Cooperation in Wireless Networks: Principles and Applications - Real Egoistic Behavior is to Cooperate!

Chapter 2 COOPERATIVE COMMUNICATIONS (p. 29)

Fundamental Limits and Practical Implementation

Arnab Chakrabarti
Rice University

Ashutosh Sabharwal
Rice University

Behnaam Aazhang
Rice UniversityAbstract:
This chapter summarizes theoretically achievable gains and the construction of practical codes for user-cooperation. Most of these results relate to the relay channel, which is a three-terminal channel that captures the essence of usercooperation and serves as one of the primary building blocks for cooperation on a larger scale. In investigating the fundamental limits of relaying, we present information-theoretic results on the achievable throughput of relay channel in mutual-information terms.

We also include results on Gaussian channels, and for the practically important case of half-duplex relaying. In the domain of relay coding, we specifically discuss pragmatic code constructions for half as well as full-duplex relaying, using LDPC codes as components.

Keywords: wireless communication, user cooperation, relay, broadcast, multiple access, decode-and-forward, estimate-and-forward, amplify-and-forward, information theory, coding, LDPC, max-flow min-cut

1. Introduction

Cooperative communication is one of the fastest growing areas of research, and it is likely to be a key enabling technology for efficient spectrum use in future. 1 The key idea in user-cooperation is that of resource-sharing among multiple nodes in a network. The reason behind the exploration of user-cooperation is that willingness to share power and computation with neighboring nodes can lead to savings of overall network resources.

Mesh networks provide an enormous application space for user-cooperation strateges to be implemented. In traditional communication networks, the physical layer is only responsible for communicating information from one node to another. In contrast, usercooperation implies a paradigm shift, where the channel is not just one link but the network itself.

The current chapter summarizes the fundamental limits achievable by cooperative communication, and also discusses practical code constructions that carry the potential to reach these limits. Cooperation is possible whenever the number of communicating terminals exceeds two. Therefore, a three-terminal network is a fundamental unit in usercooperation.

Indeed, a vast portion of the literature, especially in the realm of information theory, has been devoted to a special three-terminal channel, labeled the relay channel. The focus of our discussion will be the relay channel, and its various extensions. In contrast, there is also a prominent portion of literature devoted to cooperation as viewed from a network-wide perspective, which we will only briefly allude to.

Our emphasis is on user-cooperation in the domain of wireless communication, and the fundamental limits that we discuss are information theoretic in nature. In this regard, we first bound the achievable rates of relaying using mutual information expressions involving inputs and outputs of the cooperating nodes. We then investigate relaying in the context of Gaussian channels, and summarize known results for well-known relaying protocols.