Venkatesh T.G.

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.

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.

We present an analytical model for the access during the Service Periods (SP) of the IEEE 802.11ad Hybrid Medium Access Control protocol. As a performance measure of this protocol, we derive the worst case and average delay faced by the SP packets. We show that as the arrival rate of the SP packets increases, the delay increases linearly till a point, beyond which it grows exponentially. Further, we extend the model to variable length of beacon interval, and random allocation of SPs to the nodes. We show how a network designer can do optimal allocation of the SP and the CBAP duration to achieve a tradeoff between SP delay and CBAP throughput. We further extend our analysis for the case of heterogeneous system. Our analytical results are compared with simulation and the results show a good match.

In cognitive radio network (CRN), the secondary user (SU) opportunistically access the wireless channels whenever they are free from the licensed/primary user (PU). Even after occupying the channel, the SU has to sense the channel intermittently to detect reappearance of PU, so that it can stop its transmission and avoid interference to PU. Frequent channel sensing results in the degradation of SU's throughput whereas sparse sensing increases the interference experienced by the PU. Thus, optimal sensing interval policy plays a vital role in CRN. In the literature, optimal channel sensing strategy has been analyzed for the case when the ON-OFF time distributions of PU are exponential. However, the analysis of recent spectrum measurement traces reveals that PU exhibits heavy-tailed idle times which can be approximated well with hyper-exponential distribution (HED). In this paper, we deduce the structure of optimal sensing interval policy for channels with HED OFF times through Markov decision process. We then use dynamic programming framework to derive suboptimal sensing interval policies. A new multishot sensing interval policy is proposed and it is compared with existing policies for its performance in terms of number of channel sensing and interference to PU.

Stochastic delay differential equations play an important role in modelling scientific and engineering systems. In this paper the partial differential equation satisfying the time evolution of the probability density function of the state variable governed by the stochastic delay differential equation (SDDE) with additive white noise and coloured noise perturbation is obtained under small time delay and small correlation time approximation using path integral formalism. This partial differential equation reduces to a Fokker–Planck equation for particular linear and non-linear SDDE. Fokker–Planck equation is then solved with reflecting boundary conditions to get the analytical solutions for Stationary Probability Density Function (SPDF). The analytical solutions for SPDF is compared with the SPDF obtained using simulation. Further the Mean First Passage Time (MFPT) of the bistable system is calculated for various values of time delay and noise strength and compared with that of the simulation. The MFPT for the time delayed system for the case of cubic potential driven by Gaussian and Levy noise as well as asymmetric bistable potential driven by a noise and periodic driving force has been investigated.

In this letter, we analyze the delayed mobile data offloading scheme by modeling it as an alternating regenerative process. We use a novel way for deriving transmission delay and offloading efficiency, by analyzing the stochastic process characterizing the virtual waiting time. Furthermore, we propose a delayed offloading scheme with balking to enhance quality of experience. The balking scheme is characterized based on 1) current WLAN status; 2) number of packets queued for transmission; and 3) packet deadline. The proposed scheme achieves reduction in mean transmission delay without sacrificing much of the offloading efficiency. The performance of the scheme is mathematically analyzed using matrix-geometric techniques and validated through simulation.