Sharmistha Maitra

Bio: Sharmistha Maitra is an academic researcher from University of Denver. The author has contributed to research in topics: Synchronization & Data synchronization. The author has an hindex of 1, co-authored 1 publications receiving 5 citations.

Time Synchronization Protocol with Minimum Message Communication for High Latency Networks

Abstract: Time synchronization is a critical component of the infrastructure of wireless sensor networks (WSN). In a high latency environment such as underwater, traditional approaches to time synchronization have limited accuracy. A new method is describe for time synchronization that takes into account clock skew, clock offset, and also propagation delay. Minimum message communication is used as a performance measure of the quality of this new time synchronization protocol.

A receiver synchronized slotted Aloha for underwater wireless networks with imprecise propagation delay information

Abstract: In a wireless network, where propagation delay is high but known, slotted Aloha (S-Aloha) is synchronized with respect to the receiver's time slots. Since the transmitter knows the propagation delay to its receiver, after a frame is generated, the transmitter introduces a suitable delay before its transmission, such that the frame arrives exactly in a slot at the receiver. However, in an underwater wireless network, due to significantly less signal propagation speed, the channel dynamics has a significant effect on the time dispersion of propagation speed. Due to this uncertainty in propagation speed, even if the transmitter-receiver distance is exactly known, it is likely that a perfect synchronization at the receiver is not possible. In this paper, we first show that, even a little-less-than-perfect synchronization at the receiver reduces the throughput of receiver synchronized S-Aloha (RSS-Aloha) to that of pure Aloha. We modify the RSS-Aloha for underwater by accommodating the error in delay estimate while deciding the receiver-end slot size. Via probabilistic analysis, supported by simulations, we show that our proposed modified protocol offers a gradual increase in throughput as the propagation delay uncertainty decreases. We also show that the throughput of our proposed modified protocol is consistently higher compared to the transmitter synchronized S-Aloha when operating under the same propagation delay uncertainty. However, when the uncertainty is high, delay performance of the modified RSS-Aloha remains poorer than that of the transmitter synchronized S-Aloha in a system with smaller nodal communication range.

New Reservation Multiaccess Protocols for Underwater Wireless Ad Hoc Sensor Networks

Abstract: In a wireless network, where propagation delay is high and communications are sporadic, some kind of reservation protocol is generally used. Reservation access protocols were proposed earlier in earth stations-to-satellite communication with known propagation delay. However, optimality of the number of access slots with respect to the system performance parameters, such as system utilization, blocking probability, and delay, were not thoroughly studied. Besides, the effect of propagation delay uncertainty, which predominantly happens in underwater communications, are yet to be addressed. In this paper, we first analyze the system performance in many-to-one multiaccess data transfer scenario in underwater wireless ad hoc sensor networks with a fixed number of access slots and with the assumption of perfect propagation delay information. We propose two system state aware dynamic approaches to suitably adjust the number of access slots, and investigate the optimum slotting strategy to maximize the system utilization. Next, by accounting the propagation delay uncertainty, we relook into the optimality criteria on the number of access slots, where we apply a modified receiver-synchronized slotted Aloha principle to maximize the access performance. Via mathematical analysis, supported by discrete event simulations, we show that the system utilization and blocking probability performances with our proposed dynamic reservation protocols are consistently better compared to the competitive reservation protocols with fixed as well as variable access slots. Further, we conduct NS3 simulations to study the protocol performances under more realistic channel and traffic conditions, which also demonstrate that the proposed optimized dynamic slotting offers a much better system utilization performance compared to a similar underwater reservation multiaccess protocol.

A clock synchronization independent localization scheme for underwater wireless sensor networks

Abstract: Almost all the existing range-based localization schemes for underwater wireless sensor networks (UWSNs) assume that clocks of different nodes are well synchronized. However, clock synchronization is very challenging in UWSNs. In this paper, we design a clock synchronization independent localization scheme (CSILS). Instead of trying to estimate distances between unknown nodes and anchor nodes, we design a scheme for obtaining the distance differences based on the local clocks. In order to convert the distance differences into node's location, we propose distance-difference-based maximum likelihood estimation (D-D-BMLE). Finally, we give some suggestions on how to deploy anchor nodes Simulation results show that CSILS works well without clock synchronization when anchor nodes are deployed reasonably. Compared to a clock-synchronization-based localization scheme, CSILS improves localization accuracy with a proper level of localization coverage and communication overhead in terms of mobility model of water currents.

Adaptive Filtering Algorithm in Clock Calibration System

Abstract: Clock bias is a vital element in clock calibration system. Aiming at effectively eliminating noises contained in clock bias and improving accuracy of time synchronization, we propose an adaptive algorithm. In this adaptive algorithm, ensemble empirical mode decomposition (EEMD) is employed to decompose original data to different intrinsic mode functions (IMFs). For accurately abandoning IMFs which contain plentiful noises, principal components analysis (PCA) is proposed. Eigenvalues of all IMFs are ascertained on the bias of PCA. Then, bias between adjacent eigenvalues is used to evaluate level of noises contained in each IMF. Finally, residual IMFs are used to reconstruct the signal. Clean signal with adding noises is used to evaluate this improved algorithm. Consequence indicates that this improved algorithm is effective. We design a clock calibration experiment, where the time signal is transferred through a wireless channel. The adaptive algorithm is used to dispose bias, which is acquired in clock calibration experiment. Consequence also demonstrates that this algorithm can effectively and adaptively eliminate noises contained in actual data.