Study on modulation techniques for downlink chanel in Li-Fi
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TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI
STUDY ON MODULATION TECHNIQUES FOR
DOWNLINK CHANEL IN LI-FI
Nguyen Ngoc Tan1
Hanoi Metropolitan University
Abstract: Light-Fidelity (Li-Fi) is considered as a fully optical networked communication
with the capability of bidirectional transmission. Li-Fi is a subset of Visible Light
Communications (VLC) using visible light to modulate mobile data which offer many
advantages in indoor environment. This paper is aimed to provide a comprehensive
knowledge to the available modulation techniques which is utilized for downlink channel
in VLC networks and particularly in Li-Fi. These modulation schemes are clarified and
then grouped for the clearly and throughout vision in the paper. Advantages and
disadvantages of them are also given out adequately and compared to each other.
Keywork: Light-Fidelity (Li-Fi), Visible Light Communications, Optical Wireless
Modulation Techniques
1. INTRODUCTION
Recently, mobile wireless data is represented to be growing exponentially. For
example, it is reported only 0.9 EB per month to be used in 2012, however, the figure of
global mobile data traffic is 11.2 EB per month in 2017 [1]. Another example on YouTube
is approximately 140 views on YouTube per person on the earth (over 1 trillion views) in
2011, 72 hours of video are uploaded to YouTube every minute while 25% of global
YouTube views come from mobile devices [2]. These examples lead to Radio Frequency
(RF) spectrum shortage which has a limit available spectrum of under 10 GHz. A
promising solution is exploited an extremely large amount of visible light bandwidth which
is approximately 320 THz (430 – 750 THz). Light-Fidelity (Li-Fi) is an optical wireless
network which takes the feature of well-known Visible Light Communication networks by
using light emitting diodes (LED) [3]. Therefore, Li-Fi is consider as nm-wave
communication. Li-Fi with enhanced capacity provides the necessary connectivity to
1 Nhận bài ngày 17.04.2016; gửi phản biện và duyệt đăng ngày 10.05.2016
Liên hệ tác giả: Nguyễn Ngọc Tân; Email: [email protected]
TẠP CHÍ KHOA HỌC SỐ 4/2016
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realise the Internet-of-Thing (IoT), and contribute to the key performance indicators for the
5th generation and beyond [3].
Due to basing on electromagnetic radiation for information transmission similar to RF
networks, hence typical modulation techniques in RF communication might be transferred
in VLC systems, particularly in Li-Fi with suitable modifications such as pulse modulation
techniques On-Off Keying (OOK) [4]-[7], Pulse Width Modulation (PWM) [7], Pulse
Position Modulation (PPM) [6]-[8] which uses single-carriers for data transmission.
However, they are restricted to transmit data at high speed due to narrow-band
interference. OFDM is an efficient modulation technique for high speed data
communication through bandlimited channels, and is being widely used in RF which might
be applied in Li-Fi with the change of modulated data type [9]-[12]. There are several
different forms of OFDM for IM/DD systems: asymmetrically clipped optical OFDM
(ACO-OFDM) [13], DC biased optical OFDM (DCO-OFDM) [6], and other forms based
on ACO-OFDM and DCO-OFDM [13]. Moreover, Li-Fi also takes a number of advanced
and specific modulation schemes as Color Shift Keying (CSK) [19]-[21], Optical Spatial
Modulation (OSM) [22]-[23], Hadamard Code Modulation (HCM) [24]-[25] which
consider other requirements such as power efficiency and the complexity.
The rest of this paper is organized as follows. Section II describes conventional
modulation techniques which catalogued into two kinds of carriers consisting of single-
carrier modulation and multi-carrier modulation schemes. Advanced modulation
techniques for Li-Fi is introduced in section III. Finally, conclusions are drawn in section
IV.
2. CONVENTIONAL MODULATION TECHNIQUES
In this section, a number of conventional digital modulation techniques typically
utilized in VLC, especially in VLC are summarized and discussed. As the same way to RF
communication, Li-Fi also modulates baseband electronic signals onto higher frequency
carriers, but in visible light spectrum. Therefore, all generally RF modulation techniques
are also able to apply to Li-Fi. However, almost VLC systems are realized as an IM/DD
system, which means that the modulated signal has to be both real value and unipolar.
