Research On Visible Light Communication Systems With Non-orthogonal Multiple Access Technology

Non-orthogonal multiple access(NOMA)has been studied in visible light communication(VLC)sys-tems such as direct current biased optical orthogonal frequency division multiplexing(DCO-OFDM)for high-er spectral efficiency.This thesis mainly focuses on the effect of clipping and schemes of power allocation for the NOMA with DCO-OFDM(NOMA-DCO-OFDM)system.Firstly,we give an overview of VLC systems and power domain NOMA technology.As DCO-OFDM occupies the higher spectral efficiency in VLC systems,downlink transmission scenario of NOMA-DCO-OFDM and OMA-DCO-OFDM systems is considered.Combing NOMA with DCO-OF-DM brings higher spectral efficiency,along with new problems such as clipping noise.Existing works lack an analytical char-acterization of the clipping noise in NOMA for VLC systems.Thus,we analyze the impact of nonlinear double-sided clipping process by applying the Bussgang Theorem.Additionally,we select two typical VLC channels:AWGN and dispersive channel to evaluate achievable rates by the optimization of DC-offset and power allocation.Then,in the flat case,where the channel frequency responses of all the subcarriers are the same,the achievable rate region of the NOMA-DCO-OFDM system shows obvious difference with different DC-offset under fixed electric power limitation,which shows that the influence of clipping noise can not be neglected.The issue of obtaining the boundary of the achievable rate region is transformed into two optimization prob-lems.Moreover,different electric power constraints and clipping levels influence the achievable rate region of NOMA-DCO-OFDM system.Simulation results prove that the NOMA-DCO-OFDM system is superior in achievable rates to the OMA-DCO-OFDM system.Finally,we study on the sum rate of NOMA-DCO-OFDM with fixed DC-offset under frequency selective channel.Firstly,a VLC dispersive channel model with closed expression is established based on the delay spread.In this channel,three power allocation schemes are discussed in terms of sum-rate optimization.Iterative water-filling scheme transfers sum rate maximization problem to specific user rate maximization problem with minimum rate constraints.UD-FTPA scheme apply fractional transmit power allocation(FTPA)algorithm with uniform power allocation among subcarriers.FTPA-WF scheme performs FTPA algorithm based on average channel gains of users.Numerical results are provided to demonstrate the reliability and superiority of our proposed schemes.Based on the downlink NOMA-DCO-OFDM system,the thesis mainly achieves the following:1)the achievable rate expression with the effect of clipping;2)the determination of the boundary of achievable rate region under the AWGN channel;3)verification of its superiority to the OMA-DCO-OFDM system;4)three joint optimization schemes for subcarrier power allocation and inter-user power allocation to maximize sum rate under frequency selective channel.