IEEE C802.16x-07/NNN
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 more bursts cannot be further saved when the buffer is
full. In case of buffer overflow, that is reception of more (erroneously decoded) data than the buffer capability,
the HARQ performance drastically 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  ∞).
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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 avoided 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
The scheduling limitation actually 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 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 a pure 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 C802.16x-07/NNN
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
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HARQ buffer management protocol
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
2. Smart handling of buffer overflow events, like on-the-fly allocation of more buffering resources
3. Refrain from using part of the buffer for soft-bits than were not transmitted yet by the HARQ-IR
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
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IEEE C802.16x-07/NNN
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 H-ARQ downlink buffer size limitation”, Intel, 2008-07-07
4