FSO RF_OUT RXR

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Optics & Laser Technology 42 (2010) 682–692

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Optics & Laser Technology

0030-39

doi:10.1

n Corr

E-m

ghassem

acb@uo

journal homepage: www.elsevier.com/locate/optlastec

Scintillation effect on intensity modulated laser communicationsystems—a laboratory demonstration

W.O. Popoola a,n, Z. Ghassemlooy a, C.G. Lee b, A.C. Boucouvalas c

a Optical Communications Research Group, NCRLabs, School of CEIS, Northumbria University, Newcastle upon Tyne, UKb Department of Electronic Engineering, Chosun University, South Koreac Telecommunication Science and Technology Department, University of Peloponnese, Tripoli, Greece

a r t i c l e i n f o

Article history:

Received 16 June 2009

Received in revised form

28 October 2009

Accepted 14 November 2009Available online 22 December 2009

Keywords:

Laser communications

Atmospheric turbulence

Free-space optics

92/$ - see front matter & 2009 Elsevier Ltd. A

016/j.optlastec.2009.11.011

esponding author.

ail addresses: [email protected]

[email protected] (Z. Ghassemlooy),

p.gr (A.C. Boucouvalas).

a b s t r a c t

This paper shows the impact of atmospheric turbulence-induced fading on the symbol decision position

in the on-off keying (OOK) and the binary phase shift keying (BPSK) subcarrier intensity modulated

(SIM) laser communication link. Weak turbulence is simulated in the laboratory using a chamber

equipped with heating elements and fans. We have shown that in atmospheric turbulence, it is

advantageous to employ modulation schemes such as pulse time and subcarrier intensity modulations

that do not directly impress data on the optical irradiance as is the case with the OOK. For the

OOK-modulated laser communication system, atmospheric turbulence imposes complexity on the

symbol decision subsystem and by extension places a limit on the achievable bit error rate (BER)

performance.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Terrestrial laser communication technology commonly taggedfree-space optics (FSO) continues to attract attention because ithas the full potential to adequately complement the widely usedradio frequency (RF) technologies within the access network [1,2].There has been a steady rise in the number of vendors,telecommunication service providers, businesses and institutionsthat now deploy FSO technology within their networks [2]. Thisincreased acceptability is the direct consequence of severalsuccessful field trials in different parts of the globe. There are anumber of challenges facing FSO links including line-of-sightrequirement, continual transmitter/receiver alignment and signalattenuation due to absorption, scattering and shimmer of theoptical signals [3,4]. Absorption of optical signals is because of thepresence of water particles and carbon dioxide within theatmosphere, whereas scattering is due to fog and haze, as wellas rain and snow. The nature of scattering depends on the opticalwavelength and the size of scattering particles. Dense fog remainsthe most deleterious weather effect, resulting in over 100 dB/kmattenuation coefficient [2,3]. It consequently limits the achievablelink range (distance) to about 500 metres [2]. For FSO links

ll rights reserved.

c.uk (W.O. Popoola), fary.

[email protected] (C.G. Lee),

installed over a longer path length, the traffic will have to berouted via alternative/back-up links (such as the microwave ormillimetre wave) at a reduced data rate in the event of thick fog.The attenuation due to rain and snow is much lower because theraindrop size (200–2000 mm) is significantly larger than thetypical FSO beam size. A typical rainfall of 2.5 cm/h couldresult in an attenuation of 6 dB/km while the attenuation due tosnow can be as low as 3 dB/km and up to 30 dB/km duringblizzard [2].

