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

3. 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

4. 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

5. 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

6. 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.

7. 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

8. 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)

9. 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.

10. 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.

11. 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

12. 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.

13. 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)

14. 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

15. 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

16. 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

17. 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.

18. 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.

19. 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

20. 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.

21. 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.

22. 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)

23. Experiment Results • Augmented NS-2 simulation • Real topology of CUWiN + Random topology Visualization of 2D coordinate system Throughput performance 20 % throughput improvement obtained !

24. Experiment Results • NS-2 Simulation • Star topology with multiple wireless paths • Transmission range: 100m, Interference range 220m

25. Experiment Results • Throughput performance 27 ~ 30 % throughput improvement obtained !

26. 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.