- Email: [email protected]
Performance optimization of RoF systems using 120° hybrid coupler for OSSB signal against third order intermodulation
Optics Communications 376 (2016) 30–34
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Performance optimization of RoF systems using 120° hybrid coupler for OSSB signal against third order intermodulation Parvin Kumar a,n, Sanjay Kumar Sharma a, Shelly Singla b a b
Krishna Institute of Engineering and Technology, Ghaziabad, U.P. 201206, India Indus Institute of Engineering and Technology, Kinana, Haryana 126114, India
art ic l e i nf o
a b s t r a c t
Article history: Received 11 February 2016 Received in revised form 3 May 2016 Accepted 5 May 2016
The performance of radio over fiber (RoF) system with dual drive Mach Zehender modulator has been optimized against third order intermodulation distortion by using 120° hybrid coupler in transmission system. Signal to Noise Distortion ratio (SNDR) has been evaluated and a performance comparison is also drawn for the systems based on 90° and 120° hybrid coupler in both noise and intermodulation distortion dominant environment. The SNDR is efficiently improved by employing 120° hybrid coupler in noise dominant and intermodulation distortion dominant environment. An improvement of 4.86 dB is obtained in the maximum SNDR with 120° hybrid coupler is obtained over at 20 km optical fiber length compared with a 90° hybrid coupler based system. A significant reduction of third order intermodulation power at receiver has also been observed with 120° hybrid coupler. & 2016 Elsevier B.V. All rights reserved.
Keywords: Optical single sideband (OSSB) Radio over fiber (RoF) Dual electrode Mach Zehender modulator (DE-MZM) Third order intermodulation (IM3) Signal to Noise Distortion ratio (SNDR)
1. Introduction The Radio over Fiber (RoF) system has become a promising technique for the fifth generation wireless communication. It forms the backbone of wireless networks as it has capability to achieve multi-gigabit per second data rate. It supports bandwidth intensive applications, simplicity and cost-effectiveness since it centralizes resources at the central station. Its base stations consist only of an optical-to-electrical converter, radio frequency amplifiers and antennas. But, the RoF system proposes the redesign of existing architectures [1–3]. Optical signals can be modulated either directly or externally. The direct modulation suffers from certain shortcomings like chirp effect, instability, power fluctuations etc. [4,5]. All these issues can be controlled by external modulation. The majorly used external modulation is Mach Zehender modulator. It eliminates chirp effect and produces better modulated light signal compared to direct modulation method [6]. In external-modulation scheme, the conventional optical double sideband (ODSB) signal can degrade the received RF signal power due to fiber chromatic dispersion drastically. For overcoming the power degradation, the optical single sideband (OSSB) modulation is considered as bandwidth efficient intensity modulation scheme and is used for the optical generation and distribution of the RoF n
Corresponding author. E-mail addresses: [email protected] (P. Kumar), [email protected] (S. Singla). http://dx.doi.org/10.1016/j.optcom.2016.05.004 0030-4018/& 2016 Elsevier B.V. All rights reserved.
signal modulated with data and achieving long distance transmission [7,8]. The third order intermodulation (IM3) is one of the practical and decisive problem in high-quality services. This phenomenon is serious in RoF systems because third order intermodulation components increase significantly faster than fundamental components as an input signal power increases. In general, the level of IM3 is significantly higher than that of noise in the high data rate services. Thus, an entire system performance can be very sensitive and severely degraded by IM3 [6–10]. It is highly desirable to evaluate performance of the system including intermodulation errors. To overcome this performance degradation, many researchers have used a variety of technologies such as a decrease of IM3 [8] by a power control of an input signal [6] and EDFA utilization [11]. However, the EDFA utilization may not be efficient when IM3 power is considerably higher than the noise level because it amplifies the IM3 component as well as the fundamental signal in an optical link [12]. Performance improvement can be obtained for a RoF system with 120° hybrid coupler [14–16]. Analysis has been carried out for RoF system based on OSSB by using 120° hybrid coupler against intermodulation distortion in the present paper. In addition, Simulations have also been carried out to evaluate the performance and draw comparison between the techniques based on 90° and 120° hybrid coupler. Results have been obtained for noise and distortion dominant environment, to have a better understanding of the considered system.