Another channel effect of note is the shimmer of thepropagating light wave. This effect is prominent in clear atmo-sphere when the attenuation is low and most deleterious in long-range links. Light shimmer is due to a combination of factorsincluding light refraction, wind, cloud cover and atmosphericturbulence. Atmospheric turbulence results in the randomfluctuation of the intensity of the optical radiation that istraversing the atmospheric channel due to heated air whichcreates temperature variations among different air pockets [4].This is otherwise referred to as scintillation and it is similar ineffect to the fading experienced in RF systems except that it iscaused by the atmospheric turbulence as against multipathpropagation/channel frequency selectivity as is the case in RF. Itis typical for turbulence-induced deep fades to last for up to 100milliseconds. If unmitigated this could lead to link outage, highBER and a huge power penalty.

The coherence of the optical (laser) beam is affected by theatmospheric turbulence and thus changes the profile of its power

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W.O. Popoola et al. / Optics & Laser Technology 42 (2010) 682–692 683

distribution function. The theoretical study of the impact ofturbulence on the availability and performance of an FSO link hasseen a tremendous attention in literature [4–8]. Moreover, variousmethods to mitigate its effects have also been proposed [7]. In hispaper, we demonstrate and compare via a laboratory-basedexperiment, the scintillation effect on the threshold level of anFSO link that uses the OOK signalling format and the BPSK-SIMscheme. The paper is arranged as follows: the theory of atmo-spheric turbulence and the experimental set-up are discussed inSections 2 and 3, respectively. The effect of scintillation on theOOK and BPSK-SIM-based FSO links is highlighted in Sections 4and 5, respectively while the concluding remarks are given inSections 4.

2. Theory of atmospheric turbulence

Atmospheric turbulence results from random fluctuation ofthe atmospheric refractive index n along the path of a wavetraversing the atmosphere. This refractive index fluctuation is thedirect product of random variations in atmospheric temperaturealong the wave’s path. The randomly changing atmospherictemperature is a function of the altitude h and the wind speed

Fig. 1. (a) Block diagram of the experimental set-

v. Scintillation—fading of the received signal—degrades theperformance of long-range (41 km) atmospheric optical com-munication systems. The relationship between the temperature ofthe atmosphere and its refractive index variation is given by [9]:

n¼ 1þ77:6ð1þ7:52� 10�3l�2Þ

P

Te� 10�6

ð1Þ

where P is the atmospheric pressure in millibars, and Te is thetemperature in Kelvin. For all engineering applications, the rate ofchange of the index of refraction with respect to temperature isgiven as:

�dn=dTe ¼ 7:8� 10�5P=T2e ð2Þ

This expression points out the dependence of the index ofrefraction fluctuation on the changes in the channel temperature.Near sea level, �dn/dTeffi10�6[9].

The turbulence atmosphere can be described as containingloosely packed eddies/prisms of varying sizes and refractiveindices. The smallest eddy size lo is called the turbulence innerscale, with a value of a few millimetres, while the outer scale ofturbulence Lo has its value running to several meters. According tothe Taylor’s ‘frozen-in’ model, the temporal variation in statisticalproperties of the turbulent atmosphere is caused by the airmass

up, and (b) Turbulence simulation chamber.

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W.O. Popoola et al. / Optics & Laser Technology 42 (2010) 682–692684

movement. Also, the turbulent eddies are fixed and only vary withthe wind moving perpendicularly to the direction of thetraversing wave. The temporal coherence time to of atmosphericturbulence is known to be in the order of millisecond. This value isvery large compared to the duration of a data symbol. The channelis therefore static for more than one symbol duration and theatmospheric turbulence thus result in slow signal fading.

The strength of the irradiance fluctuation in a turbulentmedium is given by the variance of the log intensity, l (alsocalled the Roytov parameter sl

2) and the transverse coherencelength of turbulence is denoted by ro. Over the range lor

ffiffiffiffiffilLp

rLo

these parameters are defined as [10]:

s2l ¼ 2:25ð2p=lÞ7=6

Z 10

C2n ðxÞðx=LÞ5=6

ðL�xÞ5=6dx ð3Þ

ro �ffiffiffiffiffiffilLp

ð4Þ

where Cn2 is the refractive index structure constant (which

characterizes the strength of refractive index variation in themedium). A commonly used model for Cn

2 is the Hufnagel–Valley(H–V) model described by the following [5]:

C2n ðhÞ ¼ 0:00594ðv=27Þ2ð10�5hÞ10expð�h=1000Þþ2:7� 10�16expð�h=1500Þ

þ Aexpð�h=100Þ ð5Þ

Table 1Main parameters used in the experiment.

