Analysis of the Impact of P-Ratio on BER, Q-Factor and OSNR of Radio over Fiber (RoF) System

doi:10.4236/opj.2011.14029

Optics and Photonics Journal, 2011, 1, 179-188

doi:10.4236/opj.2011.14029 Published Online December 2011 (http://www.SciRP.org/journal/opj)

Analysis of the Impact of P-Ratio on BER, Q-Factor and

OSNR of Radio over Fiber (RoF) System

Vishal Sharma1, Amarpal Singh2, Ajay K. Sharma3

1Shaheed Bhagat Singh College of Engineering and Technology, Ferozepur, India

2Beant College of Engineering and Technology, Gurdaspur, India

3Dr. B.R. Ambedkar NIT, Jalandhar, India

E-mail: [email protected]

Received September 15, 2011; revised October 15, 2011; accepted November 6, 2011

Abstract

The impact of p-ratio i.e. ratio of carrier power to total received RF power at control office (CO) on Bit error

rate (BER), Q- performance parameter and Optical signal noise ratio (OSNR) of RoF communication system

is theoretically analyzed using angular full-linewidth at half maximum (FWHM) of Lorentzian shape for the

RF oscillator in this paper. As reduction in Laser linewidth improves the performance of RoF system, the RF

oscillator linewidth at FWHM also plays an important role in improving the performance of RoF system.

Keywords: RoF System, OSNR Parameter, BER, Q-Parameter, Fiber Dispersion, Matlab 7.0

1. Introduction

For realization of future high performance integrated

networks, broadband distribution & access networks and

to meet the increasing demand of multimedia services

with a guaranteed quality of service, RoF technology

comes out as the most promising technology. RoF tech-

nology combines the capacity of optical networks with

the flexibility and mobility of wireless networks. Reduc-

tion in complexity at the antenna site, reduction in in-

stallation cost of access networks, possibility of dy-

namically allocation of radio carriers to different an-

tenna sites, transparency and scalability are the few ad-

vantages of RoF technology. These systems have several

advantages including lower attenuation compared to the

coaxial cable, higher bandwidth, immunity to the RF

interference and durability [1-4]. Modulation technique

is one of the most significant processes in RoF system

where the RF electrical signal is applied to modulate the

optical carrier. RoF modulation methods can be catego-

rized into two main groups: direct modulation and exter-

nal modulation. Direct modulation, a simple technique,

directly modulates the amplitude of the laser beam but

suffers from a laser-frequency chirp effect that degrades

severely the performance of the system. However, this

can be eliminated by using the external-modulation

scheme [5] used to modulate the phase of the optical

carrier. Furthermore, the conventional optical double

sideband (ODSB) external modulation scheme degrades

the received RF signal power due to fiber chromatic dis-

persion drastically. For frequency range beyond 5 GHz

external modulation is needed for higher speeds. No

doubt such systems are capable of meeting the future

requirements of High speed high data transfer services

but some degradations effects the performance of such

systems such as fiber dispersion and modulator’s non-

nearity. For overcoming this power degradation, an opti-

cal single sideband (OSSB) external modulation scheme

is employed [5]. The effect of phase noises from a laser

and an oscillator on OSSB-RoF system is analyzed and

discussed [6]. Barry and Lee [7] and Salz [8] also ana-

lyzed the performance of coherent optical systems with

laser phase noise by utilizing a wiener process as coher-

ent detection provides better sensitivity than that of di-

rect detection. A number of techniques have been in-

vented to analyze and improve the performance of RoF

system by mitigating these problems. Minimization of

fiber dispersion using OSSB technique, reduction of

laser linewidth [9] and using external modulators are

the few important techniques. In this paper, we have

analyzed theoretically the impact of p-ratio over per-

formance parameters of RoF communication system us-

ing angular full-linewidth at half maximum (FWHM) of

Lorentzian shape of the RF oscillator at different optical

V. SHARMA ET AL.

180

span and responsivity. In section II we present the theo-

retical analysis of OSNR for a radio over fiber (RoF)

communication system. Section III presents results and

discussion for system. Finally, section IV concludes the

paper findings.

