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Research On OFDM Modulation Technique In Visible Light Communication Systems

Posted on:2019-06-10Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y YangFull Text:PDF
GTID:1318330545462605Subject:Information and Communication Engineering
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Recently,green techniques such as light-emitting diode(LED)have developed fast and drawn increasing attention due to its low power consumption,low cost and strong durability.Based on this,visible light communication(VLC)has been proposed.Utilizing the fast flick of LEDs,VLC provides both illuminations and short distance communications.Taking the prominent contradiction between the requirement for high-speed communications and the limited bandwidth of LEDs,studying multicarrier modulation scheme such as OFDM is significant.The research work is supported by Huawei innovation foundation project "Research on Prototype and Key Technologies of Visible Light Communication"(A2015264)and BUPT Excellent Ph.D.Students Foundation "Research on relay assisted uplink VLC-OFDM communications"(CX2016313).Based on optical OFDM schemes,this dissertation studies spectrally efficient OFDM scheme to achieve high speed VLC.Furthermore,this dissertation investigates the illumination-compatible OFDM schemes and uplink OFDM schemes that can guarantee eye safety.The detailed content is summarized as follows:First of all,to solve the spectrum efficiency loss problem existed in optical OFDM schemes,a ?-law compand based OFDM scheme,termed as ?-OFDM,has been proposed.In ?-OFDM,an algorithm is designed for frequency OFDM signals to obtain symmetrical time domain OFDM signals after IFFT(Inverse Fast Fourier Transform),and the real and imaginary parts of which are separated to obtain real signals.Meanwhile,a modified ?-law compand is employed in VLC systems to get appropriate LED drive current.This dissertation verifies that the proposed OFDM scheme has higher spectral efficiency when compared to conventional high energy OFDM schemes.The dissertation also proves that the proposed scheme has lower PAPR when compared to conventional high spectral efficiency OFDM schemes,thus it obtains less clipping noise and higher energy efficiency.Then,to take illumination requirements of single LED systems into consideration,an enhanced DCO-OFDM(eDCO-OFDM)scheme has been proposed.eDCO-OFDM utilizes multiple scaling factors to control the positive and negative parts of signals,respectively.In this way,it can take full advantages of the dynamic range of LEDs and improve the effective SNR at the receiver.Utilizing Bussgang theorem to study the clipping noise of the system,the proposed scheme obtains optimal scaling factor design op-eDCO-OFDM and suboptimal design fa-eDCO-OFDM.op-eDCO-OFDM is able to obtain the best BER(Bit Error Rate)performance at the cost of high computational complexity.Meanwhile,fa-eDCO-OFDM can effectively mitigate system clipping noise under the constraint of optical power,thus obtaining performance superior to conventional DCO-OFDM scheme.Next,to take illumination requirements of multi-LED systems into consideration,this dissertation studies OFDM scheme that utilizes spatial domain resource to achieve dimming control.In scenario with densely deployed LEDs,we propose spatial dimming OFDM(SD-OFDM)scheme,which changes the LED spatial state to obtain target optical power.When compared to analogue dimming scheme,SD-OFDM can obtain lower chromaticity shift;when compared to digital dimming scheme,SD-OFDM has lower complexity.In scenario with scattered LEDs,we proposed Hybrid Dimming OFDM(HD-OFDM)which simultaneously achieves dimming control and information transmission by modulating the spatial state of LEDs.HD-OFDM guarantees the uniform illumination by designing a mapping principle at the transmitter.Meanwhile,it proposed a maximum a posterior receiver to avoid amplifying channel noise.Simulation results indicate that HD-OFDM can obtain higher data rate under the constraint of BER,by utilizing the spatial degree of freedom.Finally,to solve the eye safety problem in uplink VLC communications,an uplink OFDM modulation based on relay is proposed.Through performance analysis,this dissertation first studies the system model of AF(Amplify and Forward)and DF(Decode and Forward)based relay OFDM system.By analyzing the clipping noise at the source node and the relay node,we obtain the effective SNR of AF and DF system.Through formulating an optimizing problem,we obtain the configuration that minimizes the source optical power by exhaustive search.Due to the high complexity and limitations of exhaustive search,we simplify the optimization problem and obtain a sub-optimal configuration.This dissertation verifies that the relay-assisted uplink OFDM can achieve reliable communications under the constraint of eye safety.Meanwhile,the performance of the obtained sub-optimal configuration is able to approximate that of optimal configuration,thus achieving low-complexity and safe uplink VLC.
Keywords/Search Tags:Visible Light Communications, orthogonal frequency division multiplexing, multiple input multiple output, relay system, dimming control
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