A Layered Hybrid ARQ Scheme for Scalable Video Multicast over Wireless Networks Zhengye Liu, Joint work with Zhenyu Wu

Outline Motivations & challenges Review of error protection approaches Layered hybrid ARQ Operating point selection in multiple user scenario  A general game theoretic framework in operating point selection  Layered hybrid ARQ in video multicast Conclusion

Motivations & Challenges Motivation of video multicast over WLANs  Utilize bandwidth efficiently Challenges  Error protection mechanisms are needed Fading, channel interference, …  Heterogeneity of channel conditions Different channel conditions Overall system performance Individual user fairness S C C C R R R S C C C R unicast multicast

Packet Loss Pattern Burst packet losses Difficult to predict

Review of error protection approaches Retransmission  Inappropriate in multicast scenario FEC  Constant throughput and bounded delay  Throughput is reduced in the good state Adaptive FEC  Prediction of channel conditions in the future Hybrid ARQ  [Majumdar 02]

Hybrid ARQ Scheme Generate parity packets Send source packets Send parity packets until all lost source packets can be recovered S C ACK

Hybrid ARQ Scheme Use bandwidth efficiently Should have sufficient bandwidth

Layered Hybrid ARQ Scheme Encode a video into multiple layers  Temporal scalability Transmit packets from more important layers to less important layers For each layer, transmit source packets first and then parity packets, based on hybrid ARQ Given a total transmission bandwidth, provide unequal protection  Protect more important layers  Selectively drop source packets from less important layers  No overall rate expansion

An Illustration S C

Performance Evaluation Single user scenario  Only one user in the multicast group Comparison  Hybrid ARQ with single layer video (single hybrid ARQ)  Layered hybrid ARQ

Simulation Setup (1) H.264 codec JM11 Football (720x480, 30 frame/sec) Average bitrate: 1400 kbps  Fix QPs Temporal scalability in H.264 Layer 1Layer 2 Layer 3

Simulation Setup (2) RS coding (255, k) for each layer Frame copy in decoder Total transmission bandwidth: 2200 kbps Packet loss pattern: two-state Markov model

Packet Receive Ratio Percentage of received/recovered source packets over the total encoded source packets Layered hybrid ARQ can provide unequal protection for different layers All packets from I and P frames can be received Most packets from Bs frames can be received

Average Channel Induced MSE Layered hybrid ARQ can outperform hybrid ARQ significantly in received video quality

MSE Frame by Frame (p=30%)

Demo Packet loss rate: 30% Single hybrid ARQ vs. Layered hybrid ARQ

Layered Hybrid ARQ in Multiple User Scenario Heterogeneity of channel conditions Different preferred configurations (operating points) of video multicast S C2 C1 ?

How to Select Operating Point? Worst case  Based on the user with the worst channel condition Play a game  Play “lottery” among users C1C2 Parity packet from layer 1 Source packet from layer 2 50% C1C2 Parity packet from layer 1 Source packet from layer 2

Is This Game Fair? Two players, each owning a car, play lottery with each other If a player wins the game, he/she can win the car from the other player Player 1 Player 2 50% vs. 50%?99% vs. 1% What are the probabilities for a fair game? C1C2 Parity packet from layer 1 Source packet from layer 2 λ 1 vs. λ 2 ?

Nash Bargaining Game Proposed by John Nash in 1950 A cooperative game  Players have perfect knowledge of each other Proved the existence of Nash bargaining solution (NBS) for this game  Unique solution  Pareto optimal No other solution produces better utility for one player without hurting another player  Fair in the sense of cooperative game Satisfy the axioms of fairness

Formulation of Nash Bargaining Game Player:  N users in a multicast group Strategy:  M operating points, s m  Mixed game with mixed strategy S = λ 1 s 1 + λ 2 s 2 +,…,+ λ M s M Preference:  The utility of each strategy for user i, u i (s m ).  Mixed utility U i = λ 1 u i (s 1 ) + λ 2 u i (s 2 ) +,…,+ λ M u i (s M ) Initial utility:  d i, user would like to at least achieve if they enter the game  U i >d i, otherwise user i will not enter the game Nash bargaining solution (NBS):  λ*=(λ 1, λ 2,…, λ M )  Users consider it as a fair setting of the lottery

An Example Player:  Two users Strategy:  Three operating points, s m =“transmit a packet from layer m”  Mixed game with mixed strategy, p m is the probability that a packet from layer m will be chosen S = λ 1 s 1 + λ 2 s 2 + λ 3 s 3 Preference:  u i (s m ): how much payoff user i can get when a packet from layer m will be sent  The anticipation of payoff from the lottery (mixed game) U i = λ 1 u i (s 1 ) + λ 2 u i (s 2 ) + λ 3 u i (s 3 ) Initial utility:  d i C1C2 Parity packet from layer 1 Source packet from layer 2

Utility If user i is requesting layer m, then only a packet from layer m is useful. w m should represent the importance of a packet from layer m on video quality  Use a channel distortion model to obtain w m C1 Parity packet from layer 1 u 1 (s 1 ) = w 1, u 1 (s 2 ) = 0, u 1 (s 3 ) =0 C2 Source packet from layer 2 u 2 (s 1 ) = 0, u 2 (s 2 ) = w 2, u 2 (s 3 ) =0

Channel Distortion Model of Temporal Scalable Video Channel distortion model of single layer video Channel distortion model of temporal scalable video w 1 =1400, w 2 =650, w 3 =150

Initial Utility Guarantee that the expected U i >d i  A flexible control parameter  Select a higher d i, if the “system” gives more protection to user i User i subscribes more premium service It is more urgent for user i to win the game If user i is requesting a packet from layer m C1 Parity packet from layer 1 C2 Source packet from layer 2 d 1 >d 2 α=2 d 1 =w 1 /2, d 2 =w 2 /4

Obtain NBS A Nash bargaining game  Player, strategy, preference (utility), and initial utility Solve an optimization problem  Exhaustive search for small M  Convex programming for a large M

Procedure of operating point selection Trace the state for each user  From which layer the user is requesting a packet  Based on the ACKs sent from the receivers Play Nash bargaining game Obtain u i (s m ) and d i Obtain the NBS λ*=(λ 1, λ 2,…, λ m ) Given λ*, play lottery to select a packet for sending

Performance Evaluation (1) Lead to NBS optimality and fairness in microscopic view (packet level) The macroscopic affect of a strategy on received video qualities  Overall performance: The majority of users are more likely to obtain their preferred operating points than the minority of users  Individual fairness: No individual user is denied access to the multicasting system or overly penalized  Flexibility: Can be tuned to satisfy different requirements.

Performance Evaluation (2) Comparison  Worst case  Nash bargaining game Investigate the impact of initial utility d i on system performance Higher d i leads to more protection to user i α=2, 4, and 100 By using a smaller α, guarantee a better basic video quality for bad channel users

Simulation Setup N users totally N-1 users are in a good channel condition (p=1%) One user is in a bad channel condition (p=30%) N=2, 4, 8, 16, 32

Average MSE (a) Good channel user (b) Bad channel user

Summery Worst caseNash bargaining Overall performance Ignore the majority of users Adapt the operating point to the majority of users Individual fairness The good channel users are overly penalized No user is overly penalized FlexibilityChange α to satisfy different requirements

Conclusion Propose a layered hybrid ARQ scheme for video delivery over WLANs Propose a game theoretic framework in operating point selection for video mulitcast Examine the game theoretic framework with the proposed layered hybrid ARQ

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