2. Theoretical Analysis of OSNR in Rof

System

In a simple and compact architecture of RoF system as

shown in Figure 1, the data is up-converted by a RF os-

cillator and optically modulated by a laser diode (LD) by

means of a Mach Zehnder (MZM) external modulator in

a control office (CO). The output signal of the MZM is

transmitted via a standard single-mode fiber (SSMF) and

detected by a PD to generate the photocurrent at a base

station (BS). This photocurrent goes through a band-pass

filter (BPF) and an amplifier to be launched into a wire-

less channel in the BS. The wireless channel makes sig-

nals vulnerable to amplitude and phase distortion. A user

terminal (UT) amplifies and filters the received signal to

detect the transmitted RF signal. Finally, the data are

extracted through RF demodulation.

In this paper, an optical single sideband (OSSB) signal

is generated by using a DEMZM external modulator to-

gether with a phase shifter to overcome the power deg

radation due to fiber dispersion. This RF signal is opti-

cally modulated by the LD with a Dual electrode MZM

external modulator. The optically modulated signal is

transmitted to the PD and the photocurrent corresponding

Figure1. Overall downlink architecture [6] of a RoF system.

to the transmitted RF signal is extracted by the BPF. First,

the optical signals from the laser and the RF oscillator

are modeled as follows:



() .exp()

() .cos()

LDLD LD

RFRFRF RF

x

tA jt

x

tVj tt





(1)

where A and VRF define amplitudes from the LD and the

RF oscillator, ωLD and ωRF define angular frequencies

of the signals from the LD and the RF oscillator, and

φLD(t) and φRF(t) are phase-noise processes. φLD(t) is

characterized by a Wiener process [7].

0

()'( )dLD LD

t





 (2)

The time derivative φ’

LD(t) is not flat at low frequen-

cies due to 1/f noise. The white phase noise, however, is

the principal cause for line broadening and is associated

with quantum fluctuations [8]. Thus, φ'

LD(t) can be mod-

eled as a zero-mean white Gaussian process with a power

spectral density (PSD) function as

'() 2π

L

DS



LDv



 (3)

where, ΔvLD defines a laser line-width. Unlike the

phase-noise process from the laser, the phase-noise

process φRF(t) from the RF oscillator is difficult since

the spectrum consists of various noises like flicker

frequency, a white frequency noise, and a white phase

noise. After optically modulating xRF(t) by xLD(t) with a

DE-MZM, the output signal of the DE-MZM is repre-

sented [6] as (see Equation (4))

where ()

RF

x

t

 denotes the phase-shift version of()

RF

x

t,

πdc

VV





and π

2

RF

V



Vdefine a normalized dc

and ac value, V



is the switching voltage of the

DEMZM, LMZM is the insertion loss of the DEMZM, and

θ is the phase shift by the phase shifter. Note that the

input RF signals into the DEMZM are () 2

RF

xt and

()2

RF

xt

 rather than ()

RF

x

t and ()

RF

x

t

because the

RF signal is 3dB attenuated by utilizing the power split ter.

By controlling the phase shifter, the output signal can be the

OSSB or the optical dual sideband (ODSB) signal. Among

the two signals, only the OSSB signal will be dealt with in

this paper since the ODSB signal suffers from fiber chro-

matic dispersion severely and requires double bandwidth









.

ππ

()() ()

ππ

(0,)expπ.exp.

22

.exp π() πcos( )

2

exp( )πcos( )

LD

MZMRF RF

MZM

LDLDRF RF

LD LDRFRF

Lxtxt xt

Et jj

VV

AL jttt t

jt tt t



 

 





2



 







 



 







 







(4)

181

V. SHARMA ET AL.

than that of the OSSB signals. For generating the OSSB

signal, θ and γ are set to π/2 and 1/2, respectively. By

using Equation (4) and the mentioned conditions, the

OSSB signal at the DE MZM can be modeled as follows:



π

(0,)exp( )exp(πcos( ))

2

π

exp( )exp(πcos( ))

2

(0,)exp( )cosπcos( )sinπcos (

2

MZM

LD LDRF RF

LD LDRFRF

MZM

LDLDRF RFRF RF

AL

Etj ttjtt

jt tjt t

AL

Etj ttttjtt

 



 















 



















