Performance of Free Space Optical Communication Link Under

Foggy Weather Regarding Different Wavelengths

Cheng Bai

, Jingyu Wang, Shaohua Zhou, Ruijian Rao and Tao Wang

College of Information and Communication, National University of Defense Technology, Wuhan, China

Keywords: Free Space Optic, Foggy Weather, Attenuation, Operating Wavelength.

Abstract: The stochastic influence of weather conditions on atmospheric channel limits the availability and stability of

the free space optical (FSO) system. Foggy weather is the main challenge of free space optical communication

system. Based on the attenuation model of laser in foggy weather, this paper simulates and analyses the

influence of fog on transmission performance objectives such as bit error rate and quality factor of FSO system

with different operating wavelengths. The simulation results show that the scattering effect of fog particles is

related to the transmission wavelength, the lower the operating wavelength, the greater the fog attenuation,

and the wavelength window of 1550nm is more suitable to FSO system.

1 INTRODUCTION

Free Space Optics (FSO) is a communication method

in which the laser beam uses the atmosphere as the

transmission medium to establish a connection

between the transmitter and receiver and transmits

high bandwidth data. FSO combines many features of

wireless communication and optical communication,

and can realize large capacity broadband data

transmission, which has the characteristics of

flexibility in networking, no spectrum authorization,

outstanding anti-electromagnetic interference ability

and good confidentiality. However, free space optical

communication is vulnerable to atmospheric

turbulence and path loss such as rain, fog and cloud,

and its availability needs to be further improved.

The main factors affecting free space optical

communication link are absorption, scintillation and

scattering. An obvious defect of FSO link in

troposphere is its sensitivity to weather conditions.

Various weather conditions such as rain, fog, snow

and haze, will have varying degrees of impact on

optical transmission (Nebuloni, 2005). Small changes

in atmospheric transmission path may lead to changes

and distortions in laser beam. Some unpredictable

environmental factors may even produce very strong

attenuation in the optical links, which will eventually

affect the transmission performance of FSO systems,

https://orcid.org/0000-0003-3180-6667

and even the communication failure and interruption

will be caused. It can be said that the performance of

FSO system is highly dependent on the weather

conditions at the deployment site. Therefore, the

weather of the deployment site should be investigated

in advance before the actual deployment of FSO

system.

The stochastic influence of weather conditions on

atmospheric channel limits the availability and

stability of FSO system, which leads to the fact that

this system has not been widely used. This paper

focuses on the influence of foggy weather on FSO

system. Fog is considered to be the main challenge of

FSO system (Madhuri et. al., 2018). Under the

conditions of dense marine fog and medium

continental fog, optical signal attenuation can even

reach 480dB/km (Khan and Muhammad, 2012) and

130dB/km (Flecker et. al., 2006). Fog particles are

spherical in shape and have radii varying between

0.01 and 15µm, depending on geographical location

(Muhammad and Sheikh, 2007). Mie scattering is

dominant in foggy weather, which resulting in optical

attenuation. The beam in free space is most easily

attenuated by fog drops, which makes fog become a

key factor in the attenuation of optical power and

irradiance. Flecker et. al. made a detailed analysis of

the experimental test results of the continental city

Graz and the coastal city Nice, and compared the

different characteristics of the marine fog and the

Bai, C., Wang, J., Zhou, S., Rao, R. and Wang, T.

Performance of Free Space Optical Communication Link Under Foggy Weather Regarding Different Wavelengths.

DOI: 10.5220/0011946200003612

In Proceedings of the 3rd International Symposium on Automation, Information and Computing (ISAIC 2022), pages 385-390

ISBN: 978-989-758-622-4; ISSN: 2975-9463

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

385

continental fog in optical attenuation (Flecker et. al.,

2006).

According to the different application scenarios,

the FSO system adopts various wavelengths for

communication transmission. The selection of its

operating wavelength is mainly based on the

attenuation of optical signals in the atmosphere.

