Showing papers by "Milica Stojanovic published in 2003"
15 Apr 2003
TL;DR: This paper considers several aspects in the design of shallow water acoustic networks that maximize throughput and reliability while minimizing power consumption.
Abstract: Underwater acoustic networks are generally formed by acoustically connected ocean bottom sensor nodes, autonomous underwater vehicles (AUVs), and surface stations that serve as gateways and provide radio communication links to on-shore stations. The QoS of such networks is limited by the low bandwidth of acoustic transmission channels, high latency resulting from the slow propagation of sound, and elevated noise levels in some environments. The long-term goal in the design of underwater acoustic networks is to provide for a self-configuring network of distributed nodes with network links that automatically adapt to the environment through selection of the optimum system parameters. This paper considers several aspects in the design of shallow water acoustic networks that maximize throughput and reliability while minimizing power consumption.
Keywords:
underwater;
acoustic;
communications;
network;
media access control (MAC);
routing;
ad hoc;
Seaweb
234 citations
15 Apr 2003
TL;DR: Current research is focusing on the design of efficient modulation and coding schemes, adaptive signal processing algorithms for equalization and diversity combining, multiple-access communication methods and network protocols suited for low bandwidth, long propagation delays and strict power requirements encountered in the underwater environment.
Abstract: The need for underwater wireless communications exists in off-shore oil industry, environmental monitoring, speech transmission between divers, control of autonomous underwater vehicles, and mapping of the ocean floor for detection of objects and discovery of new resources. Wireless underwater communications can be established by transmission of acoustic waves. However, the communication channels have limited bandwidth, and often cause severe multipath dispersion and time-variability. Despite these limitations, research efforts of the 90's have culminated in the development of underwater acoustic modems that are capable of transmitting information at rates on the order of several kilobits per second systems over varying distances. The emerging scenario is that of an underwater communication network consisting of both stationary and mobile nodes. Current research is focusing on the design of efficient modulation and coding schemes, adaptive signal processing algorithms for equalization and diversity combining, multiple-access communication methods and network protocols suited for low bandwidth, long propagation delays and strict power requirements encountered in the underwater environment.
Keywords:
underwater communications;
acoustic communications;
bandwidt-efficient modulation;
adaptive equalization;
diversity combining;
reduced-complexity receivers;
phase-coherent detection;
channel estimation;
multipath propagation;
multiuser detection;
underwater networks
189 citations
TL;DR: It is found that there is a fading rate range where each method provides a good tradeoff between performance and overhead and it is concluded that the MMSE per carrier decision-directed detector with RLS estimation combines good performance in low to moderate fading rates, robustness in parameter variations, and relatively low complexity and overhead.
Abstract: Multicarrier code-division multiple access (MC-CDMA) combines multicarrier transmission with direct sequence spread spectrum. Different approaches have been adopted which do not assume a perfectly known channel. We examine the forward-link performance of decision-directed adaptive detection schemes, with and without explicit channel estimation, for MC-CDMA systems operating in fast fading channels. We analyze theoretically the impact of channel estimation errors by first considering a simpler system employing a threshold orthogonality restoring combining (TORC) detector with a Kalman channel estimator. We show that the performance deteriorates significantly as the channel fading rate increases and that the fading rate affects the selection of system parameters. We examine the performance of more realistic schemes based on the minimum mean square error (MMSE) criterion using least mean square (LMS) and recursive least square (RLS) adaptation. We present a discussion which compares the decision-directed and pilot-aided approaches and explores the tradeoffs between channel estimation overhead and performance. We find that there is a fading rate range where each method provides a good tradeoff between performance and overhead. We conclude that the MMSE per carrier decision-directed detector with RLS estimation combines good performance in low to moderate fading rates, robustness in parameter variations, and relatively low complexity and overhead. For higher fading rates, however, only pilot-symbol-aided detectors are appropriate.
148 citations
01 Jan 2003
TL;DR: An autonomous network of underwater vehicles is considered in which there is no central node, but the vehicles communicate in a distributed manner over multiple hops, and the focus of network design is on a scalable multiple access technique applicable to varying coverage areas as well as varying number of vehicles.
