Underwater acoustic channels normally have low data rate, long propagation delay, severe multipath effect, and time varying fading. Cooperative transmission is a new wireless communication technique in which diversity gain is achieved by utilizing relay nodes as virtual antennae. In this paper, we investigate cooperative transmission techniques for underwater acoustic communications. First, we study the performance of several cooperative transmission schemes, originally designed for radio communications, in an underwater scenario. Second, by taking advantage of the low propagation speed of sound, we design a new wave cooperative transmission scheme. In this scheme, the relay nodes amplify the signal received from the source node, and then forward the signal immediately to the destination. The goal is to alter the multipath effect at the receiver. Third, we derive the performance upper bound for the proposed wave cooperative transmission scheme. The simulation results show that the proposed wave cooperative transmission has significant advantages over the traditional direct transmission and the existing cooperative transmission schemes originally designed for radio wireless networks.

The paper focuses on the design of remote sensing and detecting systems for the underwater environment around estuary areas. The behavior of estuary water conditions is studied and the environmental impacts on the performance of underwater sensing and detecting systems are investigated. Sensor coverage variations are determined in particular. Environmental data is extracted from the NYHOPS ocean model with periodic calibration from fixed CTD stations in the Hudson River. The result shows that careful planning is required for sensor placement in an estuary area.

A number of emerging Internet applications, such as video-conferencing and live multimedia broadcasts, must rely on scheduling algorithms in switches and routers to guarantee performance and an acceptable level of quality of service. Fairness in packet schedulers is an intuitively desirable property with practical value; fair schedulers are a critical component of QoS mechanisms that seek to guarantee end-to-end delay bounds. Popular measures of the fairness achieved by packet schedulers are based on bounds, such as the relative fairness bound which captures the maximum possible difference between the normalized service received by any two flows. We argue that such measures do not capture the actual fairness achieved at most instants of time, and therefore, do not represent a true measure of the ability of a scheduler to successfully deliver end-to-end quality for real-time applications. We borrow from the field of economics and propose a new measure of fairness based on the Gini index. This measure captures the instantaneous fairness of a scheduler and, unlike other measures based on bounds, also captures the fairness of the scheduler in its handling of flows during idle periods. Using real traces of gateway and video traffic, we present a simulation-based evaluation of several well-known schedulers based on the Gini index. We also present a qualitative analysis of the phenomena underlying the observed results.

Fair scheduling algorithms are an important component of most QoS mechanisms designed to support the performance guarantees required by real-time applications. In this paper, we present greedy fair queueing (GrFQ), a novel scheduler based on a greedy strategy of reducing the maximum difference in normalized service received by any two flows at each transmission boundary. We prove that the GrFQ scheduler achieves a better bound on the normalized lag than other known schedulers. We further propose a simplified version of the scheduler, called GrFQ-lite, which avoids the emulation of a fluid flow system and has a per-packet work complexity of O(1) in the computation of the timestamps. Borrowing from the field of economics, we use the Gini index as a measure of instantaneous fairness. Using real gateway traffic traces, we show that the GrFQ scheduler achieves better fairness than any other known scheduler at virtually all instants of time. We further show that the GrFQ-lite scheduler achieves equivalent or better fairness than other known schedulers including those that are significantly more computationally intensive in their emulation of the ideally fair fluid flow system.