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The paper studies the performance of a free space optical communication link, which consists of a Gaussian Laser beam employing direct-current-biased optical frequency division multiplexing (DCO-OFDM) modulation, to carry information. While optical multi-carrier modulation is capable of alleviating multi-path fading, higher transmit power by virtue of dc-bias, helps the optical beam in partially off-setting signal intensity loss due to beam divergence in free space channel. In order to reduce the overall transmitted power apart from avoiding nonlinearities due to high peak-to-average-power-ratio (PAPR), we utilize a PAPR-dependent dc-bias, to generate a DCO OFDM light beam from a pre-clipped electrical baseband OFDM signal. The radiated optical beam encounters turbulence-induced impairments like beam-spreading, beam-wander, etc., which affect the signal-to-noise ratio (SNR) of the received data symbols. Further signal pre-clipping and optical source linearity generate clipping noise components resulting in compromised receiver performance or reduction of span length, to achieve target bit rate error (BER). In this paper, we derive an analytical model for evaluating the performance of the free-space optical (FSO) link, by modeling various impairments as noise variances and characterizing atmospheric turbulence by refractive index structure parameter under weak- and strong-turbulent conditions. Numerical results are obtained by varying system design parameters in the model, which graphically provide useful insight on the power penalties and various trade-offs involved in operating the link on a longer span, at a higher data rate, and with reduced transmitted power through pre-clipping while ensuring desired signal BER. [ABSTRACT FROM AUTHOR]

Free Space Optical (FSO) communication technology has gained popularity because of its high bandwidth, which supports high-speed data transmission. Despite its merits over Radio Frequency technology, the FSO system performance is restricted by the combined impact of beam divergence, beam scintillation, and beam-wander effects triggered by atmospheric turbulence. A Gamma–Gamma probability density function (PDF) channel model is employed to model the irradiance fluctuations due to turbulence. In this paper, we evaluate the performance metrics of a FSO link employing the Direct Current-biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) modulation technique for different turbulent conditions. In our paper, we determine the DC-bias, which is proportional to peak-to-average-power ratio (PAPR) and a preclipping factor of n c l i p (< 1) in order to reduce the transmitted optical power of the DCO-OFDM signal. We evaluate the average bit error rate (BER) and outage probability of the FSO link by considering the impact of system design parameters related to the OFDM signal, laser beam, and atmospheric turbulence on the received signal. Numerical simulation results show that, depending on the chosen values of PAPR and preclipping, an FSO link can experience either a power saving or a power penalty, for meeting the BER requirement. It is further demonstrated that trade-offs in performance at various levels exist between data transmission rate (QAM size), system power efficiency (preclipping), signal-PAPR and link-span to ensure target signal quality (BER and Outage). Under turbulence, the link with a longer span not only incurs a power penalty but also suffers from a reduced operating range by 3–4 dB. [ABSTRACT FROM AUTHOR]