Optics Communications 419 (2018) 120–124
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Optics Communications
journal homepage: www.elsevier.com/locate/optcom
A CNA-based blind equalization scheme for ACO-OFDM in VLC system
Jing He
a,
*, Rui Zhang
a
, Jin Shi
a
, Zuo Chen
a
, Yun Cheng
b
a
College of Computer Science and Electronic Engineering, Hunan University, Changsha, Hunan, China
b
Department of Information Science and Engineering, Hunan University of Humanities, Science and Technology, Loudi, Hunan 417000, China
A R T I C L E I N F O
Keywords:
Visible light communication (VLC)
Blind equalization
Constant norm algorithm (CNA)
Constant modulus algorithm (CMA)
Asymmetrically clipped optical orthogonal
frequency division multiplexing
(ACO-OFDM)
A B S T R A C T
In this paper, a blind equalization scheme based on the constant norm algorithm (CNA) is proposed for asymmet-
rically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) in visible light communication
(VLC) system. At the receiver, the CNA can be used to improve system spectral efficiency due to the free of
training symbols. The simulation results indicate that for ACO-OFDM signal, the CNA-based blind equalization
scheme achieves better mean square error (MSE) performance than that of constant mode algorithm (CMA).
Moreover, the CNA-based blind equalization scheme outperforms the CMA scheme in the term of bit error ratio
(BER) under same data rate in VLC system.
1. Introduction
Currently, due to the rapidly growing demand for bandwidth of
end-users, the spectrum congestion has become a serious issue in
radio frequency (RF) technologies. Visible light communication (VLC)
technology has been paid more attention due to its broad bandwidth,
unlicensed visible light spectrum and inherent security [1–3].
In VLC system, advanced modulation format such as orthogonal
frequency division multiplexing (OFDM) is widely used due to its
some advantages as high spectral efficiency and robustness to inter-
symbol interference (ISI) [4–7]. Intensity modulation is applied in VLC
system, in order to generate real-valued signals, Hermitian symmetry is
imposed on all subcarriers [8,9]. Furthermore, to obtain non-negative
signals, some schemes are proposed, such as DC-biased optical OFDM
(DCO-OFDM) [10] and asymmetrically clipped optical OFDM (ACO-
OFDM) [11]. In DCO-OFDM, the positive signal is obtained by adding a
DC bias, which will suffer from the loss of energy efficiency [9]. For
ACO-OFDM, the non-negative signal is obtained by clipping at zero,
and transmitting only positive parts of OFDM waveforms [12,13]. It has
lower spectral efficiency due to its special frame construction. However,
the ACO-OFDM signals retain the properties that make OFDM resilient
in a dispersive or multipath channel. And an additional bias is not
required for ACO-OFDM. Thus, it is more power-efficient than that of
DCO-OFDM.
For VLC system, the high peak-to-average power ratio (PAPR)
of OFDM in combination with the non-linear characteristics of light
emitting diodes (LEDs) will seriously impact bit error rate (BER)
performance. The technique focusing on PAPR reduction and non-
linear mitigation is proposed in VLC system, such as pilot-assisted
*
Corresponding author.
E-mail addresses: hnu_jhe@hotmail.com, jhe@hnu.edu.cn (J. He).
methods [14]. However, it requires a certain training sequence so that
it will reduce data rate. In addition, blind equalization technique is
proposed [15]. It aims to recover distorted signals adaptively without
the use of training sequence. And it can improve system bandwidth-
efficiency. As a classical blind equalization scheme, the constant mode
algorithm (CMA) with low complexity is proposed [16]. Moreover, the
convergence speed of CMA is analyzed [17]. It is clear that CMA-
type algorithms are constellation dependent. In order to apply to any
constellation, constant norm algorithm (CNA) is proposed [18]. It shows
that the performance of CNA scheme can outperform that of the CMA.
In this paper, a CNA-based blind equalization scheme is proposed
for ACO-OFDM in VLC system. The initialization value and step size
parameter are analyzed to search the optimal values that can achieve
faster convergence speed. It can improve system spectral efficiency. It
is due to without an additional training sequence. Simulation results
illustrate that the CNA-based blind equalization scheme achieves better
mean square error (MSE) performance than CMA in ACO-OFDM VLC
system. In addition, BER performances of CNA-based, CMA-based blind
equalization scheme and pilot-aided equalization scheme are compared
for ACO-OFDM signal in VLC system.
2. Principle
2.1. ACO-OFDM generation scheme
Fig. 1 shows the data allocation and zero clipping in ACO-OFDM.
The input serially frequency domain data 𝑋 = [𝑋
0
, 𝑋
1
, 𝑋
2
, … 𝑋
𝑁∕4−1
] is
initially converted into parallel data sequence. In ACO-OFDM, only the
https://doi.org/10.1016/j.optcom.2018.02.065
Received 22 December 2017; Received in revised form 1 February 2018; Accepted 26 February 2018
Available online 24 March 2018
0030-4018/© 2018 Elsevier B.V. All rights reserved.