1. Network coding-based retransmission schemes for E-MBS with and without MS feedback Document Number: IEEE C802.16m-09/0072 Date Submitted: 2009-01-05 Source: Hua Xu, Steven Xu, Naveen Arulselvan, Suresh Kalyanasundaram Voice: +1 847 632 2176 Motorola E-mail: hua.xu; steven.xu;hua.xu; steven.xu; naveen.a; Suresh.Kalyanasundaram@motorola.com@motorola.com Venue: IEEE 802.16m-08/052: Call for Contributions and Comments on Project 802.16m System Description Document (SDD) Contributions on topic: “PHY aspects of enhanced MBS” Base Contribution: Purpose: For 802.16m discussion, adoption into the SDD and eventual adoption for standardization Notice: This document does not represent the agreed views of the IEEE 802.16 Working Group or any of its subgroups. It represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.16. Patent Policy: The contributor is familiar with the IEEE-SA Patent Policy and Procedures: and.http://standards.ieee.org/guides/bylaws/sect6-7.html#6http://standards.ieee.org/guides/opman/sect6.html#6.3 Further information is located at and.http://standards.ieee.org/board/pat/pat-material.htmlhttp://standards.ieee.org/board/pat

2. Mobile broadcast services such as mobile TV, live sports/ news-casting are expected to be popular in 16m networks Retransmission schemes have been suggested to improve reliability in 16m E- MBS, but it is challenging due to following design issues –An uplink feedback channel is needed –UL feedback overhead shall be minimized Introduction

3. Network-coding based retransmission Error packets tend to be distributed among the various cell users –If user 1 receives packet Q1 in error and Q2 correctly, while user 2 receives packet Q2 in error and packet Q1 correctly, then transmitting just will enable both users to decode their respective error packets correctly We propose network-coding based retransmission schemes that works effectively with/without an uplink feedback channel: –Basic scheme: does not require UL feedback –Basic with shared feedback: using common feedback channel (TBD) –Limited enhanced scheme: requires unicast UL feedback to report number of errors –Full enhanced scheme: requires unicast UL feedback to report the incorrect packets UL Unicast feedback –Cannot expect unicast feedback is always presented in all kinds of MBS services, –but it can exist for some type of MBS usage scenarios Interactive TV games, Pervasive multiplayer game, mobile TV for file downloading with voting, betting Broadcast Services, auction Broadcast Services, trading Broadcast Services, etc.

4. Basic scheme Does NOT require UL feedback channel Working procedure –Base-station transmits N packets : Q= {Q 1, Q 2,…., Q N } –After the N packets are transmitted, there is a retransmission interval of R packet-slots (R<N) –Consider a fixed permutation rule PR:Q->P. The N-packet permuted sequence P is sent out by the base-station as follows: in the 1 st retransmission slot, in the 2 nd retransmission slot in the R th retransmission slot … …

5. Example Eg: 10 users, 100 packets and 5 retransmissions allowed. Say, identity permutation was applied. Transmitter sends Packet Numbers (XORed with each other) 1-20, 21-40, 41-60, 61-80, 81-100 respectively, in the 5 attempts Packets that are to be network-coded and transmitted in a single slot are shown using the same color If 2 or more packets of the same color are in error, they cannot be corrected Left-hand column shows the number of errors that can be corrected among the total number of errors reported by the user 20 errors in all will be corrected using only 5 retransmissions

6. Variation to the Basic scheme Performance takes a slight hit in case error rates are very high or retransmission interval is tight. Two variations to the basic scheme are suggested for these regimes 1.Truncated sequence version: –d (a random number) packets are dropped from the original N packets. –The truncated packet sequence Q N-d is permuted as before to obtain the retransmission sequence P N-d. –This is transmitted in R slots, (N-d)/R packets at a time. –Mobile complexity is still low, but header size of the network coded packet increases. 2.Random Linear Code version: –N/R packets are selected from the original N packets for each retransmission –A linear combination of these N/R packets are sent over the broadcast channel –Individual user may have to wait till all retransmissions are over before starting to decode –Mobile-side complexity and header overhead are higher.

7. Performance benefits Simulation results show that about 80% of errors could be corrected, as compared to only 20% correction using a traditional approach. Note that the scheme works equally well when users have differing packet error rates.

8. Basic with Common Feedback channel Voice-vote mechanism to estimate the number of users NACKing a particular packet Idle-mode MS can participate without the need for C- RNTI BS will schedule network-coding based retransmissions using NACK count for each packet Exact details are TBD

9. Limited enhanced scheme Require unicast UL feedback channel to report the number of errors Working procedure –Base-station transmits N packets to K users: P= {Q 1, Q 2,…., Q N } –After the N packets are transmitted, there is a retransmission interval of R packet-slots (R<N) –Let P 1, P 2,.. P R be the set of packets that are to be network coded and sent in each retransmission slot Each retransmitted packet P i is the XOR of exactly N/R original packets Each original packet Q i appear only once in the network-coded retransmission sequence {P i } The permutation rule PR:Q->P is fixed and known to all users –Limited Feedback: Every user j informs the number of errors in the constituent packets of the network-coded packet P i : N(j,P i ) Total number of errors to be corrected from the retransmitted packet P i is N(P i ) = Maximum uplink feedback needed = KR log 2 (N/R) –Based on error count, algorithm at the BS will send retransmission packets with appropriate network coding

10. Full enhanced scheme Requires unicast UL feedback channel to report all the erroneous packets of a MS For the K MSs in the system, the set of erroneous packets are denoted by E 1, E 2, …E K Designing the optimal Retransmission scheme with an interval equal to max i |E i | is known to be NP-complete – S. El Rouayheb et. al [IT workshop 07] Sub-optimal memoryless decoding heuristic can easily implemented at the BS

11. Summary Network coding can effectively correct errors, even without UL feedback. If unicast UL feedback is present to report the number of errors, limited enhanced feedback can be used to improve efficiency. The performance can be further improved if unicast UL feedback is present to report the incorrect MBS bursts.

12. Suggested SDD text Section 14.3.4 E-MBS Operation with HARQ retransmission Details on E-MBS Operation with HARQ retransmission is FFS. BS may use network coding based retransmission. Dependent on whether BS or network provides a unicast feedback channel for MS, BS may use one of following network-coding based retransmission schemes * Basic scheme: no feedback channel is required * Limited enhanced scheme: MS uses unicast feedback channel to notify BS the number of errors for each sequence. * Full enhanced scheme: MS uses unicast feedback channel to notify BS the incorrect MBS data bursts.