Time Delay Estimation in Mobile Sensors for Underwater Networking

The time synchronization between any two sensor nodes in an Ad-hoc Underwater Sensor Networks (UWSNs) could be destroyed due to motion of these wireless sensors which induced Doppler shift. This synchronization obstacle can be sorted out by exploiting the mobility between sensor nodes. In the proposed system, the time delay between sensor nodes in both divergence and convergence scenarios are estimated based on estimating the time scaling factor. An improvement is introduced in terms of packet structure in order to challenge the channel effect and accurate estimation over the speed up to ±2 m/s. To verify the proposed system robustness, different levels of the nodes speeds have been considered in the simulation. Obtained results show that the proposed system is robust against severs channel conditions. Keywords: UWSNs, time delay, time synchronization

Underwater localization and node mobility estimation

In this paper, localizing a moving node in the context of underwater wireless sensor networks (UWSNs) is considered. Most existing algorithms have had designed to work with a static node in the networks. However, in practical case, the node is dynamic due to relative motion between the transmitter and receiver. The main idea is to record the time of arrival message (ToA) stamp and estimating the drift in the sampling frequency accordingly. It should be emphasized that, the channel conditions such as multipath and delay spread, and ambient noise is considered to make the system pragmatic. A joint prediction of the node mobility and speed are estimated based on the sampling frequency offset estimation. This sampling frequency offset drift is detected based on correlating an anticipated window in the orthogonal frequency division multiplexing (OFDM) of the received packet. The range and the distance of the mobile node is predicted from estimating the speed at the received packet and reused in the position estimation algorithm. The underwater acoustic channel is considered in this paper with 8 paths and maximum delay spread of 48 ms to simulate a pragmatic case. The performance is evaluated by adopting different nodes speeds in the simulation in two scenarios of expansion and compression. The results show that the proposed algorithm has a stable profile in the presence of severe channel conditions. Also, the result shows that the maximum speed that can be adopted in this algorithm is 9 km/h and the expansion case profile is more stable than the compression scenario. In addition, a comparison with a dynamic triangular algorithm (DTN) is presented in order to evaluate the proposed system

Robust OFDM Modem for a Swarm of Underwater Autonomous Vehicles

International audienc

Multibranch Autocorrelation Method for Doppler Estimation in Underwater Acoustic Channels

In underwater acoustic (UWA) communications, Doppler estimation is one of the major stages in a receiver. Two Doppler estimation methods are often used: the cross-ambiguity function (CAF) method and the single-branch autocorrelation (SBA) method. The former results in accurate estimation but with a high complexity, whereas the latter is less complicated but also less accurate. In this paper, we propose and investigate a multibranch autocorrelation (MBA) Doppler estimation method. The proposed method can be used in communication systems with periodically transmitted pilot signals or repetitive data transmission. For comparison of the Doppler estimation methods, we investigate an orthogonal frequency-division multiplexing (OFDM) communication system in multiple dynamic scenarios using the Waymark simulator, allowing virtual UWA signal transmission between moving transmitter and receiver. For the comparison, we also use the OFDM signals recorded in a sea trial. The comparison shows that the receiver with the proposed MBA Doppler estimation method outperforms the receiver with the SBA method and its detection performance is close to that of the receiver with the CAF method, but with a significantly lower complexity

Development of an acoustic communication link for micro underwater vehicles

PhD ThesisIn recent years there has been an increasing trend towards the use of Micro Remotely Operated Vehicles (μROVs), such as the Videoray and Seabotix LBV products, for a range of subsea applications, including environmental monitoring, harbour security, military surveillance and offshore inspection. A major operational limitation is the umbilical cable, which is traditionally used to supply power and communications to the vehicle. This tether has often been found to significantly restrict the agility of the vehicle or in extreme cases, result in entanglement with subsea structures. This thesis addresses the challenges associated with developing a reliable full-duplex wireless communications link aimed at tetherless operation of a μROV. Previous research has demonstrated the ability to support highly compressed video transmissions over several kilometres through shallow water channels with large range-depth ratios. However, the physical constraints of these platforms paired with the system cost requirements pose significant additional challenges. Firstly, the physical size/weight of transducers for the LF (8-16kHz) and MF (16-32kHz) bands would significantly affect the dynamics of the vehicle measuring less than 0.5m long. Therefore, this thesis explores the challenges associated with moving the operating frequency up to around 50kHz centre, along with the opportunities for increased data rate and tracking due to higher bandwidth. The typical operating radius of μROVs is less than 200m, in water < 100m deep, which gives rise to multipath channels characterised by long timespread and relatively sparse arrivals. Hence, the system must be optimised for performance in these conditions. The hardware costs of large multi-element receiver arrays are prohibitive when compared to the cost of the μROV platform. Additionally, the physical size of such arrays complicates deployment from small surface vessels. Although some recent developments in iterative equalisation and decoding structures have enhanced the performance of single element receivers, they are not found to be adequate in such channels. This work explores the optimum cost/performance trade-off in a combination of a micro beamforming array using a Bit Interleaved Coded Modulation with Iterative Decoding (BICM-ID) receiver structure. The highly dynamic nature of μROVs, with rapid acceleration/deceleration and complex thruster/wake effects, are also a significant challenge to reliable continuous communications. The thesis also explores how these effects can best be mitigated via advanced Doppler correction techniques, and adaptive coding and modulation via a simultaneous frequency multiplexed down link. In order to fully explore continuous adaptation of the transmitted signals, a real-time full-duplex communication system was constructed in hardware, utilising low cost components and a highly optimised PC based receiver structure. Rigorous testing, both in laboratory conditions and through extensive field trials, have enabled the author to explore the performance of the communication link on a vehicle carrying out typical operations and presenting a wide range of channel, noise, Doppler and transmission latency conditions. This has led to a comprehensive set of design recommendations for a reliable and cost effective link capable of continuous throughputs of >30 kbits/s