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Chapter 10. User Cooperative Communications. Outline. Introduction Relay Channels User-Cooperation in Wireless Networks Multi-Hop Relay Channel Summary. Introduction. User cooperative communication is a form of communication in which users work together to deliver their data.
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Chapter 10 User Cooperative Communications
Outline • Introduction • Relay Channels • User-Cooperation in Wireless Networks • Multi-Hop Relay Channel • Summary
Introduction • User cooperative communication is a form of communication in which users work together to deliver their data. • By relaying each others data, multiple independent copies of the data are received at the destination. • Processing of multiple independent copies of the signal reduces the probability of error. • Diversity acquired improves channel reliability and saves resources.
Introduction • Diversity in Communication is an effective way to tackle fading and improve reliability. • Diversity is obtained over time and frequency by means of coding and interleaving. It can also be obtained via repeated 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 of cooperative transmission where users act as relays to help each other.
Introduction • Cognitive users opportunistically exploit spectrum holes to improve spectrum utilization. • Three types of holes, • White holes, primary users inactive. • Gray holes, primary users work with low power. • Black holes, primary users work with high power. • User cooperation in cognitive systems further improves utilization of spectrum. • Secondary users cooperate to efficiently use the available holes. • Secondary users may cooperate with primary users to create more holes. • Multi-hop relaying, by cognitive users, to exploit gray holes.
The Relay Channel: Introduction • The relay channel is the basic building block for cooperative systems. • Early applications to tackle the curvature of the earth, path loss and irregular terrains. • The use of satellite systems motivated the extensive work on relay channels during 70’s. • Relaying helps improving resource utilization: • Extend transmission range. • Increase the throughput. • Improve reliability.
A General Three-Node Relay Channel: Model 1/2 • A three-node relay channel consists of • A source node, S. • A Destination node, D. • A relay node, R. • Two approaches to process the received signal at the relay • Amplify-and-forward (AF): sends a scaled copy of the received noisy signal. • Decode-and-forward (DF): First try to encode the received signal. If successful, re-encode and transmit.
A General Three-Node Relay Channel: Coding 1/2 • Regular encoding/sliding window decoding. • Encoding (the source and the relay nodes) • Message w is divided into B blocks w1,w2 . . .wBtransmitted in B + 1 time slots. • In time slot i, the source sends x(wi ) and the relay sends x(wi−1). • A constant sequence is sent by the relay n time slot 1 and by the source in time slot B + 1.
A General Three-Node Relay Channel: Coding 2/2 • Regular encoding/sliding window decoding (continued). • Decoding (the relay node) • Starts at the end of transmission of time slot 1 (decoding window size= 1). • At the end of transmission of time slot i, time slots i is used to decode wi. • Decoding (the destination node) • Starts at the end of transmission of time slot 2 (decoding window size= 2). • At the end of transmission of time slot i, time slots i − 1 and time slot i are combined to decode wi−1. • Advantages: • Simple. • Limited delay. • Achieves maximum rate. • Can be extended to multi-hop relaying.
A General Three-Node Relay Channel: Achievable Rate 1/4 • Capacity of the general relay channel is still unknown. • Mutual information is considered for performance measure. • 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 pmf’sp(y, yR|x, xR) for each (x, xR, y, yR) 2 X × XR × Y × YR.
A General Three-Node Relay Channel: Achievable Rate 2/4 • Two cases: • Non-cooperative relaying: No source-destination link. • Cooperative relaying: Fully connected network. • Achievable rate for non-cooperative relaying
A General Three-Node Relay Channel: Achievable Rate 3/4 • Cooperative relaying is possible if, • The network is fully connected. • The receiver is capable of processing multiple signals. • Achievable rate for cooperative relaying
A General Three-Node Relay Channel: Achievable Rate 4/4 • Notes on the achievable rate: • In both cooperative and non-cooperative relaying, the maximum rate is bounded by the source-relay channel. • When the source-relay channel is good, cooperative relaying achieves higher rate.
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 changes on channel status. • Performance improved by exploiting diversity.
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 and D for a period (1 − t). • Then, if successfully received, R retransmits w to D for a period t.
Wireless relay channel:Achievable Rate • Achievable rate for the wireless relay channel with half-duplex constraint on the relay:
Wireless relay channel: Maximizing Transmission Rate 1/2 • To make relaying efficient, the right time allocation must be used.
Wireless relay channel:Outage Probability 3/3 • Outage probability, and outage capacity, for the wireless relay channell with arbitrary time allocation can only be computed numerically. • Optimum operation by choosing to minimizes P. • opt can only be found numerically. • Sub-optimal operation using tight bounds.
User-Cooperation in Wireless Networks: Introduction • 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 no dedicated relays. • With changing topology and non-centralized nature, user-cooperative communication is particularly useful 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.
Two-User Cooperative Network: 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
Cooperative Wireless Network:Introduction • Hypothesis: • Randomly positioned nodes, arranged into source-destination pairs. • Slow changing topology. • Fixed peak power constraint on transmitters. • Network is partially known to users, • Each source knows other nodes within the range to its destination. • Rules for cooperation: • No more than two users are allowed to cooperate. • Partner selected such that both partners get higher mutual information. • Cooperation time is allocated similarly for both partners. • For a given pair of partners, time allocation is chosen to maximize the minimum rate.
Cooperative Wireless Network:Useful User • Definitions • User B is a useful user for user A if user A with user B as a relay can achieve a higher rate than direct transmission. • Harmful user: If User B is not a useful user, then it is a harmful user.
Cooperative Wireless Network:Constructive Partnership • Only constructive partnership is allowed. • User A and user B form a constructive partnership only if their mutual information increase after cooperation.
Multi-hop Relay Channel:Introduction • Multi-hop relaying is one way to employ multiple relays to serve a single channel. • First used in telecommunications, in 1940’s. First application focused on extending transmission range. • Recently used to increase throughput and improve reliability. • When more than two users are allowed to cooperate, partnership can take different forms (e.g. multi-hop relaying) with different degrees of complexity. • Multi-hop relaying is also useful for cognitive users to exploit gray holes.
Multi-hop Relay Channel:Model 2/2 • Example: 5-hop relay channel
Chapter 10 Summary • User cooperative communication offers an alternative to obtain some of the advantages of spatial diversity. • User-cooperation helps cognitive user get the most from available resources. • The philosophy of user cooperation is based on the theory of the relay channel. • Both the general and the wireless relay channels are discussed. • Model and results for the relay channel are extended to a two-user cooperative setup and eventually applied to a multi-user wireless network. • Finally, the three-node model is expanded to a M-hop relay channel.
