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

Contending Area C A B D

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

How CS Threshold Controls Contending Area B D C A F E Signal Strength CS Threshold distance

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.

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.

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

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

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

Analytical Model Assume uniformly distributed dense network

Interference Model worst case 1st tier interference

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

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

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)

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

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

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)

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

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

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

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)

Simulations Modified ns-2 simulator MAC: IEEE 802.11 DCF with fixed CW = 31 PHY: IEEE 802.11 a 54, 36, 18, 9 Mbps Circular topology with N transmitter-receiver pairs More results for random topologies

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)

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)

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