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

1. Physical Carrier Sensing and Spatial Reuse in Multirate and Multihop Ad Hoc Networks Hongqiang Zhai and Yuguang Fang Wireless Networks Laboratory Wireless Information Networking Group Department of Electrical and Computer Engineering University of Florida Presented by Zheng Zeng On the Physical Carrier Sense in Wireless Ad Hoc Networks On the Physical Carrier Sense in Wireless Ad Hoc Networks

2. Outline Introduction Factors affecting the optimum carrier sensing range Carrier sensing range and spatial reuse in multirate and multihop ad hoc networks Simulation studies

3. Carrier Sense Virtual carrier sensing (VCS) is not enough Collision of MAC frames leads to failure of VCS VCS can not reserve channel time outside of communication range (hidden terminal problem) Physical carrier sensing (PCS) can be more effective than VCS in avoiding collision PCS range can be much larger than communication range. PCS does not require correctly decoding of packets Large physical carrier sensing range may decrease spatial reuse ratio More users will be required to keep silent in the carrier sensing range even their transmission may not lead to collision. (exposed terminal problem) Carrier sense is an important way to control media access. It can alleviate the collision problem and allow spatial reuseCarrier sense is an important way to control media access. It can alleviate the collision problem and allow spatial reuse

4. Challenges in Wireless Multi-rate and Multi-hop Ad Hoc Networks Multiple channel rates have different SINR Receiver sensitivity Multi-hop flows needs more attention One-hop flows were the focus of previous studies How to effectively utilize multiple channel rates Which channel rate should be used?

5. Multiple Rates in IEEE 802.11a/g

6. Outline Introduction Factors affecting the optimum carrier sensing range The hexagon model Tradeoff between exposed terminal problem and hidden terminal problem Carrier sensing range and strategies for bidirectional handshakes. Carrier sensing range and spatial reuse in multirate and multihop ad hoc networks Simulation studies

7. Notations Carrier sensing radius dc : two transmitters must be at least dc away from each other Transmission radius dt : a transmitter must be at most dt away from its intended receiver Carrier sensing radius ratio X=dc/dt RXth : the signal power level at the receiver must be larger than RXth Carrier sense threshold CSth Tcs= RXth/CSth =

8. Hexagon model

9. Total number of concurrent transmissions M Achievable data rate rd and channel rate rc Aggregate throughput Numerator,??,denominator,?? Numerator,??,denominator,??

10. Achievable channel rate Aggregate throughput

11. Case study of IEEE 802.11 X changes in a large range for different values of SINR Optimum carrier sensing thresholds for different channel rates are very close to each other considering both SINR and receiver sensitivity. bracketbracket

12. Exposed terminal problem The derived carrier sensing threshold may be too conservative. Exposed terminals: Interference range: any point has a closer distance to B than (X-1)In the dt is in the interference range of B Exposed-area ratio:

13. Carrier sensing range for Bidirectional handshakes The interference model discussed focuses on one-way DATA transmissions. IEEE 802.11 MAC protocol adopts a two-way DATA/ACK handshake. The ACK of one transmission may destroy another transmission. Two carrier sensing strategies: Strategy I: forbid a node from transmitting if it senses a busy channel (for DATA transfer) Strategy II: allow a node to transmit at any situations (for ACK transfer)

14. A simple example A and D are out each other�s CS range. When A is sending DATA to B, D starts sending DATA to C. When A->B transmission ends successfully, B will send ACK to A, which may interfere with C�s reception from D.

15. New hexagon model To alleviate these problems, it is necessary to be more conservative to set the CS range. In the worst case, the receivers of six concurrent transmissions are dt closer to N0 than their corresponding senders. Denote the new value of X as

16. New hexagon model s Numerical results show that can be well approximated by X+1 with less than 1% error when SINR is larger than -3dB The cost is to aggravate the exposed terminal problem and sacrifice the spatial reuse in a more general topology.

