135 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
Outage Probability of Energy Harvesting Relay-aided Cooperative Networks Over Rayleigh Fading Channel
Energy harvesting technique is a potential way for relay node energy supply
in cooperative networks in terms of deployment flexibility and maintain charge
reduction. Unlike traditional power source, relay node in this case may run out
of energy with certain probability, which can degrade the benefit from
relay-aided cooperative transmission. In this paper, we concentrate on the
outage behavior of cooperative networks aided by energy harvesting relay node
in slow fading channel, and attempt to derive the closed-form expression of
outage probability of proposed cooperative protocol. Compared with traditional
direct transmission protocol, two conclusions are derived: 1) the diversity
gain cannot be increased excepting the extreme case that energy-exhausted
probability is zero; 2) a multiplicative gain for improving system performance
can be obtained in terms of minimizing outage probability.Comment: 5 pages, 3 figure
Energy Efficiency with Proportional Rate Fairness in Multi-Relay OFDM Networks
This paper investigates the energy efficiency (EE) in multiple relay aided
OFDM system, where decode-and-forward (DF) relay beamforming is employed to
help the information transmission. In order to explore the EE performance with
user fairness for such a system, we formulate an optimization problem to
maximize the EE by jointly considering several factors, the transmission mode
selection (DF relay beamforming or direct-link transmission), the helping relay
set selection, the subcarrier assignment and the power allocation at the source
and relays on subcarriers, under nonlinear proportional rate fairness
constraints, where both transmit power consumption and linearly rate-dependent
circuit power consumption are taken into account. To solve the non-convex
optimization problem, we propose a low-complexity scheme to approximate it.
Simulation results demonstrate its effectiveness. We also investigate the
effects of the circuit power consumption on system performances and observe
that with both the constant and the linearly rate-dependent circuit power
consumption, system EE grows with the increment of system average channel-to
noise ratio (CNR), but the growth rates show different behaviors. For the
constant circuit power consumption, system EE increasing rate is an increasing
function of the system average CNR, while for the linearly rate-dependent one,
system EE increasing rate is a decreasing function of the system average CNR.
This observation is very important which indicates that by deducing the circuit
dynamic power consumption per unit data rate, system EE can be greatly
enhanced. Besides, we also discuss the effects of the number of users and
subcarriers on the system EE performance.Comment: 35 pages, 15 fihures, submitted to IEEE Journa
The Deterministic Time-Linearity of Service Provided by Fading Channels
In the paper, we study the service process of an independent and
identically distributed (\textit{i.i.d.}) Nakagami- fading channel, which is
defined as the amount of service provided, i.e., the integral of the
instantaneous channel capacity over time . By using the Characteristic
Function (CF) approach and the infinitely divisible law, it is proved that,
other than certain generally recognized curve form {or a stochastic process},
the channel service process is a deterministic linear function of time
, namely, where is a constant determined
by the fading parameter . Furthermore, we extend it to general
\textit{i.i.d.} fading channels and present an explicit form of the constant
service rate . The obtained work provides such a new insight on the
system design of joint source/channel coding that there exists a coding scheme
such that a receiver can decode with zero error probability and zero high layer
queuing delay, if the transmitter maintains a constant data rate no more than
. Finally, we verify our analysis through Monte Carlo simulations.Comment: 17 pages, 5 figure
Efficient Signal-Time Coding Design and its Application in Wireless Gaussian Relay Networks
Signal-time coding, which combines the traditional encoding/modulation mode
in the signal domain with signal pulse phase modulation in the time domain, was
proposed to improve the information flow rate in relay networks. In this paper,
we mainly focus on the efficient signal-time coding design. We first derive an
explicit iterative algorithm to estimate the maximum number of available codes
given the code length of signal-time coding, and then present an iterative
construction method of codebooks. It is shown that compared with conventional
computer search, the proposed iterative construction method can reduce the
complexity greatly. Numerical results will also indicate that the new
constructed codebook is optimal in terms of coding rate. To minimize the buffer
size needed to store the codebook while keeping a relatively high efficiency,
we shall propose a combinatorial construction method. We will then consider
applications in wireless Gaussian relay networks. It will be shown that in the
three node network model, the mixed transmission by using two-hop and direct
transmissions is not always a good option
LRC: Dependency-Aware Cache Management for Data Analytics Clusters
Memory caches are being aggressively used in today's data-parallel systems
such as Spark, Tez, and Piccolo. However, prevalent systems employ rather
simple cache management policies--notably the Least Recently Used (LRU)
policy--that are oblivious to the application semantics of data dependency,
expressed as a directed acyclic graph (DAG). Without this knowledge, memory
caching can at best be performed by "guessing" the future data access patterns
based on historical information (e.g., the access recency and/or frequency),
which frequently results in inefficient, erroneous caching with low hit ratio
and a long response time. In this paper, we propose a novel cache replacement
policy, Least Reference Count (LRC), which exploits the application-specific
DAG information to optimize the cache management. LRC evicts the cached data
blocks whose reference count is the smallest. The reference count is defined,
for each data block, as the number of dependent child blocks that have not been
computed yet. We demonstrate the efficacy of LRC through both empirical
analysis and cluster deployments against popular benchmarking workloads. Our
Spark implementation shows that, compared with LRU, LRC speeds up typical
applications by 60%.Comment: 9 page
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
Optimum Transmission Policies for Energy Harvesting Sensor Networks Powered By a Mobile Control Center
Wireless energy transfer, namely RF-based energy harvesting, is a potential
way to prolong the lifetime of energy-constrained devices, especially in
wireless sensor networks. However, due to huge propagation attenuation, its
energy efficiency is regarded as the biggest bottleneck to widely applications.
