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OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
Optical-wireless-optical full link for polarization multiplexing quadrature amplitude/phase modulation signal transmission Xinying Li,1 Jianjun Yu,1,* Nan Chi,1 and Junwen Zhang1,2 1
Department of Communication Science and Engineering, Fudan University, Shanghai 200433, China 2
ZTE (TX) Inc., Morristown, New Jersey 07960, USA *Corresponding author: [email protected]
Received August 19, 2013; revised October 2, 2013; accepted October 5, 2013; posted October 14, 2013 (Doc. ID 195913); published November 12, 2013 We propose and experimentally demonstrate an optical wireless integration system at the Q-band, in which up to 40 Gb∕s polarization multiplexing multilevel quadrature amplitude/phase modulation (PM-QAM) signal can be first transmitted over 20 km single-mode fiber-28 (SMF-28), then delivered over a 2 m 2 × 2 multiple-input multipleoutput wireless link, and finally transmitted over another 20 km SMF-28. The PM-QAM modulated wireless millimeter-wave (mm-wave) signal at 40 GHz is generated based on the remote heterodyning technique, and demodulated by the radio-frequency transparent photonic technique based on homodyne coherent detection and baseband digital signal processing. The classic constant modulus algorithm equalization is used at the receiver to realize polarization demultiplexing of the PM-QAM signal. For the first time, to the best of our knowledge, we realize the conversion of the PM-QAM modulated wireless mm-wave signal to the optical signal as well as 20 km fiber transmission of the converted optical signal. © 2013 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.2840) Heterodyne; (060.2920) Homodyning; (060.4230) Multiplexing; (060.5625) Radio frequency photonics. http://dx.doi.org/10.1364/OL.38.004712
The high-speed optical wireless integration system may be used to provide emergency service when large-capacity long-haul optical cables are broken during natural disasters such as earthquakes and tsunami [1]. In order to realize the high-speed optical wireless integration system, the wireless links need to be developed to match the capacity of high-speed fiber-optic communication systems, while preserving transparency to bit rates and modulation formats. Due to wider bandwidths and higher frequencies, wireless delivery in millimeter-wave (mm-wave) bands is expected to provide multigigabit mobile data transmission and has been intensively studied in the research community [2–8]. Moreover, high-speed wireless mm-wave generation by the photonic technique effectively promotes seamless integration of wireless and fiber-optic networks. Recently, we have experimentally demonstrated 100G and 400G optical wireless integration systems adopting polarization multiplexing multilevel quadrature amplitude/ phase modulation (PM-QAM), photonic mm-wave generation, and advanced digital signal processing (DSP) [6–8]. However, in our previous schemes, the generated highspeed PM-QAM modulated wireless mm-wave signal is demodulated in the electrical domain and has limited radio-frequency (RF) cable transmission distance at such frequency band. Furthermore, the electrical demodulation of the high-speed PM-QAM modulated wireless mm-wave signal will become more complicated with the increase of the transmission bit rate and mm-wave carrier frequency. A RF transparent photonic mm-wave demodulation technique is proposed in [9] based on coherent detection and baseband DSP, and offers the advantage of converting the QAM modulated wireless mm-wave signal into the optical baseband signal. The converted optical baseband signal can be directly transmitted in a fiber-optic network. However, the transmitted mm-wave signal is single-polarized and the demonstrated optical wireless integration system 0146-9592/13/224712-04$15.00/0
