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.

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 802.16 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.

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 802.16 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.

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 802.16 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. 802.16 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.

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.

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.

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.

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.

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.

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

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.

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.

HARQ parameters

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.

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 802.16 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.

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

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)

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

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.

Simplicity Different Levels of Abstraction Bit level Subcarrier level Block level

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

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

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

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

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 802.16 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.

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

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

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

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

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

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