## Sarkar, Saswati

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Publication Arbitrary Throughput Versus Complexity Tradeoffs in Wireless Networks using Graph Partitioning(2006-11-10) Ray, Saikat; Sarkar, SaswatiSeveral policies have recently been proposed for attaining the maximum throughput region, or a guaranteed fraction thereof, through dynamic link scheduling. Among these policies, the ones that attain the maximum throughput region require a computation time which is linear in the network size, and the ones that require constant or logarithmic computation time attain only certain fractions of the maximum throughput region. In contrast, in this paper we propose policies that can attain any desirable fraction of the maximum throughput region and require a computation time that is independent of the network size. First, using a combination of graph partitioning techniques and lyapunov arguments, we propose a simple policy for tree topologies under the primary interference model that requires each link to exchange only 1 bit information with its adjacent links and approximates the maximum throughput region using a computation time that depends only on the maximum degree of nodes and the approximation factor. We subsequently develop a framework for attaining arbitrary close approximations for the maximum throughput region in arbitrary networks and interference models and use this framework to obtain any desired tradeoff between throughput guarantees and computation times for a large class of networks and interference models. Specifically, given any ε > 0, the maximum throughput region can be approximated in these networks using a computation time that depends only on the maximum node degree and ε.Publication A Framework for Optimal Battery Management for Wireless Nodes(2003-02-01) Sarkar, Saswati; Adamou, MariaThe focus of this paper is to extend the lifetime of a battery powered node in wireless context. The lifetime of a battery depends on both the manner of discharge and the transmission power requirements. We present a framework for computing the optimal discharge strategy which maximizes the lifetime of a node by exploiting the battery characteristics and adapting to the varying power requirements for wireless operations. The complexity of the optimal computation is linear in the number of system states. However, since the number of states can be large, the optimal strategy can only be computed offline and executed via a table lookup. We present a simple discharge strategy which can be executed online without any table lookup and attains near maximum lifetime.Publication Channel Assignment Algorithms Satisfying Cochannel and Adjacent Channel Reuse Constraints in Cellular Mobile Networks(2002-09-01) Sarkar, Saswati; Sivarajan, Kumar NImproved channel assignment algorithms for cellular networks were designed by modeling the interference constraints in terms of a hypergraph [1]. However, these algorithms only considered cochannel reuse constraints. Receiver filter responses impose restrictions on simultaneous adjacent channel usage in the same cell or in neighboring cells. We first present some heuristics for designing fixed channel assignment algorithms with a minimum number of channels satisfying both cochannel and adjacent channel reuse constraints. An asymptotically tight upper bound for the traffic carried by the system in the presence of arbitrary cochannel and adjacent channel use constraints was developed in [2]. However, this bound is computationally intractable even for small systems like a regular hexagonal cellular system of 19 cells. We have obtained approximations to this bound using the optimal solutions for cochannel reuse constraints only and a further graph theoretic approach. Our approximations are computationally much more efficient and have turned out to track very closely the exact performance bounds in most cases of interest.Publication Fairness and Throughput Guarantees with Maximal Scheduling in Multi-hop Wireless Networks(2006-04-01) Sarkar, Saswati; Chaporkar, Prasanna; Kar, KoushikWe investigate the fairness and throughput properties of a simple distributed scheduling policy, maximal scheduling, in the context of a general ad-hoc wireless network. We design a fully distributed algorithm that combines a token generation scheme with maximal scheduling policy so as to attain maxmin fair rates within the feasible region of maximal scheduling. We next present throughput guarantees of maximal scheduling that quantify the performance loss of each session due to the use of local information based scheduling. We show that the performance loss for each session depends on the maximum “interference degree” in its neighborhood. We also demonstrate that the performance penalties can not be localized any furtherPublication Economy of Spectrum Access in Timy Varying Multichannel Networks(2010-10-01) Khouzani, M.H.R.; Sarkar, SaswatiWe consider a wireless network consisting of two classes of potentially mobile users: primary users and secondary users. Primary users license frequency channels and transmit in their respective bands as required. Secondary users resort to unlicensed access of channels that are not used by their primary users. Primaries impose access fees on the secondaries which depend on access durations and may be different for different primary channels and different available communication rates in the channels. The available rates to the secondaries change with time depending on the usage status of the primaries and the random access quality of channels. Secondary users seek to minimize their total access cost subject to stabilizing their queues whenever possible. Our first contribution is to present a dynamic link scheduling policy that attains this objective. The computation time of this policy, however, increases exponentially with the size of the network. We next present an approximate scheduling scheme based on graph partitioning that is distributed and attains arbitrary trade-offs between aggregate access cost and computation times of the schedules, irrespective of the size of the network. Our performance guarantees hold for general arrival and primary usage statistics and multihop networks. Each secondary user is, however, primarily interested