| With the rapid development of wireless communication technology and the increasing demand for high-speed,reliable and secure communication,visible light communication（VLC）technology is gradually becoming an area of great interest.Compared to traditional radio frequency communication（RF）,VLC uses visible light emitted by light emitting diode（LED）as a carrier and has the advantages of high bandwidth,low energy consumption and resistance to electromagnetic interference.Optical Orthogonal Frequency Division Multiplexing（O-OFDM）technology is widely used in VLC systems because of its high spectrum utilisation and low cost.O-OFDM modulation-based visible light communication has a high peak-to-average power ratio（PAPR）,which is susceptible to LED non-linearities and leads to deterioration of system performance.Therefore,how to effectively suppress PAPR has become an important research direction in visible light communication technology.Signal distortion-type PAPR suppression techniques such as amplitude limiting and voltage spreading can effectively suppress PAPR in O-OFDM systems,but can lead to signal distortion that affects system performance.In this thesis,on the one hand,a precoded O-OFDM adaptive symbol decomposition serial transmission system（PCO-OFDM-ASDST）is proposed to address the shortcomings of the conventional symbol decomposition（ASDST）method.On the other hand,a precoded O-OFDM adaptive linear companding（PCO-OFDM-ALC）system is also proposed in this thesis to address the deterioration of the bit-error-rate（BER）performance of VLC systems by conventional linear companding（ALC）,as follows:1.Constructing a precoded O-OFDM adaptive symbol decomposition serial transmission system.The modulated signals are precoded to initially adjust the PAPR performance by reducing the non-periodic autocorrelation between the signals.The adaptive symbol decomposition technique is then used to decompose O-OFDM symbols into multiple sub-decomposed symbols to keep each decomposed symbol within the LED linear operating interval for the purpose of suppressing LED non-linear effects.An expression for the theoretical signal-to-noise ratio（SNR）of the system under multipath channels is derived,and the average PAPR,BER,number of symbol decompositions and optical power performance are analysed by Monte Carlo simulation.The results show that compared to the symbol decomposition serial transmission system,the symbol variance of PCO-OFDM-ASDST is7d Bm lower than that of ASDST at a BER of 10-4 compared to the 4QAM modulation of ACO-OFDM system;and the information rate can be improved by 10Mbit·s-1 at 64QAM when the symbol variance is 20d Bm.PCO-OFDM ASDST has a lower peak-to-average ratio than ASDST,and reduces the limiting noise and the number of symbol decompositions to achieve better BER performance and higher information rate.2.Construction of a precoded O-OFDM adaptive linear spreading system.The conventional linear spreading transform can reduce the peak-to-average ratio of O-OFDM signals,but this brings about signal distortion problems.In order to further suppress the O-OFDM peak-to-average ratio and improve the signal distortion caused by linear spreading,this thesis introduces a precoding technique to suppress the signal peaks by reducing the signal autocorrelation.The O-OFDM symbols are pre-determined to be grouped using adaptive linear compression and amplitude reduction for the large signal only,while the other group of symbols is left untreated.The corresponding decompression and symbol merging is performed at the receiver side to recover the original signal.Analysis of symbol compression,PAPR,and BER performance by Monte Carlo simulation shows that the PCO-OFDM-ALC system reduces the signal peak by approximately 0.6 V compared to the ALC system.for the DCO-OFDM system,the PCO-OFDM-ALC system improves by 1.6 compared to the ALC system when the compression and spreading factor is 0.7 and the CCDF is 0.4 d B peak-to-average ratio optimization.At a spreading factor of 0.3,the BER of the PCO-OFDM-ALC system drops to approximately 1.1×10-4,a reduction of two orders of magnitude compared to the ALC system. |
