OFDM-based Modulation Schemes In Visible Light Communication Systems

With the rapid development of wireless communication technology,mobile devices and demands for wireless data grow exponentially,which has caused the spectrum crisis.Visible light communication(VLC)has drawn significant attention due to its operation in rich license-free bandwidths.VLC meets the requirements of green communication and has potential to offer high speed data rate by employing techniques such as MIMO,OFDM,and spatial modulation.OFDM based modulation schemes in VLC systems are mainly studied due to its robustness to ISI and ICI.DCO-OFDM,ACO-OFDM,DHT-DCO-OFDM,and DHT-ACO-OFDM are studied and compared in IM/DD based VLC systems.Hermitian symmetry reduces the spectral efficiency of DCO-OFDM and ACO-OFDM systems,while DHT are used in DHT-DCO-OFDM and DHT-ACO-OFDM to improve system performance.Simulation results verify that DHT-DCO-OFDM and DHT-ACO-OFDM systems with smaller constellation size show the same BER performance with DCO-OFDM and ACO-OFDM systems.Also,systems with DHT show lower complexity.QCM,NHS-OFDM,OSM-O-OFDM,NDC-OFDM,and GLIM-OFDM are introduced and studied in MIMO-VLC systems.BER performance of NDC-OFDM,OSM-DCO-OFDM,and GLIM-OFDM are verified by simulation.MIMO and spatial modulation can be employed in VLC systems to increase transmission rates and generate better performance.A novel OFDM scheme based on DHT and optical spatial modulation(dubbed as SH-OFDM)is proposed in this paper.Real constellation and DHT are combined with optical spatial modulation(OSM)to transmit bipolar signals by the index of two LEDs.ZF and MAP estimators are employed in the receiver.Simulation results verify that MAP estimator shows better BER performance than ZF estimator.However,MAP estimator owns higher computing complexity due to its vast searching space.Simulation results show that with the increasing of constellation size and the decreaing of LED distance,SH-OFDM and P-SH-OFDM show worse BER performance.SH-OFDM owns higher energy efficiency than NHS-OFDM and shows higher spectral efficiency than NDC-OFDM with the same constellation size.It is also verified by simulation that the SH-OFDM system with smaller constellation size shows the same BER performance and lower complexity with NDC-OFDM system.Additionally,P-SH-OFDM exhibits better BER performance and high design flexibility than GLIM-OFDM.To conclude,the proposed SH-OFDM shows plenty of advantages and improves the system performance as a novel optical spatial modulation scheme for VLC.