Venkatesh T.G.

This paper presents an method of simulating the Single Machine Connected to Infinite Bus (SMIB) power system with load modeled as the random quantity. This paper investigates the effect of random load variation in the power system stability. The stability margin index being studied is theMean First Passage Time (MFPT) of the power system from the stable operating point to the closest unstable equilibrium point for small magnitude random load perturbations. The variation of MFPT for change in the loading and damping of the system, Intensity and correlation time of the stochastic load are studied. The theoritical expression used for calculation for calculation of the MFPT in [1] is shown to have limitations for calculations of MFPT for systems with less energy barrier and wide ranges of damping. ©2010 IEEE.

Cognitive Radio is a promising technology for solving spectrum scarcity problem. Design of medium access control (MAC) protocols for Cognitive Radio Networks (CRN) is challenging due to the complexity involved in sensing and accessing the channel. Assuming Primary Users (PUs) to be passive, protocols based on 802.11 MAC has been proposed in the literature. In this paper, we propose a Cognitive Radio MAC protocol for Active primary users (CRMA) and compare it with the existing Cognitive Radio MAC protocol for Passive primary users (CRMP). Unlike CRMP, in the CRMA protocol PUs are assumed to be active, in the sense that PUs are aware of the SUs. PUs signal their intention to transmit over the channel through beacons before transmitting. This minimal change in the functionality of the PUs not only improves the spectrum utilization of the SUs but also decreases the interference to PUs. The performance of both the protocols are compared in terms of throughput, delay, interference to PUs, number of RTS collisions and number of dropped packets of SUs through extensive simulation. We also investigate the effect of packet length, transmission opportunity (TXOP) duration and block acknowledgement. Simulation results show that our proposed CRMA protocol gives better performance as compared to CRMP under certain conditions. © 2013 IEEE.

We consider a multichannel Cognitive Radio Network (CRN), where secondary users sequentially sense channels for opportunistic spectrum access. In this scenario, the Channel Selection Algorithm (CSA) allows secondary users to find a vacant channel with the minimal number of channel switches. Most of the existing CSA literature assumes exponential ON-OFF time distribution for primary user's (PU) channel occupancy pattern. This exponential assumption might be helpful to get performance bounds; but not useful to evaluate the performance of CSA under realistic conditions. An in-depth analysis of independent spectrum measurement traces reveals that wireless channels have typically heavy-tailed PU OFF times. In this paper, we propose an extension to the Predictive CSA framework and its generalization for heavy tailed PU OFF time distribution, which represents realistic scenarios. In particular, we calculate the probability of channel being idle for hyper-exponential OFF times to use in CSA. We implement our proposed CSA framework in a wireless test-bed and comprehensively evaluate its performance by recreating the realistic PU channel occupancy patterns. The proposed CSA shows significant reduction in channel switches and energy consumption as compared to Predictive CSA which always assumes exponential PU ON-OFF times. Through our work, we show the impact of the PU channel occupancy pattern on the performance of CSA in multichannel CRN.

In Cognitive Radio Network (CRN) secondary users (SUs) opportunistically access the channel whenever it is not being used by primary users (PUs). The CRN Medium Access Control ( MAC) protocol needs to sense the channel to find out the availability of spectrum. In this paper we propose a hybrid sensing mechanism for SUs to access the channel in CRN environment. In the proposed hybrid sensing mechanism the SU sense the channel periodically when it is used by PU while the SU sense the channel continuously whenever the channel is used by the SU. We investigate the role of this hybrid sensing mechanism wherein PUs have a preemptive priority over SUs. Later we study the performance of our MAC protocol with respect to the following metrics: MeanWaiting Time (MWT) and Queue Length Distribution (QLD). We study the variation of these metrics as a function of sensing period (τ ), SU arrival rate (λs), SU service rate (μs), and PU traffic intensity rate (ρPU). From simulation results, we observe that there is a trade-off between MWT and sensing energy while considering periodic sensing. We finally suggest the type of sensing mechanism to be used for the channel access, based on the traffic intensity of PU.

