Cooperative Techniques for Wireless Ad Hoc and Cellular Networks
Federico Librino · Padua Research Archive (University of Padova) · 2010
The impact of fading and other impairments in wireless channels can be counteracted by leveraging communication diversity and introducing cooperative paradigms, where third-party nodes contribute to assist the communication. Cooperation can be implemented in a variety of schemes, and with a number of different purposes. In this work, we investigate how different cooperative techniques can be practically applied in different environments. In the first part, we study the performance improvement offered by Coded Cooperation in wireless Ad Hoc networks. Our analysis aims at clarifying the impact of cooperation from a network-wide perspective, and a cooperative protocol is designed to illustrate a practical implementation. Cooperation is shown to be effective both locally and globally. On the one side, it can strengthen a single link, when one or more cooperators help the source in delivering the packet to its destination; on the flip side, cooperators may be exploited to find out alternative routes to deliver a packet to a far destination, thus avoiding bad links or crowded areas of the network. A cooperative scheme based on Coded Cooperation is applied to a network whose nodes are equipped with multiple antennas. MIMO technology in fact makes it easier to coordinate the nodes involved in each communication, since multiple concurrent transmissions can be decoded. In our scheme, a distributed form of HARQ is implemented, which is able to react to interference variations and channel fading by exploiting both coding gain and spatial diversity. The second part discusses the use of relays in wireless CDMA cellular networks for uplink transmissions, with particular attention put to the constraints and the standard parameters that may affect the effectiveness of cooperation in a realistic scenario. Simple cooperative techniques like Amplify & Forward and Decode & Forward are tested. The goal is to identify some design directions that should be used when applying these cooperative techniques: cooperators positions and capabilities, in fact, have an impact on the obtained performance. Moreover, carrier assignment is important to keep interference under control. Our investigation shows that the coverage area can be increased through use of cooperation, or equivalently, that power saving is possible when a mobile terminal is helped by a relay. In order to derive a theoretical limit to the capacity of these cooperative systems, which requires combinatorial calculations and the derivation of the areas covered by one or more Stations, an iterative algorithm has been developed and is also presented in the last part of this work.