Anisoplanatism in airborne laser communication

Abstract

Airborne laser-communication systems require special considerations in size, complexity, power, and weight. We reduce the variability of the received signal by implementing optimized multiple-transmitter systems to average out the deleterious effects of turbulence. We derive the angular laser-beam separation for various isoplanatic and uncorrelated (anisoplanatic) conditions for the phase and amplitude effects. In most cases and geometries, the angles ordered from largest to smallest are: phase uncorrelated angle (equivalent to the tilt uncorrelated angle), tilt isoplanatic angle, phase isoplanatic angle, scintillation uncorrelated angle, and scintillation correlation angle (Theta(psiind) > Theta(TA) > Theta(0) > Theta(chiind) > Theta(chic)). Multiple beams with angular separations beyond Theta(chic) tend to reduce scintillation variations. Larger separations such as Theta(TA) reduce higher-order phase and scintillation variations and still larger separations beyond Theta(psiind) tend to reduce the higher and lower-order (e.g. tilt) phase and scintillation effects. Simulations show two-transmitter systems reduce bit error rates for ground-to-air, air-to-air, and ground-to-ground scenarios.

Near the ground laser communication system: anisoplantic studies based on the PSF measurements

“…There are two ways to improve this undesired condition: increase the diversity of the signal to average out the effects or adaptively compensate for the turbulence in real time. 15 Multiple-transmitter systems increase the diversity of the signal and average out the deleterious effects of turbulence without bulky, complicated AO systems, making it an appropriate choice for airborne laser communications. 15 Understanding the degree of anisoplanatism present in the multiple-transmitter systems, and its impact on FSO system performance is crucial.…”

“…15 Multiple-transmitter systems increase the diversity of the signal and average out the deleterious effects of turbulence without bulky, complicated AO systems, making it an appropriate choice for airborne laser communications. 15 Understanding the degree of anisoplanatism present in the multiple-transmitter systems, and its impact on FSO system performance is crucial. Previous research on multiple transmitters, found in the literature, has focused on satellite communications, in which the turbulence present over a short part of the propagation path, or constant-turbulence-strength paths.…”

Inter-aperture correlation in MIMO free space optical systems

“…The correlation width assumption is used in [10][11][12][13][14] for receiver diversity systems to guarantee independent channel coefficients. Further the authors of [10,13,15] use correlation width also for determining the transmit aperture separation assuming the reciprocity of the transmit/receive sides. The correlation width is also used to determine the size of a single larger receive aperture used for aperture averaging.…”

Isoplanatic angle in finite distance: theory analysis on measurement feasibility

“…For this case, starlight propagating through turbulence can be seen as plane waves, and θ 0 is the correlation angle of the whole layer atmospheric turbulence. In many applications [4,5], θ 0 of a finite propagation path (i.e., propagation through partiallayer atmospheric turbulence), which we named as isoplanatic angle in finite distance, is wanted. However, θ 0 in a finite distance can't be measured simply by using scintillation of a synthetic beacon, for beacon light can't be seen as plane waves.…”

“…The form of the filter function depends on the received aperture and distributed beacon, and has the form [4] F a γκ 4J 1 γκD∕2∕γκD 2 ;…”