SDD text proposal for IEEE 802.16m HARQ and FEC Document Number:
IEEE C802.16m-08/606 Date Submitted: 2008-07-07 Source: Tom
[email protected] Yuval [email protected] Olga
[email protected] Intel Corp. Venue: Call for Contributions
on Project 802.16m System Description Document (SDD): Hybrid ARQ
(PHY aspects) Base Contribution: Purpose: Accept changes for SDD
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Slide 2 2 Key points Select 802.16e CTC as the FEC (Forward Error
Correction) scheme Extensions for performance improvement: Minimal
code-rate , and optional extension to Support additional block
lengths HARQ-IR (Incremental redundancy) Flexible mother code rate
Constellation re-arrangement with version per symbol Variable
re-transmission size (adaptive or static) Exploit frequency
selectivity in re-transmission How many MCSs we need, and how to
describe them Rate matching to accommodate variable number of
symbols and pilots Padding is a waste of resources => Continuous
code-rate instead of MCS specification Slide 3 3 FEC selection We
suggest to use only 802.16e CTC coding scheme (section 8.4.9.2.3.1)
for all data traffic and maps An MS must implement 802.16e CTC for
backward compatibility, and designing a fresh new FEC scheme will
double the HW CTC/LDPC decoder has considerable hardware complexity
compared to overall PHY Convolutional Code used for FCH backward
compatibility only Good performance/complexity tradeoff for the
relevant block lengths (48-4800 bits) Slide 4 4 Minimal code rate
We considered a simple low-complexity extension of 16e code to
minimal code rate instead of Higher coding gain improves
performance in low SNR range by ~0.2- 0.4dB For QAM modulation
(higher SNR range), re-transmission of bits using constellation
re-arrangements performs better than sending more parity bits Since
the overall system performance improvement of rate is small,
minimal code-rate = is recommended See Optional code-rate support
(1)Optional code-rate support (1) Slide 5 5 FEC block length
granularity Block lengths in 802.16e, including IR HARQ (tables 524
and 525): 6, 9, 12, 18, 24, 27, 30, 36, 48, 54, 60, 120, 240, 360,
480, 600 Data block size (bytes) Granularity: finer granularity of
supported block length (2-3 bytes for small packets), to reduce the
padding waste Maximal block size: the marginal gain from larger
block size is negligible, and implementation (memory) complexity is
high, so we propose 600 bytes (4800 bits) as maximal block size For
new lengths, new interleavers should be designed However, because
of tail-biting, the length cannot be a integer multiple of 7 We
propose a method to generate new block lengths from larger CTC
block, called known padding: 1.Select a larger block size 2.Add
pair of zeros to the message as padding (spread them among the
message bits) to get the larger block length 3.Diminish the
systematic padding bits from the sequence of encoded bits, as well
as the parity bits from the same step in the trellis 4.Transmit
only the remaining bits Simulation results show that this method is
approx. equivalent to design of a smaller block length. Slide 6 6
HARQ-IR (1) We propose to support only HARQ-IR (incremental
redundancy) for traffic channel Chase-combining is a special case
Non-HARQ is a special case Each transmission can start in various
positions in the sequence of mother code bits This sequences length
is defined by burst size and Mother-Code-Rate (MCR) Support
flexible MCR for memory limited, high throughput users Compared to
chase combining, HARQ-IR requires more buffering in receiver, i.e.
supports lower throughput for same buffer size By changing MCR we
can allow high throughput when required with smaller robustness and
robust transmission at low throughput E.g. support MCR = , ,
Without flexible MCR the throughput achieved with given buffer size
would reduce when changing to code rate 1/3 Slide 7 7 HARQ-IR (2) -
CoRe Constellation re-arrangement (CoRe) change the bits mapping in
re- transmission for 16-QAM, 64-QAM The gain is from improving the
LSB quality and equalizing the LLR quality of the constellation
Only 2 versions (switch LSB and MSB) achieve most of the gain with
minor overhead Version is selected for each QAM symbol (rather than
same version over the packet) and can be changed within a single
(re) transmission as depicted below For MCR=, CCR= Slide 8 8 CoRe
and Mother-Code-Rate (1) Slide 9 9 CoRe and Mother-Code-Rate (2)
Slide 10 10 Variable re-trans size HARQ-IR In.16e the size of the
re- transmission is the same as the original transmission (same
MCS). For high SE we see better performance with smaller re-
transmission size (static) Two types: Static: constantly set
smaller size of re-transmission Simple to design, define and
simulate, ~8% SE improvement Adaptive: set the re-transmission size
according to channel state or another feedback More complicated
design. Up to ~20% SE improvement by information theoretic analysis
Yet to be explored We expect higher gain from opportunistic
scheduling with smaller re-transmission Slide 11 11 Frequency
diversity in re-trans. For slowly-changing frequency-selective
channel, it is important to change the mapping of symbols to
subcarriers in re-transmissions to improve frequency diversity In
case of chase-combining (I.e. high MCR), the symbols position shall
be changed in re-transmission (E.g. by starting point selected by
SPID) Slide 12 12 How many MCSs we need? On one hand With HARQ the
spectral efficiency is not very sensitive to MCS selection It shows
that we have enough MCSs in 16e On the other hand We have a rough
granularity of resource allocation Slot size is selected according
to considerations like: channel estimation, diversity etc. With
small number of MCSs a lot of padding is needed Zero-padding is
expensive It waste system resources (energy, BW) It enlarges the
error probability (detection error on unimportant parity bits)
Slide 13 13 An alternative almost continuous CR We suggest to
support finer resolution of burst size (to avoid padding) while
keeping the same slot size Need more MCSs. Or: define MCS in an
alternative manner Allocation fixes the number of resources
(slots/LRU) Describe in the map the message size An exponential
scale of possible sizes Only a few bits for the length of short
messages. More bits for the larger ones. => almost continuous
code rate selection is supported Slide 14 14 16m proposal versus
16e and WiMAX 1.0 ItemWiMAX 1.016e IR16m proposedComment /
Motivation H-ARQ schemeChaseIR Higher SE Mother code rate1/3..
