Generation and transmission of 3 × 3 w-band multi-input multi-output orthogonal frequency division multiplexing-radio-over-fiber signals using micro-ring resonators S. E. Alavi,1,* I. S. Amiri,2 H. Ahmad,2 A. S. M. Supa’at,3 and N. Fisal1 1
UTM MIMOS CoE in Telecommunication Technology Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Malaysia 2
Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia 3
Lightwave Communication Research Group, Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia *Corresponding author: [email protected]
Received 22 July 2014; revised 29 September 2014; accepted 29 September 2014; posted 28 October 2014 (Doc. ID 217512); published 24 November 2014
Using the micro-ring resonator (MRR) system, the single and multi-carriers at frequencies of f 1 192.898, f 2 192.990, f 3 193.1, f 4 193.315, and f 5 193.537 THz with a free spectral range (FSR) of 92, 110, 215, and 222 GHz, respectively, are generated to be suitable for a radio-over-fiber (RoF) system based on multi-input multi-output (MIMO) with orthogonal frequency division multiplexing (OFDM). Demonstrated are the concepts of all-optical MIMO signal generation and its transmission over a 50 km single mode fiber (SMF) optical link and an up to 3 m wireless link. Sixty-four multi-carriers are used in the all-optical generation of three MIMO W-Band RF signals, where the single carriers (f 3 − f 5 ) transport the signals over the RoF link. The bit error rate (BER) of the overall system performance is discussed; thus, the transmission of MIMO signals is feasible for up to an SMF path 50 km long and a wireless distance of 3 m. OCIS codes: (060.2310) Fiber optics; (060.2330) Fiber optics communications. http://dx.doi.org/10.1364/AO.53.008049
1. Introduction
Currently, the 35 GHz spectrum at 100 GHz of WBand (75–110 GHz) is viewed as one of the promising paths toward multi-Gbps wireless services. Radioover-fiber (RoF) technology is deployed in wireless networks to provide the capacity for and quality of service, leading to a combination of the flexibility and mobility of wireless access networks with the capacity of optical networks [1]. A.
W-Band MIMO-OFDM-RoF
The multiple-input multiple-output (MIMO) configuration is another solution to increase the data rate that deploys the spatial diversity antenna systems
[2] and is widely used today in all wireless communication systems, such as WiMax (IEEE 802.16), long-term evolution (LTE), and fourth-generation cellular technology [3]. Accordingly, employing MIMO technology can effectively increase the spectral efficiency and the performance of the 100 GHz wireless communications as well. Therefore, the combination of RoF and MIMO can enhance system efficiency. Wireless signal transportation in the RoF-MIMO system encounters several impairments, such as the dispersion effects in the fiber link and the multipath fading in the wireless link [4]. The spectral efficient orthogonal frequency division multiplexing (OFDM) transmission can eliminate the intersymbol 1 December 2014 / Vol. 53, No. 34 / APPLIED OPTICS
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interference (ISI) caused by dispersive channels [5]. By converting the channel into several flat, fading subchannels, OFDM has the advantage of robustness over the wireless frequency-selective fading channels and has inherent high chromatic dispersion tolerance in optical fibers. Exploiting diversity in both the space and the frequency domains, the MIMO-OFDM combination could result in exceptional system performance [6]. By integrating MIMO-OFDM with RoF, it is possible to provide very spectrally efficient data transmission and, thereby, meet the high-speed requirements of future generations of wireless systems [7]. In MIMOOFDM-RoF communication systems, multiple antenna arrays are distributed around a micro/femto cell and connected to a central base station via an optical fiber [8]. In MIMO, several radio channels are transported between the central and remote units with the same radio carrier frequency [9]. Separate RoF signals with the same frequency can be transported on the same fiber although on separate optical wavelengths [4]. B.
All-Optical OFDM
The OFDM system includes an inverse fast Fourier transform (IFFT) block at the transmitter and a fast Fourier transform (FFT) block at the receiver where these blocks are usually implemented in the electrical domain enabled by high-speed digital-signalprocessing (DSP) devices. These devices, however, are challenging both commercially and technically. In this regard, all-optical techniques are getting more attention and are being investigated to reduce the challenges of the electrical domain. These techniques are based on the optical generation and processing of OFDM signals using passive optical devices [10,11]. Transmission of an all-optical OFDM is implemented first by generating multiple optical subcarriers, then separating them with optical devices and, last, modulating each subcarrier separately [12,13]. Therefore, optical carrier generation is the basic building block to implement OFDM transmission fully in the optical domain. Nonlinear light behavior inside a micro-ring resonator (MRR) occurs when a strong pulse of light is put into the ring system [14]. The properties of a ring system can be modified via various control methods [15]. Ring resonators can be used as filter devices, where high-frequency (THz) soliton signals can be generated using suitable system parameters [16]. The series of MRRs connected to an add/drop filter system is used in many applications in optical communication and signal processing to generate single and multi-carriers [17–19]. Therefore, the application of the MRR system in this work is twofold. First, MRR is utilized to generate multiple wavelengths for transporting several MIMO-RoF signals over a single fiber and, second, to generate multicarriers for the W-Band 3 × 3 MIMO wireless communication systems. The required wavelengths for the proposed MIMO-OFDM-RoF system architecture 8050
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are generated, and the overall view and concepts of all optical MIMO signal generation and its transmission over a single mode fiber (SMF) optical link are analyzed by simulation and modeling using experimental and practical parameters. To overcome RF power degradation because of fiber dispersion, the optical single sideband with a carrier (OSSB + C) modulation technique is implemented [20]. The bit error rate (BER) calculation of overall system performance is discussed last. 2. Theoretical Background
The system of THz frequency band generation is shown in Fig. 1. The bright soliton and Gaussian beam are introduced into the input and add ports of the system. MRRs are made up of a waveguide by fabrication technology. The III/V semiconductors (InGaAsP/InP) on the basis of an InP with a direct bandgap are used to fabricate the ring resonator. The filtering process of the input soliton pulses is performed via the MRRs, where frequency band ranges from 191 to 196 THz can be obtained. The input optical field (Ei1 ) of the bright soliton pulse and the add optical field (Ei2 ) of the Gaussian beam are given by Ei1 t A sech
T iz exp − iω0 t ; T0 2LD
Ei2 t E0 exp
iz − iω0 t : 2LD
(1)
(2)
A, E0 , and z are the optical field amplitudes and the propagation distance, respectively. T is the soliton pulse propagation time, T t − β1 × z, where β1 and β2 are the coefficients of the linear and second order terms of the Taylor expansion of the propagation constant. LD T 20 ∕jβ2 j is the dispersion length of the soliton pulse. The frequency shift of the soliton is ω0. When the soliton peak intensity jβ2 ∕ΓT 20 j is given, then T o is known. T o is the initial propagation time. For the soliton pulse, a balance should be achieved between the dispersion length (LD ) and the nonlinear length LNL 1∕ΓΦNL , hence
Fig. 1. Schematic diagram of a modified add/drop filter system connected to the MRRs.