17. Optimal Carrier Sensing Range The optimal carrier sensing range with a radius dc*=Xdt must balance the impact of both collisions and spatial reuse, where

18. Outline Introduction Factors affecting the optimum carrier sensing range Carrier sensing range and spatial reuse in multirate and multihop ad hoc networks How to set carrier sensing threshold How to choose next hops for multihop flows Simulation studies

19. CS threshold for multirate 802.11 MAC protocol multirate 802.11 MAC protocol should adopt a single carrier sensing threshold for all channel rates for three reasons. a single carrier sensing threshold keeps the Physical/MAC protocols simple. As discussed above, the optimum carrier sensing thresholds do not change much for different channel rates. A single threshold will not sacrifice the performance much. Third, multiple carrier sensing thresholds may introduce additional collisions.

20. a transmit-receive pair A and B, which have a large carrier sensing range corresponding to a certain channel rate, senses an idle channel and then A transmits the DATA frames. During the transmission period, another transmitter C in the previous transmitter�s sensing range also senses an idle channel due to a smaller carrier sensing range. The new transmission from C may introduce a collision at the previous intended receiver B.

21. Utilize the Multirate Capability Tradeoff between different channel rates Higher channel rates achieve short transmission delay Lower channel rates have longer transmission distances and travel less hops from the source to the destination Problem How to select an appropriate forwarding node with an appropriate channel rate

22. BDiP bandwidth distance product (BDiP): for each hop the achievable data rate rd times the hop distance dh at that hop. end-to-end delay te2e: hop transmission delay th :

23. BDiP per meter transmission delay tm: When the path is a regular chain where each hop has the same distance and the total path length is dp): To minimize the end-to-end delay, we should select the candidate with the highest value of BDiP as the next hop if other conditions are the same.

25. End-to-End Throughput of A Regular Chain Spatial reuse and end-to-end throughput Concurrent transmitting nodes must be at least dc (maximum carrier sense distance) away from each other. Let N denote the hop distance between two nearest concurrent transmission along the path Let Smax denote the maximum e2e throughput for a chain with a common hop distance dh.

26. Outline Introduction Factors affecting the optimum carrier sensing range Carrier sensing range and spatial reuse in multirate and multihop ad hoc networks Simulation studies Optimal carrier sensing range Evaluation of BDiP as a routing metric

27. Simulation setup Ns2 extensions Interferences are added up to determine SINR Incoming signal is regarded decodable if the SINR is high enough even when the senses a busy channel at the first bit. Support multiple channel rates with different requirements of SINR and receiver sensitivity Ns2 settings Uniform transmit Power Two ray ground model IEEE 802.11a system parameters for SINR requirement Discrete channel rate: 54, 36, 18, and 6Mbps

28. Simulation setup Simulated scenarios 1000m X 1000m 150 randomly distributed nodes A single channel rate is used in each simulation TCP traffic One-hop flow scenario Each node randomly selects one neighbor as the destination Multihop flow scenario There are 20 TCP flows with randomly chosen sources and destinations Source-destination distance ranges from 500m to 600m

29. Observations The aggregate throughput achieves the maximum value when CSth is in the range of [-76,-61]dBm. All channel rates have more or less the same optimum carrier sensing threshold A higher channel rate generates a higher throughput The optimal CSth is inconsistent with derived value.

30. Observations The aggregate throughput achieves the maximum value when CSth is around -91dBm Multihop forwarding has a significant impact on the optimum carrier sensing range. All channel rates have the same optimum carrier sensing threshold A higher channel rate does not necessary generate a higher throughput

31. Evaluation of BDiP Random chain topology Source-destination distance is 2000 meters. 28 other nodes are randomly placed along the line between the source and destination. Three routing algorithms Adr: Min-hop routing algorithm Next hop is the farthest reachable node, and the highest achievable rate is used. Ard: Max-rate routing algorithm Next hop is the farthest node among those with a same highest channel rate ABDiP: Max-BDiP routing algorithm Next hop is the node with the highest value of BDiP Simulation results Maximum end-to-end throughputs are achieved when CSth=-95 ~ -91 dBm Compared with Adr, Ard improve throughput by 14% and ABDiP improves throughput by 27%.

32. Conclusion Different channel rates have similar optimal carrier sensing thresholds. Multi-hop property must be considered to decide the optimum carrier sensing threshold. Higher channel data rate does not necessarily generate higher throughput. A new efficient routing metric BDiP is proposed to select next hop and channel rate by considering both multi-rate capability and multi-hop property.