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Viral Communications, MIT Media Lab
Fulu Li
26 Jan. 2005
Advisors: Andrew Lippman and David Reed
Cooperative Multicast in Wireless Networks
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Cooperative Multicast in Wireless Networks
Outline of the Talk
Cooperative Routing in Wireless Networks
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Cooperative Multicast in Wireless Networks
Why Multicast?
Multicast is an elementary service and many applications need it
Info. gathering in wireless sensor networks Event Notification Systems
Resource discovery, paging, etc. One-to-many content delivery in P2P networks, e.g., exchange namecard info. at a conference
Cellular phone-based teleconference/game among a group of people
(a possible emerging application) The bottom line: it allows efficient use of network resources
(multiple users share the data on air)
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The problemIn a large and dense wireless network, how the wireless devices
can efficiently communicate with each other given the resourceconstraints, for example the power supplies. ( most of the
wireless devices are equipped with limited power supplies,
e.g., batteries).
We address the energy efficiency problem in wireless comm.
by exploringcooperation among nodes in the network.
Philosophy: as a single node, it may NOT be able to accomplish
anything, butcollaboratively a number of nodes may be able
to achieve a big thing (more resource savings).
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Wired vs Wireless Multicast
Wired Multicast: the total cost is simply the sum of the subcost
to reach each of the destination nodes individually.
Wireless Multicast (the one-hop case): the total cost is the maximum
subcost to reach each of the destination nodes individually.
Assuming that omni-directional antenna is used. This is often referred to as
Wireless Multicast Advantage (WMA)
In our treatment of cooperative multicast in wireless networks, we assume multi-hop routes,
e.g., relaying info. by intermediate nodes may save power than transmitting over a large dist.
A Simple Example
(a) (b) (c)
Src
AB
A
B
Src
A
B
Src
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Complexity Issues
Cooperative Multicast in Wireless Networks
With a source node and (N-1) intended destination nodes, the
number of possible multicast trees are
)2( NN
An NP-hard problem to find the minimum energymulticast tree.
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A Randomized Optimization Approach (cross-entropy method)
Cooperative Multicast in Wireless Networks
We define the one-step transition matrix , where N is the number of destination nodes
and denotes the probability that there is a transmission from node i to node j. The basic idea is thatif it performs well in the previous round, it will have high transmission probability in the next round.
)(, )( NNjiqQ =
ijq
00 .20 .40 .60 .8
1
124
47
70
93
1
16
1
39
1
62
1
85
2
08
The or ig ina l se q. o f t ra ns . proba bi li ties ( t=0)
Pij
00 .20 .40 .60 .8
1
124
47
70
93
116
139
162
185
208
T h e s e q u e n c e o f t r a n s it io n p r o b a b i lit ie s ( t = 1 )
Pij
00 .20 .40 .60 .8
1
124
47
70
93
116
139
162
185
208
T h e s e q . o f t r a n s it io n p r o b a b i lit ie s ( t = 4 )
Pij
An Illustration of the evolving of the transition probability matrix
(it quickly converges after several rounds: with some Pijs converging to 1s and others to 0s.)
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Empirical Results
Cooperative Multicast in Wireless Networks
11.05
1.11.151.2
1.251.3
1.351.4
10 30 50 70 90
Number of nodes in randomly-
generated networks
Norm
alizedtreepower
RTO
BIP
MST
Average normalized tree power by RTO, BIP and MST over 100 random
instances with varying number of nodes in the network
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Control Msg Exchanges
CM Protocol Components
Data packet transmission
Once the multicast tree structure is finalized, data packets are transmitted along the
tree structure. Only those relaying nodes relay data packets to its child nodes. Packets
are not destined to a given node will be discarded.
Control msgs are exchanged among local nodes to form an energy-efficient multicasttree structure such that every intended destination is covered and the total required
energy is the least.
Periodic UpdateEach node periodically updates the forwarding table information based on current channel
conditions, network topologies, node conditions (failure due to battery low, etc.), node
joins/leaves, etc. for reliability and adaptability.
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Power Combining Notation
Signals from multiple transmitters are combined either coherently (in phase)
or incoherently (out of phase using Rake receiver) at the receiver in order toachieve a desired signal-to-noise (SNR) ratio, e.g., get a decent signal.
Cooperative Routing in Wireless Networks
A Case for Power Combining (cars in between cities on highway, the cell tower is unsupportable)
Far-away receiver
Far-away receiver
A B
Uplink
(a) (b)
A B
Downlink
C DC D
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The Power Combining
(a) The shape of the signal )2cos()(1 ttx =
(b) The aggregated signal of and , where
)(1 tx
)(2 tx)
22cos()(2
+= ttx
(c) The aggregated signal of and , where
)(1 tx
)(2 tx )22cos()(2 += ttx
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Cooperative Routing Notation
It combinesroute selection andtransmit diversity
Cooperative Routing in Wireless Networks
An Illustration of Cooperative Routing
Src
Dst
A
B C
D
E
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Cooperative Routing Contd
Empirical Results
1
1.1
1.2
1.3
1.4
1.5
20 40 60 80 100
number of nodes in the netw ork
Meannormalizedpath
power
CSP
CAN
USP
Observation: As more nodes added in the network, more power
savings can be achieved for the cooperative approach comparedwith non-cooperative approaches
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Cooperative Routing Contd
Empirical Results
Observation: as more nodes added in the network, the presentedapproach achieves more fairness/load balance
0.2
0.24
0.28
0.32
0.36
0.4
20 40 60 80 100 120 140 160
number of nodes in the netw ork
STD
CSP
CAN
USP
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We addresscooperative multicast andcooperative routing in
wireless networks.
Emerging applications need efficient implementation of multicast
and power combining in next generation wireless networks
Our study suggests that the presented approaches tend to make the
networkmore efficient and more scalable from both energy
conservation and fairness/load balance standpoints in wireless
networks
Conclusion
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Snapshots of the network prototype
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Snapshots of the network prototype
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Thank you!