4,129 research outputs found
Orbital Angular Momentum Waves: Generation, Detection and Emerging Applications
Orbital angular momentum (OAM) has aroused a widespread interest in many
fields, especially in telecommunications due to its potential for unleashing
new capacity in the severely congested spectrum of commercial communication
systems. Beams carrying OAM have a helical phase front and a field strength
with a singularity along the axial center, which can be used for information
transmission, imaging and particle manipulation. The number of orthogonal OAM
modes in a single beam is theoretically infinite and each mode is an element of
a complete orthogonal basis that can be employed for multiplexing different
signals, thus greatly improving the spectrum efficiency. In this paper, we
comprehensively summarize and compare the methods for generation and detection
of optical OAM, radio OAM and acoustic OAM. Then, we represent the applications
and technical challenges of OAM in communications, including free-space optical
communications, optical fiber communications, radio communications and acoustic
communications. To complete our survey, we also discuss the state of art of
particle manipulation and target imaging with OAM beams
Efficient use of space-time clustering for underwater acoustic communications
Underwater acoustic (UWA) communication channels are characterized by the spreading of received signals in space (direction of arrival) and in time (delay). The spread is often limited to a small number of space-time clusters. In this paper, the spacetime clustering is exploited in a proposed receiver designed for guard-free orthogonal frequency-division multiplexing (OFDM) with superimposed data and pilot signals. For separation of space clusters, the receiver utilizes a vertical linear array (VLA) of hydrophones, whereas for combining delay-spread signals within a space cluster, a time-domain equalizer is used. We compare a number of space-time processing techniques, including a proposed reduced-complexity spatial filter, and show that techniques exploiting the space-time clustering demonstrate an improved detection performance. The comparison is done using signals transmitted by a moving transducer, and recorded on a 14-element non-uniform VLA in sea trials at distances of 46 km and 105 km
Low probability of detection underwater acoustic communications
Low probability of detection (LPD) underwater acoustic communications are an essential
requirement for command and control of autonomous underwater vehicles (AUV) or
submarines performing covert missions, avoiding their detection while communicating.
Based on low power signals, these covert communications may also extend the autonomy
of battery-operated AUVs, and contribute to reducing the impacts of the environmental
noise level on marine life. The present thesis aims to develop LPD communications
based on modeled and real data from three shallow water experiments. Thus, a superimposed
training method has been proposed. A bitstream is created superimposing a
long probe to the message before transmission. Computationally simple, the algorithm
explores temporal diversity to increase the processing gain and uses a Wiener filter for
equalization. Experimental results presented bit-error rates (BER) < 10−2 for signal-tonoise
ratios (SNR) < −8 dB. To understand the effects of coastal upwelling phenomena
over low SNR communications, a study compares the acoustic propagation for different
sound speed profiles using a propagation model and analyzes data from the BioCom’19
experiment, performed off Cabo Frio Island (Brazil). Temporal and spatial coherence of
low power signals propagating in this harsh environment are estimated, and both a criterion
for multichannel combining and a double Wiener filter to improve equalization are
presented. Passive time-reversal (pTR) techniques have been widely employed for communications.
To address the pTR channel mismatch due to the environmental variability
between the probe and the data transmissions, this work proposes a superimposed training
pTR (STpTR) approach for single and multichannel systems. Despite the high noise
levels, varying from -5 to +6 dB, the STpTR combined with a Wiener filter achieved
BER < 10−2, for bit rates up to 220 bps. To improve covert communications for AUVs,
this work also presents a study about vector sensor multichannel combining. Using the
STpTR approach, results from an experiment on the coast of Algarve/Portugal indicate
that combining the pressure and particle velocity channels of a vector sensor may provide
an average SNR and mean squared-error gain of up to 9.4 and 3.1 dB, respectively,
compared to the pressure channel. Therefore, a better understanding of the environment
combined with the superimposed training pTR using a vector sensor may improve the
LPD communication system’s performance and robustness while keeping covertness.No ´últimos anos, os trabalhos de investigação sobre comunicações acústicas submarinas
com baixa probabilidade de deteção (BPD) tem sido incentivados pela indústria, pelos
governos, e pela própria academia em razão de suas m´múltiplas aplicações. Na ´área militar, as
comunicações BPD permitem que submarinos e veículos autónomos possam se comunicar sem
serem detectados. Na ´área civil, permitem a economia de energia de sensores alimentados
por baterias, aumentando o tempo de funcionamento, bem como contribui para reduzir os
impactos sobre a vida marinha causados pelos altos n´ıveis de ru´ıdo submarino, entre outras
aplica¸c˜oes. Neste contexto, esta tese pretende desenvolver comunica¸c˜oes BPD utilizando um
modelo de propaga¸c˜ao ac´ustica e dados obtidos a partir de trˆes experimentos em ´aguas rasas.
