LADAR detection statistics in the presence of pointing errors

The probability of detection of optically rough targets with pulsed LADAR systems that use direct detection is considered. It is assumed that the LADAR operates under conditions of both unintentional pointing offset bias (i.e., bore-sight error) andjitter. Under these conditions the probabilities of detection of targets in both the near field and the far field of the collecting aperture (i.e., for resolved, partially resolved, and unresolved targets) and for both large and small photoelectron counts are derived, and in many cases of practical interest accurate, elementary analytic approximations that are useful for parametric system studies are obtained. A number of technical references are appended, in which some of the key results are derived. In particular, an interesting new mathematical result involving the complementary incomplete gamma function and an analytic expression for the probability distribution function of a signal photoelectron count obeying BoseEinstein statistics (such as that arising from unresolved targets) immersed in Poisson noise is derived.     

Performance improvement of optical wireless communication through fog with a decision feedback equalizer

Optical wireless communication (OWC) systems use the atmosphere as a propagation medium. However, a common problem is that from time to time moderate cloud and fog emerge between the receiver and the transmitter. These adverse weather conditions impose temporal broadening and power loss on the optical signal, which reduces the digital signal-to-noise ratio (DSNR), produces significant intersymbol interference (ISI), and degrades the communication system's bit error rate (BER) and throughput. We propose and investigate the use of a combined adaptive bandwidth mechanism and decision feedback equalizer (DFE) to mitigate these atmospheric multipath effects. Based on theoretical analysis and simulations of DSNR penalties, BER, and optimum system bandwidths, we show that a DFE improves the outdoor OWC system immunity to ISI in foggy weather while maintaining high throughput and desired low BER.     

Average BER performance of FSO SIM-QAM systems in the presence of atmospheric turbulence and pointing errors

This paper presents the exact average bit error rate (BER) analysis of the free-space optical system employing subcarrier intensity modulation (SIM) with Gray-coded quadrature amplitude modulation (QAM). The intensity fluctuations of the received optical signal are caused by the path loss, atmospheric turbulence and pointing errors. The exact closed-form analytical expressions for the average BER are derived assuming the SIM-QAM with arbitrary constellation size in the presence of the Gamma–Gamma scintillation. The simple approximate average BER expressions are also provided, considering only the dominant term in the finite summations of obtained expressions. Derived expressions are reduced to the special case when optical signal transmission is affected only by the atmospheric turbulence. Numerical results are presented in order to illustrate usefulness of the derived expressions and also to give insights into the effects of different modulation, channel and receiver parameters on the average BER performance. The results show that the misalignment between the transmitter laser and receiver detector has the strong effect on the average BER value, especially in the range of the high values of the average electrical signal-to-noise ratio.     

Performance study of terrestrial multi-hop OFDM FSO communication systems with pointing errors over turbulence channels

Abstract

The free-space optical communication systems attract significant research and commercial interest the last few years, due to their high performance and reliability characteristics along with their, relatively, low installation and operational cost. Moreover, due to the fact that these systems are using the atmosphere as propagation path, their performance is varying according to its characteristics. Here, we present the performance analysis of a serially relayed radio-on-free-space-optical (RoFSO) communication system which employs the orthogonal frequency division multiplexing technique, with a quadrature amplitude modulation scheme, over atmospheric turbulence channels modelled by either the Gamma–Gamma or the Gamma distribution model. For this RoFSO communication link, we derive closed-form mathematical expressions for the estimation of its average bit error rate and outage probability, taking into account the relays’ number, the atmospheric turbulence and the pointing errors effect. Furthermore, for realistic parameter values, numerical results are presented using the derived mathematical expressions, which are verified through the corresponding numerical simulations.     

Evaluation of coherence interference in optical wireless communication through multiscattering channels

Optical wireless communication has been the subject of much research in recent years because of the increasing interest in laser satellite-ground links and urban optical wireless communication. The major sources of performance degradation have been identified as the spatial, angular, and temporal spread of the propagating beam when the propagation channel is multiscattering, resulting in reduced power reception and intersignal interference, as well as turbulence-induced scintillations and noise due to receiver circuitry and background illumination. However, coherence effects due to multipath interference caused by a scattering propagation channel do not appear to have been treated in detail in the scientific literature. We attempt a theoretical analysis of coherence interference in optical wireless communication through scattering channels and try to quantify the resultant performance degradation for different media. We conclude that coherence interference is discernible in optical wireless communication through scattering channels and is highly dependent on the microscopic nature of the propagation medium.     