It is critical to find appropriate transmission policies to improve the global
energy efficiency in this kind of systems. To this end, this paper focuses on
the sensor networks scenario, where a mobile control center powers the sensors
by RF signal and also collects information from them. Two related schemes,
called as harvest-and-use scheme and harvest-store-use scheme, are
investigated, respectively. In harvest-and-use scheme, as a benchmark, both
constant and adaptive transmission modes from sensors are discussed. To
harvest-store-use scheme, we propose a new concept, the best opportunity for
wireless energy transfer, and use it to derive an explicit closed-form
expression of optimal transmission policy. It is shown by simulation that a
considerable improvement in terms of energy efficiency can be obtained with the
help of the transmission policies developed in this paper. Furthermore, the
transmission policies is also discussed under the constraint of fixed
information rate. The minimal required power, the performance loss from the new
constraint as well as the effect of fading are then presented.Comment: 30 pages, 5 figures. The short version was presented in part at 2015
IEEE International Conference on Communicatio
Weighted Sum-Throughput Maximization for MIMO Broadcast Channel: Energy Harvesting Under System Imperfection
In this work, a MIMO broadcast channel under the energy harvesting (EH)
constraint and the peak power constraint is investigated. The transmitter is
equipped with a hybrid energy storage system consisting of a perfect super
capacitor (SC) and an inefficient battery, where both elements have limited
energy storage capacities. In addition, the effect of data processing circuit
power consumption is also addressed. To be specific, two extreme cases are
studied here, where the first assumes ideal/zero circuit power consumption and
the second considers a positive constant circuit power consumption where the
circuit is always operating at its highest power level. The performance of
these two extreme cases hence serve as the upper bound and the lower bound of
the system performance in practice, respectively. In this setting, the offline
scheduling with ideal and maximum circuit power consumptions are investigated.
The associated optimization problems are formulated and solved in terms of
weighted throughput optimization. Further, we extend to a general circuit power
consumption model. To complement this work, some intuitive online policies are
presented for all cases. Interestingly, for the case with maximum circuit power
consumption, a close-to-optimal online policy is presented and its performance
is shown to be comparable to its offline counterpart in the numerical results.Comment: 30 page
Directivity-Beamwidth Tradeoff of Massive MIMO Uplink Beamforming for High Speed Train Communication
High-mobility adaption and massive Multiple-input Multiple-output (MIMO)
application are two primary evolving objectives for the next generation high
speed train (HST) wireless communication system. In this paper, we consider how
to design a location-aware beamforming for the massive MIMO system in the high
traffic density HST network. We first analyze the tradeoff between beam
directivity and beamwidth, based on which we present the sensitivity analysis
of positioning accuracy. Then, in order to guarantee a high efficient
transmission, we derive an optimal problem to maximize the beam directivity
under the restriction of diverse positioning accuracies. After that, we present
a low-complexity beamforming design by utilizing location information, which
requires neither eigen-decomposing (ED) the uplink channel covariance matrix
(CCM) nor ED the downlink CCM (DCCM). Finally, we study the beamforming scheme
in future high traffic density HST network, where a two HSTs encountering
scenario is emphasized. By utilizing the real-time location information, we
propose an optimal adaptive beamforming scheme to maximize the achievable rate
region under limited channel source constraint. Numerical simulation indicates
that a massive MIMO system with less than a certain positioning error can
guarantee a required performance with satisfying transmission efficiency in the
high traffic density HST scenario and the achievable rate region when two HSTs
encounter is greatly improved as well.Comment: This paper has been accepted for future publication in IEEE ACCESS.
arXiv admin note: substantial text overlap with arXiv:1702.0212
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