Communication techniques and coding for atmospheric turbulence channels

Abstract

In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. In this paper, we describe several communication techniques to mitigate turbulence-induced intensity fluctuations, i.e., signal fading. These techniques are applicable in the regime in which the receiver aperture is smaller than the correlation length of the fading, and the observation interval is shorter than the correlation time of the fading. We assume that the receiver has no knowledge of the instantaneous fading state. The techniques we consider are based on the statistical properties of fading, as functions of both temporal and spatial coordinates. Our approaches can be divided into two categories: temporal domain techniques and spatial domain techniques.In the spatial domain techniques, one must employ at least two receivers to collect the signal light at different positions or from different spatial angles. Spatial diversity reception with multiple receivers can be used to overcome turbulence-induced fading. When it is not possible to place the receivers sufficiently far apart, the fading at different receivers is correlated, reducing the diversity gain. We describe a ML detection technique to reduce the diversity gain penalty caused by such fading correlation.In the temporal domain techniques, one employs a single receiver. When the receiver knows only the marginal statistics of the fading, a symbol-by-symbol ML detector can be used to optimize performance. When the receiver also knows the temporal correlation of the fading, maximum-likelihood sequence detection (MLSD) can be employed, yielding a further performance improvement, but at the cost of very high complexity. We describe two reduced-complexity implementations of the MLSD, which 364 Xiaoming Zhu and Joseph M. Kahn make use of a single-step Markov chain model for the fading correlation in conjunction with per-survivor processing. Next, we also investigate the performance of using error-control coding and pilot symbol-assisted detection schemes through atmospheric turbulence channels.

Nanosatellite optical downlink experiment: design, simulation, and prototyping

“…It has been shown that the optical channel from LEO to ground is characterized by long fades on the order of milliseconds that result in burst errors for the data. 22,72,93,94 These burst errors cause multiple correlated errors in one codeword, and the fades are long enough such that the number of errors in one codeword will exceed the error correcting capabilities of the code. Linear block codes, such as RS codes, do not perform well in the presence of burst errors because entire codewords may be lost.…”

“…55 (11) fades when paired with forward error correction, and for this reason, NODE will use a block interleaver. 22,72,93,94 We chose a fade depth of 1 s since a common design practice for interleavers is to have the depth of the interleaver be 100 to 1000 times the coherence time of the channel fading process, which, as stated earlier, is on the order of milliseconds. Since NODE will use PPM orders of 16, 32, 64, and 128, the modulator will encode 4, 5, 6, and 7 bits per PPM symbol, respectively.…”

Suppressing amplitude fluctuations of the wave propagating in a randomly inhomogeneous medium

“…These fluctuations become stronger with the appearance of multipath propagation and random caustics [8]. Such conditions increase the risks of failures in high-speed communication systems at optical and millimeter frequencies [2,9,10]. The main remedy for fading is a diversity reception [11,12].…”

“…When a wave propagates in inhomogeneous media such as a turbulent atmosphere or the ionosphere, focusing and defocusing induce wave amplitude fluctuations which are often referred to as fading or amplitude scintillations [2,3]. The literature on theoretical and experimental investigations into scintillations is already quite voluminous (e.g.…”

Noise Suppression in the Signal Spectral Induced by Atmospheric Turbulence on the FSO (Free-Space Optical) Communications

“…Each WDM channel has a specific center frequency or signal spectral, which is at a 100 GHz spacing (ITU-T G.694.1). For WDM channels that are propagated through atmospheric turbulence, and hence are based on the fourth order of the strong fluctuation theory, each signal spectral modulates noise (Prasad, 2005;Zhu et al, 2007;Weyrauch, 2007). Thus, maximum noise is accumulated depending on the scale of turbulence and optical link distance on the WDM of the FSO.…”