Low complexity DCO-FBMC visible light communication system

International Journal of Electrical and Computer Engineering (IJECE)
Vol. 10, No. 1, February 2020, pp. 928~934
ISSN: 2088-8708, DOI: 10.11591/ijece.v10i1.pp928-934  928
Journal homepage: http://ijece.iaescore.com/index.php/IJECE
Low complexity DCO-FBMC visible light
communication system
Abdullah Ali Qasim1
, M. F. L Abdullah2
, Husam Noman Mohammedali3
, Rahmat Bin Talib4
Mohammed N. Nemah5
, Ahmed T. Hammoodi6
1,2,4,6
Department Communication Engineering, University Tun Hussein Onn Malaysia, Malaysia
1,3,5
Engineering Technical College-Najaf, Al-Furat Al-Awsat Technical University, Iraq
Article Info ABSTRACT
Article history:
Received Feb 25, 2019
Revised Sep 30, 2019
Accepted Oct 8, 2019
Filter Bank Multicarrier (FBMC) is a new waveform candidate in
the visible light communication system (VLC). FBMC is a distinctive kind of
multi-carrier modulation that can be regarded as an alternative to orthogonal
frequency Division Multicarrier (OFDM) with CP (cyclic prefix). DCO-
FBMC (DC-bias optical FBMC) has recently been used in VLC, because it
overcomes all defects in the optical-OFDM system and has high spectral
efficiency. But at the same time the traditional DCO-FBMC suffers from high
complexity due to the use of Hermitian Symmetry for real signal, by using 2N-
point subcarrier IFFT (Inverse Fast Fourier Transformer) in the modulator,
and the output is N-point subcarrier FFT (Fast Fourier Transform) in
the demodulator. In this paper, for the first time, the possibility of minimizing
complexity and generating a real signal without the use of Hermitian
Symmetry or any other technique has been verified. The proposed technology
provides 50% of the size of the IFFT / FFT and this results in a significant
reduction in power consumption and occupied chip area.
Keywords:
Complexity
DCO-FBMC
Hermitian symmetry
Real signal
VLC
Copyright © 2020 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Abdullah Ali Qasim
Department Communication Engineering,
University Tun Hussein Onn Malaysia,
Parit raja, 86400 Batu pahat, Johor, Malaysia.
Email: alzubydea@gmail.com, alzubydea@atu.edu.iq
1. INTRODUCTION
VLC has attracted high interest from researchers because of its unlicensed bandwidth, the fact that
these systems use visible light that has no electronic interference or health risk. Intensity modulation at
the transmitter and direct detection at receiver (IM/DD) is customarily applied in VLC systems, because of its
simplicity [1]. Therefore using the technique mentioned above, this confirms that the signal transmitted through
the light source (light emitting diode (LED) or laser diode (LD)) is in the form of optical power [2],
indicating that it is a positive light signal [3]. The wavelength electromagnetic spectrum range in VLC from
400 to 700 nm [4].
One of the most common problems facing this VLC is the limited bandwidth modulation, so the trend
is to use highly efficient modulation systems, to overcome this problem, many modulation schems were used,
and the most prominent of which is the OFDM technique in VLC [5]. The transmitted signal must be real to
achieve this is used Hermitian-symmetry [6, 7]. OFDM schemes such as DC bias optical OFDM
(DCO-OFDM), and the other types of schemes each scheme has its pros and cons [8, 9]. Researches has found
that OFDM suffers from limited bandwidth due to the presence of CP OFDM suffers from leakage spectrum
because of its large side lobes [5]. As in RF communication system, FBMC was considered a strong competitor
to OFDM, in the fact, FBMC is a candidate waveform for 5G [10, 11].

