Exposure to virtual environments often causes users to experience symptoms of motion sickness. An accessory manifestation of motion sickness symptoms is postural disequilibrium. If the postural disequilibrium that occurs persists beyond the time an individual is within the confines of the laboratory or system site, user safety could be compromised and products liability issues could be forthcoming. In this study, a portable, automated postural assessment system is developed that can be employed before and after exposure to a virtual reality (VR) system in order to certify that a user's balance on exiting the system is at least demonstrably as good as it was on entering. It is argued that if the "coming out" balance performance is sufficiently poorer than the "going in" balance, then the user should be retained until the pretest balance performance is regained. The results from a set of normative and validation experiments on postural equilibrium identified several reliable measures of stance that could serve as a basis for certification. Furthermore, a new automated video-based measure using only head movement showed that performance over sessions is stable and reliable. The head movement changes that occur with stimuli, such as alcohol and simulator exposure, are well behaved, predictable, and significant even with small samples. The implication is that the proposed objective measure of postural stability, in conjunction with procedures for obtaining self-reports of symptoms, can afford some measure of certification that exposure to a given VR system is without harm.

The problem of specifying the optimum quantum detector in multiple hypotheses testing is considered for application to optical communications. The quantum digital detection problem is formulated as a linear programming problem on an infinite-dimensional space. A necessary and sufficient condition is derived by the application of a general duality theorem specifying the optimum detector in terms of a set of linear operator equations and inequalities. Existence of the optimum quantum detector is also established. The optimality of commuting detection operators is discussed in some examples. The structure and performance of the optimal receiver are derived for the quantum detection of narrow-band coherent orthogonal and simplex signals. It is shown that modal photon counting is asymptotically optimum in the limit of a large signaling alphabet and that the capacity goes to infinity in the absence of a bandwidth limitation.

Basing decisions and estimates on simultaneous approximate measurements of noncommuting observables in a quantum receiver is shown to be equivalent to measuring commuting projection operators on a larger Hilbert space than that of the receiver itself. The quantum-mechanical Cramér-Rao inequalities derived from right logarithmic derivatives and symmetrized logarithmic derivatives of the density operator are compared, and it is shown that the latter give superior lower bounds on the error variances of individual unbiased estimates of arrival time and carrier frequency of a coherent signal. For a suitably weighted sum of the error variances of simultaneous estimates of these, the former yield the superior lower bound under some conditions.

A reasonable model for the atmosphere as an optical communication channel is presented and then reduced, with some approximation, to an equal-strength diversity system. By using the realistic assumption of a very large variance for the channel's random phase, the operations that the maximum-likelihood receiver must perform are determined. This resulting receiver is also a min-max receiver. Functional bounds to the error probability that results with orthogonal waveforms are determined. The variation of these bounds with the energy-to-noise ratio per diversity path and with the amount of atmospheric turbulence is discussed, and numerical results are presented for the low-rate (union-bound) performance. These results demonstrate that the error probability depends strongly on the amount of turbulence. However, if the amount of diversity can be controlled, the minimum energy-to-noise ratio per information bit for which the error probability can be made to vanish is independent of the turbulence. This minimum is just that which would exist in the absence of turbulence.

Research in coding theory has resulted in the determination of bounds, as a function of the rate of communication, on the probability of error that can be attained over a memoryless transmission facility. These results are reviewed, and their implication with regard to the evaluation of modulation and demodulation systems is discussed. The objective is to describe some of the interrelations that affect the performance of a communication system used with probabilistic coding.