Key Technologies And Performance Analysis Of Digital Coherent Optical Wireless Communication Systems

Optical wireless communication(OWC)technology employs optical carriers for information transmission,which can provide higher data rates than radio-frequency(RF)wireless communications and address the growing scarcity of RF spectrum resources.However,the outdoor OWC system,e.g.,the free space optical(FSO)communication system,is impaired by atmospheric attenuation,pointing errors,and atmospheric turbulence induced fading,which degrades the bit-error rate(BER)performance.While the indoor OWC system,e.g.,the visible light communication system,using the white light emitting diode(LED)for illumination and communication simultaneously,the data rate of which is limited by the modulation bandwidth of the LED.In order to solve these problems,this paper introduces the digital coherent detection technology,which originates from the optical fiber communication system,to the outdoor and indoor OWC systems,respectively.The average BER performance is studied for the phase shift keying(PSK)system under the combined influence of pointing errors,atmospheric turbulence and phase estimation error.Performances of the mixed RF/FSO transmission system based on amplify-and-forward(AF)and decode-and-forward(DF)relay protocols are also investigated,respectively.Furthermore,an experiment is carried out to verify the proposed indoor high speed OWC system.The main work and contributions of this paper are as follows:1.Traditionally,a phase-locked loop is used to recover the carrier phase in the heterodyne detection system,and the phase estimation error follows the Tikhonov distribution.While digital signal processing algorithms are used for carrier phase estimation in the digital coherent detection system,and the phase estimation error can be approximated as a Gaussian random variable with zero mean.In Chapter 3,the average BER performance is studied for a coherent FSO communication system employing PSK with the Mth-power phase estimation method and decision-aided maximum likelihood scheme,respectively.Closed-form expressions for the average BER,considering the combined effects of the Málaga(M)turbulence fading,pointing errors,and phase estimation errors,are derived in terms of Meijer's G function.Numerical and Monte Carlo(MC)simulation results are presented to verify the derived expressions.The results show that the phase estimation error has a minor influence on the coherent FSO system compared to the atmospheric turbulence conditions and pointing errors when state-of-the-art distributed-feedback semiconductor lasers are used as the transmitter and LO,but the phase estimation error will have a great impact on the average BER performance and lead to an unrecoverable error floor when other lasers with large linewidths are employed.2.In Chapter 4,performance analyses are presented for a fixed gain AF-based mixed RF/FSO dual-hop transmission system with subcarrier intensity modulation,and a DF-based mixed RF/FSO dual-hop transmission system with digital coherent detection,respectively.The RF path is modeled by Beaulieu-Xie fading,while the FSO hop is characterized by the M distributed turbulence with pointing errors.Firstly,closed-form expressions and infinite series representations for the probability density function(PDF)and the cumulative distribution function(CDF)of the signal-to-noise ratio(SNR)of the system whose channel gain follows BeaulieuXie fading are derived.Secondly,end-to-end performance analysis for the AF-based RF/FSO system is carried out.Novel and exact analytical expressions for the CDF,the PDF,and the moment generating function(MGF)of the overall SNR are derived in terms of Meijer's G function,followed by the accurate infinite series expressions of the outage probability,the average BER,and the ergodic capacity(average channel capacity)in terms of extended generalized bivariate Meijer's G function.Asymptotic analysis for the CDF,the MGF,the outage probability,and the average BER is also provided.As a special case,the exact infinite series expressions are also derived for the mixed Beaulieu-Xie and Gamma-Gamma dual-hop transmission system.Thirdly,the digital coherent detection technology is introduced to the DF-based mixed RF/FSO system,and end-to-end performance analysis is carried out to obtain the corresponding accurate analytical expressions as well.At last,MC simulations are performed to verify these derived expressions.3.In Chapter 5,we investigate a coherent optical wireless communication system,which is able to support tens of gigabits per second(Gb/s)data transmission rate and suitable for high speed indoor interconnection.The modulated optical signal is diverged by a diffuser,and it is collected and coupled into a digital coherent receiver.The transmission model is analyzed and simulated by taking reflection into account.We also experimentally demonstrate the indoor coherent optical wireless system at 25 Gb/s over 1 m transmission distance.The numerical and experiment results show that the indoor coherent optical wireless system is a feasible solution and can achieve high link data rates for indoor interconnection.