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WiMAX Hybrid ARQimplementation in NS-2
SeungWoon Kim
Jeonghoon Mo
Information and Communications University{swkim,jhmo} AT icu.ac.kr
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Overview
• 802.16 Hybrid ARQ
• Modeling HARQ gain
• HARQ impact on TCP
• Future Work and Plan
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Overview
• 802.16 Hybrid ARQ
• Modeling HARQ gain
• HARQ impact on TCP
• Future Work and Plan
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802.16 Hybrid ARQ Overview
• FEC + ARQ
– Chase Combining (CC) or Incremental Redundancy (IR)
– Stop and Wait ARQ
• Feedback– Dedicated Fast Feedback Channel
• Retransmission
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Hybrid ARQ DL Operation
1. BS Transmits Data
2. BS informs the location of ACK Channel
3. MS sends feedback (ACK/NACK) to the BS using the ACK Channel.
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BS Transmits Data
• HARQ DL-MAP IE– Defines 2D region for
HARQ in a frame.
• HARQ mode:– Chase Combining– IR with CTC, IR with CC– MIMO Chase, MIMO IR
HARQ, MIMO IR HARQ for CC, MIMO STC
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BS Transmits Data
• HARQ DL-MAP Chase Sub-burst IE– Each subburst IE corresponds to
one subburst
– RCID: reduced CID
– ACID: HARQ Channel ID (4bit)
– AI_SN: ARQ Identifier Seq. Number (1bit)
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BS informs the loc. of ACK Channel
• HARQ ACK region is informed to MS using HARQ ACKCH region allocation IE.
• One ACK occupy a half slot
– To transmit 16 ACKs, 8 slots are needed.
Burst#1
(x1,y1)
(x2,y2)
time
Frequency
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MS sends the ACK/NACK
• Synchronous ACK Channel– HARQ_ACK_DELAY can be one, two or three (UCD)
– MS returns ACK/NACK after HARQ_ACK_DELAY frame.
• Predetermined location– The order of ack channels is the HARQ enabled bursts order
in DL MAP.
– MS needs to know the order of its burst in among HARQ enable DL_MAP_IEs in DL_MAP.
• ACK Channel Encoding– The 1 bit ACK/NACK information is encoded into length 3
code words over 8-ary alphabet.
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Hybrid ARQ UL Operation
1. BS grants Trx Chance. (HARQ UL_MAP)
2. MS transmits Data
3. BS sends ACK/NACK
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Ack Transmission for UL Data
• Two methods– Explicit Transmission by HARQ ACK-IE (bitmap)
– Implicit Transmission using AI_SN
• Explicit HARQ ACK-IE
• Implicit Transmission – IF AI_SN field in UL subburst-IE is toggled, ACK
– Otherwise, NACK (MS retransmits data)
• Fixed delay between : – HARQ_ACK_DELAY for UL_burst, 1, 2, or 3 frame time
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HARQ Buffer Capability
• The maximal number of data bits the SS is able to store DL/UL HARQ
• Two parameters– Number of bits per channel
• Total number of bits that SS may buffer per channel
– Aggregation flag
• If flag == 1, buffer can be shared;
• otherwise, it cannot be shared
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HARQ vs. MAC ARQ
HARQ MAC ARQHigh Complexity Receiver Simple Layer 2 Operation
Stop and Wait ARQ Selective Feedback
High Feedback Overhead Low Control Overhead
Packets out of Order Packets in Order
CC/IR gain No CC/IR Gain
Dedicated Ack Channel and fixed delay
No Dedicated Channel and Long Variable Delay
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Overview
• 802.16 Hybrid ARQ
• Modeling HARQ gain
• HARQ impact on TCP
• Future Work and Plan
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Modeling HARQ in NS-2
•How much is the HARQ gain?