Single-carrier Modulation
On-Off Keying
On-Off Keying (OOK) is the most implemented modulation technique for an IM/DD
system because of its simplicity. In the scheme, a bit one is represented by an optical
intensity pulse that occupies the entire or part of the bit duration and a bit zero is
represented by the absence of an optical pulse. The optical pulse can be formed into two
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TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI
types, non-return-to-zero (NRZ) and return-to-zero (RZ) with duty cycle 0.5 [4] which
is illustrated in Fig. 1. Because OOK sequentially turns on and off the LED to transmit
data, it has the capability to support dimming. The use of RZ pulses having a duty cycle
increases the bandwidth requirement by a factor of 1/ . Due to the increased
noise associated with this expanded bandwidth is outweighed by the 1/ increase in peak
optical power, however, it decreases the average power requirement. NRZ-OOK is
represented as a modulation scheme with a good compromise between the power
requirement and bandwidth efficiency. N. Fujimoto, et al. represented OOK-NRZ based
systems which achieved high data rate of 477 Mbps [5] and 614 Mbps [6] in 2012 and
2013. However, this method is not efficiency in term of illumination control and suffers
flicker.
Pulse Position Modulation
PPM is a potential candidate for the pulse modulation techniques and is considered as
an orthogonal modulation technique [6] due to utilizing an L-PPM symbol including a
pulse of constant power occupying one slot duration within L possible time slots while the
rest of it is empty. In other words, information is modulated to the position of the pulse
which corresponds to the decimal value of the M-bit (2M = L) input data as shown in Fig. 1.
In PPM, the duration for a time slot Ts-PPM is shorter than the OOK bit duration Tb-OOK in
order to gain the same throughput as OOK which is:
TbOOK
L
M
TsPPM
(1)
The transmit pulse shape for L-PPM is given by [21]:
1
for t m 1 TsPPM ,mTsPPM
x t
PPM
(2)
0 elsewhere
Fig. 1. Comparison of time waveforms between OOK and PPM.
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There is a considerable increment in system complexity when employs PPM compared
to OOK since the receiver requires both slot and symbol synchronization in order to
demodulate the signal [7]. The main drawback of the PPM scheme is the decrease of
bandwidth efficiency in order to trade off power efficiency by lager constellation sizes.
Moreover, L-PPM is a vulnerable scheme for Inter-Symbol Interference (ISI) over
multipath channels which restricts it to high data rate systems [7]. Coming from its power
efficiency, nevertheless, PPM has still been used widely for OWC systems, particularly in
VLC, and handheld devices where lower power consumption is one of the primary key
factor in designing [6]. VPPM is a simple modification of PPM combining PPM and PWM
(Pulse Width Modulation) to provide dimming support by adjusting the width of signal
pulses corresponding to required brightness levels. In order to achieve improved power
efficiency as well as bandwidth efficiency, Differential PPM (DPPM) which removes all
the redundant slots following an occupied pulse by a PPM symbol, is proposed in [8].
Multi-carrier Modulation
SCM schemes are not preferred for high-speed data transmission because of non-linear
signal distortion at LED front-end and Inter-Symbol Interference (ISI) result of the
frequency
selectivity
in
dispersive
optical
wireless
channels. In order to improve more bandwidth efficiency, MCM is considered to replace
conventional SCM with an energy penalty. OFDM which is a realisation of MCM, has
been widely used in high-speed wired and wireless communication systems since it has the
capability to handle ISI and its high spectrum efficiency [9]-[12]. In order to gain data rate
up to 1Gbps or higher via OWC systems, OFDM is also considered as an attractive
technique. The standard OFDM in RF systems cannot applied directly to OWC systems,
however, due to OWC systems are based on Intensity Modulation/ Direct Detection
(IM/DD) which means the transmitted optical signals must be unipolar. Therefore, the
design of the conventional OFDM need to modify before applied to IM/DD optical
wireless communication systems. There are a number of innovated OFDM versions
proposing for IM/DD systems involved Asymmetrically Clipped Optical OFDM (ACO-
OFDM) [14], DC biased optical OFDM (DCO-OFDM) [15]-[16], and other forms relies on
the conventional form of ACO-OFDM and DCO-OFDM.