Int J Elec & Comp Eng ISSN: 2088-8708 
Low complexity DCO-FBMC visible light communication system (Abdullah A. Qasim)
929
According to Fourier transforms properties, for traditional OFDM, if the discrete input sequence of
the modulator has its first and center coefficients null, and if it presents Hermitian symmetry with respect to its
center, then the OFDM time modulated signal is real [12], while existing an OFDM instituted modulation,
FBMC inherits from many features related to traditional OFDM systems [13]. Nevertheless, the Hermitian
symmetry of the frequency symbols ends in a duplicated size of FFT/IFFT components [14]. Consequently,
to modulate N frequency symbols, 2N-point IFFT/FFT transforms are required [15]. This increases
the complexity of the system described. In [16] and [17] the authors presented a new technique to solve this
problem in the OFDM system and obtain a real signal without resorting to the use of Hermitian symmetry.
In this paper, first time investigated the new DCO-FBMC system in the VLC system without using
Hermitian symmetry and get real signal compatible with IM/DD to reduce the complexity of the DCO-FBMC
system and less power consuming. Moreover, the performance of the new technology has been demonstrated
to have a bit error rate (BER) where it is found to do the same performance as conventional technology.
2. CONVENTIONAL DCO-FBMC IN VLC
FBMC is one of the most promising modulation techniques in the field of optical communications,
because it provides high spectral efficiency besides other advantages. Like any modulate methods of optical
communication technology, the outgoing signal must be real and positive [18]. In this kind of systems serial
data are input to the division of data in parallel and are set in frequency of which usually employ offset
quadrature amplitude modulation (OQAM). In order to obtain real FBMC signal the input frequency symbols
to the IFFT are forced to have Hermitian symmetry [10]. The FBMC transmitted signal can be expressed
as follows:
𝑥(𝑛) = ∑ ∑ 𝑎 𝑚,𝑛 𝑔 𝑚,𝑛(𝑡)
2𝑁−1
𝑚=0
+∞
𝑛=−∞
(1)
where, m is the time index; and n is the subcarrier index; ɑm,n is the symbol (massage) being transmitted; N is
the number of subcarriers; and g(t) is the synthesis function which maps ɑm,n into the signal space; gm,n(t) is
the shifted version in time and frequency as (2) [19].
𝑔 𝑚,𝑛(𝑡) = ℎ(𝑛 − 𝑚𝒯0)𝑒
𝑗2𝜋/𝑁𝑘(𝑛−
𝐿 𝑝−1
2
)
𝑒 𝑗Φ 𝑚,𝑛 (2)
where, τ0 is the symbol spacing in time; Lp is the length of the pulse shaping filter (LP=K*N); K is
the overlapping factor; h(n) is the pulse shaping filter (known prototype filter).k is the subcarrier index.
ℎ(𝑛) = 1 + 2 ∑(−1) 𝑘
𝐻𝑘 cos(2𝜋
𝑘𝑛
𝐿 𝑃
)
𝐾−1
𝑘=1
(3)
Hk is set of coefficients related to K [20].
As a result of the use of Hermitian symmetric and the properties of the Inverse Fast Fourier
Transformer (IFFT), the imaginary signal is excluded and the outgoing signal is real as shown in Figure 1.
Figure 1 shows that M = N / 2. This means that only half of the sub-carriers are used, while the other half is
excluded as a result of the use of Hermitian symmetry. It is needed 2N size symbol FFT to get N size.
The system will suffer from high energy consumption and occupy higher chip area [21].
Figure 1. IFFT signal after hermitian symmetry showed the output for real signal and imaginary signal

 ISSN: 2088-8708
Int J Elec & Comp Eng, Vol. 10, No. 1, February 2020 : 928 - 934
930
3. NEW TECHNIQUE DCO-FBMC
The use of Hermitian symmetry in modulation systems was necessary to generate a real signal,
because this is a major condition in VLC, but there are disadvantages of this technique including increasing
the complexity, which leads to doubling the size of IFFT/ FFT. This motivation prompted researchers in [8, 9]
to find a new technique for generating a real signal in the OFDM system without using Hermitian symmetry.
In this research, the same technique was implemented in the FBMC technique, but in different steps as follows.
3.1. Design new optical DCO-FBMC transmitter
The transmitted design of FBMC as shown in Figure 2, the incoming symbols are processed into
the OQAM block where each symbol is filtered using the pulse shaping filter in (2) and each sub-carrier real
symbol (am,n) which corresponds to the real part or the imaginary part of the complex FBMC. The transmitted
signal must be a real signal, but the signal coming out of the IFFT and passing from P/S (parallel to serial
converter) is a complex one (consisting of a real part and an imaginary part), and hence the imaginary part
must be converted into a real part. The complex signal must be separated into two parts; real and imaginary
part, then Juxtaposed technique is done which means laying the imaginary part in sequence with the real part.
Figure 2. New DCO-FBMC optical transmitter block diagram
The frequency vector, ɑm,n, is directly inputted to an N-point IFFT as in traditional complex FBMC
systems since no Hermitian symmetry limitation is decreed, the IFFT output signal is complex and can be
expressed in (4).
𝑥(𝑛) = ∑ ∑ 𝑎 𝑚,𝑛 𝑔 𝑚,𝑛(𝑡)
𝑁−1
𝑚=0
+∞
𝑛=−∞
(4)
The complex time signal can be expressed precisely in (5). This technique is applied after P/S block.
𝑥(𝑛) = 𝑎(𝑛) + 𝑗𝑏(𝑛), 𝑛 = 1,2,3, … . 𝐾 ∗ (𝑁 − 1) (5)
where ɑ and b are respectively the real and imaginary components of x(n). The KN-point real and imaginary
parts of the generated in the time domain complex signal are Juxtaposed after P/S, this process generated
2-KN real FBMC in (6).
𝑥2𝐾𝑁 = {
𝑎(𝑛) 𝑛 = 0, … . . , 𝐾(𝑁 − 1)
𝑏(𝑛 − 𝐾𝑁) 𝑛 = 𝐾𝑁, … . . ,2𝐾(𝑁 − 1)
(6)
Figure 3 shows the real signal and the imaginary signal after their separation before applying the new
technique. After that, the imaginary juxtaposed real signal in the new technique is applied as in Figure 4, thus,
it is noted that the signal length is 2KN. The signal’s emerging after the application of juxtaposed technology
is bipolar, for one of the most important conditions of the signal must be positive, and that is a unipolar signal.
DC-biased optical FBMC is one of the simplest and primitive approaches that were proposed to generate
unipolar FBMC system compatible with IM/DD systems. The DC bias desired to assure non-negativity is
equivalent to the absolute value of the negative greatest amplitude of the bipolar FBMC signal. the method is
to employ a DC-bias Bdc proportional to the root square of electric power (σ) as in (7) [22].
𝐵 𝑑𝑐 = 𝑏𝜎 (7)
where b is the clipping factor and σ2
is the variance of x(n) defied as,
𝜎2
= 𝛦{𝑥2
(𝑛)} (8)

