47,883 research outputs found
Distributed Linear Convolutional Space-Time Coding for Two-Relay Full-Duplex Asynchronous Cooperative Networks
In this paper, a two-relay full-duplex asynchronous cooperative network with
the amplify-and-forward (AF) protocol is considered. We propose two distributed
space-time coding schemes for the cases with and without cross-talks,
respectively. In the first case, each relay can receive the signal sent by the
other through the cross-talk link. We first study the feasibility of cross-talk
cancellation in this network and show that the cross-talk interference cannot
be removed well. For this reason, we design space-time codes by utilizing the
cross-talk signals instead of removing them. In the other case, the self-coding
is realized individually through the loop channel at each relay node and the
signals from the two relay nodes form a space-time code. The achievable
cooperative diversity of both cases is investigated and the conditions to
achieve full cooperative diversity are presented. Simulation results verify the
theoretical analysis.Comment: 11 pages, 7 figures, accepted by IEEE transactions on wireless
communication
On Asynchronous Communication Systems: Capacity Bounds and Relaying Schemes
abstract: Practical communication systems are subject to errors due to imperfect time alignment among the communicating nodes. Timing errors can occur in different forms depending on the underlying communication scenario. This doctoral study considers two different classes of asynchronous systems; point-to-point (P2P) communication systems with synchronization errors, and asynchronous cooperative systems. In particular, the focus is on an information theoretic analysis for P2P systems with synchronization errors and developing new signaling solutions for several asynchronous cooperative communication systems. The first part of the dissertation presents several bounds on the capacity of the P2P systems with synchronization errors. First, binary insertion and deletion channels are considered where lower bounds on the mutual information between the input and output sequences are computed for independent uniformly distributed (i.u.d.) inputs. Then, a channel suffering from both synchronization errors and additive noise is considered as a serial concatenation of a synchronization error-only channel and an additive noise channel. It is proved that the capacity of the original channel is lower bounded in terms of the synchronization error-only channel capacity and the parameters of both channels. On a different front, to better characterize the deletion channel capacity, the capacity of three independent deletion channels with different deletion probabilities are related through an inequality resulting in the tightest upper bound on the deletion channel capacity for deletion probabilities larger than 0.65. Furthermore, the first non-trivial upper bound on the 2K-ary input deletion channel capacity is provided by relating the 2K-ary input deletion channel capacity with the binary deletion channel capacity through an inequality. The second part of the dissertation develops two new relaying schemes to alleviate asynchronism issues in cooperative communications. The first one is a single carrier (SC)-based scheme providing a spectrally efficient Alamouti code structure at the receiver under flat fading channel conditions by reducing the overhead needed to overcome the asynchronism and obtain spatial diversity. The second one is an orthogonal frequency division multiplexing (OFDM)-based approach useful for asynchronous cooperative systems experiencing excessive relative delays among the relays under frequency-selective channel conditions to achieve a delay diversity structure at the receiver and extract spatial diversity.Dissertation/ThesisPh.D. Electrical Engineering 201
Working collaboratively on the digital global frontier
An international online collaborative learning experience was designed and implemented in preservice teacher education classes at the University of Calgary, Canada and the University of Southern Queensland, Australia. The project was designed to give preservice teachers an opportunity to live the experience of being online collaborators investigating real world teaching issues of diversity and inclusivity. Qualitative research was conducted to examine the complexity of the online collaborative experiences of participants. Redmond and Lock’s (2006) flexible online collaborative learning framework was used to explain the design and the implementation of the project. Henri’s (1992) content analysis model for computer-mediated communication was used for the online asynchronous postings and a constant comparative method of data analysis was used in the construction of themes. From the findings, the authors propose recommendations for designing and facilitating collaborative learning on the digital global frontier
Asynchronous Orthogonal Differential Decoding for Multiple Access Channels
We propose several differential decoding schemes for asynchronous multi-user
MIMO systems based on orthogonal space-time block codes (OSTBCs) where neither
the transmitters nor the receiver has knowledge of the channel. First, we
derive novel low complexity differential decoders by performing interference
cancelation in time and employing different decoding methods. The decoding
complexity of these schemes grows linearly with the number of users. We then
present additional differential decoding schemes that perform significantly
better than our low complexity decoders and outperform the existing synchronous
differential schemes but require higher decoding complexity compared to our low
complexity decoders. The proposed schemes work for any square OSTBC, any
constant amplitude constellation, any number of users, and any number of
receive antennas. Furthermore, we analyze the diversity of the proposed schemes
and derive conditions under which our schemes provide full diversity. For the
cases of two and four transmit antennas, we provide examples of PSK
constellations to achieve full diversity. Simulation results show that our
differential schemes provide good performance. To the best of our knowledge,
the proposed differential detection schemes are the first differential schemes
for asynchronous multi-user systems.Comment: To appear in IEEE Transactions on Wireless Communication
A case study in online formal/informal learning: was it collaborative or cooperative learning?
Developing skills in communication and collaboration is essential in modern design education, in order to prepare students for the realities of design practice, where projects involve multidisciplinary teams, often working remotely. This paper presents a learning activity that focusses on developing communication and collaboration skills of undergraduate design students working remotely and vocational learners based in a community makerspace. Participants were drawn from these formal and informal educational settings and engaged in a design-make project framed in the context of distributed manufacturing. They were given designer or maker roles and worked at distance from each other, communicating using asynchronous online tools. Analysis of the collected data has identified a diversity of working practice across the participants, and highlighted the difficulties that result from getting students to work collaboratively, when not collocated. This paper presents and analysis of participants’ communications, with a view to identify whether they were learning collaboratively, or cooperatively. It was found that engaging participants in joint problem solving is not enough to facilitate collaboration. Instead effective collaboration depends on symmetry within the roles of participants and willingness to share expertise through dialogue. Designing learning activities to overcome the challenges that these factors raise is a difficult task, and the research reported here provides some valuable insight
OFDM based Distributed Space Time Coding for Asynchronous Relay Networks
Recently Li and Xia have proposed a transmission scheme for wireless relay
networks based on the Alamouti space time code and orthogonal frequency
division multiplexing to combat the effect of timing errors at the relay nodes.
This transmission scheme is amazingly simple and achieves a diversity order of
two for any number of relays. Motivated by its simplicity, this scheme is
extended to a more general transmission scheme that can achieve full
cooperative diversity for any number of relays. The conditions on the
distributed space time block code (DSTBC) structure that admit its application
in the proposed transmission scheme are identified and it is pointed out that
the recently proposed full diversity four group decodable DSTBCs from precoded
co-ordinate interleaved orthogonal designs and extended Clifford algebras
satisfy these conditions. It is then shown how differential encoding at the
source can be combined with the proposed transmission scheme to arrive at a new
transmission scheme that can achieve full cooperative diversity in asynchronous
wireless relay networks with no channel information and also no timing error
knowledge at the destination node. Finally, four group decodable distributed
differential space time block codes applicable in this new transmission scheme
for power of two number of relays are also provided.Comment: 5 pages, 2 figures, to appear in IEEE International Conference on
Communications, Beijing, China, May 19-23, 200
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
- …