Power measurement in digital wireless communication systems through parametric spectral estimation

Power measurement in digital wireless communication systems often suffers from poor repeatability, usually accompanied by a low accuracy. To face the problem, the use of parametric spectral estimators is investigated in this paper. In particular, a new method is proposed, which first estimates the power spectral density (PSD) of the analyzed signal through Burg's solution, and then evaluates the power by applying straightforward measurement algorithms to the estimated PSD. The results of a number of experiments, carried out on both laboratory and actual signals peculiar to digital wireless communication systems, assess the efficacy and reliability of the method. Moreover, a comparison of the achieved performance to that offered by an alternative measurement solution, already proposed by the authors and based on nonparametric PSD estimation, shows that the method allows for a significant reduction of measurement time, while exhibiting the same repeatability.     

Estimation of laser beam pointing parameters in the presence of atmospheric turbulence

The problem of estimating mechanical boresight and jitter performance of a laser pointing system in the presence of atmospheric turbulence is considered. A novel estimator based on maximizing an average probability density function (pdf) of the received signal is presented. The proposed estimator uses a Gaussian far-field mean irradiance profile, and the irradiance pdf is assumed to be lognormal. The estimates are obtained using a sequence of return signal values from the intended target. Alternatively, one can think of the estimates being made by a cooperative target using the received signal samples directly. The estimator does not require sample-to-sample atmospheric turbulence parameter information. The approach is evaluated using wave optics simulation for both weak and strong turbulence conditions. Our results show that very good boresight and jitter estimation performance can be obtained under the weak turbulence regime. We also propose a novel technique to include the effect of very low received intensity values that cannot be measured well by the receiving device. The proposed technique provides significant improvement over a conventional approach where such samples are simply ignored. Since our method is derived from the lognormal irradiance pdf, the performance under strong turbulence is degraded. However, the ideas can be extended with appropriate pdf models to obtain more accurate results under strong turbulence conditions.     

Modified moment-matching method for estimating pointing parameters in the presence of atmospheric turbulence

An accurate pointing system is required in free-space optical (FSO) communication links. Low energy-transmission efficiency caused by pointing errors would decline the communication system's performance. The statistics of the detected signal or return signal values could be used to estimate the pointing parameters, whereas atmospheric turbulence brings in serious challenges. A modified moment-matching estimation method is presented in this paper. The irradiance fluctuation caused by the atmospheric turbulence is considered, and the probability density function (PDF) in a weak turbulence condition is assumed to be lognormal. This modified approach is evaluated with wave-propagation simulation data and shows significant improvement over the conventional approach. The estimation accuracy and the properties of this new approach are also discussed. Although our method is based on lognormal irradiance PDF under a weak turbulence condition, the irradiance PDF would tend to be lognormal with aperture averaging effect under moderate to strong turbulence, and the ideas can be extended with appropriate PDF models to satisfy different conditions.     

Interferometer errors due to the presence of fringes

Phase-shifting interferometry permits analysis of complex interferograms. However, the measurement accuracy is reduced as the number of fringes is increased. The wave-front from a defocused spherical surface is used to demonstrate this degradation for several different transmission reference objectives.     

Process data reconciliation in the presence of non-uniform measurement errors

The goal of flow rates reconciliation was to adjust measured values and estimate unmeasured streams so as to balance both measured and unmeasured values, identify gross errors and detect leaks and losses. Thus, data reconciliation plays a key role in the monitoring of industrial plants for the early detection of critical events which might cause environmental and economic damages, and it is,therefore, an essential component of any clean technology process. Consequently, any method that improves the accuracy of the reconstructed data by considering more realistic assumptions on the statistical nature of the data can add considerably to the overall reliability of the process. The reconciliation procedure is statistical in nature and requires adequate information on the structure of the random errors of the flow rates measured. A frequent assumption is the homoscedasticity and the independence of the errors affecting different streams. This assumption leads to efficient algorithms based on advanced linear algebra decompositions, such as QR or Singular Value Decomposition, but it frequently leads to biased estimates, especially when the values of flow rates vary over two or more orders of magnitude. The goal of this article was to show the importance of considering general heteroscedasticity when reconciling flow rates. Errors are supposed to be normally distributed according to \(\varepsilon_{\text{i}} \cong N(0,\,\sigma_{0} L_{\text{i}}^{ 2\eta } )\) , where \(L_{\text{i}}\) is the measurement of the ith flow rate and \(\theta\)  =  \(\left\{ {\theta |\sigma_{0} ,\eta } \right\}\) is a set of two parameters to be estimated along with the adjustments to the measured flow rates. Therefore, the overall variance–covariance is characterised by 3 parameters \(\sigma_{0} ,\eta\) and the correlation factor among measurement errors ρ. The algorithm here proposed is based on conditional optimality, and it carries out the whole optimisation in terms of the parameters \(\theta\) only, the unknown adjustments being expressed at each iteration as functions of \(\theta\) .     