LD LNL , where Γ n2 × k0 , is the length scale over which dispersion—or nonlinear effects—makes the beam become wider or narrower. The total refractive index of the system is given by n n0 n2 I n0
n2 P; Aeff
−αLL LL 4 −jkn 2
E3 EL × E2 × e
;
(7)
p −α 1 − γ × 1 − κ5 − 1 − γ × e 2 LL −jkn LL p −α ; EL E2 × 1 − 1 − γ × 1 − κ 5 × e 2 LL −jkn LL
(3)
(8)
2 αL αL 3 p 3∕2 p L 2 L 3 −2 2 −2 κ E x κ E − jk x y y × κ E − jk e e κ E 3 4 L i1 n 1 2 3 4 L i2 n 2 2 2 7 L 6 1 p αL αL Et1 −x2 y2 κ 3 Ei2 e− 2 − jkn 4 5; L 2 −2 2 1 − x y y E e − jk 2 1 2
Et2 x2 y2 Ei2
p p αL L L L x2 κ 3 κ 3 × κ4 EL Ei1 e− 2 −jkn2 x13∕2 y1 y2 κ4 EL Ei2 e−α2 −jkn 2 2 : αL L 1 − x2 y1 y2 EL e− 2 −jkn 2 2
where n0 and n2 are the linear and nonlinear refractive indices, respectively. I and P are the optical intensity and power, respectively. The effective mode core area is given by Aeff. The normalized output can be expressed as [21] Eout t2 E t 1 − γ in 2
3
2
1 − 1 − γx κ 5: × 41 − pp 2 pp 1 − x 1 − γ 1 − κ 4x 1 − γ 1 − κ sin2 φ2 (4) κ is the coupling coefficient, and x exp−αL∕2 represents a round-trip loss coefficient; the fractional coupler intensity loss is γ, φ φ0 φNL , where, φ0 kLn0 and φNL kLn2 jEin j2 are the linear and nonlinear phase shifts, k 2π∕λ is the wave number, and L and α are the waveguide length and linear absorption coefficient, respectively. Referring to Fig. 1, the equations of the modified add/drop system are given by the following [22,23]: E1
i p h p p 1 − γ × E3 × 1 − κ 3 j κ3 × Eout2 ;
(5)
i p p h p 1 − γ × E1 × 1 − κ 4 j κ4 × Ei2 ;
(6)
E2
Table 1.
Rad 153 μm κ5 0.7
(9)
n2
L
(10)
Et1 and Et2 are the through and drop port outputs of the add/drop system. Here, LL 2πRL , where RL is the radius of the ring on the left side of the add/drop within the ring. system, and p p EL is the electric field 1 − γ , x2 1 − γ, y1 1 − κ3 and y2 xp1 1 − κ4 and L 2πRad . The through and drop port outputs of the system result in generating the single and multi-carriers used for the W-Band MIMOOFDM-RoF system. 3. Results and Discussion
The fixed parameters of the system are listed in Table 1. As shown in Fig. 2(a), the input bright soliton and Gaussian beam with a power of 1 W are inserted into the system. The output intensity powers from R1 and R2 are shown in Figs. 2(b) and 2(c). The inner signals of the modified add/drop filter system are shown in Figs. 2(d)–2(f), where the soliton pulses range from 191 to 196 THz. As shown in Fig. 2(f), the few soliton pulses can be localized within the modified add/drop filter system. The output jE3 j2 passes through the coupler κ 3, where the throughput output expressed by jEt1 j2 shows localized ultrashort soliton pulses at frequencies of f 1 192.898, f 2 192.990, f 3 193.1, f 4 193.315, and f 5 193.537 THz with FSR of 92, 110, 215, and 222 GHz, respectively, shown in Fig. 2(g). The drop port output expressed by jEt2 j2 is shown in Fig. 2(h). Because of the short bandwidth input Gaussian beam in the add port of the system, the multi-soliton pulses ranging from 192.936 to
Fixed Parameters of the MRR System
R1
R2
RL
κ1
κ2
κ3
κ4
12 μm n0 3.34
8 μm n2 m2 W−1 2.2 × 10−17
4 μm Aeff 1 μm2 0.50
0.04 Aeff 2 μm2 0.25
0.7 Aeff L μm2 0.10
0.02 α dB mm−1 0.5
0.05 γ 0.1
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Fig. 2. Results of frequencies generation: (a) input, (b) output R1 W∕μm2 , (c) output R2 W∕μm2 , (d) jE1 j2 mW∕μm2 , (e) jE2 j2 mW∕μm2 , (f) jE3 j2 mW∕μm2 , (g) jEt1 j2 mW∕μm2 , (h) jEt2 j2 mW∕μm2 range from 192.936 to 193.044, (i) 64 multi-carriers generation range from 192.9725 to 193.0075, (j) flattened 64 multi-carriers using the GFF with FSR 546 MHz.
