Selecting Transmit Powers and Carrier Sense Thresholds in CSMA

1
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

2
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?

3
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

4
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

5
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

6
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

7
A Two-Link Setup
B
D
A
C
power
S
CS Threshold
distance
8
A Two-Link Setup
B
D
A
C
power
S
I
distance
9
Analytical Results

• Collisions are doubly bad
• Waste channel resources now
• Waste channel resources upon retransmission
• Intuitively, to prevent collisions
• large transmit power gt 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

10
Notation

• pt transmit power
• pcs carrier sense threshold
• g channel gain on the link
• ? required SINR
• ? thermal noise
• ß the constant product
• k number of worst-case interferers assumed

11
The Equations
12
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

13
ns-2 Simulation Setup

• PHY layer was modified to be more accurate
• RTS/CTS disabled physical carrier sensing
  dominant
• ? set to 0 to explore upper limit in spatial
  reuse
• UDP traffic, heavily loaded
• Topologies consisting of randomly placed links

14
Sample Topology
15
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 ß held constant, k a free
  parameter
• Dynamic k (next slide)

16
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 ltgt Minimum transmit power
• Algorithm used is heuristic

17
Throughput Comparisons
18
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

19
Fairness Comparisons
20
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

21
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

22
The End

• Thanks for you attention!
• Questions?