| With the arrival of the 5G era,the demand for higher communication data rates has greatly enhanced the requirements for mobile communication networks.Since the radio frequency bands employed by the wireless communication networks have been fully developed,it is necessary to find other frequency bands to meet the fast-growing communication requirement in the future.Visible Light Communication(VLC)has the potential to supplement or even replace the existing wireless communication access network,since the visible light band possesses extremely high frequency and abundant unrestricted spectrum resources.Orthogonal frequency division multiplexing(OFDM)technology has been applied to visible light communication because of the high spectral efficiency and flexible modulation.OFDM-based precoding schemes can effectively alleviate the system penalty due to bandwidth limitation of devices in VLC systems without requiring channel conditions,and so attract widespread attention.However,VLC still faces some problems.Firstly,the communication coverage is limits by the restricted emitting power;Secondly,VLC system strongly rely on the light of sight(Lo S)link,which is easy to be obstructed and can cause interrupt in communications.To solve the above problems,this paper studies the relay technology to improve the performance and the coverage of OFDM based VLC system.The following researches focus on two scenarios,which realize full-duplex communication between nodes and improve the performance of the far user in a point-to-multipoint transmission network,respectively.1.In the first scenario,two source nodes need to communicate with each other via the relay node because the distance between two nodes is too far or a barrier blocks the Lo S link of two nodes.The traditional half-duplex mode with time division multiplexing limits the system performance.We employ physical-layer network coding(PNC)technology based on amplify and forward mode to reduce the time slots required for information exchange,thus improving the system throughput.We focus on the asymmetric two-way-relay(TWR)network in this thesis,which is more suitable to the practical application.We theoretically analyze the capacity advantages of PNC compared with the straight network coding and the traditional relay schemes,and the capacity differences between the asymmetric TWR network and the symmetric TWR network.Subsequently,we prove that PNC can effectively improve the system throughput compared with half duplex schemes in a 2.16Gbit/s VLC asymmetric TWR experiment.We also study the performance of traditional OFDM and precoded OFDM in detail in the PNC system,as well as the effects of peak-to-average power ratio,length of cyclic prefix,and the noise at the relay on system performance.The results show that orthogonal-circulant-transform precoded OFDM scheme has the best performance in this scene.In addition,we find that the maximum power condition adopted by the PNC system would suffer from performance penalty in the asymmetric TWR because of residual interference,thus is not the best power allocation scheme.The study provides experimental support for promoting full-duplex capacity in VLC systems with PNC.2.In the second scenario,we propose the OFDM-Non-orthogonal multiple access(NOMA)cooperative relay communication technology,when the transmitter node needs to transmit signal in a point-to-multipoint transmission network but the far user’s performance is poor due to the far distance or the blocked Lo S link.In this case,the transmitter sends an OFDM-NOMA signal to two users.The near user,on the one hand,receives its own data,and on the other hand,demodulates the signal of the far user utilizing the characteristics of the NOMA signal and transmits it to the far user as a relay to improve the performance of the far node.This technology takes full advantage of the characteristic that the near user intrinsically contains the information of the far user,thus enabling the seamless combination of OFDM-NOMA and the cooperative relay technology.For the first time,we verify the performance advantages of OFDM-NOMA cooperative relay scheme in a 1.84Gbit/s VLC experiment,and determine the optimal receiving angle of the far user. |