193.044 THz are generated. In this study, the range of 192.9725–193.0075 THz is selected and used to generate 64 multi-carriers at a 3 mW threshold, as shown in Fig. 2(i), which results in a 35 GHz bandwidth applicable to W-Band signal generation. Figure 2(j) shows the generated uniform carrier signals using the gain flattening filter (GFF) system. The schematic diagram of the W-Band, MIMOOFDM-RoF system is shown in Fig. 3. At the transmitter central office (TCO), the generated carriers (f 1 − f 5 ) are separated with a demultiplexer. Also, f 1 and f 2 are used to generate W-band signals, while the other three carriers (f 3 , f 4 , and f 5 ) are assigned for the optical modulating of three MIMO signals separately using Mach–Zehnder (MZM) optical modulators. The generated multi-carriers centered at f 2 192.990 THz, illustrated in Fig. 2(j), are used to generate an all-optical OFDM signal. In this regard, multi-carriers are first separated by the splitter, then 52 out of 64 are modulated using QAM. The
QAM signal is generated using a pulse pattern generator (PPG). Afterward, to imitate the IFFT block at the transmitter and an FFT at the receiver, an array waveguide grating (AWG) is used [13]. Spectra of the modulated optical subcarriers are overlapped using the AWG, which results in one optical OFDM channel band, shown in Fig. 4(a). Then the generated optical OFDM signal is multiplexed by the wavelength located at f 1 . The distance between the single subcarrier f 1 and the center of the generated optical OFDM (f 2 ) is 92 GHz, which is the RF region for W-Band transmission. After beating the photo diode, the W-Band RF OFDM signal, shown in Fig. 4(b), is generated. Using the RF coupler with a coupling factor of 1∶3, the OFDM signal is divided to three equal WBand MIMO signals (X 1 , X 2 , and X 3 ) with the same RF carrier frequency as f RF f 2 − f 1 92 GHz. Transporting the W-Band MIMO signals over a single fiber will encounter severe power degradation because of fiber chromatic dispersion; therefore, the optical single side band (OSSB)+Carrier modulation technique [20] is implemented using the Mach– Zehnder modulators (MZMs) to modulate the MIMO signals (X 1 , X 2 , and X 3 ) on the three assigned carriers f 3 –f 5, respectively, as illustrated in Fig. 4(c). The optical band-pass filters (BPF) are used to suppress one sideband of each modulated signal to generate OSSB + C signals. To transmit the MIMO signals, the lower side band (LSB) of X 2 and X 3 and the upper side band (USB) of X 1 are filtered using the BPF. Therefore, the LSB of X 1 and the USB of X 2 and X 3 are multiplexed, and the result is shown in Fig. 4(d). Now, the multiplexed signals are amplified by an Erbium doped fiber amplifier (EDFA), and are transmitted through the SMF to the transmitter antenna base station (TABS). The TABS is composed of a demultiplexer, PIN photodetectors (PDs) with 0.7 A∕W responsivity, electrical band pass filters (E-BPFs), amplifiers, and multiple transmitter antennas (Tx1, Tx2, and Tx3) for MIMO applications.
Fig. 3. System setup. 8052
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Fig. 5. (a) Received W-Band signal spectrum, (b) Downconverted W-Band signal for two optical links as B2B and 50 km.
Fig. 4. (a) All optical OFDM channel band, (b) W-Band OFDM signal spectra, (c) X 1 , X 2 , X 3 , optical modulated spectrum, (d) Multiplexed spectrum of X 1 -LSB and X 2 -USB and X 3 -USB, PSD: power spectral density.
