2018 Asia Communications and Photonics Conference (ACP)

A combinding PAPR reduction method for DCO-

OFDM UOWC
Jurong Bai Hang Dai Yi Yang
School of Electronic Engineering School of Electronic Engineering School of Electronic Engineering
Xi'an University of Posts and Xi'an University of Posts and Xi'an University of Posts and
Telecommunications Telecommunications Telecommunications
Xi'an, China Xi'an, China Xi'an, China
jurongbai@163.com couragedh@163.com yangyi@xupt.edu.cn

Abstract—Optical orthogonal frequency division multiplexing Many techniques for reducing PAPR are listed in [5]. The
(OFDM) has been adopted in underwater optical wireless clipping method brings nonlinear distortion of OFDM signals
communication (UOWC) system, to achieve high data rate and that results in serious in-band noise and BER increase [6].
huge bandwidth, as well as to deal with the problem of inter Many iteratives are required in the PTS method, which
symbol interference. However, it has the problem of high peak-
increases its complexity [7]. Only part of the total subcarriers
to-average power ratio (PAPR) which will result in serious
performance deterioration and increase equipment costs. In this in the TR method are used for data transmission, the rest
paper, an SLM method is combined with μ-law companding subcarriers are reserved to carry the peak reduction signals.
method to reduce the PAPR of UOWC system, which reduce the Since this part of the subcarriers does not carry useful
signal PAPR by 6.8dB and is superior than both the traditional information, it brings low spectrum efficiency [8]. μ-law
SLM and μ-law companding methods. As for the system bit error companding is a non-linear transformation, which amplifies
rate (BER), the proposed method has better performance than small signals, and compresses large signals to reduce PAPR
the μ-law companding method. With reasonable tradeoff with the [9]. In [10], selective mapping (SLM) is a non-distorting
BER, the proposed method has faster convergence than the method. By reducing the correlation between input sequences,
traditional SLM method.
the modulation data have multiple forms of phases. Through
Keywords—Orthogonal frequency division multiplexing, comprehensive analysis, the signal with the lowest PAPR was
underwater optical wireless communication, peak-to-average ratio, selected for transmission.
selective mapping, companding In this paper, the SLM and μ-law companding schemes are
combined to reduce PAPR of O-OFDM signal in UOWC.
I. INTRODUCTION Compared with the traditional schemes, the PAPR of the O-
OFDM signal is reduced with much faster convergence.
Underwater wireless optical communication (UOWC)
research is a hot topic in the field of communication systems. II. SYSTEM MODEL
UOWC has the advantages of low power consumption and
high security [1]. Optical orthogonal frequency division Blue-green light is less attenuated in seawater channels than
multiplexing (O-OFDM) is a multi-carrier modulation other optical bands, and has the advantages of strong
technology that can be applied in various water environments. penetrating power, high speed, high efficiency, and strong anti-
interference ability. Therefore, the blue-green laser
It converts a set of high-speed serial data into a low-speed
transmission signal is most adopted in the seawater channel.
parallel data stream, so that each sub-carrier is orthogonal to
The optical properties of absorption and scattering can cause
each other for multiplexing. O-OFDM technology is good at severe attenuation, as illustrated in Figure 1 [11]. The total
anti-multipath interference. Its main idea is to convert the attenuation coefficient of seawater is the sum of seawater
frequency domain signal into the time domain signal through absorption coefficient and scattering coefficient. Absorption is
IFFT at the transmitting end. Its orthogonality makes O- a process in which photons lose energy and are converted into
OFDM system have high spectrum utilization. However, due other forms, which causes the propagation energy of light
to the superposition of multi-carrier signals, high peaks are continuously decrease, and limit the link distance of UOWC.
generated, resulting in excessive demands on the linear power Scattering is caused by changes in photon propagation
amplifier and increasing the industrial cost [2]. To solve this directions. In the UOWC system, the size of the aperture that
problem, many research have been take. absorbs photons is constant, therefore, the number of photons
The optical properties of seawater are mainly related to received by the receiver is reduced due to the effect of
three factors: purity of seawater, dissolved substances, and scattering. Moreover, the time at which the photons reach the
suspended particles [3]. Reference [4] studied the performance receiving surface may be different, and multipath dispersion
of UOWC systems, describes the efficient and reliable may occur [12]. Impurities in seawater have an absorption
characteristics of optical channels and comprehensively effect on light, and laser signals form a beam spread when
considered the performance of the transmitter and receiver applied in a seawater channel. When transmitting at a close
parameters under different water qualities.
Scientific Research Program Funded by Shaanxi Provincial Education
Department (16JK1702).

