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Research On The High Capacity Indoor Visible Light Communication For The Next Generation Of Wireless Communication Systems

Posted on:2019-07-02Degree:DoctorType:Dissertation
Country:ChinaCandidate:K J JiaFull Text:PDF
GTID:1368330599475539Subject:Communication and Information System
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Wireless communications technology greatly facilitates people's lives and the development of the society.However,wireless communications technology face a new challenge introduced by the development of Mobile Internet and Internet of Things.The demand of communication capacity and the shortage of spectrum as a communication resource have become the most troublesome problem-s.As a promising candidate approach,optical wireless communication(OWC)expands the wireless spectrum into the new and largely unexplored visible light spectrum and presents the potential to meet the growing demand of future wireless communications.The advantages of a visible light com?munication(VLC)include among others:vast amount of unused bandwidth,no licensing fees,and no interference with sensitive electronic systems.VLC can be used as a complementary technology to radio technology.But the development of VLC face many challenges,such as a lack of agreed upon channel model,causing inter-symbol interference(ISI)for multipath propagation,the low mod-ulated bandwidth and non-linear effects of LED,and the channel matrix is non-invertible due to the high correlation between the wireless channel links.Aiming at improving the communication ca-pacity and spectrum efficiency,we research the multipath channel model,multi carrier-code division multiple access(MC-CDMA)system,multiple-input multiple-output orthogonal frequency dividion multiplexing(MIMO-OFDM)system,and non-orthogonal multiple access(NOMA)in this thesis.The main researches and contributions are as follows:Firstly,we present a multipath channel model,in which the impulse response for a transmitter and receiver pair of an indoor optical wireless channels with Lambertian reflectors is evaluated by recursive method.The start time is defined first for the discrete multipath channel model.In single input single output(SISO)system,the distance between the transmitter and receiver pair is divided by velocity of light to obtain the start time.Considering the time dispersion in a MIMO system,the time delay of the optical signal between the centroid of PDs and LEDs array is treated as the start time.Furthermore,we assume that all signals arriving at the PD with a delay of more than half of the symbol period after the first signal contribute to ISI.Thus,the discrete time interval Tsp is defined as the half of the signal duration.The channel gain of the 1st path can be estimated by summing all the impulse response before Tsp from the beginning of the start time,and the channel gain of the 2nd path can be estimated by summing all the impulse response in integral intervals from Tsp to 2Tsp,and so on.This proposed multipath channel model,which considers the transmitted symbol period and decreases the effects of time dispersion,offers the research platform for evaluting the multipath effects.Secondly,a new optical ACO-MC-CDMA system which is a combination of CDMA and asym-metrically clipped optical orthogonal frequency division multiplexing(ACO-OFDM)is proposed in highly hostle indoor optical wireless channel.ACO-MC-CDMA can support a multiplicity of users within the same bandwidth,resist the interference of ambient lights,combat the ISI and meet the needs of an IM/DD system is proposed to be utilized in indoor wireless communications.In a ACO-MC-CDMA system,in order to fix a certain electrical signal-to-noise ratio in the receiver,pre-scaling op-eration is applied before carrying out IFFT operation,which can keep the average power of the output of IFFT constant.The signal after carrying out IFFT has approximately Gaussian distribution.Fur-thermore,the ACO-MC-CDMA signals need to be clipped at the double-sided level in order to keep the forward drive current through a LED inside the linear region.Clipping causes the clipped signal to shrink and also adds an additive non-Gaussian noise.For large FFT size,the no-Gaussian clipping noise is transformed into a Gaussian noise with zero-mean and the variance remains unchanged after FFT operation.In addition,theoretical expressions of signal-to-noise ratio(SNR)for the orthogonal-ity restoring combining(ORC),maximum ratio combining(MRC)and equal gain combining(EGC)are derived under both light-of-sight(LOS)and Ceiling-Bounce channel model,respectively.The validity of the model is verified by means of Monte Carlo bit-error-ratio(BER)simulations.The sim-ulation results show that the proposed ACO-MC-CDMA systems outperform the Flip-OFDM-CDMA and U-OFDM-CDMA systems.Thirdly,the VLC-MIMO system studied in this thesis transmits multiple data streams modu-lated by using DCO-OFDM and ACO-OFDM,respectively.Moreover,theoretical expressions of signal to interference plus noise ratio(SINR)for Zero-Forcing(ZF)and Minimum Mean Square Er-ror(MMSE)are derived under the VLC-MIMO multipath channel model.The validity of the model is verified by means of Monte Carlo bit-error-ratio(BER)simulations.In order to ensure comparabil-ity,the mean emitted optical power is the same for each scenario.Hence,we evaluate the BER at the receiver side with regard to the mean emitted total electrical power Es against the power spectral den-sity N0 of AWGN.Consequently,we can take into account the specific channel differences caused by the particular position of receiver within a room.In order to denote the power penalty induced by multipath effects,a power attenuation coefficient is defined for different particular position.The results show that the multipath effects dramatically impact on the BER performance.The receiver requires more SNR to achieve the same BER for the more multipath effects.Moreover,The direct current(DC)biasing can dramatically impact on the BER performance of MIMO-DCO-OFDM.Lastly,NOMA-DCO-OFDM system is proposed in this thesis,which is a combination of NOMA technology and DCO-OFDM.We analyze the relationship between the sum of the power allocation factor for each subchannel and the variance of the output of IFFT.Furthermore,considering a linear limited range of the realistic LED,theoretical expressions of signal to interference plus noise ratio(SINR)are derived under the VLC-MIMO multipath channel model.We also study the effect of the half power semi-angles of LED,the field of views(FOV)of the PD and the decay factors of gain ratio power allocation(GRPA)and static power allocation(SPA)on the sum rate.We assume that the transmitters are random placed within a room,and the channel gain is also random.The sum rate is a random variable when simulation is performed everytime.Then,the average sum data rate is defined as the average value of the sum data rate by repeated simulations.The results show that the parameters of half power semi-angles,FOV and the power allocation can impact on the sum data rate.These parameters can offer new degrees of freedom to boost the NOMA performance.
Keywords/Search Tags:Visible Light Communication, Multipath Channel Model, Optical Orthogonal Frequency Division Multiplexing, Multi Carrier-Code Division Multiple Access, Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing
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