8,899 research outputs found
The Impact of QoS Constraints on the Energy Efficiency of Fixed-Rate Wireless Transmissions
Transmission over wireless fading channels under quality of service (QoS)
constraints is studied when only the receiver has channel side information.
Being unaware of the channel conditions, transmitter is assumed to send the
information at a fixed rate. Under these assumptions, a two-state (ON-OFF)
transmission model is adopted, where information is transmitted reliably at a
fixed rate in the ON state while no reliable transmission occurs in the OFF
state. QoS limitations are imposed as constraints on buffer violation
probabilities, and effective capacity formulation is used to identify the
maximum throughput that a wireless channel can sustain while satisfying
statistical QoS constraints. Energy efficiency is investigated by obtaining the
bit energy required at zero spectral efficiency and the wideband slope in both
wideband and low-power regimes assuming that the receiver has perfect channel
side information (CSI). In both wideband and low-power regimes, the increased
energy requirements due to the presence of QoS constraints are quantified.
Comparisons with variable-rate/fixed-power and variable-rate/variable-power
cases are given. Energy efficiency is further analyzed in the presence of
channel uncertainties. The optimal fraction of power allocated to training is
identified under QoS constraints. It is proven that the minimum bit energy in
the low-power regime is attained at a certain nonzero power level below which
bit energy increases without bound with vanishing power
Multihop Diversity in Wideband OFDM Systems: The Impact of Spatial Reuse and Frequency Selectivity
The goal of this paper is to establish which practical routing schemes for
wireless networks are most suitable for wideband systems in the power-limited
regime, which is, for example, a practically relevant mode of operation for the
analysis of ultrawideband (UWB) mesh networks. For this purpose, we study the
tradeoff between energy efficiency and spectral efficiency (known as the
power-bandwidth tradeoff) in a wideband linear multihop network in which
transmissions employ orthogonal frequency-division multiplexing (OFDM)
modulation and are affected by quasi-static, frequency-selective fading.
Considering open-loop (fixed-rate) and closed-loop (rate-adaptive) multihop
relaying techniques, we characterize the impact of routing with spatial reuse
on the statistical properties of the end-to-end conditional mutual information
(conditioned on the specific values of the channel fading parameters and
therefore treated as a random variable) and on the energy and spectral
efficiency measures of the wideband regime. Our analysis particularly deals
with the convergence of these end-to-end performance measures in the case of
large number of hops, i.e., the phenomenon first observed in \cite{Oyman06b}
and named as ``multihop diversity''. Our results demonstrate the realizability
of the multihop diversity advantages in the case of routing with spatial reuse
for wideband OFDM systems under wireless channel effects such as path-loss and
quasi-static frequency-selective multipath fading.Comment: 6 pages, to be published in Proc. 2008 IEEE International Symposium
on Spread Spectrum Techniques and Applications (IEEE ISSSTA'08), Bologna,
Ital
Throughput Analysis of Buffer-Constrained Wireless Systems in the Finite Blocklength Regime
In this paper, wireless systems operating under queueing constraints in the
form of limitations on the buffer violation probabilities are considered. The
throughput under such constraints is captured by the effective capacity
formulation. It is assumed that finite blocklength codes are employed for
transmission. Under this assumption, a recent result on the channel coding rate
in the finite blocklength regime is incorporated into the analysis and the
throughput achieved with such codes in the presence of queueing constraints and
decoding errors is identified. Performance of different transmission strategies
(e.g., variable-rate, variable-power, and fixed-rate transmissions) is studied.
Interactions between the throughput, queueing constraints, coding blocklength,
decoding error probabilities, and signal-to-noise ratio are investigated and
several conclusions with important practical implications are drawn
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