Impact of Optical Preamplifier Beat Noise on Inter-Satellite Coherent
Optical Communication System
Xiangnan Liu, Chengyong Xiang, Liang Zhang and Yingfei Li
Beijing Research Institute of Telemetry, South Dahongmen Road 1, Feng Tai District, Beijing, China
Keywords: Optical Preamplifier, Coherent Optical Communication, ASE Noise.
Abstract: The performance of Inter-Satellite coherent optical communication systems is partly limited by the beat
noise between the amplified spontaneous emission (ASE) noises and local oscillator. In order to improve the
optical detection sensitivity in inter-satellite optical communication links, a model of inter-satellite coherent
optical communication is established. And the origin of noise in this system with optical pre-amplifier is
investigated. The impact of optical pre-amplifier key parameters on performance of inter-satellite coherent
optical communication systems is discussed. Besides, bit error rate (BER) versus signal power, noise figure
and gain are numerically calculated. The results illustrated that it is effective to improve the optical
detection sensitivity and degrade the bit error rate using an optical pre-amplifier with low noise figure.
1 INTRODUCTION
Optical communication in space is an attractive and
available alternative to classical RF communication.
Optical space communication systems have the
potential for substantially higher data rates than RF-
based solutions with similar onboard mass, volume
and power consumption. Besides, optical space
communication does not require frequency
regulation and provides inherently secure data links
by means of a high beam directivity (Björn Gütlich,
2013; Patricia Martin-Pimentel, 2014).
Several optical space communication missions have
been successfully demonstrated and verified in the
recent past, and are today used in commercial
system (F. Heine, 2015; Daniel Troendle, 2014;
Mark Gregory, 2012).
Nowadays, inter-satellite coherent optical
communication draws more research interests as a
coherent BPSK data transmission at a data rate of
5.625 Gbps has been performed in-orbit between
satellite NFIRE and satellite TerraSAR-X (B.
Smutny, 2008).
In the inter-satellite coherent optical communication
system, the optical preamplifier could provide the
higher gain and detection sensitivity. However, as
the introduction of optical preamplifier, which
enhances the system performance but brings a new
challenge. That is the beat noise between the
amplified spontaneous emission (ASE) noises from
optical preamplifier and local oscillator (LO),
reducing the received signal-to-noise ratio (SNR)
(P. C. Becker, 1999).
As far as we know, there are not many literatures
examining the impact of beat noise on Inter-Satellite
Coherent Optical Communication System
systematically.
In this paper, we first describe the mathematical
expression of beat noise between the ASE noise and
LO in an inter-satellite coherent receiver.
Subsequently, the impacts of different parameters on
the system’s performance are investigated.
2 SYSTEM SETUP AND NOISE
ANALYSIS
2.1 System Setup
A system setup of an inter-satellite coherent optical
communication link is showed in Figure 1. The
coherent receiver is homodyne BPSK synchronous
receiver. In the inter-satellite link, the main noise to
impair the system includes the beat noise induced by
LO and ASE noises, shot noise, thermal noise, and
background noise. Throughout this paper, all the
parameters used in analysis are listed in Table 1
unless specified.
48
Liu X., Xiang C., Zhang L. and Li Y..
Impact of Optical Preamplifier Beat Noise on Inter-Satellite Coherent Optical Communication System.
DOI: 10.5220/0005570900480051
In Proceedings of the 6th International Conference on Optical Communication Systems (OPTICS-2015), pages 48-51
ISBN: 978-989-758-116-8
Copyright
c
2015 SCITEPRESS (Science and Technology Publications, Lda.)
Figure 1: System setup of an inter-satellite coherent
optical communication link.
Table 1: Parameters used in the calculation.
Optical Wavelength(λ) 1550nm
Range(Z) 40000km
Transmit Power(P
t
) 30dBm
Transmit telescope Diameter(D
t
) 15cm
Receive telescope Diameter(D
r
) 30cm
Transmit/ Receive efficiency(τ
T
) 0.8
Thermal Noise temperature(T) 300K
R 50
RIN -145dB/Hz
Optical Bandwidth (B
o
) 10GHz
Electrical Bandwidth (B
e
) 7.5GHz
coupler efficiency(η
c
) 0.8
The input signals to a coherent receiver consist of
signal E
s
(t), LO E
lo
(t) and ASE noise E
sp
(t)
(N.A.OLSSON, 1989; Hongbo Lu, 2010;
J.G.Proakis, 2000), which are represented by:
() ()
s
=cos+
srs
Et GP t
ω
ϕ
(1)
() ()
=cos+
lo lo lo lo
Et P t
ω
ϕ
(2)
() ()
0
=-
=cos[2(+)+]
M
sp s k
kM
Et N k t
δυ π υ δυ
ϕ
(3)
()
0
1
sp
NnG h
υ
=−
(4)
0
2MB
δυ
= (5)
where P
r
and P
lo
represent power of signal and LO
respectively,
ϕ
s
and
ϕ
LO
denote optical phase of
signal and LO respectively , G is amplifier gain, n
sp
is noise figure, h is the Planck constant, ν is optical
carrier frequency.
ϕ
k
is a random phase for each
component of spontaneous emission. In equation 5,
B
o
is optical Bandwidth and δv is an arbitrarily small
frequency width and is chosen so that M is an
integer value.
The recovered inphase and quadrature signals can be
easily shown as
()
() () ()
() () ()
()
2
2
++
1
=+
4
-+-
ssplo
Ish
ssplo
Et E t E t
it R i t
Et E tE t