931
This is defined in [15] as a bias of 10𝑙𝑜𝑔10(𝑘2
+ 1)𝑑𝐵.the DC-bias
𝑥 𝐷𝐶(𝑛) = 𝑥(𝑛) + 𝐵 𝑑𝑐 (9)
Figure 5 shows the new propose DCO-FBMC at 13.6dB negative clipping.
Figure 3. Output real and imaginary FBMC signal after separated
Figure 4. Imaginary signal juxtaposed to real signal at length 2LP
Figure 5. Add DC-bias optical to the signal at (13.6dB)
3.2. Design new dco-FBMC receiver
When the optical signal is received at the photodetector, the optical signal is converted to an electrical
signal. Then the first step is to remove the DC-bias effect and then the real signal is separated from
the imaginary part according to (10).
𝑦(𝑛) = {
𝑎 𝑟(𝑛) = 𝑦2𝐾𝑁(𝑛), 𝑛 = 0, . , 𝐾(𝑁 − 1)
𝑏 𝑟(𝑛) = 𝑦2𝐾𝑁(𝐾𝑁 + 𝑛) 𝑛 = 0, . . , 𝐾(𝑁 − 1)
(10)

 ISSN: 2088-8708
932
where ɑr and br are respectively the real and imaginary components of received signal y(n) then reshaped
the complex then pass the signal to analysis filter bank demodulation as the conventional FBMC system all
this shows in the block diagram in Figure 6.
Figure 6. New design DCO-FBMC receiver block diagram
4. COMPUTIONAL COMPLEXITY
The FBMC technology is a complex technique, but it has advantages that distinguish it from the rest
of the systems used in VLC. A new technique called Juxtaposing is proposed to reduce this complexity and
obtain a real signal instead of using the traditional techniques as shown in Figure 7 and Figure 8, to calculate
the complexity divided into two parts multiplications and additions using the Radix-2FFT algorithm [23].
Figure 7 and Figure 8 illustrate the comparison between the complexity of new technology and traditional
technology calculated according to equation (11) and (12) for new techniques, and (13) and (14) for
conventional [24, 25].
𝐶 𝑚𝑛𝑒𝑤 = 𝑁(𝑙𝑜𝑔2( 𝑁
2⁄ ) − 3) + 8 + 4(𝑁𝐾 + 1) (11)
𝐴 𝑎𝑛𝑒𝑤 = 3𝑁(𝑙𝑜𝑔2( 𝑁
2⁄ ) − 1) + 8 + 4(𝑁𝐾 − 𝑁 + 1) (12)
𝐶 𝑚𝑐𝑜𝑛 = 2𝑁(𝑙𝑜𝑔20(𝑁) − 3) + 8 + 4(2𝑁𝐾 + 1) (13)
𝐴 𝑎𝑐𝑜𝑛 = 6𝑁(𝑙𝑜𝑔2(𝑁) − 1) + 8 + 4(𝑁𝐾 − 𝑁 + 1) (14)
Figure 7. FBMC multiplication complexity Figure 8. FBMC additional complexity
where, Cmnew, Aanew ,Canew, and Aacon respectively are multiplications and additional calculation of new and
conventional DCO-FBMC technique. The Figures explained that the new technology offers a complexity of
about 50% less than conventional technology. When the FFT / IFFT size increases, both power consumption
and the occupied chip area are increased.

933
5. BER PERFORMACE
Figure 9 shows the simulation results for the BER of conventional technology and new proposed
DCO-FBMC technology as a signal to noise ratio (SNR) in dB function. These simulators were made under
DCO-FBMC of 1000 symbols for various M-QAM in AWGN constellations from 4QAM to 256QAM.
Therefore when using N size for FFT / IFFT to the performance of the term SNR will provide good indications
of DCO-FBMC where the results are the same as the conventional technique.
Figure 9. The BER of the function SNR dB of the New DCO-FBMC and conventional method
(new DCO-FBMC IFFT size 512) and the (Conventional DCO-FBMC IFFT size 1024) for different M-QAM
6. CONCLUSION
In this research, new technology was created to generate a real signal from DCO-FBMC compatible
with IM/DD. The proposed technique consists of generating a traditional complex FBMC signal and then
extracting the real and imaginary parts juxtaposed with them in the time domain to get a real FBMC signal.
This, in turn, led to a significant reduction in the complexity and thus to reduce power consumption and
occupied chip area. It was also explained that the performance of the proposed FBMC technology in terms of
BER is the same as the performance of traditional technology.
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