3 research outputs found
Diversity-Multiplexing Tradeoff of Asynchronous Cooperative Diversity in Wireless Networks
Synchronization of relay nodes is an important and critical issue in
exploiting cooperative diversity in wireless networks. In this paper, two
asynchronous cooperative diversity schemes are proposed, namely, distributed
delay diversity and asynchronous space-time coded cooperative diversity
schemes. In terms of the overall diversity-multiplexing (DM) tradeoff function,
we show that the proposed independent coding based distributed delay diversity
and asynchronous space-time coded cooperative diversity schemes achieve the
same performance as the synchronous space-time coded approach which requires an
accurate symbol-level timing synchronization to ensure signals arriving at the
destination from different relay nodes are perfectly synchronized. This
demonstrates diversity order is maintained even at the presence of asynchronism
between relay node. Moreover, when all relay nodes succeed in decoding the
source information, the asynchronous space-time coded approach is capable of
achieving better DM-tradeoff than synchronous schemes and performs equivalently
to transmitting information through a parallel fading channel as far as the
DM-tradeoff is concerned. Our results suggest the benefits of fully exploiting
the space-time degrees of freedom in multiple antenna systems by employing
asynchronous space-time codes even in a frequency flat fading channel. In
addition, it is shown asynchronous space-time coded systems are able to achieve
higher mutual information than synchronous space-time coded systems for any
finite signal-to-noise-ratio (SNR) when properly selected baseband waveforms
are employed
Strategic Versus Collaborative Power Control in Relay Fading Channels
Relaying is often advocated for improving system performance by enhancing spatial diversity in wireless networks. Relay nodes make contributions to improving the sourcedestination link quality by sacrificing their own energy. In this paper, we address the issue of energy tradeoff made by relay nodes between transmitting their own data and forwarding other nodes\u27 information in fading channels. Assuming channel state information (CSI) on fading amplitudes is perfectly known to both transmitters and receivers, we propose two power control and relaying policies. One is based on a strategic motivation, where each node functions as a relay and minimizes its own energy expenditure while meeting the outage probability requirement of all nodes. The second approach is based on complete collaboration, where the total energy consumption across all nodes is minimized. Numerical results demonstrate a significant impact of CSI on energy saving in relaying as compared with the relaying scheme without power control. In most cases, collaborative relaying dominates over the non-cooperative strategic one in the sense that the former not only minimizes total energy but also reduces individual energy expenditure of all nodes. This implies once forwarding and relaying is adopted across various nodes, exchanging of CSI becomes crucial, and collaborative energy minimization rather than the non-cooperative strategic approach should be pursued. © 2006 IEEE