Parameters Values

Laser type Modulatable Beta-Tx

Laser diode wavelength 850 nm

Laser maximum output power 3 mW

Laser beam size at aperture 5 mm�2 mm

Laser beam divergence o5 mrad

PIN detector (SFH203PFA) switching time 0.5 ms

PIN detector responsivity @ 850 nm 0.59 A/W

Modulation type OOK and BPSK-SIM

Optical band-pass filter 750–1100 nm

Turbulence simulation chamber 140�30�30 cm3

Carrier signal (sinusoidal) frequency 1 MHz

Carrier signal (sinusoidal) amplitude 150 mVp–p

Modulating signal (square wave) amplitude 50–200 mVp–p

Modulating signal (square wave) pulse duration 25 ms

Ambient temperature 25 1C

-0.05 -0.04 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.040

2

4

6

8

10

12

14

16

Signal level (V)

Bin

Siz

e

Histogram of mean signal - no scintillation

Gaussian fit

Gaussian fitMean = -0.0012Variance = 5e-5

Fig. 2. Received signal distribution without scintillation.

where A is taken as the nominal value of Cn2(0) at the ground in

m�2/3. Generally, the structure parameter is assumed constant fora horizontal link and ranges from 10�15 m�2/3 for weak to10�12 m�2/3 for strong turbulence regimes.

Based on the single scattering process that characterises weakturbulence and by assuming that the log intensity l of beamtraversing the turbulent atmosphere is normally distributed, thatis l�Nð�s2

l =2;s2l Þ, then the probability density function (pdf) of

the laser beam intensity becomes:

pðIÞ ¼1ffiffiffiffiffiffiffiffiffiffiffiffi

2ps2l

q 1

Iexp �

lnln I=Io

� �þs2

l =2� �2

2s2l

( )ð6Þ

where Io is the mean received intensity without turbulence. Thenormalised variance of the intensity s2

N is derived as follows:

s2N ¼ E½I2��ðE½I�Þ2=ðE½I�Þ2 ¼ expðs2

l Þ�1 ð7Þ

The turbulence model described by (6) is the lognormalturbulence, it is only valid for weak turbulence regime withs2

l o1:2: For s2N Z1:2, saturation sets in and the model no longer

holds. Turbulence-induced irradiance fluctuation can enter sa-turation due to one or a combination of increased Cn, link lengthand reduced wavelength. Also, when multiple scatterings areexperienced especially in longer link ranges, the incident wavebecomes increasingly incoherent and log-normal model becomesinvalid. Another model which has a wider range of validity butlacks the mathematical simplicity of lognormal is the gamma–gamma turbulence model [11]. Moreover, in the limit of strongirradiance fluctuations (i.e. in saturation regime and beyond)where the link length spans several kilometres, the number ofindependent scatterings becomes large [9]. In this regime,irradiance fluctuation is believed to follow the negative exponen-tial distribution [9].

3. Laboratory simulation of turbulence

An FSO link typically consists of a transmitter and receiverseparated by the atmospheric channel. Here we setup anexperimental system, shown in Fig. 1, with the aim of observingthe effect of atmospheric turbulence on the laser beampropagating through the channel. The optical source used in theexperiment is a 850 nm ‘modulatable Beta-Tx’ laser source with amaximum output power of 3 mW. The near-infrared wavelength

0.85 0.9 0.95 1 1.05 1.1 1.150

0.5

1

1.5

2

2.5

Signal level (V)

Bin

Siz

e

With scintillation

Lognormal fitMean =1Variance = 9e-3

Lognormal fit

Fig. 3. Received signal distribution with scintillation.

