Free Space Optics (FSO) Links in Singapore: Scintillation Effects

Ananda Ekaputera Sidarta

School of Electrical & Electronic Engineering Nayang Technological University

Nanyang Avenue, Singapore 639798 e-mail: H977168@ntu.edu.sg

Keywords: FSO link, scintillation, refractive index, peak-to-peak value, scintillation indices.

Abstract Free Space Optics (FSO) can provide an effective line-of-sight, wireless, and high bandwidth communication between two places. The range of FSO links operating in temperate regions is limited by fog, but in tropical regions, rain is expected to be the limiting factor. This is one of the reasons why the Infocomm Development Authority of Singapore (IDA) conducted a three months trial in Singapore. This project will mainly study scintillation effect for rain and non-rain period that is due to variation of refractive index of air. Ultimately, a better understanding of scintillation effect of Singapore tropical environment on FSO performance can be achieved.

1. Introduction As a communication system, FSO uses air as its medium. The transmission process using FSO is relatively simple. It only needs a laser transmitter and a receiver. Each FSO system uses a high-power optical source such as laser plus a telescope that transmits light through the atmosphere to another telescope which acts as a receiver. An FSO link refers to a pair of FSO telescope, each aiming a laser beam at the other. Hence one telescope has duplex capability to act as a laser transmitter as well as a receiver [1]. Most of the cases, these telescopes are installed on the top of the building.

Most of the FSO equipments operate in two frequency bands 780nm−900nm and 1500−1600nm. Because they use invisible infrared light which is corresponding to frequencies in the order of terahertz, they will not interfere with RF equipments which operate in the gigahertz band. In addition, the very narrow laser beam used makes it unlikely that nearby FSO systems would interfere with one another. Hence, unlike RF, the frequencies used by FSO do not have to be assigned or actively regulated

Both FSO and radio system use air as medium. However, FSO is categorized as optical system because:

i. FSO uses laser beam which can transmit at a speed up to 2.7 Gbps and even higher which is not possible to obtain using radio system.

ii. It does not need to buy expensive spectral license (no license is necessary).

Nowadays, FSO systems are widely used especially to connect one part of the city to the other remote area or to support Metropolitan Area Network (MAN). It can also be used to extend metropolitan-area fibre rings to connect to the new outside-networks.

Figure 1. FSO telescope is installed on the roof.

2. Problems Identification The only major disadvantage of FSO is the effect of atmospheric (air) itself on the laser beam transmitted. These effects can be classified into two: attenuation of laser power and fluctuation of laser power due to laser beam deformation [2]. Attenuation consists of absorption and scattering of the laser light photos by the different molecules in the atmosphere, such as water droplets, haze, dust, and organic materials. Some of the important causes of attenuation are discussed:

i. Absorption will occur when suspended water molecules in the terrestrial atmosphere extinguish photons. This causes a decrease in the power attenuation (or power loss) of photon beam. Further, absorption occurs in certain wavelengths than others, which means it depends on atmospheric condition. Most free space laser transmission wavelengths are primarily chosen to get very low absorption losses.

ii. Scattering is caused by particles in the atmosphere where their size is relatively smaller than a wavelength. The primary difference between scattering and absorption is that in scattering, there is no power loss for every photon transmitted and only a directional redistribution of energy that may cause significant intensity loss at the receiver.

Water droplets mentioned earlier can be in the form of rain, snow, or fog. Among these three, fog is the main cause of attenuation [2]. Fog with low visibility could introduce losses of several hundreds dB/km. The primary way to tackle this is by shortening the link-space and using multi path systems. Multi path send the same information over multiple optical points which will increase the possibility that any one path will be able to penetrate the fog. Historical visibility data is available for every season in each region of the cities which can be used as consideration. In tropical countries such as Singapore, rain is our major concern. The other thing which must be taken into account is scintillation.

2.1. Scintillation Effect Laser beam deformation is caused by the changes in the refraction index of medium (free space or air). It is mainly due to scintillation which affects the laser beam propagation and ultimately causes fluctuation at the receiver end signal. It occurs because of atmospheric turbulence that produces many temporary areas that we call pockets or Fresnel zone. Atmospheric turbulence (such as wind) produces temporary pockets of air with different temperatures, different densities, and hence will produce different refractive index. As the turbulence is random, these pockets are continuously being created and destroyed. Look at Figure 2.

Figure 2. Scintillation is caused by variation of refractive index of air which can produce signal fluctuation at the receiver.

Figure 2 shows how the laser beam will randomly bend and cannot reach the destination. Scintillation effect depends on the air pockets size. If the size is big, then the laser beams will randomly bend and cannot reach the destination. If the size is smaller, ray bending and diffraction will cause distortion at the receiver. This is what makes the signal fluctuates.

2.2. Scintillation Indices The amplitude of received signal (i.e. laser beam) can fluctuate about the mean level in certain manner. To study scintillation, certain parameters which are called scintillation indices are used. One of the parameters is peak-to-peak scintillation which is simply taken from the maximum and the minimum value for certain period. It is a very easy to represent characteristics of scintillation. Other important indices are S4 and SI (scintillation index). For SI, generally Pmax is taken as the third peak down from the highest peak that occurred in a given period and Pmin is similarly the third peak up from the smallest peak. However, in certain extent, S4 is more accurate to describe scintillation. The parameters are taken within specific period, 1 minute being typical. Mathematically:

meandeviation standard4

x

x ==m

S σ …(1)

)1()( 22

x −∑−∑

=nn

xxnσ …(2)

minmaxminmax

PPPPSI

+−

= …(3)

2.3. Experimental Setup Starting February 2002, IDA Tech Group embarked on a trial at Sengkang estate where three FSO links were installed. From these, physical tests (to study rain and scintillation effect, including window glass test, spray test, and filter test) were conducted for three months. Observations were done for 1 km (1000m) open ground link. The data obtained were then collected and MATLAB® was used to analyze them. As a whole, basically, there were three different MATLAB source codes: 1. Daily analysis (diurnal) for every month for three

months. MATLAB read the whole column of LaserInputLevel obtained for each day and then plotted on the screen.

