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									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 Control frames sent at maximum power to communicate information about transmission powers and interference margins



PCDC, POWMAC [Muqattash03, Muqattash04]




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
B

A

C

D

power

S
CS Threshold

distance

A Two-Link Setup
B

A

C

D

power

S I
distance

Analytical Results


Collisions are doubly bad


Waste channel resources now



Waste channel resources upon retransmission
large transmit power => small CS threshold



Intuitively, to prevent collisions




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 =  1 pt g pcs = −  k

 

The Role of k


Analysis uses collocation approximation


A potential interferer sees same gain to both transmitter and receiver The local topology around the link



The value of k accounts for:
 

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 Transmit power held constant, CS threshold a free parameter Our scheme with β held constant, k a free parameter



Fixed Tx Power




Static k




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:



∑ 
n 2

xi

i= 1

n ∑ xi2
i= 1

n

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?


								
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