Abstract: In recent years, free space optical (FSO) communication has gained a considerable interest amongst the researchers due to certain benefits it has to offer over RF links such as availability of large unlicensed bandwidth, high data rates, negligible electromagnetic interference, ease of installation etc. However the conventionally used FSO systems use On-Off Keying (OOK) as the modulation technique. Such a link requires high levels of transmit power. In this paper, we have suggested the use of QPSK modulation technique which allows us to reap the benefits of coherent detection. Our calculations show that QPSK is more inert to the changes in atmospheric conditions like temperature, air pressure, humidity, aerosol concentration etc.

Keywords- FSO, QPSK, Air Pressure, Temperature.

I. INTRODUCTION

In last few decades, the data requirements for various applications like Defense, Research, Education, Monetary services, Entertainment etc has grown exponentially. A major problematic situation of rapidly depleting usable licensed bandwidth has risen. Researchers are striving hard to develop techniques to efficiently an optimally utilize the available bandwidth. Satellites have moved on to Ka- Band in RF domain for higher bandwidth. However the data requirements keep on rising with advent of new technologies like digital direct to home television, high speed internet requirements, online video conferencing, online movie streaming at the theatres etc. Thus, we are in need of communication links which provide sufficient data rates. One of the techniques to resolve this problem is Free Space Optical Communication link. Since, we are moving on to optical domain, the

frequencies of the signal lie in THz range which gives high bandwidth (upto 5-10 GHz).With this high Bandwidth available, data rates in range of Gbps can be achieved. With the growing interest in this technology, many systems are available today implementing OOK modulation technique. In OOK, a Laser beam is modulated by a data bit stream. If the data bit is 1, high power pulse is sent and if the data bit is 0, no pulses are sent. OOK has been implemented mainly because of its extremely less complexity. However in this paper, we have thrown light on certain defects in using OOK modulation technique.

To design a wireless optical link for conveying data between a Geo-synchronous satellite and a ground station, we have chosen following design parameters:

1) Probability of Error (BER) = 102) Link margin = 6 dB3) Data rate = 3 Gbps4) Link Distance= 40000km

The design of this laser communication system is based on the data provided in [1] which provides a comprehensive treatment of laser communication systems operating in free space. Some of this data is empirical/ and or statistical and cannot be generated theoretically.

II. SIMULATION PARAMETERSThe following transmitter/receiver parameters have been assumed for the simulation.

Distance= 40000 km Wavelength=1550nm Receiver sensitivity for OOK modulation= 100 photons/bit Transmitter optical loss(˜ T )= -10dB

Jokhakar Jignesh D. U. Sripati Muralidhar Kulkarni

. M. Tech (Research), E&C dept., NITK. Prof., E&C dept., NITK Prof., E&C dept., NITK.

Mangalore, India Mangalore, India Mangalore, India

jokhakarjignesh@gmail.com sripati_acharya@yahoo.co.in mkuldce@gmail.com

Performance of QPSK Modulation for FSO Geo-Synchronous Satellite Communication Link

under Atmospheric Turbulence.

978-1-4673-5149-2/13/$31.00 ©2013 IEEE

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Receiver optical loss(˜ R) = -10dB Transmitter Divergence Angle ( ) = 2.24 ×10 rad Transmitter Antenna Gain ( )= = +105dBReceiver diameter (D) = 1m

III. IMPORTANT DEFINITIONS

Link margin= P P … (1)

i.e. Link margin (dB) = P(received)(dB)-P(required)(dB) P required = R … (2)

= πDλ

= +129dB … (3)

(dB) = - [ (dB) + ˜ T (dB) + ˜ ATM (dB) + S (dB) + (dB) + ˜ R (dB)] … (4)

Space loss(S) = λπL … (5)

where,

= Required Transmitted power, = Received Power, N = Receiver sensitivity (photons/bit), R= required data rate (bps), h= Planck’s constant= 6.626×10 , c=speed of light wave=3×10 m/s, =wavelength of light beam employed.

IV. ADVANTAGES OF COHERENT DETECTION

The coherent detection technique yields a gain of 10-20dB in receiver sensitivity [2] (depending on input capacitance of the detector, ambient temperature and frequency of signal) as compared to non coherent detection. Another factor affecting the receiver sensitivity is the quantum efficiency. These factors result in the variation of the receiver sensitivity gain in the range of 10dB – 20dB.The noise degradation can be reduced by using Laser diodes emitting signals with narrow spectral line widths. For this purpose, DFB-LD (Distributed Feedback laser diodes) or DBR-LDs (Distributed Bragg Reflector laser diodes) are used. The noise density affects the signal to noise ratio of the detector, which eventually affects the gain. For high receiver sensitivities, low spectral line-width is required. Thus, the use of DFB-LDs and DBR-LDs is recommended. The Coherent QPSK Receiver is shown in Fig. 1. The detailed explanation of the QPSK receiver is given inreference [3]. The Balanced Optical Detector configuration provides additional 6dB gain to the signal. Refer [3]. The difference in polarization of the received signal and the local oscillator signal can give rise to imperfect demodulation. Hence, the polarization of both signals needs to be maintained

similar. Faraday polarization mirrors are used to meet this requirement [10].

