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In this paper, we derive upper and lower bounds as well as a simple closed-form approximation for the capacity of the continuous-time, bandlimited, additive white Gaussian noise channel in a three-dimensional free-space electromagnetic propagation environment subject to constraints on the total effective antenna aperture area of the link and a total transmitter power constraint. We assume that the communication range is much larger than the radius of the sphere containing the antennas at both ends of the link, and we show that, in general, the capacity can only be achieved by transmitting multiple spatially-multiplexed data streams simultaneously over the channel. Furthermore, the lower bound on capacity can be approached asymptotically by transmitting the data streams between a pair of physically-realizable distributed antenna arrays at either end of the link. A consequence of this result is that, in general, communication at close to the maximum achievable data rate on a deep-space communication link can be achieved in practice if and only if the communication system utilizes spatial multiplexing over a distributed MIMO antenna array. Such an approach to deep-space communication does not appear to be envisioned currently by any of the international space agencies or any commercial space companies. A second consequence is that the capacity of a long-range free-space communication link, if properly utilized, grows asymptotically as a function of the square root of the received SNR rather than only logarithmically in the received SNR.

This paper discusses the characteristics of a space-to-space multiple-input, multiple-output (MIMO) communication channel that distinguish it from more common terrestrial MIMO communication channels. These characteristics imply that the channel matrices for space communication channels have a particularly simple structure that leads to statistical characteristics that are both predictable and readily controllable using clusters of satellites as distributed communication nodes. Furthermore, the extremely high cost of launching mass into space introduces a constraint into the channel capacity equation that leads to a spectral-efficiency vs. energy-efficiency tradeoff for space MIMO communication that is fundamentally different from the tradeoff that is generally considered applicable to terrestrial MIMO communication systems., Comment: arXiv admin note: text overlap with arXiv:1601.03664

The characteristics of a space-to-space multiple-input, multiple-output (MIMO) communication channel that distinguish it from terrestrial MIMO communication channels are summarized and discussed primarily from an information-theoretic viewpoint. The implications of these characteristics for the design and application of future space-based communication systems are also discussed, and it is shown that in general, either energy-efficient or spectrally-efficient communication in space can only be achieved using a distributed MIMO architecture.

In this paper, a new lower bound on spectral efficiency in the low-power wideband regime is derived and utilized to develop an improved estimate of the behavior of spectral efficiency above but in the vicinity of the Shannon limit.

In this paper, we present a unified approach to function approximation in reproducing kernel Hilbert spaces (RKHS) that establishes a previously unrecognized optimality property for several well-known function approximation techniques, such as minimum-norm interpolation, smoothing splines, and pseudo-inverses. We consider the problem of approximating a function belonging to an arbitrary real-valued RKHS on R^d based on approximate observations of the function. The observations are approximate in the sense that the actual observations (i.e., the true function values) are known only to belong to a convex set of admissible observations. We seek a minimax optimal approximation for the function that minimizes the supremum of the RKHS norm on the error between the true function and the chosen approximation subject only to the conditions that the true function belongs to a uniformly bounded uncertainty set of functions that satisfy the constraints on the observations and that the approximation is a member of the RKHS. We refer to such a solution as a minimax robust reconstruction. We characterize the solution to the minimax robust reconstruction problem and show that it is equivalent to solving a straightforward convex optimization problem. We demonstrate that a minimax robust reconstruction will generally be more stable than an approximation based on interpolation through a nominal set of observations and that, subject to some mild regularity conditions on the convex set of admissible observations, the minimax robust reconstruction is unconditionally stable. We motivate our results by characterizing the minimax robust reconstruction for several specific convex observational models and discuss relationships with other approaches to function approximation.

We consider the problem of adaptive modulation for wideband DS-CDMA Rayleigh fading channels with imperfect channel state information (CSI). We assume a multidimensional signal subspace spanned by a collection of random spreading codes (multicoding) and study the effects of both the subspace dimension and the probability distribution of the transmitted symbols on the mutual information between the channel input and output in the presence of uncertainty regarding the true state of the channel. We develop approximations for the mutual information as well as both upper and lower bounds on the mutual information that are stated explicitly in terms of the dimension of the signal constellation, the number of resolvable fading paths on the channel, the current estimate of channel state, and the mean-squared-error of the channel estimate. We analyze these approximations and bounds in order to quantify the impact of signal dimension and symbol distribution on system performance.

The predominate traffic patterns in a wireless sensor network are many-to-one and one-to-many communication. Hence, the performance of wireless sensor networks is characterized by the rate at which data can be disseminated from or aggregated to a data sink. In this paper, we consider the data aggregation problem. We demonstrate that a data aggregation rate of O(log(n)/n) is optimal and that this rate can be achieved in wireless sensor networks using a generalization of cooperative beamforming called cooperative time-reversal communication.

A series of exact analytical solutions is developed for a capsule with two parachutes to verify CAPDYN and FAST results for terminal conditions. The case described herein is for the capsule and two parachutes acting as a fixed system in terminal descent. In this simplest case, the density is modeled as a constant defined by the initial release altitude. The analytical solution predicts the system terminal dynamic pressure, velocity, and altitude for comparison with CAPDYN and FAST simulation results.

The Orion capsule is designed to land under a nominal configuration of three main parachutes; however, the system is required to be fault tolerant and land successfully if one of the main parachutes fails to open. The Capsule Parachute Assembly System (CPAS) Team performed a series of drop tests in order to characterize the performance of the system with two main parachutes. During the series of drop tests, several distinct dynamical modes were observed. The most consequential of these is the pendulum mode. Three other modes are benign: flyout (scissors), maypole, and breathing. The actual multi-body system is nonlinear, flexible, and possesses significant cross-coupling. Rather than perform analysis of this highly complex system directly, we conduct analysis of each dynamical mode observed during flight, based on first principles. This approach is analogous to traditional aircraft flight dynamics analysis in which the full nonlinear behavior of the airframe is decomposed into longitudinal dynamics (phugoid and short-period modes) and lateral dynamics (spiral, roll-subsidence, and dutch-roll modes). This analysis is intended to supplement multi-body nonlinear simulations in order to provide further insight into the system.

Motion resembling that of a pendulum undergoing large-amplitude oscillation was ob- served during a series of flight tests of an unoccupied Orion Capsule Parachute Assembly System (CPAS) drop-test vehicle. Large excursions away from vertical by the capsule could cause it to strike the ground or ocean at a large angle with respect to vertical, with an undesirable attitude with respect to heading, or with a large horizontal or vertical speed. These conditions are to be avoided because they would endanger the occupants of the capsule in an actual mission. Pendulum motion is intimately related to a parachute’s aerodynamic normal force coefficient, which is a nonlinear function of angle of attack. An analytical investigation of the dynamics of pendulum motion is undertaken with the aid of a simplified model of the physical system and the assumption that the normal force coefficient is a linear function of angle of attack in the neighborhood of a value corresponding to stable equilibrium. The analysis leads to a simple relationship for the location of a pivot point, which provides insights that are consistent with previous studies.