**3.7. Optical link budget**

To calculate the FSO link budget several parameters taken into account as geometric loss, link margin, received power and bit error rate. The received power should be grater less the transmitted power from the source and equal the transmitted power minus total loss. In the basic free-space channel the optical field generated at the transmitter propagates only with

an associated beam spreading loss. For this system the performance can be determined directly from the power flow. The signal power received PRx [W] depends on the transmit power PTx [W], transmit and receive antenna gains GTx, GRx, and the total loss [36]

$$P\_r = P\_t + G\_{T\chi} + G\_{R\chi} \text{ - total loss} \tag{36}$$

Table (5) shows the values of transmitted output power for diffuse and tracked topology (Data obtained from [7]).

In the indicated reference, they presented an expression to calculate the link distance L achievable from direct line propagation:


**Table 5.** Values of transmitted output power for diffuse and tracked topology from [7].

$$L = \frac{\rho\_\text{\textsuperscript{P\_\text{\tiny A}}} \mathcal{T}\_1 \mathcal{T}\_2}{2 \pi \mathcal{P}\_{\text{rw}} \text{(1 - \cos \ \varnothing)}}\tag{37}$$

Here, Pt represent the optical output power from the transmitter (in mW), Ar is the active area of the photodetector, T <sup>1</sup> is the transmittivity of the transmitter filter, T <sup>2</sup> is the transmittivity of the filter at the receiver, Prm is the optical power required (in mW) to obtain a specific carrierto-noise ratio at the receiver, and ∅ is the half angle of the energy related by optical source. From this expression, they calculate achievable distances (depending on the FOV), which in their case covered a range of between 10 and 20 m.

#### **3.8. BER and SNR**

sions of lognormal field amplitude variance depend on the nature of the electromagnetic wave

In this chapter, we do not take into account the atmospheric turbulence, because its influence in Yemeni climate could be negligible. That means the effect of the turbulence is too small contrary to visibility and geometric loss. Therefore, we have taken into account only the total

An FSO communication system is influenced by atmospheric attenuation, which limits their performance and reliability. The atmospheric attenuated by fog, haze, rainfall, and scintillation has a harmful effect on FSO system. The majority of the scattering occurred on the laser beam is Mie scattering. This scattering is due to the fog and haze aerosols existed at the atmosphere and can be calculated through visibility. FSO attenuation at thick fog can reach values of hundreds dB. Thick fog reduces the visibility range to less than 50 m, and it can affect on the performance of FSO link for distances. The rain scattering (non-selective scattering) is inde‐ pendent on wavelength, and it does not introduce significant attenuation in wireless infrared links, it affects mainly on microwave and radio systems that transmit energy at longer wavelengths. There are three effects on turbulence: scintillation, laser beam spreading and laser beam wander. Scintillation is due to variation in the refractive index of air. If the light is traveled by scintillation, it will experience intensity fluctuations. The geometric loss depends on FSO components design such as beam divergence, aperture diameter of both transmitter and receiver. The total attenuation depends on atmospheric attenuation and geometric loss. To reduce total attenuation, the effect of geometric loss and atmospheric attenuation is small, as FSO system must be designed. The following section explores the simulation results of

To calculate the FSO link budget several parameters taken into account as geometric loss, link margin, received power and bit error rate. The received power should be grater less the transmitted power from the source and equal the transmitted power minus total loss. In the basic free-space channel the optical field generated at the transmitter propagates only with

an associated beam spreading loss. For this system the performance can be determined directly from the power flow. The signal power received PRx [W] depends on the transmit power PTx

Table (5) shows the values of transmitted output power for diffuse and tracked topology (Data

In the indicated reference, they presented an expression to calculate the link distance L


[W], transmit and receive antenna gains GTx, GRx, and the total loss [36]

Pr = Pt + GTx + GRx

traveling in the turbulence and on the link geometry.

186 Contemporary Issues in Wireless Communications

attenuation depending on visibility, and geometric loss.

geometric loss and total attenuations for Yemeni climate.

**3.7. Optical link budget**

obtained from [7]).

achievable from direct line propagation:

Both SNR and BER are used to assess the quality of communication systems. BER performance depends on the average received power, the scintillation strength, and the receiver noise. With an appropriate design of aperture averaging, the received optical power could be increased and the effect of the scintillation can be dumped. With turbulence, the SNR is expressed as follows [37]:

$$\text{SNR} = \left\{ 0.31 \, \text{C}\_n^2 \, \text{k}^{\frac{\gamma}{6}} \, \text{I}^{\frac{11}{6}} \right\}^{\text{-1}} \tag{38}$$

For FSO links with an on off keying modulation scheme in BER can be written as

$$BER = \frac{\exp\{-\text{SNR} \mid 2\}}{\text{(2\pi SNR)}^{0.5}} \tag{39}$$

In our model, we have assumed that the surface area of the photo detector is large enough so that the effective SNR includes the beam spreading effect, thus the effective SNR is defined as

$$SNR\_{eff} = \frac{SNR}{1 + 1.33\sigma\_i^2 \left[\frac{2I}{k\omega(l)^2}\right]^5} \Bigg|\tag{40}$$

The performance and reliability of FSO communication systems are affected and limited by atmospheric attenuation. It has a harmful effect by haze, rainfall, fog, and scintillation has a harmful effect of FSO system. The majority of the scattering occurred to the laser beam is due to the Mie scattering. This scattering is due to the fog and haze aerosols existed at the atmos‐ phere. This scattering is calculated through visibility. FSO attenuation at thick fog can reach values of hundreds dB. Thick fog reduces the visibility range to less than 50 m, and it can affect on the performance of FSO link for distances as small. The rain scattering (non-selective scattering) is wavelength independent and it does not introduce a significant attenuation in wireless IR links, it affect mainly on microwave and radio systems that transmit energy at longer wavelengths.

There are three effects on turbulence: scintillation, laser beam spreading and laser beam wander. Scintillation is due to variation in the refractive index structure of air, so if the light traveling through scintillation, it will experience intensity fluctuations. The Geometric loss depends on FSO components design such as beam divergence, aperture diameter of both transmitter and receiver. The total attenuation depends on atmospheric attenuation and Geometric loss. In order to reduce total attenuation, FSO system must be designed so that the effect of geometric loss and atmospheric attenuation is small.
