This paper investigates the approximation of the Bit Error Rate of a coherent optical focal plane array
receiver for PPM signals in the presence of atmospheric turbulence. Analysis of the expressions needed to
obtain the Bit Error Rate (BER) for the real system under study in the laboratory is shown, specifically an
approach using the Saddle-Point approximation of the Marcum-Q function and a procedure is
further discussed to approximate the Marcum-Q function using the cumulative distribution function (cdf) of
the Nakagami-m distribution. It is shown that by approximation of the Marcum-Q function by means of
the cdf of the Nakagami-m distribution, the numerical value of the probability of error can be readily
computed.
The performance of a coherent optical focal plane array receiver for PPM signals under atmospheric turbulence is investigated and applications of this system are addressed. The experimental demonstration of this project has already been explained in previous publications. This article shows a more exhaustive analysis of the expressions needed to obtain the Bit Error Rate (BER) for the real system under study in the laboratory, specifically an approach using the Saddle-Point approximation of the Marcum-Q function. Selected experimental results of this system are described and compared with theoretical BER expressions, and array combining gains are presented. Receiver sensitivity in terms of photons per bit (PPB) is examined; BER results are shown as a function of signal to noise ratios (SNR) as well as a function of photons per bit.
The performance of a coherent free-space optical communications system operating in the presence of turbulence is
investigated. Maximum Likelihood Detection techniques are employed to optimally detect Pulse Position Modulated
signals with a focal-plane detector array, and reconstruct the turbulence-degraded signals. The experimental
demonstration of this project and results may be divided in three parts; two of which have already been explained in
previous publications [1]. This latest paper shows the final experimental results, including investigation of
performance of the Coherent Optical Receiver Experiment (CORE) performed at the laboratory facilities at JPL. Bit
Error Rate (BER) is presented for single and multichanel optical receivers, where quasi-shot noise limited performance
is achieved, under simulated turbulence conditions using non-coherent post-detection processing techniques.
Theoretical BER expressions are compared with experimental obtained BER results and array combining gains are
presented. Receiver sensitivity in terms of photons per bit (PPB) is examined; BER results are shown as a function of
signal to noise ratios, (SNR), as well as a function of photons per symbol, and photons per bit.
In free-space optical communications, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received signal. We propose to use focal-plane arrays to collect optical signals from different spatial modes simultaneously, and then recombine them optimally. Experimental setup for proof-of-concept coherent adaptive array detection experiment using 32-Pulse Position Modulated (PPM) signals under atmospheric turbulence has been completed. Adaptive combining of experimentally obtained heterodyned PPM signals with pulse-to-pulse coherence, in the presence of simulated atmospheric turbulence is demonstrated. The adaptively combined PPM signals are phased up via a Least-Mean-Square (LMS) algorithm suitably optimized to operate with PPM in the presence of additive shot-noise, and detected via a maximum likelihood software receiver. Convergence study of the algorithm is presented and results with simulated PPM signals and real PPM signals experimentally obtained at the laboratory are presented.
Adaptive combining of experimentally obtained heterodyned pulse position modulated (PPM) signals with pulse-to-pulse coherence, in the presence of simulated spatial distortions resembling atmospheric turbulence, is demonstrated. The adaptively combined PPM signals are phased up via an LMS algorithm suitably optimized to operate with PPM in the presence of additive shot-noise. A convergence analysis of the algorithm is presented, and results with both computer simulated and experimentally obtained PPM signals are analyzed.
KEYWORDS: Signal detection, Signal to noise ratio, Heterodyning, Sensors, Homodyne detection, Modulation, Signal processing, Receivers, Oscillators, Error analysis
The performance of a coherent free-space optical communications system is investigated. Bit Error Rate (BER) performance is analyzed, and laboratory equipment and experimental setup used to carry out these experiments at the Jet Propulsion Laboratory are described. The key components include two lasers operating at 1064 nm wavelength for use with coherent detection, a 16 element (4X4) focal plane detector array, and data acquisition and signal processing assembly needed to sample and collect the data and analyze the results. Combining of the signals is accomplished using the least-mean-square (LMS) algorithm. Convergence of the algorithm for experimentally obtained signal tones is demonstrated in these initial experiments.
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