Research On SIM-OFDM Scheme For Visible Light Communications

Visible light communication(VLC)has recently become one hot topic in the area of mo-bile communications owing to its appealing advantages of alleviating the crowded radio fre-quency(RF)spectrum,providing illumination during the data transmission,energy saving,etc.In order to mitigate inter-symbol interference(ISI)and boost data rates,orthogonal frequen-cy division multiplexing(OFDM)is widely applied in VLC.Modifications of conventional OFDM schemes for transmitting VLC signals have been an important research branch in the field of VLC.Based on some existing VLC OFDM schemes including direct-current-biased optical OFDM(DCO-OFDM),asymmetrically clipped optical OFDM(ACO-OFDM),pulse-amplitude-modulated discrete multi-tone modulation(PAM-DMT)and flipped OFDM(Flip-OFDM),we further integrate them with the subcarrier-index modulation OFDM(SIM-OFDM)to enhance the transmission performance of VLC OFDM systems in this paper.The main con-tributions of this paper are summarized as follows:Firstly,we propose an enhanced hybrid asymmetrically clipped optical orthogonal frequen-cy division multiplexing(EHACO-OFDM)scheme.To improve the spectral efficiency and bit error rate(BER)performance of the ACO-OFDM system,EHACO-OFDM uses ACO-OFDM on the odd subcarriers,PAM-DMT on the imaginary parts of even subcarriers,and DCO-OFDM on the real parts of even subcarriers.Since all the subcarriers are utilized for data modulation,EHACO-OFDM can achieve higher spectral efficiency than existing VLC OFDM schemes.Meanwhile,since no DC-bias is added on ACO-OFDM or PAM-DMT path,EHACO-OFDM can transmit the signals with only a small DC bias,i.e.,the power efficiency is high.In addi-tion to the conventional detection scheme,an iterative receiver using the pairwise clipping and pairwise averaging is also designed to improve the BER performance of the EHACO-OFDM system.We analyze the computational complexity of the proposed EHACO-OFDM receivers.Simulation results show that the proposed EHACO-OFDM scheme can significantly improve the spectral efficiency and BER performance with only a small increase of computational com-plexity.Secondly,we propose an optical SIM-OFDM(OSIM-OFDM)scheme and analyze its per-formance.To improve the performance of the conventional DCO-OFDM system,we com-bine DCO-OFDM with SIM-OFDM to yield the concept of OSIM-OFDM.Besides,we analyze the minimum Euclidean distance(MED),system design criterion,spectral efficiency,peak-to-average power ratio(PAPR)performance and computational complexity of the OSIM-OFDM scheme.Simulation results verify that the proposed OSIM-OFDM scheme outperforms the con-ventional DCO-OFDM scheme in terms of BER and PAPR.Thirdly,we propose an SIM-based ACO-OFDM(SACO-OFDM)scheme and analyze it-s PAPR performance.To improve the transmission performance of the ACO-OFDM system,we combine it with SIM to yield the concept of SACO-OFDM.Then,we propose an enhanced SACO-OFDM(ESACO-OFDM)modulation scheme which takes advantage of the unutilized even subcarriers for PAM-DMT.Besides,we analyze the PAPR performance,and the complex-ity of the proposed SACO-OFDM and ESACO-OFDM schemes.Simulation results verify that the proposed ESACO-OFDM scheme outperforms the conventional ACO-OFDM scheme in terms of both BER and PAPR.Finally,we propose an optical SIM-based Flip-OFDM modulation scheme(OSIM-Flip-OFDM)and a PAPR reduction method.To enhance the performance of Flip-OFDM,we com-bine it with SIM to yield the concept of OSIM-Flip-OFDM.Then,we propose an improved OSIM-Flip-OFDM(ISIM-Flip-OFDM)scheme to reduce the PAPR of OSIM-Flip-OFDM.Sim-ulation results verify that when the number of subcarriers is large and the ratio of the active sub-carrier number to the total subcarrier number exceeds a certain threshold,the proposed OSIM-Flip-OFDM scheme can outperform the conventional Flip-OFDM scheme with a small increase of computational complexity.