79,598 research outputs found
Fundamental Constraints on Multicast Capacity Regions
Much of the existing work on the broadcast channel focuses only on the
sending of private messages. In this work we examine the scenario where the
sender also wishes to transmit common messages to subsets of receivers. For an
L user broadcast channel there are 2L - 1 subsets of receivers and
correspondingly 2L - 1 independent messages. The set of achievable rates for
this channel is a 2L - 1 dimensional region. There are fundamental constraints
on the geometry of this region. For example, observe that if the transmitter is
able to simultaneously send L rate-one private messages, error-free to all
receivers, then by sending the same information in each message, it must be
able to send a single rate-one common message, error-free to all receivers.
This swapping of private and common messages illustrates that for any broadcast
channel, the inclusion of a point R* in the achievable rate region implies the
achievability of a set of other points that are not merely component-wise less
than R*. We formerly define this set and characterize it for L = 2 and L = 3.
Whereas for L = 2 all the points in the set arise only from operations relating
to swapping private and common messages, for L = 3 a form of network coding is
required
Lithopanspermia in Star Forming Clusters
This paper considers the lithopanspermia hypothesis in star forming groups
and clusters, where the chances of biological material spreading from one solar
system to another is greatly enhanced (relative to the field) due to the close
proximity of the systems and lower relative velocities. These effects more than
compensate for the reduced time spent in such crowded environments. This paper
uses 300,000 Monte Carlo scattering calculations to determine the cross
sections for rocks to be captured by binaries and provides fitting formulae for
other applications. We assess the odds of transfer as a function of the
ejection speed and number of members in the birth aggregate. The odds of any
given ejected meteroid being recaptured by another solar system are relatively
low. Because the number of ejected rocks per system can be large, virtually all
solar systems are likely to share rocky ejecta with all of the other solar
systems in their birth cluster. The number of ejected rocks that carry living
microorganisms is much smaller and less certain, but we estimate that several
million rocks can be ejected from a biologically active solar system. For
typical birth environments, the capture of life bearing rocks is expected to
occur 10 -- 16,000 times per cluster (under favorable conditions), depending on
the ejection speeds. Only a small fraction of the captured rocks impact the
surfaces of terrestrial planets, so that only a few lithopanspermia events are
expected (per cluster).Comment: 27 pages including 5 figures; accepted to Astrobiolog
Immunocompetence in Hydra. Epithelial cells recognize self-nonself and react against it
The evolution of effective immunologic defense mechanisms in multicellular organisms involves the ability of host cells to distinguish betweeen self and nonself and to react appropriately to eliminate foreign tissue. By producing interspecies grafts we have obtained evidence that immunorecognition followed by incompatibility reactions occur in Hydra. Our results demonstrate that epithelial cells of Hydra recognize and phagocytose foreign hydra cells, indicating that they are the effector cells in the incompatibility reactions. This observation is consistent with the idea that immunocompetence appeared early in the evolution of multicellular organisms
Interference Mitigation Through Limited Receiver Cooperation: Symmetric Case
Interference is a major issue that limits the performance in wireless
networks, and cooperation among receivers can help mitigate interference by
forming distributed MIMO systems. The rate at which receivers cooperate,
however, is limited in most scenarios. How much interference can one bit of
receiver cooperation mitigate? In this paper, we study the two-user Gaussian
interference channel with conferencing decoders to answer this question in a
simple setting. We characterize the fundamental gain from cooperation: at high
SNR, when INR is below 50% of SNR in dB scale, one-bit cooperation per
direction buys roughly one-bit gain per user until full receiver cooperation
performance is reached, while when INR is between 67% and 200% of SNR in dB
scale, one-bit cooperation per direction buys roughly half-bit gain per user.
The conclusion is drawn based on the approximate characterization of the
symmetric capacity in the symmetric set-up. We propose strategies achieving the
symmetric capacity universally to within 3 bits. The strategy consists of two
parts: (1) the transmission scheme, where superposition encoding with a simple
power split is employed, and (2) the cooperative protocol, where
quantize-binning is used for relaying.Comment: To appear in IEEE Information Theory Workshop, Taormina, October
2009. Final versio
Cloned interstitial stem cells grow as contiguous patches in hydra
The migration of interstitial cells was analyzed during the growth of stem cell clones in vivo. The spatial distribution of cloned cells was analyzed at a time by which extensive migration of interstitial cells could have occurred. All interstitial cell clones were found to form large contiguous patches of cells. The results indicate that there is little migration of large interstitial cells in undisturbed tissue during normal growth. This finding is surprising since numerous grafting experiments have shown extensive migration of these cells. The implications of finding nonrandomly distributed stem cells are discussed
Interference Mitigation Through Limited Receiver Cooperation
Interference is a major issue limiting the performance in wireless networks.
Cooperation among receivers can help mitigate interference by forming
distributed MIMO systems. The rate at which receivers cooperate, however, is
limited in most scenarios. How much interference can one bit of receiver
cooperation mitigate? In this paper, we study the two-user Gaussian
interference channel with conferencing decoders to answer this question in a
simple setting. We identify two regions regarding the gain from receiver
cooperation: linear and saturation regions. In the linear region receiver
cooperation is efficient and provides a degrees-of-freedom gain, which is
either one cooperation bit buys one more bit or two cooperation bits buy one
more bit until saturation. In the saturation region receiver cooperation is
inefficient and provides a power gain, which is at most a constant regardless
of the rate at which receivers cooperate. The conclusion is drawn from the
characterization of capacity region to within two bits. The proposed strategy
consists of two parts: (1) the transmission scheme, where superposition
encoding with a simple power split is employed, and (2) the cooperative
protocol, where one receiver quantize-bin-and-forwards its received signal, and
the other after receiving the side information decode-bin-and-forwards its
received signal.Comment: Submitted to IEEE Transactions on Information Theory. 69 pages, 14
figure
- âŠ