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 2018 Asia Communications and Photonics Conference (ACP)

range through the seawater channel, the delay of channel A. SLM Scheme
propagation can be ignored [13]. The main idea of the SLM method [5] is to copy OFDM
As illustrated in Fig.1, Pi (λ ) is the incident light power. input signal X = [ X1 , X 2 ,, X N −1 ] into U groups with a length of
Pa (λ ) is the part that is absorbed. Ps (λ ) is the part being N for transmission. The U-group data are multiplied by
different phase factors bu = [b1u , b2u ,, bNu −1 ] , in order to find a
scattered. The transmission formula of the optical channel in
the seawater channel is [14]: suitable phase factor. The phase factor should be as diverse as
possible. The smaller the correlation between the phase factors,
Pr = Pt ⋅  ar2 /( d ⋅ tan θ + at ) 2  ⋅ e − c ( λ )⋅d (1) the better the PAPR reduction performance for the OFDM
ar is the radius of the receiving antenna. Pt is the power signal obtained by multiplying the multipath signal with the
phase factor. Then IFFT is performed to obtain {x1 ,x 2 ,,xU } 。
emitted by the light source. Pr is the power received. θ is the
The one with the smallest PAPR is selected for transmission.
divergence angle of the light source beam. d is the underwater The PAPR reduction performance differs with the number of
optical communication distance. c represents the total U.
attenuation of the absorption and the scattering coefficients.
The phase factor is expressed as:
And λ is the laser wavelength.
Intensity modulation direct detection (IM/DD) is adopted bun = e jϕu ,n ,0 ≤ n ≤ N − 1
in the UOWC system, which requires the modulation signal to  (5)
be a positive and real number [15]. It is necessary to convert ϕ u ,n∈ [0,2π ],1 ≤ u ≤ U
the modulated bipolar complex signals into unipolar real
signals. The specific steps are: first go through the Hermitian B. Companding Scheme
Symmetry operation [8]. Next, the unipolar signal is obtained The traditional companding method is to amplify the small
by adding a DC bias to remove the negative part. Then, the signal with the large signal remains unchanged, which
transmission signal is ready for data transmission. increases the average power of the signal, thus reduces the
PAPR. Here we choose an improved μ-law companding
transform method [9], which amplifies small signals and
compresses large signals.At the same time, the C-transform of
the transmitted signal is given by:
Axn μ
Sn = C { xn } = ln(1 + xn ) (6)
ln(1 + μ ) xn A
Fig. 1. Geometry of inherent optical property.
where A is the average amplitude of the OFDM symbol, and it
is also a turning point of the C transform. μ is a constant,
III. PAPR REDUCTION usually less than 5.
OFDM signal is formed by operating IFFT to the QPSK The receiving end performs a C inverse transform on the
signal. The expression of the OFDM symbol in the time received signal according to:
domain is: A′xn  [ n A′ ] 
r ln(1+ μ )

yn = C −1 {rn } =  e − 1 (7)
1 N −1 j 2π kn μ rn  
xn =
N
 X ke
k =0
LN
, n = 0,1,, N − 1 (2)
where rn is the received signal after passing through the
where X k is the QPSK symbol expressed as {1, -1, j, -j}. L channel, and A′ is the average amplitude of rn .
denotes the oversampling rate. N is the number of subcarriers. C. Proposed Scheme
Due to the superposition of multiple orthogonal subcarriers, In this section, the system block diagram of SLM-μ
peaks will be generated. PAPR is defined as the peak power Companding, its theoretical analysis and improved design are
divided by the average power: depicted. Figure 2 shows the process of applying the proposed
SLM-companding to UOWC system.
PAPR(dB)=10log10
{ }
max xn
2

(3) b1

E{ x }
IFFT
2 2
b
Companding

n IFFT
min PAPR

Symmetry
Hermitian

P/S DAC
mapping

+DC CP
Seclect
QPSK

Data in
LD

Set a peak-to-average ratio threshold PAPR0 , and use b U

IFFT UOWC
CCDF to indicate the probability that the PAPR of the OFDM Channel

signal exceeds this threshold. The mathematical expression is:

Companding
Data× phase
QPSK De-

Amplifier
ADC S/P
mapping

Data out
-CP DC
Inverse

Photo-
FFT

detector
CCDF =Pr (PAPR ≥ PAPR0 ) (4)

Fig. 2. Schematic diagram of the UOWC system with SLM-companding.

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 2018 Asia Communications and Photonics Conference (ACP)

IV. SIMULATION companding methods. With greater μ, the PAPR reduction

In the simulation work, the number QPSK modulation is performance is better, when μ=3, the proposed method can
adopted and the number of subcarriers is 1024, of OFDM reduce signal PAPR by 6.8dB compared with the original
signal is 1000, μ=3. And blue-green laser is selected for signal.
transmission in the seawater channel.
ACKNOWLEDGMENT
Figure 3 shows the PAPR produced by the SLM-
companding scheme. It can be seen that the proposed algorithm The authors would like to thank every anonymous
can significantly reduce the PAPR compared with the reviewer for his/her detailed comments. The authors would
traditional SLM and μ-Companding transform methods. also like to thank the support by the Scientific Research
Compared with the SLM algorithm, the PAPR is reduced by Program Funded by Shaanxi Provincial Education
3.9 dB, which has 1.1 dB more reduction than the companding Department (16JK1702).
method. The PAPR is reduced by 6.8dB compared with the
original OFDM signal. In addition, it can be seen that as the μ REFERENCES
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