Este trabalho apresenta um m´etodo de treinamento superposto para comunica¸c˜oes submarinas
em um ambiente com baixa rela¸c˜ao sinal/ru´ıdo, e demonstra sua aplica¸c˜ao para
comunica¸c˜oes BPD. Computacionalmente simples, o m´etodo foi desenvolvido para funcionar
com um ´único projetor acústico, transmitindo com baixa potência, e um hidrofone, sem o
ganho de um arranjo de sensores distribuídos no espaço. Antes da transmissão, uma longa
sequência de comprimento m´máximo ´e somada `a mensagem para efeitos de equalização e sincronismo.
Os dois sinais são binários, modulados em fase e possuem 2047 bits. Porém,
possuem amplitudes diferentes. Na realidade, a amplitude do sinal de treinamento ´e ligeiramente
superior `a da mensagem. Em um ambiente com baixa rela¸c˜ao sinal ru´ıdo, um sinal de
treinamento mais forte permite ocultar a mensagem a ser transmitida, bem como melhorar
o ganho para a estima¸c˜ao da resposta impulsiva e para a sincroniza¸c˜ao do sistema. A mensagem
´e composta por 3 bits nulos e 4 sequˆencias de 511 bits. Delimitados por uma curta
sequˆencia de comprimento m´aximo de 31 bits, para dupla sincroniza¸c˜ao, os pacotes de dados
possuem 480 bits e transportam o seguinte pangrama: (The Quick Brown Fox Jumps Over
the Lazy Dog 0123456789!@#$). O m´etodo explora a diversidade temporal do canal, transmitindo
a mesma sequˆencia diversas vezes para aumentar o ganho de processamento do sinal
e implementar a corre¸c˜ao de erros atrav´es da soma coerente dos sinais. A resposta impulsiva
do canal ´e estimada pela transformada r´apida de Hadamard, e a equaliza¸c˜ao do sinal ´e feita
por um filtro de Wiener. A remo¸c˜ao da interferˆencia causada pelo sinal de treinamento ´e
realizada pelo m´etodo “hyperslice cancellation by coordinate zeroing (HCC0)”, e a seguir a
mensagem ´e decodificada. Resultados obtidos a partir de um experimento em ´aguas rasas,
utilizando uma fonte e um ´único hidrofone, apresentaram taxas de erro de bit menores que
10−2, para relações sinal/ruído inferiores a −8 dB.
A ressurgência costeira ´e um fenómeno oceanográfico dinâmico que modifica, profundamente,
a estratificação de temperatura do oceano, influenciando diretamente na propagação
acústica. Por outro lado, os crescentes n´níveis de ruído antropogénico não apenas reduzem
o desempenho dos sistema de comunicação, corrompendo a informação transmitida, mas tamb´em afetam a vida marinha. Para compreender os efeitos da ressurgˆencia costeira sobre
as comunica¸c˜oes com baixa rela¸c˜ao sinal/ru´ıdo, um estudo analisa os dados do experimento
BioCom’19, realizado nas proximidades da Ilha do Cabo Frio, Rio de Janeiro (Brasil). As
respostas impulsivas do canal e a propaga¸c˜ao ac´ustica, para diferentes perfis de velocidade do
som, foram estimadas usando o modelo de propaga¸c˜ao ac´ustica “Monterey-Miami Parabolic
Equation model (MMPE)”. Al´em disso, o desempenho do sistema de comunica¸c˜oes foi correlacionado
com os perfis de temperatura durante o experimento. Os resultados obtidos
indicam uma significativa redu¸c˜ao da energia ac´ustica nos receptores durante a ressurgˆencia,
degradando o desempenho do sistema.