Lower code rate Variable M.C.R Coding schemeCTC Good performance +
backward compatibility Symbol mappingGray Co-Re Better Performance
Concatenation ruleYes, modulation dependent NoneYes, modulation
independent FEC block granularity~6BCoarse~2-3B for small blocks
Padding overhead versus Map overhead tradeoff Variable re-trans
sizeNo Yes Max FEC block size (bits) 4804800 Performance v.s
complexity tradeoff Slide 15 15 Text proposal for SDD (1) Insert
the following text into Physical Layer clause (Chapter xx in [IEEE
802.16m-08/003r1]) ------------------------------- Text Start
------------------------------- 11.x Channel coding and Hybrid-ARQ
11.x.1 Block diagram And repetition? Slide 16 16 Text proposal for
SDD (2) 11.x.2 Modulation In IEEE 802.16m each data tone is
modulated with QPSK, 16-QAM or 64-QAM constellation, and carries 2,
4 or 6 coded bits respectively. For 16-QAM and 64-QAM the mapping
of bits to constellation changes in HARQ re-transmission, and is
selected by CoRe-version. There are 2 possible mappings, between
which the LSB and MSB bits are switched. 11.x.3 Encoding 11.x.3.1
Coding scheme IEEE 802.16m uses the CTC code rate defined in
802.16e for encoding all traffic and control channels, except UL
control channels and FCH in backward compatible mode. The coding
scheme is extended to support additional FEC block sizes. The
maximal block size is 4800 information bits. The resolution of
block sizes for small blocks will be 16-24 bits. The definition of
the code rate in the map is TBD and will support granularity of
16-24 bits for small PDUs. 11.x.3.2 Rate matching In IEEE 802.16m
the number of subcarriers in a logical allocation (LLRU) varies due
to e.g. varying number of pilots and shortened subframes. The
amount of coded bits is adapted to the allocation size by using a
flexible channel code-rate. The bit- selection takes an arbitrary
number of mother coded bits for transmission according to the
allocation size. Slide 17 17 Text proposal for SDD (3) 11.x.3.3
Repetition Repetition is done when the number of transmitted bits
is larger than the number of mother- coded bits (total number of
information and parity bits generated by FEC encoder). The coded
bits selection is done cyclically over the buffer of encoded
message. 11.x.3.4 Concatenation rules and interface to resource
allocation Concatenation rules are based on number of information
bits and do not depend on the structure of resource allocation
(number of LLRUs and their size). Concatenation rules include 2
parts: partition of the message bits, and partition of the
allocated tones. 11.x.4 HARQ IEEE 802.16m implements Incremental
Redundancy HARQ. The transmitted coded bits are selected from the
mother-code bits by the bit-selection that depends on the
re-transmission number. The SPID determines the starting points and
the bits are taken sequentially and cyclically. The mother code
rate (MCR, between 1 and 1/3) may be controlled by the BS in order
to trade-off buffer requirements, throughput and robustness. When
the mother code rate is higher than 1/3, only the first n k /MCR
encoder outputs are considered for HARQ. Slide 18 18 Text proposal
for SDD (4) For each transmitted bit the CoRe-version is selected
by the number of transmission of this bit. A possible mechanism is
to indicate the version of the first bit transmitted in each
(re)transmission, and flip the version when returning from the last
encoded bit to the first. The number of subcarriers used in each
retransmission may vary in a fixed or adaptive way (adaptive H-ARQ
is for further study). IEEE 802.16m will support changing the
allocation of coded bits to subcarriers between retransmissions
(the specific mechanism is TBD) -------------------------------
Text End ------------------------------- Slide 19 19 Backup slide
Slide 20 20 CoRe and mother-code-rate (1) Slide 21 21 CoRe and
mother-code-rate (2) Slide 22 22 CoRe and mother-code-rate (3)
Slide 23 23 Optional code-rate support (1) Proposal: decrease
minimal code rate to to improve coding gain Design method: Optimal
third polynomial given the previous ones Pros: Simple
low-complexity change (same trellis, modify only LLR calculation)
same decoder Fully backward compatible with 16e CTC (as a feature
subset) Improvement of maps and uplink implies better coverage
Cons: Performance improvement is relatively small (0.2-0.3 dB)
compared to code-rate [some simulations showed higher gain]
Negligible improvement for high spectral-efficiency cases
(16/64-QAM), even with HARQ-IR Slide 24 24 Optional code-rate
support (2) Green- code rate , Red code rate 1/3, Blue code rate .
A gain of 0.4dB is obtained when the code rate is decreased from
1/3 to A gain of 0.4dB is obtained when the code rate is decreased
from 1/3 to Slide 25 25 Optional code-rate support (3) X1X1 X2X2 +
--- For the X parity bit: 0x3, equivalently 1+D Slide 26 26
Optional code-rate support (4) Sub-packet generation X 1 subblock X
2 subblock