Although lasers at 1060nm have very low attenuation

values in atmospheric transmission, few FSO systems

currently operate at this wavelength due to the

difficulty of finding optical transmission devices

working at this wavelength. Optical receiving and

transmitting components with operating wavelength

of 850nm have low manufacturing cost and small

attenuation. Some low-cost FSO systems mainly

work in this wavelength window (Dautov et. al.,

2017). The wavelength window of 1550nm also has a

wider range of low loss window, which is convenient

for the transmission of WDM signals. Therefore, this

wavelength window has become another major

option of FSO system. Gebhart et. al. provide

comparisons of fog attenuation effect of different

wavelengths, however the influence of fog

attenuation effect on the transmission performance of

FSO system has yet to be further investigated

(Gebhart et. al., 2005). Madhuri et. al. analysed the

optical power and quality factor of FSO system

operating at 1310nm, but lacked the comparative

analysis of the system operating at 850nm and

1510nm (Madhuri et. al., 2020).

In order to improve the design of free space

optical communication systems, this paper primarily

concentrates on the internal parameters which are

transmission wavelength, bit rate, quality factor, link

range for the performance analysis of the terrestrial

FSO link under fog weather conditions. Two different

wavelengths (850nm, 1550nm) are used in order to

find the most appropriate operating wavelength for

transmission of signals. OptiSystem software is used

to simulate all the above parameters. The

organization of this paper is as follows: The effect of

foggy weather condition on FSO system and the

models of fog attenuation are discussed in Section 2.

Following that, the simulation experimental system

of the free space optical communication is set up in

Section 3. The results are presented and discussed in

the next section.

2 ATTENUATION MODEL

The attenuation of fog can be calculated both

theoretically and empirically. The theoretical

research comes from the Mie scattering theory.

Because the shape, direction and complex chemical

composition of fog particles cannot be predicted

prior, it is very complicated and time-consuming to

calculate the attenuation coefficient by means of the

Mie scattering theory. To overcome this, an empirical

model is usually used to predict the attenuation effect

of fog on laser propagation based on actually

measured visibility value (Anandkumar and

Sangeetha, 2021).

The earliest empirical model used by the

researchers to calculate fog attenuation is the Kruse

model, which is convenient in the narrow wavelength

range of 785-1550 nm. The attenuation coefficient is

expressed as

𝛽

(

𝜆

)

.













(1)

where, β(λ) represents the atmospheric attenuation

coefficient, V is the measured visibility value (km), λ

represents the optical wavelength (nm), and q

represents the size distribution parameter of scattered

particles, and its common values can be obtained

according to the Kruse model (Alkholidi and Altowij,

2014).

𝑞=

1.6, 𝑉≥50𝑘𝑚

1.3, 6𝑘𝑚≤𝑉<50𝑘𝑚

0.585𝑉

/

, 𝑉≤6𝑘𝑚

(2)

The formula for calculating fog attenuation

coefficient using the Kruse model is related to

visibility, which is more convenient and practical in

the narrow wavelength range of 785-1550 nm.

However, in the strong fog or haze weather with

visibility less than 6km, there is a certain deviation in

the Kruse model, and the Kim model (Kim et. al.,

2001) modifies the q value in the Kruse model.

𝑞=

⎩

⎪

⎨

⎪

⎧

1.6, 𝑉≥50𝑘𝑚

1.3, 6𝑘𝑚≤𝑉<50𝑘𝑚

0.16 𝑉+ 0.34, 1𝑘𝑚≤𝑉<6𝑘𝑚

𝑉− 0.5, 500𝑚≤𝑉<1𝑘𝑚

0, 𝑉<500𝑚

(3)

The Kim model is an extension of the Kruse

model, which helps to obtain better accuracy in low

visibility. In order to better predict the attenuation

effects of different types of fog, Bouchet et. al.

proposed the Advection model and the Radiation

model respectively for advection fog and radiation

fog in the atmospheric (Bouchet et. al., 2005).

𝛾

_

(𝜆)=

..



(4)

ISAIC 2022 - International Symposium on Automation, Information and Computing

386

𝛾

_

(𝜆)=

.



..



