978-1-5090-0690-8/16/$31.00 © 2016 IEEE
A Relay Node Placement for Free Space Optical
Communication System Under Multi-user
Environments
Genfa Zhang, Jun Yan, Bingcheng Zhu, Wei-ping Zhu
College of Telecommunications and Information Engineering
Nanjing University of Posts and Telecommunications, Nanjing China 210003
Abstract—In this paper, a relay node placement for free-space
optical (FSO) communication system are proposed for different
multi-user and communication service requirements. First, the
relay problem is formulated as the constraint multi-objective
optimization framework. The outage probability function of each
user, the obstacle and feasible region are described as the objec-
tive function and the constraint function respectively. Then, the
multi-objective optimization problem is transformed into a sin-
gle-objective optimization problem, according to each different
communication service requirement. At last, the optimal position
of each relay node can be obtained using the linear weight pa-
rameter method. The simulation test are carried out under dif-
ferent practical environments.
Keywords—relay node placement; free-space optical communi-
cation; multi-hop; multi-objective optimization;
I.
I
NTRODUCTION
Free-space optical (FSO) communication is an optical
communication technology that uses laser beams or light-
emitting diodes (LEDs) to transfer data in free space. Because
of high bit rates, immunity to electromagnetic interference,
ease of deployment,inherent security and protocol transparency,
it has received much attention for academic and industry in
recent years. FSO can be utilized in a wide range of services
from short-distance outdoor communication to long-distance
deep space communications[1].
Since the turbulence-induced fading [2] in a clear weather
condition and the laser pointing error [3] in sunny day become
the main adverse factor in FSO communication systems, many
previous works have been done to mitigate these problem. As
we know, the above two negative effects on the FSO system
increase with the distance. So, when the distance of source and
destination reaches tens or even hundreds of kilometers, single-
hop FSO communication system is neither economic nor realis-
tic. Thus, multi-hop relay FSO communication system which
converting a long link into multiple short links is considered as
a best solution. This system has been proved that it can in-
crease the relative diversity order of the end-to-end perfor-
mance [4][5]. So, multi-hop relaying scheme has become a
research hot now.
In previous works, many researchers paid more attentions
to model and performance analysis about the FSO relay-
assisted system. Due to the complex turbulence fading channel,
little closed-form error rate expressions have been derived [6].
Until 2008, the authors in [4] obtained a closed-form outage
probability of FSO multi-hop decode-and-forward (DF) sys-
tems over lognormal channels. It shown that the fading severity
of relay-assisted FSO systems depends on the distance of each
hop in atmospheric turbulence channels, So, some techniques
have been studied to utilize the hop to improve the system per-
formance.
In [5], the authors rigorously showed that the hop distance
should be equals each other in order to minimize the outage
probability. So, some previous works have been turned to de-
termine the optimal relay position. In [7], some available relay
node placements have been proposed under the link obstacles
and the infeasible regions, according to the constraint functions
model of the obstacles and the feasible regions. However, this
first work on relay placement does not consider a practical sce-
nario with multi-user and different service requirements.
In practical transmission scenarios, under the FSO relay
communication system, the different kinds of communication
service are always required due to the followings. First, many
relay nodes can be utilized as the service node for information
utilization. Second, many user may have different communica-
tion services for multi-user conditions. Like the basic idea of
the cognitive radio system, some nodes only utilized as the
secondary users (SU) in the system, so the communication ser-
vice may be lower than the primary user ( destination user)
(PU). For another, the service time for each users may also
different. Some relay node only require little time to infor-
mation utilization than the destination user. Therefore, it is of
importance to search for the best available locations for relay
nodes placement under the above multi-user scenarios.
In this paper, we consider an intensity-modulation direct-
detection (IM/DD) FSO multi-hop decode-and-forward (DF)
system. When a relay node receives the incoming signal, after
decoding, it forwards the remodulated signal to the next node.
This process repeats at each relay node until the signal reaches
its destination. We propose an optimization framework to con-
sider the problem of optimal relay node placement on a 2-
dimensional (2D) plane under the multi-user condition. To the
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