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Fog is a highly dispersive medium at optical wavelengths, and the received pulse waveform may suffer significant distortion. Thus it is desirable to have the impulse response of the propagation channel to recover data transmitted through fog. The fog particle density and the particle size distribution both strongly influence the channel impulse response, yet it is difficult to estimate these parameters. We present a method using a dual-wavelength free-space optical system for estimating the average particle diameter and the particle number density and for approximating the particle distribution function. These parameters serve as inputs to estimate the atmospheric channel impulse response using simulation based on the modified vector radiative transfer theory. The estimated channel response is used to design a minimum mean-square-error equalization filter to improve the bit error rate by correcting distortion in the received signal waveform due to intersymbol interference and additive white Gaussian noise. OCIS codes: 010.1300, 010.3310, 010.5620, 060.2605, 200.2605, 290.1090.

We consider the design of iteratively decoded bit-interleaved space--time coded modulation (BI-STCM) over fast Rayleigh-fading channels with [N.sub.t] transmit and [N.sub.r] receive antennas. We propose the design criterion to achieve the largest asymptotic coding gain inherited in the constellation labeling. In particular, for orthogonal space--time block codes, the labeling design criterion reduces to maximizing the (-[N.sub.t][N.sub.[r])th power mean of the complete set of squared Euclidean distances associated with all "error-free feedback" events in the constellation. Based on this power mean criterion, we show that the labeling optimization problem falls into the category of quadratic assignment problems for constellations of any shape and with an arbitrary number of transmit and receive antennas. For a set of practical values of [N.sub.t] and [N.sub.r], we present optimal labeling maps for 8-PSK, 16-QAM, and 64-QAM constellations. Index Terms--Bit-interleaved coded modulation (BICM), constellation labeling, multiple antennas, optimal code search, quadratic assignment problem (QAP), space--time coding.

We consider coded modulation with generalized selection combining (GSC) for bandwidth-efficient-coded modulation over Rayleigh fading channels. Our results show that reception diversity with generalized selection combining can conveniently trade off system complexity versus performance. We provide a number of new results by calculating the cutoff rate, and by deriving analytical upper bounds on symbol-interleaved trellis-coded modulation (TCM) and bit-interleaved-coded modulation (BICM) with GSC. All are verified by simulation. We show that our new bounds on TCM with GSC, which includes maximum ratio combining and selection combining as special cases, are tighter than the previously derived bounds. A new asymptotic analysis on the pairwise error probability, which can be used as a guideline for designing coded modulation over GSC channels, is also given. Finally, we show that BICM with iterative decoding (BICM-ID) can achieve significant coding gain over conventional coded modulation in a multiple-receiving-antenna channel. Index Terms--Bandwidth-efficient-coded modulation, BICM, generalized selection combining (GSC), I-Q trellis-coded modulation (TCM), Rayleigh fading channels, reception diversity.

We have suggested bit-interleaved coded modulation with soft decision iterative decoding (BICM-ID) for bandwidth-efficient transmission over Gaussian and fading channels. Unlike trellis coded modulation, BICM-ID has a small free Euclidean distance but large diversity order due to bit interleaving. With iterative decoding, soft bit decisions can be employed to significantly improve the conditional intersignal Euclidean distance. This leads to a large coding gain, comparable to that of turbo TCM, over both Gaussian and Rayleigh fading channels with much less system complexity. We address critical design issues to enhance the decoding performance and provide the analytical bounds on the performance with an ideal feedback assumption. We investigate the performance characteristics of BICM-ID through extensive simulations and show that at high signal to noise ratios, the performance of BICM-ID converges to the performance assuming error-free feedback. Index Terms--BICM, coded modulation, digital communications, iterative decoding, turbo codes.

Small formed elements and gas bubbles in flowing blood, called microemboli, can be detected using Doppler ultrasound. In this application, a pulsed constant-frequency ultrasound signal insonates a volume of blood in the middle cerebral artery, and microemboli moving through its sample volume produce a Doppler-shifted transient reflection. Current detection methods include searching for these transients in a short-time Fourier transform (STFT) of the reflected signal. However, since the embolus transit time through the Doppler sample volume is inversely proportional to the embolus velocity (Doppler-shift frequency), a matched-filter detector should in principle use a wavelet transform, rather than a short-time Fourier transform, for optimal results. Closer examination of the Doppler shift signals usually shows a chirping behavior apparently due to acceleration or deceleration of the emboli during their transit through the Doppler sample volume. These variations imply that a linear wavelet detector is not optimal. We apply linear and quadratic time-frequency and time-scale detectors to a set of noise-corrupted embolus data. Our results show improvements of about 1 dB using the time-scale detectors versus an STFT-based detector signifying that embolus detection is best approached as a time-scale problem. A time-scale-chirp detector is also applied and is found to have the overall best performance by about 0.5-0.7 dB while coming fairly close (about 0.75 dB) to a theoretical upper bound. Index Terms - Doppler ultrasound, microemboli, time-frequency, time-scale, wavelet detection.

