1. Hangguan Shan, Member, IEEE, Ho Ting Cheng, Student Member, IEEE, and Weihua Zhuang, Fellow, IEEE Cross-Layer Cooperative MAC Protocol in Distributed Wireless Networks IEEE Transactions on Wireless Communications, Vol. 10, No. 8, August 2011

2. Outline Introduction Cross-layer MAC Protocol Design Probabilities of Successful Cooperation and Direct Transmission Numerical Results Conclusions 2

3. Introduction The existence of channel variations (or fading) is one of the big challenges that affects the capacity of wireless networks. MIMO technology is effective to meet the challenges Limited radio spectrum Mitigate channel impairments Limited physical size Cost constraints 3

4. Introduction Cooperative communications By utilizing the broadcasting nature of wireless transmissions, some nodes can act as helpers. Deliver the information from a source node to a destination node. 4

5. Introduction It is not clear to what extent the cooperation gain. Physical layer Advanced transmission techniques. Higher layer Exploring effective signaling overhead. 5

6. Introduction To facilitate cooperative communications, we need to address two issues: 1) when to cooperate. 2) whom to cooperate with, if cooperation is beneficial. 6

7. Goals 7 To devise an efficient and effective MAC protocol that can exploit beneficial node cooperation. Reducing the single overhead. Increasing the cooperative gain.

8. Preliminary 8 Physical layer preliminaries Nodes in both networks operate in half-duplex mode. ℛ is the rate set supported by applying adaptive modulation and coding at the physical layer SD H R c1 R c2 Repetition-based two-timeslot cooperation

9. Preliminary 9 MAC layer preliminaries Single-channel fully-connected wireless network. IEEE 802.11DCF (CSMA/CA) Effective payload transmission rate (EPTR) W : The payload length of a data packet T P : The times needed to transmit T O : The payload and overhead of the packet

10. Cross-layer MAC Protocol Design 10 Source Destination Optimal helper Other helper candidates Non-helper NAVRTS Random Backoff SIFS CTS SIFS NAV (RTS) NAV (RTS) NAV (RTS) NAV max(GI+MI) RTH NAV max(MI) Busy Medium Busy Medium Data SIFS Data SIFS Data SIFS ACK NAV (RTH) NAV (RTH) NAV (HI) HI GI MI HI GI : Group indication MI : Member indication HI : Helper indication

11. Cross-layer MAC Protocol Design Cooperative Region (CR) The EPTR with cooperation is always larger than that without cooperation. 11 R 1 : (in R) denote the transmission rate of direct transmission the source to the destination. SD H R c1 R1R1 R c2

12. Analysis of Payload and Overhead Transmission Times 12 Case 1 :Direct Transmission The payload and overhead transmission time

13. Analysis of Payload and Overhead Transmission Times 13 Case 2 :Cooperative transmission is set to be triggered, but no HI signal is detected after an RTS/CTS exchange. The payload and overhead transmission time

14. Analysis of Payload and Overhead Transmission Times 14 Case 3 :Cooperative transmission The payload and overhead transmission time

15. Analysis of Payload and Overhead Transmission Times 15 Case 4 : Cooperative transmission Potential helpers re-contention. The payload and overhead transmission time

16. Analysis of Payload and Overhead Transmission Times 16 Case 5 :If a re-transmission of an RTH packet fails, a source node initiates direct transmission. The payload and overhead transmission time

17. Cooperation Region Determination and Protocol Parameter Setting 17 Deciding the optimal CR The signaling overhead control at the MAC layer The cooperative rate allocation at the physical layer To maximize the average EPTR provided by the CR.

18. Cooperation Region Determination and Protocol Parameter Setting 18 EPTR

19. Cooperation Region Determination and Protocol Parameter Setting 19 A two-phase decomposition method to determine the CR and to set the protocol parameters. Without considering contention collisions Taking account of possible contention collisions

20. Optimal Grouping 20 Maximum M

21. Optimal Grouping 21 Maximum L

22. Group-based backoff mechanism P O N RQT K JL SD C M I GHNF E GNF I H E GFN –Re-contention (K) –Inter-group contention (G) –Intra-group contention (M) F

23. Probabilities of Successful Cooperation and Direct Transmission Let () and () respectively denote the probability of successful cooperation and that of direct transmission. 23 The probability density function of. th,0 : The SNR threshold for any control packet in the MAC protocol.

24. Probabilities of Successful Cooperation and Direct Transmission 24 Let () and () respectively denote the probability of successful cooperation and that of direct transmission. :The node set of potential helpers, including all nodes in the network except the source node and the destination node. One helper n helpers B i : The event that there exists at least one helper candidate G l : The events that the maximal CCTR appearing in the helper selection equals to I n : The number of potential helpers with the maximal CCTR is M max (i): The maximal number of CCTRs K l (i): The minislot number for the ℎ largest CCTR in the CR

25. Numerical Results DF based distributed space-time coding, set = 1 and = 1024 bytes. Model the channel with joint log-distance path loss and Rice fading Larger -factor means a better channel condition. The path-loss exponent is set to be 3.8. Nodes in the network are randomly deployed in a circular area. 25

26. Numerical Results 26 Ten traffic flows Packets in each traffic flow arrive according to a Poisson process with mean rate 10 packets per second. The simulations for 30 runs and average the results, where each simulation run sustains a network time of 50 seconds.

27. Numerical Results 27 Network performance

28. Numerical Results 28 Network performance

29. Numerical Results 29 Transmission Probability

30. Numerical Results 30 Transmission Probability

31. Numerical Results 31 Transmission Probability

32. Numerical Results 32 Transmission Probability

33. Conclusions To unearth benefits of cooperative communications in a distributed wireless network, When to cooperate Whom to cooperate with To improve link utilization and thus increase network throughput, optimal grouping of helpers The signaling overhead minimization is considered. A greedy algorithm for protocol refinement is devised. 33