1. On the Physical Carrier Sense in Wireless Ad-hoc Networks
CS598 Advanced Topics in Wireless Networks
On the Physical Carrier Sense in Wireless Ad-hoc Networks
Xue Yang and Nitin H. Vaidya
Department of Electrical and Computer Engineering
and Coordinated Science Laboratory
University of Illinois at Urbana-Champaign

2. Contending Area C A B D

3. Spatial Contention Resolution
Control the “contending area” by adapting carrier-sensing threshold
Goal:
Problem considered in the paper:
Derive an analytical model that determines the optimal carrier-sensing range, assuming a fixed transmission power
Study how MAC layer overheads affect determination of the carrier sense range
Contending area Desired contention level

4. How CS Threshold Controls Contending AreaB D C A F E
Signal Strength
CS Threshold
distance

5. Bandwidth-(in)dependent MAC Overheads
Channel time consumed by physical layer convergence procedure (PLCP) preamble and header (192 ms in IEEE802.11b).
Inter-frame space (DIFS, SIFS, etc) and the backoff durations
Bandwidth-dependent MAC overhead
Overhead associated with transmission failures (e.g., retransmission)
A smaller fraction of channel capacity is wasted in MAC overhead, if the channel operates at a lower rate.

6. Impact of Carrier Sense Range on Bandwidth-Dependent Overhead
When the carrier sense range is reduced from D to D’,
#contending stations inside the carrier sense reduces
Less retransmissions and less bandwidth-dependent overhead result.
Interference level from concurrent transmissions outside the carrier sense range increases, which is accounted for in the data rate.

7. How CS Threshold Controls Contending Area
Larger CS threshold leads to smaller contending area
Less nodes compete the channel in time  less collisions B D C A F E
Signal Strength
CS Threshold
distance

8. Transmission Rate Needs to Be Adjusted Accordingly
Larger CS threshold leads to higher interference
Transmission rate depends on Signal-to-Interference-Noise Ratio B D C A F E
Signal Strength
CS Threshold
distance

9. Benefits of Smaller Contending Area
A smaller fraction of channel capacity is wasted in MAC overhead, if the channel operates at a lower rate.
More spatial reuse
At the cost of lower transmission rate
Optimal CS Threshold
Low rate links
High rate links

10. Analytical Model
Assume uniformly distributed dense network

11. Interference Model worst case 1st tier interference

12. SINR at the Receiver Unvaried settings Other notations
Transmission power P
Receiving signal threshold RXth
Other notations
Ө : path loss coefficient
D : carrier sense range
R : maximum transmission range
X : D/R

13. Achievable Channel Rate
Use Shannon Capacity as the best achievable rate between a transmitter/receiver pair

14. Multiple Concurrent Transmissions
When there is only one transmitter in the network, the contending area
When many transmitters exist, the triangle area is shared by 3 concurrent transmitters.
The area consumed by each transmitter is proportional to D² (represented as As)

15. Network Aggregate Throughput
When the MAC overhead is not considered:
where

16. Network Aggregate Throughput
When the MAC overhead is considered:
a (seconds): rate-independent overhead
b (bits): rate-dependent overhead
c (bits): payload size Oi

17. Metric for Rate-independent Overhead
Let
Oi represents the relative ratio of the wasted channel spectrum in rate-independent overhead over the sum of the rate-dependent overhead and the payload
(unit: hertz / bps)

18. Derivation of Bandwidth-dependent Term b
The number of contending stations M given a carrier sense range of D is (where k is the #nodes in the transmission area)
The average number of collisions per transmission cycle, E(Nc) is

19. Derivation of Bandwidth-(In)dependent Term a and b
The bandwidth dependent term is
The bandwidth independent term is
Oi can be represented as

20. Optimal Carrier Sense Range
Oi = 0, b = 0
Oi: rate-independent overhead
b: rate-dependent overhead
k: node density
X: ratio between carrier sense range and transmission range
Oi = 0.5, b = 0
Optimum value of X
Oi = 0.5, b > 0, k = 5
Oi = 0.5, b > 0, k = 20
Note: larger carrier sense threshold  smaller carrier sense range
Path loss coefficient Ө

21. MAC Overhead Affects Optimal CS Threshold
Oi = 0, b = 0
Oi = 0.5, b = 0
Increasing MAC overhead
Normalized aggregate throughput
Normalized aggregate throughput
Oi = 0.5, b > 0, k = 5
Oi = 0.5, b > 0, k = 20 β
= CSth / Rx th (dB)

22. Simulations Modified ns-2 simulator
MAC: IEEE DCF with fixed CW = 31
PHY: IEEE a
54, 36, 18, 9 Mbps
Circular topology with N transmitter-receiver pairs
More results for random topologies

23. Optimal CS Threshold Increases Optimal Transmission Rate Decreases with Transmitter Density
Rate=54 Mbps, β>= -38 dB
Rate = 18 Mbps, β = -6 dB
N = 3
Aggregate Throughput (Mbps)
Rate=36 Mbps, β>= -24 dB
N = 32
N = 8 β
= CSth / Rx th (dB)

24. Transmission Success Probability and Concurrent Transmissions
Number of concurrent transmissions
18 Mbps
18 Mbps
54 Mbps
36 Mbps
36 Mbps
54 Mbps β
= CSth / Rx th (dB) β
= CSth / Rx th (dB)

25. Related Work
[ZhuWCMC04], [VasanInfocom05], [NadeemInfocom05] propose schemes to adapt the CS threshold to improve the spatial reuse
Improve spatial reuse only, not the medium access efficiency.
Designed for a pre-defined transmission rate
Unable to fully explore the spatial reuse when transmission rate can be adjusted