1. WiMAX Hybrid ARQ implementation in NS-2
SeungWoon Kim
Jeonghoon Mo
Information and Communications University {swkim,jhmo} AT icu.ac.kr
Today, I would like to present the progress of HARQ module implementation for Wimax ns-2 simulator, and the research plan on the Impact analysis of WiMAX Hybrid ARQ on TCP.

2. Overview 802.16 Hybrid ARQ Modeling HARQ gain HARQ impact on TCP
Future Work and Plan
Before going into the detailed simulator implementation, I’ll present a brief introduction to IEEE Hybrid ARQ.
And then, I’ll tell you how I’m going to implement HARQ.
%And then, I’ll introduce the research issue related to HARQ and TCP.
Finally, I’ll end up with future work and plan.
Let’s Move on to the next page.

3. Overview 802.16 Hybrid ARQ Modeling HARQ gain HARQ impact on TCP
Future Work and Plan
Before going into the detailed simulator implementation, I’ll present a brief introduction to IEEE Hybrid ARQ.
And then, I’ll tell you how I’m going to implement HARQ.
%And then, I’ll introduce the research issue related to HARQ and TCP.
Finally, I’ll end up with future work and plan.
Let’s Move on to the next page.

4. 802.16 Hybrid ARQ Overview FEC + ARQ Feedback Retransmission
Chase Combining (CC) or Incremental Redundancy (IR)
Stop and Wait ARQ
Feedback
Dedicated Fast Feedback Channel
Retransmission
At first, I’ll present a brief introduction to H-ARQ.
- Hybrid Automatic Repeat Request is an error control technique at Link layer,
and is combination of mac layer ARQ and physical layer FEC.
- When there’s any error in a packet, first try to correct the error by FEC, and then retransmit the packet by ARQ
%- ARQ recovers a frame by retransmitting the whole frame repeatedly based on ACK or NACK.
%It’s effective when the channel is lossy and burst bit errors occur.
%- FEC recovers a frame by its error correcting procedure.
%When spare bit errors are usual, it’s easy to recover the frame, and it reduces the number of retransmission.
%But when there are bit errors in burst and when it’s hard to recover with the few information bits, it’s better to retransmit the whole packet by ARQ.
provides FEC and ARQ, and Hybrid ARQ as well. Chase Combining and Incremental Redundancy is provided at the standard, but we are using Chase Combining only. And, it’s simple stop and wait arq.
The main difference from ARQ is that HARQ uses dedicated Fast Feedback channel to send ACK or NACK. For the DL data, UL Ack is located in the stating position of UL subframe, and for UL data, AI_SN or Ack bitmap is located in the map.

5. Hybrid ARQ DL Operation
BS Transmits Data
BS informs the location of ACK Channel
MS sends feedback (ACK/NACK) to the BS using the ACK Channel.
This figure shows how DL HARQ operates.
When there’s a data to send from BS to MS,
BS transmits DL-MAP notifying the position of the data, and at the same frame HARQ data is sent to MS.
At the same frame, BS allocates ACK channel for the MS,
then after “HARQ ACK Delay for DL burst” frame, MS sends feedback to the BS using the ACK channel.
Each operation is like followings.
When receiving nack, the transmission process repeats again until MS sends ACK or the retransmission number reaches to the max_retransmission_count.

6. 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
To transmit Data, BS should inform MSs that where it’ll send the data by giving them HARQ DL-MAP IE.
A DL-MAP IE defines 2D region for HARQ in a frame, and among the several HARQ mode, Chase Combining is currently used. For each subburst in the HARQ region, DL HARQ sub-burst IE is added at the end of the MAP IE.

7. 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)
That subburst IE has different format with respect to the HARQ mode. This figure is for Chase combining.
RCID is for reducing the space,
and ACID is for HARQ channel ID, and there can be maximum 16 HARQ channels. A connection can use one or more HARQ channels.
Packets are sent in stop and wait manner at each channel.
AI_SN is for sending ACK bit for UL data , and when it toggles from 0 to 1, 1 to 0, on successful transmission.

8. 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.
time
(x1,y1)
Burst#1
Frequency
(x2,y2)
This is the send step.
After sending DL data, BS informs the location of ACK region by sending HARQ ACKCH region allocation IE.
One ack occupies half slot, so transmit 16 ACKs, 8 slots are needed.

9. MS sends the ACK/NACK Synchronous ACK Channel Predetermined location
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.
MS sends ACK through the synchronous ack channel which is notified by ACKCH region allocation IE
When MS receives DL burst at ith frame, it should send ack at (i+HARQ_ACK_DELAY) th frame.
Ack bits are in the order of the burst in the Downlink.

