Showing papers by "Milica Stojanovic published in 2014"

Adaptive OFDM Modulation for Underwater Acoustic Communications: Design Considerations and Experimental Results

Abstract: In this paper, we explore design aspects of adaptive modulation based on orthogonal frequency-division multiplexing (OFDM) for underwater acoustic (UWA) communications, and study its performance using real-time at-sea experiments. Our design criterion is to maximize the system throughput under a target average bit error rate (BER). We consider two different schemes based on the level of adaptivity: in the first scheme, only the modulation levels are adjusted while the power is allocated uniformly across the subcarriers, whereas in the second scheme, both the modulation levels and the power are adjusted adaptively. For both schemes we linearly predict the channel one travel time ahead so as to improve the performance in the presence of a long propagation delay. The system design assumes a feedback link from the receiver that is exploited in two forms: one that conveys the modulation alphabet and quantized power levels to be used for each subcarrier, and the other that conveys a quantized estimate of the sparse channel impulse response. The second approach is shown to be advantageous, as it requires significantly fewer feedback bits for the same system throughput. The effectiveness of the proposed adaptive schemes is demonstrated using computer simulations, real channel measurements recorded in shallow water off the western coast of Kauai, HI, USA, in June 2008, and real-time at-sea experiments conducted at the same location in July 2011. We note that this is the first paper that presents adaptive modulation results for UWA links with real-time at-sea experiments.

CAPTURE: A Communications Architecture for Progressive Transmission via Underwater Relays With Eavesdropping

Abstract: As analysis of imagery and other science data plays a greater role in mission execution, there is an increasing need for autonomous marine vehicles to transmit these data to the surface Communicating imagery and full-resolution sensor readings to surface observers remains a significant challenge Yet, without access to the data acquired by an unmanned underwater vehicle (UUV), surface operators cannot fully understand the mission state of a vehicle This paper presents an architecture capable of multihop communication across a network of underwater acoustic relays In concert with an abstracted physical layer, CAPTURE provides an end-to-end networking solution for communicating science data from autonomous marine vehicles Automatically selected imagery, SONAR, and time-series sensor data are progressively transmitted across multiple hops to surface operators To incorporate human feedback, data are transmitted as a sequence of gradually improving data “previews” Operators can request arbitrarily high-quality refinement of any resource, up to an error-free reconstruction The results of three diverse field trials on SeaBED, OceanServer, and Bluefin AUVs, with drastically different software architectures, are also presented

Differential Decoding for SFBC OFDM Systems in Underwater MIMO Channels

Abstract: We investigate the use of differential space fre- quency block codes (SFBCs) with orthogonal frequency division multiplexing (OFDM) over underwater acoustic channels. While SFBC efficiently exploits spatial transmit diversity, differentially coherent detection eliminates the need for extensive signal pro- cessing required for channel tracking. System performance is demonstrated using real data transmitted in the 12 !26 kHz acoustic band from a vehicle moving at 0.5 !2 m/s and received over a 100 m shallow water channel, using 4-QAM and a varying number of carriers ranging from 128 to 2048. Performance results demonstrate the advantage of the differentially coherent SFBC detection over the conventional, coherent SFBC detection which suffers from imperfect channel estimation.

Coxeter Groups as Automorphism Groups of Solid Transitive 3-simplex Tilings

Abstract: In the papers of I.K. Zhuk, then more completely of E. Molnar, I. Prok, J. Szirmai all simplicial 3-tilings have been classified, where a symmetry group acts transitively on the simplex tiles. The involved spaces depends on some rotational order parameters. When a vertex of a such simplex lies out of the absolute, e.g. in hyperbolic space H3, then truncation with its polar plane gives a truncated simplex or simply, trunc-simplex. Looking for symmetries of these tilings by simplex or trunc-simplex domains, with their side face pairings, it is possible to find all their group extensions, especially Coxeter’s reflection groups, if they exist. So here, connections between isometry groups and their supergroups is given by expressing the generators and the corresponding parameters. There are investigated simplices in families F3, F4, F6 and appropriate series of trunc-simplices. In all cases the Coxeter groups are the maximal ones.

Estimation and tracking of time-varying channels in OFDM systems

Abstract: We propose a method for channel estimation in orthogonal frequency division multiplexing (OFDM) systems. Unlike the conventional sample-spaced and sub-sample-spaced methods, which target the taps of an equivalent discrete-delay channel response, this method targets the physical propagation paths in a continuous-delay domain. Numerical results quantify its benefits over the conventional least-squares, orthogonal matching pursuit, and mixed-norm methods. Assuming that channel is slowly varying from one OFDM block to another, we also develop an adaptive algorithm for channel tracking. Performance is illustrated through simulation.

Packet scheduling for underwater acoustic sensor network localization

Abstract: This article considers the problem of packet scheduling for localization in an underwater acoustic sensor network where sensor nodes are distributed randomly in an operating area. Our goal is to minimize the localization time, and to do so we consider two packet transmission schemes, namely collision-free, and collision-tolerant. Through analytical results and numerical examples the performances of these schemes are shown to be comparable. In general, for small packet length (as is the case for a localization packet) and large operating area (above 3km in at least one dimension), the performances of the collision-tolerant protocol is superior to its collision-free counterpart. At the same time, the anchors work independently of each other, and this feature simplifies the implementation process.

Creating an eap/esp core textbook: focus on acquiring knowledge in english rather than about english

Abstract: This paper discusses some of the issues involved in creating an EAP/ESP core textbook, focusing especially on choosing textbook content as one of the main factors contributing to higher student motivation and better language acquisition. It further offers a reflection on EAP/ESP textbook titles and dwells on some other problems related to designing and teaching textbook material.

