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1.
Hong Kwang Yeo Sharif B.S. Adams A.E. Hinton O.R. 《Oceanic Engineering, IEEE Journal of》2002,27(1):17-27
This paper presents multiple access interference cancellation techniques based on joint implementation of spatial-beamforming and multiuser detection strategies for coherent underwater acoustic communication network. Performances of adaptive multiuser detection strategies based on weighted parallel, successive, and recursive successive interference cancellation techniques are compared using experimental sea-trial data. Results show that the receiver structure adopting weighted recursive successive interference cancellation (RSIC) exhibits robustness in extracting useful data for weak users in the presence of co-channel interference from strong users. In addition, this type of structure simultaneously suppresses the summed interference effects contributed by weaker users toward the strongest user. The RSIC structure is therefore a potential candidate for multiple access interference suppression in coherent shallow water acoustic communication systems 相似文献
2.
High-throughout multiple-access communication networks are being considered for use in underwater acoustic channels. Bandwidth limitations of underwater acoustic channels require receivers to process broad-band communications signals in the presence of several active users. To deal with the resulting multiple-access interference in addition to high intersymbol interference, the spatial variability of ocean multipath is exploited in a multichannel multiuser receiver. Two configurations of such a receiver, a centralized and a decentralized one, are presented in fully adaptive modes of operations. While greatly reducing intersymbol and multiple-access interference, spatial diversity implies high increase in adaptive multiuser receiver complexity. To reduce the complexity of the optimal multichannel combiner, spatial structure of multipath is exploited. The complexity of resulting adaptive decentralized multichannel multiuser receiver is reduced at almost no cost in performance. Comparison of proposed multichannel receivers in an experimental shallow water channel demonstrates superior performance of spatial signal combining. The use of multiple input channels is shown to provide high level of tolerance for the near-far effect in both centralized and decentralized receivers. Decentralized receiver with reduced-complexity combining is found to satisfy the performance/complexity trade-off required for practical receiver realization in shallow water networks 相似文献
3.
《Oceanic Engineering, IEEE Journal of》1996,21(4):528-536
Adaptive decision feedback equalization (DFE) has recently been used to enable high-rate data transmission through shallow-water acoustic channels. This adaptive receiver successfully tracks and suppresses intersymbol interference due to a dispersive multipath channel. However, acoustic modems which are used for network applications must also contend with interference due to cochannel signals from proximal modems. In this work, we propose and evaluate a multiuser receiver with cochannel interference suppression. The advantages of this multiuser receiver in the presence of strong cochannel interference are shown by a performance comparison to a bank of the single-user DFEs described above. Conclusions are supported in part by the demodulation of experimental data for two simultaneous cochannel signals and by a steady-state performance analysis 相似文献
4.
An underwater acoustic local area network (ALAN) provides multipoint-to-point telemetry between many high-rate, ocean-bottom sensors and a central, surface-deployed receiver in the 10-30 kHz vertical acoustical channel. Ocean-bottom modems initiate the transmission process by requesting data channel time slots via a common narrow-band request channel. Request packets overlap in time and frequency in this channel, and the throughput and average transmission delay rely heavily on the successful resolution of the request packet collisions. This paper presents the design, analysis, and experimental demonstration of a request channel receiver capable of resolving collisions between several asynchronous and cochannel packets. The receiver algorithm differs from standard capture schemes (by demodulating the data from both strong and weak transmitters), conventional spread-spectrum receivers (by overcoming the near-far problem), and existing multiple-access demodulation techniques (by adapting to the number of interfering signals, and the unknown phase, Doppler, amplitude, and timing of each signal in the collision). The receiver demodulates the collided packets by decision-directed techniques through a novel method of estimating the interference for each user which minimizes error propagation due to inaccurate tentative decisions. An inwater experiment illustrates that this technique is extremely desirable for collision resolution in underwater acoustic local area networks, and also for underwater autonomous vehicles with both sidescan sonar as well as acoustic telemetry links 相似文献
5.