DCO – OFDM
A typical model of DCO-OFDM is described in Fig. 2 [11]. After being converted
from serial to parallel and mapped by a conventional modulation such as M-QAM, the
input data has become a complex data signal vector where its size is then doubled by
Hermitian symmetry in order to create a N complex sample vector X = [X0, X1, …, XN-1]:
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TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI
N
Xn XN* n for 0 n
(3)
2
where N is the number of subcarriers used for OFDM, and specific elements X0 and
XN/2 are set to zero. Then the complex vector X becomes the input of IFFT which is
transformed from discrete frequency domain to discrete time domain to obtain a real signal
vector x. The k-th sample in time domain of vector x is given as shown in [12]:
N 1
1
j2kn
N
(4)
xk
X exp
n
N
n0
where Xn represents the n-th subcarrier of the complex Hermitian symmetry vector X.
Fig. 2. A DCO-OFDM system.
Due to Hermitian symmetry and specific elements are zero, there are only (N/2 – 1)
subcarrier carrying unique data. The real value signal vector x is then converted from
parallel to serial and added a cyclic prefix (CP) by a DAC convertor and a low pass filter
(LPF) block to be ready for adding a suitable DC bias. After that, all remaining negative
peaks are clipped in order to create the transmitting signal xDCO(t). The main drawback of
DCO-OFDM is large power consumption for eliminating negative peaks due to OFDM
signals have a very high Peak-to-Average Power ratio (PAPR). Therefore, instead of using
a large DC bias, a moderate DC level is used [17] while the other remaining negative
components are clipped:
(5)
BDC E x t
where µ is a proportionality constant and BDC is defined as a bias of 10log10(µ2 + 1)
[11]. The optimum clipping level depends on the signal constellation at the input of IFFT.
For practical BERs, large constellations such as 256 QAM require very high SNRs, hence
clipping noise must be low and therefore BDC must be large. This also leads to the
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{ }
increment of the optical power due to . Finally, the transmitting signal
xDCO(t) is emitted by an optical modulator which converts directly proportional the input
electrical current to the intensity of optical signal.
In order to recovery the information at the receiver, the received optical
signal is
first converted to the electronical signal by PDs. A same process as in the conventional
OFDM is applied after this step. In DCO-OFDM, both odd and even subcarriers are used
to carry data. With only few negative peaks exceeded the DC-bias level are clipped,
therefore, DCO-OFDM obtains high bandwidth efficiency compared with ACO-OFDM.
ACO – OFDM
The ideal of ACO-OFDM is cutting data carried on even subcarriers while maintains
odd subcarriers for transmission. The Fig. 3 shows the front-end of ACO-OFDM as same
as DCO-OFDM. Firstly, information is also converted from serial to parallel and mapped
by a conventional modulation. Output of this block is performed to obtain Hermitian
symmetry feature as defined in (3).
Fig. 3. A ACO-OFDM system.
In ACO-OFDM, the input signal vector X is eliminated all even subcarriers elements
and remain only odd subcarriers elements before being taken to the IFFT, X = [0, X1, 0,
X3, …, XN-1]. After performing the IFFT, the resulting time domain signal x is real and has
the anti-symmetry property as represented in [18]:
xk xk
N
2
(6)
Before being converted to optical intensity by an optical modulator such as the LED or
LD, the remaining negative elements of the electrical signal x(t) are clipped to the signal
xACO(t) without the DC bias addition process as DCO-OFDM. It is noted that the ACO-
OFDM would be not have any loss of information because of the anti-symmetry property.
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Similar to the receiver process of DCO-OFDM, the information is also recovered by
the FFT. There is only one difference to DCO-OFDM which only data symbols on odd
subcarriers are demodulated.
