Title: Improving Spatial Reuse through Tuning Transmit Power, Carrier Sense Threshold, and Data Rate in Mul
1Improving Spatial Reuse through Tuning Transmit
Power, Carrier Sense Threshold, and Data Rate in
Multi-hop Wireless Networks
- Tae-Suk Kim
- Hyuk Lim
- Jennifer C. Hou
- Dept. of Computer Science
- UIUC
- ACM MobiCom 2006
2Contents
- Background and Motivations
- Related Works
- Network Capacity Analysis
- Analysis on Tuning Parameters
- Proposed Power and Rate Control (PRC) Algorithm
- Simulation Study
- Conclusions
3- Background and Motivations
- Related Works
- Network Capacity Analysis
- Analysis on Tuning Parameters
- Proposed Power and Rate Control (PRC) Algorithm
- Simulation Study
- Conclusions
4Background and Motivations
- Multi-hop network capacity depends on
- Achievable channel capacity at each individual
wireless link - Level of spatial reuse
- Tradeoff between the two factors
- Increasing one factor inevitably causes
decreasing the other - Physical carrier sensing employed in IEEE 802.11
MAC - Carrier sense threshold
- Determines the minimum distance, termed as the
carrier sense range, between any pair of
transmitters - The other control knob
- Wireless medium is shared, and the shared range
is determined by both the transmit power and the
carrier sense threshold - One can control the level of spatial reuse by
adjusting the transmit power or the carrier sense
threshold
5Background and Motivations (contd)
- Two questions
- What is the relation between the transmit power
and the carrier sense threshold? - Does increasing transmit power have the same
effect as increasing the carrier sense threshold? - Our contributions
- Analytic model that expresses the network
capacity as a function of the transmit power and
the carrier sense threshold - Spatial reuse depends only on the ratio of the
transmit power and the carrier sense threshold - Several advantages of tuning the transmit power
over tuning the carrier sense threshold - Analyze the number of power levels required to
achieve the same control granularity as afforded
by tuning the carrier sense threshold - Localized power and rate control (PRC) algorithm
- Each transmitter dynamically determines its
transmit power and data rate adapting to the
interference level that it perceives.
6- Background and Motivations
- Related Works
- Network Capacity Analysis
- Analysis on Tuning Parameters
- Proposed Power and Rate Control (PRC) Algorithm
- Simulation Study
- Conclusions
7Related Works
- Carrier sense threshold adjustment
- Level of spatial reuse is controlled by varying
the carrier sense threshold - Yang and Vaidya 1 is perhaps the first to
address, with the data rate issue figured in, the
impact of physical carrier sense on spatial reuse
in multi-hop wireless networks. They also propose
a heuristic algorithm, called Dynamic Spatial
Backoff (DSB). - Power control
- For the purpose of spatial reuse and capacity
optimization (PCMA, PCDC, POWMAC, etc.) - Not consider the effect of carrier sense
threshold on the network capacity - Analysis of the relation between the transmit
power and the carrier sense threshold - Vaidya et al. 8 also analyze the relation
between the transmit power and the carrier sense
threshold in determining the network capacity. - They conclude that transmitters should keep the
product of their transmit power and carrier sense
threshold fixed at a constant.
8- Background and Motivations
- Related Works
- Network Capacity Analysis
- Analysis on Tuning Parameters
- Proposed Power and Rate Control (PRC) Algorithm
- Simulation Study
- Conclusions
9Interference Model
- Assumptions
- Nodes are randomly and uniformly distributed in
an area U with reasonably high node density ?. - Distance between a transmitter and a receiver, R,
is given - Path-loss radio propagation model
- Perfect MAC protocol
- Interference level and SINR at a receiver
- Consider the transmission between TX0 and RX0
that are R away from each other - Transmit power Ptx, Carrier sense threshold Tcs
- Carrier sense range D nodes concurrently
transmitting with TX0 must be at least D away
from TX0 and each other
10Interference Model (contd)
- The worst-case interference, I, as perceived at
RX0 - Corresponding SINR at RX0
11Network Capacity as a Function of Transmit Power
and Carrier Sense Threshold
- Network Capacity
- where ,
is the area of the system, and is the area
consumed by each transmitter (
). - where is a constant.
From the fact that , - where is a constant.
12- Background and Motivations
- Related Works
- Network Capacity Analysis
- Analysis on Tuning Parameters
- Proposed Power and Rate Control (PRC) Algorithm
- Simulation Study
- Conclusions
13Benefits of Power Control
- Example 1
- Consider a transmission TX RX under the
assumption that all node use the same transmit
power Ptx. Let M denote the number of concurrent
transmissions. The SINR at RX is then - RX may endure the decrease of its SINR while
sustaining ri, hoping more concurrent
transmissions can be accommodated. - CST tuning
- Adjusting SINR depends on node distribution
around the TX-RX!!! - The same scenario using the power control.
- How many number of power levels are needed to
achieve the same control granularity as tuning
the carrier sense threshold?
14Benefits of Power Control (contd)
- Example 2
- TX1 can increase its transmit power up to the
point where it sustains a higher data rate r3
while not depriving the other concurrent
transmission TX2 RX2 of the data rate r2. - Tuning carrier sense threshold
- TX1 can achieve the rate r3 only when TX1
decreases Tcs to the degree that TX2 is included
within the carrier sense range of TX1. - Tuning carrier sense threshold can not achieve
the same object!
