International Journal on Communications (IJC) Volume 3, 2014 www.seipub.org/ijc

1

Effects of Backscattering and Atmospheric Scintillation on OFDM Based Optical Wireless Communication 1S. R. Sabuj, 2Md. Mahfujul Hasan and 3Md. Jakir Hossain

1Department of EEE, Bangladesh University, Dhaka, Bangladesh 2Department of CSE, Metropolitan University, Sylhet, Bangladesh 3Department of CSE, Metropolitan University, Sylhet, Bangladesh 1saifuriict@gmail.com; 2mahfuz_dk@yahoo.com; 3md.jakir.sub@gmail.com Abstract

Orthogonal frequency division multiplexing (OFDM) based optical wireless communication (OWC) is a viable technology and has been widely applied in indoor as well as outdoor communication. Atmospheric scintillation and backscattering are the main impairments in OWC. Orthogonal Frequency Division Multiplexing (OFDM) has been widely used in the optical wireless communication. Nevertheless, due to both frequency offset and phase noise, the performance of OFDM can severely be affected. The combined effect of atmospheric scintillation, backscattering, frequency offset and phase noise has been investigated in OFDM based OWC. As the value of backscattering power and atmospheric scintillation increase, the BER also increases and thus degradation of performance occurs.

Keywords

Atmospheric Scintillation; Backscattering; Frequency Offset; Optical Wireless Communication; Orthogonal Frequency Division Multiplexing and Phase Noise

Introduction

The demand of portable wireless communication device is increasing when the available bandwidth is limited. Optical wireless communications (OWC) offer a feasible alternative to radio frequency (RF) commu-nication for indoor as well as outdoor applications [D. Kedar and S. Arnon 2003, R. M S R. Pir and M. M. Hasan 2011]. The main reasons for suitability of an OWC are enormous amount of unregulated bandwidth, no license requirement, low cost transceivers and no interference with sensitive electronic system [D. Tsonev, S. Sinanovic and H. Haas 2012, Z. Ghassemlooy 2003].

The OFDM system is characterized by superior flexibility due to high spectral efficiency, high toler-ance to multi-path interference, channel dispersion and frequency-selective fading [I. B. Djordjevic, B. Vasic and M. A. Neifeld 2007, E. Vanin 2011]. Subcarrier

frequency offset and phase noise cause a number of impairments including attenuation of the received OFDM symbols and inter carrier interference (ICI) which degrade the BER of the system.

The main challenge for OFDM-OWC system is the performance degradation caused by the atmospheric scintillation, scattering and backscattering. Scintillation is fluctuation in the intensity and phase of the received signal [G. Yan, W. Min and D. Weifeng 2011]. The light is backscattered from a transmitted beam to the receiver of the same transceiver. Then the overlap occurs between the propagating beam and the receiver [D. Kedar and S. Arnon 2005]. The effect of crosstalk due to backscattered light is one of the numbers of interfe-rence that degrade the signal. Debbie Kedar et al. (2005) have analyzed the crosstalk effect of aerosol backscatter on the performance of a wavelength division multiplexed (WDM) OWC system [D. Kedar and S. Arnon 2005].

In this paper, we have established an analytical model for OFDM based OWC. The performance of the system in the presence of backscattered light, atmospheric scintillation, frequency offset and phase noise has been evaluated in terms of BER.

System Model

The block diagram of an OFDM based optical wireless communication system is shown in fig. 1. Input bits are first encoded by using suitable modulation technique like - BPSK, QPSK or M-QAM. After the serial to parallel converter (S/P), the duration of input bits is increased. The transmitted OFDM signal after IFFT at the transmitter [Y.-S.Li, H.-G.Ryu, J.-W.Li, D.-Y.Sun, H.-Y. Liu, L. –J. Zhou and Y. Wu 2008] can be expressed as,

21 ( )

0( )

N j knNk

ks n d e

π−

== ∑ for 0 ≤ n ≤ N-1 (1)

www.seipub.org/ijc International Journal on Communications (IJC) Volume 3, 2014

2

FIG. 1 OFDM BASED OWC SCHEME (A) TRANSMITTER (B) RECEIVER

Where, j = 1− , N is the total number of subcarriers, kd is data symbol over the kth subcarrier. After IFFT,

the data symbols are serialized using a parallel to serial converter (P/S), then converted to analog via the digital to analog converter (DAC). An infrared emitter is used as optical wireless transmitter to generate optical signal before the transmission of the signal.

