A Layered Hybrid ARQ Scheme
for Scalable Video Multicast
over Wireless Networks
Zhengye Liu,
Joint work with
Zhenyu Wu
Outline
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
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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
S
R
C
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R
S
R
R
C
C
unicast
C
C
C
multicast
Challenges

Error protection mechanisms are needed


Fading, channel interference, …
Heterogeneity of channel conditions



Different channel conditions
Overall system performance
Individual user fairness
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
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
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Generate parity packets
Send source packets
Send parity packets until all lost source packets can be
recovered
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1
2
3
4
5
1
2
3
C
ACK
Hybrid ARQ Scheme
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
Use bandwidth efficiently
Should have sufficient bandwidth
Layered Hybrid ARQ Scheme

Encode a video into multiple layers

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
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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
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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 1
Layer 2
Layer 3
Simulation Setup (2)
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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
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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
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
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

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Based on the user with the worst channel condition
Parity packet
from layer 1
Source packet
from layer 2
C1
C2
Play a game

Play “lottery” among users
Parity packet
from layer 1
C1
50%
Source packet
from layer 2
50%
C2
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 2
Player 1
50% vs. 50%?
C1
99% vs. 1%
Source packet
from layer 2
Parity packet
from layer 1
C2
λ1 vs. λ2 ?
What are the probabilities for a fair game?
Nash Bargaining Game
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
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
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Player:

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N users in a multicast group
Strategy:

M operating points, sm
 Mixed game with mixed strategy
S = λ1s1 + λ2s2 +,…,+ λMsM

Preference:


The utility of each strategy for user i, ui(sm).
Mixed utility
Ui = λ1ui(s1) + λ2ui(s2) +,…,+ λMui(sM)

Initial utility:



di, user would like to at least achieve if they enter the game
Ui>di, otherwise user i will not enter the game
Nash bargaining solution (NBS):


λ*=(λ1, λ2,…, λM)
Users consider it as a fair setting of the lottery
An Example
C1

Source packet
from layer 2
C2
Player:


Parity packet
from layer 1
Two users
Strategy:


Three operating points, sm=“transmit a packet from layer m”
Mixed game with mixed strategy, pm is the probability that a packet from layer m
will be chosen
S = λ1s1 + λ2s2 + λ3s3

Preference:


ui(sm): 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)
Ui = λ1ui(s1) + λ2ui(s2) + λ3ui(s3)

Initial utility:

di
Utility


If user i is requesting layer m, then only a packet from
layer m is useful.
C1
Parity packet
from layer 1
u1(s1) = w1, u1(s2) = 0, u1(s3) =0
C2
Source packet
from layer 2
u2(s1) = 0, u2(s2) = w2, u2(s3) =0
wm should represent the importance of a packet from
layer m on video quality

Use a channel distortion model to obtain wm
Channel Distortion Model of Temporal Scalable
Video

Channel distortion model of single layer video

Channel distortion model of temporal scalable video
w1=1400, w2=650, w3=150
Initial Utility

Guarantee that the expected Ui>di
 A flexible
control parameter
 Select a higher di, 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
C1

Parity packet
from layer 1
C2
Source packet
from layer 2
d1>d2
α=2
d1=w1/2,
d2=w2/4
If user i is requesting a packet from layer m
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


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Play Nash bargaining game
Obtain ui(sm) and di
Obtain the NBS λ*=(λ1, λ2,…, λm)
Given λ*, play lottery to select a packet for
sending
Performance Evaluation (1)
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
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 di on system
performance

Higher di 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
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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 case
Overall
Ignore the majority
performance of users
Individual
fairness
Flexibility
The good channel
users are overly
penalized
Nash bargaining
Adapt the operating
point to the majority of
users
No user is overly
penalized
Change 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
Thanks!