Selecting Transmit Powers and Carrier Sense Thresholds in CSMA Jason Fuemmeler, Nitin Vaidya, Venugopal Veeravalli ECE Department & Coordinated Science Lab University of Illinois at Urbana-Champaign WICON 2006 Boston, MA August 3, 2006 Funded in part by NSF and by a NSF Graduate Research Fellowship

Medium Access Control  The wireless channel  All nodes share same medium  Nodes can interfere with one another  Channel can support multiple transmissions if separated in space (spatial reuse)  Medium Access Control (MAC) is needed to use the channel effectively  Question: How can we design MAC protocols to maximize spatial reuse?

Power Control  Power control can be used to increase spatial reuse  MAC protocols utilizing power control must perform a balancing act  Must maintain desired SINR at each receiver  Need interference margin at each receiver to maintain this SINR  Increasing transmit power increases interference margin  But increasing transmit power increases interference to other transmissions

Previous Research  PCMA [Monks01]  Busy tones sent on out-of-band channel to communicate current interference margins  PCDC, POWMAC [Muqattash03, Muqattash04]  Control frames sent at maximum power to communicate information about transmission powers and interference margins  Transmission power selection strategies in these protocols left unjustified

Previous Research  CS Threshold Selection in [Zhu04]  Does not address selection of transmit powers  In our work, we address both transmit power and carrier sense threshold selection in the IEEE protocol

Physical Carrier Sensing  We primarily consider physical carrier sensing  How it works:  Node is allowed to transmit only if channel is idle  Channel assumed to be idle only if total power seen at its location is less than carrier sense (CS) threshold  Idle channel should mean that transmitting will not cause a collision

A Two-Link Setup A B D C power distance S CS Threshold

A Two-Link Setup A B D C power distance S I

Analytical Results  Collisions are doubly bad  Waste channel resources now  Waste channel resources upon retransmission  Intuitively, to prevent collisions  large transmit power => small CS threshold  Analysis of collision prevention yields that the product of the transmit power and the CS threshold should remain constant throughout the network  Bounds the amount of interference one link can pose to another

Notation  p t : transmit power  p cs : carrier sense threshold  g: channel gain on the link  γ: required SINR  η: thermal noise  β: the constant product  k: number of worst-case interferers assumed

The Equations

The Role of k  Analysis uses collocation approximation  A potential interferer sees same gain to both transmitter and receiver  The value of k accounts for:  The local topology around the link  Any error introduced by the collocation approximation  For k sufficiently large, collisions will be prevented on the link

ns-2 Simulation Setup  PHY layer was modified to be more accurate  RTS/CTS disabled – physical carrier sensing dominant  η set to 0 to explore upper limit in spatial reuse  UDP traffic, heavily loaded  Topologies consisting of randomly placed links

Sample Topology

Schemes Considered  Fixed Rx Power  Power at receiver held constant, CS threshold a free parameter  Fixed Tx Power  Transmit power held constant, CS threshold a free parameter  Static k  Our scheme with β held constant, k a free parameter  Dynamic k (next slide)

Dynamic k Scheme  Each link adjusts its value of k dynamically  Uses transmission failures as feedback  Attempts to find minimum value of k such that collisions are prevented on that link  Minimum k Minimum transmit power  Algorithm used is heuristic

Throughput Comparisons

Fairness Issues  Our scheme does lead to some unfairness  Links with high CS thresholds get to transmit more often  In general, short links are given preference  Could perhaps mitigate unfairness by having short links voluntarily lower CS threshold  Fairness Measure:

Fairness Comparisons

Conclusions  Analyzed collision prevention conditions  Concluded that product of transmit power and CS threshold should remain constant throughout network  Simulation results indicate increased spatial reuse

Future Research  More detailed simulations  Comparisons with non based schemes  Understand interactions with virtual carrier sensing  Better justified algorithm for adjustment of k  Mitigation of unfairness

The End Thanks for you attention! Questions?