Physical Carrier Sensing and Spatial Reuse in Multirate and Multihop Wireless Ad Hoc Networks

1. Physical Carrier Sensing and Spatial Reuse in Multirate and Multihop Wireless Ad Hoc Networks Hongqiang Zhai and Yuguang Fang Dept of Electrical & Computer Engineering University of Florida Presented by Tae Hyun Kim

2. Contents • Problem Statement • Analysis for optimum CS distance • Interference models • … • End-to-end throughput • Simulation • Conclusion • Some comments

3. Problem Statement • To find optimum CS (Carrier Sense) distance that maximizes throughput • Considered factors • MAC overhead (frame headers and IFSs) • Bidirectional handshaking • Multirate – different TX ranges, RX sensitivities and required SINR • Multihop – forwarding, hidden/exposed nodes, and random topology

4. Analysis for Optimum CS distance • Notations • Two worst case interference models • Multiple CS distances for multiple rates? • Exposed/hidden node problems • Bidirectional handshaking intensifies interference • End-2-end throughput

5. Notations Payload size Frame header size Fixed-lengthoverhead

6. Two Interference Models • Worst case of 6 interferers

7. Two Interference Models (cont.) • Non-overlapping area of each TX • # of concurrent transmissions • Aggregate throughput • Thus, given SINR, we can find optimum CS distance while fixing transmission distance dt

8. Two Interference Models (cont.) • Optimum CS distance is, • SINR plays major role other than protocol overhead ≈ 10-10 ≈ 10-7

9. Two Interference Models (cont.) • 6 interferers scenario may be too conservative • Worst case of only ONE strong interferer • Compute dc’using same method • Optimum CS area reduces to 25~89% of 6 interferers • As shorter CS distance may greatly increase spatial reuse, we may be allowed to decrease dc

10. Multiple CS distances for Multiple Rates? • Optimum X in interference model varies much, given SINR requirement • Fortunately,RX sensitivities vary much, too • Recall

11. Multiple CS distances for Multiple Rates? • Optimum CS distances and thresholds by 6 interferers model • Single CS distance is sufficient to maximize throughput

12. Multiple CS distances for Multiple Rates? • We do have more reasons for single CS distance • High complexity to adapt multiple CS distances for multiple rates • Mobility, distance, channel fading, etc. • Multiple CS distances may introduce additional collisions

13. Exposed/Hidden Nodes • Exposed node problem • Nodes that are unnecessarily shut up • Let’s define interference range • (X-1)dt=dc-dtfrom receiver • Exposed-area ratio • E.g.) By using 6 interferers model,54 Mbps δ=0.24, 0.56 when X=10, 5, γ>3 Exposed area

14. Exposed/Hidden Nodes (cont.) • Shorter dc could • Alleviate exposed nodes problem • Achieve higher spatial reuse • Have potentially larger hidden nodes • Hidden node problem • A’s TX is not sensed by C • C may interfere TX from A to B • Increase collisions • Large CS distance can reduce hidden nodes

15. Exposed/Hidden Nodes (cont.) • Summary • Tradeoff between degrees of exposed nodes and hidden nodes

16. Bidirectional Handshaking • Bidirectional handshaking incurs • Packet collision by immediate ACK • Receiver blocking (permanent link failure) • Packet collision by immediate ACK • After successfully receiving DATA, ACK is transmitted without CS • RTS may mitigate this as following CTS is sent when channel is idle A B C D DATA DATA ACK

17. Bidirectional Handshaking (cont.) • Receiver blocking • Before transmitting either CTS or DATA, CS is performed • If there is nearby on-going transmission, receiver never replies to RTS • MAC decides that link has been broken A B C D DATA RTS A receiver does not replyas channel is busy

18. Bidirectional Handshaking (cont.) • Receivers of previous interferers become new interferers closer to yellow receiver • Modified 6 interferers model • Intuitively, larger dc required to prevent interferers from transmitting

19. Bidirectional Handshaking (cont.) • Compute optimum CS distance, again • For SINR > - 3dB, • Thus,

20. Bidirectional Handshaking (cont.) • This solution, • Sacrifices spatial reuse • Increases potential exposed nodes • Incurs MAC contention • But, this also reduces • Potential hidden terminals • Packet collision by immediate ACKs • Receiver blocking

21. Optimum CS distance • Summary of previous observations • Tradeoff between larger and smaller dc • For protocol stability, larger dc might be better • Optimum CS distance is determined by optimum X* • Simulation study will find μ

22. Multihop flow consideration • End-2-end throughput • Conditions for maximized spatial reuse along the path • Distance between TXs be less than dc • Not corrupting each other’s packet • N – # of hops between nearest concurrent TXs • 1/N – spatial reuse ratio of a multihop flow • Then, throughput upperbound is N=3 A B C D E F Hop distance

23. Multihop flow consideration (cont.) • Upperbound for one multihop flow throughput • Observations • Higher rate does not necessarily generate higher throughput IF MAC overhead is taken into account

24. Multihop flow consideration (cont.) • Consider interference from nearby concurrent transmissions in a regular chain topology • Achievable maximum E2E throughput • This is proportional to BDiP ( ) • May not be maximum in general topology (?!) dc' dc'-dt A B C D E F

25. Simulation • Modified Ns-2: cumulative interference • 150 nodes in 1000m x 1000 m area • To obtain “one hop” optimum CS distance • Observations • Maximum throughput when 60<CSth*<70 dBm • For some high rates, CSth* < RXse; starving flows exist • Max throughput can be sustained with some starving flows RX sensitivities for different rates

26. Simulation (cont.) • Optimum CS distance for multihop flows • Observations • CSth* for single hop does not work well • CSth* ~ 91 dBm (smaller CS distance) • Single CS distance could be optimal • Higher rates do not necessarily generate higher throughput Optimum CSth* CSth < RXse randomly selected 20 TCP connections with 500~600 E2E distance

27. Conclusion • This paper analyzes impact of CS distance to throughput from various perspectives • Found optimum CS distance • Single CS distance is sufficient • dc* may be less than dc due to conservativeness of 6 interferers model • dc * ~ dc + dt due to bidirectional handshaking • dc * = μ(dc / dt + 1) • dt = dh to get maximum E2E throughput • μ can be found by simulation according to network setup

28. Comments • Based on simplified interference models and extreme cases • Through analysis no relationships between any factors are drawn – only some intuitions • Ignorance on random access MAC overhead – much larger than frame header and IFSs overhead • Packet with higher rate has more overhead proportion, thus penalizing higher rates • Hard to compute BDiP • Does 801.11 do CS before sending CTS and DATA?? • It does not. Nevertheless, receiver blocking can happen due to virtual CS • If rate is adapted, then single CSth* may not be a good strategy

29. THANK YOU!ANY QUESTIONS?

30. Backup slides

31. Multihop flow consideration (cont.) • Worst case model for condition (2) • Equivalent to bidirectional handshaking model • We have, • Bound for one multihop flow throughput

32. Multihop flow consideration (cont.) • Spatial reuse ratio = Ns-2 default: SINR=10 dB, γ=4 Spatial reuse ratio = 1/3