P. Kumar et al. / Optics Communications 376 (2016) 30–34
2. Principle and analysis
E90°(0, t ) =
Fig. 1.1 shows the diagram of OSSB RoF system based on a Dual Electrode Mach Zehender modulator (DE-MZM). With a proper dc bias, the phase difference between optical signals in the upper and lower arms of the DE-MZM is controlled. In a 90° phase shift method, the phase difference between the optical components is nπ⁄2 ∅0 while for 120° hybrid coupler based design, the phase difference is nπ⁄3 ∅0 . ∅0=π /3, the 1st and þ2nd order sideband are simultaneously suppressed because these sidebands have a 180° and 180° phase difference respectively and are destructively inferred when combined at the output port of the DE-MZM. As a result, an OSSB signal with both of the 1st order sideband and þ2nd order sideband suppressed is generated. Similarly, when ∅0= − π /3, an OSSB signal with both of the þ1st order sideband and 2nd order sideband suppressed is generated [14–16]. The input signals from the laser diode and the RF oscillator are modeled as:
xD (t )=V0 exp ( jωD t )
2
π {expj[ +βπ (cosω1t + cosω2t )] 2
+ expj[βπ (sinω1t + sinω2t )]}
E120° (0, t ) =
⎡π ⎤ L att V0 e jωD t ⎧ ⎨ exp j ⎢ +βπ ( cos ω1t +cos ω2 t ) ⎥ ⎣3 ⎦ 2 ⎩ ⎪
⎡ ⎛ ⎛ ⎛ 2π ⎞ 2π ⎞ ⎞ ⎤ ⎫ +exp j ⎢ βπ ⎜ cos ⎜ ω1t + ⎟+cos ⎜ ω2 t + ⎟ ⎟ ⎥ ⎬ ⎝ ⎠ ⎝ 3 3 ⎠⎠⎦⎭ ⎣ ⎝
⎪
(5)
Using the above equations and defined functions, the output of DD-MZM can be represented in terms of Bessel function as: E 90° (0, t )=
E120° (0, t )=
⎧ L att V0 e jωD t ⎪ ⎨j ⎪ 2 ⎩
2
⎫ ⎪ Jn (βπ ) e jn ( ω m t ) ⎬ ⎪ ⎭ m = 1 n =−∞ 2
n =∞
∏ ∑
j n Jn (βπ ) e jnω m t +
m = 1 n =−∞
n =∞
∏ ∑
L att V0 e jωD t 2 2 n =∞ 2 n =∞ ⎧ ⎫ ⎪ jn ω m t + 2π ⎪ jπ 3 ⎬ ×⎨ e 3 ∏ ∑ j n Jn (βπ ) e jnω m t + ∏ ∑ j n Jn (βπ ) e ⎪ ⎪ ⎩ ⎭ m = 1 n =−∞ m = 1 n =−∞
(
(1) E 90° (0, t )=
xr1 (t )=VRFO cos ( ω1t )
LattV0e jωDt
31
)
(6)
L att V0 e jωD t ⎡ 2 ⎣ j J0 (βπ )+j2J0 (βπ ) J1 (βπ ) ( cos ω1t +cos ω2 t ) 2
{
−2J0 (βπ ) J2 (βπ ) ( cos 2ω1t +cos 2ω2 t )−j 4J1 (βπ )
(2)
× J2 (βπ ) ( cos ω1t ∙ cos 2ω2 t +cos ω2 t ∙ cos 2ω1t )
xr2 (t )=VRFO cos ( ω2 t )
}
−4J12 (βπ ) cos ω1t ∙ cos ω2 t
(3)
{
+ J02 (βπ )+j2J0 (βπ ) J1 (βπ ) ( sin ω1t +sin ω2 t )
where, xD (t ) is the optical signal from a laser and xr1 (t ), xr2 (t ) the input RF modulated signals allocated to one user in the SCM group. V0 , VRFO are the amplitudes from the laser diode and RF oscillator respectively. ωD , ω1, ω2 are the angular frequencies of the signals. The output signal of the DE-MZM is expressed as: ⎧ ⎡π ∼ ⎡ xr1 (t )+∼ xr 2 (t ) π ( xr1 (t )+xr 2 (t ) ) ⎤ L V e jωD t ⎪ ⎥ +exp j ⎢ ⎨ exp j ⎢ γ0 π + EC (0,t )= att 0 ⎢ ⎢ ⎥ 2 ⎪ 2 Vs 2 Vs ⎣ ⎦ ⎣ ⎩
(
) ⎤⎥ ⎫⎪⎬ ⎥⎪ ⎦⎭
+2J0 (βπ ) J2 (βπ ) ( cos 2ω1t +cos 2ω2 t ) +j 4J1 (βπ )× J2 (βπ ) ( cos 2ω1t ∙ sin ω2 t +sin ω1t ∙ cos 2ω2 t ) − 4J12 (βπ ) sin ω1t ∙ sin ω2 t ⎤⎦
}
E120° (0, t) =
L att V0 e jωD t 2 ×
{
π 2
3 e j 6 J0 (βπ )−j2 3 J0 (βπ ) J1 (βπ )
( cos ω1t+cos ω2 t )−j4 EC (0,t )=
e jωD t
L att V0 2
{ exp j ⎡⎣ γ π+βπ ( cos ω t+cos ω t )⎤⎦ +exp j ⎡⎣ βπ ( cos ( ω t +θ )+cos ( ω t +θ ) ) ⎤⎦ } 1
0
1
where, γ0=
VDC , Vs
( cos ω1t∙ cos 2ω2 t+cos ω2 t∙ cos 2ω1t )
2
2
switching voltage of DEMZM respectively. L att =10−LDM /20 is the attenuation of DEMZM due to insertion loss LDM . ∼ xr2 (t ) xr1 (t ) and ∼ are the phase-shifted versions of xr1 (t ) , xr 2 (t )respectively. θ is the VRFO 2 Vs
.
By setting the values of θ and B = q0 b as (π /2, 1/2) and (2π /3, 1/3), the OSSB signals are generated using 90° and 120° hybrid coupler respectively. Thus, the output of DDMZM based on 90° and 120° hybrid coupler can be expressed as: DE-MZM
Optical Fiber
Laser Diode
RF Inputs bias Voltage
RF Signal Output
DC Bias
Photodetector
Fig. 1.1. Block diagram of OSSB RoF system based on a dual electrode Mach Zehender modulator.
} (7)
(4)
VDC , Vs are the normalized dc bias voltage and
phase shift and β =
π
3 e j 6 J1 (βπ )× J2 (βπ )
After the transmission over a fiber length, L km, the signal can be represented by using Schrodinger equation [13] as
∼ E (L, t )=exp LD E (0, t )
( )
∼ λ2D ∂ 2 α ∙ − D= − j 4πc ∂t 2 2
(8)
where L is the length of the optical fiber, D̃ is the dispersion operator, D is the dispersion parameter, λ is the optical wavelength, c is the light velocity and α is the fiber loss. A square law model is used to obtain a photocurrent at the output of photodetector. The photocurrent i(t) can be obtained from Eq. (8) as
i (t )=R E (L, t ) 2 +n (t )
i (t )=i f1 (t )+i f 2 (t )+i2f1− f 2 (t )+i2f 2− f1 (t )+i0 (t )+n (t )
(9)
where, i f1 (t ) and i f2 (t ) represent the fundamental components of
32
P. Kumar et al. / Optics Communications 376 (2016) 30–34
