A novel approach for simultaneous millimeter wave generation and high bit rate data transmission for Radio over Fiber (RoF) systems

Optik 125 (2014) 6347–6351

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A novel approach for simultaneous millimeter wave generation and high bit rate data transmission for Radio over Fiber (RoF) systems M. Baskaran ∗ , M. Ganesh Madhan Department of Electronics Engineering, Madras Institute of Technology Campus, Anna University, Chennai 600044, India

a r t i c l e

i n f o

Article history: Received 11 October 2013 Accepted 27 May 2014 Keywords: Radio over Fiber (RoF) Stimulated Brillouin Scattering (SBS) Optical millimeter (mm)-wave Mach–Zehnder modulator Laser diode

a b s t r a c t We propose a novel method for simultaneous transmission of OC-192 (9.95328 Gbps) digital data and 60 GHz RF generation in a Standard Single Mode Fiber (SSMF) link utilizing Stimulated Brillouin Scattering (SBS). The system comprises of a 1550 nm DFB Laser diode, Mach Zehnder modulator (MZM), 50 km SSMF and Optical receiver. The receiver includes laser diode for optical pump, a regenerator for data retrieval and a RF bandpass filter for RF generation. This system requires minimum number of RF and optical components for the generation of 60 GHz RF. The remotely generated 60 GHz RF signal may be used for wireless transmission of data. The entire link is simulated in Optisystem software to analyze the system performance. © 2014 Elsevier GmbH. All rights reserved.

1. Introduction The need for broadband wireless network for multimedia traffic is ever increasing. Radio over Fiber (RoF) has received considerable interest in recent years, as it provides RF transmission for broadband wireless communications, phased array antenna, millimeter wave (MMW) imaging [1–3], and radar applications. Future wireless networks require microwave signals in the range of 60 GHz as the transmission medium [4]. For example, the Federal Communication Commission (FCC) has given an uncontiguous 7 GHz spectrum ranging from 57 to 63 GHz (60 GHz band), as a license free band. The transmission of RF signal over fiber is preferable because of the enormous bandwidth that offered by optical fiber which is up to few THz [5]. Different services such as high definition video, high bit rate data could be transferred via a single fiber link as digital data or modulated RF signal. The high optical bandwidth also enables high speed signal processing to be implemented. Optical techniques for microwave generation have gained importance in the recent days, due to the requirement of low cost and simple remote access point, for Pico and micro cells applications. Optoelectronic oscillators are commonly used for microwave generation [6]. Other schemes include mode locked laser based optical comb generator which is capable of generating microwave signals of high spectral purity in the frequency range of 1–10 GHz [7,8].

∗ Corresponding author. E-mail addresses: [email protected] (M. Baskaran), [email protected] (M. Ganesh Madhan). http://dx.doi.org/10.1016/j.ijleo.2014.07.012 0030-4026/© 2014 Elsevier GmbH. All rights reserved.

Mode-locking techniques offer low phase noise signal but make frequency tuning difficult. Further, Optical heterodyning schemes are also used for microwave signal generation [9,10]. Stimulated Brillouin Scattering (SBS) has been mostly recognized as one of the causes degrading the system performance in fiber-optic networks, due to the effect that the signal energy is transferred to the backscattering signal. It manifests through the generation of a backward propagating Stokes’ wave that carries most of the input energy, once the Brillouin threshold is reached. Therefore, SBS imposes limitations on the amount of optical power that can be launched into the fiber without degrading the signal quality. The detrimental effects of SBS has been extensively studied in digital and sub carrier-modulated transmission systems [11]. However, SBS has some beneficial characteristics such as frequency selective amplification that can be utilized in microwave and millimeter wave photonics applications. Recently Nawawi and Idrus reported a scheme of 40 GHz RF generation using SBS [12]. In their approach, the Non Linear characteristics of MZM have been utilized to derive the harmonics of the given RF sinusoidal signal. A 10 GHz signal source has been used to provide fundamental RF for the MZM. The required RF signal should be one of the harmonics resulting from MZM nonlinearity, which is generated by amplifying it using SBS process in the nonlinear fiber link. A high powered pump source, oscillating at a frequency 11 GHz higher than the required RF sideband is utilized at the receiver. Gross et al demonstrated the generation of microwave and millimeter wave by heterodyning the output modes of a dual-wavelength fiber laser based on Stimulated Brillouin Scattering [13]. Optical heterodyning of two lasers offers a simple