Reliable,with high data rate,acoustic communication in time-varying,multipath shallow water environment is a hot research topic recently.Passive time reversal communication has shown promising results in improvement of the system performance.In multiuser environment,the system performance is significantly degraded due to the interference among different users.Passive time reversal can reduce such interference by minimizing the cross-correlated version of channel impulse response among users,which can be realized by the well-separated users in depth.But this method also has its shortcomings,even with the absence of relative motion,the minimization sometimes may be impossible because of the time-varying environment.Therefore in order to avoid the limitation of minimizing the cross-correlated channel function,an approach of passive time reversal based on space-time block coding (STBC) is presented in this paper.In addition,a single channel equalizer is used as a post processing technique to reduce the residual symbol interference.Experimental results at 13 kHz with 2 kHz bandwidth demonstrate that this method has better performance to decrease bit error rate and improve signal to noise ratio,compared with passive time reversal alone or passive time reversal combined with equalization. 相似文献
6.
Merlina N. Andalecio 《Ocean & Coastal Management》2011,54(10):760-770
This paper examines an approach that integrates the judgment and perceptions of coastal resource users in prioritizing the criteria and indicators for fisheries management impact evaluation. Five criteria with corresponding indicators were identified and selected namely, acceptability, biotic diversity, economic performance, enforceability, and equity. The weights of importance of criteria and indicators were determined through the Analytic Hierarchy Process applied in a case study with 23 resource user groups (categorized as fishers, local government unit, non-government organization, private sector and women) in San Miguel Bay, Philippines.The results of this study revealed that overall, acceptability is the most important criterion in evaluating the impacts of fisheries management strategy in San Miguel Bay, especially for the local government units and fishers groups. The best measure of acceptability criterion are the indicators resource users participation in fisheries management process, level of awareness of resource users in fisheries resource management and number of fishers who belong to an organization. Ten groups have consistency ratios between 0.0 and 0.08 which are within the acceptable level of 0.10. Conversely, most groups (78%) have consistency ratios less than 0.26. Also, the results of the preference analysis are remarkable because among the representative groups, most fishers groups exhibited consistencies in their judgments (average consistency ratios of 0.06-0.08). Economic performance indicators were consistently judged across groups indicating that individuals present higher convergence of views toward economic objective. 相似文献
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8.
Although multiple-input multiple-output(MIMO) underwater acoustic(UWA) communication has been intensively investigated in the past years, existing works mainly focus on open-water environment. There is no work reporting MIMO acoustic communication in under-ice environment. This paper presents results from a recent MIMO acoustic communication experiment which was conducted in Bohai Gulf during winter. In this experiment, high frequency MIMO signals centered at 10 kHz were transmitted from a two-element source array to a four-element vertical receiving array at 1 km range. According to the received signal of different array elements, MIMO acoustic communication in under-ice environment suffers less effect from co-channel interference compared with that in open-water environment. In this paper, time reversal followed by a single channel decision feedback equalizer is used to process the experimental data. It is demonstrated that this simple receiver is capable of realizing robust performance using fewer hydrophones(i.e. 2) without the explicit use of complex co-channel interference cancelation algorithms, such as parallel interference cancelation or serial interference cancelation. 相似文献
9.
A large increase in the reliability of shipboard or stationary underwater acoustic telemetry systems is achievable by using spatially distributed receivers with aperture sizes from 0.35 to 20 m. Output from each receiver is assigned a quality measure based on the estimated error rate, and the data, weighted by the quality measure, are combined and decoded. The quality measure is derived from a Viterbi error-correction decoder operating on each receiver and is shown to perform reliability in a variety of non-Gaussian noise and jamming environments and reduce to the traditional optimal diversity system in a Gaussian environment. The dynamics of the quality estimator allow operation in the presence of high-power impulsive interference by exploiting the signal and noise differential travel times to individual sensors. The spatial coherence structure of the shallow water acoustic channel shows relatively low signal coherence at separations as short as 0.35 m. Increasing receiver spacing beyond 5 m offers additional benefits in the presence of impulsive noise and larger-scale inhomogeneities in the acoustic field. A number of data transmission experiments were carried out to demonstrate system performance in realistic underwater environments 相似文献
10.
The 60-Hz electromagnetic fields in the Atlantic Ocean off Montauk Point, NY, and from Block Island Sound to the harbor in New London, CT, have been measured to determine the levels of interference to an extremely low frequency (ELF) receiver. Such data are needed to specify how close to the coastline an ELF receiver can come before 60-Hz external interference will degrade system performance. The data indicate that degradation to the ELF receiver from 60-Hz external interference will not occur offshore, but may occur within the confines of New London Harbor. 相似文献
11.
