Browsing by Subject "Asymptotic analysis"

Now showing 1 - 8 of 8

Open Access
Advanced asynchronous random access protocols
(2020-08) Akyıldız, Talha
Fifth generation wireless systems and beyond require linking an enormous number of simple machine type devices leading to a new wave of interest in massive machine type communications (mMTC). Diﬀerent from the human-centric communication systems, mMTCs are composed of a large number of devices where each user node generates small data blocks sporadically in an unpredictable manner. In such scenarios, traditional multiple access schemes, e.g., time division multiple access or frequency division multiple access, are not suitable because resource allocation and scheduling based approaches cannot be conveniently adopted due to the required complexity and latency, motivating the use of uncoordinated random access (RA) protocols and making asynchronous ALOHA-like solutions ideal candidates for such applications. In this thesis, we consider the design and analysis of advanced asynchronous RA protocols for diﬀerent settings. We ﬁrst study contention resolution ALOHA (CRA) and irregular repetition ALOHA (IRA) protocols with regular and irregular repetition rates on the collision channel where collisions are resolved through successive interference cancellation. We also propose concatenation of packet replicas with some clean parts with IRA, named irregular repetition ALOHA with replica concatenation (IRARC). Secondly, we introduce energy harvesting (EH) into the framework with the motivation of self-sustainability, and study RA protocols with EH nodes. Finally, we propose a generalization of IRA with packet length diversity to improve the system performance further. We present asymptotic analyses of all the proposed RA protocols, and determine the optimal repetition distributions to maximize the system throughput. We also provide a comprehensive set of numerical results for both asymptotic and practical scenarios to further demonstrate the eﬀectiveness of the proposed approaches.
Open Access
Asymptotic analysis of contention resolution ALOHA with replica concatenation
(Institute of Electrical and Electronics Engineers Inc., 2019) Akyıldız, Talha; Demirhan, U.; Duman, Tolga M.
In this paper, we present an asymptotic performance analysis of contention resolution ALOHA (CRA) on the collision channel for both regular and irregular repetition rates. In addition, we consider an improvement to CRA by merging the clean parts of replicas in partial collisions and extend our analysis to this scenario. Specific designs of repetition distributions based on the new analysis show that the optimized solutions for irregular repetition slotted ALOHA (IRSA) perform well in both CRA and the enhanced scheme.
Open Access
Asymptotic analysis of reliability for switching systems in light and heavy traffic conditions
(Birkhäuser, Boston, 2000) Anisimov, Vladimir V.; Limnios, N.; Nikulin, M.
An asymptotic analysis of flows of rare events switched by some random environment is provided. An approximation by nonhomogeneous Poisson flows in case of mixing environment is studied. Special notions of S-set and “monotone” structure for finite Markov environment are introduced. An approximation by Poisson flows with Markov switches in case of asymptotically consolidated environment is proved. An analysis of the 1st exit time from a subset is also given. In heavy traffic conditions an averaging principle for trajectories with Poisson approximation for flows of rare events in systems with fast switches is proved. The method of proof is based on limit theorems for processes with semi-Markov switches. Applications to the reliability analysis of state-dependent Markov and semi-Markov queueing systems in light and heavy traffic conditions are considered
Open Access
Energy harvesting irregular repetition ALOHA with replica concatenation
(IEEE, 2021) Akyıldız, Talha; Demirhan, U.; Duman, Tolga M.
In this paper, we consider an asynchronous random access scheme called irregular repetition ALOHA (IRA) as a generalization of contention resolution ALOHA (CRA) with varying repetitions. We present an asymptotic performance analysis of CRA and IRA on the collision channel for regular and irregular repetition rates. We also propose an improvement by merging the clean parts of packet replicas in partial collisions, and extend our analysis to this scenario as well. Specific designs of repetition distributions based on the new analysis show that the optimized solutions of irregular repetition slotted ALOHA (IRSA) perform well in both IRA and the enhanced scheme, and they considerably outperform the regular repetition distributions. We also introduce energy harvesting (EH) to both schemes as a practical and sustainable adaptation, where users are able to harvest energy and store it in their finite-capacity batteries. We model the battery state by a discrete-time Markov chain and derive an optimal transmission policy to maximize the asymptotic performance of the system. We provide comprehensive numerical results for both practical and asymptotic scenarios to verify the validity of the proposed analyses, and illustrate the benefits of the proposed systems.
Open Access
A high-frequency based asymptotic solution for surface fields on a source-excited sphere with an impedance boundary condition
(Wiley-Blackwell Publishing, 2010-10-05) Alisan, B.; Ertrk V. B.
A high-frequency asymptotic solution based on the Uniform Geometrical Theory of Diffraction (UTD) is proposed for the surface fields excited by a magnetic source located on the surface of a sphere with an impedance boundary condition. The assumed large parameters, compared to the wavelength, are the radius of the sphere and the distance between the source and observation points along the geodesic path, when both these points are located on the surface of the sphere. Different from the UTD-based solution for a perfect electrically conducting sphere, some higher-order terms and derivatives of Fock type integrals are included as they may become important for certain surface impedance values as well as for certain separations between the source and observation points. This work is especially useful in the analysis of mutual coupling between conformal slot/aperture antennas on a thin material coated or partially coated sphere.
Open Access
Irregular repetition slotted ALOHA with energy harvesting nodes
(IEEE, 2019-09) Demirhan, Umut; Duman, Tolga M.
We propose an irregular repetition slotted ALOHA (IRSA) based uncoordinated random access scheme for energy harvesting (EH) nodes. Specifically, we consider the case in which each user has a battery that is recharged with harvested energy from the environment in a probabilistic manner. We analyze this scheme starting with a unit-sized battery at the nodes and extend the analysis to the case of a finite-sized battery. For both scenarios, we derive the asymptotic throughput expressions and obtain the optimized probability distributions for the number of packet replicas of the users. We demonstrate that for the case of IRSA with EH nodes, these optimized distributions perform considerably better than the alternatives, including slotted ALOHA (SA), contention resolution diversity slotted ALOHA (CRDSA), and IRSA, which do not take into account the EH process for both asymptotic and finite frame length scenarios.
Open Access
Last PhD supervised by Professor Kouyoumjian: Extended UTD by Dr. Buyukdura
(IEEE, 2011) Altıntaş, Ayhan
While he is a Professor Emeritus at Ohio State, Professor Kouyoumjian supervised a thesis work by Merih Buyukdura. They first derived a dyadic Green's function for a PEC wedge using spherical wave functions and employed asymptotic approximation. They also derived the extended UTD in which higher order terms in the diffraction matrix are predicted. The thesis was defended in 1984. In this presentation, a brief discussion of edge waves as derived from the asymptotic expansion of dyadic Green's function in terms of spherical functions will be made and afterwards the derivation of extended UTD diffraction coefficients will be given. © 2011 IEEE.
Open Access
Stability analysis of switched time-delay systems
(IEEE, 2008-12) Yan, P.; Özbay, Hitay
This paper addresses the asymptotic stability of switched time delay systems with heterogenous time invariant time delays. Piecewise Lyapunov-Razumikhin functions are introduced for the switching candidate systems to investigate the stability in the presence of infinite number of switchings. We provide sufficient conditions in terms of the minimum dwell time to guarantee asymptotic stability under the assumptions that each switching candidate is delay-independently or delaydependently stable. Conservatism analysis is also provided by comparing with the dwell time conditions for switched delay free systems. © 2008 IEEE.