Ground-high altitude platform-satellite laser communication system and method based on anisotropic non-Kolmogorov turbulence
A ground-high altitude platform-satellite (G-H-S) laser communication system and method based on anisotropic non-Kolmogorov turbulence are provided, and the system includes a ground transmitting terminal, a HAP, a satellite terminal, a ground-HAP uplink and an HAP-satellite uplink. When laser beams are transmitted upwards from the ground transmitting terminal and through the HAP, the laser beams are affected light intensity scintillation, beam drift, and angle of arrival fluctuation, and are transmitted through vacuum channels and received by receiving antenna of the satellite terminal. The HAP is located at stratosphere, and an anisotropic non-Kolmogorov turbulence model is suitable for analyzing turbulence effect in stratosphere. Thus, the anisotropic non-Kolmogorov turbulence model is used to research transmission characteristics of the laser beams. Meanwhile, a hexagon quadrature amplitude modulation (HQAM) is used in the G-H-S laser communication system, which optimizes system performances on basis of improving communication quality, and enhances stability of communication links.
1 . A ground-high altitude platform-satellite (G-H-S) laser communication system based on anisotropic non-Kolmogorov turbulence, comprising: a ground transmitting terminal, a high-altitude platform (HAP), and a satellite terminal;
wherein the ground transmitting terminal comprises a laser transmitter and a Mach-Zehnder (MZ) modulator, the laser transmitter is configured to transmit laser beams to the MZ modulator, the MZ modulator is configured to modulate the laser beams to obtain laser signals with different intensities after receiving the laser beams and transmit the laser signals to the HAP;
wherein the HAP is a geostationary satellite as a relay station, the HAP is located at stratosphere, and the HAP is configured to analyze atmospheric turbulence parameters during transmitting the laser beams from the ground transmitting terminal to the HAP by using an anisotropic non-Kolmogorov turbulence model of the stratosphere, and the atmospheric turbulence parameters indicate an effect of an anisotropy factor on transmission characteristics of the laser beams; and the HAP comprises a multi-aperture receiver, and the multi-aperture receiver is configured to receive the laser signals from branches of the multi-aperture receiver, perform in-phase processing on the laser signals to obtain processed laser signals, combine the processed laser signals by using an equal gain combination (EGC) method to obtain combined laser signals, and transmit the combined laser signals to the satellite terminal through a vacuum channel; and
wherein the satellite terminal comprises an optical receiving antenna, a photodetector, a preamplifier and a demodulator, the optical receiving antenna is configured to receive the combined laser signals, and send the combined laser signals to the photodetector, and the photodetector is configured to convert the combined laser signals to electrical signals, the preamplifier is configured to amplify the electrical signals to obtain amplified electrical signals, and the demodulator is configured to demodulate the amplified electrical signals to obtain original information;
wherein an uplink between the ground transmitting terminal and the HAP, and an uplink between the HAP and the satellite terminal are free-space optical communication (FSO) links, and the uplink between the ground transmitting terminal and the HAP, and the uplink between the HAP and the satellite terminal are configured to apply a hexagon quadrature amplitude modulation (HQAM) to relieve an atmospheric turbulence effect, to thereby achieve data communication.
2 . The G-H-S laser communication system as claimed in claim 1 , wherein the HAP is located at an altitude of 17 to 32 kilometers (km) in the stratosphere.
3 . The G-H-S laser communication system as claimed in claim 1 , wherein a power-law spectrum of anisotropic non-Kolmogorov turbulence in the stratosphere is expressed as follows:
ϕ
n
(
κ
,
h
)
=
A
(
α
)
C
~
n
2
(
h
)
exp
(
κ
2
κ
m
2
)
(
κ
2
+
κ
0
2
)
α
/
2
,
0
≤
κ
<
∞
,
3
<
α
<
4
;
wherein
C
~
n
2
(
h
)
represents a generalized structural constant in the anisotropic non-Kolmogorov turbulence, K represents a spatial frequency, h represents an altitude of the anisotropic non-Kolmogorov turbulence, n represents an atmospheric refractive index, a represents a power-law index,
κ
m
=
C
(
α
)
l
0
,
κ
0
=
2
π
L
0
,
C
(
α
)
=
[
(
2
π
3
)
Γ
(
5
-
0
.
5
α
)
A
(
α
)
]
1
/
(
α
5
)
,
A
(
α
)
=
0.25
π
-
2
Γ
(
α
-
1
)
cos
(
0
.
5
πα
)
,
l 0 represents an inner scale of the anisotropic non-Kolmogorov turbulence, and L 0 represents an outer scale of the anisotropic non-Kolmogorov turbulence.
4 . The G-H-S laser communication system as claimed in claim 1 , wherein the satellite terminal is a lower Earth orbit (LEO) satellite, and the satellite terminal is configured to perform ground communication, observation, and navigation.
5 . The G-H-S laser communication system as claimed in claim 1 , wherein a principle of the HQAM is as follows: original data is divided into two paths at a transmitter, a speed of data in each path is changed to half of a speed of the original data, the data in each path is converted to a multilevel signal through a bit encoder, the multilevel signal in each path is multiplied with a pair of orthogonal modulation carriers to obtain a modulated signal, and the modulated signals in the two paths are summed and then output.
6 . The G-H-S laser communication system as claimed in claim 5 , wherein the HQAM applies a bidirectional modulator to decompose the multilevel signal in each path into two components including an in-phase and a quadrature-phase, modulate the in-phase and the quadrature-phase of the multilevel signal onto the orthogonal modulation carriers respectively, to thereby transmit a plurality of data symbols, and an expression of the modulated signal s(t) is expressed as follows:
s
(
t
)
=
2
T
(
A
i
cos
(
w
t
)
+
B
j
sin
(
w
t
)
)
;
A
i
=
±
(
2
i
-
1
)
;
B
j
=
±
(
2
i
-
1
)
;
wherein A i and B j represent amplitudes of the in-phase and the quadrature-phase, respectively; w represents an angular frequency, i and j are positive integers, and T represents a period.