3. ADVANCED MODULATION TECHNIQUES
Li-Fi is represented as an excessively high data rate network model for indoor
environment. This leads to the challenge of designing advanced modulation techniques in
order to increase data rate on downlink channels as well as mitigate Inner-Symbol
Interferences (ISI) caused by dispersive channel. Recently, there are several novel
modulation schemes proposing to Visible Light Communication networks involved Color
Shift Keying (CSK), Optical Spatial Modulation (OSM), Hadamard Coded Modulation
(HCM) and variants of these schemes.
Color shift keying
Color Shift Keying (CSK) exploits the inherent feature of Visible Light
Communication that is the color of light in order to modulate data. The basic operation of
CSK is outlined in the standard IEEE 802.15.7 published in 2011 [27] and simply
described in Fig. 4. CSK is represented to use for the PHY III standard which allows data
rates ranging from 12 to 96 Mbit/s. As illustrated in the figure, incoming data is mapped on
to the instantaneous chromaticities of the colored LEDs which are defined in the CIE 1931
(Fig. 5). Color table in Fig. 5 the x-y coordinates of the chromaticities of the mapped bits.
These coordinates are passed through a x-y to RGB convertor to control three Blue, Green,
and Red monochromatic color LEDs emitting different color intensities while maintain a
constant average perceived color for common illumination purpose [3].
Fig. 4. Color Shift Keying system.
At the receiver side, color optical signals are filtered by three Blue, Green and Red
lens before detected by Photodiodes. These electronic signals are then decoded to recovery
data by a RGB-to-Data convertor. Because of guarantee of maintaining a constant
luminous flux, CSK would be not affected by flicker over an extremely wide frequency
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range of visible light. Moreover, CSK also provides a dimming method for VLC networks
by changing the current which drives the light source. However, CSK takes a disadvantage
due to the complexity at both transmitter and receiver sides and cost efficiency. There are
two variants of CSK involving Color Intensity Modulation (CIM) [20] and Metameric
Modulation (MM) [21]. MM can gain higher energy efficiency and provide further control
of the color quality. However, it requires an additional and independently controlled green
LED. CIM is proposed to maximize the communication capacity and can achieve the
upper-bound capacity for both orthogonal and non-orthogonal optical channels using
inverse source coding.
Fig. 5. CIE 1931 color space target color and corresponding RGB constellation.
Optical Spatial Modulation
The key design of Optical Spatial Modulation (OSM) scheme is high spectral
efficiency while high power efficiency is still guaranteed [22]. An optical MIMO system
model consisting of Nt transmitters and Nr receivers is consider to implement the OSM
scheme (Fig. 6). The decisive ideal of the OSM is activating only one transmitter at instant
time while all other elements are set to zero. Thereby, the transmitted data when applies
the OSM scheme, is not suffered by ISI. Moreover, incoming bits are not only modulated
by conventional modulation techniques, but also mapped to transmitters‟ indexes.
Fig 6. Diagram of the operation of Optical Spatial Modulation.
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TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI
The process of the OSM is simply described as following: A coming bit sequence is
first separated into n-bit groups q(k). Each group q(k) is then split into two bit parts. The
first part is conveyed by a conventional modulation scheme such as M-ary PAM [23]. The
intensities Im of the M-ary PAM used for the OSM scheme are given by:
2I
Im
m for m 0,1,...,M 1
(7)
M 1
where I is the mean optical intensity emitted. Hence, a spectral efficiency achieved by
M-ary PAM is log2(M) bit/s/Hz.
Meanwhile, the other one is mapped to LEDs‟ indexes as illustrated in the case of the
four transmitters MIMO model in Fig. 7. Thus, the bit group q(k) is now become the signal
vector x(k) consisting of Nt elements. Therein xl having the intensity Im is at the position of
the activating transmitter. The total data rate is increased by the contribution of mapping
on LEDs‟ indexes which is log2(M) + log2(Nt) = log2(MNt) bit/s/Hz.
Fig 7. Illustration of mapping binary data to transmitters’ indexes.
