Adaptive Subcarrier PSK Intensity Modulation in Free Space Optical Systems

We propose an adaptive transmission technique for free space optical (FSO) systems, operating in atmospheric turbulence and employing subcarrier phase shift keying (S-PSK) intensity modulation. Exploiting the constant envelope characteristics of S-PSK, the proposed technique offers efficient utilization of the FSO channel capacity by adapting the modulation order of S-PSK, according to the instantaneous state of turbulence induced fading and a pre-defined bit error rate (BER) requirement. Novel expressions for the spectral efficiency and average BER of the proposed adaptive FSO system are presented and performance investigations under various turbulence conditions and target BER requirements are carried out. Numerical results indicate that significant spectral efficiency gains are offered without increasing the transmitted average optical power or sacrificing BER requirements, in moderate-to-strong turbulence conditions. Furthermore, the proposed variable rate transmission technique is applied to multiple input multiple output (MIMO) FSO systems, providing additional improvement in the achieved spectral efficiency as the number of the transmit and/or receive apertures increases.Comment: Submitted To IEEE Transactions On Communication

MIMO free-space optical communication employing subcarrier intensity modulation in atmospheric turbulence channels

In this paper, we analyse the error performance of transmitter/receiver array free-space optical (FSO) communication system employing binary phase shift keying (BPSK) subcarrier intensity modulation (SIM) in clear but turbulent atmospheric channel. Subcarrier modulation is employed to eliminate the need for adaptive threshold detector. Direct detection is employed at the receiver and each subcarrier is subsequently demodulated coherently. The effect of irradiance fading is mitigated with an array of lasers and photodetectors. The received signals are linearly combined using the optimal maximum ratio combining (MRC), the equal gain combining (EGC) and the selection combining (SelC). The bit error rate (BER) equations are derived considering additive white Gaussian noise and log normal intensity fluctuations. This work is part of the EU COST actions and EU projects

Performance analysis of FSO using relays and spatial diversity under log-normal fading channel

The performance analysis of free space optical communication (FSO) system using relays and spatial diversity at the source is studied in this paper. The effect of atmospheric turbulence and attenuation, caused by different weather conditions and geometric losses, has also been considered for analysis. The exact closed-form expressions are presented for bit error rate (BER) of M-ary quadrature amplitude modulation (M-QAM) technique for multi-hop multiple-input single-output (MISO) FSO system under log-normal fading channel. Furthermore, the link performance of multi-hop MISO and multi-hop single-input and single-output (SISO) FSO systems are compared to the different systems using on-off keying (OOK), repetition codes (RCs) and M-ary pulse amplitude modulation (M-PAM) techniques. A significant performance enhancement in terms of BER analysis and SNR gains is shown for multi-hop MISO and multi-hop SISO FSO systems with M-QAM over other existing systems with different modulation schemes. Moreover, Monte-Carlo simulations are used to validate the accuracy and consistency of the derived analytical results. Numerical results show that M-QAM modulated multi-hop MISO and multi-hop SISO FSO system with relays and spatial diversity outperforms other systems while having the same spectral efficiency of each system.Comment: 4 pages, 4 figures, 4th International Conference on Electrical Energy Systems (ICEES), Feb. 7-9, 2018, SSNCE, Chennai, TN, INDI

BPSK subcarrier intensity modulated free-space optical communications in atmospheric turbulence

Free-space optical communications (FSO) propagated over a clear atmosphere suffers from irradiance fluctuation caused by small but random atmospheric temperature fluctuations. This results in decreased signal-to-noise ratio (SNR) and consequently impaired performance. In this paper, the error performance of the FSO using a subcarrier intensity modulation (SIM) based on a binary phase shift keying (BPSK) scheme in a clear but turbulent atmosphere is presented. To evaluate the system error performance in turbulence regimes from weak to strong, the probability density function (pdf) of the received irradiance after traversing the atmosphere is modelled using the gamma-gamma distribution while the negative exponential distribution is used to model turbulence in the saturation region and beyond. The effect of turbulence induced irradiance fluctuation is mitigated using spatial diversity at the receiver. With reference to the single photodetector case, up to 12 dB gain in the electrical SNR is predicted with two direct detection PIN photodetectors in strong atmospheric turbulence