Massive machine type communication (mMTC) is a key use case that is expected to be supported by 5G NR. One of the key challenges in 5G NR communications for mMTC use case is to enhance the initial access procedure to overcome the problem of congestion and overloading from mMTC devices. The intermittent blockages in the mmWave frequency range of 5G further degrade the random access performance significantly. 5G NR introduces the concept of numerology where multiple types of sub-carrier spacing is allowed. To circumvent the problem of random access performance enhancement, we propose a blockage aware adaptive numerology to maximize the number of devices that can be connected to the cellular network. In this paper, the problem of selecting optimal numerology for random access is modelled as a contextual multi-arm bandit framework while taking the blockages and their mobility pattern in the cell at a given time into account. We consider the upper confidence bound (UCB) action selection method to take the uncertainty of rewards of numerologies at a given instance into account. The proposed adaptive numerology solution maximizes the number of devices joining the network per RACH occasion and also minimizes the average joining time of a device when compared to fixed numerology schemes.

This paper presents an approach of analyzing power system voltage stability based on system potential. This framework provides a simple and better way of understanding voltage stability by visualizing the changes in topological structure of the system potential with respect to the change in power system loading. This paper also investigates the effect of random load variation in the power system stability. The stability margin index being studied is the Mean First Passage Time of the power system from the stable operating point to the closest unstable equilibrium point for small magnitude random load perturbations. The variation of MFPT for change in the loading of the system, Intensity and correlation time of the stochastic load are studied initially for an SMIB system. © 2011 Elsevier Ltd. All rights reserved.

We consider the nonlinear non-Markovian stochastic process associated with the damped nonlinear dynamical system driven by Ornstein-Uhlenbeck noise. An approximate Fokker-Planck-type equation governing the above stochastic process is derived using the path-integral approach. The stationary probability density function (SPDF) of the above process is then computed using the matrix continued fraction method. The SPDF compares favorably with the corresponding digital simulation results obtained by us. © 2000 The American Physical Society.

Reuse distance is an important metric for analytical estimation of cache miss rate. To find the miss rate of a particular cache, the reuse distance profile has to be measured for that particular level and configuration of the cache. Significant amount of simulation time and overhead can be reduced if we can find the miss rate of higher level cache like L2 cache from the RD profile with respect to a lower level cache (i.e., cache that is closer to the processor) such as L1. The objective of this paper is to give an analytical method to find the miss rate of L2 cache for various configurations from the RD profile with respect to L1 cache. We consider all three types of cache inclusion policies namely (i) Strictly Inclusive, (ii) Mutually Exclusive and (iii) Non-Inclusive Non-Exclusive policy. We first prove some general results relating the RD profile of L1 cache to that of L2 cache. We use probabilistic analysis for our derivations. We validate our model against simulations, using the multi-core simulator Sniper with the PARSEC and the SPLASH benchmark suites.

Cognitive radio networks (CRN) rely upon dynamic spectrum access that allows spectrum to be used efficiently. The role of the medium access control (MAC) protocol in CRN is to distribute efficiently the spectrum among the users. We propose a MAC protocol for cognitive wireless local area network (WLAN) that allows an unlicensed secondary user (SU) to opportunistically access and transmit data on the licensed channel owned by the primary user (PU). The proposed MAC protocol is an adaptation of IEEE 802.11 distributed coordination function. Extensive performance evaluation of the cognitive WLAN MAC protocol has been carried out as follows. First, a Markov chain model of the cognitive WLAN MAC protocol is developed. By solving our model we derive saturated throughput, packet delay, channel busy probability, conditional collision probability of SU, and interference to PU. Extension of our model for the case of multiple PUs under lightly loaded condition is also carried out. Further, we analyze the energy efficiency of the cognitive WLAN MAC protocol. We calculate the optimal sensing time interval for the SUs and the corresponding sensing energy incurred by the SUs. Effect of imperfect channel sensing on the performance of the proposed protocol is investigated. We validate our model by comparing the analytical results with simulation.

In this paper we study the role of control/status channel in MAC protocol of cognitive radio networks. To this end we consider three channel access schemes of IEEE 802.11 DCF namely (i) the Basic Access mechanism that uses no control channel and (ii) RTS-CTS mechanism, that uses a control channel for contention and (iii) cooperative Basic Access scheme that uses a dedicated channel for status update. We calculate and compare different metrics like throughput, mean delay, dropped packets etc of secondary users through extensive simulation.