We describe an efficient procedure to calculate the probability of error [P.sub.e] for a quadrature amplitude modulation (QAM) communications system operating over a channel that introduces distortion, interference, and noise. The method is an extension of the saddlepoint integration technique introduced by Helstrom to efficiently evaluate [P.sub.e] for one-dimensional pulse-amplitude modulation (PAM) systems with intersymbol interference (ISI) and crosstalk. We consider the effects of noise, random carrier phase offset, ISI, and crosstalk between the I and Q channels. The error probability is written as a double Laplace inversion integral and can be easily applied to any rectangular constellation. This integral is calculated by extending the saddlepoint integration technique to two complex dimensions. Results are presented for QAM systems with 16, 64, and 256 symbols. The technique can be directly extended to environments with cochannel interference consisting of other QAM signals. Index Terms - Crosstalk, digital communications, error probability, ISI, QAM, saddlepoint integration.

An iterative method based on Wiener-Hopf equation offers a space-time joint FIR prefilter and postfilter design for multiple-input multiple-output (MIMO) digital communications. The prefilter manages the channel input signal spectrum and reduces the error rate, whereas the postfilter curbs ISI and the cochannel interference. A directed random search method minimizes mean square error and obtains tap coefficients to offer a near-optimal solution for the joint filters. The advantage of joint pre/post filter design for the case of ocean acoustic channels with spatially diverse antenna arrays is discussed.

A theoretical and numerical study of the acoustic field intensity within a curved flow conduit having 1) diameter similar to the wavelength of the interrogating frequency and 2) speed of sound mismatch with the surrounding medium is presented. The field intensity is shown to vary significantly and in a monotonic fashion across the flow conduit. The resulting insonation of emboli transiting through the Doppler sample volume is explored with a Monte Carlo study of the behavior of the embolus to blood power ratio (EBR). The numerical simulation findings are shown to be in good agreement with previously reported experimental results. A method is explored for estimating embolus diameter when this refraction artifact is present, and shown to yield excellent results when applied to experimental data. Further work toward clinical application of these results is discussed.

An experiment to verify a theory describing ultrasound backscattering from emboli in flowing blood is presented. The theoretical predictions for the backscattered power versus embolus size are based on the measurement of the embolus to blood ratio (EBR) of backscattered acoustic power. This tool is necessary for in vivo clinical application because it removes the need to characterize attenuation and reflection loss in heterogeneous tissue. The experiment presented utilizes a customized ultrasound pulse Doppler that is capable of interrogating a sample volume with two different frequencies concurrently. A flow circuit including a 3.6-mm-diameter conduit in polyacrylamide gel in which emboli are observed with the dual frequency Doppler is described. The flow within the circuit has acoustic backscatter coefficient similar to blood due to a calibrated concentration of 31.1-[Mu] diameter polystyrene microspheres. Polystyrene micro-sphere 'emboli' having nominal diameters of 161 and 239 [Mu] are placed in this flow loop and time series Doppler shift signatures are recorded. These signatures are investigated and a refraction artifact hypothesis is proposed to explain systematic deviation of the signatures away from theoretically expected results. Results show that gross discrimination of embolus size is feasible.

The Pade approximation (PA) method is used to analyze the detection performance of single and multiple pulse radar systems operating in K-distributed clutter and thermal noise. Simple approximations for false-alarm and detection probabilities are obtained, using lower order moments for the detection decision statistic. Both envelope and squaring detector laws are considered, with noncoherent integration, for independent and correlated K clutter. The target is assumed to be pulse-to-pulse Rayleigh fading. The methods are a substantial application of the PA methods we have recently published.

The analysis of radar detection systems often requires extensive knowledge of the special functions of applied mathematics, and their computation. Yet, the moments of the detection random variable are often easily obtained. We demonstrate here how to employ a limited number of exactly specified moments to approximate the probability density and distribution functions of various random variables. The approach is to use the technique of Pade approximations (PA) which creates a pole-zero model of the moment generating function (mgf). This mgf is inverted using residues to obtain the densities.

The reliability testing of fault-tolerant computer systems with multi-phased missions is investigated. Such systems are difficult to characterize due to the varying nature of their mean-time-before-failure and error handling property. Several approaches and a computer program for conducting efficient reliability analysis are presented along with Markov models for systems whose configuration or success criteria may be altered.