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.