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

Scope Link and MAC layer for fading channels Two parts: –Cooperative Scheduling –Cooperative ARQ

Introduction Challenges in wireless domain: –Fading –Interference –Limited bandwidth Potentials: –Again, fading –Spatial diversity Sample fading process

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

Introduction: Spatial diversity Using independent transmission paths to increase: –Capacity –Reliability –Both Examples: –Multiple antenna systems –Cooperative communications –Multiuser diversity

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

Cooperative ARQ: Basic idea Let neighbor nodes assist the retransmission trials Transmission X

Cooperative ARQ: Basic idea Let neighbor nodes join retransmission NAK Negative or positive ACK

Cooperative ARQ: Basic idea Let neighbor nodes join retransmission Retransmission

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

Cooperative ARQ: System model Network model A single cooperation group

Cooperative ARQ: Basic scheme Sender and receiver nodes perform their normal operations.

Cooperative ARQ: Basic scheme Neighbor nodes: 1.Decode and store a copy of each frame. 2.Drop the frame if ACK is received. 3.Transmit the frame in NAK is received. Neighbors cooperate if –They will to cooperate –They have enough resources

Cooperative ARQ: Analytical model Fading channel model

Cooperative ARQ: Analytical model Three steps: –Model cooperation of a single node –Combine multiple nodes into a super node –Obtain the protocol model

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.

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

Cooperative ARQ: The protocol model The cooperation group is either in Transmission state (T) or Retransmission state (R). O(k-1)P(k)N(k)O(k) TGCT TGNCT TBCR TB R RGCT RG T RBCT RB 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

Cooperative ARQ: The protocol model

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:

Cooperative ARQ: Application of the model For a packet with n p fragments: Delay jitter:

Cooperative ARQ: Simulations Parameters Carrier freq.2400 MHz Maximum Doppler freq. shift11 Hz Frame duration5 ms Channel simulationJake’s model Sampling rate of fading channel8000 sample/s

Cooperative ARQ: Simulations The definition of the normalized inverse fading margin Normalized inverse fading margin:

Cooperative ARQ: Simulation results: Normalized throughput N=2 (number of neighbor nodes)

Cooperative ARQ: Simulation results: Normalized throughput Lp=-1 dB N=2

Simulation results: Delay and Jitter N=2 n p =20

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