Nidhi Hegde

Capacity Gains of Some Frequency Reuse Schemes in OFDMA Networks

The downlink capacity of cellular networks is known to be strongly limited by inter-cell interference. In order to mitigate this interference, a number of frequency reuse schemes have recently been proposed. In this paper, we compare the potential capacity gains of these schemes accounting for the random nature of traffic.

Sojourn times in finite-capacity processor-sharing queues

Motivated by the need to develop simple parsimonious models for evaluating the performance of wireless data systems, we consider finite-capacity processor-sharing systems. For such systems, we analyze the sojourn time distribution, which presents a useful measure for the transfer delay of documents such as Web pages. The service rates are allowed to be state-dependent, to capture the fact that the throughput in wireless data systems may vary with the number of active users due to scheduling gains or channel collisions. We derive a set of linear equations for the Laplace-Stieltjes transforms of the sojourn time distributions, conditioned on the number of users upon arrival. This set of equations is solved, and the resulting LSTs are inverted, resulting in a phase-type distribution for the unconditional sojourn time. Numerical results are provided, and two types of approximations are proposed that substantially reduce the computational effort.

Mobility-Driven Scheduling in Wireless Networks

The design of scheduling policies for wireless data systems has been driven by a compromise between the objectives of high overall system throughput and the degree of fairness among users, while exploiting multi-user diversity, i.e., fast-fading variations. These policies have been thoroughly investigated in the absence of user mobility, i.e., without slow fading variations. In the present paper, we examine the impact of intra- and inter-cell user mobility on the trade-off between throughput and fairness, and on the suitable choice of alpha-fair scheduling policies. We consider a dynamic setting where users come and go over time as governed by random finite-size data transfers, and explicitly allow for users to roam around. It is demonstrated that the overall performance improves as the fairness parameter alpha is reduced, and in particular, that proportional fair scheduling may yield relatively poor performance, in sharp contrast to the standard scenario with only fast fading. Since a lower alpha tends to affect short-term fairness, we explore how to set the fairness parameter so as to strike the right balance between overall performance and short-term fairness. It is further established that mobility tends to improve the performance, even when the network operates under a local fair scheduling policy as opposed to a globally optimal strategy. We present extensive simulation results to confirm and illustrate the analytical findings.

Blocking in large mobile cellular networks with bursty traffic

We consider large cellular networks. The traffic entering the network is assumed to be correlated in both space and time. The space dependency captures the possible correlation between the arrivals to different nodes in the network, while the time dependency captures the time correlation between arrivals to each node. We model such traffic with a Markov-Modulated Poisson Process(MMPP).It is shown that even in the single node environment, the problem is not mathematically tractable. A model with an infinite number of circuits is used to approximate the finite model. A novel recursive methodology is introduced in finding the joint moments of the number of busy circuits in different cells in the network leading to accurate determination of blocking probability. A simple mixed-Poisson distribution is introduced as an accurate approximation of the distribution of the number ofbusy circuits.We show that for certain cases, in the system with an infinite number of circuits in each cell, there is no effect of mobility on the performance of the system. Our numerical results indicate that the traffic burstiness has a major impact on the system performance. The mixed-Poisson approximation is found to be a very good fit to the exact finite model. The performance of this approximation using few moments is affected by traffic burstiness and average load. We find that in a reasonable range of traffic burstiness, the mixed-Poisson distribution provides a close approximation.

On the impact of soft hand-off in cellular systems

We present a model for soft, hand-off in wireless cellular networks. In such networks, due to overlapping cells, hand-offs are not instantaneous and multiple channels may be occupied by a single mobile for a non-zero freeze time period. We provide a mathematical model of wireless cellular networks with soft hand-offs. We examine different performance measures and show that freeze time may have a major impact on the system performance if the mobility rate is not negligible. Both exact and approximate formulations are given. Different fixed-point approximation methods are used to reduce the complexity of the exact solution. Various performance measures such as new and hand-off blocking and probability of a call dropout are carefully examined.

ACM Sigmetrics Performance Evaluation Review: A New Series on Diversity

This editorial announces a new series on diversity in the ACM Sigmetrics Performance Evaluation Review (PER). In several upcoming and future issues we will feature invited articles on diversity from authors in the performance evaluation community, but also from the larger Computing Science (CS) community. The articles will touch various aspects in CS including K-12 and post-secondary education, graduate studies, academic recruitment, industry perspectives, harassment issues, and gender, ethnicity, and racial bias.

Rate region of multi-channel mesh data networks with rate control

Mesh networking has recently been advocated as an efficient and low-cost approach for providing high speed access to the Internet. Before such networks are able to support high-bandwidth data transfers in reality, a number of challenges must be addressed including routing, channel and radio interface assignments, radio resource management, and MAC scheduling. Our focus in this paper is to provide guidelines on the design of optimal radio resource management and distributed MAC scheduling algorithms. To this aim, we develop a general analytical model to characterize the performance of such networks. This task comprises many major difficulties: analyzing the behavior of distributed MAC schedulers in multi-hop networks, accounting for the interaction of links through interference and quantifying the impact of underlying congestion control algorithms. We present a unified approach partially based on decoupling arguments to address these challenges. Specifically, we provide approximations of the rate region of the network. Based on the derived results, we can propose a set of design rules for mesh multi-channel networks. For example, we investigate how to parametrize the coverage of the RTS/CTS signalling messages in IEEE 802.11-based networks.