A Dynamic Surface Gateway Placement Scheme for Mobile Underwater Networks.

Deployment of surface-level gateways holds potential as an effective method to alleviate high-propagation delays and high-error probability in an underwater wireless sensor network (UWSN). This promise comes from reducing distances to underwater nodes and using radio waves to forward information to a control station. In an UWSN, a dynamic energy efficient surface-level gateway deployment is required to cope with the mobility of underwater nodes while considering the remote and three-dimensional nature of marine space. In general, deployment problems are usually modeled as an optimization problem to satisfy multiple constraints given a set of parameters. One previously published static deployment optimization framework makes assumptions about network workload, routing, medium access control performance, and node mobility. However, in real underwater environments, all these parameters are dynamic. Therefore, the accuracy of performance estimates calculated through static UWSN deployment optimization framework tends to be limited by nature. This paper presents the Prediction-Assisted Dynamic Surface Gateway Placement (PADP) algorithm to maximize the coverage and minimize the average end-to-end delay of a mobile underwater sensor network over a specified period. PADP implements the Interacting Multiple Model (IMM) tracking scheme to predict the positions of sensor nodes. The deployment is determined based on both current and predicted positions of sensor nodes, which enables better coverage and shorter end-to-end delay. PADP uses a branch-and-cut approach to solve the optimization problem efficiently, and employs a disjoint-set data structure to ensure connectivity. Simulation results illustrate that PADP significantly outperforms a static gateway deployment scheme.

A hybrid path-oriented code assignment CDMA-based MAC protocol for underwater acoustic sensor networks.

Due to the characteristics of underwater acoustic channel, media access control (MAC) protocols designed for underwater acoustic sensor networks (UWASNs) are quite different from those for terrestrial wireless sensor networks. Moreover, in a sink-oriented network with event information generation in a sensor field and message forwarding to the sink hop-by-hop, the sensors near the sink have to transmit more packets than those far from the sink, and then a funneling effect occurs, which leads to packet congestion, collisions and losses, especially in UWASNs with long propagation delays. An improved CDMA-based MAC protocol, named path-oriented code assignment (POCA) CDMA MAC (POCA-CDMA-MAC), is proposed for UWASNs in this paper. In the proposed MAC protocol, both the round-robin method and CDMA technology are adopted to make the sink receive packets from multiple paths simultaneously. Since the number of paths for information gathering is much less than that of nodes, the length of the spreading code used in the POCA-CDMA-MAC protocol is shorter greatly than that used in the CDMA-based protocols with transmitter-oriented code assignment (TOCA) or receiver-oriented code assignment (ROCA). Simulation results show that the proposed POCA-CDMA-MAC protocol achieves a higher network throughput and a lower end-to-end delay compared to other CDMA-based MAC protocols.

[Application of near-infrared absorption spectrum scanning techniques in gas quantitative measurement].

A practical gas sensing system utilizing absorption spectrum scanning techniques was developed. Using the narrow-band transmission of a fiber tunable filter (TOF) and wavelength modulation technique, the so-called cross-sensing effects of the traditional spectrum absorption based gas sensor were reduced effectively and thus the target gas was detected sensitively and selectively. In order to reduce the effects of nonlinearity of TOF on the measurement results and improve the system stability in operation, the reflection spectrum of a reference FBG was monitored and employed to control the modulation region and center of TOF wavelength precisely. Moreover, a kind of weak signal detecting circuits was developed to detect the weak response signal of the system with high sensitivity. The properties of the proposed system were demonstrated experimentally by detection of acetylene. Approximate linear relationships between the system responses and the input acetylene concentrations were demonstrated by experiments. The minimum detectable acetylene of 5 x 10(-6), with signal-noise ratio of 3, was also achieved by experiments.