Performance analysis and modeling: atmospheric turbulence and crosstalk of WDM-FSO network

— A wavelength division multiplexing (WDM) access network using high-speed free-space optical (FSO) communication for the distribution link is proposed. This Paper investigates terrestrial atmospheric of WDM-FSO communication systems operating under the influence of turbulence-induced scintillation, beam spreading, optical interchannel crosstalk, amplified spontaneous emission noise and pointing errors.On-off keying-non – return-to-zero and digital pulse position modulation are the modulation schemes used for the calculations.


II. Network structure
A diagram of the proposed network is shown in Fig.1, the optical orthogonal codes (OOCs) are also used for this network. Each K users on the same wavelength use distinct OOCs from the same family, and a user corresponds to a single optical network unit (ONU) so both terms are used interchange ably in this analysis. In the downstream, coded signals are transmitted on multiple wavelengths from the optical line terminator (OLT) via a feeder fibre and distributed to individual ONUs by FSO link. At the ONUs, upstream coded signals are transmitted through a short fibre length and from a transmitting lens (TL) to the corresponding collecting lens (CL) at the remote node using the FSO link.

III. Upstream transmission
Multiple ONUs limited by the number of OOCs share a dedicated wavelength and establish a point-topoi t upstream transmission link with the OLT (seeFi.1b).The operating wavelengths are assumed to be set around 1550 nm on the C-band of the ITUT grid, thus benefiting from the low signal attenuation and developed optical device technology in those range of wavelengths. Each group of ONUs transmitting on a fixed wavelength uses the same laser transmitters operating at a set central wavelength. Optically encoded signals from the ONUs located a t homes, buildings or kerb are transmitted upstream through a turbulent FSO link to the remote node. The average optical received power at the OLT photodiode from an ONU on the desired signal wavelength and an ONU on the crosstalk wavelength are respectively written as [18][19][20][21][22][23][24][25][26][27][28][29][30].

Where and are the transmit power of an ONU on the desired signal wavelength and an ONU on the crosstalk Wavelength respectively, and The single polarization ASE power spectral density (PSD) at the OLT photodetector inputs from an ONU on the desired signal wavelength and an ONU on the crosstalk wavelength are respectively written as [18 -30].
( ) ( )

Where G and NF are the optical amplifier gain and noise figure respectively, h is
Plank's constant, and are the optical frequencies of the desired signal and crosstalk signal respectively.

IV. Downstream transmission
The downstream architecture is similar to the upstream but the system operation is different. Each wavelength at the OLT has a separate laser source which transmits the signal for the group of ONUs on its wavelength. As shown in Fig.1a, the signal on each wavelength is split into the number of ONUs on the wavelength and separately encoded with the OOC for each ONU before being recombined for wavelength multiplexing [18][19][20][21][22][23][24][25][26][27][28][29][30]. The multiplexed signals propagates through the feeder fibre to the remote node for optical amplification and demultiplexing before each wavelength"s signal is split into each ONUs signal and further transmitted through the FSO link for decoding and reception [30][31][32][33][34][35].

V. TURBULENCE MODELING
Atmospheric scintillation occurs due to thermally induced refractive index changes of the air along the optical link, causing rapid fluctuation of signal irradiance at the receiver, reduction in degree of coherence of the optical signal [34], and potentially poor bit error rate(BER).The gamma-gamma(GG) distribution is widely used for characterizing the whole range of turbulence effects ,i.e., weak, moderate, and strong, not only because closed form expressions exist but also because of their direct dependence on turbulence parameters and the closeness to experimental results [14,16,[18][19][20][21][22][23][24][25][26][27][28][29][30].

VI. BER Analysis
In its most general form,under the assumption of independent signal and crosstalk channels (e.g.,as in the upstream), the average (turbulence-accentuated) BER [for given fixed transmitter powers for the signal and crosstalk [18].

VII. Pulse Position Modulation
In LOS OWC links where the requirement for the bandwidth is not of a major concern, PPM with its significantly better power efficiency seems to be the most attractive option for a range of applications PPM is an orthogonal modulation technique and a member of the pulse modulation family (see Figure 10). The PPM modulation technique improves on the power efficiency of OOK but at the expense of an increased bandwidth requirement and greater complexity.
In order to achieve the same throughput as OOK, PPM slot duration is shorter than the OOK bit duration by a factor L/M that is [16].
The transmit pulse shape for L-PPM is given by [16].

VIII. FSO Link Performance under the Effect of Atmospheric Turbulence.
Atmospheric turbulence is known to cause signal fading in the channel. There are many different types of modulation schemes that are suitable for optical wireless communication systems. The effect of atmospheric turbulence-induced fading on the following techniques: on-off keying (OOK), pulse position modulation (PPM) and phase shift keying pre-modulated subcarrier intensity modulation [16]. The classical modulation technique used for FSO is OOK [1,2]. This is primarily because of the simplicity of its design and implementation. The PPM requires no adaptive threshold and is predominantly used for deep space free-space optical communication links because of its enhanced power efficiency compared to the OOK signaling [5][6][7][8][9][10].

X. Conslusion
We analysed A hybrid fibre and FSO WDM network and the effects of turbulence-accentuated interchannel crosstalk , it could be seen that DPPM systems required lower optical power copmared to OOK systems. It can be deduced from this analysis tha tinterchannel crosstalk,turbulence-induced scintillation, and ASE noises are dominant causesof system degradation, especially in the upstream transmission, causing the BER to increase by several orders of magnitude.