)

ππ

exp( )cosπcos( )sinπcos( )

22

LD LDRFRFRFRF

jt tt tjtt

  

















 





















(5)



02

1

21

0

02

1

2

cos πcos( )(π)2(π)cos2( )

sin πcos(( )2(1)(π)cos(21)[( )]

cos πsin( )(π)2 (π)cos2( )

sin πsin( )2

RF RFnRF RF

n

RF RFnRF RF

n

RF RFnRF RF

n

RF RFn

ttJJ ntt

ttJn tt

ttJJ ntt

tt J

 

 

 



















 



 





1

0

(π)cos(21)[( )]

RF RF

n

ntt











Now by using the Jacobi’s expansion for cos- and sin-terms and after simplification, Equation (5) can be written as



01

π

(0, )..(π).exp( )2(π) exp()()

4

SMZMLD LDLDRFLDRF

EtALJjt tJjtt tt



 





 





 



(6)

By neglecting the high-order components of the Bessel

function since the value of π



in the Bessel function is

very small due to the fact that π

R

F in general. The

output signal at the DEMZM is transmitted via the SSMF

experiencing different group delays, due to the fiber

chromatic dispersion, at a different wavelength. After the

transmission of

VV

F

iber

L km, the signal at the end of the

SSMF becomes



.

20

0

1

01 2

0

(,)10 (π)

2(π)

π

exp()exp()()

4(π)

fiber Fiber

L

SMZM add

LD LDLDRFLDRF

ELt ALLJ

J

jt tjtt tt

J



   







 

 

 



 

 





 

 

(7)

where Ladd denotes an additional loss in the optical link,

f

iber



is the SSMF loss,

F

iber

L is the transmission dis-

tance of the SSMF, 0



and





define group delays for a

center angular frequency of

L

D



and an upper sideband

frequency of

L

DRF



, 1



and 2



are phase-shift

parameters for specific frequencies due to the fiber

chromatic dispersion. For performance evaluation of RoF

system, we determine and analyze OSNR performance

parameter by employing the ratio between the signal

power and the noise power by using an autocorrelation

and a PSD- function. By using a square-law model, the

photocurrent i (t) can be obtained from (7) as follows:





22

110 2

()(,)2cos(()()())

SRFLDLDRF

itE LtABttt1





















  (8)

where

.

1

20

101

0

2

1

2(π)

10( π), ,

(π)

1

Fiber Fiber

L

MZM add

J

AAL LJJ

B



 





 



where,



defines the responsivity of the PD and 2

.is

the square-law detection. From Equation (8), the auto-

correlation function ()

I

R



is obtained as

() ().()

I

Ritit





(9)

V. SHARMA ET AL.

182

1

2

1

2

24 22

2

11

1

2cos( ),

()

2cos( ),

e

LD RF

RF

I

RF

e

RB

Ae



 





1

















 





(10)

where,

L

D

v and

R

F

v are the laser- and RF oscilla-

tor- line-width respectively and 2(2π)

L

D

vLD



 and

2(2π)

R

F

vRF



 define the angular full line-width at

half maximum (FWHM) of the Lorentzian shape for the

laser and the RF oscillator, respectively. The2t



, termed

as total line-width, is not given as 2π2π

L

DR

vv

as 2ππ

L

D

vv

RF



 . The term 1



is the differential delay

due to the fiber chromatic dispersion and is dependent on

the optical wavelength



, carrier frequency

R

F

f

, fiber

dispersion parameter, D and optical length,

F

ibe

Lr and

mathematically, given [10] as

2

1

R

F

Fiber

f

DL c



  (11)

where, c is the light velocity. The PSD function of the

photocurrent is given by the Fourier transform of (8).

The PSD function SI (f) can be written as

F

but



 



11

1

24

1

2

22

2

11

21

22

2

12

2

()

() ()

2cos2π() 4

()

2π() (2)2π()

4π.()( )

cos2π() sin2π(

2π()

et

t

I

II

RF RF

RF RFtRF

LD LDRFRF

tt RF

RF RF

Sf

Sf FRA

effe

Bf

ff ff

ff

eff ff

ff

 





 





 







 

 

 



 





1

)()

R

FR

Gf f



 

 

 

 

 F

(12)

The first term of Equation (12) represents a dc compo-

nent, the second and third is the broadening effects due

to the fiber chromatic dispersion and the line-widths of

the laser and the RF oscillator. By using Equation (12),

the received RF carrier power

I

P is approximately rep-

resented as follows [6]

 

1 1

1

2

2 2

22 2

24 2111

122

22

22 2

2

2()d

2..cos(2π() 4.

