104 research outputs found
A Scalable Hybrid MAC Protocol for Massive M2M Networks
In Machine to Machine (M2M) networks, a robust Medium Access Control (MAC)
protocol is crucial to enable numerous machine-type devices to concurrently
access the channel. Most literatures focus on developing simplex (reservation
or contention based)MAC protocols which cannot provide a scalable solution for
M2M networks with large number of devices. In this paper, a frame-based Hybrid
MAC scheme, which consists of a contention period and a transmission period, is
proposed for M2M networks. In the proposed scheme, the devices firstly contend
the transmission opportunities during the contention period, only the
successful devices will be assigned a time slot for transmission during the
transmission period. To balance the tradeoff between the contention and
transmission period in each frame, an optimization problem is formulated to
maximize the system throughput by finding the optimal contending probability
during contention period and optimal number of devices that can transmit during
transmission period. A practical hybrid MAC protocol is designed to implement
the proposed scheme. The analytical and simulation results demonstrate the
effectiveness of the proposed Hybrid MAC protocol
Game among Interdependent Networks: The Impact of Rationality on System Robustness
Many real-world systems are composed of interdependent networks that rely on
one another. Such networks are typically designed and operated by different
entities, who aim at maximizing their own payoffs. There exists a game among
these entities when designing their own networks. In this paper, we study the
game investigating how the rational behaviors of entities impact the system
robustness. We first introduce a mathematical model to quantify the interacting
payoffs among varying entities. Then we study the Nash equilibrium of the game
and compare it with the optimal social welfare. We reveal that the cooperation
among different entities can be reached to maximize the social welfare in
continuous game only when the average degree of each network is constant.
Therefore, the huge gap between Nash equilibrium and optimal social welfare
generally exists. The rationality of entities makes the system inherently
deficient and even renders it extremely vulnerable in some cases. We analyze
our model for two concrete systems with continuous strategy space and discrete
strategy space, respectively. Furthermore, we uncover some factors (such as
weakening coupled strength of interdependent networks, designing suitable
topology dependency of the system) that help reduce the gap and the system
vulnerability
Fast Functionalization with High Performance in the Autonomous Information Engine
Mandal and Jarzynski have proposed a fully autonomous information heat
engine, consisting of a demon, a mass and a memory register interacting with a
thermal reservoir. This device converts thermal energy into mechanical work by
writing information to a memory register, or conversely, erasing information by
consuming mechanical work. Here, we derive a speed limit inequality between the
relaxation time of state transformation and the distance between the initial
and final distributions, where the combination of the dynamical activity and
entropy production plays an important role. Such inequality provides a hint
that a speed-performance trade-off relation exists between the relaxation time
to functional state and the average production. To obtain fast
functionalization while maintaining the performance, we show that the
relaxation dynamics of information heat engine can be accelerated significantly
by devising an optimal initial state of the demon. Our design principle is
inspired by the so-called Mpemba effect, where water freezes faster when
initially heated.Comment: 14 pages, 3 figures; all comments are welcom
Experimental Testing and modeling of a Dual-Fired LiBr-H2O Absorption Chiller
An LiBr-H2O chiller was modified to utilize heat sources from natural gas combustion and/or from hot fluid. This was achieved by replacing the original gas-fired generator with a dual-fired generator. Steam was used as the hot fluid. The generator of the chiller can be powered by each source separately or both sources simultaneously. Experimental investigation was performed to obtain capacity and coefficient of performance (COP) of the original chiller and the modified chiller. During the experiments, the modified chiller was powered solely by steam, natural gas, or both. There was a significant increase in COP and capacity when steam was used as a heat source. The tests using natural gas resulted in performance similar to the original chiller. The experimental conditions were closely modeled by a numerical program
Decentralized Multi-Charger Coordination for Wireless Rechargeable Sensor Networks
International audienceWireless charging is a promising technology for provisioning dynamic power supply in wireless rechargeable sensor networks (WRSNs). The charging equipment can be carried by some mobile nodes to enhance the charging flexibility. With such mobile chargers (MCs), the charging process should simultaneously address the MC scheduling, the moving and charging time allocation, while saving the total energy consumption of MCs. However, the efficient solutions that jointly solve those challenges are generally lacking in the literature. First, we investigate the multi-MC coordination problem that minimizing the energy expenditure of MCs while guaranteeing the perpetual operation of WRSNs, and formulate this problem as a mixed-integer linear program (MILP). Second, to solve this problem efficiently, we propose a novel decentralized method which is based on Benders decomposition. The multi-MC coordination problem is then decomposed into a master problem (MP) and a slave problem (SP), with the MP for MC scheduling and the SP for MC moving and charging time allocation. The MP is being solved by the base station (BS), while the SP is further decomposed into several sub-SPs and being solved by the MCs in parallel. The BS and MCs coordinate themselves to decide an optimal charging strategy. The convergence of proposed method is analyzed theoretically. Simulation results demonstrate the effectiveness and scalability of the proposed method
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