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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. Contents. Introduction Interference in multiple-hop wireless networks

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a coordinate based approach for exploiting temporal spatial diversity in wireless mesh networks

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

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

interference in mesh network

Inter flow-interference

Intra flow-interference

Interference in Mesh Network
  • Interferences
    • Inter flow interference: interference between difference flows.
    • Intra flow (self) interference: interference between consecutive wireless links inthe sameflow
mitigating interference
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
intra flow interference

Interference Range

Interference Range

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.
temporal spatial diversity
Temporal-Spatial Diversity
  • What if we schedule packet transmissions as follows:

sender

W

Y

Z

A

B

C

D

Packets in node A’s queue

Packet Transmission at node A

case 1

time t

case 2

time t

assumption
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)
issues to be considered
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.
topology discovery
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.
topology discovery cont d

distance  level of interference

Example of 3D representation

Topology Discovery (cont’d)
  • Procedures:

Pairwise RSS Measurements

Singular Value Decomposition (SVD)

Cartesian Coordinate System

Distance Metric

RSS Prediction

topology discovery cont d notations

Pairwise RSS Measurements

Topology Discovery (cont’d)- Notations
  • M(GN) : the set of neighbor nodes that can directly communicate with GN and GN itself.
  • The RSS measurements are represented by the p*p 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.
topology discovery cont d13

SVD (singular value decomposition)

Pairwise RSS Measurements

Topology Discovery (cont’d)

Cartesian Coordinate System

  • Project the p-dimensional space into a new q-dimensional space.
      • Example of PCA (Principal Component Analysis)
topology discovery cont d14

Pairwise RSS Measurements

Topology Discovery (cont’d)

Cartesian Coordinate System

  • Example (cont’d)
    • SVD of matrix D
    • Calculate coordinates of hosts in two-dimensional coordinate system
topology discovery cont d15

Pairwise RSS Measurements

Topology Discovery (cont’d)

Cartesian Coordinate System

  • Find the optimal scaling factor α*that minimizes the following function

is 0.6

  • The new coordinate of a node is written by
topology discovery cont d16

Pairwise RSS Measurements

Topology Discovery (cont’d)

Cartesian Coordinate System

  • Determining coordinates for nodes that are two hops away

j

GN

k

i

i’

i

k

Transmission range

issues to be considered17
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.
transmission scheduling

Pairwise RSS Measurements

Transmission Scheduling

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.

transmission scheduling cont d
Transmission Scheduling (cont’d)
  • 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.

.

.

.

.

Select morepkts dependingon SNR.

Queue of a node

issues to be considered20
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.
packet transmission

Frame

Frame

ACK

ACK

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.

DIFS

BACKOFF

SIFS

SIFS

SIFS

busy

Congested node

Two nodes belongingto the same setof least interferencenodes.

experiment results
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)
experiment results23
Experiment Results
  • Augmented NS-2 simulation
    • Real topology of CUWiN + Random topology

Visualization of 2D coordinate system

Throughput performance

20 % throughput improvement obtained !

experiment results24
Experiment Results
  • NS-2 Simulation
    • Star topology with multiple wireless paths
    • Transmission range: 100m, Interference range 220m
experiment results25
Experiment Results
  • Throughput performance

27 ~ 30 % throughput improvement obtained !

conclusion
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.