| With the rapid development of artificial intelligence,internet of things and other technologies,as well as the high popularity of intelligent terminal devices,the demand for mobile data traffic is growing dramatically,while the available spectrum resource is increasingly short.Therefore,the radio frequency(RF)wireless communication is facing an extremely tight crisis of spectrum resource.However,visible light communication(VLC)has ultra-wide spectrum with no need for authorization,which can provide illumination and communication simultaneously and has become a beneficial supplement and strong competitor to RF.Orthogonal frequency division multiplexing(OFDM)is introduced to VLC to effectively overcome the limited modulation bandwidth and linear range of the light-emitting diode(LED),significantly improving the data rate.Unlike Fourier transform,Hartley transform can greatly reduce the hardware cost and computational complexity for the identity of forward and inverse transforms and omitting the calculation of imaginary part.The advantages of reflexivity and avoiding complex operation are more obvious in the field of real-valued applications.Compared with the traditional discrete Fourier transform(DFT)-based schemes,using discrete Hartley transform(DHT)to realize OFDM systems is more suitable for the VLC systems where real signals are transmitted.And the ability of removing the Hermitian symmetry restriction at the input can make more improvement in the spectral efficiency of optical index modulation systems.This thesis focuses on studying the DHT-based optical OFDM(O-OFDM)systems and proposing more efficient schemes by employing LED index modulation in the space domain and subcarrier index modulation in the frequency domain.The transmitter schemes and receiver detections are presented to decrease the hardware cost and computational complexity,as well as enhance the spectral efficiency and reliability.And the performance superiority of the proposed schemes is verified by analysis and simulation.Detailed contents are as follows:(1)The DHT-based O-OFDM scheme with LED index modulation is proposed.Firstly,the system model of the proposed scheme is given,which can adopt advanced two-dimensional high-order modulation at the input.The polarities of bipolar signals are indexed and transmitted separately by LEDs to avoid spectral efficiency loss caused by the single polarization process.Then,two modified receiver detections are proposed to improve the reliability for the symmetric and asymmetric placements of LEDs and photodetectors(PDs).Besides,the bit error rate(BER)performance of the proposed scheme is analyzed by statistical mathematical method.Finally,the proposed scheme is compared with other optical multiple input multiple output(MIMO)-OFDM systems,and the simulation results show that the proposed scheme can achieve better peak-to-average power ratio(PAPR)and BER performance with lower hardware cost and computational complexity.(2)The complex DHT-based O-OFDM scheme with LED index modulation is proposed.Firstly,the feasibility of using complex DHT to realize O-OFDM systems is derived.Then,the system model of the proposed scheme is given,which can directly perform orthogonal transformation on the input complex sequence without complex multiplication or addition operation,successfully converting the complex field to the real field for processing.In addition,the BER performance of the proposed scheme is analyzed and compared with other optical MIMO-OFDM systems.Furthermore,a variety of optical MIMO-OFDM systems are comprehensively compared in terms of system limitations,spectral efficiency,power efficiency,etc.Finally,the simulation results indicate that the proposed scheme has more advantages in system design flexibility and BER performance.(3)The DHT-based O-OFDM schemes with subcarrier index modulation(IM)are proposed.Firstly,the performance improvement of O-OFDM systems by utilizing the IM technique is analyzed and verified.Then,the transmitter models of the proposed schemes are given.And a reduced complexity maximum likelihood(ML)detection is proposed according to the specificity of optical signals,which can obtain the same optimal performance as the ML detection but with significantly reduced computational complexity.Besides,the spectral efficiency and BER performance of the optical DHT-OFDM-IM systems are analyzed in detail.Finally,the simulation results show that the optical DHT-OFDM-IM schemes can achieve better performance in terms of spectral efficiency,energy efficiency,PAPR and BER performance with lower complexity than the DFT-based counterparts and the traditional O-OFDM systems.Especially at the optimal points of spectral efficiency in case of n =32,compared with the traditional O-OFDM systems,the spectral efficiency improvement of optical DHT-OFDM-IM systems is 5 times that of the DFT-based counterparts,in the meantime the energy efficiency improvement is 1.62 times.(4)The more spectral efficient DHT-based dual-mode(DM)optical OFDM-IM schemes are proposed.Firstly,the transmitter models and the designs of dual-mode pulse amplitude modulation(PAM)constellations are given.Then,two modified detection algorithms for DM-OFDM-IM systems are raised,which can achieve higher reliability with lower complexity.Furthermore,the spectral efficiency improvement and BER performance are analyzed.And the BER performance is compared by calculating the minimum distance between transmitted symbol vectors of each subblock,and verified by Monte Carlo simulation.Finally,the simulation results demonstrate that,compared with the single-mode optical DHT-OFDM-IM systems and the DFT-based DM-OFDM-IM systems,the DHT-based DM-OFDM-IM schemes make notable improvement in terms of spectral efficiency and BER performance.Especially for the optimal points of spectral efficiency in case of n =32,compared with the traditional O-OFDM systems,the spectral efficiency improvement of the DHT-based DM-OFDM-IM schemes is about twice that of the single-mode optical DHT-OFDM-IM systems and the DFT-based counterparts. |
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