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is chosen because cheap matching photodetectors are readilyavailable at this wavelength. The laser source has its biased pointfactory-set midway along the linear section of its outputcharacteristic curve. In order for this laser source to operatewithin its linear region the peak-to-peak amplitude of the inputelectrical signal must not exceed 500 mV. With this in mind, theOOK signalling is generated by directly modulating the laseroutput with a bipolar square wave signal.

The BPSK modulated subcarrier on the other hand is generatedby using a MC 1496 multiplier integrated circuit. The subcarrieroscillator is a sinusoidal signal of frequency 1 MHz while themodulating signal is a bipolar binary data sequence. In thisexperiment, the aim is to investigate the scintillation effect on thesymbol detection threshold level, as such, less emphasis will beplaced on the data rate. For fair comparison however, bothmodulation techniques under consideration will be operated atthe same data rate. The output impedance of the BPSK modulatoris over 100 kO at the 1 MHz centre frequency. This is far higher

Fig. 4. OOK-modulated laser received signal waveform and its distribution for

than the 50 O input impedance of the laser source. To ensureoptimum power transfer therefore, an impedance matchingnetwork is inserted between the subcarrier BPSK modulator andthe laser source.

The turbulence simulation, whose schematic diagram is shownin Fig. 1(b) has the following dimensions: 140 cm�30 cm�30cm. The scintillation effect due to atmospheric turbulence issimulated by exploiting the dependence of the channel index ofrefraction on the temperature variations. The turbulence simula-tion process involves blowing cold and hot air into the chamber atdifferent locations as illustrated in Fig. 1(b). The direction of theair is such that it is transverse to the optical beam’s direction oftravel. The cold air is set at room temperature of �25 1C and hotair covers a temperature range of 25–95 1C. Using a series of airvents, additional temperature control is achieved thus ensuring atemperature gradient between the source and the detector. Fourtemperature sensors are used to measure the instantaneoustemperatures at different positions along the length of the

50 mVp–p input signal: (a) without turbulence, and (b) with turbulence.

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chamber. The whole experiment is conducted in a dark room toreduce the effect of ambient light to a barest minimum. A similarapproach for simulating turbulence has been reportedly used in[12,13].

The optical receiver is composed of a telescope which focusesthe incoming optical radiation onto a SFH203PFA PIN photodiode.The photodiode has an in-built optical band-pass filter with aspectral bandwidth of 750–1100 nm to limit the backgroundradiation. The trans-impedance amplifier (TIA) that follows usesthe LT6202 operational amplifier. The output of the TIA is about2.93 V at the bias point of the laser. In the case of OOK-modulatedinput signal, the output of the TIA is directly fed into anoscilloscope and then to the computer for processing using the‘LabView SignalExpress’ data logging software. For the BPSK-SIMcase, a coherent demodulator which is based on the same MC1496multiplier is used and it shares the same oscillator with the BPSKmodulator. The TIA output is here used as the modulating input


signal to the multiplier. In order to remove the out-of-bandsignals, the output of the coherent demodulator is fed into a 10th-order low-pass filter. The output of the filter is then captured,logged and processed by using the oscilloscope with the datalogging software.

Since the emphasis in this experiment is on the threshold levelrather than the data rate, the constraint imposed on theachievable data rate by the filter will hence have no impact onthe result of the experiment. The parameters used in theexperiment are summarised in Table 1 and the picture ofthe experimental set-up is given in Appendix I. To evaluate thestrength of the turbulence generated via this set-up, the intensityof laser is directly modulated by a 1 MHz 200 mV peak-to-peak(p–p) sinusoidal signal and the mean values of the received signalwithout the locally generated turbulence logged over a period oftime. The process is then repeated with the turbulence simulatorin operation and the mean values acquired as well. While carrying

100 mV p–p input signal: (a) without turbulence, and (b) with turbulence.