Fig.1 Coherent Receiver system

FOR optical QPSK, Bit Error Rate (BER) is given by [4] :

BER=erfc( n N ) … (6)

By substituting BER= 10-9 in equation (6), we get required =19 photons/bit. The Receiver sensitivity for OOK systems

is 100 photons/bit[1]. From these statements, we conclude that QPSK has a huge sensitivity gain over OOK. From Equation (2), we can infer that QPSK requires less power to be received at the receiver detector to maintain the BER of 10-9 and data rate of 3 Gbps.

V. ATMOSPHERIC PARAMETERS EFFECTS

As the signal travels through the turbulent atmosphere, it undergoes degradation due to atmospheric absorption, scattering and scintillations. These effects cause random fluctuations in the signal strength. This effect is termed as signal fading. The primary cause of scintillation is the random change in refractive index of the medium due to changes in atmospheric temperature. Absorption and Scattering are

Balanced Detector Configuration

Optical Hybrid

Local Oscillator

L= ·Polarization Control Faraday Mirrors

Polarization Control Faraday Mirrors

Received Signal

S= · Ф S+L*

S*-L

S+jL*

S*-jL

A

B

A= 4 · · · cos Ф B= 4 · · · sin Ф

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responsible for reduction in power level, whereas Scintillation is responsible for the probabilistic change in the channel attenuation. The received signal can be expressed as following:

Y=h.X+n ... (7)

Where, ‘Y’ is the received signal power, ‘X’ is the transmitted signal power, ‘h’ is the attenuation and ‘n’ is the noise power added during the transmission. The absorption, scattering and scintillation effects characterize the factor ‘h’. `h’ can be characterized as a probabilistic parameter whose mean amplitude depends on the attenuation and scattering effect and the variance depends on the scintillation effect. ‘h’ can be defined as gamma-gamma distributed.[5]

Scattering effect in consideration here is the Rayleigh scattering due to various scattering components present in the atmosphere of the earth such as gases, aerosols, dust, fog, smoke etc. Due to these anomalies present, the light beam is diverted from its track by an angle called as the forward scattering angle. As far as the long haul links are considered, multiple forward scattering takes place. The collective scattering effect gives us a collective forward scattering angle. Statistical data shows that this total forward scattering angle for 1550nm light beam lies between 5 to 11.5 degrees (0.0872rad to 0.2rad). Upto this range of forward scattering angles, the scattered beam can be exploited along with the unscattered beam. The reason for this is multiple forward scattering. If the scattering angle goes beyond this range, the beam suffers extremely high losses in power level.

The required transmitted power can now be calculated considering the Scattering effect by equation (8). The 6dB link margin is compensated by the required 6dB gain achieved from balanced optical receiver configuration [3]. The G-APD gain is included in the calculation.

P =P Q GAPD G β ′ L ...(8)

Where, = Receiver aperture radius=0.75m GAPD = APD gain = 30dB G = Power gain given to the beam before Transmission.

θ = √ ; where ρ = L F θF= total forward scattering angle β ′=effective attenuation coefficient= β +β β =attenuation coefficient

β can be considered as the Space Loss attenuation

given by λπL .

=scattering coefficient

Here, is the power required to maintain the BER of 10-9. P Q= N RWhere, R is the data rate i.e. 3 Gbps and is 19 for QPSK from equation (6). ‘h’ is the Planck’s constant (6.626×10-34). The value of can be considered as optical Rayleigh scattering coefficient and can be calculated using equation (9). [6] β =24× N× × ...(9)

Where, N= no. of density of aerosol molecules ( ; n= refractive index of the air, δ= depolarization factor.

The refractive index(n) is in reality affected by the temperature (T), air pressure in Torr. (p) and water vapour pressure in Torr.(f). This dependence is given by [6] n 1 n 1 . . f 5.722 0.0457 10

…(10)

Where,

˜ = wave no. ( n = Standard refractive index of air with 330ppm CO .

It can be calculated from equation (11).

(n -1) 10 = . . … (11)

For practical calculations, we use instead of n in equation (9).The value of water vapour pressure can be calculated using equation (12) [7].

f =10A BC T ... (12)

Where, A= 8.07131, B= 1730.63, C= 233.426

Reference [8] gives the value of depolarization factor δ.

