6,805 research outputs found
High Speed Railway Wireless Communications: Efficiency v.s. Fairness
High speed railways (HSRs) have been deployed widely all over the world in
recent years. Different from traditional cellular communication, its high
mobility makes it essential to implement power allocation along the time. In
the HSR case, the transmission rate depends greatly on the distance between the
base station (BS) and the train. As a result, the train receives a time varying
data rate service when passing by a BS. It is clear that the most efficient
power allocation will spend all the power when the train is nearest from the
BS, which will cause great unfairness along the time. On the other hand, the
channel inversion allocation achieves the best fairness in terms of constant
rate transmission. However, its power efficiency is much lower. Therefore, the
power efficiency and the fairness along time are two incompatible objects. For
the HSR cellular system considered in this paper, a trade-off between the two
is achieved by proposing a temporal proportional fair power allocation scheme.
Besides, near optimal closed form solution and one algorithm finding the
-optimal allocation are presented.Comment: 16 pages, 6 figure
Wireless Information and Energy Transfer for Two-Hop Non-Regenerative MIMO-OFDM Relay Networks
This paper investigates the simultaneous wireless information and energy
transfer for the non-regenerative multipleinput multiple-output orthogonal
frequency-division multiplexing (MIMO-OFDM) relaying system. By considering two
practical receiver architectures, we present two protocols, time switchingbased
relaying (TSR) and power splitting-based relaying (PSR). To explore the system
performance limit, we formulate two optimization problems to maximize the
end-to-end achievable information rate with the full channel state information
(CSI) assumption. Since both problems are non-convex and have no known solution
method, we firstly derive some explicit results by theoretical analysis and
then design effective algorithms for them. Numerical results show that the
performances of both protocols are greatly affected by the relay position.
Specifically, PSR and TSR show very different behaviors to the variation of
relay position. The achievable information rate of PSR monotonically decreases
when the relay moves from the source towards the destination, but for TSR, the
performance is relatively worse when the relay is placed in the middle of the
source and the destination. This is the first time to observe such a
phenomenon. In addition, it is also shown that PSR always outperforms TSR in
such a MIMO-OFDM relaying system. Moreover, the effect of the number of
antennas and the number of subcarriers are also discussed.Comment: 16 pages, 12 figures, to appear in IEEE Selected Areas in
Communication
Probing the Metal Enrichment of the Intergalactic Medium at Using the Hubble Space Telescope
We test the galactic outflow model by probing associated galaxies of four
strong intergalactic CIV absorbers at --6 using the Hubble Space Telescope
(HST) ACS ramp narrowband filters. The four strong CIV absorbers reside at
, , , and , with column densities ranging from
cm to cm. At , we
detect an i-dropout Ly emitter (LAE) candidate with a projected impact
parameter of 42 physical kpc from the CIV absorber. This LAE candidate has a
Ly-based star formation rate (SFR) of 2
yr and a UV-based SFR of 4 yr. Although we cannot
completely rule out that this -dropout emitter may be an [OII] interloper,
its measured properties are consistent with the CIV powering galaxy at
. For CIV absorbers at and , although we detect two
LAE candidates with impact parameters of 160 kpc and 200 kpc, such distances
are larger than that predicted from the simulations. Therefore we treat them as
non-detections. For the system at , we do not detect LAE candidates,
placing a 3- upper limit of SFR
yr. In summary, in these four cases, we only detect one plausible CIV
source at . Combining the modest SFR of the one detection and the three
non-detections, our HST observations strongly support that smaller galaxies
(SFR yr) are main sources of
intergalactic CIV absorbers, and such small galaxies play a major role in the
metal enrichment of the intergalactic medium at .Comment: Accepted for Publications in ApJ
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