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M. Baskaran, M. Ganesh Madhan / Optik 125 (2014) 6347–6351

Fig. 1. RF generation scheme in a nonlinear fiber network.

configuration, but the phase noise characteristics of the generated signal is poor due to the mismatching of the phases of the two freerunning lasers. Shee et al. reported the generation of a millimeter wave carrier from a multi wavelength Brillouin-erbium fiber laser (BEFL) [14]. Jiaxin Ma et al. explained the generation of a 60 GHz millimeter wave using 20 GHz double side band (DSB) optical mmwave [15]. A 64 GHz optical millimeter (mm)-wave was generated via a nested LiNbO3 Mach–Zehnder modulator and a 8 GHz local oscillator [16]. Techniques for 60 GHz dual-tone optical millimeter (mm) wave generation are reported by quadrupling a radiofrequency local oscillator via single dual-electrode Mach–Zehnder modulator [17]. Most of the schemes reported till now are either complex or require a RF source at the transmitter for remote generation of RF signal. In this paper, we propose a novel scheme where the RF signal is generated in the receiver, due to high data rate modulation and SBS. This approach does not require RF source. The proposed RF generation system is shown in Fig. 1. In this scheme, we transmit OC-192 standard, 9.953 Gbps NRZ data and generate RF (FRF ) signal, which is derived from the frequency component of the data signal. The pump signal frequency is adjusted, so that the gain due to SBS exactly matches the required frequency given by ωRF + ωLD + 11 GHz. By this scheme, the required RF sideband is amplified and detected at the receiver. This approach generates RF signal only during the presence of digital data thereby increasing the efficiency of the system. The data is decoded in the receiver with the help of a data regeneration unit, since the data pulse would have undergone considerable distortion due to elimination of its frequency component. The regenerated 9.953 Gbps digital data and RF signal are available in the receiver simultaneously. Data can be further transmitted through wireless channel after appropriate modulation. The unique feature of the scheme is that it requires very less components when compared to the conventional optical based microwave generation schemes. 2. Principle

Fig. 2. PN sequence at 9.953 Gbps in different generation modes. Table 1 PRBS generation modes. S. no.

PN sequence generator mode

Power at 60 GHz (dBm)

1. 2.

Order (n = 2–30) Probability of number of ones in the PN sequence (0.1–0.9) Alternate ones and zeros

−46.172 to −38.864 −50.16 to −38.007

3.

Tw is the global parameter (Time window) and Br is the bit rate parameter. The number of bits generated is NG . nl and nt represent number of leading zeros and the number of trailing zeros. A simulation is carried using Optisystem software, where 9.953 Gbps data is generated and its corresponding frequency spectrum is plotted in Fig. 2. Different PN sequence generation modes are provided and the power at 60 GHz is found out and tabulated in Table 1. From this theory and simulations, it evident that significant amount of RF power is present at 60 GHz due to digital data at 9.953 Gbps. This result in the availability of 60 GHz RF component in the sidebands of optical carrier after external modulation. External modulators are necessary for millimeter-wave RoF transmission since the modulation cut-off frequencies of commercial lasers are below 10s of GHz in the case of direct modulation. Two types of external modulators are commonly used in various millimeterwave RoF systems. They are Mach–Zehnder modulator (MZM) and Electro Absorption Modulator (EAM). We consider a MZM in this study. The output optical intensity of a MZM is given by [19]



