An experimental demonstration of a quantum-optimal receiver for optical binary signals, developed as a joint effort by the Jet Propulsion Laboratory and the California Institute if Technology, is described in this article. A brief summary of the classical, quantum-optimal, and quantum near optimal solutions to detecting binary signals is first presented. The components and experimental setup used to implement the receivers is then discussed. Experimental performance and results for both optimal and near-optimal receivers are presented and compared to theoretical limits. Finally, experimental shortcomings are discussed along with possible solutions and future direction.
The results of field experiments designed to demonstrate key enabling concepts of optical array receivers are presented,
in a field environment similar to actual operating conditions. The theoretical framework and analytical evaluation of
optical array receivers have been developed and documented in previous articles. This article describes the
implementation and field evaluation of a two-element optical array receiver incorporating photon-counting, signal
conditioning, high-speed digital sample distribution, adaptive delay compensation and sample combining operations
required for array reception of intensity-modulated optical pulse-position modulated (PPM) signals. Data collected in the
field was processed offline to determine communications performance of a two-element optical array, and demonstrate
the inherent advantages of optical array reception.
The conceptual design, theoretical performance, and experimental verification of a two-telescope optical array receiver currently under development at the Jet Propulsion Laboratory, is described in this paper. A brief summary of optical communications theory for array reception of pulsed laser signals is developed, and the impact of coding discussed. The development of the optical detection, array processing, and data-acquisition assemblies required for experimental demonstration is described, and preliminary results obtained in a field environment are presented and evaluated.
An optical receiver concept for binary signals with performance approaching the quantum limit at low average signal energies is developed and analyzed. A conditionally nulling receiver that reaches the quantum limit in the absence of background photons has been devised by Dolinar, however this receiver requires ideal optical combining and complicated real-time shaping of the local field, hence tends to be difficult to implement at high data rates. A simpler nulling receiver that approaches the quantum limit without complex optical processing, suitable for high-rate operation has been suggested earlier by Kennedy. Here we formulate a vector receiver concept that incorporates the Kennedy receiver together with a physical beamsplitter, but also utilizes the reflected signal component to improve signal detection. It is found that augmenting the Kennedy receiver with classical coherent detection at the auxiliary beamsplitter output, and optimally processing the vector observations, always improves on the performance of the Kennedy receiver alone, significantly so at low average photon rates. This is precisely the region of operation where modern codes approach channel capacity. It is also shown that the addition of background radiation has little effect on the performance of the coherent receiver component, suggesting a viable approach for near quantum-limited performance in high background environments.
KEYWORDS: Receivers, Telescopes, Space telescopes, Turbulence, Sensors, Signal detection, Optical arrays, Optical communications, Staring arrays, Chemical elements
An optical array receiver concept is developed and analyzed. It is shown that for ground-based reception, the number of array elements can be increased without any performance degradation, as long as the array telescope diameters exceed the coherence-length of the atmosphere. Maximum likelihood detection of turbulence-degraded signal fields is developed for the case of pulse-position modulated (PPM) signals observed in the presence of background radiation. Performance of optical array receivers is compared to single-aperture receivers with diameters ranging from 4 to 8 meters, both in the presence of turbulence and in a turbulence-free environment such as space. It is shown that in the absence of atmospheric turbulence, single-aperture receivers outperform receiver arrays when
significant background radiation is present. However, it is also
shown that for ground-based reception of deep-space signals, the number of array elements can be as great as several thousand without incurring any performance degradation relative to a large single-aperture receiver.
A new technique for evaluating the performance of quantum signals observed in the presence noise is described and evaluated. The quantum theory for detecting coherent-state signals has been developed previously, however the quantum "signal plus noise" problem has received little attention due to its complexity. Here we develop a discrete approximation to the coherent-state representation of signal-plus-noise density operators, and present solutions to optimum receiver performance in terms of quantum "measurement states" whose performance is optimized via generalized rotations in Hilbert space. An efficient algorithm for carrying out the required numerical optimization is described and applied to binary signals observed in the presence of noise, for which exact results are available for comparison. The algorithm is then applied to the detection of ternary signals observed in the presence of noise, a previously unsolved problem, and the performance of the optimum receiver characterized.
The fundamental performance limits and channel capacity of optical communications systems operating over the free space channel will be examined using quantum detection theory. The performance of the optimum quantum receiver for on-off keying (OOK) and optical binary phase shift keying (BPSK) is first examined as a pure state (no noise) problem. The classical capacity of the binary symmetric channel for these two modulation schemes is then evaluated for the optimum quantum receiver by making use of the concept of quantum measurement states. The performance of M-ary pulse position modulation, which requires a product state representation, is evaluated along with the performance of certain 'dense signal sets.' Performance comparisons with classical techniques shows over 5 dB improvement in some cases when quantum detection is employed. As a further application of the quantum detection theory, the capacity of the binary channel with on-off keyed modulation and quantum detection is evaluated, and shown to exceed the capacity obtained with classical photon counting.
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