一种基于真实身份的BBS系统实名保障技术 总被引：1，自引：0，他引：1
用户身份的真实性一直是BBS系统管理的重要方面之一。采用SAML协议,将跨域单点登录中的认证技术引入到BBS系统中,从而实现了用户采用真实身份登录BBS系统,保证了BBS用户的实名。并以多个高校的BBS系统作为样例,建立起基于真实身份的示范BBS系统。 相似文献
12.
The life duration of underwater cooperative network has been the hot topic in recent years. And the problem of node energy consuming is the key technology to maintain the energy balance among all nodes. To ensure energy efficiency of some special nodes and obtain a longer lifetime of the underwater cooperative network, this paper focuses on adopting precoding strategy to preprocess the signal at the transmitter and simplify the receiver structure. Meanwhile, it takes into account the presence of Doppler shifts and long feedback transmission delay in an underwater acoustic communication system. Precoding technique is applied based on channel prediction to realize energy saving and improve system performance. Different precoding methods are compared. Simulated results and experimental results show that the proposed scheme has a better performance, and it can provide a simple receiver and realize energy saving for some special nodes in a cooperative communication. 相似文献
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14.
The purpose of this paper is to develop a decision-feedback equalizer (DFE) using a fixed set of parameters applicable to most shallow oceans with minimal user supervision (i.e., a turn key system). This work is motivated by the superior performance [bit error rate (BER)] of the multichannel DFE compared with other methods, such as passive-phase conjugation (PPC), at the same time noting its sensitivity to different acoustic environments. The approach is to couple PPC, utilizing its adaptability to different environments, with a single-channel DFE. This coupling forms an optimal processor for acoustic communications in theory, but it has never been implemented in practice. By coupling with DFE, the method achieves the same spatial diversity as conventional multichannel DFE, without requiring a large number of receivers as does PPC. The correlation-based DFE in terms of the autocorrelation functions of the channel impulse responses summed over the receiver channels (the Q function) is derived. This paper shows in terms of waveguide physics, further supported by real data, the many desirable features of the Q function that suggest, given adequate sampling of the water column, a general applicability of the correlation-based equalizer to different environments, irrespective of the sound speed profiles, bottom properties, and source-receiver ranges/depths. This property can be expected to hold approximately for a small number of receivers with spatial diversity. This paper demonstrates the robustness of the new equalizer with moving source data despite the range change (which modifies the impulse response) and symbol phase change due to time-varying Doppler. 相似文献
15.
《Ocean & Coastal Management》2003,46(6-7):487-505
The European Commission supports the development of a European Virtual Coastal and Marine Data Warehouse called CoastBase that aims to improve data and information search and exchange. This paper discusses a study of user requirements and defines relevant and obtainable data and information within the CoastBase project. Thematic as well as technical considerations were taken into account in the study. CoastBase aims to satisfy a variety of users. Potential users interested in data and information on various aggregation levels are represented within the CoastBase consortium. They described their requirements, initially unguided but later guided by a few broad questions about their present work and about the potential role of CoastBase in facilitating it. The results were grouped and summarised. Institutional user groups were formed based on the focus of the organisations on European, regional, national, and local issues, respectively. Individual users were grouped according to their function as Decision & policy maker, Policy advisor & project manager, Researcher, or Database administrator & programmer, respectively. The study has shown that the individual user groups in particular differed in their preferences for aggregation level of the data and information to be extracted from CoastBase. Implications, in the form of envisioned dissimilar use of the CoastBase system are discussed. Both institutional and individual users had different preferences for topic and geographical area to be covered in the CoastBase prototype. However, the study provided a useful synthesis for discussion in plenary, and led to identification of two thematic scenarios for development of the prototype, which are Eutrophication in the North Sea, and Planning in the North Sea and Mediterranean coastal zone. In addition, a comprehensive list of requirements and recommendations for the technical development and evaluation of the system had been compiled from the user requirements. This list, which was approved by the consortium, is included in the paper. 相似文献
16.