(6)
()
() () ()
() () ()
()
2
2
++j
1
=+
4
-+-j
ssp lo
Qsh
ssplo
Et E t E t
it R it
Et E t E t





(7)
2.2 Noise Analysis
In the coherent receiver, the variance of the beat
noises between LO and ASE noise is given by
2
-0
=4
lo sp lo e
NRPNB
(8)
where R is the responsivity, B
e
is electrical
bandwidth.
The variance of shot noise is written as
()
0
=2 + +
s
hot e o r lo
NeRBNBGPP
(9)
where e is electron quantity.
The variance of thermal noise is given by
=4
th b e
L
N
kTB R
(10)
where k
b
is Boltzmann constant, R
L
is equivalent
resistance, T is temperature, and RIN is relative
intensity noise.
For the inter-satellite link, the background radiation
is a shot noise with variance as
()
=2 +
bg e sky sun
NeRBPP
(11)
where P
sky
and P
sun
represent the background
irradiance of the Sun and the sky respectively.
Consequently, the electrical SNR in the considered
coherent receiver has the expression as
2
-
+++
rlo
lo sp shot th bg
RGPP
SNR
NNNN
=
(12)
To investigate the BER of BPSK for homodyne
detection, its analytical BER is calculated by
equation (13), where the probability density function
p(Δ
ϕ
) of phase noise is assumed to be Gaussian-
distributed with mean zero.
()()()
-
1
cos
2
BER erfc SNR p d
π
π

=⋅ΔΔΔ

(13)
3 NUMERICAL RESULTS
First we calculate the SNR of system versus
preamplifier gain with noise figure as parameter. In
the calculation, the received power P
r
is -40dBm and
LO power P
lo
is 10dBm. As shown in Figure 2, the
SNR of the system is monotonically increasing as
the surge of preamplifier gain, but gradually reach
saturation. This can be mainly attributed to both the
ImpactofOpticalPreamplifierBeatNoiseonInter-SatelliteCoherentOpticalCommunicationSystem
49
received power and the LO-ASE beat noise increase
as the surge of preamplifier gain. It is not used for
G=10dB because of the SNR is less than 6dB. And it
is also no enhancement for SNR while the
preamplifier gain reaches to 30dB.
Figure 2: SNR versus gain of preamplifier with different
noise figure. (NF=5dB red line, NF=4.5dB magenta line,
NF=4dB blue line, NF=3dB black line).
Then we calculate the BER versus preamplifier gain
with noise figure as parameter. The results are
shown in Figure 3.The BER of the system is
decreasing as preamplifier gain increasing, but
gradually reach a minimum. It is worthy to know
that the BER is 10
-10
, 10
-9
, 10
-8
for the NF of 4dB,
4.5dB and 5dB, respectively when the gain larger
than 30dB. The BER will be increasing an order as
the NF grows up 0.5dB.
Figure 3: BER versus gain of preamplifier with different
noise figure. (NF=5dB red line, NF=4.5dB magenta line,
NF=4dB blue line, NF=3dB black line).
We set noise figure at the value of 4.5dB, and
calculate BER versus receive power P
r
with
preamplifier gain as parameter. As the results shown
in Figure 4, the sensitivity will be enhanced 1.5dB
when the BER is 10
-9
for comparing the gain of
20dB and 30dB. It is possible to enhance the
sensitivity via increasing the preamplifier gain with
the same noise figure to maintain BER.
Figure 4: BER versus received power with different gain.
(G=20dBm red line, G=25dB magenta line, G=30dB blue
line, G=35dB black line).
BER versus NF with different gain are calculated
and illustrated in Figure 5. The BER is
monotonically increasing as the surge of NF. It is
possible to decrease the BER via increasing the gain
if the NF maintains constant.
Figure 5: BER versus NF with different gain. (G=20dBm
red line, G=25dB magenta line, G=30dB blue line,
G=35dB black line).
OPTICS2015-InternationalConferenceonOpticalCommunicationSystems
50
As shown in Figure 6, it also concludes that we
could enhance the sensitivity via increasing the
preamplifier gain if the BER maintains constant.
Figure 6: Received power Pr versus gain with different
noise figure. (NF=5dB red line, NF=4.5dB magenta line,
NF=4dB blue line, NF=3dB black line).
We last calculate BER versus the rms phase error
with noise figure as parameter. And we set the gain
at the value of 30dB. The results are shown in Figure
7. In order to maintain BER at 10
-9
and the rms
phase error less than 0.175rad, it shows that the
noise figure should less than 4.5 dB if the gain is
30dB.
Figure 7: BER versus rms phase error with different noise
figure. (NF=5dB red line, NF=4.5dB magenta line,
NF=4dB blue line, NF=3dB black line).
4 CONCLUSIONS
We have studied the impact of optical preamplifier
ASE beat noise on an inter-satellite coherent optical
communication system. It is numerically shown that
the presence of preamplifier would enhance the
received sensitivity at a certain LO power level.
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ImpactofOpticalPreamplifierBeatNoiseonInter-SatelliteCoherentOpticalCommunicationSystem
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