Statistical analysis of fading power vectors for real-time atmospheric channel emulation

Abstract

The design and testing of a free-space optical communication system requires assessment of the impact of random fluctuations in received power from a laser beam transmitted over an atmospheric channel. A number of methods for generating fading power vectors for in-lab emulation of an atmospheric channel have previously been reported. These techniques include spectral shaping and filtering of a signal from a normally distributed pseudo-random number generator, full wave optics simulations with random phase screens, and pre-recorded measurements from experimental free-space links. In this work, a statistical analysis of atmospheric fading is presented with the goal of producing a practical engineering model suitable for generating synthetic fade vectors in real-time for long-duration receiver testing with channel interleaving. Specifically, a parametric model is developed for turbulence-induced fade on space-to-ground links with large-aperture receivers, including aperture-averaging and the effects of aperture size on the instantaneous coupling efficiency for mode-limited receivers. In particular, we analyze the probability density function and temporal power spectrum for fluctuations of the coupling efficiency for few-mode fibers in a range of turbulence conditions.

Performance of a real-time photon counting optical receiver in the presence of emulated channel fading

“…8 A power law roll-off of 40 dB/decade (𝛼 = 4 ) is chosen to model the psd of the aperture-averaged fluctuations in the large-aperture regime. 7 [7] Scintillation fade vectors for the following tests are generated using a cutoff frequency 𝑓 𝑐 = 220 Hz corresponding to a half-width at half maximum (HWHM) atmospheric correlation time 𝜏 𝐶 = 1 ms.…”

“…𝑀 < (𝐷/𝑟 0 ) 2 , the amount of light from the telescope which is collected by the fiber at a given instant fluctuates depending primarily on the atmospheric turbulence-induced distortion of the phase of the optical field across the telescope aperture. 7 The cutoff frequency for coupling-induced fade can be estimated from the Greenwood frequency 𝑓 𝐺 together with 𝐷/𝑟 0 .…”

“…In this work, we use a cutoff frequency 𝑓 𝑐 = 6 Hz corresponding to a Greenwood frequency of 30 Hz with 𝐷/𝑟 0 = 6 and a roll-off of 30 dB/decade (𝛼 = 3). 7 .…”

“…The coupling efficiency for two fibers (10-mode and 69-mode) and turbulence conditions (𝐷/𝑟 0 = 2,4,6 and 9) was modeled using Monte Carlo simulations with Kolmogorov phase screens and simulated tilt compensation representing a fast-steering mirror (FSM). 7 [7] The fiber modes are modeled using LP modes for a step-index fiber (due to pairing of certain LP modes with the same cutoff frequency 𝑉 𝑐 , coupling into the 70-mode photonic lantern is modeled using a 𝑉-number producing a set of 69 scalar modes). The numerical results were then used to model the pdf based on a fit to an appropriate distribution.…”