the photocurrent, i2f1− f2 (t ) and i2f2− f1 (t ) represent IM3 components, i0 (t ) represent other spurious terms including second order intermodulation and n(t) represents additive noise. The second order intermodulation terms are neglected as they can be eliminated by using symmetrical DEMZM. So, we focussed on fundamental frequency and IM3 components. In order to evaluate SNDR of the considered system, it is necessary to derive the power of a fundamental signal and IM3 from a photocurrent at a photodetector and by utilizing Jn (βπ ) ≈ (βπ )n /2n n! for βπ ≪ 1. The fundamental frequency component power using can be expressed as
⎞2 αL 2 αL 2 ⎛ ⎞ ⎛1 P 90°, f1 = ⎜ 10− 10 L att V02 R J03 (βπ ) J1 (βπ ) ⎟ = ⎜ (βπ ) 10− 10 LDM V02 R⎟ , ⎝ ⎠ ⎝2 ⎠
SNDR 90°
SNDR120° =
PRFO = αL 2 ⎛ ⎞2 ⎜ 3 .10− 10 L att V02 R J03 (βπ ) J1 (βπ ) ⎟ ⎝ ⎠
⎛ 3 ⎞2 αL =⎜ (βπ ) 10− 10 L2DM V02 R⎟ , for βπ ≪ 1 ⎝ 2 ⎠
(10)
⎞2 αL 2 ⎛ ⎞2 ⎛ 1 P 90°,2f1 − f 2 = 2. ⎜ 10− 10 L att V02 R⎟ ⎜ (βπ )3⎟ c90 ⎝ ⎠ ⎝ 16 ⎠
c90 = 5 − 3. cos ( θ 2f1 − f 2 )
⎞2 αL 2 ⎛ ⎞2 ⎛ 3 (βπ )3⎟ c120 P 120°,2f1 − f 2 = ⎜ 10− 10 L att V02 R⎟ ⎜ ⎝ ⎠ ⎝ 16 ⎠
θf =
πLDλ2f 2 c
(11)
(12)
where λ is the wavelength of laser diode, c is the speed of light. By taking into account that the frequencies (f1 , f2 ) of the signals are much higher than the difference
( f1−f2 ). The c90 and c120 can
be approximated as:
⎛ 2πLDλ2f 2 ⎞ c90=5 − 3 cos ⎜ ⎟ ⎝ ⎠ c ⎛ 2πLDλ2f 2 ⎞ c120=2 − cos ⎜ ⎟ ⎝ ⎠ c
(13)
SNDR is evaluated to obtain the performance of the system. From Eq. (10) and (11), SNDR can be obtained as
SNDR =
2 − αL 2 2 ⎞ 3 π10 10 Latt V0 R ⎟ PRFO ⎟ 2Vπ
⎠ ⎤ ⎡ ⎛ 3 − αL 2 2 ⎞2 π 10 10 Latt V0 R 3 3 ⎟ C120o ⎥ PRFO 2 ⎢⎢ 256 ⎜⎜ + PN 3 ⎟ ⎥ Vπ ⎝ ⎠ ⎦ ⎣
(14)
2 VRFO 2
PN = Nth + Nshot = 4kT + qRV02
Where, R is the responsivity of the photodetector. It is worthy noted that P f1=P f2 and in case of OSSB fundamental components are tough against fiber dispersion. Thus, the IM3 power P2f1 − f2 can be given as
c120 = 2 − cos ( θ 2f1 − f 2 )
⎛ 2 ⎜⎜ ⎝
where,
for βπ ≪ 1
P 120°, f1 =
⎛ − αL 2 2 ⎞2 π10 10 Latt V0 R ⎟ PRFO 2 ⎜⎜ ⎟ 2Vπ ⎝ ⎠ = ⎡⎛ ⎞⎤ ⎛ 3 − αL 2 2 ⎞2 ⎢ 1 π 10 10 Latt V0 R ⎟ ⎟⎥ 3 o 2 ⎢ ⎜⎜ 128 ⎜⎜ C ⎟ 90 ⎟ ⎥ PRFO + PN Vπ3 ⎝ ⎠ ⎢⎣ ⎝ ⎠ ⎦⎥
P f1+P f 2 PN +P 2f1− f 2+P 2f 2− f1
The SNDR for the system based on 90° and 120° hybrid coupler are represented as
k = 1.38 × 10−23 J/K is the Boltzmann constant, q = 1.6 × 10−19 C is the electron charge, T = 290 K is the absolute temperature, PRFO is the input signal power. Thus, it is observed that the SNDR and IM3 power are sensitive to fiber dispersion and the input signal power.
3. Results and discussion The presented analysis has been utilized to carry out numerical simulations for SNDR, third order intermodulation and fundamental power of 90° and 120° Hybrid Coupler OSSB RoF transmission system. The results have been reported by taking values of parameters such as wavelength of laser diode ¼1550 nm, power of laser diode ¼0 dB m, switching voltage of DD-MZM ¼ 2.5 V, responsivity ¼ 0.4, DD-MZM insertion loss ¼6 dB, optical fiber loss¼ 0.2 dB/km, dispersion parameter¼ 16 ps/km nm and optical fiber length ¼ 20 km. Fig. 1.2 (a) and (b) shows the SNDR as well as the fundamental power and IM3 power components as a function of the frequency of the input RF signal modulated in a noise-dominant situation where the noise level is much higher than IM3 for considered systems. The SNDR is found to be almost flat as the fundamental signals are robust against fiber dispersion due to the utilization of OSSB signals. Additionally, the power of IM3 fluctuates within a range of 5.99 dB m for 90° hybrid coupler based system and 4.75 dB m for 120° hybrid coupler based system, because the IM3 components are generated due to phase shift in frequency components. It can be seen in Fig. 1.2 (a) that the peak value of SNDR, the fundamental, and IM3 power for 90° hybrid coupler based system are 94.25 dB, 102.87 dB m, and 187.91 dB m respectively. While in Fig. 1.2 (b) for 120° hybrid coupler based system, SNDR is found to be 99.12 dB, 98.10 dB m, and 189.15 dB m respectively. An improvement of 4.86 dB in SNDR is observed for 120° hybrid coupler based system and a suppression by 1.25 dB m in IM3 is observed by 120° hybrid coupler based system over conventional 90° hybrid coupler based system. As the input signal power increases, the IM3 power increases rapidly. In such a case, the distortion due to IM3 is a dominant factor degrading the SNDR of the system as shown in Fig. 1.3. Therefore, the shape of the SNDR curve is strongly dependent on the IM3 curve and SNDR also fluctuates within a range of 5.99 dB m for 90° hybrid coupler based system and 4.75 dB m for
P. Kumar et al. / Optics Communications 376 (2016) 30–34
33
Fig. 1.2. SNDR, the fundamental, and IM3 power variations as a function of the frequency of the input RF signal modulated by the OSSB incorporating a 10-dBm input signal power and a 20-km fiber in a noise-dominant situation when the noise level is much higher than IM3.
