Cross-Layer Cooperative MAC Protocol in Distributed Wireless Networks

Hangguan Shan, Member, IEEE, Ho Ting Cheng, Student Member, IEEE, and Weihua Zhuang, Fellow, IEEECross-Layer Cooperative MAC Protocol inDistributed Wireless NetworksIEEE Transactions on Wireless Communications, Vol. 10, No. 8, August 2011OutlineIntroductionCross-layer MAC Protocol DesignProbabilities of Successful Cooperation and Direct TransmissionNumerical ResultsConclusions2IntroductionThe 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 challengesLimited radio spectrum Mitigate channel impairmentsLimited physical sizeCost constraints

3TXRXIntroductionCooperative communicationsBy utilizing the broadcasting nature of wireless transmissions, some nodes can act as helpers.Deliver the information from a source node to a destination node.

4TXTXRXIntroductionIt is not clear to what extent the cooperation gain.Physical layerAdvanced transmission techniques.Higher layerExploring effective signaling overhead.

55IntroductionTo facilitate cooperative communications, we need to address two issues: 1) when to cooperate. 2) whom to cooperate with, if cooperation is beneficial.

6Goals7To devise an efficient and effective MAC protocol that can exploit beneficial node cooperation.Reducing the single overhead.Increasing the cooperative gain.

Preliminary8Physical layer preliminariesNodes in both networks operate in half-duplex mode. is the rate set supported by applying adaptive modulation and coding at the physical layerSDHRc1Rc2Rc2

Repetition-based two-timeslot cooperationPreliminary9MAC layer preliminariesSingle-channel fully-connected wireless network. IEEE 802.11DCF (CSMA/CA)Effective payload transmission rate (EPTR)

W : The payload length of a data packet TP : The times needed to transmit TO : The payload and overhead of the packetCross-layer MAC Protocol Design10SourceDestinationOptimal helperOther helper candidatesNon-helperNAVRTSRandomBackoffSIFSSIFSCTSSIFSNAV(RTS)NAV(RTS)NAV(RTS)

NAVmax(GI+MI)

RTHNAVmax(MI)BusyMediumBusyMediumDataSIFSDataSIFSDataSIFSACKNAV(RTH)NAV(RTH)NAV(HI)HIGIMIHIGI : Group indicationMI : Member indicationHI : Helper indicationCross-layer MAC Protocol DesignCooperative Region (CR)The EPTR with cooperation is always larger than that without cooperation.

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R1 : (in R) denote the transmission rate of direct transmission the source to the destination. SDHRc1R1Rc2Rc2Analysis of Payload and Overhead Transmission Times12Case 1 :Direct TransmissionThe payload and overhead transmission time

Analysis of Payload and Overhead Transmission Times13Case 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

Analysis of Payload and Overhead Transmission Times14Case 3 :Cooperative transmissionThe payload and overhead transmission time

Analysis of Payload and Overhead Transmission Times15Case 4 : Cooperative transmission Potential helpers re-contention.The payload and overhead transmission time

Analysis of Payload and Overhead Transmission Times16Case 5 :If a re-transmission of an RTH packet fails, a source node initiates direct transmission.The payload and overhead transmission time

Cooperation Region Determination and Protocol Parameter Setting17Deciding the optimal CRThe signaling overhead control at the MAC layerThe cooperative rate allocation at the physical layer

To maximize the average EPTR provided by the CR.Cooperation Region Determination and Protocol Parameter Setting18EPTR

Cooperation Region Determination and Protocol Parameter Setting19A two-phase decomposition method to determine the CR and to set the protocol parameters.

Without considering contention collisionsTaking account of possible contention collisions

Optimal Grouping20

Maximum MOptimal Grouping21

Maximum LGroup-based backoff mechanismPONRQTKJLSDCMIGHNFEGNFIHEGFNRe-contention (K)Inter-group contention (G)Intra-group contention (M)F

Probabilities of Successful Cooperation and Direct TransmissionLet () 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.Probabilities of Successful Cooperation and Direct Transmission24Let () 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 helpern helpersBi: The event that there exists at least one helper candidateGl: The events that the maximal CCTR appearing in the helper selection equals to

In : The number of potential helpers with the maximal CCTR is Mmax(i): The maximal number of CCTRsKl(i): The minislot number for the largest CCTR in the CRNumerical ResultsDF 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.25Numerical Results26Ten traffic flows Packets in each traffic flow arrive according to a Poisson process with mean rate 10 packets per second. The simulationsfor 30 runs and average the results, where each simulation run sustains a network time of 50 seconds.

Numerical Results27Network performance

Numerical Results28Network performance

Numerical Results29Transmission Probability

Numerical Results30Transmission Probability

Numerical Results31Transmission Probability

Numerical Results32Transmission Probability

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