3,950 research outputs found
Performance Bounds for Bi-Directional Coded Cooperation Protocols
In coded bi-directional cooperation, two nodes wish to exchange messages over
a shared half-duplex channel with the help of a relay. In this paper, we derive
performance bounds for this problem for each of three protocols.
The first protocol is a two phase protocol were both users simultaneously
transmit during the first phase and the relay alone transmits during the
second. In this protocol, our bounds are tight and a multiple-access channel
transmission from the two users to the relay followed by a coded broadcast-type
transmission from the relay to the users achieves all points in the two-phase
capacity region.
The second protocol considers sequential transmissions from the two users
followed by a transmission from the relay while the third protocol is a hybrid
of the first two protocols and has four phases. In the latter two protocols the
inner and outer bounds are not identical, and differ in a manner similar to the
inner and outer bounds of Cover's relay channel. Numerical evaluation shows
that at least in some cases of interest our bounds do not differ significantly.
Finally, in the Gaussian case with path loss, we derive achievable rates and
compare the relative merits of each protocol in various regimes. This case is
of interest in cellular systems. Surprisingly, we find that in some cases, the
achievable rate region of the four phase protocol sometimes contains points
that are outside the outer bounds of the other protocols.Comment: 15 page
Observation of vortex-antivortex pairing in decaying 2D turbulence of a superfluid gas
In a two-dimensional (2D) classical fluid, a large-scale flow structure
emerges out of turbulence, which is known as the inverse energy cascade where
energy flows from small to large length scales. An interesting question is
whether this phenomenon can occur in a superfluid, which is inviscid and
irrotational by nature. Atomic Bose-Einstein condensates (BECs) of highly
oblate geometry provide an experimental venue for studying 2D superfluid
turbulence, but their full investigation has been hindered due to a lack of the
circulation sign information of individual quantum vortices in a turbulent
sample. Here, we demonstrate a vortex sign detection method by using Bragg
scattering, and we investigate decaying turbulence in a highly oblate BEC at
low temperatures, with our lowest being , where is the
superfluid critical temperature. We observe that weak spatial pairing between
vortices and antivortices develops in the turbulent BEC, which corresponds to
the vortex-dipole gas regime predicted for high dissipation. Our results
provide a direct quantitative marker for the survey of various 2D turbulence
regimes in the BEC system.Comment: 8 pages, 8 figure
- …