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- Linking acoustic communications and network performance. Integration and experimentation of an underwater acoustic networkPublication . Caiti, A.; Grythe, K.; Hovem, J. M.; Jesus, S. M.; Lie, A.; Munafò, Andrea; Reinen, Tor Arne; Silva, A.; Zabel, F.Underwater acoustic networks (UANs) are an emerging technology for a number of oceanic applications, ranging from oceanographic data collection to surveillance applications. However, their reliable usage in the field is still an open research problem, due to the challenges posed by the oceanic environment. The UAN project, a European-Union-funded initiative, moved along these lines, and it was one of the first cases of successful deployment of a mobile underwater sensor network integrated within a wide-area network, which included above water and underwater sensors. This contribution, together with a description of the underwater network, aims at evaluating the communication performance, and correlating the variation of the acoustic channel to the behavior of the entire network stack. Results are given based on the data collected during the UAN11 (May 2011, Trondheim Fjord area, Norway) sea trial. During the experimental activities, the network was in operation for five continuous days and was composed of up to four Fixed NOdes (FNOs), two autonomous underwater vehicles (AUVs), and one mobile node mounted on the supporting research vessel. Results from the experimentation at sea are reported in terms of channel impulse response (CIR) and signal-to-interference-plus-noise ratio (SINR) as measured by the acoustic modems during the sea tests. The performance of the upper network levels is measured in terms of round trip time (RTT) and probability of packet loss (PL). The analysis shows how the communication performance was dominated by variations in signal-to-noise ratio, and how this impacted the behavior of the whole network. Qualitative explanation of communication performance variations can be accounted, at least in the UAN11 experiment, by standard computation of the CIR and transmission loss estimate.
- Arrival-based equalizer for underwater communication systemsPublication . Ijaz, S.; Silva, A.; Jesus, S. M.One of the challenges in the present underwater acoustic communication systems is to combat the underwater channel effects which results in time and frequency spreading of the transmitted signal. The time spreading is caused by the multipath effect while the frequency spreading is due to the time variability of the underwater channel. The passive Time Reversal (pTR) equalizer has been used in underwater communications because of its time focusing property which minimizes the time spreading effect of the underwater channel. In order to compensate for the frequency spreading effect, an improved version of pTR was proposed, called Frequency shift passive time reversal (FSpTR). FSpTR tries to compensate for the frequency spreading by applying a frequency shift in the estimated channel impulse response (IR). In the multipath environment, multiple replicas of the transmitted signal reaches the receiver through different paths where each path is affected differently by environmental variations. In such cases, a single frequency shift fails to compensate for the environmental variations on each path, resulting in degradation in the performance. In this paper, an arrival-based equalizer is proposed to compensate for the environmental variations on each path. The concept of beamforming is integrated with FSpTR equalizer, in this paper, to compensate each arrival separately for the environmental variations. The proposed equalizer is tested with the real data and the results showed that the proposed approach outperforms pTR and FSpTR equalizers and provides a mean MSE gain of 4.9 dB and 4.2 dB respectively.
- Network nodes for ocean data exchange through submarine fiber optic cable repeatersPublication . Martins, M. S.; Cruz, Nuno A.; Silva, António; Ferreira, Bruno; Zabel, Fred; Matos, Tiago; Jesus, Sergio; Pinto, André; Pereira, Eduardo; Matos, Aníbal; Faria, Carlos; Tieppo, Marcos; Goncalves, L. M.; Rocha, João; Faria, JoãoAs humanity progresses and globalization advances, humanized environment and associated systems increase in complexity and size. In earth systems, oceans represent an essential element of equalization and normal functioning. Ocean-atmospheric interactions are nowadays believed to be at the heart of all earth vital signs and climatic behaviours, and therefore are essential to accurate monitoring and understanding of earth systems. The work presented is a preliminary result of the K2D- Knowledge and Data from the Deep to Space, project which addresses the challenge of creating underwater network nodes to provide power and communication to land through the submarine fiber optic cable repeaters. The N2ODE system will consist of a set of subsystems that will allow continuous monitoring and interaction with fixed and mobile underwater devices.
- Compensating for source depth change and observing surface waves using underwater communication signalsPublication . Ijaz, S.; Silva, A.; Jesus, S. M.Underwater communications impose great challenges due to the unpredictable changes in the environment. In order to accommodate for these changes, equalizers are used to track the Impulse Responses (IRs) and compensate the intersymbol interference (ISI). Source and array depth shifts are one of the major contributing factors to continuous amplitude and phase changes in IR. These changes magnify the problem of data processing in which correlation between successive received signals is involved, e.g. passive Time Reversal (pTR) where a probe signal is sent ahead of the data for post cross-correlation. In this paper, an environment based algorithm is used for pTR equalization, where an appropriate frequency shift of the estimated IR compensates for the geometric changes such as source and array depth shifts. We have applied this Frequency Shift pTR equalizer (FSpTR) on real data collected from Underwater Acoustic Barrier 2007 (UAB’07) sea trial having 1000 baud BPSK signaling at carrier frequency of 6.25 KHz with a sudden source depth change of 0.5 m at various known instants of time. The results illustrate that a considerable gain can be attained using the environment based equalizer in the presence of source and/or array depth changes. Moreover by close analysis of the FSpTR results we have detected surface wave motion through the frequency shifts caused by the array depth variations.
- OFDM demodulation in underwater time-reversed shortned channelsPublication . Gomes, João; Silva, A.; Jesus, S. M.This work addresses the problem of OFDM transmission in dispersive underwater channels where impulse responses lasting tens of miliseconds cannot be reliably handled by recently proposed methods due to limitations of channel estimation algorithms. The proposed approach relies on passive time reversal for multichannel combining of observed waveforms at an array of sensors prior to OFDM processing, which produces an equivalent channel with a shorter impulse response that can be handled much more easily. A method for tracking the narrowband residual phase variations of the channel after Doppler preprocessing is proposed. This is a variation of an existing technique that can improve the spectral efficiency of OFDM by reducing the need for pilot symbols. This work also examines techniques to handle sparse impulse responses and proposes a channel estimation method where an l1 norm is added to the standard least-squares cost function to transparently induce sparseness in the vector of channel coefficients. Algorithms are assessed using data collected during the UAB’07 experiment, which was conducted in Trondheim fjord, Norway, in September 2007. Data were transmitted with bandwidths of 1.5 and 4.5 kHz, and recorded at a range of about 800 m in a 16-hydrophone array. Significant multipath was observed over a period of at least 30 ms.
- A Demonstrator for future fiber-optic active SMART repeatersPublication . Cruz, Nuno A.; Silva, António; Zabel, Friedrich; Ferreira, Bruno; Jesus, Sergio; Martins, Marcos S.; Pereira, Eduardo; Matos, Tiago; Viegas, Ruben; Rocha, João; Faria, JoãoThe deep-sea environment still presents many challenges for systematic, comprehensive data acquisition. The current generation of SMART cables incorporates low-power sensors in long-range telecommunication cables to improve knowledge of ocean variables, aid in earthquake and tsunami warnings, and enhance coastal protection. The K2D Project seeks to expand SMART cables' capabilities by increasing the diversity of sensors along deep water cables, integrating active devices, and leveraging mobile platforms like deep-water AUVs, thereby improving spatial coverage and advancing ocean monitoring technology. This paper discusses a demonstration of these capabilities, focusing on the description of the main building blocks developed along the project, with results from a sea deployment in September 2023.