At the TABS, the optical downstream is demultiplexed to the three signals and, afterward, three modulated optical signals are converted directly to electrical signals (X 11 , X 12 , and X 13 ) using the PDs. Based on the allocated RF frequency, the electrical signals are filtered using the BPFs. Next, the MIMO OFDM signals (X 11 , X 12 , and X 13 ), which are the same as the transmitted signals (X 1 , X 2 , and X 3 ) with little noise, will be propagated wirelessly using W-band 3 × 3 MIMO T x antennas. The 3 × 3 MIMO Array antenna has been designed to operate at 92 GHz. It must be noted that the array type of antenna is chosen to enhance the gain values. In addition, it was found that the MIMO array antenna covers a footprint of 45 mm × 45 mm, radiates a fixed beam in the boresight direction, and achieves the 10dB impedance bandwidth of 35.7 GHz from 74.3 to 110 GHz with the maximal realized gain of 23.66 dBi at 92.5 GHz. Here, the W-Band RF signals are propagated through the MIMO channel, and they are received by wireless end users. For instance, Fig. 5(a) has shown the power spectrum density (PSD) of the Tx1-Rx1 wireless link. At the receiver antenna base station, the amplified received RF signal is upconverted using a distributed feedback (DFB) laser to process the received signal optically. The upconverted signal is transmitted to the receiver central office (RCO) through 2 m SMF. At RCO, the AWG is used to implement the FFT function optically. The demodulation is performed, and the BER
is calculated. The spectrum of the down-converted W-Band signal for two optical links as B2B and 50 km, while the wireless distance was fixed at 3 m, also shown in Fig. 5(b). Figure 5(a) shows the amplified version of the received W-Band signal spectrum versus the transmitted signal for Tx1-Rx1 antennas over a wireless link. In this figure, we have shown the amplified version of the received signal that is going to be processed in the RCO (RCO). Figure 5(b) also shows the down-converted W-Band signal for two optical links as B2B and 50 km. As shown in this figure, the baseband down-converted OFDM signal has approximately a 17.5 GHz band that could be applied to a W-Band short range communication link. The performance of the system setup was tested based on two optical lengths that were back to back (B2B) and 50 km. As per the FEC-limit, the 3.8 × 10−3 of the BER [shown with a dashed line in Figs. 6(a)
Fig. 6. (a) BER, eye and constellation diagram of 3 × 3 MIMORoF with B2B optical link and (b) with 50 km optical link. 1 December 2014 / Vol. 53, No. 34 / APPLIED OPTICS
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and 6(b)] is the threshold for a successful transmission. In the case of B2B, as shown in Fig. 6, the total optical power level after amplification is adjusted with a variable optical attenuator (VOA). It is clear for different wireless transmitters (Tx1, Tx2, Tx3, MIMO) that there is an acceptable BER variation for a 3 m MIMO wireless distance. Therefore, the transmission of MIMO W-Band signals is feasible for a B2B optical link and a wireless distance of 3 m. Further investigation of system performance is conducted based on a 50 km optical link. As illustrated in Fig. 6(b), the system performance in four circumstances is investigated, which are individual MIMO streams (Tx1, Tx2, and Tx3) and the combined MIMO signal (Tx1 Tx2 Tx3). At the optical power related to the BER threshold, the eye and constellation diagram of this optical link confirms acceptable signal transmission and recovery. In conclusion, a series of MRRs incorporating a modified add/drop system is used to generate a W-Band RF signal. Single and multi-carriers optical soliton pulses are generated. The range of 192.9725 to 193.0075 THz is selected to generate 64 multicarriers with a specific FSR of 546 MHz, which are used in all optical generation of the three MIMO W-Band RF signals. The single carriers are used in the optical transportation of the three separate MIMO signals over an RoF link. The overall view and concepts of all optical MIMO signal generation and its transmission over a 50 km SMF optical link and up to 3 m 3 × 3 MIMO wireless link was analyzed thoroughly. The authors express their gratitude to the Ministry of Higher Education (MOHE), Malaysia and Research Management Center (RMC) of Universiti Teknologi Malaysia (UTM) for the financial support of this project under Research Grant No. R.J130000.7823.4L145. I. S. Amiri and H. Ahmad acknowledge the financial support from the University Malaya/MOHE under Grant Number UM.C/625/1/HIR/MOHE/SCI/29.
6. 7.
8.
9. 10.
11. 12.
13.
14.
15. 16. 17.
18. 19.
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using a reflective semiconductor optical amplifier and single‐arm Mach–Zehnder modulator,” Microw. Opt. Technol. Lett. 54, 1954–1958 (2012). G. L. Stuber, J. R. Barry, S. W. Mclaughlin, Y. Li, M. A. Ingram, and T. G. Pratt, “Broadband MIMO-OFDM wireless communications,” Proc. IEEE 92, 271–294 (2004). L. Deng, X. Pang, Y. Zhao, M. B. Othman, J. B. Jensen, and D. Zibar, “2 × 2 MIMO-OFDM Gigabit fiber-wireless access system based on polarization division multiplexed WDM-PON,” Opt. Express 20, 4369–4375 (2012). T. Yamakami, T. Higashino, K. Tsukamoto, and S. Komaki, “An experimental investigation of applying MIMO to RoF ubiquitous antenna system,” in Microwave Photonics (IEEE, 2008), pp. 201–204. C.-X. Wang, X. Hong, X. Ge, X. Cheng, G. Zhang, and J. Thompson, “Cooperative MIMO channel models: a survey,” IEEE Commun. Mag. 48(2), 80–87 (2010). Y. Benlachtar, P. M. Watts, R. Bouziane, P. Milder, D. Rangaraj, and A. Cartolano, “Generation of optical OFDM signals using 21.4 GS/s real time digital signal processing,” Opt. Express 17, 17658–17668 (2009). Q. Yang, S. Chen, Y. Ma, and W. Shieh, “Real-time reception of multi-gigabit coherent optical OFDM signals,” Opt. Express 17, 7985–7992 (2009). H. Chen, M. Chen, and S. Xie, “All-optical sampling orthogonal frequency-division multiplexing scheme for high-speed transmission system,” J. Lightwave Technol. 