Study on modulation techniques for downlink chanel in Li-Fi

) 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
F
ˆ argmax ( , ) argmin
x
p x H H
y
x
x y y x (8) 
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 
TẠP CHÍ KHOA HỌC SỐ 4/2016 101 
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 2log log
t
GSM a
a
N
N M
N

 (9) 
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. 
102 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
x (1 )N N
N
 uH u H (10) 
where 
NH is the complement of NH . The components of the signal vector u are 
assumed being modulated signal by a M-ary Pulse Amplitude Modulation (PAM), where 
 { 
 } . The equation (10) is then rewritten as 
following: 
  
1
x 0,1,1,...,1
2
N N
N
N
 u H H (8) 
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 h x 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 T T
N N
N
 v yH yH (12) 
TẠP CHÍ KHOA HỌC SỐ 4/2016 103 
Fig. 9. Block diagram of the HCM receiver using IFWHT 
For an ideal non-dispersive channel with impulse response as defined in [24]: 
1 0
0 0
k
h k
k
 (13) 
The decoded data can be rewritten as following: 
 
1
1,1,1,....,1
2
N
v u n (14) 
where 
1 T T
N N
N
 n = n H H is a 1 × N noise vector with independent components. The 
BER of M-PAM HCM for non-dispersive AWGN channels can be calculated from (34): 
2
2 22
2
1 3
BER
log 1
HCM
N clip
PM NQ
M M M  
 (15) 
where γ represents the penalty in SNR due to the pulse shaping, 2
N is the variance of 
the additive Gaussian noise at the receiver and 2
clip is the variance of the clipping noise. 
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: 
,0,...,0 1DCNb 
v u n - (16) 
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 
for average optical powers higher than 18 dBm and 20.3 dBm for 2
n = −30 dBm and 
2
n = 
−20 dBm, respectively. Both HCM and DCR-HCM shows the capability to gain a lower 
104 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.