120° hybrid coupler based system. If the input RF signal power is the same, then the fluctuation range is a constant. However, the curves fluctuate more rapidly as the frequency increases. It can be seen in Fig. 1.3 (a) that the peak value of SNDR, the fundamental, and IM3 power for 90° hybrid coupler based system are 56.01 dB, 82.87 dB m, and 127.91 dB m respectively. While in Fig. 1.3 (b) for 120° hybrid coupler based system, SNDR is found to be 62.03 dB, 78.10 dB m, and 129.15 dB m respectively. An improvement of 6.02 dB in SNDR and 1.25 dB m in IM3 is observed by 120° hybrid coupler based system over conventional 90° hybrid coupler based system. Further, for better and close investigation, SNDR is depicted in terms of the input signal power and frequency in Fig. 1.4. The input RF signal frequency has been swept from 0.1 to 25 GHz. When IM3 is dominant with respect to the noise level, the SNDR curves are almost of the same shape, except for the decibel level. As the power of IM3 decreases, the SNDR level shifts and the shape of SNDR follows the characteristics of the fundamental component. For 90° hybrid coupler based system, it can be seen in Fig. 1.4 (a) that the peak value of SNDR has a value of 71.44, 72.51, 68.02, 58.39, and 48.40 dB at input signal power of 10, 5, 0, 5 and 10 dB m respectively. Similarly, for 120° hybrid coupler based system, SNDR values are observed as 76.30, 77.57, 73.92, 64.44, and
54.42 dB at input signal power of 10, 5, 0, 5 and 10 dB m respectively. Thus, a minimum improvement of 4.86 dB is observed in the peak value of SNDR while adopting 120° Hybrid Coupler in comparison with 90° Hybrid Coupler.
4. Conclusion Analysis and simulation have been carried out for the intermodulation effects due to DEMZM and performance has been studied by evaluating SNDR for OSSB signal based on 90° and 120° hybrid coupler based systems in an RoF systems. Fiber dispersion has also been considered in the analysis. The influence of the input RF power and modulation frequency have also been investigated to understand and maximize the overall system performance. For OSSB signals, the fundamental frequency components are found tolerant against dispersion while IM3 signal fluctuates with a 5.99 dB range for 90° hybrid coupler and 4.75 dB range for 120° hybrid coupler based systems. An improvement of 4.86 dB in SNDR is observed by using 120° hybrid coupler based system over conventional 90° hybrid coupler based system in a noise-dominant situation and an improvement of 6.02 dB is observed in SNDR by 120° hybrid coupler based system over conventional 90° hybrid
Fig. 1.3. SNDR, the fundamental, and IM3 power variations as a function of the frequency of the input RF signal modulated by the OSSB incorporating a 10-dB m input signal power and a 20-km fiber in a distortion-dominant situation when the IM3 is much higher than noise level.
34
P. Kumar et al. / Optics Communications 376 (2016) 30–34
Fig. 1.4. SNDR as a function of the power and frequency of the input RF signal modulated by OSSB incorporating a 20-km fiber.
coupler based system in an intermodulation distortion dominant situation. Also, a reduction of 1.25 dB m in IM3 is observed by 120° hybrid coupler based system over conventional 90° hybrid coupler based system in both noise and intermodulation distortion dominant situation Thus, Utilization of 120° hybrid coupler would greatly improve the performance of RoF links where the intermodulation distortion needs to be controlled.
References [1] V.A. Thomas, M. El-Hajjar, L. Hanzo, Performance improvement and cost reduction techniques for radio over fiber communications, IEEE Commun. Surv. tutor. 17 (2) (2015) 627–670. [2] H. Ogawa, D. Polifko, S. Banda, Millimeter-wave fiber optics systems for personal radio communication, IEEE Trans. Microw. Theory Tech. 40 (12) (1992) 2285–2292. [3] J.L. Corral, J. Marti, J.M. Fuster, General expressions for IM/DD dispersive analog optical links with external modulation or optical upconversion in a Mach–Zehnder electrooptical modulator, IEEE Trans. Microw. Theory Tech. 49 (10) (2001) 1968–1976. [4] H.J. Zhou, W. Chen, Z. Meng, Optical single sideband-frequency generation with carrier totally suppressed for Brillouin distributed fiber sensing, Opt. Commun. 285 (2012) 4391–4394. [5] A. Faniuolo, G. Tartarini, P. Bassi, Effects of directly modulated laser chirp on the performance of radio over fiber systems, in: Proceedings of the International Topical meeting on Microwave Photonics, 2003, 〈http://dx.doi.org/10. 1109/MWP.2003.1422900〉.
[6] G.H. Smith, D. Novak, Overcoming chromatic-dispersion effects in fiberwireless systems incorporating external modulators, IEEE Trans. Microw. Theory Tech. 45 (8) (1997) 1410–1415. [7] S. Singla, S.K. Arya, Simulative investigation for third order-IM terms in multitone RoF system, Int. J. Light Electron Opt: Opt. 125 (14) (2014) 3756–3758. [8] T.S. Cho, K. Kim, Optimization of radio-on-fiber systems employing ODSB signals by utilizing a dual electrode Mach-Zehnder modulator against IM3, IEEE Photonics Technol. Lett. 18 (9) (2006) 1076–1078. [9] S.K. Kim, W. Liu, Q.L. Pei, H. Fetterman, Nonlinear intermodulation distortion suppression in coherent analog fiber optic link using electro-optic polymeric dual parallel Mach-Zehnder modulator, Opt. Expr. 19 (8) (2011) 7865–7871. [10] F. Assadourian, Intermodulation distortion and efficiency analysis of multicarrier repeaters, IEEE Trans. Commun. 8 (1) (1960) 68–71. [11] T. Akiyama, K. Inagaki, Y. Mizuguchi, Intermodulation distortion by PD in heterodyne detection fibre-optic millimetre-wave links, Electron. Lett. 35 (17) (1999) 1478–1480. [12] T.S. Cho, B. Kwon, Performance optimization of radio-on-fiber systems employing erbium doped fiber amplifier for optical single sideband signals considering intermodulation distortion, Photonic Netw. Commun. 25 (2013) 73–78. [13] G.P. Agrawal, Nonlinear Fiber Optics, Academic, San Diego, CA, 1989. [14] M. Xue, S. Pan, Y. Zhao, Optical single-sideband modulation based on a dualdrive MZM and a 120° hybrid coupler, J. Lightwave Technol. 32 (19) (2014) 3317–3323. [15] P. Kumar, S.K. Sharma, S. Singla, Performance analysis of an OSSB RoF link using 90° and 120° hybrid coupler, Opt. Commun. (2015), http://dx.doi.org/ 10.1016/j.optcom.2015.09.081i, ISSN: 0030-4018. [16] P. Kumar, S.K. Sharma, S. Singla, Performance improvement of RoF transmission link by using 120 degree hybrid coupler in OSSB generation, Wirel. Netw. Springer J. (2015), http://dx.doi.org/10.1007/s11276-015-1123-9, ISSN: 15728196 (electronic version).
