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M. H. Ahmed and Salama IkkiMemorial University

      Newfoundland, Canada

Chapter 3

To Cooperate or Not to Cooperate? That Is the Question!

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Outline

Introduction

System Overview of Cooperative-Diversity Systems

Benefits of Cooperative-Diversity Systems

Major Challenges of Cooperative-Diversity Systems

Discussion and Conclusion

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Relayed transmission is a promising technique for improving the quality of wireless communications.

Its advantages relative to direct link communication include: Ease of implementation and good scalability Increased connectivity Robustness to changing channel conditions, and Reduced operating power levels

The relays are classified into: Regenerative: fully decode the received signal Non- regenerative: forward an amplified version of the received signal

Introduction

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Relaying Techniques

Amplify-and-Forward

I.Relays simply amplify the received signal (including the desired signal and added noise as well.)

II.It is difficult to implement in TDMA systems because it requires the storage of a large amount of analog data.

III.AF suffers from the noise amplification problem which can degrade the signal quality, particularly at low SNR

Overview of Cooperative-Diversity Systems

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Relaying Techniques

Decode-and-Forward

Fixed Decode-and-forward

I.Removes the noise by decoding the received signals and then regenerating and re-encoding the signal to be forwarded to the destination.

II.DF suffers from the error propagation problem which may occur if the relay incorrectly detects/decodes a message and forwards this incorrect information to the destination.

Overview of Cooperative-Diversity Systems

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Relaying Techniques

Decode-and-Forward

Adaptive Decode-and-forward

I.The relay forwards the signal to the destination only if it is able to decode the signal correctly.

II.The correct decoding can be checked using some error detection check or SNR threshold.

III.A special case is that when the relay detects the signal but does not decode it. In this case the scheme is called detect-and-forward

Overview of Cooperative-Diversity Systems

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Relaying Techniques Other Relaying Techniques

I.Hybrid AF and DF: the relay switches between AF and DF depending on the channel conditions

II.Demodulate-and-forward: the relay demodulates (without detecting) the received signal and then forwards the signal to the destination.

III.Incremental Relaying: limit the cooperation to necessary conditions

IV.Best-Relay Selection: is used to improve the resource utilization. In this case, the best relay only is selected to forward the signal to the destination.

Overview of Cooperative-Diversity Systems

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Combining Techniques Maximum Ratio Combining (MRC)

I.The received signals are weighted by the optimal weights then, the weighted signals are added coherently.

II.The main drawback of the MRC technique is that it requires full knowledge of the channel state information (CSI).

Overview of Cooperative-Diversity Systems

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Combining Techniques Other Combining Techniques

I.Optimum Combining (OC) that maximizes SNR in interference-limited networks.

II.Generalized Selection Combining (GSC), where N of M relayed signals are selected based on some signal-quality criterion.

III.Cooperative Space-Time Coding relays cooperate with the source to form a space-time code (e.g., Alamouti scheme).

Overview of Cooperative-Diversity Systems

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Combining Techniques Other Combining Techniques

I.Superposition Transmission: each node sends its own signal to the BS. While each node sends its own signal, it listens to the transmitted signal of its partner. Of course, this implies a full duplex transmission, which is implemented using orthogonal channels using CDMA codes.

II.Cooperative Beamforming, In this scheme each cooperating node multiplies the signal (to be forwarded) by a specific weight so that the cooperating nodes can form a beam directed towards the destination.

Overview of Cooperative-Diversity Systems

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Signal-Quality Improvement

1. Diversity Gain2. Better Links3. Virtual Array Gain

Benefits of Cooperative-Diversity Systems

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Benefits of Cooperative-Diversity Systems

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Reduced Power : If we trade the signal-quality improvement with the transmission power, the total transmission power of cooperative diversity can be reduced to be significantly less than the transmission power of the traditional direct transmission for the same end-to-end SNR or received power levels

Benefits of Cooperative-Diversity Systems

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Better Coverage: The extension the signal coverage and communication range to remote users experiencing large path-loss by utilizing the signal-quality improvement (in terms of higher SNR and received power). Also, if the relay locations are carefully chosen, cooperative diversity can overcome large shadowing that may exist in the direct link due to blockage by large objects

Benefits of Cooperative-Diversity Systems

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Benefits of Cooperative-Diversity Systems

Capacity Gain:

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Resources Over-Utilization: cooperative-diversity systems need M+1 channels to send one message from the source to the destination using M relays

1. Best-Relay Selection2. Incremental Relaying 3. Superposition modulation

Additional Delay : When cooperative-diversity systems (with M relays) use TDMA as the access scheme, M+1 time slots are needed to send the signal from the source to the destination.

Major Challenges of Cooperative-Diversity Systems

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Complexity Signal Combining and Detection Channel Estimation Relaying Protocols Synchronization Resource Management

Unavailability of Cooperating Nodes: there is no guarantee that other nodes will be always available or willing to cooperate Security Threats: serious challenge in securing users’ data simply because different users share their data with each other

Major Challenges of Cooperative-Diversity Systems

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Discussion and Conclusion

It has been shown that cooperative diversity brings several benefits including better signal quality, reduced transmission power, better coverage and higher capacity.

Cooperative diversity has some drawbacks and challenges including resource over-utilization, additional delay, implementation complexity, unavailability of cooperating nodes and security threats.

It should be emphasized that the implementation of cooperative diversity is shown to be challenging due to the various complexity issues

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Discussion and Conclusion

Reduced-complexity algorithms and protocols are needed to simplify the implementation of cooperative diversity in order to be able to harvest the benefits of cooperative diversity.

Effective measures and schemes are required to increase users’ motivation for cooperation and to enhance information security and service authenticity in cooperative diversity.

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Thanks