for the UVLC system using high-order QAM modulation.
The environment in UVLC is more complex, which needs
advanced modulation formats to resist the loss of complex
environments, make the full use of modulated bandwidth,
achieve high-speed transmission, as well as obtain a stable
and reliable system.
Table
2 summarizes some achievements in the UVLC
systems in recent years. Recent research in LED UVLC
has mostly focused on utilizing different modulation
techniques to increase both the data transmission rate
and the link distance. In 2010, Doniec et al. of the National
University of Singapore achieved 0.6 Mb/s over 9 m by
using six blue LEDs as a transmitter, an avalanche photo-
diode (APD) as a receiver and digital pulse integration
modulation (DPIM) as the modulation format
[30]
.In
2014, Cossu et al. utilized two LED arrays and the
non-return-to-zero (NRZ) 8 b/10 b format to achieve
12.5 Mb/s over a 2.5 m distance. In 2016, Xu et al. from
Zhejiang University adopted OFDM modulation formats
and a compact blue LED, and finally obtained a data rate
of 161 Mb/s over 2 m. In 2017, a data rate of 200 Mb/s
system was achieved by Tian et al. in Fudan University.
They selected OOK, a simple modulation format, and
combined it with a μLED to transmit signals over 5.4 m.
In 2018, Kong et al. in Zhejiang University proposed an
underwater wireless optical communication (UWOC) sys-
tem using an arrayed transmitter/receiver and optical
superimposition-based pulse amplitude modulation with
4 levels (PAM4). The bit error rate (BER) under the
FEC threshold can be reached for the 12.288 Mb/s
PAM4 signal after transmitting through a 2 m underwater
channel. In the same year, Wang et al. in Fudan University
proposed an underwater VLC system utilizing QAM-DMT
and multi-PIN reception to do the maximum-ratio combin-
ing (MRC) receiving. The data rate of 2.175 Gb/s transmis-
sion over 1.2 m was achieved successfully.
As a promising solution, the Volterra series-based
equalizer plays significant role in mitigating the nonline-
arity in the VLC
[31]
. In our group’s previous work, a blind
post-equalization scheme called the cascaded Volterra
modified multi-modulus algorithm was employed and
demonstrated to compensate for linearity and mitigate
the LED nonlinearity in the CAP modulation-based
VLC system
[32]
.
We choose three advanced modulation formats which
are CAP, orthogonal frequency-divisi on multiplexing
(OFDM) and DFT-S OFDM modulation for UVLC. To
the best of our knowledge, the performance comparison
of these three modulation formats has not been reported,
especially for UVLC. Such comparison is of great value
considering the requirements of high-speed and valid
transmission for underwater applications.
In this Letter, we present a comprehen sive comparison
of advanced modulation formats including CAP, OFDM,
and DFT-S OFDM. We also discuss the corresponding
digital signal processing (DSP). For each format, a
post-equalizer structu re consisting of two cascaded stages
is suggested. For the first time, a post-equalizer based on
novel recursive least square (RLS)- Volterra is presented
to mitigate the nonlinear effect and improve the system
performance of UVLC. The Volterra is used to compen-
sate the nonlinearity and the RLS algorithm is used to up-
date the tap coefficients of Volterra. Then these three
typical advanced modulation formats are all realized at
the same bit rate of 3 Gbit/s. The comparison is carried
out to evaluate the performance of each modulation for-
mat in terms of peak-to-average power ratio (PAPR),
nonlinear equalization, LED current, optical power, and
peak-to-peak voltage. Finally, CAP-64 and DFT-S
OFDM are successfully achieved over 1.2 m of UVLC
transmission under a hard-decision forward error
correction (HD-FEC) threshold of 3.8 × 10
−3
. However,
Table 2. Summary of Research Results of LED UVLC Systems
Transmitter
Modulation
Formats Equalization Receiver Data Rate
Distance
(m)
Authors and
Research Group Year
Six blue LEDs DPIM / APD 0.6 Mb/s 9 Doniec et al., National
University of Singapore
[30]
2010
Two-LED
arrays
NRZ
8 b/10 b
/ APD 12.5 Mb/s 2.5 Cossu et al., Scuola
Superiore Sant’Anna
VTeCIP, Italy
[33]
2014
Compact blue
LED
OFDM Pre/Post PIN 161 Mb/s 2 Xu et al., Zhejiang
University
[34]
2016
μLED OOK / PIN/
APD
200 Mb/s 5.4 Tian et al., Fudan
University
[35]
2017
Two blue LEDs PAM4 / MPPC
(contain
SPADs)
12.288 Mb/s 2 Kong et al., Zhejiang
University
[36]
2018
Blue silicon
substrate LED
QAM-DMT Pre/ Post PIN 2.175 Gb/s 1.2 Wang et al., Fudan
University
[37]
2018
COL 16(12), 120603(2018) CHINESE OPTICS LETTERS December 10, 2018
120603-3