Linear profiles monitoring in the presence of nonnormal random errors

Profile monitoring is a technique to test the stability of the relationship between a response variable and explanatory variables over time. The most relevant linear profile monitoring methods have been constructed using the normality assumption. However, the normality assumption could be violated in many quality control applications. In this study, we consider a situation in which the random errors in a linear profile model follow a skew‐normal distribution. The skew‐normal distribution is a generalized version of the normal distribution. A new Shewhart‐type chart and exponentially weighted moving average (EWMA) chart, named the Shewhart^R and EWMA^R charts, respectively, are constructed based on residuals to monitor the parameters of linear profile model. The simulation results show that the multivariate EWMA chart is sensitive to the normality assumption and that the proposed Shewhart^R and EWMA^R charts have good ability to detect big and small‐to‐moderate process shifts, respectively. An example using photo mask techniques in semiconductor manufacturing is provided to illustrate the applications of the Shewhart^R and EWMA^R charts.     

Experimental comparison of performance degradation from terahertz and infrared wireless links in fog

We describe a lab setup for analyzing impairments of terahertz (THz) and infrared (IR) free space links caused by local refraction index changes in the signal's propagation paths that could be induced by turbulence, particles, humidity, etc. A THz signal comprising a 2.5 Gb/s data load modulated on a carrier at 625 GHz, is launched through a weather emulating chamber, detected, and its performance analyzed. An IR beam at 1.5 um wavelength carrying the same data load is superposed with the THz beam, propagating through the same weather conditions and also performance analyzed. We modulate the IR channel with a usual non-return-to-zero (NRZ) format but use duobinary coding for driving our THz source, which enables signaling at high data rate and higher output power. As both beams pass through the same channel perturbations and as their degradations are recorded simultaneously we can simultaneously compare the weather impact on both. We investigate scintillation and fog attenuation effects for the THz and IR signals by measuring bit error rates (BER), signal power, and phase front distortions.     

Optical tomography in the presence of void regions

There is a growing interest in the use of near-infrared spectroscopy for the noninvasive determination of the oxygenation level within biological tissue. Stemming from this application, there has been further research in the use of this technique for obtaining tomographic images of the neonatal head, with the view of determining the levels of oxygenated and deoxygenated blood within the brain. Owing to computational complexity, methods used for numerical modeling of photon transfer within tissue have usually been limited to the diffusion approximation of the Boltzmann transport equation. The diffusion approximation, however, is not valid in regions of low scatter, such as the cerebrospinal fluid. Methods have been proposed for dealing with nonscattering regions within diffusing materials through the use of a radiosity-diffusion model. Currently, this new model assumes prior knowledge of the void region location; therefore it is instructive to examine the errors introduced in applying a simple diffusion-based reconstruction scheme in cases in which there exists a nonscattering region. We present reconstructed images of objects that contain a nonscattering region within a diffusive material. Here the forward data is calculated with the radiosity-diffusion model, and the inverse problem is solved with either the radiosity-diffusion model or the diffusion-only model. The reconstructed images show that even in the presence of only a thin nonscattering layer, a diffusion-only reconstruction will fail. When a radiosity-diffusion model is used for image reconstruction, together with a priori information about the position of the nonscattering region, the quality of the reconstructed image is considerably improved. The accuracy of the reconstructed images depends largely on the position of the anomaly with respect to the nonscattering region as well as the thickness of the nonscattering region.     

Image reconstruction in optical tomography in the presence of coupling errors

Image reconstruction in optical tomography is a nonlinear and generally ill- posed inverse problem. Noise in the measured surface data can give rise to substantial artifacts in the recovered volume images of optical coefficients. Apart from random shot noise caused by the limited number of photons detected at the measurement site, another class of systematic noise is associated with losses specific to individual source and detector locations. A common cause for such losses in data acquisition systems based on fiber-optic light delivery is the imperfect coupling between the fiber tips and the skin of the patient because of air gaps or surface moisture. Thus the term coupling errors was coined for this type of data noise. However, source and detector specific errors can also occur in noncontact measurement systems not using fiber-optic delivery, for example, owing to local skin pigmentation, hair and hair follicles, or instrumentation calibration errors. Often it is not possible to quantify coupling effects in a way that allows us to remove them from the data or incorporate them into the light transport model. We present an alternative method of eliminating coupling errors by regarding the complex-valued coupling factors for each source and detector as unknowns in the reconstruction process and recovering them simultaneously with the images of absorption and scattering. Our method takes into account the possibility that coupling effects have an influence on both the amplitude and the phase shift of the measurements. Reconstructions from simulated and experimental phantom data are presented, which show that including the coupling coefficients in the reconstruction greatly improves the recovery of absorption and scattering images.     