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Performance optimization of RoF systems using 120° hybrid coupler for OSSB signal against third order intermodulation Parvin Kumar a,n, Sanjay Kumar Sharma a, Shelly Singla b a b
Krishna Institute of Engineering and Technology, Ghaziabad, U.P. 201206, India Indus Institute of Engineering and Technology, Kinana, Haryana 126114, India
art ic l e i nf o
a b s t r a c t
Article history: Received 11 February 2016 Received in revised form 3 May 2016 Accepted 5 May 2016
The performance of radio over fiber (RoF) system with dual drive Mach Zehender modulator has been optimized against third order intermodulation distortion by using 120° hybrid coupler in transmission system. Signal to Noise Distortion ratio (SNDR) has been evaluated and a performance comparison is also drawn for the systems based on 90° and 120° hybrid coupler in both noise and intermodulation distortion dominant environment. The SNDR is efficiently improved by employing 120° hybrid coupler in noise dominant and intermodulation distortion dominant environment. An improvement of 4.86 dB is obtained in the maximum SNDR with 120° hybrid coupler is obtained over at 20 km optical fiber length compared with a 90° hybrid coupler based system. A significant reduction of third order intermodulation power at receiver has also been observed with 120° hybrid coupler. & 2016 Elsevier B.V. All rights reserved.
Keywords: Optical single sideband (OSSB) Radio over fiber (RoF) Dual electrode Mach Zehender modulator (DE-MZM) Third order intermodulation (IM3) Signal to Noise Distortion ratio (SNDR)
1. Introduction The Radio over Fiber (RoF) system has become a promising technique for the fifth generation wireless communication. It forms the backbone of wireless networks as it has capability to achieve multi-gigabit per second data rate. It supports bandwidth intensive applications, simplicity and cost-effectiveness since it centralizes resources at the central station. Its base stations consist only of an optical-to-electrical converter, radio frequency amplifiers and antennas. But, the RoF system proposes the redesign of existing architectures [1–3]. Optical signals can be modulated either directly or externally. The direct modulation suffers from certain shortcomings like chirp effect, instability, power fluctuations etc. [4,5]. All these issues can be controlled by external modulation. The majorly used external modulation is Mach Zehender modulator. It eliminates chirp effect and produces better modulated light signal compared to direct modulation method [6]. In external-modulation scheme, the conventional optical double sideband (ODSB) signal can degrade the received RF signal power due to fiber chromatic dispersion drastically. For overcoming the power degradation, the optical single sideband (OSSB) modulation is considered as bandwidth efficient intensity modulation scheme and is used for the optical generation and distribution of the RoF n
Corresponding author. E-mail addresses: [email protected] (P. Kumar), [email protected] (S. Singla). http://dx.doi.org/10.1016/j.optcom.2016.05.004 0030-4018/& 2016 Elsevier B.V. All rights reserved.
signal modulated with data and achieving long distance transmission [7,8]. The third order intermodulation (IM3) is one of the practical and decisive problem in high-quality services. This phenomenon is serious in RoF systems because third order intermodulation components increase significantly faster than fundamental components as an input signal power increases. In general, the level of IM3 is significantly higher than that of noise in the high data rate services. Thus, an entire system performance can be very sensitive and severely degraded by IM3 [6–10]. It is highly desirable to evaluate performance of the system including intermodulation errors. To overcome this performance degradation, many researchers have used a variety of technologies such as a decrease of IM3 [8] by a power control of an input signal [6] and EDFA utilization [11]. However, the EDFA utilization may not be efficient when IM3 power is considerably higher than the noise level because it amplifies the IM3 component as well as the fundamental signal in an optical link [12]. Performance improvement can be obtained for a RoF system with 120° hybrid coupler [14–16]. Analysis has been carried out for RoF system based on OSSB by using 120° hybrid coupler against intermodulation distortion in the present paper. In addition, Simulations have also been carried out to evaluate the performance and draw comparison between the techniques based on 90° and 120° hybrid coupler. Results have been obtained for noise and distortion dominant environment, to have a better understanding of the considered system.
