A review of visible light communication (VLC) technology

Novel Unipolar Optical Modulation Techniques for Enhancing Visible Light Communication Systems Performance

“…Those 3 vectors are applied to the Hermitian symmetry and data arrangement block that outputs the parallel data vectors 𝑋 𝑜𝑑𝑑 𝑖,𝑗 (𝑘), 𝑋 𝑒𝑣𝑒𝑛 𝑠𝑝𝑒𝑐𝑖𝑎𝑙 (𝑘), and 𝑋 𝑒𝑣𝑒𝑛 (𝑘) respectively. The symbols of those vectors are arranged as illustrated in ( 8)- (10). The data symbols are sorted, such that the output of the IFFT blocks are real bipolar signals.…”

“…The vectors in (8-a) - (10), are processed as follows; Firstly, the data symbols of 𝑋 𝑜𝑑𝑑 𝑖,𝑗 (𝑘) applied to the first IFFT block are arranged according to the ACO-OFDM technique in which only the odd subcarriers carry data and the positions of the even subcarriers are set to zero so the resultant output will be asymmetric bipolar signal 𝑥 𝑜𝑑𝑑 𝑖,𝑗 (𝑛). Then, 𝑥 𝑜𝑑𝑑 𝑖,𝑗 (𝑛) signal is applied to a splitter to output 𝑥 𝑖 𝑜𝑑𝑑 (𝑛) and 𝑥 𝑗 𝑜𝑑𝑑 (𝑛) signals.…”

“…Thus, the proposed technique is named as special symmetric and asymmetric clipping optical technique. Moreover, a zero clipping process takes place to convert 𝑥 𝑠𝑝𝑒𝑐𝑖𝑎𝑙 (𝑛) into a unipolar real signal 𝑥 𝑠𝑝𝑒𝑐𝑖𝑎𝑙 (𝑛) ̅̅̅̅̅̅̅̅̅̅̅̅̅̅ as in, Thirdly, the data symbols of 𝑋 𝑒𝑣𝑒𝑛 (𝑘) applied to the third IFFT block are applied to some even subcarriers in an order as illustrated in (10), this arrangement leads the output of the IFFT to be symmetric bipolar signal 𝑥 even (𝑛) as discussed in [29], and shown in Fig. 11.…”

“…Moreover, those techniques must take into consideration the required high data rate. Single subcarrier techniques such as pulse position modulation (PPM), on-off keying (OOK), and binary phase shift keying (BPSK) satisfy both the real-valued and unipolar criteria but unfortunately, they imply low data rates because of its low modulation order (M=2) [10]. On the other hand, higherorder modulation techniques such as M-ary pulse amplitude modulation (M-PAM), M-ary phase shift keying (M-PSK), and M-ary quadrature amplitude modulation (M-QAM) achieve high data rates but cannot be directly used in VLC systems as they output complex data.…”

“…One of these adaptations is applying the Hermitian symmetry on the transmitted data at the input of the IFFT [12,13], which results in converting the data into real form at the cost of reducing the data rate to half compared to the rate of the traditional OFDM (real and imaginary). The issue of converting the bipolar signal into a unipolar one was the main motivation for many researchers [5,[8][9][10][11]. So, different modified OFDM systems were under investigation taking into consideration SE, power efficiency (PE), bit error rate (BER), and nonlinearity distortion.…”