Cooperative communications with real-world constraints

Abstract

In modern society information and communications technologies play a key role. In this context, wireless communications are practically important. In this work, we explore cooperative communications techniques like distributed space-time codes and relaying to enhance the capabilities of wireless communications. At the beginning we discuss background material related to wireless communications. Specifically, we describe how the wireless channel can be measured and modeled, how the fast fading component of a channel can be characterized in indoor-to-indoor or outdoor-to-indoor environments, and we give a brief summary of OFDM and of basic concepts of cooperative communications. The first set of contribution is concerned with cooperative communications under real-world constraints. It has been found that the use of space-time codes is beneficial in multiple-input multiple-output systems to exploit diversity. We consider the use of distributed space-time codes over an OFDM system and characterize the performance subject to real-world constraints like timing and frequency offsets. We further consider a particular class of relaying schemes in order to characterize their behavior over real- world channels. For one of the relaying schemes considered we propose a time allocation strategy that improves the end-to-end spectral efficiency. We also evaluate relaying and user cooperation strategies in a multi-cellular environment under realistic conditions. Including the effect of interference in our study renders the results relevant to real-world operation. We present a simulation study that uses a standard compliant IEEE 802.11p system to characterize the performance achieved by amplify-and-forward and decode-and-forward relaying in mobile scenarios. We consider a static relay that collaborates in the communication between a mobile source and a mobile destination. The second main contribution of this work is a novel scheme for two-hop relay networks. The proposed scheme referred to as transmit-outage pre-equalization encompasses three different scenarios: single carrier, OFDM, and two-way relay networks. The proposed method uses channel state information at the source to pre-equalize the source-relay channel provided it is not in outage; it has multiple advantages with respect to existing schemes: it achieves a better performance than existing algorithms even when they are assumed to have perfect channel state information. Furthermore, channel estimation is facilitated with our new scheme. We characterize the proposed scheme analytically and via simulations.