5 research outputs found
Interference Cancellation at the Relay for Multi-User Wireless Cooperative Networks
We study multi-user transmission and detection schemes for a multi-access
relay network (MARN) with linear constraints at all nodes. In a MARN, sources, each equipped with antennas, communicate to one
-antenna destination through one -antenna relay. A new protocol called
IC-Relay-TDMA is proposed which takes two phases. During the first phase,
symbols of different sources are transmitted concurrently to the relay. At the
relay, interference cancellation (IC) techniques, previously proposed for
systems with direct transmission, are applied to decouple the information of
different sources without decoding. During the second phase, symbols of
different sources are forwarded to the destination in a time division
multi-access (TDMA) fashion. At the destination, the maximum-likelihood (ML)
decoding is performed source-by-source. The protocol of IC-Relay-TDMA requires
the number of relay antennas no less than the number of sources, i.e., . Through outage analysis, the achievable diversity gain of the proposed
scheme is shown to be . When {\small}, the proposed scheme achieves the maximum
interference-free (int-free) diversity gain . Since concurrent
transmission is allowed during the first phase, compared to full TDMA
transmission, the proposed scheme achieves the same diversity, but with a
higher symbol rate.Comment: submitted to IEEE Transaction on Wireless Communicatio
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
Virtual-MIMO systems with compress-and-forward cooperation
Multiple-input multiple-output (MIMO) systems have recently emerged as one of the most
significant wireless techniques, as they can greatly improve the channel capacity and link reliability
of wireless communications. These benefits have encouraged extensive research on a
virtual MIMO system where the transmitter has multiple antennas and each of the receivers has
a single antenna. Single-antenna receivers can work together to form a virtual antenna array and
reap some performance benefits of MIMO systems. The idea of receiver-side local cooperation
is attractive for wireless networks since a wireless receiver may not have multiple antennas due
to size and cost limitations.
In this thesis we investigate a virtual-MIMO wireless system using the receiver-side cooperation
with the compress-and-forward (CF) protocol. Firstly, to perform CF at the relay, we propose
to use standard source coding techniques, based on the analysis of its expected rate bound and
the tightness of the bound. We state upper bounds on the system error probabilities over block
fading channels. With sufficient source coding rates, the cooperation of the receivers enables
the virtual-MIMO system to achieve almost ideal MIMO performance. A comparison of ideal
and non-ideal conference links within the receiver group is also investigated. Considering the
short-range communication and using a channel-aware adaptive CF scheme, the impact of the
non-ideal cooperation link is too slight to impair the system performance significantly.
It is also evident that the practicality of CF cooperation will be greatly enhanced if a efficient
source coding technique can be used at the relay. It is even more desirable that CF cooperation
should not be unduly sensitive to carrier frequency offsets (CFOs). Thus this thesis then
presents a practical study of these two issues. Codebook designs of the Voronoi VQ and the
tree-structure vector quantization (TSVQ) to enable CF cooperation at the relay are firstly described.
A comparison in terms of the codebook design complexity and encoding complexity
is presented. It is shown that the TSVQ is much simpler to design and operate, and can achieve
a favourable performance-complexity tradeoff. We then demonstrate that CFO can lead to significant
performance degradation for the virtual MIMO system. To overcome it, it is proposed
to maintain clock synchronization and jointly estimate the CFO between the relay and the destination.
This approach is shown to provide a significant performance improvement.
Finally, we extend the study to the minimum mean square error (MMSE) detection, as it has
a lower complexity compared to maximum likelihood (ML) detection. A closed-form upper
bound for the system error probability is derived, based on which we prove that the smallest
singular value of the cooperative channel matrix determines the system error performance. Accordingly,
an adaptive modulation and cooperation scheme is proposed, which uses the smallest
singular value as the threshold strategy. Depending on the instantaneous channel conditions,
the system could therefore adapt to choose a suitable modulation type for transmission and an
appropriate quantization rate to perform CF cooperation. The adaptive modulation and cooperation
scheme not only enables the system to achieve comparable performance to the case with
fixed quantization rates, but also eliminates unnecessary complexity for quantization operations
and conference link communication
The 1992 Goddard Conference on Space Applications of Artificial Intelligence
The purpose of this conference is to provide a forum in which current research and development directed at space applications of artificial intelligence can be presented and discussed. The papers fall into the following areas: planning and scheduling, control, fault monitoring/diagnosis and recovery, information management, tools, neural networks, and miscellaneous applications