Efficient Interference-Aware TDMA Link Scheduling for Static Wireless Networks

1. Efficient Interference-Aware TDMA Link Scheduling for Static Wireless Networks • Outline • Introduction—TDMA • Assumptions • Interference Models • Implementation of Interference free algorithms • Results • Conclusion • References Oluwasoji O. Omiwade

2. Wireless Networks • Major Problem: Reduction of capacity due to interference • Caused by simultaneous transmissions • Solution: Use multiple channels/multiple Radios? • No. Can only alleviate; Cannot eliminate • What Then?

3. TDMA • Two ways to solve: • Utilize a random access and MAC layer scheme • Carefully construct a transmission schedule (TDM)

4. Assumptions • Time is slotted • Time is synchronized • Time slots assigned between [1, T] • All links must be interference aware: no scheduled transmission results in collision(s). • Complexity of finding optimal TDMA? • NP Complete.

5. Paper's Uniqueness • Previous Work: • Assumed Unit Disk Graph (UDG) • General Graph Model • Both don't capture the true wireless network features • General Graph: 2 nodes must be within communicable distance • UDG: Better; • But communication still impossible if barrier or path fading occurs

6. Paper Contributions • Main: Provide centralized and distributed link scheduling algorithms • Other Contributions • More realistic model: No UDG or General Graph • Both weighted and unweighted flow • Some links demand more than other • Theoretical Perfomance Guarantee for centralized and distributed algorithms • Layer Independence

7. Transmission/Interference • Our graph is directed • Transmission Range: ti • Nove vj receives signal from vi if |vi-vj| <= ti • Interference Range: ri • |vi-vj| <= ri and vj is not the intended recipient • Typically ri = 2-4 times vi

8. Some Definitions and Assumptions • There is only one radio • Primary Interference: receiving from a source while sending • Secondary Interference: receiving from 2 or more sources • Our model must combat both of these • Two models proposed • Fixed Protocol Interference Model (fPrIM) • RTS/CTS Model

9. Wireless Network Interference Models • Fixed Power Protocol Interference Model (fPrIM) • RTS/CTS Model

10. Fixed Power Protocol Interference Model (fPrIM) • Each node vi has its own fixed transmission power • Each node also has an interference range • Transmission from vi to vj successful if |vk-vj| > rk for every node vk • Only if transmitting in the same time slot!!!

11. RTS/CTS Model • Similar but RTS and CTS, not power, govern interference • i.e Interference Region Determined by RTS/CTS

12. Concrete Differentiation • How are they different • In fPrIM BA and CD can be assigned the same, but not in RTS/CTS

13. Network Assumptions • The comm links are predetermined. E.g through AODV • Or can be predicted from routes • Comm network G (V, E) • Derive Conflict Graph: F(G,R), F(G,f) • R-->RTS/CTS; f-->fPrIM • Why a Conflict free graph? • Will help us create a TDMA schedule

14. Conflict Graph (CG) • Model interference as CG • Link in G is represented by vertex in CG • Edge in CG if the two links interfere • ‘Clique’ in CG • Clique = set of links that interfere with each other • e.g. AED, ADC, ABC • Cliques are local structures • Only one link in a clique may be active at once Courtesy of Smart-Nets Rsrch; UC-Berkeley

15. Problem • To schedule without any interference • F(G,D2): Conflict Graph for RTS/CTS • F(G,P): Conflict Graph for fPrIM • We will find algorithms that schedule without interference, so that throughput is maximized • Maximum throughput ==> Smallest T • Why smallest T?

16. Problem: Interference-free Link Scheduling • This problem is essentially the vertex coloring problem (VCP). • Similar Algorithms for VCP are applicable • Centralized Scheduling • Distributed Scheduling • Centralized faster [by a constant factor] • But distributed sends less messages [desirable]

17. Algorithms • We start out simple:

18. Algorithm 1 • Centralized algorithm that performs very well. • Needs no more than the product of a constant factor and the minimum schedule period.

19. Problems • If ti equals ri, then Algorithm1 may not be close to optimal anymore • Performance is Degraded • Algorithm 2 solves this problem

20. Algorithm 2

21. Something More Interesting • A Distributed Coloring Algorithm:

22. Fast Distributed Algorithm • “Fast” in terms of messages not complexity

23. Results for the Presented Algorithms • Simulation Studies • Links were allowed to have weights: traffic load measures • Number of Nodes: 40 to 200 • One AP • About 500 meters x 500 meters • ti's randomly picked between 90 and 100 meters; where • Ri = Product(rand(1.5,2), ti) • 100 randomly sampled networks reported

24. Blur • Be prepared for blurred fonts in the results • Don't worry; I'll explain the results

25. Results: No Traffic Load Information

26. No Traffic Load Information

27. Non Uniform Traffic Load

28. Non Uniform Traffic Load

29. Conclusion • Wireless networks are essential • Communication in a mesh, sensor benefit greatly from link scheduling • Link scheduling can be • Centralized • Distributed • Centralized is faster, but not preferable

30. References • Whei Zhao Wang et al. Efficient Interference-Aware TDMA Link Scheduling for Static Wireless Networks. MobiCom '06. • Quality of Service for Flows in Ad-Hoc Networks. SmartNet Research Group. Dept of EECS, UC Berkeley