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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
Three-dimensionality of space and the quantum bit: an information-theoretic approach
It is sometimes pointed out as a curiosity that the state space of quantum
two-level systems, i.e. the qubit, and actual physical space are both
three-dimensional and Euclidean. In this paper, we suggest an
information-theoretic analysis of this relationship, by proving a particular
mathematical result: suppose that physics takes place in d spatial dimensions,
and that some events happen probabilistically (not assuming quantum theory in
any way). Furthermore, suppose there are systems that carry "minimal amounts of
direction information", interacting via some continuous reversible time
evolution. We prove that this uniquely determines spatial dimension d=3 and
quantum theory on two qubits (including entanglement and unitary time
evolution), and that it allows observers to infer local spatial geometry from
probability measurements.Comment: 13 + 22 pages, 9 figures. v4: some clarifications, in particular in
Section V / Appendix C (added Example 39
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