At the receiver side, an optimal SM detector is utilized to estimate the signal vector
̂(k) from the electronic signal converted from the received optical signal by Photodiodes
(PDs) [28]. The estimation is relied on the Maximum-Likelihood (ML) principle which
decides the estimated signal vector ̂ by minimizing the Euclidean distance between the
actual received signal y and all potential received signals:
2
ˆ
(8)
x argmax py (y x,H) argmin y Hx
F
x
x
where py is the probability density distribution of the received signal y conditioned on
the transmitted signal x and the channel matrix H. It is clearly seen that OSM do not only
achieve higher data rate over conventional modulations and mitigate ISI, but also it
addresses the power efficiency by the requirement of activating only one transmitter at
instant time compared to other MIMO models. Comparison to OOK modulation, OSM
achieves Bit Error Rate (BER) slightly better. Another factor is also considered is
computational complexity at the receiver. OSM requires fewer mathematical operations
than Repetition Coding (RC) in order to detect transmitted data [18]. It takes only 3MNr
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operations, while is the operations required by RC. By using the
transmitters‟ indexes for data modulation, however, Bit Error Rate (BER) of OSM is
affected by coherence among transmitters (LEDs). It means that the distances between
LEDs must be sufficiently far in order to guarantee estimating exactly what LED is used to
transmit data at instant time. Beside, OSM offers only a logarithmic increase of the data
rate with the number of transmitters. This might limit OSM to be implemented for practical
number of LEDs using for illumination in any room. The last disadvantage of OSM is
channel knowledge which must be well known for data detection, it might lead complexity
constraints on the channel estimation unit [29]. From the perspective of increasing spectral
efficiency, Generalized Spatial Modulation (GSM) in VLC is also proposed in [30].
Instead of fixing the number of transmitter as an exponential of two, GSM is a generalized
form of SM which actives Na (0 < Na < Nt) transmitters simultaneously at any time. Hence,
the data rate of GSM is increased as following:
2
Nt
GSM log
N log M
(9)
a
2
Na
Another application of SM is proposed in [31] to obtain positive and real-valued
signals for OFDM in VLC. The proposed method solves the DC-bias problem in DCO-
OFDM and get a higher spectral efficiency than ACO-OFDM [31] called NDC-OFDM.
The authors added a SM mapper behind the IFFT block to separate positive and negative
value OFDM signals into two LED transmitters. In other words, the sign of the OFDM
symbols is represented by the index of the corresponding LED.
Hadamard Coded Modulation
OFDM is represented as a high-dimensional modulation technique for high data-rate
transmission that has been widely adapted to many modern broadband communications
and standards, however, suffer source, channel and amplifier nonlinearities due to its high
peak-to-average ratio (PAPR) [24]. OFDM signals with large peaks are then clipped by the
peak optical power constraint of the optical sources. In VLC systems, due to high average
optical powers are required for illumination, some symbols of OFDM might suffer for
signal clipping [25]. Mohammad Noshad, et al. are introduced an alternative modulation
technique to OFDM called Hadamard Code Modulation (HCM) which uses the fast Walsh-
Hadamard transform (FWHT) to modulate data. The proposed modulation scheme uses
binary Hadamard matrixes to encode the input data stream, which has the same complexity
as the FFT in OFDM, Nlog2 N, where N is the size of the Hadamard matrix. HCM achieves
a same BER compared to OFDM, while can provide brighter illumination levels for VLC
systems because of its low PAPR.
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TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI
Fig. 8. Block diagram of the HCM transmitter using FWHT
A Hadamard matrix of order N which is modified by replacing 0 for -1 elements in the
original {-1, 1} Hadamard matrix [32], is denoted by HN. The transmitted vector x is
obtained from the input data vector as shown below:
1
(10)
x
uH (1u)H
N
N
N
where
is the complement of HN . The components of the signal vector u are
HN
assumed being modulated signal by a M-ary Pulse Amplitude Modulation (PAM), where
{
} . The equation (10) is then rewritten as
following:
1
N
x u
H H
0,1,1,...,1
(8)
N
N
2
N
Only N-1 rows of the matrix HN which have a weight of N/2, are used to modulate
data, while the first row of the Hadamard matrix which all values are one, is ignored.