A coerˆencia temporal e espacial dos sinais de baixa potˆencia transmitidos no experimento
BioCom’19 foi estimada, e um crit´erio para combina¸c˜ao dos sinais, provenientes dos m´ultiplos
hidrofones, foi proposto. Utilizando dados de um arranjo piramidal e um arranjo vertical
linear, de 4 hidrofones cada, a coerˆencia foi estimada antes e depois do filtro de Wiener
para mostrar o impacto do multicaminhamento sobre a taxa de erro de bit. Os resultados
mostram que a coˆerencia temporal pode ser utilizada como crit´erio para combinar sinais
consecutivos em um mesmo canal, enquanto a diversidade espacial permite a combina¸c˜ao de
m´ultiplos canais do arranjo de sensores. Sequˆencias cuja coerˆencia temporal esteja acima
de um limite pr´e-definido s˜ao somadas. A coerˆencia espacial entre canais foi estimada e
comparada em termos da taxa de erro de bit. Para diferentes taxas de transmiss˜ao, as taxas
de erro de bit est˜ao em concordˆancia com a evoluc˜ao da coerˆencia espacial. Quanto mais
elevada a coerˆencia, melhor o desempenho e menor a taxa de erro de bit.
Um duplo filtro de Wiener para melhorar a equaliza¸c˜ao dos sinais de baixa potˆencia,
durante a ressurgˆencia, tambem foi proposto. Utilizando dados dos 4 hidrofones de um
arranjo piramidal, as respostas impulsivas foram estimadas para observar a variabilidade
das condi¸c˜oes de propaga¸c˜ao. Em uma condi¸c˜ao de perfil de temperatura isot´ermico, as
respostas impulsivas apresentaram multicaminhamento curto, com chegadas mais fortes nos
receptores. `A medida que a ressurgˆencia ocorria, foram observadas quedas abruptas de
temperatura superiores a 10◦C, na posi¸c˜ao dos hidrofones, acarretando uma forte refra¸c˜ao da
onda sonora para o fundo marinho. Em consequˆencia, sinais mais fracos foram observados
nos hidrofones. Os resultados obtidos com dados do BioCom’19 mostram que, para uma
rela¸c˜ao sinal/ru´ıdo variando entre −3.9 e 7.3 dB, o duplo filtro de Wiener forneceu um
ganho do erro m´edio quadr´atico de at´e 2.8 dB, comparado com o filtro de Wiener simples.
As t´ecnicas de tempo reverso passivo (TRP) tem sido amplamente empregadas nas
comunica¸c˜oes submarinas. Por´em, as r´apidas altera¸c˜oes das condi¸c˜oes de propaga¸c˜ao em
canais submarinos variantes no tempo, durante as transmiss˜oes da sequˆencia de treinamento
e da mensagem, degradam o desempenho das t´ecnicas TRP de equaliza¸c˜ao. Assim, esse
trabalho prop˜oe um m´etodo de TRP utilizando treinamento superposto, para sistemas com
um ou m´ultiplos sensores. O m´etodo proposto utiliza uma sequˆencia de treinamento, com
o efeito Doppler corrigido, para estimar o canal e realizar o TRP. O m´etodo compara 3
differentes estrat´egias para melhorar a performance do sistema de comunica¸c˜ao: a diversidade
temporal devido aos sinais idˆenticos transmitidos continuamente, a diversidade espacial
devido aos 2 arranjos de hidrofones, piramidal e linear, com 4 hidrofones cada um, al´em da
combina¸c˜ao dos 2 arranjos (8 hidrofones). Neste m´etodo, a t´ecnica de TRP com treinamento
superposto minimiza o multicaminhamento e realiza a corre¸c˜ao de erros atrav´es da soma
coerente dos diferentes canais. A interferˆencia intersimb´olica residual ´e removida pelo filtro
de Wiener. Resultados obtidos com dados do experimento BioCom’19 mostram que o m´etodo
proposto pode fornecer um ganho do erro médio quadrático de até 1.62 dB para canais independentes, e 3.13 dB, para canais combinados, comparativamente ao m´etodo sem o TRP,
usando apenas o filtro de Wiener. Neste contexto, o m´etodo de TRP utilizando treinamento
superposto alcan¸cou taxas de erro de bit < 10−2 para uma rela¸c˜ao sinal ru´ıdo, na banda de
transmiss˜ao, variando entre −5 a +6 dB.