27, 4848–4854 (2009). Z. Wang, K. S. Kravtsov, Y.-K. Huang, and P. R. Prucnal, “Optical FFT/IFFT circuit realization using arrayed waveguide gratings and the applications in all-optical OFDM system,” Opt. Express 19, 4501–4512 (2011). M. Spyropoulou, N. Pleros, and A. Miliou, “SOA-MZI-based nonlinear optical signal processing: a frequency domain transfer function for wavelength conversion, clock recovery, and packet envelope detection,” IEEE J. Quantum Electron. 47, 40–49 (2011). I. S. Amiri, R. Ahsan, A. Shahidinejad, J. Ali, and P. P. Yupapin, “Characterisation of bifurcation and chaos in silicon microring resonator,” IET Commun. 6, 2671–2675 (2012). S. Lin and K. B. Crozier, “Planar silicon microrings as wavelength-multiplexed optical traps for storing and sensing particles,” Lab Chip 11, 4047–4051 (2011). I. S. Amiri, A. Nikoukar, and J. Ali, “GHz frequency band soliton generation using integrated ring resonator for WiMAX optical communication,” Opt. Quantum Electron. 46, 1165– 1177 (2013). I. S. Amiri, J. Ali, and P. P. Yupapin, “Enhancement of FSR and finesse using add/drop filter and PANDA ring resonator systems,” Int. J. Mod. Phys. B 26, 1250034 (2012). I. S. Amiri, S. E. Alavi, and J. Ali, “High capacity soliton transmission for indoor and outdoor communications using integrated ring resonators,” Int. J. Commun. Syst. (to be published). C. Hong, C. Zhang, M. Li, L. Zhu, L. Li, and W. Hu, “Singlesideband modulation based on an injection-locked DFB laser in radio-over-fiber systems,” IEEE Photon. Technol. Lett. 22, 462–464 (2010). S. Hanim, J. Ali, and P. Yupapin, “Dark soliton generation using dual Brillouin fiber laser in a fiber optic ring resonator,” Microw. Opt. Technol. Lett. 52, 881–883 (2010). I. S. Amiri, S. E. Alavi, S. M. Idrus, A. Nikoukar, and J. Ali, “IEEE 802.15.3c WPAN standard using millimeter optical soliton pulse generated by a panda ring resonator,” IEEE Photon. J. 5, 7901912 (2013). S. E. Alavi, I. S. Amiri, S. M. Idrus, A. S. M. Supa’at, J. Ali, and P. P. Yupapin, “All optical OFDM generation for IEEE802.11a based on soliton carriers using microring resonators,” IEEE Photon. J. 6, 1 (2014).
UTM MIMOS CoE in Telecommunication Technology Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Malaysia 2
Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia 3
Lightwave Communication Research Group, Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia *Corresponding author: [email protected]
Received 22 July 2014; revised 29 September 2014; accepted 29 September 2014; posted 28 October 2014 (Doc. ID 217512); published 24 November 2014
Using the micro-ring resonator (MRR) system, the single and multi-carriers at frequencies of f 1 192.898, f 2 192.990, f 3 193.1, f 4 193.315, and f 5 193.537 THz with a free spectral range (FSR) of 92, 110, 215, and 222 GHz, respectively, are generated to be suitable for a radio-over-fiber (RoF) system based on multi-input multi-output (MIMO) with orthogonal frequency division multiplexing (OFDM). Demonstrated are the concepts of all-optical MIMO signal generation and its transmission over a 50 km single mode fiber (SMF) optical link and an up to 3 m wireless link. Sixty-four multi-carriers are used in the all-optical generation of three MIMO W-Band RF signals, where the single carriers (f 3 − f 5 ) transport the signals over the RoF link. The bit error rate (BER) of the overall system performance is discussed; thus, the transmission of MIMO signals is feasible for up to an SMF path 50 km long and a wireless distance of 3 m. OCIS codes: (060.2310) Fiber optics; (060.2330) Fiber optics communications. http://dx.doi.org/10.1364/AO.53.008049
1. Introduction
Currently, the 35 GHz spectrum at 100 GHz of WBand (75–110 GHz) is viewed as one of the promising paths toward multi-Gbps wireless services. Radioover-fiber (RoF) technology is deployed in wireless networks to provide the capacity for and quality of service, leading to a combination of the flexibility and mobility of wireless access networks with the capacity of optical networks [1]. A.
W-Band MIMO-OFDM-RoF
The multiple-input multiple-output (MIMO) configuration is another solution to increase the data rate that deploys the spatial diversity antenna systems
[2] and is widely used today in all wireless communication systems, such as WiMax (IEEE 802.16), long-term evolution (LTE), and fourth-generation cellular technology [3]. Accordingly, employing MIMO technology can effectively increase the spectral efficiency and the performance of the 100 GHz wireless communications as well. Therefore, the combination of RoF and MIMO can enhance system efficiency. Wireless signal transportation in the RoF-MIMO system encounters several impairments, such as the dispersion effects in the fiber link and the multipath fading in the wireless link [4]. The spectral efficient orthogonal frequency division multiplexing (OFDM) transmission can eliminate the intersymbol 1 December 2014 / Vol. 53, No. 34 / APPLIED OPTICS
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interference (ISI) caused by dispersive channels [5]. By converting the channel into several flat, fading subchannels, OFDM has the advantage of robustness over the wireless frequency-selective fading channels and has inherent high chromatic dispersion tolerance in optical fibers. Exploiting diversity in both the space and the frequency domains, the MIMO-OFDM combination could result in exceptional system performance [6]. By integrating MIMO-OFDM with RoF, it is possible to provide very spectrally efficient data transmission and, thereby, meet the high-speed requirements of future generations of wireless systems [7]. In MIMOOFDM-RoF communication systems, multiple antenna arrays are distributed around a micro/femto cell and connected to a central base station via an optical fiber [8]. In MIMO, several radio channels are transported between the central and remote units with the same radio carrier frequency [9]. Separate RoF signals with the same frequency can be transported on the same fiber although on separate optical wavelengths [4]. B.