Oirginal Transmission
Retransmission
Block1 Error
Error
Block 1’’ Error???Block 1’’ = Block1 + Block 1’
Block 1’
Receiver may or may not recover the error
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Error Computation in Rel. 2.1
• On receiving a packet
i, 0itotal_subcarrier, calculate
1) Interferencepower[i]
2) signalpower[i]
3)
Block1 Block2 Block3
PacketMax block size
Packet size in slot
For each block,
indexk = f (modulation, block_size)
k = f(mobility model, indexk)
Bernoulli toss with pk = BLERk to determine Block error
Then, PER = 1-(1- pk)
Last block size
)cepower[i]interferen
r[i]signalpowe(log10
SINR[i]
10
k
k
blocksubcarrier
i
iSINR
k eEESM__#
0
][
)_#log(k
kkk blocksubcarrier
EESMSIR
),(_ kkk SIRindexlookuptableBLER
…
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Important Factors
• HARQ Region
• Modulation and Coding Rate– Each subburst can have different modulation and
coding.
• # of subchannels that data is using
• Subchannelization Method: PUSC, FUSC, AMC
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Simplicity
• Different Levels of Abstraction
– Bit level
– Subcarrier level
– Block level
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Bit Level Abstraction
• Code block: [c1, c2, …, cN]
• SNR computation per each code symbol
• Due to Adaptive modulation and coding, a group of code in the same subcarrier may be separated into different subcarriers in the retransmission.
• High Complexity
C1 C2 C3 C4 C1 C2
1st Transmission16QAM
2nd TransmissionQPSK
C3 C4
4 codes in one subcarrier block 2 codes in one subcarrier block
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Subcarrier Level Abstraction
• Keep Track of SNR per subcarrier
• Use EESM or some other methods to have representative values
• High Complexity
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Block Level Abstraction
Block1 Block 1’ Block1’’
SNR(1)BLER(1)
SNR(1’)BLER(1’)
Given
SNR(1’’)BLER(1’’)
SNR(1’’) = f(SNR(1), SNR(1’), BLER(1), BLER(1’’))
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Block Level Abstraction
• Would like to find combined SINR c(n), combined
block error rate BLERc(n)
• Compute BLER from c(n)
n
i
nc in
1
1 )()(
# Retransmission
BLER
Combined SINR 10,
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Overview
• 802.16 Hybrid ARQ
• Modeling HARQ gain
• HARQ impact on TCP
• Future Work and Plan
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TCP over Wireless Channel
• TCP is sensitive to losses.
• Factors affecting TCP performance– FEC factors
• Recovering Power
– ARQ factors• Fragmentation Size
• Maximum try of Retransmission
• Coding rate when with FEC
• Treatment of crucial fragment, especially the last fragment of a TCP packet
– TCP factors • AWND size
• RTO, RTT and CWND size
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A Few Related Work
• Retransmission helps TCP performance. What is the best retransmission strategy? [1]
– Retrx. with Fixed Limit
– Persistent Retransmission
– Adaptive Retransmission
• Video and HARQ [2]
– Modeled the channel to Multistate Markov Chain (MSMC)
• more accurate than two-state Gilbert-Elliott model
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Retrx. # vs. TCP Thput [1]
• Persistent Retransmission is the best
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Related Work [2]
• Showed HARQ improves Residual Packet Error Rate (RPER) and TCP throughput– Error correction codes are beneficial in some
range of SNR, and with high mobility
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Future Work
• HARQ Implementation to Release 2.1
– By early November, alpha version
– By late November, beta tested version
• Study the implication of HARQ on TCP performance– By December, 2007
– TCP timeout, packet error rate, throughput, delay
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References
[1] F. Vacirca et al, “Optimal Design of Hybrid FEC/ARQ Schemes for TCP over Wireless Links with Rayleigh Fading”, IEEE TRANSACTIONS ON MOBILE COMPUTING, VOL. 5, NO. 4, APRIL 2006
[2] Hang Liu and Magda El Zarki, “Performance of H.263 Video Transmission over Wireless Channels Using Hybrid ARQ,” IEEE JSAC, Dec. 1997