115 research outputs found
On the Sum of Order Statistics and Applications to Wireless Communication Systems Performances
We consider the problem of evaluating the cumulative distribution function
(CDF) of the sum of order statistics, which serves to compute outage
probability (OP) values at the output of generalized selection combining
receivers. Generally, closed-form expressions of the CDF of the sum of order
statistics are unavailable for many practical distributions. Moreover, the
naive Monte Carlo (MC) method requires a substantial computational effort when
the probability of interest is sufficiently small. In the region of small OP
values, we propose instead two effective variance reduction techniques that
yield a reliable estimate of the CDF with small computing cost. The first
estimator, which can be viewed as an importance sampling estimator, has bounded
relative error under a certain assumption that is shown to hold for most of the
challenging distributions. An improvement of this estimator is then proposed
for the Pareto and the Weibull cases. The second is a conditional MC estimator
that achieves the bounded relative error property for the Generalized Gamma
case and the logarithmic efficiency in the Log-normal case. Finally, the
efficiency of these estimators is compared via various numerical experiments
Adaptive Power Control for Single and Multiuser Opportunistic Systems
In this dissertation, adaptive power control for single and multiuser opportunistic
systems is investigated. First, a new adaptive power-controlled diversity combining
scheme for single user systems is proposed, upon which is extended to the multiusers
case. In the multiuser case, we first propose two new threshold based parallel multiuser
scheduling schemes without power control. The first scheme is named on-off
based scheduling (OOBS) scheme and the second scheme is named switched based
scheduling (SBS) scheme. We then propose and study the performance of thresholdbased
power allocation algorithms for the SBS scheme. Finally, we introduce a unified
analytical framework to determine the joint statistics of partial sums of ordered RVs
with i.i.d. and then the impact of interference on the performance of parallel multiuser
scheduling is investigated based on our unified analytical framework
An Accurate Sample Rejection Estimator of the Outage Probability With Equal Gain Combining
We evaluate the outage probability (OP) for L-branch equal gain combining (EGC) receivers operating over fading channels, i.e., equivalently the cumulative distribution function (CDF) of the sum of the L channel envelopes. In general, closed form expressions of OP values are out of reach. The use of Monte Carlo (MC) simulations is not a good alternative as it requires a large number of samples for small values of OP. In this paper, we use the concept of importance sampling (IS), being known to yield accurate estimates using fewer simulation runs. Our proposed IS scheme is based on sample rejection where the IS density is the truncation of the underlying density over the L dimensional sphere. It assumes the knowledge of the CDF of the sum of the L channel gains in closed-form. Such an assumption is not restrictive since it holds for various challenging fading models. We apply our approach to the case of independent Rayleigh, correlated Rayleigh, and independent and identically distributed Rice fading models. Next, we extend our approach to the interesting scenario of generalised selection combining receivers combined with EGC under the independent Rayleigh environment. For each case, we prove the desired bounded relative error property. Finally, we validate these theoretical results through some selected experiments
Design and performance evaluation of RAKE finger management schemes in the soft handover region
We propose and analyze new finger assignment/management techniques that
are applicable for RAKE receivers when they operate in the soft handover region.
Two main criteria are considered: minimum use of additional network resources and
minimum call drops. For the schemes minimizing the use of network resources, basic
principles are to use the network resources only if necessary while minimum call drop
schemes rely on balancing or distributing the signal strength/paths among as many
base stations as possible. The analyses of these schemes require us to consider joint
microscopic/macroscopic diversity techniques which have seldom been considered before
and as such, we tackle the statistics of several correlated generalized selection
combining output signal-to-noise ratios in order to obtain closed-form expressions for
the statistics of interest. To provide a general comprehensive framework for the assessment
of the proposed schemes, we investigate not only the complexity in terms of
the average number of required path estimations/comparisons, the average number
of combined paths, and the soft handover overhead but also the error performance of
the proposed schemes over independent and identically distributed fading channels.
We also examine via computer simulations the effect of path unbalance/correlation as
well as outdated/imperfect channel estimations. We show through numerical exam ples that the proposed schemes which are designed for the minimum use of network
resources can save a certain amount of complexity load and soft handover overhead
with a very slight performance loss compared to the conventional generalized selection
combining-based diversity systems. For the minimum call drop schemes, by
accurately quantifying the average error rate, we show that in comparison to the
conventional schemes, the proposed distributed schemes offer the better error performance
when there is a considerable chance of loosing the signals from one of the
active base stations
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