Abstract: An autonomous network of underwater vehicles is considered in which there is no central node, but the vehicles communicate in a distributed manner over multiple hops. The focus of network design is on a scalable multiple access technique which is applicable to varying coverage areas as well as varying number of vehicles. The proposed scheme relies on grouping the adjacent vehicles into clusters, and using time-division multiple access within each cluster. Interference among clusters is managed by assigning different spreading codes to adjacent clusters, while scalability is achieved by spatial reuse of the codes. Network operation begins with an initialization phase, and moves on to continuous maintenance during which mobility is managed. Performance is quantified through measures of connectivity, successful transmission rate, average delay and energy consumption. Simulation analysis is used to obtain optimal cluster size and transmission power for a network with given density of vehicles.
138 citations
01 Jan 2003
TL;DR: An experimental system, based on discrete cosine transform and Huffman entropy coding for video compression, and variable rate M-ary QAM, achieves bit rates as high as 150 kbps, which are sufficient for real-time transmission of compressed video.
Abstract: A high bit rate acoustic link for video transmission over an underwater channel is investigated. The key to achieving this objective lies in two approaches: use of efficient data compression algorithms and use of high-level bandwidth-efficient modulation methods. Currently available video encoding standards allow video transmission at bit rates as low as 64 kbps. While this rate is still above the limit of commercially available acoustic modems, prototype acoustic modems based on phase coherent modulation/detection have demonstrated successful transmission up to 30 kbps over a deep-water vertical path. To bridge the final gap and provide acoustic transmission capability needed for near real-time video, we focus on the use of high-level bandwidth-efficient modulation methods. An experimental system, based on discrete cosine transform and Huffman entropy coding for video compression, and variable rate M-ary QAM was implemented. Phase-coherent detection is accomplished by decision-directed synchronization and adaptive equalization. System performance is demonstrated experimentally, using 25000 symbols/sec at a carrier frequency of 75 kHz over a short vertical path. Excellent results were obtained using modulation methods of 16, 32 and 64-QAM, thus achieving bit rates as high as 150 kbps, which are sufficient for real-time transmission of compressed video.
86 citations
01 Jan 2003
TL;DR: The design and testing of an underwater acoustic ad hoc network using OPNET Modeler/Radio and a genetic algorithm based routing optimization that maximizes the network’s lifetime is employed.
Abstract: In this paper, we discuss the design and testing of an underwater acoustic ad hoc network using OPNET Modeler/Radio. The network is intended for the long-term monitoring of a selected ocean area. The data flow of the network is mainly towards a master node, which is responsible for collecting data generated by sensor nodes. When the network is first deployed, an initialization algorithm is executed and preliminary routes are determined. Since the network nodes are battery powered, an important measure of effectiveness is power efficiency. Therefore, in addition to the initialization algorithm, a genetic algorithm based routing optimization that maximizes the network’s lifetime is employed. Simulation results are presented.
26 citations
01 Jan 2003
TL;DR: In this paper, the authors proposed a scalable multiple access technique for underwater vehicle networks, which is applicable to varying coverage areas as well as varying number of vehicles, by grouping the adjacent vehicles into clusters, and using time-division multiple access within each cluster.
Abstract: An autonomous network of underwater vehicles is considered in which there is no central node, but the vehicles communicate in a distributed manner over multiple hops. The focus of network design is on a scalable multiple access technique which is applicable to varying coverage areas as well as varying number of vehicles. The proposed scheme relies on grouping the adjacent vehicles into clusters, and using time-division multiple access within each cluster. Interference among clusters is managed by assigning different spreading codes to adjacent clusters, while scalability is achieved by spatial reuse of the codes. Network operation begins with an initialization phase, and moves on to continuous maintenance during which mobility is managed. Performance is quantified through measures of connectivity, successful transmission rate, average delay and energy consumption. Simulation analysis is used to obtain optimal cluster size and transmission power for a network with given density of vehicles.
01 Jan 2003
TL;DR: In this paper, an adaptive channel estimation and filtering algorithm is proposed for underwater acoustic channel acquisition, which employs feedback of pre-cursor and postcursor inter-chip interference to provide a channel estimate from which the code timing is acquired, and also uses the channel estimate to filter the signal prior to making the acquisition decision.
Abstract: Acquisition of the code timing in DS/SS systems is addressed for time-varying frequency-selective underwater acoustic channels. Acquisition algorithm based on adaptive channel estimation and filtering is proposed. The algorithm employs feedback of pre-cursor and post-cursor inter-chip interference to provide a channel estimate from which the code timing is acquired. The channel estimate is also used to filter the signal prior to making the acquisition decision. Channel estimation/filtering procedure eliminates the need for explicit equalization, and enables a computationally efficient implementation. Simulation results show excellent performance in low-SNR regime, as well as in the presence of multiple-access interference and fast fading.