Popt (t) = Popt,in 1 −

The principle behind this approach in the combined effect of harmonics in digital modulation and simultaneous amplification using SBS gain in SMF link. From the signals theory, it is well known that for a data coded in NRZ format of rate N Gbps the nulls occur at N GHz and its harmonic. In the case of 10 Gbps standards transmission, the actual data rate is 9.953 Gbps and the nulls corresponds to frequency of 9.953 GHz and its harmonics. This can be observed in the spectrum of the data signal however spectral components corresponding to 10 GHz and its harmonics are not eliminated. Hence, it is clear that spectral components of a NRZ data signal are translated around the optical carrier frequency, when it modulates an optical carrier. As a first step, harmonics of a Pseudo Noise (PN) sequence at 9.953 Gbps is examined under different modes. These bit sequences represent the random data. A Pseudo Random Binary Sequence (PRBS) generator generates a sequence of N bits [18]: N = Tw Br

(1)

NG = N−ni−nt

(2)

−48.745

∞ 



k

(−1) j2k + 1(m) cos(2k + 1)wc t

(3)

k=0

√ where m = 2uo /v ; uo is the rms amplitude of the modulating signal and v is the voltage half wave arm. In single mode optical fibers, the SBS signal is downshifted by a frequency of 11 GHz from the pump signal. The backscattered power is expressed by the following equation [20].



Is (0) = Is (L) exp Leff =

Po gB Leff

1 − exp(−˛L) ˛

Aeff



− ˛L

(4)

(5)

where L is length of the fiber, gB is the Brillouin gain, Po is the input power, Aeff is the effective area of the SMF and ˛ is the attenuation of the fiber. This SBS effect along the fiber is used to amplify the required sideband signal. The frequency selective amplification of the 60 GHz sideband is ensured by adjusting the pump frequency so that the SBS gain is maximum for the 60 GHz sideband. The photo

M. Baskaran, M. Ganesh Madhan / Optik 125 (2014) 6347–6351

Fig. 3. Detailed block diagram of SBS based millimeter wave generation.

Fig. 4. The optical spectrum after 50 km fiber link.

detector generates the RF signal with the aid of band pass filter. This generated RF can be used for further wireless transmission. A 1550 nm DFB laser diode with linewidth of 10 MHz is used as the optical source. A Mach Zender Modulator ensures modulation of the optical carrier by a 9.953 Gbps NRZ data source. The bias point of the MZM is adjusted so that its nonlinear effects are minimized. The optical spectral output of the MZM shows the optical carrier frequency and harmonics due to pulse modulation. These signals are injected into a 50 km long standard single mode optical fiber (SMF) link. The output from the optical fiber is fed to port 1 of a 3-port circulator and a pump laser source is provided at the third port for creating SBS gain in the SMF. The output of the circulator is obtained at port 2, which is fed to the photo detector. The electrical signal is obtained and passed through a RF band pass filter with bandwidth of 5 GHz and center frequency of 60 GHz. A regenerator is utilized to generate the digital data to its original form. Thus the retrieved data can be used for further data transmission.

Fig. 5. RF power spectrum at 18 dBm pump power.

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3. Simulation results The entire link shown in Fig. 3, is implemented in Optisystem software and simulated. The simulation parameters are tabulated in Table 2. The length of fiber and pump power at the receiver side are fixed as 50 km and 18 dBm respectively. The received optical spectrum after the fiber link is shown in Fig. 4. The sidebands corresponding to the digital data and the amplified sideband are clearly visible. The RF Spectrum (Fig. 5) shows the output obtained after the received signal is passed through the band pass filter (BPF). The required RF signal at 60 GHz with a power of about 7.703 dBm is obtained for a pump power of 16 dBm. To analyze the generation scheme, the time domain waveform of the 60 GHz RF is plotted for two different pump levels i.e. 5 dBm and 20 dBm. From the results, it is observed that as the pump power Table 2 Simulation parameters. Parameter

Values

Bit rate Time window Sample rate Sequence length Samples per bit Number of samples Sensitivity Resolution Fiber length Optical power of CW laser Linewidth of CW laser Wavelength of CW laser Dispersion of optical fiber Attenuation of optical fiber Effective area of optical fiber, Aeff Brillouin gain, gB Brillouin linewidth Photodiode responsivity

