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Cognitive Radio Communications and Networks: Principles and PracticeBy A. M. Wyglinski, M. Nekovee, Y. T. Hou (Elsevier, December 2009) 1

Chapter 10

User Cooperative Communications


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Outline Introduction

Relay Channels

User-Cooperation in WirelessNetworks

Multi-Hop Relay Channel

Summary


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Introduction

User cooperative communication is a form ofcommunication in which users work together todeliver their data.

By relaying each others data, multipleindependent copies of the data are received atthe destination.

Processing of multiple independent copies of

the signal reduces the probability of error. Diversity acquired improves channel reliability

and saves resources.


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Cognitive Radio Communications and Networks: Principles and PracticeBy A. M. Wyglinski, M. Nekovee, Y. T. Hou (Elsevier, December 2009)

Introduction

Diversity in Communication is an effective way to tacklefading and improve reliability.

Diversity is obtained over time and frequency by meansof coding and interleaving. It can also be obtained viarepeated transmission.

Number of ways to obtain spatial diversity Multiple-input multiple-output (MIMO) antenna systems.

Cooperative transmission through relaying

User cooperative transmission.

User cooperative transmission is a special case ofcooperative transmission where users act as relays tohelp each other.

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Cognitive Radio Communications and Networks: Principles and Practice

By A. M. Wyglinski, M. Nekovee, Y. T. Hou (Elsevier, December 2009)

Introduction

Cognitive users opportunistically exploit spectrum holesto improve spectrum utilization.

Three types of holes,

1. White holes, primary users inactive.

2. Gray holes, primary users work with low power.

3. Black holes, primary users work with high power.

User cooperation in cognitive systems further improvesutilization of spectrum.

Secondary users cooperate to efficiently use the available

holes.

Secondary users may cooperate with primary users tocreate more holes.

Multi-hop relaying, by cognitive users, to exploit grayholes.

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The Relay Channel:Introduction

The relay channel is the basic building block forcooperative systems.

Early applications to tackle the curvature of the

earth, path loss and irregular terrains.

The use of satellite systems motivated theextensive work on relay channels during 70s.

Relaying helps improving resource utilization:

Extend transmission range. Increase the throughput.

Improve reliability.

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A General Three-Node Relay

Channel: Model 1/2 A three-node relay channel consists of

1. A source node, S.

2. A Destination node, D.

3. A relay node, R.

Two approaches to process the received signalat the relay

1. Amplify-and-forward (AF): sends a scaled copy of thereceived noisy signal.

2. Decode-and-forward (DF): First try to encode thereceived signal. If successful, re-encode andtransmit.

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Channel: Model 2/2

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Channel: Coding 1/2

Regular encoding/sliding window decoding.

Encoding (the source and the relay nodes)

1. Message w is divided into B blocks w1,w2 ...wB

transmitted in B + 1 time slots.2. In time slot i, the source sends x(wi ) and the relay

sendsx(wi1).

3. A constant sequence is sent by the relay n time slot 1and by the source in time slot B + 1.

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Channel: Coding 2/2 Regular encoding/sliding window decoding (continued).

Decoding (the relay node)1. Starts at the end of transmission of time slot 1 (decoding window size=

1).

2. At the end of transmission of time slot i, time slots i is used to decode

wi.

Decoding (the destination node)1. Starts at the end of transmission of time slot 2 (decoding window size=

2).

2. At the end of transmission of time slot i, time slots i 1 and time slot iare combined to decode wi1.

Advantages: Simple.

Limited delay.

Achieves maximum rate.

Can be extended to multi-hop relaying.

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Channel: Achievable Rate 1/4 Capacity of the general relay channel is still unknown.

Mutual information is considered for performancemeasure.

A discrete relay channel, denoted (X XR,p(y, yR|x,xR),Y YR), consists of:

Finite sets X and XR for the source and relay inputs.

Finite sets Y and YR for the destination and relay outputs.

A collection of pmfsp(y, yR|x, xR) for each (x, xR, y, yR)2 X XR Y YR.

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Channel: Achievable Rate 2/4 Two cases:

1. Non-cooperative relaying: No source-destination link.

2. Cooperative relaying: Fully connected network.

Achievable rate for non-cooperative relaying

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Channel: Achievable Rate 3/4 Cooperative relaying is possible if,

1. The network is fully connected.

2. The receiver is capable of processing multiple signals.

Achievable rate for cooperative relaying

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Channel: Achievable Rate 4/4 Notes on the achievable rate:

In both cooperative and non-cooperative

relaying, the maximum rate is boundedby the source-relay channel.