10. Hybrid ARQ UL Operation
UL operation is similar to the DL operation.
First bs grants transmission chance by sending HARQ UL_MAP.
After MS sends data, bs sends ack or nack.
When receiving nack, the transmission process repeats again until BS sends ACK.
The difference is that ACK is going with UL-MAP IE as AI_SN bits.
BS grants Trx Chance. (HARQ UL_MAP)
MS transmits Data
BS sends ACK/NACK

11. 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
There are two methods for ack transmission in the standard, but only AI_SN is used now.
AI_SN is for sending ACK bit for UL data , and when it toggles from 0 to 1, 1 to 0, on successful transmission.
When BS receives UL burst at ith frame, it should send ack at (i+HARQ_ACK_DELAY) th frame.

12. 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
We can limit the HARQ buffer size per HARQ channel or per station.

13. HARQ parameters

14. HARQ vs. MAC ARQ HARQ MAC ARQ CC/IR gain High 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
In this table, I compared HARQ with MAC ARQ.

15. Overview 802.16 Hybrid ARQ Modeling HARQ gain HARQ impact on TCP
Future Work and Plan
Before going into the detailed simulator implementation, I’ll present a brief introduction to IEEE Hybrid ARQ.
And then, I’ll tell you how I’m going to implement HARQ.
%And then, I’ll introduce the research issue related to HARQ and TCP.
Finally, I’ll end up with future work and plan.
Let’s Move on to the next page.

16. Modeling HARQ in NS-2 How much is the HARQ gain? Block1 Error
Oirginal Transmission
Block1
Error
Retransmission
Block 1’
Error
In the real system, it’s really simple to get the combined code after retransmission, but in the simulator, we need a model for combinig.
Block 1’’ = Block1 + Block 1’
Block 1’’
Error???
Receiver may or may not recover the error 16

17. Error Computation in Rel. 2.1
Packet size in slot
On receiving a packet
Packet
Max block size
Last block size …
Block1
Block2
Block3
For each block,
indexk = f (modulation, block_size)
k = f(mobility model, indexk)
i, 0itotal_subcarrier, calculate
Interferencepower[i]
signalpower[i]
Bernoulli toss with pk = BLERk to determine Block error
Then, PER = 1-(1- pk)

18. 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

19. Modeling Objectives Simple Not too complicated Close to Real World
Error is small
In the real system, it’s really simple to get the combined code after retransmission, but in the simulator, we need a model for combinig.

20. Simplicity Different Levels of Abstraction Bit level Subcarrier level
Block level

21. 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
2nd Transmission
QPSK
1st Transmission
16QAM
Bit Level Abstraction: Virtual Decoding
Data bits -> Encoded codeblock
The SINR value of combined c(i) after n transmission = sum_{i=1,…,n} ^{n-1} (s(i,t),t)
(c1, …, cN are bits? Or each of them is a codeword?)
To have SINR(SIR?),
C1 C2
C3 C4
C1 C2 C3 C4
4 codes in one subcarrier block
2 codes in one subcarrier block

22. Subcarrier Level Abstraction
Keep Track of SNR per subcarrier
Use EESM or some other methods to have representative values
High Complexity

23. Block Level Abstraction
SNR(1’’)
BLER(1’’)
SNR(1)
BLER(1)
SNR(1’)
BLER(1’)
SNR(1’’) = f(SNR(1), SNR(1’), BLER(1), BLER(1’’))
Given

24. Block Level Abstraction
Would like to find combined SINR c(n), combined block error rate BLERc(n)
Compute BLER from c(n)
Combined SINR
Reference, nokia paper
# Retransmission
BLER

25. Overview 802.16 Hybrid ARQ Modeling HARQ gain HARQ impact on TCP
Future Work and Plan
Before going into the detailed simulator implementation, I’ll present a brief introduction to IEEE Hybrid ARQ.
And then, I’ll tell you how I’m going to implement HARQ.
%And then, I’ll introduce the research issue related to HARQ and TCP.
Finally, I’ll end up with future work and plan.
Let’s Move on to the next page.

26. 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
AWND size
bigger awnd makes the queue size longer and makes the retransmission delay longer
small awnd lowers the throughput since the packets feeded are not sufficient
RTO, RTT and CWND size
affected by HARQ scheme and determines the total performance

27. 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

28. Retrx. # vs. TCP Thput [1]
Persistent Retransmission is the best

29. 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

30. 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

31. 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