Browsing by Author "Ijaz, S."
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- Accounting for source depth variations in time-reversal communicationsPublication . Jesus, S. M.; Ijaz, S.; Silva, AntónioPassive Time Reversal (pTR) is a low complexity receiver scheme that uses multichannel probing for time signal refocusing, thus reducing time spreading and improving inter-symbol interference. Recognizing that signals travelling through different paths are subject to arrival-angle-related Doppler displacements, this letter proposes a further improvement to pTR that applies correcting frequency shifts optimized for beams formed along each path arrival angle. The proposed equalizer is tested with real data and the results show that the proposed approach outperforms both pTR and the modified pTR channel combiners providing an MSE gain of 4.9 dB and 4.2 dB, respectively.
- An environmental equalizer for underwater acoustic communications Tested at Hydralab IIIPublication . Silva, A.; Zabel, F.; Mertins, C.; Ijaz, S.; Jesus, S. M.It is known that small changes in source and receiver locations can cause significant changes in underwater acoustic channel impulse responses. At HYDRALAB III an underwater acoustic experiment was conducted to show that a source depth-shift causes a frequency-shift in the channel impulse response and that such behavior can be used to implement an environmental-based equalizer for underwater communications that compensates for the performance loss due to the source depth-shift.
- 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.
- 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.
- Doppler Domain Decomposition of the Underwater Acoustic Channel ResponsePublication . Ijaz, S.; Silva, A.; Rodríguez, O. C.; Jesus, S. M.Most underwater applications are very sensitive to environmental perturbations like source/receiver motion and surface variations. In real conditions, the transmitted signal reaches the receiver through different paths where each path is affected by different environmental perturbations. Due to the interaction with these environmental variations different Doppler is induced in each path. By observing the Doppler for the whole signal it is not possible to determine the contribution of each environmental variation. The main goal of this paper is to isolate different paths and analyse the effects of the environmental variations on each path in terms of the induced Doppler. A new technique called Time Windowed Doppler Spectrum is proposed which has been found very effective in tracking the Doppler due to each path separately. By using this technique it can be shown that it is possible to distinguish between surface reflected and direct path by observing the Doppler variations associated with each path. The surface induced Doppler was observed using this technique by analysing the temporal evolution of the surface reflected path. The surface variability effects the Doppler in terms of stretching and shortening the path between the transmitter and receiver. The data processed in this paper was acquired during CALCOMM'10 Experiment which took place in June 2010 at the south coast of Portugal.
- Measuring underwater noise with high endurance surface and underwater autonomous vehiclesPublication . Silva, A.; Matos, A.; Soares, C.; Alves, J. C.; Valente, J.; Zabel, F.; Cabral, H.; Abreu, N.; Cruz, N.; Almeida, R.; Ferreira, R. N.; Ijaz, S.; Lobo, V.This paper describes the results of AcousticRobot'13 - a noise measurement campaign that took place off the Portuguese Coast in May 2013, using two high endurance autonomous vehicles capable of silent operation (an underwater glider and an autonmomous sailing vessel) equipped with hydrophones, and a moored hydrophone that served as reference. We show that the autonomous vehicles used can provide useful measurements of underwater noise, and describe the main advantages and shortcomings that became evident during the campaign.
- Potential of vertical line array beamforming in underwater acoustic communicationsPublication . Ijaz, S.; Silva, A.; Jesus, S. M.Um dos principais tópicos atuais de pesquisa é ́ a concepçãoo de técnicas de processamento de sinal eficazes para comunicações submarinas. Este interesse é reforç ̧ado pelos desafios inerentes às condições incontroláveis, como a batimetria, a temperatura, e as variações temporais (geométricas) das profundidades da fonte e da antena de receptores. A obtenção de taxas de transmissão modestas é ainda um desafio, devido a estas variações. O presente trabalho aborda o uso potencial do formador de feixes (beamformer) no sistema de comunicações submarinas. O propósito de usar um beamformer neste trabalho é o de separar os diferentes caminhos que chegam do transmissor para o recetor, o que é motivado pelo fato de que, num ambiente real, não é possível distinguir todos os caminhos de propagação, no domínio do tempo. Neste trabalho, é usado o beamformer de atraso-e-soma, que aplica atrasos diferentes para cada hidrofone da antena vertical, e adiciona as saídas resultantes. Cada um dos atrasos é função do ângulo de chegada e da profundidade do hidrofone. O objetivo do beamformer é aplicar os atrasos opostos a cada elemento da antena vertical, para garantir a soma coerente da frente de onda numa direção particular. Ambos resultados com dados simulados e dados reais são apresentados neste trabalho, para mostrar o desempenho do beamformer. Os resultados do beamformer são comparados com os resultados da análise Doppler, para caracterizar chegadas diferentes e fazer corresponder essas chegadas às variações Doppler, mostrando que cada chegada é afetada pelas variações ambientais de uma maneira diferente, resultando numa quantidade diferente de Doppler.