adopting the photonic mm-wave demodulation technique has neither wireless nor long-haul fiber transmission [10]. It is well known that the polarization multiplexing technique is a practical solution for the future spectrally efficient high-speed optical transmission to double the capacity of a fiber link [11,12]. Thus, it is necessary for us to investigate how to realize this polarization multiplexing signal transmission in an optical wireless integration system. In this Letter, we propose and experimentally demonstrate an optical wireless integration system at the Q-band (33–50 GHz), in which up to 40 Gb∕s PM-QAM signal can be first transmitted over 20 km single-mode fiber-28 (SMF-28), then delivered over a 2 m 2 × 2 multipleinput multiple-output (MIMO) wireless link, and finally transmitted over another 20 km SMF-28. The PM-QAM modulated wireless mm-wave signal is generated based on the remote heterodyning technique, and is demodulated by the RF transparent photonic technique based on homodyne coherent detection and baseband DSP. Neither 2 m wireless delivery nor 20 20 km SMF-28 transmission causes any optical signal-to-noise ratio (OSNR) penalty. For the first time, we realize the conversion of the PM-QAM modulated wireless mm-wave signal to the optical signal as well as 20 km fiber transmission of the converted optical signal. Figure 1 shows the principle of our proposed opticalwireless-optical link for PM-QAM signal transmission. The PM-QAM modulated wireless mm-wave signal is generated based on the remote heterodyning technique [13,14], and is demodulated by the RF transparent photonic technique based on homodyne coherent detection and baseband DSP. At the transmitter central office (CO), the continuous-wavelength (CW) lightwave at λ1 is externally modulated by the transmitter data and then polarization multiplexed to generate the PM-QAM modulated © 2013 Optical Society of America
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optical baseband signal. At the transmitter base station (BS), the PM-QAM modulated optical baseband signal after fiber transmission is heterodyne beat with the CW lightwave at λ2 and upconverted to the PM-QAM modulated wireless signal at a mm-wave frequency of f RF cj1∕λ1 − 1∕λ2 j (c is the velocity of light). Then, the PMQAM modulated wireless mm-wave signal is delivered by a 2 × 2 MIMO wireless link [6], which consists of two pairs of transmitter and receiver horn antennas (HAs). At the receiver BS, the received wireless mm-wave signal externally modulates the CW lightwave at λ3 to generate the optical carrier suppression (OCS) signal, which contains a small central carrier at λ3 (if the optical carrier is not fully suppressed due to limited extinction ratio) and two PM-QAM modulated sidebands at λ3 λRF (λRF c∕f RF ). The upper sideband (or lower sideband) and the optical carrier are filtered out by the subsequent tunable optical filter (TOF), and thus only an equivalent PM-QAM modulated optical baseband signal at λ3 − λRF (or λ3 λRF ) is sent to the receiver CO after fiber transmission. At the receiver CO, the transmitter data are recovered from the PM-QAM modulated optical baseband signal by homodyne coherent detection and baseband DSP [15]. The adopted local oscillator (LO) at the receiver CO has an operating wavelength of λ3 − λRF (or λ3 λRF ). Figures 1(a)–1(c) give the schematic optical spectra after heterodyne upconversion, optical OCS modulation, and TOF, respectively. For the PM-QAM signal, the fiber transmission from the transmitter CO to the transmitter BS and from the receiver BS to the receiver CO as well as the 2 × 2 MIMO wireless delivery from the transmitter BS to the receiver BS can all be considered based on a 2 × 2 MIMO model and denoted by a 2 × 2 Jones matrix. The multiplication of three 2 × 2 Jones matrices is still a 2 × 2 matrix. Thus, the classic constant modulus algorithm (CMA) can be used at the receiver CO to realize PM-QAM signal polarization de-multiplexing. Figure 2 shows the experimental setup for the opticalwireless-optical link at the Q-band, which can in order realize up to 40 Gb∕s PM-QAM signal transmission over 20 km SMF-28, 2 m 2 × 2 MIMO wireless link, and 20 km SMF-28. Four external cavity lasers (ECLs), with linewidth less than 100 kHz and output power of 14.5 dBm, are free-running and operate at different wavelengths. At the transmitter CO, the CW lightwave from ECL1 at 1549.39 nm is modulated by a 5–12.5 Gbaud electrical binary signal with the aid of an in-phase/quadrature (I/Q)