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out the experiment without turbulence, the temperature acrossthe turbulence simulation chamber is kept the same as theambient temperature. With turbulence, the temperature at fourdifferent positions, T1–T4 in Fig. 1(b), are constantly changing.

By removing the 2.93 V DC bias voltage from the receivedsignal, we plot in Fig. 2 the distribution of the mean valueswithout turbulence. In Fig. 2 (histogram of the mean values), thebin size, when expressed as a fraction the total number ofsamples, is a representation of the pdf of the received meanvalues. The variation observed in the received signal under noscintillation is down to the innate noise (shot and thermal)associated with photodetection process. By fitting theexperimental data to a Gaussian curve, the detection noisevariance is found to be approximately 5�10�5 (V2). Fig. 3shows the mean value distribution with the simulated turbulence.In order to estimate the strength of the simulated scintillationeffect, the log-normal distribution described by (6) is fitted onto


the histogram as shown in Fig. 3. From this fit, the log-intensityvariance sl

2 is obtained to be 9�10�3 (V2). When the log-intensity variance is in the range 0osl

2r1, the turbulence isgenerally classified to be in the weak regime, the simulatedturbulence is thus very weak.

4. Scintillation effect on the OOK–FSO system

To demonstrate the effect of the weak scintillation on an OOK-modulated FSO link, a stream of low (bit ‘0’) and high (bit ‘1’)bipolar signal is used to directly modulate the laser whose outputis then transmitted through the turbulence simulation chamber.In order to recover a faithful replica of the transmitted pulse andavoid pulse shape distortion due to the rise and fall times of thedevices, the transmitted electrical square pulses are each ofduration 25 ms. This data rate restriction is down to the limited

200 mV p–p input signal: (a) without turbulence, and (b) with turbulence.

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bandwidth of the components used in the experiment. Todemonstrate the impact of turbulence on the threshold and inputsignal levels, three modulating signal amplitudes 50, 100 and200 mVp-p were used and the corresponding received signalamplitude in the absence of turbulence are �150, �300, �600mVp-p, respectively at the output of the trans-impedanceamplifier. The received signal waveforms and distributions(histograms) with and without scintillation are shown inFigs. 4–6.

From Fig. 4(a), it is observed that without turbulence, bits ‘1’and ‘0’ are clearly distinguishable from the signal distribution. Thesignal profiles for both bits are similar and are equally spaced onboth sides of the zero mark. The decision threshold point cantherefore be set at the zero mark. But in the presence ofturbulence, the signal waveform becomes heavily distorted andthe signal distributions are no longer distinguishable. The signalprofiles for bits ‘1’ and ‘0’ have now merged to become one asillustrated Fig. 4(b), making it very difficult to locate the thresholdlevel from the signal distributions.

Fig. 7. Received signal waveform for BPSK–SIM-modulated laser and its distribution fo

Increasing the input signal amplitude by a factor of two doesnot make any difference in terms of the threshold level position inthe absence of turbulence, the zero mark as seen in Fig. 5(a)remains the obvious threshold position. During turbulence, thesignal distributions for bits ‘1’ and ‘0’ are though distinguishableas shown in Fig. 5(b), there exists and overlap in the distribution.From the distributions, the threshold position that will result inminimum detection error cannot be said to be fixed at the zero, itlies between �50 mV and +50 mV and the overlap in thedistributions signifies increased error detection possibilitiescompared with no turbulence case.

With the input signal level increased four folds to 200 mVp–p,the Gaussian-like signal distributions without turbulence shownin Fig. 6(a), are still very much similar to the ones shown in Figs. 4and 5 with the threshold level best positioned at the zero mark. Inthe presence of turbulence, the signal distributions for bits ‘1’ and‘0’ become more separated, broadened with some overlap.Compared to the case with no turbulence, the overlap indicateshigher likelihood of detection error.

r 50 mV p–p modulating signal: (a) without turbulence, and (b) with turbulence.

