blocks from the buffer (requires additional storage of decoded bits)
2. Smart handling of buffer overflow events, like on-the-fly allocation of more buffering resources
3. Refrain from reserving buffer space for all of the mother-code-bits and save metrics only for actually
received coded bits
4. Trade off performance for buffer size (lossy compression of received information)
As can be observed from the above list it might be difficult and undesired to fully standardize the MS internal
buffer management. As an example, in order to fully specify the first item to the BS the MS needs to report the
size of the soft bit buffer, the size of the decoded bit buffer, and some parameters regarding the delay of
evacuating correctly received blocks. Even with this information, the BS does not have the full information in
order to decide on the maximum traffic to send, since the utilization of these buffers depends on the channel
behavior. From this reason we propose a different approach: the MS internal buffer management is not exposed
to the BS, but instead, the BS receives metrics from the MS that will allow it to manage the traffic rate. These
metrics will encompass the channel behavior as well as the specific MS implementation.
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IEEE C802.16m-09/0306r1
Buffer occupancy factor feedback
In order to allow BS to over-use the HARQ buffer correctly we suggest two kinds of feedback from MS to BS:
1. HARQ buffer size: the MS reports as a capability the number of coded soft-bits it can save in the HARQ
buffer
2. Buffer occupancy characterization: a measure of the buffer usage efficiency, that is reported periodically
or upon BS request
References
[1] IEEE P802.16 Rev2/D7, “Draft IEEE Standard for Local and Metropolitan Area Networks: Air Interface
for Broadband Wireless Access,” Oct. 2008.
[2] IEEE 802.16m-08/003r6, “The Draft IEEE 802.16m System Description Document”
[3] IEEE 802.16m-08/043, “Style guide for writing the IEEE 802.16m amendment”
[4] IEEE C802.16maint-08/112r4, “Relaxation of HARQ downlink buffer size limitation”, Intel, 2008-07-07
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IEEE C802.16m-09/0306r1
Text proposal
15.3.xx HARQ
15.3.xx.yy DL HARQ buffer management
MS reports its HARQ buffering capability as part of capability negotiation defined in 15.??? (TBD). The DL
HARQ buffering capability indicates the number coded soft-bits that can be stored for combining.
The buffer size is indicated by a value of K=0..63, where the number of soft-bits is floor(8192*2K/4).
The buffer is aggregated between all of the HARQ channels. When the sum of the number of soft-bits stored in
HARQ buffer for all HARQ channels is smaller or equal than the buffer size, the MS shall fully combine the
soft-bits stored in the buffer with newly received re-transmissions.
If the MS receives more information than its buffering capability, it shall try to decode it (whether it is HARQ or
non-HARQ) without saving the new soft bits in the HARQ buffer for combining.
The buffer shall be considered to contain n/RMC soft-bits after a first transmission, where n is the number of
information bits in the transmission, including CRCs, and RMC is the mother-code-rate, even though the actual
number of transmitted bits might be smaller.
When HARQ buffer occupancy characterization report is requested by BS, as described in section 15.???
(TBD), the MS shall report the efficiency of its HARQ buffer usage. The efficiency is defined as the ratio
between the amount of information received during the period of report and the amount of information that
could have been received, under the same channel and allocation conditions with overall degradation in spectral
efficiency not exceeding 4% compared to a situation where the soft metric buffer size is not exceeded. The
efficiency is computed by the MS by a dry run of the communication scenario during the report period
(reception, misreception, HARQ channel usage etc), assuming only the amount of traffic increases by a given
proportion. The efficiency calculation should include any degradation due to projected buffer overflows and
possible changes in receiver sensitivity, and may be interpreted as the MS recommendation for a tolerable traffic
increase, under the same conditions.
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