Received signal is affected by phase noise and frequency offset. So, it can be expressed as [Y.-S. Li, H.-G.Ryu, J.-W.Li, D.-Y.Sun, H.-Y. Liu, L. –J. Zhou and Y. Wu 2008],

[2 ( )]( ) [ ( ) ( ) ( )]} j ft nr n s n h n w n e π ϕ∆ += ⊗ + (2) Where, Δf and φ(n) are frequency offset and phase noise. s(n), h(n), w(n), r(n) are transmitted signal, channel impulse response, complex Gaussian noise and received signal respectively. At the receiver, a photodetector collects the optical signal and converts it to an electrical current. The received symbols are passed through low noise amplifier and converted from analog to digital using the analog to digital converter (ADC) and transferred by the S/P. The FFT of the received signal can be expressed as [Y.-S. Li, H.-G.Ryu, J.-W.Li, D.-Y.Sun, H.-Y. Liu, L. –J. Zhou and Y. Wu 2008],

21 [ ]

021 1 [( )( ) ( )]

0 01

0

1( ) ( )

1

N j knN

n

N N j l k n nN

l l kn l

N

l l l k kl

Y k r n eN

d H e NN

d H Q N

π

π ε ϕ

− −

=

− − − + +

= =

−

−=

= ∑

= +∑ ∑

= +∑

(3)

Where, Y(k), dl and Hl are the frequency domain

expression of r(n), d(n), h(n) . Nk is the complex AWGN. Here, ε is the normalized frequency offset and is given by ΔfT. Δf is the frequency difference between the transmitted and received carrier frequencies and T is the subcarrier symbol period. Using phase noise linear approximation method [ ]nϕ is so small that [ ]j ne ϕ can

be approximated into1 [ ]j nϕ+ .So, LQ can be defined as [Y.-S.Li, H.-G.Ryu, J.-W.Li, D.-Y.Sun, H.-Y. Liu, L. –J. Zhou and Y. Wu 2008],

21 [( )( ) ( )]

021 [( )( ) ]

0

1

0

1

1 (1 ( ))

exp[{ ( )}(1 1/ )]sin{ ( )] 1{1 ( )}

.sin[{ ( )} / ]

N j L n nN

Ln

N j L nN

n

N

n

Q eN

e j nN

j L NL j n

N L N N

π ε ϕ

π εϕ

π επ ε ϕπ ε

− + +

=

− +

=

−

=

= ∑

= +∑

= + −+

+ ∑+

(4)

Phase noise occurs random drift of received symbol its constellation diagram. Frequency offset and random phase noise signal of OFDM signal become corrupted at the receiving end. It involves two kinds of components. One component is its own subcarrier signal corrupted by common phase error and the other is ICI from adjacent subcarrier signals. Here, the cyclic prefix is not considered for the ease of analysis.

At the receiver, the direct detection (photo detector) process is mathematically equivalent to applying the squared modulus [E. H. Miguel 2010],

212

0

2 2

( )N

l l l k kl

signal ICI k

I y k d H Q N

I N

η

η ησ

−

−=

∝ = +∑

= + +

(5)

International Journal on Communications (IJC) Volume 3, 2014 www.seipub.org/ijc

3

Where, η is photodiode efficiency. The average received current from the signal and the backscattered interference are signalI and BSI respectively. In order to evaluate the statistical properties, average channel gain is assumed [V. K. Dwivedi and G. Singh 2008]

2 1lE H = and 2 2lE d d = (6)

The received signal is generated by the signal of kth subcarrier. Considering l=k, the received signal power can be represented by

2 2 220

2 2 20

[ ]. [ ].

. .

signal k k

k k

I E d E H Q

d H Q

=

= (7)

ICI is corrupted by adjacent subcarrier signal. Considering l k≠ , the ICI power is

1 2 2 22

0,

1 2 2 2

1

[ ]. [ ].

. .