P. Kumar et al. / Optics Communications 376 (2016) 30–34
2. Principle and analysis
E90°(0, t ) =
Fig. 1.1 shows the diagram of OSSB RoF system based on a Dual Electrode Mach Zehender modulator (DE-MZM). With a proper dc bias, the phase difference between optical signals in the upper and lower arms of the DE-MZM is controlled. In a 90° phase shift method, the phase difference between the optical components is nπ⁄2 ∅0 while for 120° hybrid coupler based design, the phase difference is nπ⁄3 ∅0 . ∅0=π /3, the 1st and þ2nd order sideband are simultaneously suppressed because these sidebands have a 180° and 180° phase difference respectively and are destructively inferred when combined at the output port of the DE-MZM. As a result, an OSSB signal with both of the 1st order sideband and þ2nd order sideband suppressed is generated. Similarly, when ∅0= − π /3, an OSSB signal with both of the þ1st order sideband and 2nd order sideband suppressed is generated [14–16]. The input signals from the laser diode and the RF oscillator are modeled as:
xD (t )=V0 exp ( jωD t )
2
π {expj[ +βπ (cosω1t + cosω2t )] 2
+ expj[βπ (sinω1t + sinω2t )]}
E120° (0, t ) =
⎡π ⎤ L att V0 e jωD t ⎧ ⎨ exp j ⎢ +βπ ( cos ω1t +cos ω2 t ) ⎥ ⎣3 ⎦ 2 ⎩ ⎪
⎡ ⎛ ⎛ ⎛ 2π ⎞ 2π ⎞ ⎞ ⎤ ⎫ +exp j ⎢ βπ ⎜ cos ⎜ ω1t + ⎟+cos ⎜ ω2 t + ⎟ ⎟ ⎥ ⎬ ⎝ ⎠ ⎝ 3 3 ⎠⎠⎦⎭ ⎣ ⎝
⎪
(5)
Using the above equations and defined functions, the output of DD-MZM can be represented in terms of Bessel function as: E 90° (0, t )=
E120° (0, t )=
⎧ L att V0 e jωD t ⎪ ⎨j ⎪ 2 ⎩
2
⎫ ⎪ Jn (βπ ) e jn ( ω m t ) ⎬ ⎪ ⎭ m = 1 n =−∞ 2
n =∞
∏ ∑
j n Jn (βπ ) e jnω m t +
m = 1 n =−∞
n =∞
∏ ∑
L att V0 e jωD t 2 2 n =∞ 2 n =∞ ⎧ ⎫ ⎪ jn ω m t + 2π ⎪ jπ 3 ⎬ ×⎨ e 3 ∏ ∑ j n Jn (βπ ) e jnω m t + ∏ ∑ j n Jn (βπ ) e ⎪ ⎪ ⎩ ⎭ m = 1 n =−∞ m = 1 n =−∞
(
(1) E 90° (0, t )=
xr1 (t )=VRFO cos ( ω1t )
LattV0e jωDt
31
)
(6)
L att V0 e jωD t ⎡ 2 ⎣ j J0 (βπ )+j2J0 (βπ ) J1 (βπ ) ( cos ω1t +cos ω2 t ) 2
{
−2J0 (βπ ) J2 (βπ ) ( cos 2ω1t +cos 2ω2 t )−j 4J1 (βπ )
(2)
× J2 (βπ ) ( cos ω1t ∙ cos 2ω2 t +cos ω2 t ∙ cos 2ω1t )
xr2 (t )=VRFO cos ( ω2 t )
}
−4J12 (βπ ) cos ω1t ∙ cos ω2 t
(3)
{
+ J02 (βπ )+j2J0 (βπ ) J1 (βπ ) ( sin ω1t +sin ω2 t )
where, xD (t ) is the optical signal from a laser and xr1 (t ), xr2 (t ) the input RF modulated signals allocated to one user in the SCM group. V0 , VRFO are the amplitudes from the laser diode and RF oscillator respectively. ωD , ω1, ω2 are the angular frequencies of the signals. The output signal of the DE-MZM is expressed as: ⎧ ⎡π ∼ ⎡ xr1 (t )+∼ xr 2 (t ) π ( xr1 (t )+xr 2 (t ) ) ⎤ L V e jωD t ⎪ ⎥ +exp j ⎢ ⎨ exp j ⎢ γ0 π + EC (0,t )= att 0 ⎢ ⎢ ⎥ 2 ⎪ 2 Vs 2 Vs ⎣ ⎦ ⎣ ⎩
(
) ⎤⎥ ⎫⎪⎬ ⎥⎪ ⎦⎭
+2J0 (βπ ) J2 (βπ ) ( cos 2ω1t +cos 2ω2 t ) +j 4J1 (βπ )× J2 (βπ ) ( cos 2ω1t ∙ sin ω2 t +sin ω1t ∙ cos 2ω2 t ) − 4J12 (βπ ) sin ω1t ∙ sin ω2 t ⎤⎦
}
E120° (0, t) =
L att V0 e jωD t 2 ×
{
π 2
3 e j 6 J0 (βπ )−j2 3 J0 (βπ ) J1 (βπ )
( cos ω1t+cos ω2 t )−j4 EC (0,t )=
e jωD t
L att V0 2
{ exp j ⎡⎣ γ π+βπ ( cos ω t+cos ω t )⎤⎦ +exp j ⎡⎣ βπ ( cos ( ω t +θ )+cos ( ω t +θ ) ) ⎤⎦ } 1
0
1
where, γ0=
VDC , Vs
( cos ω1t∙ cos 2ω2 t+cos ω2 t∙ cos 2ω1t )
2
2
switching voltage of DEMZM respectively. L att =10−LDM /20 is the attenuation of DEMZM due to insertion loss LDM . ∼ xr2 (t ) xr1 (t ) and ∼ are the phase-shifted versions of xr1 (t ) , xr 2 (t )respectively. θ is the VRFO 2 Vs
.
By setting the values of θ and B = q0 b as (π /2, 1/2) and (2π /3, 1/3), the OSSB signals are generated using 90° and 120° hybrid coupler respectively. Thus, the output of DDMZM based on 90° and 120° hybrid coupler can be expressed as: DE-MZM
Optical Fiber
Laser Diode
RF Inputs bias Voltage
RF Signal Output
DC Bias
Photodetector
Fig. 1.1. Block diagram of OSSB RoF system based on a dual electrode Mach Zehender modulator.
} (7)
(4)
VDC , Vs are the normalized dc bias voltage and
phase shift and β =
π
3 e j 6 J1 (βπ )× J2 (βπ )
After the transmission over a fiber length, L km, the signal can be represented by using Schrodinger equation [13] as
∼ E (L, t )=exp LD E (0, t )
( )
∼ λ2D ∂ 2 α ∙ − D= − j 4πc ∂t 2 2
(8)
where L is the length of the optical fiber, D̃ is the dispersion operator, D is the dispersion parameter, λ is the optical wavelength, c is the light velocity and α is the fiber loss. A square law model is used to obtain a photocurrent at the output of photodetector. The photocurrent i(t) can be obtained from Eq. (8) as
i (t )=R E (L, t ) 2 +n (t )
i (t )=i f1 (t )+i f 2 (t )+i2f1− f 2 (t )+i2f 2− f1 (t )+i0 (t )+n (t )
(9)
where, i f1 (t ) and i f2 (t ) represent the fundamental components of
32
P. Kumar et al. / Optics Communications 376 (2016) 30–34