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From the foregoing, it can be said that in the absence ofturbulence, the threshold level can be fixed at the zero signal levelbut this is no longer the case as soon as turbulence sets in. Duringturbulence, finding the exact decision threshold level that willminimize detection error will require the knowledge of theturbulence strength. What this means in essence is that, anadaptive threshold will be required for the OOK-modulated FSOsystem to perform optimally. The practical implementation ofsuch an FSO system with the adaptive threshold is not trivial, asthe design will involve a continual tracking of both thescintillation and noise levels. This conclusion has previously beentheoretically arrived at in [7,8]. Also from the results, increasingthe input signal/transmitted power level can be used to combatthe effect of very weak turbulence on the position of the symboldetection threshold level. Increasing the transmitted power willwork fine for short-range FSO links, for long-range systems(41 km), this approach will have a limited impact. Moreover, the

Fig. 8. BPSK–SIM-modulated laser received signal waveform and its distribution for 1

fact that the transmitted power is premium and the existence of alimit on the amount of power can be safely transmitted throughthe atmosphere further limits the use of increased power as a wayof mitigating atmospheric turbulence effect.

5. Scintillation effect on the BPSK-SIM-based FSO system

In this section, the same experiment described above isperformed for the BPSK-SIM-based FSO link. The modulatingsignal amplitude levels at the transmitter are the same as thatused under OOK modulation and the carrier signal amplitude isadjusted such that the electrical signal amplitude at the output ofthe low pass filter that follows the BPSK demodulator isapproximately the same as that for the OOK case. The waveformsand distributions of the received signals are then shown inFigs. 7–9 for the three different input signal levels.

00 mV p–p modulating signal: (a) without turbulence, and (b) with turbulence.

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With or without turbulence, it is observed from thefigures that signal distribution for bits ‘1’ and ‘0’ are similarand are equally spaced on either sides of the zero mark. Thisis in contrast to when OOK signalling is used. The thresholdposition can therefore be fixed at the zero mark and it isnot affected by scintillation. This is a clear advantage ofBPSK-SIM over the simple OOK signalling. The results of thisexperiment can also be explained in the spectral domainby noting that the scintillation noise power is concentrated invery low frequencies below 100 Hz [14,15]. This makes theOOK, which has a significant portion of its power distribu-tion within the low frequency, susceptible to scintillation asdemonstrated by the experiment. The SIM-BPSK-based FSO on theother hand does not suffer this same fate because its signalspectral distribution is shifted away from the low frequencyregion to the frequency of the subcarrier, which is 1 MHz in thisexperiment.

Fig. 9. Received signal waveform for BPSK–SIM-modulated laser and its distribution fo

To illustrate the effect of scintillation on the system BER, theanalysis presented in [7] will be followed. Based on similarelectrical SNR, the BER of BPSK-SIM and OOK-modulated FSOsystems are given by (8) and (9), respectively.

The error probability for BPSK-SIM-modulated FSO system isexpressed as:

Peffi1

p

Z p=2

0

1ffiffiffiffipp

Xm

i ¼ 1

wiexp �K2expð2ð

ffiffiffi2p

slxi�s2l =2ÞÞ

2sin2ðyÞ

!dy

ffi1ffiffiffiffipp

Xmi ¼ 1

wiQ ðKeðxi

ffiffi2p

sl�sl2=2ÞÞ; ð8Þ

where K ¼RE½I�A=ffiffiffi2p

s. In the expression for K, R is theresponsivity of the PIN photodetector, A is subcarrier signal peakamplitude, s2 is the noise variance andQ ðxÞ ¼ 0:5erfcðx=

ffiffiffi2pÞ. It

should be noted that xi and wi terms in (8), which are independent

r 200 mV p–p modulating signal: (a) without turbulence, and (b) with turbulence.

