The Mixed Transmission Technology And Performance Analysis In Wireless Optical Communication System

Optical wireless communication(OWC)has recently attracted much attention due to its multifarious advantages of wide unregulated spectrum,high date rate,high secrecy and no electromagnetic interference.It acts as a complementary technology of the traditional radio frequency(RF)communication,and be considered as an promising technology for solving the 'last mile' problem in the future wireless communication systems.Focusing on the OWC system,we in this paper first propose a new orthogonal frequency division multiplexing(OFDM)scheme,namely R-OFDM,for the indoor OWC system by employing red-green-blue-amber LED(RBA-LED).Then we investigate the power allocation strategy for multi-access in an indoor OWC system in order to achieve the maximum system capacity.Finally,we investigate the performance of the mixed RF/free space optical(RF/FSO)technique in an outdoor OWC system.The main contributions of this paper are summarized as follows:Firstly,in Chapter 2,we propose a reshaped orthogonal frequency division multiplexing(R-OFDM)scheme for the indoor OWC systems,where an LED consisting of red,green,blue,and amber chips(RGBA-LED)is adopted as the transmitter.Considering both illumination and communication requirements,the signal is adjusted by signal separation and biasing after clipping(BAC)/scaling before clipping(SBC)operations for transmitting on the RGBA-LED.We then optimize both biasing and scaling factors for the BAC and SBC operations,respectively.Correspondingly,a direct detection(DD)algorithm is developed for data recovery at receiver.Theoretical bit error rate(BER)of the quadrature amplitude modulation(QAM)for the DD algorithm is further derived in closed form.In order to improve the system performance,linear detection algorithms,like zero forcing(ZF)and minimum mean square error(MMSE)algorithms are employed for mitigating the cross-talk in multi-color VLC channels.Moreover,a low complexity parallel interference cancellation(PIC)is also proposed to eliminate these interference.Numerical results over both ideal and practical optical channels are presented to validate the advantage of the proposed R-OFDM scheme.Theoretical analysis and simulation results reveal that the R-OFDM achieves noticeable performance gain than the conventional optical OFDM scheme in term of BER.Then,in Chapter 3,we investigate the power allocation strategy for multiple user accessing in an in-door OWC system.Considering the requirements of both illumination and communication,we first analyze the total electrical power constraints and optical power constraints.After obtaining the signal to noise ratio(SNR)at receiver,we derive the system capacity in closed form.Then,we formulate the power allocation problem into an optimization problem based on the above power constraints,where the optimization goal is to maximize the system capacity.For the simultaneously transmission,the problem turns out to be a non-convex problem whose globally optimal solution is hard to obtain.To address this difficult problem,we first adopt the famous gradient projection algorithm to obtain a locally optimal solution,then we employ the powerful convex relaxation technique to achieve a high-quality solution.While for the round-robin case,we are able to derive an optimal solution in closed form by virtue of Karush-Kuhn-Tucker(KKT)conditions.Interestingly,the derived closed-form solution follows a water-filling pattern with both square root and two-side clipping operations.Numerical results reveal that the performance of the proposed power allocation strategies are superior to that of the traditional equal power allocation strategy under various settings.Finally,we investigate the average symbol error rate(SER)performance of a dual-hop mixed RF/FSO transmission scheme in an outdoor OWC system,where the RF link is used for supporting mobile communi-cation,the FSO link is adopted as the backhaul of the cellular infrastructure.The whole system works in either fixed-gain amplify-and-forwad(AF)relaying mode or variant-gain AF relaying mode.Considering non-line-of-sight(NLOS)RF transmissions and a generalized atmospheric turbulence(AT)channel,the associated end-to-end statistical features constituted of cumulative distribution function(CDF)and moment generating function(MGF)are derived in closed form.They are then used for calculating the closed-form or approximate SER of M-ary phase shift keying(PSK),differentially PSK(DPSK)and non-coherent frequency-shift keying(FSK).A range of additional asymptotic expressions are also derived for all the modulation schemes under high signal-to-noise ratios(SNR).We then obtain the diversity orders of all the modulation schemes,calcu-late the performance differences between them,and derive some helpful observations from the asymptotic expressions.Furthermore,we obtain the closed-form or approximate SER expressions for different modula-tion schemes under both K and Gamma-Gamma FSO channels in the mixed RF/FSO system from the above theoretical analysis.Finally,simulation results are presented for confirming the accuracy of the theoretical analysis and observations.