the photocurrent, i2f1− f2 (t ) and i2f2− f1 (t ) represent IM3 components, i0 (t ) represent other spurious terms including second order intermodulation and n(t) represents additive noise. The second order intermodulation terms are neglected as they can be eliminated by using symmetrical DEMZM. So, we focussed on fundamental frequency and IM3 components. In order to evaluate SNDR of the considered system, it is necessary to derive the power of a fundamental signal and IM3 from a photocurrent at a photodetector and by utilizing Jn (βπ ) ≈ (βπ )n /2n n! for βπ ≪ 1. The fundamental frequency component power using can be expressed as
⎞2 αL 2 αL 2 ⎛ ⎞ ⎛1 P 90°, f1 = ⎜ 10− 10 L att V02 R J03 (βπ ) J1 (βπ ) ⎟ = ⎜ (βπ ) 10− 10 LDM V02 R⎟ , ⎝ ⎠ ⎝2 ⎠
SNDR 90°
SNDR120° =
PRFO = αL 2 ⎛ ⎞2 ⎜ 3 .10− 10 L att V02 R J03 (βπ ) J1 (βπ ) ⎟ ⎝ ⎠
⎛ 3 ⎞2 αL =⎜ (βπ ) 10− 10 L2DM V02 R⎟ , for βπ ≪ 1 ⎝ 2 ⎠
(10)
⎞2 αL 2 ⎛ ⎞2 ⎛ 1 P 90°,2f1 − f 2 = 2. ⎜ 10− 10 L att V02 R⎟ ⎜ (βπ )3⎟ c90 ⎝ ⎠ ⎝ 16 ⎠
c90 = 5 − 3. cos ( θ 2f1 − f 2 )
⎞2 αL 2 ⎛ ⎞2 ⎛ 3 (βπ )3⎟ c120 P 120°,2f1 − f 2 = ⎜ 10− 10 L att V02 R⎟ ⎜ ⎝ ⎠ ⎝ 16 ⎠
θf =
πLDλ2f 2 c
(11)
(12)
where λ is the wavelength of laser diode, c is the speed of light. By taking into account that the frequencies (f1 , f2 ) of the signals are much higher than the difference
( f1−f2 ). The c90 and c120 can
be approximated as:
⎛ 2πLDλ2f 2 ⎞ c90=5 − 3 cos ⎜ ⎟ ⎝ ⎠ c ⎛ 2πLDλ2f 2 ⎞ c120=2 − cos ⎜ ⎟ ⎝ ⎠ c
(13)
SNDR is evaluated to obtain the performance of the system. From Eq. (10) and (11), SNDR can be obtained as
SNDR =
2 − αL 2 2 ⎞ 3 π10 10 Latt V0 R ⎟ PRFO ⎟ 2Vπ
⎠ ⎤ ⎡ ⎛ 3 − αL 2 2 ⎞2 π 10 10 Latt V0 R 3 3 ⎟ C120o ⎥ PRFO 2 ⎢⎢ 256 ⎜⎜ + PN 3 ⎟ ⎥ Vπ ⎝ ⎠ ⎦ ⎣
(14)
2 VRFO 2
PN = Nth + Nshot = 4kT + qRV02
Where, R is the responsivity of the photodetector. It is worthy noted that P f1=P f2 and in case of OSSB fundamental components are tough against fiber dispersion. Thus, the IM3 power P2f1 − f2 can be given as
c120 = 2 − cos ( θ 2f1 − f 2 )
⎛ 2 ⎜⎜ ⎝
where,
for βπ ≪ 1
P 120°, f1 =
⎛ − αL 2 2 ⎞2 π10 10 Latt V0 R ⎟ PRFO 2 ⎜⎜ ⎟ 2Vπ ⎝ ⎠ = ⎡⎛ ⎞⎤ ⎛ 3 − αL 2 2 ⎞2 ⎢ 1 π 10 10 Latt V0 R ⎟ ⎟⎥ 3 o 2 ⎢ ⎜⎜ 128 ⎜⎜ C ⎟ 90 ⎟ ⎥ PRFO + PN Vπ3 ⎝ ⎠ ⎢⎣ ⎝ ⎠ ⎦⎥
P f1+P f 2 PN +P 2f1− f 2+P 2f 2− f1
The SNDR for the system based on 90° and 120° hybrid coupler are represented as
k = 1.38 × 10−23 J/K is the Boltzmann constant, q = 1.6 × 10−19 C is the electron charge, T = 290 K is the absolute temperature, PRFO is the input signal power. Thus, it is observed that the SNDR and IM3 power are sensitive to fiber dispersion and the input signal power.
3. Results and discussion The presented analysis has been utilized to carry out numerical simulations for SNDR, third order intermodulation and fundamental power of 90° and 120° Hybrid Coupler OSSB RoF transmission system. The results have been reported by taking values of parameters such as wavelength of laser diode ¼1550 nm, power of laser diode ¼0 dB m, switching voltage of DD-MZM ¼ 2.5 V, responsivity ¼ 0.4, DD-MZM insertion loss ¼6 dB, optical fiber loss¼ 0.2 dB/km, dispersion parameter¼ 16 ps/km nm and optical fiber length ¼ 20 km. Fig. 1.2 (a) and (b) shows the SNDR as well as the fundamental power and IM3 power components as a function of the frequency of the input RF signal modulated in a noise-dominant situation where the noise level is much higher than IM3 for considered systems. The SNDR is found to be almost flat as the fundamental signals are robust against fiber dispersion due to the utilization of OSSB signals. Additionally, the power of IM3 fluctuates within a range of 5.99 dB m for 90° hybrid coupler based system and 4.75 dB m for 120° hybrid coupler based system, because the IM3 components are generated due to phase shift in frequency components. It can be seen in Fig. 1.2 (a) that the peak value of SNDR, the fundamental, and IM3 power for 90° hybrid coupler based system are 94.25 dB, 102.87 dB m, and 187.91 dB m respectively. While in Fig. 1.2 (b) for 120° hybrid coupler based system, SNDR is found to be 99.12 dB, 98.10 dB m, and 189.15 dB m respectively. An improvement of 4.86 dB in SNDR is observed for 120° hybrid coupler based system and a suppression by 1.25 dB m in IM3 is observed by 120° hybrid coupler based system over conventional 90° hybrid coupler based system. As the input signal power increases, the IM3 power increases rapidly. In such a case, the distortion due to IM3 is a dominant factor degrading the SNDR of the system as shown in Fig. 1.3. Therefore, the shape of the SNDR curve is strongly dependent on the IM3 curve and SNDR also fluctuates within a range of 5.99 dB m for 90° hybrid coupler based system and 4.75 dB m for
P. Kumar et al. / Optics Communications 376 (2016) 30–34
33
Fig. 1.2. SNDR, the fundamental, and IM3 power variations as a function of the frequency of the input RF signal modulated by the OSSB incorporating a 10-dBm input signal power and a 20-km fiber in a noise-dominant situation when the noise level is much higher than IM3.