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10 15 20 25 30 35

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

Electrical SNR (dB)

BE

R

OOK withadaptive ith

OOK, ith = 0.5

OOK, ith = 0.2

BPSK-SIMσl = 0.22

Fig. 10. BER of OOK and BPSK–SIM-based FSO in atmospheric turbulence, sl2=0.2.


of y, are the zeros and the corresponding weights of an mth orderHermite polynomial respectively.

The error probability for the OOK-modulated FSO system isexpressed as:

Pe ¼ Pð0Þ

Z1ith

Pðir=0ÞdirþPð1Þ

Z ith

0Pðir=1Þdir ð9Þ

The marginal probabilities contained in (9) are defined by:

Pðir=1Þ ¼R1

0 Pðir=1; IÞPIðIÞdI

¼

Z 10

1ffiffiffiffiffiffiffiffiffiffiffiffi2ps2p exp �

ðir�RIÞ2

2s2

( )1ffiffiffiffiffiffiffiffiffiffiffiffi

2ps2l

q 1

I

�exp� lnI=E½I�þs2

l =2� �2

2s2l

( )dI ð10Þ

Pðir=0Þ ¼1ffiffiffiffiffiffiffiffiffiffiffiffi

2ps2p expð�i2r =2s2Þ ð11Þ

The ith in (9) is the value of ir when P(ir/1)=P(ir/0). To show theimpact of turbulence on the error performance of laser commu-nications, we present in Fig. 10, the BER of OOK-based systemwith both adaptive and fixed threshold levels against thenormalised SNR=(RE[I])2/s2. In the same figure, the BER for theBPSK-SIM-based system is also shown for comparison. The figureclearly shows that using the fixed threshold level is not just sub-optimal, it also results in a BER floor. The lowest attainable BERperformance depends on the noise, scintillation and the thresholdlevel adopted. Also based on the similar electrical SNR, using OOKwith an adaptive threshold requires �6 dB SNR more than usingthe BPSK-SIM at a BER of 10�6. It should be mentioned thatimproved BER performance of subcarrier modulation is at theexpense of poor power efficiency since it suffers from a high peakto average power ratio. Depending on the application and thechoice of modulation type, there is always a trade off between theBER performance and the power efficiency.

It can therefore be inferred from the experimental andanalytical data that in turbulent atmospheric channels, an FSOsystem with the data directly impressed on the intensity of thelaser irradiance, as is the case in OOK, will suffer from symbol

decision difficulties which will eventually lead to detection errors.An alternative approach will be to consider modulation schemesthat will require no prior knowledge of turbulence in decidingwhat symbol has been received. These schemes include : i) PSKpre-modulated SIM, in which the information is not directly onthe laser intensity but on the phase of the subcarrier signal, ii)polarisation shift keying, in which the information is placed onthe polarisation state of the optical beam, iii) or pulse timemodulation techniques, such as pulse position modulation, pulsewidth modulation and pulse frequency modulation; in which theinformation is also not directly on the beam intensity but on thepulse position, pulse width and the pulse frequency, respectively.

6. Conclusion

In this paper we have demonstrated the effect of scintillationon the decision making threshold on an FSO system base on thefollowing modulation schemes: the OOK and BPSK-SIM. It has alsobeen shown that the use OOK with a fixed threshold level doeslead to sub-optimal performance and places a limit on theachievable BER performance. Subcarrier intensity modulation orpulse modulation techniques are therefore proposed for FSOsystems in atmospheric turbulence channels. Since these mod-ulation techniques, unlike the OOK, do not impress the data/information directly on the intensity of the optical radiation.Because of the short length of the atmospheric simulationchamber, we have managed to simulate very weak turbulence.Work is currently in progress to increase the channel length to beable to investigate moderate turbulence.

Appendix I

Photo of the experimental set-up.

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