2. Daily peak to peak variations for every minute and ten minutes, daily. The peak-to-peak can be obtained from the respective maximum and minimum values during the specified period, say one minute.

3. Scintillation index S4 using Equation (1) for every minute and ten minutes, daily. Average and standard deviation are taken for a specified period.

Here, it is important that the LaserInputLevel is either in dBm or in level number between 0 and 100 which is the original scale obtained from measurement. Hence, certain calibration must be done. Linear approximation is used between level of 70 to 90 (≈ −12 dBm to 2 dBm).

3. Data Analyses 3.1. Characteristic of Non-rain Period

Generally, the diurnal variation has the following pattern:

Transmitter Air pockets Receiver

1. Scintillation started with a small peak-to-peak variation in dusk and in the morning [3].

2. During midday, 10:00 to 15:00, the scintillation reached its highest peak-to-peak. This was also when the temperature reaches its highest nominal value. In the evening, the scintillation dropped again to normal, relatively the same as that during the morning period.

Typical example of non-rain period of March 4 is shown in Figure 3. The numbers for scintillation indices are the original figures obtained from measurement (not in dB). Seasonal variations for 24 hours also follow the similar pattern and it reached its highest peak during April.

3.2. Characteristic of Rain Period

Rain that occurs in Singapore can further attenuate the signal and the laser beam can be seen in Figure 4 (20 March). From Figure 4, it can be seen that:

1. There were three periods whereby the signals were attenuated severely. Among these, some were due to water-spray tests. Comparing with the rain records from the Radar, it can be known that the rain itself occurred at 14.30 pm where the signal attenuation reached level 30 (≈ −34 dBm). This rain attenuation level was considered high [4].

2. The peak-to-peak scintillation also increased during midday. Before rain (12:00 to 14:00) the peak-to-peak scintillation reached ≈ 4 dB. This can be seen either from diurnal plot or the peak-to-peak plot. During the period 12:00 to 14:00, it reached level 9 which corresponded to ≈ 4 dB.

3. On the event of the rain itself, the peak-to-peak scintillation and hence S4-values increased.

4. After rain, the peak-to-peak scintillation dropped at level 4 (≈ ½ dB).

Notice that because non-linearity of calibration graph, the rain and non-rain conversion are different. For high attenuation (low Input Level) between level 30 to 60 can be approximately ≈ −34 dBm to −18 dBm.

Another rain-period example is given in Figure 5. The rain occurred in the morning (around 9:40 am) and the peak-to-peak scintillation was again analyzed. Some interesting observations can be achieved:

1. The rain started to fall at 9:40 where the attenuation reached level 37 (≈ −31 dBm). Again this level of rain attenuation was considered high.

2. Peak-to-peak scintillation was still quite high even after the rain (beyond level 10 or ≈ 6 dB) and continued to be high until around 18:00 pm and then decreased a bit until midnight. This is unlike the previous example.

+2.0

−3.0

−6.0

−9.0 dBm

Figure 3. Diurnal variation, peak-to-peak and S4 (for 1 minute processing) on 4 March 2002 (non-rain).

+2.0 −6.0 −12.0 dBm

−6.0 −18.0 −28.5

−34.0 dBm

Figure 4. Diurnal variation, peak-to-peak and S4 (for 1 minute processing) on 20 March 2002 (rain).

+2.0 −6.0 −12.0 dBm

−6.0

−18.0

−28.5 dBm

Figure 5. Diurnal variation of FSO laser beam on 24 March 2002 (rain). The attenuation is considered high.

4. Conclusion This project discusses scintillation effect of tropical environment (Singapore) on the performance of FSO. The three month data were collected and analyzed using MATLAB. In general, some conclusions can be achieved as followed:

1. Signal fluctuations are high during midday and this can be seen from the plots. Hence, during midday, peak-to-peak scintillation tends to be higher than that in the morning and evening.

2. Rain can also affect signal attenuation. Scintillation effect during rain period and non-rain period were studied.

3. Peak-to-peak scintillation during rain could be as high as 6 dB, while the rain attenuation was as low as −34 dBm.

5. Recommendation This project can be further conducted to analyze SI-parameter variations which use third maximum and third minimum value (for certain period of time)

After getting some pictures regarding tropical environment, including rain and scintillation effect, the effect of temperature and humidity on FSO performance may also be further studied. Acknowledgment Special thank to Infocomm Development Authority (IDA) of Singapore for providing the data for this project. Moreover, thanks to Assoc. Prof. J T Ong (School of EEE) and Dr. K. I. Timothy, Research Fellow of School of EEE, who has spent times for consultations.

References [1, 2]. Kim, Isaac, et. al. Wireless optical transmission of fast Ethernet, FDDI, ATM, and ESCON protocol data using the TerraLink laser communication system. SPIE. 1998 [3, 4]. Sidarta, Ananda Ekaputera. Collection of CanoBeam Signal Plots. 2002. (A collection of MATLAB® plots of CanoBeam InputSignalLevel, including diurnal variation, peak-to-peak, and S4-parameter).

Figure 5 (cont). Peak-to-peak and S4-value (for 1 minute processing) on 24 April 2002 (rain).