120° hybrid coupler based system. If the input RF signal power is the same, then the fluctuation range is a constant. However, the curves fluctuate more rapidly as the frequency increases. It can be seen in Fig. 1.3 (a) that the peak value of SNDR, the fundamental, and IM3 power for 90° hybrid coupler based system are 56.01 dB, 82.87 dB m, and 127.91 dB m respectively. While in Fig. 1.3 (b) for 120° hybrid coupler based system, SNDR is found to be 62.03 dB, 78.10 dB m, and 129.15 dB m respectively. An improvement of 6.02 dB in SNDR and 1.25 dB m in IM3 is observed by 120° hybrid coupler based system over conventional 90° hybrid coupler based system. Further, for better and close investigation, SNDR is depicted in terms of the input signal power and frequency in Fig. 1.4. The input RF signal frequency has been swept from 0.1 to 25 GHz. When IM3 is dominant with respect to the noise level, the SNDR curves are almost of the same shape, except for the decibel level. As the power of IM3 decreases, the SNDR level shifts and the shape of SNDR follows the characteristics of the fundamental component. For 90° hybrid coupler based system, it can be seen in Fig. 1.4 (a) that the peak value of SNDR has a value of 71.44, 72.51, 68.02, 58.39, and 48.40 dB at input signal power of 10, 5, 0, 5 and 10 dB m respectively. Similarly, for 120° hybrid coupler based system, SNDR values are observed as 76.30, 77.57, 73.92, 64.44, and
54.42 dB at input signal power of 10, 5, 0, 5 and 10 dB m respectively. Thus, a minimum improvement of 4.86 dB is observed in the peak value of SNDR while adopting 120° Hybrid Coupler in comparison with 90° Hybrid Coupler.
4. Conclusion Analysis and simulation have been carried out for the intermodulation effects due to DEMZM and performance has been studied by evaluating SNDR for OSSB signal based on 90° and 120° hybrid coupler based systems in an RoF systems. Fiber dispersion has also been considered in the analysis. The influence of the input RF power and modulation frequency have also been investigated to understand and maximize the overall system performance. For OSSB signals, the fundamental frequency components are found tolerant against dispersion while IM3 signal fluctuates with a 5.99 dB range for 90° hybrid coupler and 4.75 dB range for 120° hybrid coupler based systems. An improvement of 4.86 dB in SNDR is observed by using 120° hybrid coupler based system over conventional 90° hybrid coupler based system in a noise-dominant situation and an improvement of 6.02 dB is observed in SNDR by 120° hybrid coupler based system over conventional 90° hybrid
Fig. 1.3. SNDR, the fundamental, and IM3 power variations as a function of the frequency of the input RF signal modulated by the OSSB incorporating a 10-dB m input signal power and a 20-km fiber in a distortion-dominant situation when the IM3 is much higher than noise level.
34
P. Kumar et al. / Optics Communications 376 (2016) 30–34
Fig. 1.4. SNDR as a function of the power and frequency of the input RF signal modulated by OSSB incorporating a 20-km fiber.
coupler based system in an intermodulation distortion dominant situation. Also, a reduction of 1.25 dB m in IM3 is observed by 120° hybrid coupler based system over conventional 90° hybrid coupler based system in both noise and intermodulation distortion dominant situation Thus, Utilization of 120° hybrid coupler would greatly improve the performance of RoF links where the intermodulation distortion needs to be controlled.
References [1] V.A. Thomas, M. El-Hajjar, L. Hanzo, Performance improvement and cost reduction techniques for radio over fiber communications, IEEE Commun. Surv. tutor. 17 (2) (2015) 627–670. [2] H. Ogawa, D. Polifko, S. Banda, Millimeter-wave fiber optics systems for personal radio communication, IEEE Trans. Microw. Theory Tech. 40 (12) (1992) 2285–2292. [3] J.L. Corral, J. Marti, J.M. Fuster, General expressions for IM/DD dispersive analog optical links with external modulation or optical upconversion in a Mach–Zehnder electrooptical modulator, IEEE Trans. Microw. Theory Tech. 49 (10) (2001) 1968–1976. [4] H.J. Zhou, W. Chen, Z. Meng, Optical single sideband-frequency generation with carrier totally suppressed for Brillouin distributed fiber sensing, Opt. Commun. 285 (2012) 4391–4394. [5] A. Faniuolo, G. Tartarini, P. Bassi, Effects of directly modulated laser chirp on the performance of radio over fiber systems, in: Proceedings of the International Topical meeting on Microwave Photonics, 2003, 〈http://dx.doi.org/10. 1109/MWP.2003.1422900〉.
[6] G.H. Smith, D. Novak, Overcoming chromatic-dispersion effects in fiberwireless systems incorporating external modulators, IEEE Trans. Microw. Theory Tech. 45 (8) (1997) 1410–1415. [7] S. Singla, S.K. Arya, Simulative investigation for third order-IM terms in multitone RoF system, Int. J. Light Electron Opt: Opt. 125 (14) (2014) 3756–3758. [8] T.S. Cho, K. Kim, Optimization of radio-on-fiber systems employing ODSB signals by utilizing a dual electrode Mach-Zehnder modulator against IM3, IEEE Photonics Technol. Lett. 18 (9) (2006) 1076–1078. [9] S.K. Kim, W. Liu, Q.L. Pei, H. Fetterman, Nonlinear intermodulation distortion suppression in coherent analog fiber optic link using electro-optic polymeric dual parallel Mach-Zehnder modulator, Opt. Expr. 19 (8) (2011) 7865–7871. [10] F. Assadourian, Intermodulation distortion and efficiency analysis of multicarrier repeaters, IEEE Trans. Commun. 8 (1) (1960) 68–71. [11] T. Akiyama, K. Inagaki, Y. Mizuguchi, Intermodulation distortion by PD in heterodyne detection fibre-optic millimetre-wave links, Electron. Lett. 35 (17) (1999) 1478–1480. [12] T.S. Cho, B. Kwon, Performance optimization of radio-on-fiber systems employing erbium doped fiber amplifier for optical single sideband signals considering intermodulation distortion, Photonic Netw. Commun. 25 (2013) 73–78. [13] G.P. Agrawal, Nonlinear Fiber Optics, Academic, San Diego, CA, 1989. [14] M. Xue, S. Pan, Y. Zhao, Optical single-sideband modulation based on a dualdrive MZM and a 120° hybrid coupler, J. Lightwave Technol. 32 (19) (2014) 3317–3323. [15] P. Kumar, S.K. Sharma, S. Singla, Performance analysis of an OSSB RoF link using 90° and 120° hybrid coupler, Opt. Commun. (2015), http://dx.doi.org/ 10.1016/j.optcom.2015.09.081i, ISSN: 0030-4018. [16] P. Kumar, S.K. Sharma, S. Singla, Performance improvement of RoF transmission link by using 120 degree hybrid coupler in OSSB generation, Wirel. Netw. Springer J. (2015), http://dx.doi.org/10.1007/s11276-015-1123-9, ISSN: 15728196 (electronic version).