Distributed Cooperative Communication in Large-Scale Wireless Networks

Abstract

Cooperative communication employs distributed transmission resources at the physical layer as a single radio with spatial diversity in order to increase the performance of wireless networks. However, node cooperation entails large communication overhead, and distributed protocols that eliminate or reduce the communication overhead are desirable. This dissertation proposes distributed cooperative schemes for wireless ad hoc networks and develops new methods to analyze their performance.

First, we study the behavior of distributed cooperative transmission in wireless networks for both point-to-point and broadcasting scenarios. In particular, we analyze the effect of critical network parameters on the number of nodes reached by cooperative transmission. We show that there exists a phase transition in the network behavior: if the decoding threshold is below a critical value, the message is delivered to the intended recipient(s). Otherwise, only a fraction of the nodes is reached. Our approach is based on the idea of continuum approximation, which yields closed-form expressions that are accurate when the network density is high.

We next study the optimal power allocation problem for the cooperative broadcast in dense large-scale networks. The transmission order (schedule) and the transmission powers of the relays are designed so that the message reaches the entire network with the minimum possible total power consumption. In general, finding the best scheduling in cooperative broadcast is known to be an NP-complete problem. We show that the optimal scheduling problem can be solved for dense networks, which can be expressed as a continuum of nodes.

Finally, we study the design of distributed space-time codes for cooperative communication. With few exceptions, most of the literature on the subject proposes coding rules such that either inter-node communication or a central control unit is required for code assignment. We introduce novel randomized strategies that decentralize the transmission of a space time code from a set of distributed relays. Our simple idea is to let each node transmit an independent random linear combination of the codewords that would have been transmitted by all the elements of a multi-antenna system. We show that the proposed scheme achieves the optimal diversity order.