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

http://www.crhc.uiuc.edu/wireless/

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 802.11 [Zhu04] Does not address selection of transmit powers

In our work, we address both transmit power and carrier sense threshold selection in the IEEE 802.11 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

BD

C

pow

er

distance

SCS Threshold

A Two-Link Setup

A

BD

C

pow

er

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

pt: transmit power

pcs: 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

pt pcs=

pcs=1k p tg −

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:

∑i=1

n

x i2

n∑i=1

n

xi2

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-802.11-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?