All-Optical OFDM
The OFDM system includes an inverse fast Fourier transform (IFFT) block at the transmitter and a fast Fourier transform (FFT) block at the receiver where these blocks are usually implemented in the electrical domain enabled by high-speed digital-signalprocessing (DSP) devices. These devices, however, are challenging both commercially and technically. In this regard, all-optical techniques are getting more attention and are being investigated to reduce the challenges of the electrical domain. These techniques are based on the optical generation and processing of OFDM signals using passive optical devices [10,11]. Transmission of an all-optical OFDM is implemented first by generating multiple optical subcarriers, then separating them with optical devices and, last, modulating each subcarrier separately [12,13]. Therefore, optical carrier generation is the basic building block to implement OFDM transmission fully in the optical domain. Nonlinear light behavior inside a micro-ring resonator (MRR) occurs when a strong pulse of light is put into the ring system [14]. The properties of a ring system can be modified via various control methods [15]. Ring resonators can be used as filter devices, where high-frequency (THz) soliton signals can be generated using suitable system parameters [16]. The series of MRRs connected to an add/drop filter system is used in many applications in optical communication and signal processing to generate single and multi-carriers [17–19]. Therefore, the application of the MRR system in this work is twofold. First, MRR is utilized to generate multiple wavelengths for transporting several MIMO-RoF signals over a single fiber and, second, to generate multicarriers for the W-Band 3 × 3 MIMO wireless communication systems. The required wavelengths for the proposed MIMO-OFDM-RoF system architecture 8050
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are generated, and the overall view and concepts of all optical MIMO signal generation and its transmission over a single mode fiber (SMF) optical link are analyzed by simulation and modeling using experimental and practical parameters. To overcome RF power degradation because of fiber dispersion, the optical single sideband with a carrier (OSSB + C) modulation technique is implemented [20]. The bit error rate (BER) calculation of overall system performance is discussed last. 2. Theoretical Background
The system of THz frequency band generation is shown in Fig. 1. The bright soliton and Gaussian beam are introduced into the input and add ports of the system. MRRs are made up of a waveguide by fabrication technology. The III/V semiconductors (InGaAsP/InP) on the basis of an InP with a direct bandgap are used to fabricate the ring resonator. The filtering process of the input soliton pulses is performed via the MRRs, where frequency band ranges from 191 to 196 THz can be obtained. The input optical field (Ei1 ) of the bright soliton pulse and the add optical field (Ei2 ) of the Gaussian beam are given by Ei1 t A sech
T iz exp − iω0 t ; T0 2LD
Ei2 t E0 exp
iz − iω0 t : 2LD
(1)
(2)
A, E0 , and z are the optical field amplitudes and the propagation distance, respectively. T is the soliton pulse propagation time, T t − β1 × z, where β1 and β2 are the coefficients of the linear and second order terms of the Taylor expansion of the propagation constant. LD T 20 ∕jβ2 j is the dispersion length of the soliton pulse. The frequency shift of the soliton is ω0. When the soliton peak intensity jβ2 ∕ΓT 20 j is given, then T o is known. T o is the initial propagation time. For the soliton pulse, a balance should be achieved between the dispersion length (LD ) and the nonlinear length LNL 1∕ΓΦNL , hence
Fig. 1. Schematic diagram of a modified add/drop filter system connected to the MRRs.
LD LNL , where Γ n2 × k0 , is the length scale over which dispersion—or nonlinear effects—makes the beam become wider or narrower. The total refractive index of the system is given by n n0 n2 I n0
n2 P; Aeff
−αLL LL 4 −jkn 2
E3 EL × E2 × e
;
(7)
p −α 1 − γ × 1 − κ5 − 1 − γ × e 2 LL −jkn LL p −α ; EL E2 × 1 − 1 − γ × 1 − κ 5 × e 2 LL −jkn LL
(3)
(8)
2 αL αL 3 p 3∕2 p L 2 L 3 −2 2 −2 κ E x κ E − jk x y y × κ E − jk e e κ E 3 4 L i1 n 1 2 3 4 L i2 n 2 2 2 7 L 6 1 p αL αL Et1 −x2 y2 κ 3 Ei2 e− 2 − jkn 4 5; L 2 −2 2 1 − x y y E e − jk 2 1 2
Et2 x2 y2 Ei2
p p αL L L L x2 κ 3 κ 3 × κ4 EL Ei1 e− 2 −jkn2 x13∕2 y1 y2 κ4 EL Ei2 e−α2 −jkn 2 2 : αL L 1 − x2 y1 y2 EL e− 2 −jkn 2 2
where n0 and n2 are the linear and nonlinear refractive indices, respectively. I and P are the optical intensity and power, respectively. The effective mode core area is given by Aeff. The normalized output can be expressed as [21] Eout t2 E t 1 − γ in 2
3
2
1 − 1 − γx κ 5: × 41 − pp 2 pp 1 − x 1 − γ 1 − κ 4x 1 − γ 1 − κ sin2 φ2 (4) κ is the coupling coefficient, and x exp−αL∕2 represents a round-trip loss coefficient; the fractional coupler intensity loss is γ, φ φ0 φNL , where, φ0 kLn0 and φNL kLn2 jEin j2 are the linear and nonlinear phase shifts, k 2π∕λ is the wave number, and L and α are the waveguide length and linear absorption coefficient, respectively. Referring to Fig. 1, the equations of the modified add/drop system are given by the following [22,23]: E1
i p h p p 1 − γ × E3 × 1 − κ 3 j κ3 × Eout2 ;
(5)
i p p h p 1 − γ × E1 × 1 − κ 4 j κ4 × Ei2 ;
(6)
E2
Table 1.