(5)

At this point, the calculation expression of fog

attenuation coefficient is

𝛽

(

𝜆

)



 ()

⋅𝛾(𝜆) (6)

In advection fog, the atmospheric particles move

laterally or horizontally, while in radiation fog, they

move circumferentially. Based on the particle

distribution and density of atmospheric channel, the

Bouchet model is suitable for advection fog and

radiation fog with wave length of 690-1550nm and

visibility of 0.05-1km, which not only reflects the

visibility factor, but also describes the relationship

between optical attenuation and wavelength in

different types of fog.

Figure 1: Relationship between fog attenuation coefficient

and visibility with different models.

Figure 2: Relationship between fog attenuation coefficient

and visibility with different wavelengths.

Kim, Kruse and Bouchet (advection fog and

convective fog) models are the most commonly used

empirical models to predict fog attenuation

coefficient using visibility. Figure 1 depicts the

variations of the fog attenuation coefficient with

different visibility which is calculated by using

different empirical models. It can be seen that the

attenuation coefficient is closely related to visibility.

With the decrease of visibility, the attenuation

coefficient gradually increases. This trend is more

significant when the visibility is less than 100m (i.e.

in dense foggy weather), and the attenuation

coefficient can even reach more than 100dB/km at

this time. Figure 2 further describes the influence of

different wavelengths (850nm and 1550nm) on the

fog attenuation coefficient. As the transmission

wavelength increases, the attenuation coefficient

decreases, which is more obvious in the Kruse model.

Table 1 shows the attenuation coefficients of laser

in different levels of fog calculated by the Kruse

model and the Kim model, it can be seen that in the

cast of mist, compared with the wavelength of

850nm, the optical signal operating at 1550nm suffers

less attenuation.

Table 1. Attenuation coefficient in foggy weather.

Fog

level

Visibility

(km)

The attenuation coefficient

(dB/km)

The Kruse

Model

The Kim Model

850nm 1550nm 850nm 1550nm

Mist

10 0.22 0.10 0.22 0.10

4 0.65 0.37 0.64 0.35

2 1.42 0.91 1.47 0.99

1 3.03 2.13 3.15 2.33

Heavy

fog

0.5 6.39 4.84 7.82 7.82

0.2 16.85 13.72 19.56 19.56

Dense

fog

0.05 71.23 62.58 78.24 78.24

3 SIMULATION EXPERIMENT

SYSTEM

OptiSystem software is used to simulate the wireless

optical communication system. The simulation

experiment system uses the OOK modulation scheme

is the simplest scheme and is widely applied in the

commercially available FSO systems. OOK is very

sensitive to channel turbulence. Non-Return-to-Zero

(NRZ) and Return-to-Zero (RZ) OOK models are

often used within the OOK modulation scheme due to

their easy implementation and cost effectiveness. The

NRZ

OOK modulation provides higher bandwidth

Performance of Free Space Optical Communication Link Under Foggy Weather Regarding Different Wavelengths

387

Figure 3: FSO system designed in OptiSystem.

efficiency than the RZ OOK. In order to analyse the

influence of foggy weather on FSO transmission

performance, the simulation experimental system of

the free space optical communication is set up as

shown in Figure 3.

In the simulation system, the CW Laser is used to

generate carrier optical signal. Two lasers with

different wavelengths (850nm and 1550nm) were

used to compare the influences of wavelengths on the

transmission performance of the link. The Pseudo-

Random Bit Sequence Generator generates a sequence

of input data bits at a rate of 155.520Mbps. Then, a

Non-Return-to-Zero Pulse Generator (NRZ Pulse

Generator) is used to code modulate the input data as

the modulation input of the external modulator. Then

the carrier signal is encoded by a multiplexer, and the

non-return to zero code signal generated by the Non-

Return-to-Zero Pulse Generator is loaded into the

optical carrier signal via On-Off Keying modulation

(OOK) in the external Mach-Zehnder Modulator.

Finally, the optical signal generated by the modulation

is transmitted to the receiver through the free space

optical channel (FSO Channel).

The receiving part of the simulation system is

composed of photoelectric detector, low pass filter,

error analyzer. In the experiment, the receiver

detector adopts APD photoelectric detector, which

can improve the sensitivity of the receiving system.

Low Pass Bessel Filter is used to filter the noise. The

BER analyzer and 3R Regenerator are used to test the

system transmission performance. The specific

parameters of the simulation experiment system are

shown in Table 2.