Focado em comunica¸c˜oes com baixa probabilidade de detec¸c˜ao para pequenos ve´ıculos
submarinos, este trabalho tamb´em apresenta um estudo sobre a combina¸c˜ao dos canais
de press˜ao e velocidade de part´ıcula dos sensores vetoriais. Os sensores vetoriais possuem
pequenas dimens˜oes, adequadas `a utilizac˜ao em ve´ıculos autˆonomos, e permitem obter um
ganho de diversidade para as comunica¸c˜oes BPD. Para testar o m´etodo de treinamento
superposto com o tempo reverso passivo utilizando sensores vetoriais, um experimento foi
realizado, em ´aguas rasas, na costa do Algarve/Portugal. Para reduzir a rela¸c˜ao sinal/ru´ıdo
para uma faixa de 0 a −10 dB, foi adicionado ru´ıdo gravado no experimento. Os resultados
experimentais indicam que a combina¸c˜ao dos canais podem fornecer um ganho da rela¸c˜ao
sinal/ru´ıdo e do erro m´edio quadr´atico de at´e 9.4 e 3.1 dB, respectivamente, comparados
com os resultados do sensor de press˜ao.
Portanto, as principais contribui¸c˜oes dessa tese s˜ao (i) a proposta do m´etodo de treinamento
superposto para comunica¸c˜oes com baixa probabilidade de detec¸c˜ao, (ii) a compreens
˜ao dos efeitos da ressurgˆencia costeira sobre as comunica¸c˜oes, e seus impactos sobre a
coerˆencia temporal e espacial, (iii) o m´etodo de treinamento superposto em conjunto com o
tempo reverso passivo para lidar com a varia¸c˜ao do canal entre o tempo de recep¸c˜ao do sinal
de treinamento e da mensagem, e (iv) a combina¸c˜ao dos m´ultiplos canais dos sensores vetoriais
para comunica¸c˜oes BPD. Dessa forma, uma melhor compreens˜ao do canal submarino
e a utiliza¸c˜ao dos m´etodos propostos combinados com sensores do estado da arte, como os
sensores vetoriais, se configura como um avan¸co neste campo do conhecimento, permitindo
aumentar a robustez do sistema BPD, bem como reduzir a probabilidade de detec¸c˜ao, mantendo
a ocultação das comunicações.I am also grateful for the sponsorship provided by the Brazilian Navy through the Postgraduate
Study Abroad Program, Grant No. Port.227/MB/2019
Low-complexity iterative receiver algorithms for multiple-input multiple-output underwater wireless communications
This dissertation proposes three low-complexity iterative receiver algorithms for multiple-input multiple-output (MIMO) underwater acoustic (UWA) communications. First is a bidirectional soft-decision feedback Turbo equalizer (Bi-SDFE) which harvests the time-reverse diversity in severe multipath MIMO channels. The Bi-SDFE outperforms the original soft-decision feedback Turbo equalizer (SDFE) while keeping its total computational complexity similar to that of the SDFE. Second, this dissertation proposes an efficient direct adaptation Turbo equalizer for MIMO UWA communications. Benefiting from the usage of soft-decision reference symbols for parameter adaptation as well as the iterative processing inside the adaptive equalizer, the proposed algorithm is efficient in four aspects: robust performance in tough channels, high spectral efficiency with short training overhead, time efficient with fast convergence and low complexity in hardware implementation. Third, a frequency-domain soft-decision block iterative equalizer combined with iterative channel estimation is proposed for the uncoded single carrier MIMO systems with high data efficiency. All the three new algorithms are evaluated by data recorded in real world ocean experiment or pool experiment. Finally, this dissertation also compares several Turbo equalizers in single-input single-output (SISO) UWA channels. Experimental results show that the channel estimation based Turbo equalizers are robust in SISO underwater transmission under harsh channel conditions --Abstract, page iv
Algorithms for propagation-aware underwater ranging and localization
Mención Internacional en el título de doctorWhile oceans occupy most of our planet, their exploration and conservation are one of
the crucial research problems of modern time. Underwater localization stands among the
key issues on the way to the proper inspection and monitoring of this significant part of our
world. In this thesis, we investigate and tackle different challenges related to underwater
ranging and localization. In particular, we focus on algorithms that consider underwater
acoustic channel properties. This group of algorithms utilizes additional information
about the environment and its impact on acoustic signal propagation, in order to improve
the accuracy of location estimates, or to achieve a reduced complexity, or a reduced
amount of resources (e.g., anchor nodes) compared to traditional algorithms.
