17,500 research outputs found
Energy-aware cooperative wireless networks with multiple cognitive users
In this paper, we study and analyze cooperative cognitive radio networks with arbitrary number of secondary users (SUs). Each SU is considered a prospective relay for the primary user (PU) besides having its own data transmission demand. We consider a multi-packet transmission framework that allows multiple SUs to transmit simultaneously because of dirty-paper coding. We propose power allocation and scheduling policies that optimize the throughput for both PU and SU with minimum energy expenditure. The performance of the system is evaluated in terms of throughput and delay under different opportunistic relay selection policies. Toward this objective, we present a mathematical framework for deriving stability conditions for all queues in the system. Consequently, the throughput of both primary and secondary links is quantified. Furthermore, a moment generating function approach is employed to derive a closed-form expression for the average delay encountered by the PU packets. Results reveal that we achieve better performance in terms of throughput and delay at lower energy cost as compared with equal power allocation schemes proposed earlier in the literature. Extensive simulations are conducted to validate our theoretical findings
The origin of bursts and heavy tails in human dynamics
The dynamics of many social, technological and economic phenomena are driven
by individual human actions, turning the quantitative understanding of human
behavior into a central question of modern science. Current models of human
dynamics, used from risk assessment to communications, assume that human
actions are randomly distributed in time and thus well approximated by Poisson
processes. In contrast, there is increasing evidence that the timing of many
human activities, ranging from communication to entertainment and work
patterns, follow non-Poisson statistics, characterized by bursts of rapidly
occurring events separated by long periods of inactivity. Here we show that the
bursty nature of human behavior is a consequence of a decision based queuing
process: when individuals execute tasks based on some perceived priority, the
timing of the tasks will be heavy tailed, most tasks being rapidly executed,
while a few experience very long waiting times. In contrast, priority blind
execution is well approximated by uniform interevent statistics. These findings
have important implications from resource management to service allocation in
both communications and retail.Comment: Supplementary Material available at http://www.nd.edu/~network
Modeling bursts and heavy tails in human dynamics
Current models of human dynamics, used from risk assessment to
communications, assume that human actions are randomly distributed in time and
thus well approximated by Poisson processes. We provide direct evidence that
for five human activity patterns the timing of individual human actions follow
non-Poisson statistics, characterized by bursts of rapidly occurring events
separated by long periods of inactivity. We show that the bursty nature of
human behavior is a consequence of a decision based queuing process: when
individuals execute tasks based on some perceived priority, the timing of the
tasks will be heavy tailed, most tasks being rapidly executed, while a few
experiencing very long waiting times. We discuss two queueing models that
capture human activity. The first model assumes that there are no limitations
on the number of tasks an individual can hadle at any time, predicting that the
waiting time of the individual tasks follow a heavy tailed distribution with
exponent alpha=3/2. The second model imposes limitations on the queue length,
resulting in alpha=1. We provide empirical evidence supporting the relevance of
these two models to human activity patterns. Finally, we discuss possible
extension of the proposed queueing models and outline some future challenges in
exploring the statistical mechanisms of human dynamics.Comment: RevTex, 19 pages, 8 figure
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