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

2. Scope • Link and MAC layer for fading channels • Two parts: • Cooperative Scheduling • Cooperative ARQ

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

4. Introduction:Stochastic 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.

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

6. 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

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

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

9. Cooperative ARQ: Basic idea • Let neighbor nodes join retransmission Retransmission

10. 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

11. Cooperative ARQ: System model • Network model • A single cooperation • group

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

13. 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

14. Cooperative ARQ: Analytical model • Fading channel model

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

16. 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.

17. 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

18. 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

19. Cooperative ARQ: The protocol model

20. 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:

21. Cooperative ARQ: Application of the model • For a packet with np fragments: • Delay jitter:

22. Cooperative ARQ: Simulations

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

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

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

26. Simulation results: Delay and Jitter N=2 np=20

27. 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