696 research outputs found
Content Delivery in Erasure Broadcast Channels with Cache and Feedback
We study a content delivery problem in a K-user erasure broadcast channel
such that a content providing server wishes to deliver requested files to
users, each equipped with a cache of a finite memory. Assuming that the
transmitter has state feedback and user caches can be filled during off-peak
hours reliably by the decentralized content placement, we characterize the
achievable rate region as a function of the memory sizes and the erasure
probabilities. The proposed delivery scheme, based on the broadcasting scheme
by Wang and Gatzianas et al., exploits the receiver side information
established during the placement phase. Our results can be extended to
centralized content placement as well as multi-antenna broadcast channels with
state feedback.Comment: 29 pages, 7 figures. A short version has been submitted to ISIT 201
Optimal Channel Training in Uplink Network MIMO Systems
We consider a multi-cell frequency-selective fading uplink channel (network
MIMO) from K single-antenna user terminals (UTs) to B cooperative base stations
(BSs) with M antennas each. The BSs, assumed to be oblivious of the applied
codebooks, forward compressed versions of their observations to a central
station (CS) via capacity limited backhaul links. The CS jointly decodes the
messages from all UTs. Since the BSs and the CS are assumed to have no prior
channel state information (CSI), the channel needs to be estimated during its
coherence time. Based on a lower bound of the ergodic mutual information, we
determine the optimal fraction of the coherence time used for channel training,
taking different path losses between the UTs and the BSs into account. We then
study how the optimal training length is impacted by the backhaul capacity.
Although our analytical results are based on a large system limit, we show by
simulations that they provide very accurate approximations for even small
system dimensions.Comment: 15 pages, 7 figures. To appear in the IEEE Transactions on Signal
Processin
Asymptotic Moments for Interference Mitigation in Correlated Fading Channels
We consider a certain class of large random matrices, composed of independent
column vectors with zero mean and different covariance matrices, and derive
asymptotically tight deterministic approximations of their moments. This random
matrix model arises in several wireless communication systems of recent
interest, such as distributed antenna systems or large antenna arrays.
Computing the linear minimum mean square error (LMMSE) detector in such systems
requires the inversion of a large covariance matrix which becomes prohibitively
complex as the number of antennas and users grows. We apply the derived moment
results to the design of a low-complexity polynomial expansion detector which
approximates the matrix inverse by a matrix polynomial and study its asymptotic
performance. Simulation results corroborate the analysis and evaluate the
performance for finite system dimensions.Comment: 7 pages, 2 figures, to be presented at IEEE International Symposium
on Information Theory (ISIT), Saint Petersburg, Russia, July 31 - August 5,
201
Cache-Enabled Broadcast Packet Erasure Channels with State Feedback
We consider a cache-enabled K-user broadcast erasure packet channel in which
a server with a library of N files wishes to deliver a requested file to each
user who is equipped with a cache of a finite memory M. Assuming that the
transmitter has state feedback and user caches can be filled during off-peak
hours reliably by decentralized cache placement, we characterize the optimal
rate region as a function of the memory size, the erasure probability. The
proposed delivery scheme, based on the scheme proposed by Gatzianas et al.,
exploits the receiver side information established during the placement phase.
Our results enable us to quantify the net benefits of decentralized coded
caching in the presence of erasure. The role of state feedback is found useful
especially when the erasure probability is large and/or the normalized memory
size is small.Comment: 8 pages, 4 figures, to be presented at the 53rd Annual Allerton
Conference on Communication, Control, and Computing, IL, US
Morphological Characteristics of the Carapace of the Hawksbill Turtle, Eretmochelys imbricata, from Cuban Waters
Hawksbill turtles, Eretmochelys imbricata (Linnaeus, 1766), from Cuban waters of the Caribbean were analyzed to determine the relationships between straight carapace length (SCL) and either straight carapace width (SCW) or body weight (Wt). The regression equations were SCW = 0.9136(SCL)0.951 (R2 = 0.923, n = 315) and Wt = 4.17 x 10–4(SCL)2.68 (R2 = 0.798, n = 289), respectively. The regression equations between the first costal width (C1W) and either SCW or Wt were SCW = 3.223(C1W)0.847 (R2 = 0.919, n = 156) and Wt = 1.416 x 10–2(C1W)2.426 (R2 = 0.740, n = 133), respectively. There was no difference in slopes of the C1W-SCL relationship between wild and captive raised turtles as analyzed by ANCOVA. Thus, I pooled the group data and re-calculated the C1W and SCL relationship as SCL = 4.353(C1W)0.848 (R2 = 0.953, n = 340). This result indicated that SCL measurements could be estimated based on C1W measurements and that the C1W-SCL relationship could be applied to captive raised or wild hawksbills. It is clear that the SCL-SCW and C1W-SCW relationships were more similar to the relationship in the hawksbill turtles from Puerto Rican waters than to those captured in Australian waters, although there was no significant geographic difference between specimens from the Caribbean and Australian waters
Quantized vs. Analog Feedback for the MIMO Downlink: A Comparison between Zero-Forcing Based Achievable Rates
We consider a MIMO fading broadcast channel and compare the achievable
ergodic rates when the channel state information at the transmitter is provided
by analog noisy feedback or by quantized (digital) feedback. The superiority of
digital feedback is shown, with perfect or imperfect CSIR, whenever the number
of feedback channel uses per channel coefficient is larger than 1. Also, we
show that by proper design of the digital feedback link, errors in the feedback
have a minor effect even by using very simple uncoded modulation. Finally, we
show that analog feedback achieves a fraction 1 - 2F of the optimal
multiplexing gain even in the presence of a feedback delay, when the fading
belongs to the class of Doppler processes with normalized maximum Doppler
frequency shift 0 <= F <= 1/2.Comment: Submitted to ISIT, January 2007. 5 page
On the Degrees of Freedom of time correlated MISO broadcast channel with delayed CSIT
We consider the time correlated MISO broadcast channel where the transmitter
has partial knowledge on the current channel state, in addition to delayed
channel state information (CSI). Rather than exploiting only the current CSI,
as the zero-forcing precoding, or only the delayed CSI, as the Maddah-Ali-Tse
(MAT) scheme, we propose a seamless strategy that takes advantage of both. The
achievable degrees of freedom of the proposed scheme is characterized in terms
of the quality of the current channel knowledge.Comment: 7 pages, 1 figure, submitted to ISIT 2012, extended version with
detailed proof
Degrees of Freedom of Time Correlated MISO Broadcast Channel with Delayed CSIT
We consider the time correlated multiple-input single-output (MISO) broadcast
channel where the transmitter has imperfect knowledge on the current channel
state, in addition to delayed channel state information. By representing the
quality of the current channel state information as P^-{\alpha} for the
signal-to-noise ratio P and some constant {\alpha} \geq 0, we characterize the
optimal degree of freedom region for this more general two-user MISO broadcast
correlated channel. The essential ingredients of the proposed scheme lie in the
quantization and multicasting of the overheard interferences, while
broadcasting new private messages. Our proposed scheme smoothly bridges between
the scheme recently proposed by Maddah-Ali and Tse with no current state
information and a simple zero-forcing beamforming with perfect current state
information.Comment: revised and final version, to appear in IEEE transactions on
Information Theor
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