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modulator. The electrical binary signal has a pseudorandom binary sequence (PRBS) length of 215 − 1 and is generated from a pulse pattern generator (PPG). For optical QAM modulation, the two parallel Mach–Zehnder modulators (MZMs) in the I/Q modulator are both biased at the null point and driven at the full swing. The phase difference between the upper and lower arms of the I/Q modulator is controlled at π∕2. Then, the generated optical QAM signal passes through an erbium-doped fiber amplifier (EDFA) and is polarization multiplexed by a polarization multiplexer. The polarization multiplexer includes a polarization-maintaining optical coupler (OC) to split the signal into two branches, an optical delay line (DL) to provide a 150-symbol delay, an optical attenuator to balance the power of two branches, and a polarization beam combiner (PBC) to recombine the signals. The generated PM-QAM modulated optical baseband signal is launched into 20 km SMF-28, which has 18 dB average fiber loss and 17 ps∕km∕nm chromatic dispersion (CD) at 1550 nm without optical dispersion compensation. The launched power into fiber is 0 dBm. At the transmitter BS, ECL2 at 1549.70 nm functioned as LO has 40 GHz frequency offset relative to ECL1. Two polarization beam splitters (PBSs) and two OCs are used to implement polarization diversity of the received signal and LO in the optical domain before heterodyne beating. Figure 2(a) shows the optical spectrum (0.1 nm resolution) for 50 Gb∕s bit rate after polarization diversity. 40 GHz frequency spacing exists between the signal and the LO. The power of the LO is 4 dB larger than that of
OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
the signal. It is worth noting that the X- or Y -polarization component after PBS in Fig. 2 does not mean that only X- or Y -polarization transmission signal exists at each output port of the PBS. In fact, each output port contains both X- and Y -polarization transmission signals. In this Letter, we define one output port of the PBS as the X-polarization component and the other as the Y -polarization for simplification. Two single-ended photodetectors (PDs), each with 70 GHz 3 dB bandwidth and 9 dBm input power, directly upconvert the PM-QAM modulated optical baseband signal into the PM-QAM modulated wireless mmwave signal at 40 GHz. The generated PM-QAM modulated wireless mm-wave signal is delivered over a 2 m 2 × 2 MIMO wireless link at the Q-band. Each pair of transmitter and receiver HAs has a 2 m wireless distance, the X- and Y -polarization wireless links are parallel, and two transmitter (receiver) HAs have a 10 cm wireless distance. Each HA has 25 dBi gain and a frequency range of 33–50 GHz, and is connected with an electrical amplifier (EA) with 17 GHz electrical bandwidth, 30 dB gain, and 20 dBm saturation output power. The 3 dB beam width at the input of each receiver HA is about 10° × 10°. At the receiver BS, the CW lightwave from ECL3 at 1550.08 nm is first split by a polarization-maintaining OC into two branches. Each branch is modulated by the X- or Y -polarization component of the received wireless mm-wave signal with the aid of an intensity modulator (IM). Each IM has 3 dB bandwidth of ∼36 GHz, 2.8 V half-wave voltage, and 5 dB insertion loss. Each IM is DC-biased at the minimal output for OCS modulation when the wireless mm-wave signal is turned off. The driving voltage (peak-to-peak) on each IM is 1.7 V. The input optical power into each IM is 16 dBm. A PBC is used to recombine the two modulated branches. Figure 2(b) shows the optical spectrum (0.1 nm resolution) for 50 Gb∕s bit rate after the PBC. The generated optical OCS signal has an optical carrier at 1550.08 nm and two PM-QAM modulated sidebands separated by 40 GHz from the optical carrier. The relatively large power of the optical carrier is due to a limited extinction ratio and the limited driving voltage on the IM. An EDFA is used to amplify the power of the optical OCS signal. Then, a 0.3 nm TOF is used to suppress the upper sideband and the optical carrier as well as amplified spontaneous emission (ASE) noise. The generated optical baseband signal is sent into 20 km SMF-28. The launched power into fiber is 0 dBm. Figure 2(c) shows the optical spectrum (0.1 nm resolution) for 50 Gb∕s bit rate after 20 km SMF-28 transmission. At the receiver CO, ECL4 functioning as an LO has an operating wavelength identical to that of the optical baseband signal. A polarization-diversity 90° hybrid is used to realize polarization- and phase-diversity coherent detection of the LO and the received optical signal before balanced detection [15]. The analog-to-digital conversion is realized in the real-time digital oscilloscope (OSC) with 80 GSa∕s sampling rate and 20 GHz electrical bandwidth. The baseband DSP [15] is carried out after analog-todigital conversion. First, the clock is extracted using the “square and filter” method, and then the digital signal is resampled at twice the baud rate based on the recovered clock. Second, a T/2-spaced time-domain finite impulse response (FIR) filter is used for CD compensation, where the filter coefficients are calculated from the known fiber