First, we tackle the problem of passive range estimation using the differences in the
times of arrival of multipath replicas of a transmitted acoustic signal. This is a costand
energy- effective algorithm that can be used for the localization of autonomous
underwater vehicles (AUVs), and utilizes information about signal propagation. We study
the accuracy of this method in the simplified case of constant sound speed profile (SSP)
and compare it to a more realistic case with various non-constant SSP. We also propose
an auxiliary quantity called effective sound speed. This quantity, when modeling acoustic
propagation via ray models, takes into account the difference between rectilinear and
non-rectilinear sound ray paths. According to our evaluation, this offers improved range
estimation results with respect to standard algorithms that consider the actual value of
the speed of sound.
We then propose an algorithm suitable for the non-invasive tracking of AUVs or
vocalizing marine animals, using only a single receiver. This algorithm evaluates the
underwater acoustic channel impulse response differences induced by a diverse sea
bottom profile, and proposes a computationally- and energy-efficient solution for passive
localization.
Finally, we propose another algorithm to solve the issue of 3D acoustic localization
and tracking of marine fauna. To reach the expected degree of accuracy, more sensors
are often required than are available in typical commercial off-the-shelf (COTS) phased
arrays found, e.g., in ultra short baseline (USBL) systems. Direct combination of multiple
COTS arrays may be constrained by array body elements, and lead to breaking the optimal array element spacing, or the desired array layout. Thus, the application of
state-of-the-art direction of arrival (DoA) estimation algorithms may not be possible. We
propose a solution for passive 3D localization and tracking using a wideband acoustic
array of arbitrary shape, and validate the algorithm in multiple experiments, involving
both active and passive targets.Part of the research in this thesis has been supported by the EU H2020 program under
project SYMBIOSIS (G.A. no. 773753).This work has been supported by IMDEA Networks InstitutePrograma de Doctorado en Ingeniería Telemática por la Universidad Carlos III de MadridPresidente: Paul Daniel Mitchell.- Secretario: Antonio Fernández Anta.- Vocal: Santiago Zazo Bell
Performance of Spatial Diversity DCO-OFDM in a Weak Turbulence Underwater Visible Light Communication Channel
The performance of underwater visible light communication (UVLC) system is severely affected by absorption, scattering and turbulence. In this article, we study the performance of spectral efficient DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) in combination with the transceiver spatial diversity in turbulence channel. Based on the approximation of the weighted sum of lognormal random variables (RVs), we derived a theoretical exact bit error rate (BER) for DCO-OFDM systems with spatial diversity. The simulation results are compared with the analytical prediction, confirming the validity of the analysis. It is shown that spatial diversity can effectively reduce the turbulence-induced channel fading. The obtained results can be useful for designing, predicting, and evaluating the DCO-OFDM UVLC system in a weak oceanic turbulence condition
Multicarrier communication over underwater acoustic channels with nonuniform Doppler shifts
Author Posting. © IEEE, 2008. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 33 (2008): 198-209, doi:10.1109/JOE.2008.920471.Underwater acoustic (UWA) channels are wideband in nature due to the small ratio of the carrier frequency to the signal bandwidth, which introduces frequency-dependent Doppler shifts. In this paper, we treat the channel as having a common Doppler scaling factor on all propagation paths, and propose a two-step approach to mitigating the Doppler effect: 1) nonuniform Doppler compensation via resampling that converts a "wideband" problem into a "narrowband" problem and 2) high-resolution uniform compensation of the residual Doppler. We focus on zero-padded orthogonal frequency-division multiplexing (OFDM) to minimize the transmission power. Null subcarriers are used to facilitate Doppler compensation, and pilot subcarriers are used for channel estimation. The receiver is based on block-by-block processing, and does not rely on channel dependence across OFDM blocks; thus, it is suitable for fast-varying UWA channels. The data from two shallow-water experiments near Woods Hole, MA, are used to demonstrate the receiver performance. Excellent performance results are obtained even when the transmitter and the receiver are moving at a relative speed of up to 10 kn, at which the Doppler shifts are greater than the OFDM subcarrier spacing. These results suggest that OFDM is a viable option for high-rate communications over wideband UWA channels with nonuniform Doppler shifts.B.
Li and S. Zhou are supported by the ONR YIP grant N00014-07-1-0805
and the NSF grant ECCS-0725562. M. Stojanovic is supported by the ONR
grant N00014-07-1-0202. L. Freitag is supported by the ONR grants N00014-
02-6-0201 and N00014-07-1-0229. P. Willett is supported by the ONR
grant N00014-07-1-0055
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