N

ICI l l l kl l k

N

l l ll

E d E H Q

d H Q

σ−

−= ≠

−

=

= ∑

= ∑ (8)

The noise terms are 2ASE ASEσ × , 2

ASE Sσ × and 2ASE BSσ × due

to beating of the amplified spontaneous emission (ASE) from the optical amplifier with itself, the signal and the backscatter respectively. 2

sig bsσ × is the noise due to the

mixing of the signal with the backscatter. The beat noise terms caused by the mixing of two signals can be defined as [G. P. Agrawal 1997]

[ ]22ASE ASE nq GF Bσ η υ× = ∆ (9)

[ ]22 2 nASE S R

Fq G P B

hσ η

υ× = (10)

[ ]22 2 nASE BS BS

Fq G P B

hσ η

υ× = (11)

2 21 ( )2S BS R BS

q G P Phησυ× = (12)

where, q , h ,υ , B, G , nF are electron charge, Planck’s constant, optical frequency of the received power, electronic bandwidth, optical amplifier gain and noise factor. Here backscatter is denoted as ‘BS’. ID is the threshold current which is represented by ( signalI +

) / 2BSI . PR and PBS are the received power and backscatter power. Hence, the noise current variances are given by [G. P. Agrawal 1997]

( )

2 2 210

2 2ASE ASE ASE S

Rn n

Pq G BF Fh

σ σ σ

η υυ

× ×= +

= ∆ +

(13)

( )

2 2 201

2 2ASE ASE ASE BS

BSn n

Pq G BF F

h

σ σ σ

η υυ

× ×= +

= ∆ +

(14)

( )

( )

22 2 2 2 211

22

2 ( ) 1( ) .2

ASE ASE ASE S ASE BS S BS

n R BS R BSn

q G

BF P P P PB F

h h

σ σ σ σ σ η

υυ υ

× × × ×= + + + =

+ ∆ + +

(15)

[ ]22 200 ASE ASE nq GF Bσ σ η υ×= = ∆ (16)

According to the deriving BER expression, all accompanying noise is independent and Gaussian distribution. BER of OFDM based optical wireless communication with Scintillation can be expressed as [D. Kedar and S. Arnon 2005, G. P. Agrawal 1997],

{ }

{ }

{ }

{ }

0.52 2 210

0.52 2 211

0.52 2 201

0.52 2 200

0.25.2

0.25.2

0.25.2

0.25.2

signal D

ICI s

signal BS D

ICI s

D BS

ICI s

D

ICI s

I IBER erfc

I I Ierfc

I Ierfc

Ierfc

σ σ σ

σ σ σ

σ σ σ

σ σ σ

− = + + + + − + + +

− + + + + +

(17)

Results and Discussion

According to the theoretical analysis, performance results of OFDM-OWC system are presented in the following section. The performance of OFDM-OWC system in the presence of backscattering power, atmospheric scintillation, normalized frequency offset and phase noise has been evaluated in terms of BER. The values of system parameters are given in table 1.

TABLE 1 VALUES OF NUMERICAL PARAMETERS

Parameter Typical value Electronic Bandwidth, B 500 (MHz)

Amplifier Gain, G 1000 (30 dB) Noise Figure, Fn 2.238 (4 dB)

Photodiode Efficiency, η 0.8 Optic Frequency (central), υ 1.94x1014 (Hz)

Optic Bandwidth, Δ υ 125 (GHz)

The effect of backscattering optical power on BER is shown in Fig. 2. It is seen that OFDM-OWC show better performance without backscattering optical power. Again, backscattering and atmospheric scintillation have no impact when 2 6s dBmσ ≤ . But, for

2 6s dBmσ ≥ , OFDM-OWC performs better without backscattering and atmospheric scintillation.

The plots of BER versus received power with different scintillation 2

sσ are shown in fig. 3. It is obviously

www.seipub.org/ijc International Journal on Communications (IJC) Volume 3, 2014

4

noticed that the amount of degradation in BER performance due to scintillation is very significant at a given BER. For example, at a 21.4 10BER x −= , the required receiver sensitivity is almost 6 dBm corresponding to 2 310sσ

−= where as it increases to

about 6 dBm and 8.6 dBm corresponding to 2 310sσ−=

and 2 210sσ−= respectively.