2()d d

2π() (2)2π()

.c

RF

RF RFRF

t t

RF RFRF

t

B

f

II

B

f

BB B

ff f

RF RF

I

BB B

RF RFtRF

ff f

tt

PSff

effe

PABff f

ff ff

e

 













 

 

 







 

 













11

1 1

2

242

22

11

1

4π.()( )

os 2π() sin2π()

2π()

4π.π.

(1)tantan

2

forπ1,

tt

LD LDRFRF

RF RF

tRF

RF

ff

ff ff

ff

ABB

ee

B

 

















 

 



 



 

 



 



 



 









1

242

21

11

21

4tan

π

t

ttRF

RF

and

AB

e





















(13)

In Equation (13), the last condition, 1

2

t1





 and

tRF



, is reasonable because the laser line- width is

much greater than the RF-oscillator line- width and the

fiber length is usually less than a few tens of kilometers

in RoF systems. Note that the coefficient 2 of Equation

(13) is due to the real and imaginary spectrum. The cos

and sin terms in Equation (12) are approximately equal

to 1 and 0 in the integrand when 1RF

πB1



 is satis-

fied.The received RF signal power is a function of dif-

ferential delay1



, laser- and RF oscillatorline-width and

bandwidth of electrical filter

R

F

For evaluating the total RF power excluding dc

power, we utilize Equation (10) as follows:

B.

24224

1

(0) 2

tI

PRABA2

11



 (14)

By using (14), we define the ratio p between the total

carrier power and the required power as follows [6]:

1

242

21

11

242

1

21

1

4π.

tan

π

2

π.

2tan

π

for 21

t

I

t

RF

ttRF

P

pP

AB

e

A

B

e

and











 

























(15)

183

V. SHARMA ET AL.

From (14), the required bandwidth for the p- ratio is ob-

tained as

1

2

π

tan( )

π2

t

RF

B





 ep

1

(16)

The required bandwidth increases as we need more

received signal power. Note that the required band-

width for a specific received carrier power is domi-

nantly dependent on the phase noise from the RF os-

cillator rather than that from the laser for 1

2

t







and tRF



. By using Equations (14) and (16),

OSNR is calculated as

1

242

1

0

4

.

2

20

2

0

OSNR

Signal 2

Noise Noise2.

2

(π)

1.10

2()

π.

20 tan()

2

fiberFiber

t

power t

powerpower RF

L

rf

MZM add

PAB

N

B

VAL L

V

pe

N















 















(17)

The value of Bit error rate (BER) by using the direct

relation among BER, Q-parameter and OSNR [10] can

be predicted as

2

BERError probability of Q

11

d1

2

π2

1

.

2π

x

Q

ex erf

e

Q

























　　 (18)

where,

22OSNR

Q114OSNR





3. Results and Discussion

The impact of p ratio using angular full-linewidth at half

maximum (FWHM) of Lorentzian shape for the RF os-

cillator can be easily observed graphically of the RoF

system at different optical span by varying responsivity

of photo detector. The OSNR decreases as we increases

the optical span but reduces as we decreases the p ratio

from p = 0.6 to p = 0.5 as shown in Figures 2-3. Also it

is observed that OSNR increases with increasing the re-

sponsivity (R-parameter) of the photodetector along with

by using the angular full-linewidth at half maximum

(FWHM) of Lorentzian shape the RF oscillator. The

OSNR increases about 3dBm by using the RF oscillator

linewidth at FWHM as shown in Figures 2(a) and 3(a).