For pt. I see ibid., vol. 25, no. 1, p. 62-72 (2000). A receiver for coherent communication through underwater communication channels is analyzed. The receiver performance and stability versus delay spread, Doppler spread, and signal-to-noise ratio is quantified. The stability is governed by the ill-conditioning of a correlation matrix estimate and it sets the limit on how many taps should be used for a channel with a given number of degrees of freedom. The receiver is used extensively on both simulated and real data that are Doppler spread, and good performance in these channels is verified 相似文献
17.
A combined assessment of beach occupancy and public perceptions of beach quality: A case study in the Costa Brava, Spain 总被引：1，自引：0，他引：1
Elisabet Roca Claudia Riera Míriam Villares Rosa Fragell Rosa Junyent 《Ocean & Coastal Management》2008,51(12):839-846
The main objective of this paper is to present an approach that contributes to evaluate beach quality based on a combined analysis of beach occupancy and the perception of beach users. The study area is a major Mediterranean tourist destination in NE Spain. Six beaches that fulfilled different environmental and social criteria were assessed. Sampling took place during 2004 and 2005. An analysis of beach occupancy showed that highly accessible urban beaches suffer the most from overcrowding. A comparison of the two approaches (occupancy and perception) indicates that a higher sand availability increased user satisfaction, although not as much as might have been expected because other parameters are also highly relevant in evaluating beach quality from the perspective of beach user such as physical characteristics, landscape and facilities. 相似文献
18.
Spatial processing, including beamforming and diversity combining, is widely used in communications to mitigate intersymbol interference (ISI) and signal fading caused by multipath propagation. Beamforming suppresses ISI (and noise) by eliminating multipath (and noise) arrivals outside the signal beam. Beamforming requires the signals to be highly coherent between the receivers. Diversity combining combats ISI as well as signal fading by taking advantage of the independent information in the signal. Classical (spatial) diversity requires that signals are independently fading, hence are (spatially) uncorrelated with each other. In the real world, the received signals are neither totally coherent nor totally uncorrelated. The available diversity is complex and not well understood. In this paper, we study the spatial processing gain (SPG) as a function of the number of receivers used, receiver separation, and array aperture based on experimental data, using beamforming and multichannel combining algorithms. We find that the output symbol signal-to-noise ratio (SNR) for a multichannel equalizer is predominantly determined by the array aperture divided by the signal coherence length, with a negligible dependence on the number of receivers used. For a given number of receivers, an optimal output symbol SNR (OSNR) is achieved by spacing the receivers equal to or greater than the signal coherence length. We model the SPG in decibels as the sum of the noise suppression gain (NSG, equivalent to signal-to-noise enhancement) and the ISI suppression gain (ISG, equivalent to signal-to-ISI enhancement) both expressed in decibels; the latter exploits the spatial diversity and forms the basis for the diversity gain. Data are interpreted using the modeled result as a guide. We discuss a beam-domain processor for sonar arrays, which yields an improved performance at low-input SNR compared to the element-domain processor because of the SNR enhancement from beamforming many sensors. 相似文献
19.
In this paper, we study the temporal resolution of a time-reversal or passive-phase conjugation process as applied to underwater acoustic communications. Specifically, we address 1) the time resolution or the pulse width of a back-propagated time-compressed pulse as compared with the original transmitted pulse; 2) the effectiveness of temporal focusing as measured by the peak-to-sidelobe ratio of the back-propagated or phase-conjugated pulse (both pulse elongation and sidelobe leakages are causes of intersymbol interference and bit errors for communications); 3) the duration of temporal focusing or the temporal coherence time of the underwater acoustic channel; and 4) the stability of temporal focusing as measured by the phase fluctuations of successive pulses (symbols). Binary phase-shift keying signals collected at sea from a fixed source to a fixed receiver are used to extract the above four parameters and are compared with simulated results. Mid-frequency (3-4-kHz) data were collected in a dynamic shallow-water environment, exhibiting high temporal fluctuations over a scale of minutes. Despite this, the channel is found to be highly coherent over a length of 17 s. As a result, only one probe signal is used for 17 s of data. The bit error rate and variance of the symbol phase fluctuations are measured as a function of the number of receivers. They are of the same order as that calculated from the simulated data. The agreement suggests that these two quantities could be modeled for a communication channel with high coherence time. The phase variance can be used to determine the maximum data rate for a phase-shift keying signal for a given signal bandwidth and a given number of receivers. 相似文献