Vikram Srinivasan
Email Address
elevs@nus.edu.sg
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Publication Coverage game in wireless sensor networks(2006) Ai, X.; Srinivasan, V.; Tham, C.-K.; ELECTRICAL & COMPUTER ENGINEERINGIn a wireless sensor network, we deploy a number of energy constrained nodes in an area to fulfill some monitoring task for a period. There exists a trade-off between network lifetime and coverage performance. We investigate this trade-off by maximizing the average coverage performance during a given network lifetime. Our main idea is to divide all the nodes in the network into several disjoint groups, and at each time only one group are in the active mode. To maximize the average coverage, we want the overlapped sensing area in each group to be minimized. Furthermore, to achieve this objective distributively we formulate this group division and coverage maximizing problem into a game theory model and our desired solution in this model is a Nash Equilibrium strategy profile. Finally, we apply a hill-climbing Nash Equilibrium convergence idea into our coverage game and let each node converge to an approximate Nash Equilibrium in a coherent local way. Our simulation results show that from a random start point we can realize convergence in only 20 to 45 iterations when number of nodes in the network increases from 80 to 170. What's more, the coverage performance provided by Nash Equilibrium solution is 96.43% and 95.49% to a derived upper bound in the high density grid and random networks respectively. © 2006 IEEE.Publication Matching queries in a network(2012-09-18) MEHUL, MOTANI; VIKRAM, SRINIVASAN; ELECTRICAL & COMPUTER ENGINEERINGA method of matching queries in a hybrid infrastructure/infrastructure-less network, the network comprising pluralities of first and second type communication devices respectively, the method comprising placing a first query by a user on one of the first type devices and forwarding the query via infrastructure based communication to one of the second type devices; forwarding, depending on a category of the first query, the first query from the one second type device to one or more first type devices via infrastructure based communication; and relaying the first query from each of one or more first type devices to one or more neighboring first type devices via infrastructure-less communication.Publication Localized recursive estimation in energy constrained wireless sensor networks(2006) Wang, B.; Chua, K.C.; Srinivasan, V.; ELECTRICAL & COMPUTER ENGINEERINGThis paper proposes a localized recursive estimation scheme for parameter estimation in wireless sensor networks. Given any parameter of a target occurring at some location and time, a number of sensors recursively estimate the parameter by using their local measurements of the parameter that is attenuated with the distance between a sensor and the target location and corrupted by noise. Compared with centralized estimation schemes that transmit all encoded measurements to a sink (or a fusion center), the recursive scheme needs only to transmit the final estimate to a sink. When the sink is faraway from the sensors and multihop communications have to be used, using localized recursive estimation can help to reduce energy consumption and reduce network traffic load. A sensor sequence with the fastest convergence rate is identified, by which the variance of estimation error reduces faster than all other sequences. In the case of adjustable transmission power, a heuristic has been proposed to find a sensor sequence with the minimum total transmission power when performing the recursive estimation. Numerical examples have been used to compare the performance of the proposed scheme with that of a centralized estimation scheme and have also shown the effectiveness of the proposed heuristic. © 2006 ACADEMY PUBLISHER.Publication Quality of service in ad hoc and sensor networks(2007-06) Chiasserini, C.-F.; Srinivasan, V.; ELECTRICAL & COMPUTER ENGINEERINGPublication Information coverage for wireless sensor networks(2005-11) Wang, B.; Wang, W.; Srinivasan, V.; Chua, K.C.; ELECTRICAL & COMPUTER ENGINEERINGCoverage is a very important issue in wireless sensor networks. Current literature defines a point to be covered if it is within the sensing radius of at least one sensor. In this paper we argue that this is a conservative definition of coverage. This definition implicitly assumes that each sensor makes a decision independent of other sensors in the field. However, sensors can cooperate to make an accurate estimation, even if any single sensor is unable to do so. We then propose a new notion of information coverage and investigate its implications for sensor deployment. Numerical and simulation results show that significant savings in terms of sensor density for complete coverage can be achieved by using our definition of information coverage compared to that by using the existing definition. © 2005 IEEE.Publication Scheduling sensor activity for information coverage of discrete targets in sensor networks(2009-06) Wang, B.; Chua, K.C.; Srinivasan, V.; Wang, W.; ELECTRICAL & COMPUTER ENGINEERINGIn this paper, we study the problem of scheduling sensor activity to cover a set of targets with known locations such that all targets can be monitored all the time and the network can operate as long as possible. A solution to this scheduling problem is to partition all sensors into some sensor covers such that each cover can monitor all targets and the covers are activated sequentially. In this paper, we propose to provide information coverage instead of the conventional sensing disk coverage for target. The notion of information coverage is based on estimation theory to exploit the collaborative nature of geographically distributed sensors. Due to the use of information coverage, a target that is not within the sensing disk of any single sensor can still be considered to be monitored (information covered) by the cooperation of more than one sensor. This change of the problem settings complicates the solutions compared to that by using a disk coverage model. We first define the target information coverage (TIC) problem and prove its NP-completeness. We then propose a heuristic to approximately solve our problem. Simulation results show that our heuristic is better than an existing algorithm and is close to the upper bound when only the sensing disk coverage model is used. Furthermore, simulation results also show that the network lifetime can be significantly improved by using the notion of information coverage compared with that by using the conventional definition of sensing disk coverage. Copyright © 2008 John Wiley & Sons, Ltd.Publication Adaptive contact probing mechanisms for delay tolerant