Table 2. Parameters of FSO system

Parameters Value

Transmitter aperture diameter, d

(m) 0.05

Receiver aperture diameter, d

(m) 0.15

Beam diver

ence,

(

mrad

)

Transmitted o

tical

ower,

transmit

(

dBm

)

Wavelen

th, λ

(

)

850,1550

Link Range,

(km) 0.6~3

4 RESULTS AND DISCUSSIONS

Figure 4 and Figure 5 show the variation of system

transmission performance with link range under

foggy weather in different visibility. Figure 4 is the

result of Quality factor (Q-factor), and Figure 5 is the

result of average bit error rate. The greater the

visibility, the less the fog effect, and according, the

average bit error rate of the system is lower, the Q-

factor value is higher, which means the transmission

performance is good. When the visibility is 10km and

the link range is 2.7km, the requirement of average

bit error rate (BER ≤ 10

-9

, corresponding Q-factor ≥

6) can still be met. When the visibility is 1km, the

system performance degrades due to the large

attenuation. When the operating wavelength is

1550nm, the maximum available link length of the

system is 2.6km. When the operating wavelength is

850nm, the maximum available link length of the

system is only 2.35km. If the link range exceeds the

maximum available length, the transmission quality

cannot be guaranteed, and the link transmission may

even be blocked.

Figure 4: Influence of different fog weather on Quality

factor (Q-factor).

ISAIC 2022 - International Symposium on Automation, Information and Computing

388

Figure 5: Influence of different fog weather on average

BER.

Figure 6 shows the eye diagrams of two systems

with different operating wavelengths when the

visibility is 1km and link range is 2.1km. Figure 6 (a)

is a system with an operating wavelength of 850nm.

At this time, the opening eye pattern is small, the eye

height which indicating the opening size of eye is

4.20×10

-6

, the bit error rate is 3.33×10

-9

, and the Q-

factor is 5.73, which is not enough to ensure the

transmission quality of the system. Figure 6 (b) shows

the system with an operating wavelength of 1550nm,

and the opening eye pattern is better. The eye height

is 7.64×10

-6

, the corresponding bit error rate is

2.39×10

-11

, and the Q-factor is 6.50. The transmission

performance of the system is stable, and the

transmission distance of the system can be further

increased.

(a)

(b)

Figure 6: Eye diagram of two systems with different

operating wavelengths in foggy weather (a) 850nm and (b)

1550nm.

It can be seen from the simulation experiment that

the system with the operating wavelength of 1550nm

has a higher Q-factor and better transmission

performance. At the same time, the system with

1550nm operating wavelength can still meet the

requirements of system bit error rate (≤ 10

-9

) at

2.6km, while the system with 850nm operating

wavelength can no longer satisfy the requirements of

system transmission quality at 2.35km, and the

maximum transmission distance of the system is

relatively short.

5 CONCLUSIONS

As the size of fog particles is equivalent to the

operating wavelength of FSO system, foggy weather

has the most prominent influence on the atmospheric

transmission link among various weather phenomena.

The scattering effect of fog particles in the

atmosphere will lead to the attenuation of the

transmission beam, weaken the received optical

power, affect the transmission performance of the

system, and even cause the interruption of the system.

The scattering effect of fog particles is related to the

transmission wavelength. The larger the operating

wavelength is, the smaller the attenuation is. In this

case, the transmission quality of FSO system is better,

and the transmission link range is also longer.

Performance of Free Space Optical Communication Link Under Foggy Weather Regarding Different Wavelengths

389

Therefore, by predicting the effects of foggy weather

on FSO systems with different operating wavelengths

in advance, the resulting performance degradation

can be estimated and the outage probability can be

reduced.

The simulation experiment system uses the OOK

modulation scheme is the simplest scheme and is

widely applied in the commercially available FSO

systems. OOK is very sensitive to channel turbulence.

This paper studies the performance of Free Space

Optics communication method in foggy weather

taking in consideration different wavelengths while

implementing the simulation. A new practical means

to further facilitate wireless communications is

provided, and these results can be at a point of

reference, in designing a Future FSO system.

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