CD transfer function using the frequency-domain truncation method. Third, two complex-valued, 39–59-tap, T/2spaced adaptive FIR filters, based on the classic CMA, are used to retrieve the modulus of the PM-QAM signal and realize polarization demultiplexing. The subsequent step is carrier recovery, which includes frequency-offset estimation (FOE) and carrier phase estimation (CPE). The former is based on the fast Fourier transform (FFT) method, while the latter is based on the fourth-power Viterbi–Viterbi algorithm. Finally, differential decoding is used to eliminate π∕2 phase ambiguity before bit-errorratio (BER) counting. In this experiment, the BER is counted over 10 × 106 bits (10 data sets; each set contains 106 bits). Figures 3(a) and 3(b) show photos of the 2 × 2 MIMO wireless link at the Q-band, the transmitter HA array, and the receiver HA array, respectively. Figure 4 shows the BER versus the OSNR for the 20 Gb∕s PM-QAM signal transmission over the opticalwireless-optical link. Here, without fiber transmission denotes the optical signal is transmitted back to back from the transmitter CO to the transmitter BS and from the receiver BS to the receiver CO, while without wireless delivery denotes the wireless mm-wave signal is sent directly from the transmitter BS to the receiver BS without wireless boosting. The adopted CMA-tap number is 39. To our knowledge, the tap number is about 13 for commercial 100G PM-QAM products. In our proposed scheme, there exist several occurrences of optical-toelectrical and electrical-to-optical conversion, during each time of which the PM-QAM signal is separated to go through different path lengths. Thus, a large path effect is introduced and a larger tap number is required. Moreover, the tap number will also be increased with the increase of the transmission bit rate. However, the required tap number will be reduced if we can match the length of the two different paths the PM-QAM signal is separated
Fig. 3. Photos for (a) 2 × 2 MIMO wireless link at the Q-band, (b) transmitter HA array, and (c) receiver HA array.
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to during each time of optical-to-electrical or electricalto-optical conversion. Neither 2 m wireless delivery nor 20 20 km SMF-28 transmission causes any OSNR penalty. Note that the potential effects in the RF domain, such as multipath, EA saturation, and weak power at the receiver HA, can cause OSNR penalty. However, no EA saturation effect exists in our experiment because all EAs work in a linear area. No multipath effect is observed due to high directionality of our HA system. Moreover, the multipath effect, even if exists, can be compensated by long-tap CMA [8]. We also adopt a powerful EA at the transmitter side to ensure the received wireless power is large enough. Figures 5(a)–5(e), respectively, show the X-polarization constellations for the 20 Gb∕s PM-QAM signal without wireless delivery and fiber transmission before clock extraction, after clock extraction, after CMA equalization, after FOE, and after CPE, while Figs. 5(f)–5(j) show the corresponding Y -polarization constellations. Figure 6 shows the BER versus the OSNR for the PM-QAM signal transmission over the optical-wirelessoptical link with 2 m wireless delivery and 20 20 km SMF-28 transmission at bit rates of 30, 40, and 50 Gb∕s, respectively. The CMA tap number is 51 for 30 and 40 Gb∕s bit rate and 59 for 50 Gb∕s bit rate. Larger OSNR is required at a higher bit rate at the same BER. Compared to the case of 30 Gb∕s bit rate, the OSNR penalty at the BER of 3.8 × 10−3 is 1.7 and 2.8 dB for the 40 and 50 Gb∕s bit rates, respectively. There exists an error floor at the BER of 1 × 10−3 for 50 Gb∕s bit rate, which is due to the bandwidth limitation of the optical and electrical components (including four EAs connected with each HA, two IMs, and the digital OSC) and limited OSNR (the maximal value is 21 dB). Figures 7(a)–7(e) show the Y -polarization