applications(2007) Wang, W.; Srinivasan, V.; Motani, M.; ELECTRICAL & COMPUTER ENGINEERINGIn many delay tolerant applications, information is opportunistically exchanged between mobile devices who encounter each other. In order to effect such information exchange, mobile devices must have knowledge of other devices in their vicinity. We consider scenarios in which there is no infrastructure and devices must probe their environment to discover other devices. This can be an extremely energy consuming process and highlights the need for energy conscious contact probing mechanisms. If devices probe very infrequently, they might miss many of their contacts. On the other hand, frequent contact probing might be energy inefficient. In this paper, we investigate the trade-off between the probability of missing a contact and the contact probing frequency. First, via theoretical analysis, we characterize the trade-off between the probability of a missed contact and the contact probing interval for stationary processes. Next, for time varying contact arrival rates, we provide an optimization framework to compute the optimal contact probing interval as a function of the arrival rate. We characterize real world contact patterns via Bluetooth phone contact logging experiments and show that the contact arrival process is self-similar. We design STAR, a contact probing algorithm which adapts to the contact arrival process. Via trace driven simulations on our experimental data, we show that STAR consumes three times less energy when compared to a constant contact probing interval scheme. Copyright 2007 ACM.Publication Efficient cache placement in multi-hop wireless networks(2006-10) Nuggehalli, P.; Srinivasan, V.; Chiasserini, C.-F.; Rao, R.R.; ELECTRICAL & COMPUTER ENGINEERINGIn this paper, we address the problem of efficient cache placement in multi-hop wireless networks. We consider a network comprising a server with an interface to the wired network, and other nodes requiring access to the information stored at the server. In order to reduce access latency in such a communication environment, an effective strategy is caching the server information at some of the nodes distributed across the network. Caching, however, can imply a considerable overhead cost; for instance, disseminating information incurs additional energy as well as bandwidth burden. Since wireless systems are plagued by scarcity of available energy and bandwidth, we need to design caching strategies that optimally trade-off between overhead cost and access latency. We pose our problem as an integer linear program. We show that this problem is the same as a special case of the connected facility location problem, which is known to be NP-hard. We devise a polynomial time algorithm which provides a suboptimal solution. The proposed algorithm applies to any arbitrary network topology and can be implemented in a distributed and asynchronous manner. In the case of a tree topology, our algorithm gives the optimal solution. In the case of an arbitrary topology, it finds a feasible solution with an objective function value within a factor of 6 of the optimal value. This performance is very close to the best approximate solution known today, which is obtained in a centralized manner. We compare the performance of our algorithm against three candidate cache placement schemes, and show via extensive simulation that our algorithm consistently outperforms these alternative schemes. © 2006 IEEE.Publication Trade-offs between mobility and density for coverage in wireless sensor networks(2007) Wang, W.; Srinivasan, V.; Chua, K.-C.; ELECTRICAL & COMPUTER ENGINEERINGIn this paper, we study the coverage problem for hybrid networks which comprise both static and mobile sensors. We consider mobile sensors with limited mobility, i.e., they can move only once over a short distance. Such mobiles are simple and cheap compared to sophisticated mobile robots. In conventional static sensor networks, for a random deployment, the sensor density should increase as O(log L + k log log L) to provide k-coverage in a network with a size of L. As an alternative, an all mobile sensor network can provide k-coverage over the field with a constant density of O(k), independent of network size L. We show that the maximum distance that any mobile sensor will have to move is O(1 over k log 3 over 4 (kL)). We then propose a hybrid network structure, comprising static sensors and a small fraction of O(1 over (k)) of mobile sensors. For this network structure, we prove that k-coverage is achievable with a constant sensor density of O(k), independent of network size L. Furthermore, for this hybrid structure, we prove that the maximum distance which any mobile sensor has to move is bounded as O(log 3 over 4 L). We then propose a distributed relocation algorithm, where each mobile sensor only requires local information in order to optimally relocate itself and characterize the algorithm's computational complexity and message overhead. Finally, we verify our analysis via extensive numerical evaluations. Copyright 2007 ACM.Publication Altruistic cooperation for energy-efficient multi-channel MAC protocols(2007) Luo, T.; Motani, M.; Srinivasan, V.; ELECTRICAL & COMPUTER ENGINEERINGRecently, a new notion of cooperation was proposed to solve multi-channel coordination problems. When a transmit-receive pair wishes to initiate communication, neighboring nodes share their knowledge of channel usage. This helps to substantially reduce collisions and increases throughput significantly. However, it comes at the cost of increased energy consumption since idle nodes have to stay awake to overhear and acquire channel usage information. In fact this can be as high as 264% of a power-saving protocol without cooperation. In this paper, we propose a strategy called altruistic cooperation for cooperative multi-channel MAC protocols to conserve energy. The core idea is to introduce specialized nodes called altruists in the network whose only role is to acquire and share channel usage information. All other nodes, termed peers, go in to the sleep mode when idle. This strategy seems naive because it needs additional nodes to be deployed. In fact, it is unclear whether a desirable throughput-energy trade-off can be achieved and whether the cost of additional nodes can offset the performance gain. We perform a close study on this strategy in terms of three aspects: network deployment, cost efficiency, and system performance. Our study indicates that only a few additional nodes need to be deployed and cost efficiency is more than doubled in terms of a new metric called bit-price ratio that we propose. By using the strategy, a cooperative protocol is found to save up to 70% energy while not compromising throughput.