29,905 research outputs found
A Unified Relay Framework with both D-F and C-F Relay Nodes
Decode-and-forward (D-F) and compress-and-forward (C-F) are two fundamentally
different relay strategies proposed by (Cover and El Gamal, 1979).
Individually, either of them has been successfully generalized to multi-relay
channels. In this paper, to allow each relay node the freedom of choosing
either of the two strategies, we propose a unified framework, where both the
D-F and C-F strategies can be employed simultaneously in the network. It turns
out that, to fully incorporate the advantages of both the best known D-F and
C-F strategies into a unified framework, the major challenge arises as follows:
For the D-F relay nodes to fully utilize the help of the C-F relay nodes,
decoding at the D-F relay nodes should not be conducted until all the blocks
have been finished; However, in the multi-level D-F strategy, the upstream
nodes have to decode prior to the downstream nodes in order to help, which
makes simultaneous decoding at all the D-F relay nodes after all the blocks
have been finished inapplicable. To tackle this problem, nested blocks combined
with backward decoding are used in our framework, so that the D-F relay nodes
at different levels can perform backward decoding at different frequencies. As
such, the upstream D-F relay nodes can decode before the downstream D-F relay
nodes, and the use of backward decoding at each D-F relay node ensures the full
exploitation of the help of both the other D-F relay nodes and the C-F relay
nodes. The achievable rates under our unified relay framework are found to
combine both the best known D-F and C-F achievable rates and include them as
special cases
Flow Decomposition for Multi-User Channels - Part I
A framework based on the idea of flow decomposition is proposed to
characterize the decode-forward region for general multi-source, multi-relay,
all-cast channels with independent input distributions. The region is difficult
to characterize directly when deadlocks occur between two relay nodes, in which
both relays benefit by decoding after each other. Rate-vectors in the
decode-forward region depend ambiguously on the outcomes of all deadlocks in
the channel. The region is characterized indirectly in two phases. The first
phase assumes relays can operate non-causally. It is shown that every
rate-vector in the decode-forward region corresponds to a set of flow
decompositions, which describe the messages decoded at each node with respect
to the messages forwarded by all the other nodes. The second phase imposes
causal restrictions on the relays. Given an arbitrary set of (possibly
non-causal) flow decompositions, necessary and sufficient conditions are
derived for the existence of an equivalent set of causal flow decompositions
that achieves the same rate-vector region
Federal Procurement and Equal Employment Opportunity
The paper contains a discussion about what results about the quality of an estimated model can be achieved, if no probabilitic assumptions are introduced. Several technical results that illustrate possibilities and difficulties are also given
A strong negative correlation between radio loudness and optical-to-X-ray spectral index in low-luminosity AGNs
It has been argued for years that the accretion mode changes from bright
active galactic nuclei (AGNs) to low-luminosity AGNs (LLAGNs) at a rough
dividing point of bolometric Eddington ratio . In this
work, we strengthen this scenario through investigation of the relationship
between the radio loudness and the optical-to-X-ray spectral index
in LLAGNs with .
We compile from literature a sample of 32 LLAGNs, consisting 18 LINERs and 14
low Eddington ratio Seyfert galaxies, and observe a strong negative -- relationship, with large scatter in both
and . We further demonstrate that this negative correlation,
and the additional two negative relationships reported in literature (-- and -- correlations), can be
understood consistently and comprehensively under the truncated accretion--jet
model, the model that has been applied successfully applied to LLAGNs. We argue
that the scatter in the observations are (mainly) due to the spread in the
viscosity parameter of a hot accretion flow, a parameter that
potentially can serve as a diagnose of the strength and/or configuration of
magnetic fields in accretion flows.Comment: 8 pages, 3 figures, 2 tables. Accepted by MNRA
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