The performance parameter Q also increases by de-

creasing the p-ratio of the RoF system and by increasing

the R- parameter. The Q-parameter is also improving by

using the RF oscillator linewidth at FWHM as shown in

Figures 4-5. The BER of RoF system is also improving

in the same manner depending upon the parameter such

as R-parameter, p-ratio and gamma RF as shown in Fig-

ures 6-9. The results obtained and shown graphically if

Figures 2-9 reveals that the performance of the RoF

system improves if the RF oscillator is defined at angular

full-linewidth at half maximum (FWHM) with increasing

the R-parameter.

(a) (b)

V. SHARMA ET AL.

184

(c) (d)

Figure 2. OSNR vs optical length for (a,b) R = 1 and (c,d) R = 0.5 with . π

RF





(a) (b)

(c) (d)

Figure 3. OSNR vs optical length for (a,b) R = 1 and (c,d) R = 0.5 with π2

RF



.

185

V. SHARMA ET AL.

(a) (b)

(c) (d)

Figure 4. Q-parameter vs optical length for (a,b) R = 1 and (c,d) R = 0.5 with . π

RF





(a) (b)

V. SHARMA ET AL.

186

(c) (d)

Figure 5. Q-parameter vs optical length for (a,b) R = 1 and (c,d) R = 0.5 with π2

RF



.

(a) (b)

(c) (d)

Figure 6. BER vs optical length for (a,b) R = 1 and (c,d) R = 0.5 with . π

RF





187

V. SHARMA ET AL.

(a) (b)

(c) (d)

Figure 7. BER vs optical length for (a,b) R = 1 and (c,d) R = 0.5 with π2

RF



.

(a) (b)

Figure 8. BER vs OSNR for (a) R = 1 and (b) R = 0.5 withπ

RF





.

V. SHARMA ET AL.

188

(a) (b)

Figure 9. BER vs OSNR for (a) R = 1 and (b) R = 0.5 with π2

RF



.

4. Conclusions [4] J. Hecht, “Understanding Fiber Optics,” 4th Edition, Pren-

tice Hall, Upper Saddle River, 2002.

In this paper, we have analyzed the impact of p-ratio on

OSNR, Q-parameter and BER of RoF communication

system at FWHM of RF oscillator linewidth in conjunc-

tion with variable value of fiber length (LFiber) up to 100

Km. An improvement of 3 dBm in OSNR have been

observed for p-ratio = 0.5 and π2

RF



along with

R-parameter of 1. A considerable improvement in Q-

parameter and BER has also been observed for lesser

value of p-ratio.

[5] G. H. Smith and D. Novak, “Overcoming Chromatic-

Dispersion Effects in Fiber-Wireless Systems Incorpo-

rating External Modulators,” IEEE Transactions on Mi-

crowave Theory Technology, Vol. 45, No. 8, 1997, pp.

1410-1415. doi:10.1109/22.618444

[6] T.-S. Cho, C. Yu and K. Kim, “Analysis of CNR Penalty

of Radio over Fiber Systems Including the Effects of

Phase Noise from Laser and RF Oscillator,” Journal of

Lightwave Technology, Vol. 23, No. 12, 2005, pp. 4093-

4100.

[7] J. R. Barry and E. A. Lee, “Performance of Coherent

Optical Receivers,” Proceeding of Institute of Electrical

Engineering, Vol. 78, No. 8, 1990, pp. 1369-1394.

5. References

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[8] J. Salz, “Modulation and Detection for Coherent Light-

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[9] V. Sharma, A. Singh and A. K. Sharma, “Analysis the

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rier-to- Noise Ratio in Optical Single Sideband (OSSB)

RoF Transmission Systems”, Optics and Lasers in Engi-

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doi:10.1016/j.optlaseng.2009.06.012

[2] M. Sauer, A. Kobyakov, J. E. Hurley and J. George, “Ex-

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Standard and High-Bandwidth Multimode Optical Fi-

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tonics, Seoul, 12-14 October 2005, pp. 99-102.

doi:10.1109/MWP.2005.203549 [10] U. Gliese, S. Nørskov and T. N. Nielsen, “Chromatic Dis-

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Links,” IEEE Transaction of Microwave Theory Tech-

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[3] R. J. Green, “Secure Communications: The Infrared Al-

ternative,” In ICTON Mediterranean Winter Conference,

6-8 December 2007, pp. 1- 4.

doi:10.1109/ICTONMW.2007.4446906

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