9.95328 Gbps 1.28e−0.08 s 640 GHz 128 Bits 64 8192 −100 dBm 0.01 nm 50 km 2 dBm 10 MHz 1550 nm 16.75 ps/nm/km 0.2 dB/km 110 ␮m2 4.6e−11 m/W 20 MHz 0.5 A/W

Fig. 6. The magnitude of 60 GHz RF signal generated (a) 5 dBm and (b) 20 dBm.

is increased, the amplitude of the generated RF signal also increases. This behavior is shown in Fig. 6. The simulation is repeated for various values of pump powers and the generated RF power is determined. Fig. 7 shows the variations. It is observed that as the pump power crosses the SBS threshold RF power in the receiver increases exponentially. However beyond 10 dBm pump power the output RF power tends to saturate. Fig. 8 shows the 9.953 Gbps input data, distorted output of the 50 km nonlinear fiber link and the recovered data after the data

Fig. 7. RF power versus optical pump power.

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Fig. 8. (a) Input data (b) received data signal (c) retrieved signal after data recovery unit at 10 dBm pump power.

recovery unit. Simulation shows that the exact digital data could be retrieved at the receiver. The digital data undergoes significant distortion due to the effect of SBS amplification of a particular frequency component, which is the required for 60 GHz RF signal generation. This calls for a data recovery unit for overcoming the distortion introduced into the signal by the system. By this approach, the original data is recovered along with a 60 GHz RF carrier.

In order to evaluate the quality of digital transmission along with the RF generation, eye diagram of the link is evaluated under various pump powers. The eye pattern of the signal is obtained with BER analyzer option in the Optisystem tool. Fig. 9 shows the eye diagrams of the data recovered in the receiver before data recovery unit, which clearly demonstrates the suppression of the data pulse as the pump power increases linearly. The eye is almost completely closed above 20 dBm pump power, whereas the amplitude of 60 GHz RF signal reaches the maximum value as shown in Fig. 7. When the pump power is increased, the SBS gain becomes significant, and hence the amplitude of the generated RF carrier is increased. However it also leads to increase the distortion in the data, which results in the increase of BER of the system. Ultimately the quality of reception of the data becomes deteriorated. Thus the overall quality factor of the system is reduced significantly with increasing power levels. The above analysis is carried out before the data recovery unit. However the presence of data recovery unit significantly improves the data reception. It is found that until 15 dBm of optical pump power, the original data could be retrieved. Beyond that the data recovery unit fails. The simulation is repeated for the various values of pump power and the Bit Error Rate (BER) is calculated. It is perceived that as the pump power exceeds the SBS threshold, BER in the receiver increases exponentially. The BER performance with pump power is determined for various values of link distance and plotted in Fig. 10. It is observed that longer link distance increases the BER at lower pump powers. This is due the earlier onset of SBS threshold at higher link lengths. The SBS threshold reduction for longer length is in accordance with literature [20].

Fig. 9. Eye diagrams corresponding to optical pump power (a) −5 dBm, (b) 0 dBm, (c) 5 dBm, (d) 10 dBm, (e) 15 dBm and (f) 20 dBm.

M. Baskaran, M. Ganesh Madhan / Optik 125 (2014) 6347–6351

[3]

[4]

[5]

[6]

[7] [8] [9] Fig. 10. Bit Error Rate for various pump power. [10]

4. Conclusion We have demonstrated a novel scheme of millimeter wave generation for RoF system using SBS in SSMF without using electrical RF generator. This approach provides simultaneously transmission of high bit rate data and generation of 60 GHz RF signal at the receiver. The 9.953 Gbps data transmitted can be retrieved in the receiver overcoming the effects of SBS until 15 dBm pump power. The presence of data regenerator could improve the performance. The generated 60 GHz signal can be used for further downstream wireless transmission. This approach eliminates the need for a separate RF signal source which will be necessary otherwise. Also, the RF signal is generated only when the data is transmitted, resulting in significant electrical power reduction in the overall RF generation process.

[11]

[12]

[13]

[14]

[15]

[16]

[17]

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