573 research outputs found
Enhancing diversity and multiplexing gains in multi-user wireless relay systems
The demand for higher transmission rates and better quality of service in modern wireless
communications is endless. The use of multiple transmit or /and receive antennas has been
considered as one of the most powerful approaches to facilitate high -speed and high -quality
communications. However, in practical cellular systems, mobile terminals may not be able to
support a multiple- antenna setup. Thus an emerging technique called cooperative diversity is
under consideration to utilize the multi -hop relay concept to realize the advantages of multiple - antenna systems in multi -user single- antenna networks. Cooperative diversity has attracted
much interest in recent years as a very promising direction for future wireless communication
evolution.Due to the fact that in practice terminals cannot transmit and receive simultaneously (i.e. the
half -duplex limitation), the diversity improvement brought by the standard cooperative diversity
transmission protocols is in general accompanied by a multiplexing loss (equivalent to a
reduction in transmission data rate in high signal -to -nose ratio (SNR)). The purpose of this
thesis is to use advanced transmission protocols to provide both good diversity and multiplexing
performance when using the practical repetition -coded decode - and -forward (DF) relaying
strategy in uplink mobile -to -base station transmission of cellular systems.The task is fulfilled by relaxing the orthogonal channel allocation requirement of the standard
protocols and by using two relays to take turns forwarding source information to destination.
We start our analysis from an M- source two -relay one -destination network. Through
diversity -multiplexing tradeoff (DMT) analysis, we prove that for an isolated -relay scenario
and a strong -interference scenario, the considered approach effectively recovers the multiplexing
loss induced by the standard protocols while still obtaining diversity improvement over
direct source -destination transmission without considering relaying.In addition, since the optimal multiplexing gain of the considered system can be achieved by the
above approach, we study further improving diversity performance for a two -source network.
We analyze taking full advantage of the multiple- source structure, multiple -relay structure, and
the capability of affording complex signal processing at the destination (base station). For all
three cases, we prove that the diversity performance of the above approach can be enhanced
without a significant loss of multiplexing performance or using complex coding strategies at
relays. Since the good DMT performance is not affected by source -relay channel conditions,
the protocols discussed in this thesis make relaying more beneficial
Recovering Multiplexing Loss Through Successive Relaying Using Repetition Coding
In this paper, a transmission protocol is studied for a two relay wireless
network in which simple repetition coding is applied at the relays.
Information-theoretic achievable rates for this transmission scheme are given,
and a space-time V-BLAST signalling and detection method that can approach them
is developed. It is shown through the diversity multiplexing tradeoff analysis
that this transmission scheme can recover the multiplexing loss of the
half-duplex relay network, while retaining some diversity gain. This scheme is
also compared with conventional transmission protocols that exploit only the
diversity of the network at the cost of a multiplexing loss. It is shown that
the new transmission protocol offers significant performance advantages over
conventional protocols, especially when the interference between the two relays
is sufficiently strong.Comment: To appear in the IEEE Transactions on Wireless Communication
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
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