A Power Controlled Multiple Access Protocol for Wireless Packet Networks

1. A Power Controlled Multiple Access Protocol for Wireless Packet Networks Jeffrey P. Monks, Vaduvur Bharghavan, and Wen-mei W. Hwu University of Illinois Urbana-Champaign, IL 61801

2. Outline • INTRODUCTION • THE PROBLEM AND APPROACH TO THE SOLUTION • THE PCMA PROTOCOL • PERFORMANCE OF PCMA • CONCLUSION

3. A major issue in wireless networks is developing efficient medium access protocols that optimize spectral reuse CSMA/CA – fixed power controlled “on/off” collision avoidance model PCMA – Power Controlled Multiple Access, using “variable bounded power” collision suppression model power:The rate of transfer or absorption of energy per unit time in a system INTRODUCTION

4. INTRODUCTION (cont.) PCMA • Does not require the presence of base stations to manage transmission power (decentralized) • Allows a greater number of simultaneous transmissions (spectral reuse) • Improvements in aggregate channel utilization by more than a factor of 2 compared to the IEEE 802.11 protocol standard

5. THE PROBLEM AND APPROACH TO THE SOLUTION CSMA/CA in Wireless Networks A: sender B: receiver C: exposed station (within range of sender, but not receiver) D: hidden station (within range of receiver, but not sender) C A B D Time RTS CTS C doesn’t hear CTS, resumes transmission D hears CTS, defers transmission D hears ACK, resumes transmission C hears RTS, defers transmission DATA ACK

6. THE PROBLEM AND APPROACH TO THE SOLUTION (cont.) D C B A Traditional CSMA/CA protocol : A could not send to B If C reduced its transmission power such that it would be just enough for D to capture its signal then other nodes in the region could also proceed with their transmission

7. THE PROBLEM AND APPROACH TO THE SOLUTION (cont.) We can achieve PCMA by adhering to two key principles 1.The power conserving principle: each station must transmit at the minimum power level that is required to be successfully heard by its intended receiver under current network conditions 2.The cooperation principle: no station that commences a new transmission must transmit loud enough to disrupt ongoing transmissions For these purposes every station must advertise its noise tolerance

8. THE PROBLEM AND APPROACH TO THE SOLUTION (cont.) • Channel Propagation Models • The amount of transmission power required for a node to send a valid signal to its destination will depend on the gain between each source and destination. Gain (Gij): The ratio of output current power to input current power • G is proportional to 1/d2 (inside the Fresnel zone), or 1/d4 (outside the Fresnel zone) • Fresnel Zones are a series of concentric ellipsoids surrounding the radio path

9. THE PROBLEM AND APPROACH TO THE SOLUTION (cont.) • Channel Propagation Models (cont.) B A Data channel Busy tone channel • In PCMA, we assume that: • Data channel gains ≒ busy tone channel gains • GAB≒ GBA • The channel gain is stationary for the duration of the control and data packet transmissions

10. THE PROBLEM AND APPROACH TO THE SOLUTION (cont.) • Channel Propagation Models (cont.) Three basic channel effects – path loss, shadowing, multipath

11. THE PROBLEM AND APPROACH TO THE SOLUTION (cont.) • Power Constraints Pt_Max, Pt_Min : the maximum and minimum transmission powers for a transmitter on the data channel, respectively RX_Thresh, CS_Thresh : the minimum received signal power for receiving a valid packet and for sensing a carrier, respectively SIR_Tresh : minimum signal to interference ratio for which the receiver can successfully receive a packet Pti : the minimum power which a transmitter i must use to transmit a packet to a receiver j Ek : the “noise tolerance” of a receiver k

12. THE PROBLEM AND APPROACH TO THE SOLUTION (cont.) • Power Constraints (cont.) • Pt_Min ≦ Pti ≦ Pt_Max • PtiGij ≧ RX_Thresh • SIRj = PtiGij / Pnj≧ SIR_Thresh where Pnj = +Nj , Njis the thermal noise • Ek = (Prk / SIR_Thresh) – Pnk for all k, Pti≦ Ek/Gik ∴Pti≦ mink{ Ek/Gik } = Pt_boundi Prk K

13. THE PCMA PROTOCOL • PCMA Protocol Overview • request-power-to-send (RPTS) / acceptable-power-to-send (APTS) handshake VS. RTS / CTS in IEEE 802.11 • RPTS and APTS used to determine the minimum transmission power that the sender must use • Noise tolerance advertisement is periodically pulsed in busy tone channel VS. Carrier sense • The signal strength of the pulse indicates the noise tolerance

14. THE PCMA PROTOCOL (cont.) • PCMA VS IEEE 802.11 (collision avoidance)

15. THE PCMA PROTOCOL (cont.) • PCMA Protocol Steps j time i k step1 RPTS step2 APTS step3 DATA step4 step5 Send Busy Tone pulses step6 ACK step7

16. PERFORMANCE OF PCMA • IPC , PCMA , and IEEE 802.11 • Ideal power controlled protocol (IPC) • IPC is provided with perfect knowledge of the link gain between any two nodes, the noise at any potential destination , and the upper bound on a transmitter’s signalpower

17. PERFORMANCE OF PCMA (cont.) • The source node is picked randomly from the set of all nodes and the destination is picked randomly from the set of all nodes one hop away • Each data transmission between source and destination will be referred to as a flow • Flow rate refers to the number of packets sent per second

18. PERFORMANCE OF PCMA (cont.) Sending busy tone pulse for every 128 bytes The performance of PCMA is demonstrated for differing number of busy tone pulses sent per data transmission period (1, 4, 16, 64)

19. PERFORMANCE OF PCMA (cont.) The region is smaller than the transmission range. PCMA and 802.11 almost the same throughput performance

20. PERFORMANCE OF PCMA (cont.) PCMA can sent packets simultaneously in both clusters by reducing its transmission power

21. PERFORMANCE OF PCMA (cont.) • Unfair phenomenon if network load increases 1. the expected power for a source to reach its destination will increase (increasing background noise) 2. the expected power bound decrease (increasing exposed receivers) source are more likely to backoff allowing a greater number of short range transmissions unfair favoritism toward source-destination pairs sending over shorter distances

22. PERFORMANCE OF PCMA (cont.) A perfect fair protocol should like this source The fraction of total packets received by destinations in five distance ranges

23. PERFORMANCE OF PCMA (cont.) Packets lost

24. PERFORMANCE OF PCMA (cont.) • Fixed power – fair but more packets lost • Unfixed power – not fair but less packets lost Fairness is improved due to power bound increasing, but less throughput

25. PERFORMANCE OF PCMA (cont.) • Mulitpath effect on the three assumption X (dB) denotes the channel gain, X = -u with a probability ¼, u with probability ¼, 0 with probability ½,

26. PERFORMANCE OF PCMA (cont.)

27. PERFORMANCE OF PCMA (cont.)

28. CONCLUSION • PCMA allows for a greater number of simultaneous senders than 802.11 • PCMA can achieve more than a 2 times improvement in aggregate bandwidth compared to 802.11 for highly dense networks • PCMA is still a protocol design in progress, so fairness properties and performance under mobility must be ongoing work