Physical Carrier Sensing and Spatial Reuse in Multirate and Multihop Wireless Ad Hoc Networks

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Physical Carrier Sensing and Spatial Reuse in
Multirate and Multihop Wireless Ad Hoc Networks

• Hongqiang Zhai and Yuguang Fang
• Dept of Electrical Computer Engineering
• University of Florida
• Presented by Tae Hyun Kim

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Contents

• Problem Statement
• Analysis for optimum CS distance
• Interference models
• End-to-end throughput
• Simulation
• Conclusion
• Some comments

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Problem Statement

• To find optimum CS (Carrier Sense) distance that
  maximizes throughput

• Considered factors
• MAC overhead (frame headers and IFSs)
• Bidirectional handshaking
• Multirate different TX ranges, RX sensitivities
  and required SINR
• Multihop forwarding, hidden/exposed nodes, and
  random topology

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Analysis for Optimum CS distance

• Notations
• Two worst case interference models
• Multiple CS distances for multiple rates?
• Exposed/hidden node problems
• Bidirectional handshaking intensifies
  interference
• End-2-end throughput

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Notations
Payload size
Frame header size
Fixed-lengthoverhead
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Two Interference Models

• Worst case of 6 interferers

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Two Interference Models (cont.)

• Non-overlapping area of each TX
• of concurrent transmissions
• Aggregate throughput
• Thus, given SINR, we can find optimum CS distance
  while fixing transmission distance dt

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Two Interference Models (cont.)

• Optimum CS distance is,
• SINR plays major role other than protocol
  overhead

10-10
10-7
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Two Interference Models (cont.)

• 6 interferers scenario may be too conservative
• Worst case of only ONE strong interferer
• Compute dc using same method
• Optimum CS area reduces to 2589 of 6
  interferers
• As shorter CS distance may greatly increase
  spatial reuse, we may be allowed to decrease dc

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Multiple CS distances for Multiple Rates?

• Optimum X in interference model varies much,
  given SINR requirement
• Fortunately,RX sensitivities vary much, too
• Recall

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Multiple CS distances for Multiple Rates?

• Optimum CS distances and thresholds by 6
  interferers model
• Single CS distance is sufficient to maximize
  throughput

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Multiple CS distances for Multiple Rates?

• We do have more reasons for single CS distance
• High complexity to adapt multiple CS distances
  for multiple rates
• Mobility, distance, channel fading, etc.
• Multiple CS distances may introduce additional
  collisions

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Exposed/Hidden Nodes

• Exposed node problem
• Nodes that are unnecessarily shut up
• Lets define interference range
• (X-1)dt dc-dt from receiver
• Exposed-area ratio
• E.g.) By using 6 interferers model,54 Mbps?
  d0.24, 0.56 when X10, 5, ?gt3

Exposed area
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Exposed/Hidden Nodes (cont.)

• Shorter dc could
• Alleviate exposed nodes problem
• Achieve higher spatial reuse
• Have potentially larger hidden nodes
• Hidden node problem
• As TX is not sensed by C
• C may interfere TX from A to B
• Increase collisions
• Large CS distance can reduce hidden nodes

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Exposed/Hidden Nodes (cont.)

• Summary
• Tradeoff between degrees of exposed nodes and
  hidden nodes

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Bidirectional Handshaking

• Bidirectional handshaking incurs
• Packet collision by immediate ACK
• Receiver blocking (permanent link failure)
• Packet collision by immediate ACK
• After successfully receiving DATA, ACK is
  transmitted without CS
• RTS may mitigate this as following CTS is sent
  when channel is idle

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Bidirectional Handshaking (cont.)

• Receiver blocking
• Before transmitting either CTS or DATA, CS is
  performed
• If there is nearby on-going transmission,
  receiver never replies to RTS
• MAC decides that link has been broken

A
B
C
D
DATA
RTS
A receiver does not replyas channel is busy
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Bidirectional Handshaking (cont.)

• Receivers of previous interferers become new
  interferers closer to yellow receiver
• Modified 6 interferers model
• Intuitively, larger dc required to prevent
  interferers from transmitting

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Bidirectional Handshaking (cont.)

• Compute optimum CS distance, again
• For SINR gt - 3dB,
• Thus,

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Bidirectional Handshaking (cont.)

• This solution,
• Sacrifices spatial reuse
• Increases potential exposed nodes
• Incurs MAC contention
• But, this also reduces
• Potential hidden terminals
• Packet collision by immediate ACKs
• Receiver blocking

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Optimum CS distance

• Summary of previous observations
• Tradeoff between larger and smaller dc
• For protocol stability, larger dc might be better
• Optimum CS distance is determined by optimum X
• Simulation study will find µ

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Multihop flow consideration

• End-2-end throughput
• Conditions for maximized spatial reuse along the
  path
• Distance between TXs be less than dc
• Not corrupting each others packet
• N of hops between nearest concurrent TXs
• 1/N spatial reuse ratio of a multihop flow
• Then, throughput upperbound is

N3
A
B
C
D
E
F
Hop distance
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Multihop flow consideration (cont.)

• Upperbound for one multihop flow throughput
• Observations
• Higher rate does not necessarily generate higher
  throughput IF MAC overhead is taken into account

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Multihop flow consideration (cont.)

• Consider interference from nearby concurrent
  transmissions in a regular chain topology
• Achievable maximum E2E throughput
• This is proportional to BDiP (
  )
• May not be maximum in general topology (?!)

dc'
dc'-dt
A
B
C
D
E
F
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Simulation

• Modified Ns-2 cumulative interference
• 150 nodes in 1000m x 1000 m area
• To obtain one hop optimum CS distance
• Observations
• Maximum throughput when 60ltCSthlt70 dBm
• For some high rates, CSth lt RXse starving flows
  exist
• Max throughput can be sustained with some
  starving flows

RX sensitivities for different rates
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Simulation (cont.)

• Optimum CS distance for multihop flows
• Observations
• CSth for single hop does not work well
• CSth 91 dBm (smaller CS distance)
• Single CS distance could be optimal
• Higher rates do not necessarily generate higher
  throughput

Optimum CSth
CSth lt RXse
randomly selected 20 TCP connections with
500600 E2E distance
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Conclusion

• This paper analyzes impact of CS distance to
  throughput from various perspectives
• Found optimum CS distance
• Single CS distance is sufficient
• dc may be less than dc due to conservativeness
  of 6 interferers model
• dc dc dt due to bidirectional handshaking
• dc µ(dc / dt 1)
• dt dh to get maximum E2E throughput
• µ can be found by simulation according to network
  setup

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Comments

• Based on simplified interference models and
  extreme cases
• Through analysis no relationships between any
  factors are drawn only some intuitions
• Ignorance on random access MAC overhead much
  larger than frame header and IFSs overhead
• Packet with higher rate has more overhead
  proportion, thus penalizing higher rates
• Hard to compute BDiP
• Does 801.11 do CS before sending CTS and DATA??
• It does not. Nevertheless, receiver blocking can
  happen due to virtual CS
• If rate is adapted, then single CSth may not be
  a good strategy

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THANK YOU!ANY QUESTIONS?
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Backup slides
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Multihop flow consideration (cont.)

• Worst case model for condition (2)
• Equivalent to bidirectional handshaking model
• We have,
• Bound for one multihop flow throughput

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Multihop flow consideration (cont.)

• Spatial reuse ratio

Ns-2 default SINR10 dB, ?4 ?Spatial reuse
ratio 1/3