15Benefits of Power Control (contd)
- Granularity of Transit Power levels needed
- Derive a lower bound on the number of power
levels required to achieve at least the same
control granularity as carrier sense threshold
tuning. - Denote the SINR at RX as , where
. We decrease at TX
such that the carrier sense range increases from
to and includes one additional
concurrent transmitter TXc. Then, the
interference level at RX will be decreased by
amount of .
- The minimum possible SINR increase
with carrier sense tuning - The minimum possible SINR increase
with power control - Set to Ptx/k, where k denotes the number of
power levels available. - If DgtgtR, then total of five power levels should
be sufficient!
16- Background and Motivations
- Related Works
- Network Capacity Analysis
- Analysis on Tuning Parameters
- Proposed Power and Rate Control (PRC) Algorithm
- Simulation Study
- Conclusions
17Determining Power Range
- PRC algorithm
- A localized algorithm that enables each
transmitter to adapt to the interference level
that it perceives and determines its transmit
power. - The transmit power is so determined that the
transmitter can sustain the highest possible data
rate, while keeping the adverse interference
effect on the other neighboring concurrent
transmissions minimal. - Transmit power range
- Determine the minimum transmit power that ensures
that a receiver can sustain the minimum data rate
considering the worst-case where TX0 transmits
with the minimum transmit power, while its six
1st tier interfering nodes transmit with the
maximum power level. For this purpose, the SINR
level at RX0 should satisfy
18Determining Carrier Sense Threshold
- Objective if TX transmits with its minimum
transmit power, then at RX, which is the maximum
distance Rmax away from TX, the minimal date rate
of r1 can be sustained. - We determine Tcs at the transmitter to ensure the
IRX requirement is satisfied at RX. However, it
needs global knowledge of node distribution! ?
conservative but localized approach. - The most conservative scenario occurs when the
distance between RX and the interferer closest to
RX is minimized. This is when IRX is contributed
by a single interfering node TXi using the
minimum transmit power. - The minimum interference level perceived at TX
19Proposed Algorithm
- Theoretical base
- Find the maximal transmit power such that it does
not deprive the other concurrent transmissions of
sustaining their data rate.? needs global
knowledge!!! - Estimate the position of a hypothetical
interfering node TXi based on the level of ITX
under the conservative scenario. To ensure both
TX and TXi can engage in transmission
concurrently, - Decide the final transmit power
- If the data rate ri afforded by can be
achieved with a smaller power level, there is no
need to transmit with to mitigate the
interference level of other transmissions.
20Proposed Algorithm (contd)
- Pseudo codes of PRC
- Concurrent transmissions may commence and
terminate dynamically, the interference level
perceived at the receiver fluctuates with time.
?Need to monitor the variation in the
interference level. - Ns and Nf
21- Background and Motivations
- Related Works
- Network Capacity Analysis
- Analysis on Tuning Parameters
- Proposed Power and Rate Control (PRC) Algorithm
- Simulation Study
- Conclusions
22Simulation Setup
- Modified ns-2 Ver. 2.28
- The interference perceived at a receiver is the
collective aggregate interference from all the
concurrent transmissions - Each node uses physical carrier sense to
determine if the medium is free - IEEE 802.11a radios supporting 8 discrete data
rate (6 54 Mbps) - Random topology
- 3, 10, 20, 30, and 50 transmitter-receiver pairs
are randomly generated in a 300m X 300m area, and
represent sparsely, moderately, and densely
populated networks, respectively,. - Algorithms used for evaluations
- Static
- Dynamic Spatial Backoff (DSB)
- Greedy Power Control (GPC)
- Power and Rate Control (PRC)
23Simulation Results
- Sparse network
- Low effect of carrier sense range
- PRC operates in an economical manner
- Compared with Static
- Higher concurrent transmissions
- Unnecessarily high transmit power can actually
reduces the attainable level of spatial reuse
24Simulation Results (contd)
- Compared with DSB
- In spite of a smaller carrier sense range, using
a lower transmit power leads to less
interference, and enables more concurrent
transmissions. - Comparison between DSB and Static
- High carrier sense threshold, when combined with
an inappropriately tuned transmit power, can
actually impair the network throughput. - GPC
- Higher power level leads to the decrease in
spatial reuse - Aggregate throughput approaches to that of static
with the size of the network
25- Background and Motivations
- Related Works
- Network Capacity Analysis
- Analysis on Tuning Parameters
- Proposed Power and Rate Control (PRC) Algorithm
- Simulation Study
- Conclusions
26Conclusions
- We have investigated the impact of spatial reuse
on the network capacity - Derive the network capacity as a function of the
two control knobs the transmit power and the
carrier sense threshold - Show their relation (i) in the case of continuous
data rate (i.e., the channel rate follows the
Shannon capacity) and (ii) in the case of
discrete data rate - We proposed a localized power and rate control
(PRC) algorithm - Each node can adjust transmit power and data rate
dynamically based on its signal interference
level. - A transmitter determines its transmit power so
that it can sustain the highest possible data
rate, while keeping the adverse interference
effect on the other neighboring concurrent
transmissions minimal - PRC achieves up to 22 improvement in the
aggregate network throughput as compared to the
DSB algorithm