in minimizing the cost it incurs, rather than in minimizing the aggregate cost. Thus, it will schedule its transmissions so as to minimize the aggregate cost only if it perceives that the aggregate cost is shared among the users as per a fair cost sharing scheme. Using concepts from cooperative game theory, we develop a rational basis for sharing the aggregate cost among secondary sessions and present a cost sharing mechanism that conforms to the above basis.Publication An Adaptive Strategy for Maximizing Throughput in MAC layer Wireless Multicast(2004-05-24) Chaporkar, Prasanna; Bhat, Anita; Sarkar, SaswatiBandwidth efficiency of wireless multicast can be improved substantially by exploiting the fact that several receivers can be reached at the MAC layer by a single transmission. The multicast nature of the transmissions, however, introduces several design challenges, and systematic design approaches that have been used effectively in unicast and wireline multicast do not apply in wireless multicast. For example, a transmission policy that maximizes the stability region of the network need not maximize the network throughput. Therefore, the objective is to design a policy that decides when a sender should transmit in order to maximize the system throughput subject to maintaining the system stability. We present a sufficient condition that can be used to establish the throughput optimality of a stable transmission policy. We subsequently design an adaptive stable policy that allows a sender to decide when to transmit using simple computations based only on limited information about current transmissions in its neighborhood, and without using any information about the network statistics. The proposed policy attains the same throughput as the optimal offline stable policy that uses in its decision process past, present, and even future network states. We prove the throughput optimality of this policy using the suffi- cient condition and the large deviation results. We present a MAC protocol for acquiring the local information necessary for executing this policy, and implement it in ns-2. The performance evaluations demonstrate that the optimal strategy significantly outperforms the existing approaches in adhoc networks consisting of several multicast and unicast sessions.Publication A fair scheduling policy for wireless channels with intermittent connectivity(2008-03-19) Aram, A.; Kzouhani, M.H. R.; Sarkar, Saswati; Tassiulas, L.We consider a system of parallel queues with the constraint that only one queue can be served at a time. Each queue can be served through a wireless channel with intermittent connectivity. We propose a policy which serves the longest connected queue when the lengths of all connected are below a given threshold, and serves the connected queue that exceeds the threshold and has received the least service so far, otherwise. We prove that this simple policy (a) maximizes the aggregate service rate of all queues (b) maximizes the minimum service rate attained by any queue and (c) attains the stability region of the network.Publication A Framework for Routing and Congestion Control for Multicast Information Flows(2002-10-01) Sarkar, Saswati; Tassiulas, LeandrosWe propose a new multicast routing and scheduling algorithm called multipurpose multicast routing and scheduling algorithm (MMRS). The routing policy load balances among various possible routes between the source and the destinations, basing its decisions on the message queue lengths at the source node. The scheduling is such that the flow of a session depends on the congestion of the next hop links. MMRS is throughput optimal. In addition, it has several other attractive features. It is computationally simple and can be implemented in a distributed, asynchronous manner. It has several parameters which can be suitably modified to control the end-to-end delay and packet loss in a topology-specific manner. These parameters can be adjusted to offer limited priorities to some desired sessions. MMRS is expected to play a significant role in end-to-end congestion control in the multicast scenario.Publication Characterizing Temporal SNR Variation in 802.11 Networks(2008-07-01) Guha, Ratul K; Sarkar, SaswatiThe analysis and design of wireless medium access control (MAC) protocols, coding schemes, and transmission algorithms can significantly benefit from an understanding of the channel quality variation.We attempt to represent channel quality variation using a finite-state birth–death Markov model. We outline a method to compute the parameters of the model based on measured traces obtained using common wireless chipsets. Using this Markov chain, we statistically evaluate the performance based on the channel quality, long-term correlations, and burst length distributions. Such a model significantly performs better than a traditional two-state Markov chain in characterizing 802.11 networks while maintaining the simplicity of a birth–death model. We interpret the variation of the model parameters across different locations and different times. A finite-state stationary model is amenable to analysis and can substantially benefit the design of efficient algorithms and make simulations for wireless network protocols faster.Publication Can Bluetooth Succeed as a Large-Scale Ad Hoc Networking Technology?(2005-03-01) Guérin, Roch A.; Vergetis, Evangelos; Sarkar, Saswati; Rank, JacobWe investigate issues that Bluetooth may face in evolving from a simple wire replacement to a large-scale ad hoc networking technology. We do so by examining the efficacy of Bluetooth in establishing a connected topology, which is a basic requirement of any networking technology. We demonstrate that Bluetooth experiences some fundamental algorithmic challenges in accomplishing this seemingly simple task. Specifically, deciding whether there exists at least one connected topology that satisfies the Bluetooth constraints is NP-hard. Several implementation problems also arise due to the internal structure of the Bluetooth protocol stack. All these together degrade the performance of the network, or increase the complexity of operation. Given the availability of efficient substitute technologies, Bluetooth’s use may end up being limited to small ad hoc networks.