Rad 153 μm κ5 0.7
(9)
n2
L
(10)
Et1 and Et2 are the through and drop port outputs of the add/drop system. Here, LL 2πRL , where RL is the radius of the ring on the left side of the add/drop within the ring. system, and p p EL is the electric field 1 − γ , x2 1 − γ, y1 1 − κ3 and y2 xp1 1 − κ4 and L 2πRad . The through and drop port outputs of the system result in generating the single and multi-carriers used for the W-Band MIMOOFDM-RoF system. 3. Results and Discussion
The fixed parameters of the system are listed in Table 1. As shown in Fig. 2(a), the input bright soliton and Gaussian beam with a power of 1 W are inserted into the system. The output intensity powers from R1 and R2 are shown in Figs. 2(b) and 2(c). The inner signals of the modified add/drop filter system are shown in Figs. 2(d)–2(f), where the soliton pulses range from 191 to 196 THz. As shown in Fig. 2(f), the few soliton pulses can be localized within the modified add/drop filter system. The output jE3 j2 passes through the coupler κ 3, where the throughput output expressed by jEt1 j2 shows localized ultrashort soliton pulses at frequencies of f 1 192.898, f 2 192.990, f 3 193.1, f 4 193.315, and f 5 193.537 THz with FSR of 92, 110, 215, and 222 GHz, respectively, shown in Fig. 2(g). The drop port output expressed by jEt2 j2 is shown in Fig. 2(h). Because of the short bandwidth input Gaussian beam in the add port of the system, the multi-soliton pulses ranging from 192.936 to
Fixed Parameters of the MRR System
R1
R2
RL
κ1
κ2
κ3
κ4
12 μm n0 3.34
8 μm n2 m2 W−1 2.2 × 10−17
4 μm Aeff 1 μm2 0.50
0.04 Aeff 2 μm2 0.25
0.7 Aeff L μm2 0.10
0.02 α dB mm−1 0.5
0.05 γ 0.1
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Fig. 2. Results of frequencies generation: (a) input, (b) output R1 W∕μm2 , (c) output R2 W∕μm2 , (d) jE1 j2 mW∕μm2 , (e) jE2 j2 mW∕μm2 , (f) jE3 j2 mW∕μm2 , (g) jEt1 j2 mW∕μm2 , (h) jEt2 j2 mW∕μm2 range from 192.936 to 193.044, (i) 64 multi-carriers generation range from 192.9725 to 193.0075, (j) flattened 64 multi-carriers using the GFF with FSR 546 MHz.
193.044 THz are generated. In this study, the range of 192.9725–193.0075 THz is selected and used to generate 64 multi-carriers at a 3 mW threshold, as shown in Fig. 2(i), which results in a 35 GHz bandwidth applicable to W-Band signal generation. Figure 2(j) shows the generated uniform carrier signals using the gain flattening filter (GFF) system. The schematic diagram of the W-Band, MIMOOFDM-RoF system is shown in Fig. 3. At the transmitter central office (TCO), the generated carriers (f 1 − f 5 ) are separated with a demultiplexer. Also, f 1 and f 2 are used to generate W-band signals, while the other three carriers (f 3 , f 4 , and f 5 ) are assigned for the optical modulating of three MIMO signals separately using Mach–Zehnder (MZM) optical modulators. The generated multi-carriers centered at f 2 192.990 THz, illustrated in Fig. 2(j), are used to generate an all-optical OFDM signal. In this regard, multi-carriers are first separated by the splitter, then 52 out of 64 are modulated using QAM. The
QAM signal is generated using a pulse pattern generator (PPG). Afterward, to imitate the IFFT block at the transmitter and an FFT at the receiver, an array waveguide grating (AWG) is used [13]. Spectra of the modulated optical subcarriers are overlapped using the AWG, which results in one optical OFDM channel band, shown in Fig. 4(a). Then the generated optical OFDM signal is multiplexed by the wavelength located at f 1 . The distance between the single subcarrier f 1 and the center of the generated optical OFDM (f 2 ) is 92 GHz, which is the RF region for W-Band transmission. After beating the photo diode, the W-Band RF OFDM signal, shown in Fig. 4(b), is generated. Using the RF coupler with a coupling factor of 1∶3, the OFDM signal is divided to three equal WBand MIMO signals (X 1 , X 2 , and X 3 ) with the same RF carrier frequency as f RF f 2 − f 1 92 GHz. Transporting the W-Band MIMO signals over a single fiber will encounter severe power degradation because of fiber chromatic dispersion; therefore, the optical single side band (OSSB)+Carrier modulation technique [20] is implemented using the Mach– Zehnder modulators (MZMs) to modulate the MIMO signals (X 1 , X 2 , and X 3 ) on the three assigned carriers f 3 –f 5, respectively, as illustrated in Fig. 4(c). The optical band-pass filters (BPF) are used to suppress one sideband of each modulated signal to generate OSSB + C signals. To transmit the MIMO signals, the lower side band (LSB) of X 2 and X 3 and the upper side band (USB) of X 1 are filtered using the BPF. Therefore, the LSB of X 1 and the USB of X 2 and X 3 are multiplexed, and the result is shown in Fig. 4(d). Now, the multiplexed signals are amplified by an Erbium doped fiber amplifier (EDFA), and are transmitted through the SMF to the transmitter antenna base station (TABS). The TABS is composed of a demultiplexer, PIN photodetectors (PDs) with 0.7 A∕W responsivity, electrical band pass filters (E-BPFs), amplifiers, and multiple transmitter antennas (Tx1, Tx2, and Tx3) for MIMO applications.