Hence the first row is set to zero and the rate of M-PAM HCM is . The
interference of the Hadamard codewords on each other due to the fixed cross correlation
between these remaining N − 1 rows can be removed at the receiver side [33]. The received
signal is given by:
y hx n
(11)
where n is assumed an additive white Gaussian noise (AWGN) and h is the discrete
time equivalent impulse response of the channel which h = {h(k)}. The vector y is then
demodulated to the vector v by an inverse FWHT (IFWHT) as shown in Fig. 9:
1
v
yHT yHT
N
(12)
N
N
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Fig. 9. Block diagram of the HCM receiver using IFWHT
For an ideal non-dispersive channel with impulse response as defined in [24]:
1
k 0
k 0
h k
(13)
0
The decoded data can be rewritten as following:
1
v u N 1,1,1,....,1 n
(14)
2
1
n
HT HT
is a 1 × N noise vector with independent components. The
where
N
N
N
BER of M-PAM HCM for non-dispersive AWGN channels can be calculated from (34):
P2
M 1
3
N
BERHCM
Q
(15)
2 c2lip
M 2 1
M log2 M
N
N2
where γ represents the penalty in SNR due to the pulse shaping,
is the variance of
2
the additive Gaussian noise at the receiver and
is the variance of the clipping noise.
clip
The author is also introduced an improved version of HCM which reduces the DC bias
without losing information. A DC bias value bDC is added to the transmitted signal, then
the decoded vector becomes:
(16)
v NbDC ,0,...,0 un
It is clearly shown that the DC bias is only added to the first component of the
transmitted signal and has no effect on the rest of the data. The BER comparison between
ACO-OFDM using 16-QAM to modulate 128 subcarriers and HCM signals are generated
by an FWHT size N = 128 are realised in [17]-[18]. As a result, HCM achieves lower BER
2
n
n2
for average optical powers higher than 18 dBm and 20.3 dBm for
= −30 dBm and
=
−20 dBm, respectively. Both HCM and DCR-HCM shows the capability to gain a lower
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TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI
achievable BER for all spectral efficiencies tested when are compared to ACO-OFDM and
DCO-OFDM [25].
4. CONCLUSION
Li-Fi with great applications makes itself to become a potential candidate for the
architecture of the 5G network. The hottest topic attracted the most researches on it is data
modulation. The three primary keys for modulation techniques considered in Li-Fi are the
complexity, spectral and power efficiency. Through the paper, all available modulation
schemes for Li-Fi are represented and compared the benefits and also shortages of each
technique relied on these factors.
5. ACKNOWLEGMENT
I am very much grateful for the help of Department of Information Technology –
Hanoi Metropolitan University and members which fully support me to implement this
research.
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NGHIÊN CỨU CÁC KỸ THUẬT ĐIỀU CHẾ CHO KÊNH ĐƯỜNG XUỐNG
TRONG MẠNG LI-FI
Tóm tắt: Light-Fidelity (Li-Fi) được xem như một mô hình mạng không dây quang hoàn
chỉnh với khả năng truyền song công. Li-Fi là một trường hợp riêng của mạng truyền
thông sử dụng ánh sáng nhìn thấy (VLC) sử dụng ánh sáng nhìn thấy để điều chế tín hiệu
di động. Nó đạt được rất nhiều lợi ích trong môi trường truyền thông trong nhà. Mục tiêu
của bài báo là cung cấp kiến thức về các kỹ thuật điều chế có thể sử dụng cho kênh
đường xuống trong mạng VLC, và mạng Li-Fi nói riêng. Các cơ chế điều chế này được
phân loại và nhóm lại nhằm cung cấp một cái nhìn rõ ràng và xuyên suốt trong toàn bộ
bài báo. Ngoài ra, các ưu điểm và hạn chế của các kỹ thuật điều chế trên cũng được đưa
ra và so sánh với nhau.
Từ khóa: Light-Fidelity (Li-Fi), Mạng truyền thông sử dụng ánh sáng nhìn thấy, Các kỹ
thuật điều chế trong mạng quang không dây.
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