FIG. 2 EFFECT OF DIFFERENT BACKSCATTERING OPTICAL

POWER ON BER

FIG. 3 EFFECT OF ATMOSPHERIC SCINTILLATION ON BER

The fig. 3 illustrates clearly that OFDM-OWC systemrealize high speed communication when 2

sσ is

small. When 2 0.01sσ ≤ , we get lower BER. On the other

hand, 2 0.01sσ ≥ we found higher BER. This is because there exist the effects of scintillation; there is a possibility thatthe interference power becomes larger than the desired user’spower even in the regime of

high power. When 2 0.01sσ ≤ , there exist no effects of scintillation.

FIG. 4 EFFECT OF FREQUENCY OFFSET ON BER

Fig. 4 shows the comparison of the performance BER and different normalized frequency offset while phase noise is constant. As normalized frequency offset increases, BER increases dramatically. These results show that degradation of performance increases with normalized frequency offset. In normalized frequency offset 0.025 and 1.75, the values of BER in OFDM-OWC are approximately 27.9 10−× and 11.3 10−× respectively at 12 dBm.

FIG. 5 EFFECT OF PHASE NOISE ON BER

Fig. 5 shows the effect of phase noise on BER due to invariable normalized frequency offset. As the value of phase noise increases, the BER also increases and thus

International Journal on Communications (IJC) Volume 3, 2014 www.seipub.org/ijc

5

degradation of performance increases with normalized frequency offset and phase noise.

Conclusion

An analytical approach is used to evaluate the combined influence of backscattered light, atmospheric scintillation, frequency offset and phase noise in the BER performance of OFDM based OWC with IM/DD scheme. Numerical results show that the effect of backscattering is significant. Not only backscattering but also scintillation is an important factor in favor of OFDM-OWC which degrades the scheme performance. The frequency offset and phase noise has a noteworthy effect on the BER performance. OFDM has more advantages, more suitable for future optical wireless communication scheme in the removal the channel impairments.

REFERENCES

D. Kedar and S. Arnon, “Optical wireless communication

through fog in the presence of pointing errors,” Applied

Optics, vol. 42, no. 24, pp. 4946-4954, 2003.

D. Tsonev, S. Sinanovic and H. Haas “Novel unipolar

orthogonal frequency division multiplexing (U-OFDM)

for optical wireless communication,” in Proc. of Vehicular

Technology Conference (VTC 2012 – Spring), 2012, to appear.

D. Kedar and S. Arnon, “Backscattering-Induced Crosstalk in

WDM Optical Wireless Communication” Journal of

Lightwave Technology, vol. 23, no. 6, pp. 2023-2030, June

2005.

E. Vanin “Performance evaluation of intensity modulated

optical OFDM system with digital baseband distortion”

Optics Express, vol. 19, no. 5, pp. 4280-4293, 2011.

E. H. Miguel, “Fiber-based Orthogonal Frequency Division

Multiplexing Transmission Systems” M. Sc Eng. thesis,

2010.

G. Yan, W. Min and D. Weifeng “Performance Research of

Modulation for Optical Wireless Communication System”

Journal of networks, vol. 6, no. 8, pp. 1099 – 1105, August

2011.

G. P. Agrawal, Fiber-Optic Communication Systems. New

York: Wiley, 1997.

I. B. Djordjevic, B. Vasic and M. A. Neifeld, “LDPC- Coded

OFDM for Optical Communication system with direct

detection,” IEEE Journal of Selected Topics in Quantum

Electronics, vol.13, no. 5, pp. 1446 – 1454, 2007.

R. M S R. Pir and M. M. Hasan, “Performance Limitations of

a Wireless Optical Link Impaired by Backscattering and

Atmospheric Scintillation,” International Journal of

Research and Reviews in Computing Engineering, vol. 1,

no. 1, pp. 35-38, March 2011.

V. K. Dwivedi and G. Singh, “An efficient BER analysis of

OFDM systems with ICI conjugate cancellation method,”

PIERS Proceedings, Cambridge, USA, 2008.

Y.-S. Li, H.-G.Ryu, J.-W.Li, D.-Y.Sun, H.-Y. Liu, L. –J. Zhou

and Y. Wu, “ICI compensation in MISO-OFDM system

affected by frequency offset and phase noise”, Wireless

Commun., Networking and Mobile Computing (WiCOM

'08), vol. 5, no. 12, pp. 32-38, 2008.

Z. Ghassemlooy, “Indoor Optical Wireless Communication

Systems – Part I: Review,” Quantum Beam Ltd. UK, pp.

11-31, 2003.