Fig. 3. System setup. 8052
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Fig. 5. (a) Received W-Band signal spectrum, (b) Downconverted W-Band signal for two optical links as B2B and 50 km.
Fig. 4. (a) All optical OFDM channel band, (b) W-Band OFDM signal spectra, (c) X 1 , X 2 , X 3 , optical modulated spectrum, (d) Multiplexed spectrum of X 1 -LSB and X 2 -USB and X 3 -USB, PSD: power spectral density.
At the TABS, the optical downstream is demultiplexed to the three signals and, afterward, three modulated optical signals are converted directly to electrical signals (X 11 , X 12 , and X 13 ) using the PDs. Based on the allocated RF frequency, the electrical signals are filtered using the BPFs. Next, the MIMO OFDM signals (X 11 , X 12 , and X 13 ), which are the same as the transmitted signals (X 1 , X 2 , and X 3 ) with little noise, will be propagated wirelessly using W-band 3 × 3 MIMO T x antennas. The 3 × 3 MIMO Array antenna has been designed to operate at 92 GHz. It must be noted that the array type of antenna is chosen to enhance the gain values. In addition, it was found that the MIMO array antenna covers a footprint of 45 mm × 45 mm, radiates a fixed beam in the boresight direction, and achieves the 10dB impedance bandwidth of 35.7 GHz from 74.3 to 110 GHz with the maximal realized gain of 23.66 dBi at 92.5 GHz. Here, the W-Band RF signals are propagated through the MIMO channel, and they are received by wireless end users. For instance, Fig. 5(a) has shown the power spectrum density (PSD) of the Tx1-Rx1 wireless link. At the receiver antenna base station, the amplified received RF signal is upconverted using a distributed feedback (DFB) laser to process the received signal optically. The upconverted signal is transmitted to the receiver central office (RCO) through 2 m SMF. At RCO, the AWG is used to implement the FFT function optically. The demodulation is performed, and the BER
is calculated. The spectrum of the down-converted W-Band signal for two optical links as B2B and 50 km, while the wireless distance was fixed at 3 m, also shown in Fig. 5(b). Figure 5(a) shows the amplified version of the received W-Band signal spectrum versus the transmitted signal for Tx1-Rx1 antennas over a wireless link. In this figure, we have shown the amplified version of the received signal that is going to be processed in the RCO (RCO). Figure 5(b) also shows the down-converted W-Band signal for two optical links as B2B and 50 km. As shown in this figure, the baseband down-converted OFDM signal has approximately a 17.5 GHz band that could be applied to a W-Band short range communication link. The performance of the system setup was tested based on two optical lengths that were back to back (B2B) and 50 km. As per the FEC-limit, the 3.8 × 10−3 of the BER [shown with a dashed line in Figs. 6(a)
Fig. 6. (a) BER, eye and constellation diagram of 3 × 3 MIMORoF with B2B optical link and (b) with 50 km optical link. 1 December 2014 / Vol. 53, No. 34 / APPLIED OPTICS
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and 6(b)] is the threshold for a successful transmission. In the case of B2B, as shown in Fig. 6, the total optical power level after amplification is adjusted with a variable optical attenuator (VOA). It is clear for different wireless transmitters (Tx1, Tx2, Tx3, MIMO) that there is an acceptable BER variation for a 3 m MIMO wireless distance. Therefore, the transmission of MIMO W-Band signals is feasible for a B2B optical link and a wireless distance of 3 m. Further investigation of system performance is conducted based on a 50 km optical link. As illustrated in Fig. 6(b), the system performance in four circumstances is investigated, which are individual MIMO streams (Tx1, Tx2, and Tx3) and the combined MIMO signal (Tx1 Tx2 Tx3). At the optical power related to the BER threshold, the eye and constellation diagram of this optical link confirms acceptable signal transmission and recovery. In conclusion, a series of MRRs incorporating a modified add/drop system is used to generate a W-Band RF signal. Single and multi-carriers optical soliton pulses are generated. The range of 192.9725 to 193.0075 THz is selected to generate 64 multicarriers with a specific FSR of 546 MHz, which are used in all optical generation of the three MIMO W-Band RF signals. The single carriers are used in the optical transportation of the three separate MIMO signals over an RoF link. The overall view and concepts of all optical MIMO signal generation and its transmission over a 50 km SMF optical link and up to 3 m 3 × 3 MIMO wireless link was analyzed thoroughly. The authors express their gratitude to the Ministry of Higher Education (MOHE), Malaysia and Research Management Center (RMC) of Universiti Teknologi Malaysia (UTM) for the financial support of this project under Research Grant No. R.J130000.7823.4L145. I. S. Amiri and H. Ahmad acknowledge the financial support from the University Malaya/MOHE under Grant Number UM.C/625/1/HIR/MOHE/SCI/29.
6. 7.
8.
9. 10.
11. 12.
13.
14.
15. 16. 17.
18. 19.
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