Recent developments in indoor optical wireless [Optical wireless communications]

An overview of the developments in optical wireless systems viewed from the traditional communications viewpoint of transmitter, channel and receiver is presented. The trends in modulation formats that match information to the optical wireless channel are considered. This is followed by the discussion of recent transmitter and receiver innovations, particularly the utilisation of diversity transceivers. As a preliminary to the following treatment, the nature and modelling of the optical wireless channel are introduced, with particular emphasis on its unique features in terms of transmitted power constraints and non-negativity. From the examination of modulation formats, on-off-keying remains the format of choice for basic binary transmission, whereas pulse-position modulation and its derivatives are preferred for more sophisticated requirements. The recent introduction of techniques from radio systems employing subcarriers is seen to be the most promising development in modulation techniques at present. In receiver technology, quasi-diffuse systems employing multispot diffusion and angular diversity are significant developments. They offer lower path loss and less multipath dispersion, at a lower transmission power compared to 'conventional' wide-angle diffuse systems, while providing a high level of user mobility compared to line-of-sight transmission. These developments are helping optical wireless systems to fulfil their promise by adopting a philosophy inspired by the radio domain to accommodate operation within a hostile channel.     

Effects of fog on the bit-error rate of a free-space laser communication system

Free-space laser communication (lasercom) systems are subject to performance degradation when heavy fog or smoke obscures the line of sight. The bit-error rate (BER) of a high-bandwidth (570 Mbits/s) lasercom system was correlated with the atmospheric transmission over a folded path of 2.4 km. BER's of 10(-7) were observed when the atmospheric transmission was as low as 0.25%, whereas BER's of less than 10(-10) were observed when the transmission was above 2.5%. System performance was approximately 10 dB less than calculated, with the discrepancy attributed to scintillation, multiple scattering, and absorption. Peak power of the 810-nm communications laser was 186 mW, and the beam divergence was purposely degraded to 830 murad. These results were achieved without the use of error correction schemes or active tracking. An optimized system with narrower beam divergence and active tracking could be expected to yield significantly better performance.     

Performance of the MEWMA-CoDa control chart in the presence of measurement errors

This paper can be considered as an extension of the work of Tran et al (for monitoring compositional data using a multivariate exponentially weighted moving average MEWMA-compositional data [CoDa] chart) by taking into account potential measurement errors that are known to highly affect production processes. A linearly covariate error model with a constant error variance is used to study the impact of measurement errors on the MEWMA-CoDa control chart. In particular, the influence of the device parameters (σ_M,b), the number of independent observations m, and the the number of variables p are investigated in terms of the MEWMA optimal couples (r,H) as well as in terms of their corresponding ARLs. A comparison between the Hotelling-CoDa T² and the proposed chart is made in order to show that the MEWMA-CoDa chart is more efficient in detecting shifts in the presence of measurement errors. A real-life example of muesli production, using multiple measurements for each composition, is used to estimate the parameters and also to demonstrate how the MEWMA-CoDa can handle measurement errors to detect shifts in the process.     

Optical alignment-induced errors in holographic particle image velocimetry

A ray-tracing analysis of point-source imaging in the presence of optical misalignment is used to analyze relative image shift as a source of measurement error in holographic particle image velocimetry (HPIV). Although single-reference-beam HPIV is relatively insensitive to optical misalignment, dual-reference-beam systems may suffer substantial errors because of misalignments of the order of microradians. These systems are particularly sensitive to rotations of the hologram about an axis perpendicular to the film and to reconstruction beam misalignment. In a swirling flow experiment, a proposed error-compensation scheme was able to reduce uncertainty from 130% to 10% of the mean measured velocity.     

Optimization of indoor wireless communication network layouts

Radio Frequency Data Communications (RFDC)technology is rapidly becoming a critical component of many traditional industrial engineering functions including materials tracking, inventory control, warehousing, order processing, shipping and database management. As a means of moving information, RFDC has many attractive features, such as speed, accuracy, reliability, convenience and low operating costs. When implementing RFDC systems a major problem is to quickly and efficiently determine the locations where transceivers should be placed so that effective radio communication can take place. The research described in this paper addresses this issue by developing a computerized layout simulation system that incorporates heuristic optimization methods to solve the placement problem. The effectiveness of this unique automated layout methodology is demonstrated by comparing it with the current method of utilizing manual site surveys, as well as with other placement methods. The methodology and solutions are validated by field-testing at actual facilities.