247,986 research outputs found
Capacity Bounds for a Class of Interference Relay Channels
The capacity of a class of Interference Relay Channels (IRC) -the Injective
Semideterministic IRC where the relay can only observe one of the sources- is
investigated. We first derive a novel outer bound and two inner bounds which
are based on a careful use of each of the available cooperative strategies
together with the adequate interference decoding technique. The outer bound
extends Telatar and Tse's work while the inner bounds contain several known
results in the literature as special cases. Our main result is the
characterization of the capacity region of the Gaussian class of IRCs studied
within a fixed number of bits per dimension -constant gap. The proof relies on
the use of the different cooperative strategies in specific SNR regimes due to
the complexity of the schemes. As a matter of fact, this issue reveals the
complex nature of the Gaussian IRC where the combination of a single coding
scheme for the Gaussian relay and interference channel may not lead to a good
coding scheme for this problem, even when the focus is only on capacity to
within a constant gap over all possible fading statistics.Comment: 23 pages, 6 figures. Submitted to IEEE Transactions on Information
Theory (revised version
Fundamental Limits of Caching
Caching is a technique to reduce peak traffic rates by prefetching popular
content into memories at the end users. Conventionally, these memories are used
to deliver requested content in part from a locally cached copy rather than
through the network. The gain offered by this approach, which we term local
caching gain, depends on the local cache size (i.e, the memory available at
each individual user). In this paper, we introduce and exploit a second,
global, caching gain not utilized by conventional caching schemes. This gain
depends on the aggregate global cache size (i.e., the cumulative memory
available at all users), even though there is no cooperation among the users.
To evaluate and isolate these two gains, we introduce an
information-theoretic formulation of the caching problem focusing on its basic
structure. For this setting, we propose a novel coded caching scheme that
exploits both local and global caching gains, leading to a multiplicative
improvement in the peak rate compared to previously known schemes. In
particular, the improvement can be on the order of the number of users in the
network. Moreover, we argue that the performance of the proposed scheme is
within a constant factor of the information-theoretic optimum for all values of
the problem parameters.Comment: To appear in IEEE Transactions on Information Theor
Routing for Security in Networks with Adversarial Nodes
We consider the problem of secure unicast transmission between two nodes in a
directed graph, where an adversary eavesdrops/jams a subset of nodes. This
adversarial setting is in contrast to traditional ones where the adversary
controls a subset of links. In particular, we study, in the main, the class of
routing-only schemes (as opposed to those allowing coding inside the network).
Routing-only schemes usually have low implementation complexity, yet a
characterization of the rates achievable by such schemes was open prior to this
work. We first propose an LP based solution for secure communication against
eavesdropping, and show that it is information-theoretically rate-optimal among
all routing-only schemes. The idea behind our design is to balance information
flow in the network so that no subset of nodes observe "too much" information.
Interestingly, we show that the rates achieved by our routing-only scheme are
always at least as good as, and sometimes better, than those achieved by
"na\"ive" network coding schemes (i.e. the rate-optimal scheme designed for the
traditional scenario where the adversary controls links in a network rather
than nodes.) We also demonstrate non-trivial network coding schemes that
achieve rates at least as high as (and again sometimes better than) those
achieved by our routing schemes, but leave open the question of characterizing
the optimal rate-region of the problem under all possible coding schemes. We
then extend these routing-only schemes to the adversarial node-jamming
scenarios and show similar results. During the journey of our investigation, we
also develop a new technique that has the potential to derive non-trivial
bounds for general secure-communication schemes
A Guide to Using Qualitative Research Methodology
A beginner's guide to using qualitative research methodologyA guide to using qualitative research methodology developed by OCA and an external academi
Exploring the practice of feedback to students
Feedback to students is a vital but relatively underresearched area. An analysis is presented of practice in a complex modular business programme. Attention is drawn to the difficulty of such analysis, but three ways of looking at the text are presented: by modality, by area of concern and by developmental content. A conversational form of feedback with a focus on ‘implied development’ is identified and placed in the context of Bernstein’s notion of an ‘invisible pedagogy’. Such a focus may disadvantage students and the importance of reflection on feedback in the context of knowledge claims at both the module- and the programme level is stressed
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