constellations for the 50 Gb∕s PM-QAM signal with 2 m wireless delivery and 20 20 km SMF-28 transmission before clock extraction, after clock extraction, after CMA equalization, after FOE, and after CPE, respectively. In conclusion, we propose and experimentally demonstrate an optical wireless integration system at the Qband, in which up to 40 Gb∕s PM-QAM signal can be first transmitted over 20 km SMF-28, then delivered over a 2 m 2 × 2 MIMO wireless link, and finally transmitted over another 20 km SMF-28. The PM-QAM modulated wireless mm-wave signal at 40 GHz is generated based on remote heterodyning and demodulated by the RF transparent photonic technique based on homodyne coherent detection and baseband DSP. We propose a new scheme and realize the conversion of the PM-QAM modulated wireless mm-wave signal to the optical signal as well as 20 km fiber transmission of the converted optical signal. References 1. NTT Group CSR Report 2011, http://www.ntt.co.jp/csr_e/ 2011report/. 2. A. Hirata, M. Harada, and T. Nagatsuma, J. Lightwave Technol. 21, 2145 (2003). 3. X. Pang, A. Caballero, A. Dogadaev, V. Arlunno, R. Borkowski, J. S. Pedersen, L. Deng, F. Karinou, F. Roubeau, D. Zibar, X. Yu, and I. T. Monroy, Opt. Express 19, 24944 (2011). 4. A. Kanno, T. Kuri, I. Hosako, T. Kawanishi, Y. Yasumura, Y. Yoshida, and K. Kitayama, in Proceedings of the European Conference and Exhibition on Optical Communication (2012), paper We.3.B.2. 5. X. Li, J. Yu, Z. Dong, J. Zhang, N. Chi, and J. Yu, Opt. Lett. 38, 742 (2013). 6. X. Li, Z. Dong, J. Yu, N. Chi, Y. Shao, and G. K. Chang, Opt. Lett. 37, 5106 (2012). 7. Z. Dong, J. Yu, X. Li, G. K. Chang, and Z. Cao, in Proceedings of the Optical Fiber Communication Conference (2013), paper OM3D.2. 8. X. Li, J. Yu, J. Zhang, Z. Dong, F. Li, and N. Chi, Opt. Express 21, 18812 (2013). 9. R. Sambaraju, D. Zibar, R. Alemany, A. Caballero, and J. Herrera, in Proceedings of the Optical Fiber Communication Conference (2010), paper OML1. 10. R. Sambaraju, D. Zibar, A. Caballero, I. T. Monroy, R. Alemany, and J. Herrera, Photonics Technol. Lett. 22, 1650 (2010). 11. X. Zhou and J. Yu, J. Lightwave Technol. 27, 3641 (2009). 12. J. Yu and X. Zhou, IEEE Commun. Mag. 48(3), S56 (2010). 13. J. Zhang, Z. Dong, J. Yu, N. Chi, L. Tao, X. Li, and Y. Shao, Opt. Lett. 37, 4050 (2012). 14. X. Li, Z. Dong, J. Yu, J. Yu, and N. Chi, Opt. Express 21, 8808 (2013). 15. J. Yu, X. Zhou, M. F. Huang, Y. Shao, D. Qian, T. Wang, M. Cvijetic, P. Magill, L. Nelson, M. Birk, S. Ten, H. B. Matthew, and S. K. Mishra, in Proceedings of the European Conference and Exhibition on Optical Communication (2008), paper PDP: Th. 3. E. 2.
OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
Optical-wireless-optical full link for polarization multiplexing quadrature amplitude/phase modulation signal transmission Xinying Li,1 Jianjun Yu,1,* Nan Chi,1 and Junwen Zhang1,2 1
Department of Communication Science and Engineering, Fudan University, Shanghai 200433, China 2
ZTE (TX) Inc., Morristown, New Jersey 07960, USA *Corresponding author: [email protected]
Received August 19, 2013; revised October 2, 2013; accepted October 5, 2013; posted October 14, 2013 (Doc. ID 195913); published November 12, 2013 We propose and experimentally demonstrate an optical wireless integration system at the Q-band, in which up to 40 Gb∕s polarization multiplexing multilevel quadrature amplitude/phase modulation (PM-QAM) signal can be first transmitted over 20 km single-mode fiber-28 (SMF-28), then delivered over a 2 m 2 × 2 multiple-input multipleoutput wireless link, and finally transmitted over another 20 km SMF-28. The PM-QAM modulated wireless millimeter-wave (mm-wave) signal at 40 GHz is generated based on the remote heterodyning technique, and demodulated by the radio-frequency transparent photonic technique based on homodyne coherent detection and baseband digital signal processing. The classic constant modulus algorithm equalization is used at the receiver to realize polarization demultiplexing of the PM-QAM signal. For the first time, to the best of our knowledge, we realize the conversion of the PM-QAM modulated wireless mm-wave signal to the optical signal as well as 20 km fiber transmission of the converted optical signal. © 2013 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.2840) Heterodyne; (060.2920) Homodyning; (060.4230) Multiplexing; (060.5625) Radio frequency photonics. http://dx.doi.org/10.1364/OL.38.004712