Hence, the received power will vary with the variations in Temperature, Air Pressure and Humidity in atmosphere, thus,

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giving rise to scintillation effects. The effects of fog and rainfall attenuation [9] were considered and calculations were performed.

VI. SIMULATIONS

Fig. 3 shows the graph of required transmitted power ( ) vs. atmospheric air pressure plotted using equations (8-12) considering required receiver power for OOK and QPSK modulation. The Transmitter and Receiver Losses (10 dB) [1] were considered in the calculations. It can be observed that required transmission power for OOK is much larger than QPSK modulation. It can also be observed from Fig. 3 that as the temperature (T) increases the required transmission power to maintain the BER also increases. A major point to be observed from Fig. 3 is that when temperature of atmosphere increases from 5 to 25 , the required transmission power at any value of air pressure increase to a greater extent in OOK modulation as compared to QPSK modulation.

Consider the air pressure level of 1000 mbar. When the temperature of atmosphere increases from 5 to 25 , the required transmission power increases by 750mW (from 4W to 4.75W) for OOK whereas it increases only by 100mW (from 600mW to 700mW) for QPSK.

Fig. 4 shows the Graph of required transmission Power against Temperature of the atmosphere. It can be observed that Required Transmission power increases with increase in Temperature as well as air pressure. Consider Temperature (T) of 25 . As the air pressure rises from 800 mbar to 1050 mbar, the required transmission power increases by 2.75W for OOK, whereas, it rises by 350mW for QPSK.

The Rainfall and Fog attenuation is calculated from Reference [9]. Fig. 5 shows the plot of Required Transmission Power considering rainfall and Fog attenuation. The values of atmospheric parameters like aerosol density and water vapour pressure is taken from statistical data for a location in north India (cannot be disclosed).

VII. CONCLUSIONS

The required transmission power in OOK technique is higher than QPSK technique. As the temperature or the air pressure of the atmosphere increases, the required transmission power also increases to maintain a BER of 10-9 and 3 Gbps data rate. This variation in required transmission power is much higher in OOK as compared to QPSK for certain variation in either temperature or the air pressure. Thus, the effect of scintillations is more in OOK as compared to QPSK. Thus, QPSK proves to be power efficient and robust modulation technique in comparison to OOK.

VIII. RESULTS

Fig .2 Required Transmission Power vs. Atmospheric Pressure for Temperature of 5 and 25 including 4dB

Tracking Error Loss.

Fig .3 Required Transmission Power vs. temperature for Atmospheric Pressure 800mbar and 1050mbar with Tracking

Error Losses of 4dB.

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Fig .4 Required Transmission Power vs. Atmospheric Pressure for Temperature of 5 , 25 and 32 including Fog and

Rainfall attenuation and 4dB Tracking Error Loss. IX. REFERENCES

[1] “Free-space laser communication performance in the atmospheric Channel”: Arun K. Majumdar, J. Opt. Fiber. Commun. Rep. 2, 345–396 © 2005 Springer Science + Business Media Inc. (2005) [2] John M Senior; “Optical fiber communication”; third edition; chpt. 13, coherent and phase modulated systems. [3] Arkady Kaplan, Kobi Achiam “LiNB Integrated Optical QPSK and Coherent Detector.” ECIO conference paper (2003). [4] Tejbir Singh Hanzra, Gurpartap Singh“Performance of Free Space Optics in BPSK and QPSK communication systems.” IOSR Journal of Electronics and Communication Engineering, Vol. 1, Issue 3 (2012). [5] Murat Uysal, Jing Li, Meng yu. “Error Rate Performance Analysis of Coded Free Space Optical Links over Gamma-Gamma Atmospheric Turbulence Channels.” Wireless Communication, IEEE Transactions. (2006). [6] C. Frohlich, G. Shaw; “New Determination of Rayleigh Scattering in Terrestrial atmosphere.” Applied Optics, Vol. 19, Issue 11, pp. 1773-1775 (2010) [7] Wikipedia page. Search- Vapour Pressure of Water.

[8] Kenneth Cartwright, Edit J. Keminsky. “A Simple Improvement to Viterbi and Viterbi Monomial-Based Phase Estimators.”IEEE GlobeComm’06. (2006) [9] S. Sheikh. Muhammad, P. Kohldorlfer, E. Leitgeb. ”Channel modeling of Terrestrial Free Space Optical Links.” Transparent Optical Networks, 2005, Proceedings of 2005 7th International Conference. (2005). [10] AGILTRON 1310/1480/1550 Faraday mirrors datasheet.

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