A Coordinate-Based Approach for Exploiting Temporal-Spatial Diversity in Wireless Mesh Networks

1
A Coordinate-Based Approach for Exploiting
Temporal-Spatial Diversity in Wireless Mesh
Networks

• Hyuk Lim Chaegwon Lim Jennifer C. Hou
• MobiCom 2006
• Modified and Presented by Jihyuk Choi

2
Contents

• Introduction
• Interference in multiple-hop wireless networks
• Proposed approach to mitigate interference
• Topology discovery
• Transmission scheduling
• Packet transmission
• Experiment Results
• Conclusion

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Wireless Mesh Network

• A wireless network that allows wireless nodes to
  supply backhaul services to other nodes.

Soekris board
Mesh node
Wireless ad-hoc network, multihop, GN(Gateway
Node), static
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Interference in Mesh Network

• Interferences
• Inter flow interference interference between
  difference flows.
• Intra flow (self) interference interference
  between consecutive wireless links in the same
  flow

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Mitigating Interference

• To mitigate interference and maximize the network
  capacity, there are several control knobs
• Transmit power topology control.
• Carrier sense threshold trade-off between
  spatial reuse and interference level.
• Channel diversity use of non-overlapping
  channels.
• Scheduling concurrent transmissions for
  least-interference connection.
• In this paper, the authors consider the problem
  of mitigating interference and improving network
  capacity from the angle of temporal-spatial
  diversity

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Intra flow Interference

• Example of a single flow

A
B
C
D
E
F
Pk1
Pk2

• Observation
• The intra flow interference is considered as a
  self capacity limiting mechanism.
• It cannot be avoided in a single flow.

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Temporal-Spatial Diversity

• What if we schedule packet transmissions as
  follows

sender
W
Y
Z
A
B
C
D
Packets in node As queue
Packet Transmission at node A
1
2
3
4
5
6
case 1

?
time t
1
2
3
4
5
6
case 2

?
time t
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Assumption

• The authors focus on transporting downstream
  traffic at gateway nodes.
• most of the Internet accesses are intended for
  downloading large video/audio/text files
• by virtue of the way how wireless mesh networks
  operate, all the downloaded traffic is handled by
  gateway nodes
• The authors restrict the measurement area to be
  within two hops from GN (Gateway Node)

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Issues to Be Considered

• Topology discovery
• How to establish network topology to predict
  interference between nodes.
• Transmission scheduling
• How to find sets of nodes that result in the
  least inter flow interference.
• Packet transmission
• How to interleave packet transmissions for
  least-interference connections.

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Topology Discovery

• Goal to facilitate the prediction of received
  signal strength (RSS) or interference strength
  between nodes.
• RSS prediction
• Direct measurement possible between neighbor
  nodes.
• Indirect estimation Signal from a non-neighbor
  node cannot be decoded.
• Use geographic locations and path loss model.
• Use a coordinate-based network topology
  constructed with pairwise RSS measurements.

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Topology Discovery (contd)

• Procedures

Pairwise RSS Measurements
Singular Value Decomposition (SVD)
Cartesian Coordinate System
Distance Metric
RSS Prediction
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Topology Discovery (contd)- Notations
Pairwise RSS Measurements

• M(GN) the set of neighbor nodes that can
  directly communicate with GN and GN itself.
• The RSS measurements are represented by the pp
  square matrix S. ( p M(GN) )
• The ith column vector of S, which denoted by si,
  is the (-RSS)s measurement made in dBm by the ith
  node from all nodes in M(GN).
• As the sign of the RSS measurement is negated, a
  smaller value of si,j implies stronger signal
  strength.

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Topology Discovery (contd)
Pairwise RSS Measurements
Cartesian Coordinate System

• Project the p-dimensional space into a new
  q-dimensional space.
• Example of PCA (Principal Component Analysis)

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Topology Discovery (contd)
Pairwise RSS Measurements
Cartesian Coordinate System

• Example (contd)
• SVD of matrix D
• Calculate coordinates of hosts in two-dimensional
  coordinate system

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Topology Discovery (contd)
Pairwise RSS Measurements
Cartesian Coordinate System

• Find the optimal scaling factor a that minimizes
  the following function
• is 0.6
• The new coordinate of a node is written by

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Topology Discovery (contd)
Pairwise RSS Measurements
Cartesian Coordinate System

• Determining coordinates for nodes that are two
  hops away

j
GN
i
k
i
i
k
Transmission range
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Issues to Be Considered

• Topology discovery
• How to establish network topology to predict
  interference between nodes.
• Transmission scheduling
• How to find sets of nodes that result in the
  least inter flow interference.
• Packet transmission
• How to interleave packet transmissions for
  least-interference connections.

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Transmission Scheduling
Pairwise RSS Measurements
Cartesian Coordinate System
RSS Prediction

• Computing SNR between two-hop neighbor nodes to
  get least-interference nodes.

SNR
If SNR ? ?, the jth node is not an interfering
node to the ith node.
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Transmission Scheduling (contd)

• Determining the transmission order of
  least-interference nodes.
• Procedure
• Pick the first packet in the queue.
• Search up to N packets to obtain the set of
  non-interferingnodes.

Select the first pkt.
Pk1
Pk2
Pk3
Pk4
Pk5
Pk6
. . . .
Select morepkts dependingon SNR.
Queue of a node
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Issues to Be Considered

• Topology discovery
• How to establish network topology to predict
  interference between nodes.
• Transmission scheduling
• How to find sets of nodes that result in the
  least inter flow interference.
• Packet transmission
• How to interleave packet transmissions for
  least-interference connections.

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Packet Transmission

• Basic idea If a node is congested, it has to
  have a higher priority over neighbor nodes.
• Without backoff, send packets in a bulk, and take
  a longer pause (backoff) time.? of packets
  sent in the previous transmission.

SIFS
SIFS
SIFS
BACKOFF
DIFS
busy
Congested node
Two nodes belongingto the same setof least
interferencenodes.
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Experiment Results

• We focus on transporting downstream traffic at
  gateway nodes
• Gateway nodes are responsible for transporting a
  large amount of downstream traffic
• Champaign-Urbana community wireless network
  (CUWiN)

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Experiment Results

• Augmented NS-2 simulation
• Real topology of CUWiN Random topology

Visualization of 2D coordinate system
Throughput performance
20 throughput improvement obtained !
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Experiment Results

• NS-2 Simulation
• Star topology with multiple wireless paths
• Transmission range 100m, Interference range 220m

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Experiment Results

• Throughput performance

27 30 throughput improvement obtained !
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Conclusion

• A coordinate-based approach is proposed for
  representing network topology and mitigating
  interference in wireless mesh networks.
• Future work
• Topology construction with various performance
  metrics such as packet loss rate and delay.
• More experiments in a large scale mesh network.