Chapter 10 User Cooperative Communications 1
Outline Introduction Relay Channels User-Cooperation in Wireless Networks Multi-Hop Relay Channel Summary 2
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. 3
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. 4
Introduction Cognitive users opportunistically exploit spectrum holes to 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 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. 5
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. 6
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 signal at the relay 1. Amplify-and-forward (AF): sends a scaled copy of the received noisy signal. 2. Decode-and-forward (DF): First try to encode the received signal. If successful, re-encode and transmit. 7
A General Three-Node Relay Channel: Model 2/2 8
A General Three-Node Relay 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 sends x(wi 1). 3. A constant sequence is sent by the relay n time slot 1 and by the source in time slot B + 1. 9
A General Three-Node Relay 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 i are combined to decode wi 1. Advantages: Simple. Limited delay. Achieves maximum rate. Can be extended to multi-hop relaying. 10
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 s p(y, yr x, xr) for each (x, xr, y, yr) 2 X XR Y YR. 11
A General Three-Node Relay 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 12
A General Three-Node Relay 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 13
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. 14
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. 15
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. 16
Wireless relay channel: Model and Strategy 2/2 17
Wireless relay channel: Achievable Rate Achievable rate for the wireless relay channel with half-duplex constraint on the relay: 18
Wireless relay channel: Maximizing Transmission Rate 1/2 To make relaying efficient, the right time allocation must be used. 19
Wireless relay channel: Maximizing Transmission Rate 2/2 20
Wireless relay channel: Outage Probability 1/3 21
Wireless relay channel: Outage Probability 2/3 22
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. 23
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. 24
Two-User Cooperative Network: System Model 1/2 25
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 26
Two-User Cooperative Network: System Constraints 27
Two-User Cooperative Network: Optimizing Performance 28
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. 29
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. 30
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. 31
Cooperative Wireless Network: Data Link Layer 32
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. 33
Multi-hop Relay Channel: Model 1/2 34
Multi-hop Relay Channel: Model 2/2 Example: 5-hop relay channel 35
Multi-hop Relay Channel: Achievable Rate 36
Multi-hop Relay Channel: Optimal Time Allocation 37
Multi-hop Relay Channel: Outage Probability 38
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. 39