Investigations on modified OOK and adaptive threshold for wavelength division multiplexing free-space optical systems impaired by interchannel crosstalk, atmospheric turbulence, and ASE noise

This paper presents the design and analysis of a dense wavelength division multiplexing free-space optical communication (DWDM-FSO) optical fiber under the effects of noises, interchannel crosstalk for weak (WT), moderate (MT) and strong (ST) turbulence with both of the approach modified on–off keying (OOK) and an adaptive threshold modulation. The modified OOK modulation has brought a revolution because the system data capacity is enhanced by simply adding more number of channels and reducing the channel spacing without having the need of more than one FSO link. For investigation, \(\left(8\times 2.5\text{Gbit}/\text{ps}\right)\) $\left(8,\times ,2.5,\text{Gbit},/,\text{ps}\right)$ channels wavelength division multiplexing (WDM) based FSO system is proposed and performance is evaluated on widely accepted modulation schemes under WT, MT and ST turbulence conditions. We present analytical mathematical model derivation of the error floor that relies on OOK and adaptive threshold modulation for a general representation of practical DWDM-FSO links. The numerical results show the modified OOK can suppress the error floor which occurs of case signal with turbulence, interferer with no turbulence (TurbuSi, XT), decrease the noises, scintillation and enhancement TurbuSi, XT at bit error rate (BER)  \(\text{of}{\;10}^{-12}.\) $\text{of}{10}^{-12}.$ Modified OOK modulation adds a further improvement where the distance of data transmission rate increase and enhancement the BER performance which depends of average received power and is best for strong turbulence.