On capacity of a class of acoustic channels

Abstract: We use a per-path Rician fading model to analyze the capacity of acoustic channels, and provide experimental data to confirm the practicality of simulation results. Three power allocation policies are considered: water-filling, uniform power allocation across selected, channel-favored frequencies (“on-off” carriers in an OFDM system), and uniform power allocation across all carriers irrespective of the channel (“all-on”). The impact of channel estimation errors and feedback delay is taken into account. The three policies are found to differ little in terms of the achievable rate, and to suffer similar loss from imperfect channel knowledge at the receiver (about 1 bps/Hz loss from 4 bps/Hz at 20 dB for the experimental channel). Unlike water-filling and on-off power allocation, which are additionally sensitive to outdated channel feedback, all-on uniform power allocation does not require feedback and is simple to implement, thus emerging as a justified practical solution.

Random linear packet coding for broadcast networks

Abstract: Random linear packet coding is considered for efficient broadcasting in networks with large propagation delays, such as underwater acoustic networks. To additionally overcome the effects of fading, we combine packet coding with adaptive power control, whereby the transmitter adjusts its power upon receiving feedback from the receiver on the current state of the channel (locally-averaged, large-scale gain). We investigate two power adjustment rules: the worst link rule and the average link rule. In the first case, the transmit power is adjusted in accordance with the link that has the lowest channel gain, while in the second case, the power is adjusted in accordance with the average of the gains on all links. System performance is evaluated based on the average energy per bit of successfully transmitted bit of information, using (i) simulated channels, (ii) experimentally recorded gain values from the MISSION 2012 experiment, and (iii) actual network deployment from the MISSION 2013 experiment conducted off the coast of Singapore. Results indicate energy savings on the order of several dB compared to systems that do not use power control.

Collision Tolerant Packet Scheduling for Underwater Acoustic Localization.

Abstract: This article considers the joint problem of packet scheduling and self-localization in an underwater acoustic sensor network where sensor nodes are distributed randomly in an operating area. In terms of packet scheduling, our goal is to minimize the localization time, and to do so we consider two packet transmission schemes, namely a collision-free scheme (CFS), and a collision-tolerant scheme (CTS). The required localization time is formulated for these schemes, and through analytical results and numerical examples their performances are shown to be generally comparable. However, when the packet duration is short (as is the case for a localization packet), and the operating area is large (above 3km in at least one dimension), the collision-tolerant scheme requires a smaller localization time than the collision-free scheme. After gathering enough measurements, an iterative Gauss-Newton algorithm is employed by each sensor node for self-localization, and the Cramer Rao lower bound is evaluated as a benchmark. Although CTS consumes more energy for packet transmission, it provides a better localization accuracy. Additionally, in this scheme the anchor nodes work independently of each other, and can operate asynchronously which leads to a simplified implementation.

Cramér rao lower bound for underwater range estimation with noisy sound speed profile

Abstract: In this paper, the Cramer Rao bound (CRB) for range estimation between two underwater nodes is calculated under a Gaussian noise assumption on the measurements. The nodes can measure their depths, their mutual time of flight, and they have access to noisy sound speed samples at different depths. The effect of each measurement on the CRB will be analyzed, and it will be shown that for long distances, the effect of the sound speed measurement noise is dominant, and its impact depends on the positions of the nodes, actual sound speed profile, the number of sound speed samples, and the depths at which the sound speed samples are gathered.

Hierarchical underwater acoustic sensor networks with virtual transmit/receive arrays

Abstract: We consider a hierarchical underwater acoustic sensor network architecture in which the sensors and the collector stations operate in distinct layers. The hierarchical architecture is motivated by the property of the acoustic underwater transmission medium that for each transmission distance, there exists an operating frequency for which the narrow-band signal-to-noise ratio is maximised. The sensors and the collector stations are consequently allocated different operating frequencies. We assume a uniform distribution of both sensors and collector stations over the finite area of the sensing field. The sensors are organised into clusters forming virtual transmit/receive arrays. The collector stations, on the other hand, are equipped with co-located transmit/receive arrays. We adopt a communication-theoretic approach and study the interdependence of the sustainable number of hops through the network, end-to-end frame error probability, power and bandwidth allocation. The analysis is performed under the assumption of Ricean fading and interference from other nodes within the same layer of the hierarchy. We present numerical examples that illustrate the network performance and demonstrate that there are preferred operating frequencies, which ensure network operation without any cross-interference between the collector network and the sensor network. Copyright © 2012 John Wiley & Sons, Ltd.

Random linear packet coding: Joint power and rate control

Abstract: Random linear packet coding is considered for channels that experience fading and have long propagation delay, such as the underwater acoustic channels. Previously, we employed power control (adjusting the transmission power according to the channel gain) and rate control (adjusting the number of coded packets according to the channel gain) to counteract the effects of fading. For such policies, it was shown that there exists an optimal number of coded packets (when employing power control) or optimal transmission power (when employing rate control) for which the energy required per bit of information transmitted is minimized. In this paper, we present a strategy to jointly optimize the transmission power and the number of coded packets. We present two optimization criteria, i.e., (a) minimizing the average energy per bit for transmission and (b) maximizing the throughput. Given the statistics of the channel, we compute the average energy per bit and the average throughput under each criterion, to make a choice based on the desired tradeoff. In applications with limited power, minimizing the average energy per bit will be the guiding principle, while in time-critical applications maximizing the average throughput will dictate the choice.