Modeling the Effect of Transmit Power and Physical Carrier Sense in Multi-hop Wireless Networks

1. Modeling the Effect of Transmit Power and Physical Carrier Sense in Multi-hop Wireless Networks Yong Yang, Jennifer C. Hou and Lu-chuan Kung Department of Computer Science University of Illinois at Urbana-Champaign Y. Yang, J. Hou and L.-C. Kung

2. Carrier Sensing Rangedcs Carrier Sensing Range dcs Motivation • Single-hop / Single-cell wireless networks s1 r1 s2 r2 Y. Yang, J. Hou and L.-C. Kung

3. Interfernce Range din Motivation • Single-hop / Single-cell wireless networks s1 r1 s2 r2 Y. Yang, J. Hou and L.-C. Kung

4. dcsdind Tx Power, CS Threshold Motivation • Single-hop / Single-cell wireless networks • Multi-hop / Multi-cell wireless networks s1 r1 s2 r2 • Objective: study the effect of Transmit Power and Carrier Sensing on Network Throughput Y. Yang, J. Hou and L.-C. Kung

5. s1 r1 s2 r2 s3 Contributions • Model IEEE 802.11 based multi-hop networks • Incorporate important MAC/PHY attributes • Transmit Power, Carrier Sense Threshold • Consider accumulated signal effect • Analyze the effect of MAC/PHY attributes on network throughput Y. Yang, J. Hou and L.-C. Kung

6. Outline • Motivation • Contributions • Accumulated Signal Effect • Analytical Model • Effect of Tx. Power and Carrier Sensing • Conclusions and Future Work Y. Yang, J. Hou and L.-C. Kung

7. Consider a thin ring Ri outside dcs of width Dr: • Inner radius: • Signal level at s: • node together can make s sense busy • Effective contenting nodes ri Accumulated Signal Effect • Accumulated signals outside dcs may exceed CSth • Assumptions • Nodes are distributed by a Poisson point process with d • Nodes always have packet to transmit • Effective number of contenting nodes within dcs : s dcs Y. Yang, J. Hou and L.-C. Kung

8. Accumulated Signal Effect (contd.) • Thus, the total effective contenting nodes: • Equivalently, we can use an effective carrier sense threshold • Similarly, we can obtain the effective SIR threshold s dcs Y. Yang, J. Hou and L.-C. Kung

9. Idle Busy Collision Success … Model Overview • The MAC layer throughput of each sender s: • Tv: expected length of a slot • Four types of activities in a slot with respective to s • Pa: transmission attempt probability in a slot • Pc: collision probability given a transmission attempt Y. Yang, J. Hou and L.-C. Kung

10. Model: Transmission Attempt Prob. • Bianchi models the back-off process of a node as a discrete-time Markov process • The unit of time is the virtual slot time • The key assumption: for each transmission attempt, it incurs a collision with a constant and independently probability Pc • This can be applied to multi-hop networks • The only different is the virtual slot time Y. Yang, J. Hou and L.-C. Kung

11. Model: Virtual Slot Time • A virtual slot could be • Successful transmission: • Sender s transmits, and no collision • Collision: • Sender s transmits, but runs into a collision • Idle: • No node in CSsU{s} attempts to transmit • Busy: • Sender s is backing off, while at least one in CSs transmit • Expected length of a virtual slot Tsucc, Tcol, Tidleand Tbusy can be determined given MAC/PHY parameters Y. Yang, J. Hou and L.-C. Kung

12. s r Model: Collision Probability Tp • A transmission is corrupted if • Any node in CSs INrstarts to transmit at the beginning of the transmission • Any node in INr \ CSsstarts to transmit at or even after the beginning of the transmission • The collision probability: t CSs INr d din dcs Y. Yang, J. Hou and L.-C. Kung

13. Fixed Point Analysis • A set of fixed point equations: • We prove that a unique solution exists to this equations • But hard to find explicit formulas Y. Yang, J. Hou and L.-C. Kung

14. Area 1000m1000m Payload 2048B Rate 36Mbps  4 Effect of Tx Power • The per-node throughput S decreases with the increase of d • Adjust transmit power to control d • Subject to connectivity constraint Y. Yang, J. Hou and L.-C. Kung

15. d din dcs s r Effect of Carrier Sense • When dcs > d + din, the per-node throughput S decreases with the increase of dcs • The optimal dcs* exists in [0, d + din] Y. Yang, J. Hou and L.-C. Kung

16. Conclusions and Future Work • Model IEEE 802.11 based multi-hop networks • Analysis and simulation show • Use power control to decrease d to improve MAC layer throughput • Adjust carrier sensing threshold s.t. dcs < d + din • Future Work • Online power control algorithm joint with carrier sense threshold adjustment • End-to-end throughput Y. Yang, J. Hou and L.-C. Kung

17. Thank You and Questions? Y. Yang, J. Hou and L.-C. Kung