Symbol error rate performance analysis of soft-decision
decoded MPPM free space optical system over
exponentiated Weibull fading channels
Duan Zhou (周 端)
1
, Tian Cao (曹 天)
2,3,
*, Yintang Yang (杨银堂)
4
,
Jianxian Zhang (张剑贤)
1
, Ping Wang (王 平)
2,5
, and Bensheng Yang (杨本圣)
2
1
School of Computer Science and Technology, Xidian University, Xi’an 710071, China
2
State Key Laboratory of Integrated Service Networks, School of Telecommunications Engineering,
Xidian University, Xi’an 710071, China
3
Southwest China Research Institute of Electronic Equipment, Chengdu 610036, China
4
Key Laboratory of the Ministry of Education for Wide Band-Gap Semiconductor Materials and Devices,
School of Microelectronics, Xidian University, Xi’an 710071, China
5
School of Physics and Optoelectronic Engineering , Xidian University, Xi’an 710071, China
*Corresponding author: tcao2014@163.com
Received December 2, 2016; accepted February 17, 2017; posted online March 13, 2017
The symbol error rate (SER) performance of a multipulse pulse-position modulation (MPPM) free space optical
(FSO) system under the combined effect of turbulence-induced fading modeled by exponentiated Weibull (EW)
distribution and pointing errors with a soft-decision detector is investigated systematically. Particularly, the
theoretical conditional SER (CSER) of soft-decision decoded MPPM is derived. The corresponding closed-form
CSER is obtained via curve fitting with the Levenberg–Marquardt method. The analytical SER expression over
the aggregated fading channels is then achieved in terms of Laguerre integration. Monte Carlo simulation results
are also offered to corroborate the validity of the proposed SER model.
OCIS codes: 060.2605, 010.1300, 010.1330.
doi: 10.3788/COL201715.050602.
In recent years, free space optical (FSO) communication
draws an enormous amount of attention because of
its large capacity, low cost, secure transmission, and
license-free spectrum
[1,2]
. It is regarded as an alternative
approach to the radio-frequency (RF) technology and
widely accepted to solve the “last mile ” problem where
fiber optic links are not practical
[2,3]
. However, FSO links
suffer a lot from atmospheric turbulence-induced fading
caused by random refractive-index fluctuations, which
will result in the system performance degradation
[4,5]
.
In order to predict the reliability of FSO channels under
different strengths of turbulence, some mathematical
models describing the probability density function (PDF)
of the received optical scintillation have been proposed
over the years, such as lognormal (LN), gamma-gamma
(G-G), and exponentiated Weibull (EW) distributions.
Among them, EW distribution is a novel one, which is
proposed and experimentally verified by Barrios and Dios
in Refs. [
6,7]. Their study revealed that this distribution
could model the PDF of irradiance in weak-to-strong tur-
bulence regimes under all aperture-averaging conditions.
Most importantly, apart from the perfect fit to right tail
of the PDF, the left tail of EW distribution is much better
than G-G and LN distributions, which is essential in assess-
ing the outage and error performances
[7]
. Due to this advan-
tage, some works concerning the performance of an FSO
communication system over EW fading channels have been
reported in these years
[8–12]
.
Currently, intensity modulation with direct detection
(IM/DD) is a feasible transmission scheme and popularly
used in practical FSO systems, where the transmitters and
receivers only modulate and detect the intensity of the
carrier without its phase
[13]
. Among the realizations of
IM/DD, on-off keying (OOK), pulse-position modula-
tion (PPM), and multipulse PPM (MPPM) are three rep-
resentative schemes. As is known, OOK is a binary level
modulation type and has been extensively used in many
existing works because of its simplicity and easy
implementation
[14]
. However, OOK has lower energy effi-
ciency so as to satisfy the requirement of the dynamic
threshold at the receiver
[2,15]
. In order to overcome this
drawback, PPM has been proposed, which is an orthogo-
nal modulation scheme that provides a reduction in aver-
age power consumption compared to OOK, but at the
expense of an increased bandwidth requirement
[13,16]
. This
is due to the fact that the time slot when an optical pulse
takes place is narrowed to increase the amount of informa-
tion transmitted per signal block
[17]
. Although it is cer-
tainly true that a large bandwidth is easy to achieve in
the optical band, spectral efficiency, from a practical
perspective, is also a crucial design consideration because
it is directly related to the required speed of electronic cir-
cuitry in FSO systems
[2]
. MPPM, proposed by Sugiyama
and Nosu in Ref. [
17], can substantially improve the
band-utilization efficiency of optical PPM, and it could
be a tradeoff between OOK and PPM. Therefore, it is
COL 15(5), 050602(2017) CHINESE OPTICS LETTERS May 10, 2017
1671-7694/2017/050602(5) 050602-1 © 2017 Chinese Optics Letters