Chapter 10 Cooperation Link Level Retransmission in Wireless Networks M. Dianati, X. Shen, and K. Naik

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Chapter 10Cooperation Link Level Retransmission
in Wireless NetworksM. Dianati, X. Shen, and K.
Naik
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Scope

• Link and MAC layer for fading channels

• Two parts
• Cooperative Scheduling
• Cooperative ARQ

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Introduction
Sample fading process

• Challenges in wireless domain
• Fading
• Interference
• Limited bandwidth

• Potentials
• Again, fading
• Spatial diversity

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IntroductionStochastic model of flat fading
process
Power spectrum density
Complex envelope of fading process
Power spectrum density
Fading process is a non-white stochastic process
with relatively slow variations.
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Introduction Spatial diversity

• Using independent transmission paths to increase
• Capacity
• Reliability
• Both

• Examples
• Multiple antenna systems
• Cooperative communications
• Multiuser diversity

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Cooperative ARQ Motivations

• ARQ link level retransmission
• Is de facto part of wireless link layer protocols

• Cooperative ARQ uses
• Channel state info. (since fading is a non-white
  process)
• Spatial diversity

• To improve
• Throughput
• Delay

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Cooperative ARQ Basic idea

• Let neighbor nodes assist the retransmission
  trials

Transmission
X
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Cooperative ARQ Basic idea

• Let neighbor nodes join retransmission

Negative or positive ACK
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Cooperative ARQ Basic idea

• Let neighbor nodes join retransmission

Retransmission
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Cooperative ARQ Basic idea

• Assuming that the physical layer can handle
  multiple receptions, node cooperation
• Mitigates the impact of deep fading on the
  primary path from the sender to the receiver
• Improves the chance of successful retransmission

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Cooperative ARQ System model

• Network model
• A single cooperation
• group

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Cooperative ARQ Basic scheme

• Sender and receiver nodes perform their normal
  operations.

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Cooperative ARQ Basic scheme

• Neighbor nodes
• Decode and store a copy of each frame.
• Drop the frame if ACK is received.
• Transmit the frame in NAK is received.
• Neighbors cooperate if
• They will to cooperate
• They have enough resources

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Cooperative ARQ Analytical model

• Fading channel model

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Cooperative ARQ Analytical model

• Three steps
• Model cooperation of a single node
• Combine multiple nodes into a super node
• Obtain the protocol model

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Cooperative ARQ Cooperation model of a single
neighbor node

• A tagged neighbor can help if
• 1. It has correctly received the previously
  transmitted frame
• AND
• 2. Its channel to the receiver node is in good
  condition.

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Cooperative ARQ Cooperation model of multiple
neighbor node

• What if there are two neighbor nodes?
• Model as a single node with a better cooperation
  capability

• More than two neighbor nodes
• Iterative combination of all neighbor nodes into
  a super node

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Cooperative ARQ The protocol model

• The cooperation group is either in Transmission
  state (T) or Retransmission state (R).

O(k) Status of the protocol at discrete time
k P(k) Status of the primary channel N(k)
Status of the super node G Good state B Bad
state C Cooperative state NC Non Cooperative
state
O(k-1) P(k) N(k) O(k)
T G C T
T G NC T
T B C R
T B NC R
R G C T
R G NC T
R B C T
R B NC R
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Cooperative ARQ The protocol model
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Cooperative ARQ Application of the model

• Throughput

• Delay
• Definition of delay the total time required to
  transmit a single packet from the network layer
• Average delay

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Cooperative ARQ Application of the model

• For a packet with np fragments

• Delay jitter

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Cooperative ARQ Simulations
Parameters Parameters
Carrier freq. 2400 MHz
Maximum Doppler freq. shift 11 Hz
Frame duration 5 ms
Channel simulation Jakes model
Sampling rate of fading channel 8000 sample/s
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Cooperative ARQ Simulations

• The definition of the normalized inverse fading
  margin

Normalized inverse fading margin
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Cooperative ARQ Simulation results Normalized
throughput

• N2 (number of neighbor nodes)

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Cooperative ARQSimulation results Normalized
throughput

• Lp-1 dB

Lp-1 dB N2
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Simulation results Delay and Jitter

• N2
• np20

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Cooperative ARQ Summary and further direction

• Cooperation of few nodes can improve performance
  of ARQ scheme significantly.
• Cooperative ARQ is backward compatible.
• There is not much signaling or maintenance
  overhead.
• Further extensions
• Non-ideal feedback channels