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A Cooperative Diversity-Based Robust

MAC Protocol in wireless Ad Hoc

NetworksSangman Moh, Chansu Yu

Chosun University, Cleveland State UniversityKorea, Cleveland

IEEE TPDS, March 2011

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OutlineO IntroductionO Background O Cooperative Diversity MACO Performance EvaluationO Conclusion and Future Work

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IntroductionO In wireless ad hoc networks, signal fading

and interference are two major obstacles in realizing their full potential in delivering signals.

O Cooperation among the nodes is considered critically important in addressing these problems.

O It may be difficult to maximize the performance unless nodes are coordinated to cooperate at lower levels.O MAC- and PHY- layer

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Introduction O Recently, some researches in developing

cooperative MAC algorithms O Path-centric medium access O MAC-layer packet relaying

O is determined in a proactive mannerO is used to forward frames at higher bit ratesO but not enhance the communication

reliability, especially in interference-rich environment

O Cooperative communicationO enhance the link reliability

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IntroductionO Cooperative communication exploits

diversity offered by multiple users, known as multiuser or cooperative diversity.

O It dramatically improves bit error rate (BER).O More reliable transmissionO Higher throughput

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Background

OCooperative DiversityOCooperative Diversity in

Wireless Ad Hoc Networks

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Cooperative DiversityO Two types of cooperative diversity

algorithmsO Repetition-based

O Amplify-and-forwardO Decode-and-forward

O Space-time-codedO Distributed space-time coding (DSTC)O Space-time coding (STC) (multiantenna

system)O Space-time block coding (STBC)

(multiantenna system)

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DSTC

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Cooperative Diversity in Wireless Ad Hoc NetworksO Azgin et al. proposed Cooperative

MAC (C-MAC) and the corresponding routing protocols for wireless ad hoc network. (GlobeCom 2005) O Relaying start (RS)O Relay acknowledgement (RA)O Relay broadcasting (RB)O Transmission start (TS)O RTS-CTS-RS-RA-RB-TS-ACK

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Cooperative Diversity MAC

O Overview

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A Simple Cooperative scheme

O The authors proposed in ICC 2007.O Based on DCF of the IEEE 802.11

standardO If a primary link is reliable enough to

successfully transmit frames, no cooperative transmission is enabled.

O If it fails, the sender retransmits the frame but cooperatively with its partnerO When to cooperate

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A case for simple scheme

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Problems of simple scheme

O Twice longer than that without cooperation

O An RTS failure can be due to collisions/interference from other nodes’ communication

O The RTS/CTS exchange effectively eliminates the problem associated with “the hidden terminals”.

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Cooperative Diversity MAC

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State transition diagrams

Sender

Receiver

Partner

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Partner Selection and Its Propagation

O Every node proactively selects its partner by monitoring or overhearing its neighbors with respect to link quality.

O Measure and record the link quality while receiving a frame

O The neighbor with the best link quality among all neighbors is chosen as its partner.

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Why choose the neighbor with the best link quality?O Three reasons behind this choice

O Communication between a node and its partner must be highly reliable.

O A partner with the best link quality is most probably the closest node.

O It ensures that the sender and the partner share the same communication environment so that they can make a consistent decision on cooperation.

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Performance Evaluation

O Using ns-2 simulatorO 50 mobile nodes move over a square area

of 300X1500 m2

O Each simulation runs for 900 seconds. O Source-destination pairs are randomly

selected.O Mobile nodes are assumed to move

randomly with the node speed of 0-5 m/secO Pause time between moves varies from 0

to 900 seconds.

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Performance Comparison

This is due to the complex interplay among MAC- and routing-layer protocols in MANET environment

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Overhead and cooperation efficiency

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Impact of environment noise

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Effect of network traffic on packet delivery ratio

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Conclusions and Future Work

O This paper proposed CD-MAC and discusses design issues and performance benefits in wireless ad hoc network.

O When a communication link is unreliable, a sender transmits its signal together with its partner delivering the signal more reliably.

O As a future work, exploiting cooperative diversity based on multichannel interfaces will be investigated.