When the source-relay channel is good,cooperative relaying achieves higherrate.

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Wireless relay channel:Introduction

Three features distinguish the wireless systems

1. The wireless broadcast property (WBP), Orthogonal transmission to avoid interference.

Network is always fully connected.

Exploited by user cooperative networks.

2. Half-duplex constraint on wireless devices, A wireless can either listen or transmit at a given time and a

given frequency band.

When the source node is transmitting the relay listens only.

The source stays idle when relay is transmitting. 3. Channel behavior (fading),

Degraded performance due to rapid and unpredictable changeson channel status.

Performance improved by exploiting diversity.

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Wireless relay channel:

Model and Strategy 1/2

Transmission of the message w takes place in two time

instances,

First, S broadcast w to R andD for a period (1 t).

Then, if successfully received, R retransmits w to Dfor aperiod t.

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Model and Strategy 2/2

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Achievable Rate Achievable rate for the wireless relay

channel with half-duplex constraint on therelay:

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Wireless relay channel: Maximizing

Transmission Rate 1/2 To make relaying efficient, the right time

allocation must be used.

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Wireless relay channel: Maximizing

Transmission Rate 2/2

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Outage Probability 1/3

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Outage Probability 2/3

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Outage Probability 3/3 Outage probability, and outage

capacity, for the wireless relay

channell with arbitrary time allocationcan only be computed numerically.

Optimum operation by choosing tominimizes P.

opt can only be found numerically.

Sub-optimal operation using tightbounds.

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User-Cooperation in Wireless

Networks:I

ntroduction User-cooperative communication is a means to

improve performance through spatial diversity.

User-cooperative transmission can be useful for

users with single antennas and where there are nodedicated relays.

With changing topology and non-centralized nature,user-cooperative communication is particularlyuseful for MANET.

Relay channel is the basic building block. Unlike relay channels, in a user-cooperative model

each of the cooperating users has data to transmit.

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Two-User Cooperative Network:

System Model 1/2

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System Model 2/2 Two users in partnership,

User A sends wA to a destination node DA, while

user B sends wB to a destination node DB

Two relay channels: (A,B,DA) and (B,A,DB).

Achievable rate

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System Constraints

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Optimizing Performance

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Cooperative Wireless Network:I

ntroduction Hypothesis:

Randomly positioned nodes, arranged into source-destinationpairs.

Slow changing topology.

Fixed peak power constraint on transmitters. Network is partially known to users,

Each source knows other nodes within the range to itsdestination.

Rules for cooperation:

No more than two users are allowed to cooperate.

Partner selected such that both partners get higher mutualinformation.

Cooperation time is allocated similarly for both partners.

For a given pair of partners, time allocation is chosen to maximizethe minimum rate.

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Cooperative Wireless Network:

Useful User Definitions

User B is a useful user for user A if user A with userB as arelay can achieve a higher rate than direct transmission.

Harmful user: If User B is not a useful user, then it is a

harmful user.

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Constructive Partnership Only constructive partnership is

allowed.

User A and user B form a constructivepartnership only if their mutualinformation increase aftercooperation.

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Data Link Layer

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Multi-hop Relay Channel:I

ntroduction Multi-hop relaying is one way to employ multiple relays

to serve a single channel.

First used in telecommunications, in 1940s. Firstapplication focused on extending transmission range.

Recently used to increase throughput and improvereliability.

When more than two users are allowed to cooperate,partnership can take different forms (e.g. multi-hoprelaying) with different degrees of complexity.

Multi-hop relaying is also useful for cognitive users toexploit gray holes.

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Multi-hop Relay Channel:Model 1/2

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Multi-hop Relay Channel:Model 2/2

Example: 5-hop relay channel

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Multi-hop Relay Channel:Achievable Rate

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Multi-hop Relay Channel:Optimal Time Allocation

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Multi-hop Relay Channel:Outage Probability

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Chapter 10 Summary

User cooperative communication offers an alternative toobtain some of the advantages of spatial diversity.

User-cooperation helps cognitive user get the most fromavailable resources.

The philosophy of user cooperation is based on thetheory of the relay channel.

Both the general and the wireless relay channels arediscussed.

Model and results for the relay channel are extended to a

two-user cooperative setup and eventually applied to amulti-user wireless network.

Finally, the three-node model is expanded to a M-hoprelay channel.

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