The high-speed optical wireless integration system may be used to provide emergency service when large-capacity long-haul optical cables are broken during natural disasters such as earthquakes and tsunami [1]. In order to realize the high-speed optical wireless integration system, the wireless links need to be developed to match the capacity of high-speed fiber-optic communication systems, while preserving transparency to bit rates and modulation formats. Due to wider bandwidths and higher frequencies, wireless delivery in millimeter-wave (mm-wave) bands is expected to provide multigigabit mobile data transmission and has been intensively studied in the research community [2–8]. Moreover, high-speed wireless mm-wave generation by the photonic technique effectively promotes seamless integration of wireless and fiber-optic networks. Recently, we have experimentally demonstrated 100G and 400G optical wireless integration systems adopting polarization multiplexing multilevel quadrature amplitude/ phase modulation (PM-QAM), photonic mm-wave generation, and advanced digital signal processing (DSP) [6–8]. However, in our previous schemes, the generated highspeed PM-QAM modulated wireless mm-wave signal is demodulated in the electrical domain and has limited radio-frequency (RF) cable transmission distance at such frequency band. Furthermore, the electrical demodulation of the high-speed PM-QAM modulated wireless mm-wave signal will become more complicated with the increase of the transmission bit rate and mm-wave carrier frequency. A RF transparent photonic mm-wave demodulation technique is proposed in [9] based on coherent detection and baseband DSP, and offers the advantage of converting the QAM modulated wireless mm-wave signal into the optical baseband signal. The converted optical baseband signal can be directly transmitted in a fiber-optic network. However, the transmitted mm-wave signal is single-polarized and the demonstrated optical wireless integration system 0146-9592/13/224712-04$15.00/0
adopting the photonic mm-wave demodulation technique has neither wireless nor long-haul fiber transmission [10]. It is well known that the polarization multiplexing technique is a practical solution for the future spectrally efficient high-speed optical transmission to double the capacity of a fiber link [11,12]. Thus, it is necessary for us to investigate how to realize this polarization multiplexing signal transmission in an optical wireless integration system. In this Letter, we propose and experimentally demonstrate an optical wireless integration system at the Q-band (33–50 GHz), in which up to 40 Gb∕s PM-QAM signal can be first transmitted over 20 km single-mode fiber-28 (SMF-28), then delivered over a 2 m 2 × 2 multipleinput multiple-output (MIMO) wireless link, and finally transmitted over another 20 km SMF-28. The PM-QAM modulated wireless mm-wave signal is generated based on the remote heterodyning technique, and is demodulated by the RF transparent photonic technique based on homodyne coherent detection and baseband DSP. Neither 2 m wireless delivery nor 20 20 km SMF-28 transmission causes any optical signal-to-noise ratio (OSNR) penalty. For the first time, we realize the conversion of the PM-QAM modulated wireless mm-wave signal to the optical signal as well as 20 km fiber transmission of the converted optical signal. Figure 1 shows the principle of our proposed opticalwireless-optical link for PM-QAM signal transmission. The PM-QAM modulated wireless mm-wave signal is generated based on the remote heterodyning technique [13,14], and is demodulated by the RF transparent photonic technique based on homodyne coherent detection and baseband DSP. At the transmitter central office (CO), the continuous-wavelength (CW) lightwave at λ1 is externally modulated by the transmitter data and then polarization multiplexed to generate the PM-QAM modulated © 2013 Optical Society of America
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optical baseband signal. At the transmitter base station (BS), the PM-QAM modulated optical baseband signal after fiber transmission is heterodyne beat with the CW lightwave at λ2 and upconverted to the PM-QAM modulated wireless signal at a mm-wave frequency of f RF cj1∕λ1 − 1∕λ2 j (c is the velocity of light). Then, the PMQAM modulated wireless mm-wave signal is delivered by a 2 × 2 MIMO wireless link [6], which consists of two pairs of transmitter and receiver horn antennas (HAs). At the receiver BS, the received wireless mm-wave signal externally modulates the CW lightwave at λ3 to generate the optical carrier suppression (OCS) signal, which contains a small central carrier at λ3 (if the optical carrier is not fully suppressed due to limited extinction ratio) and two PM-QAM modulated sidebands at λ3 λRF (λRF c∕f RF ). The upper sideband (or lower sideband) and the optical carrier are filtered out by the subsequent tunable optical filter (TOF), and thus only an equivalent PM-QAM modulated optical baseband signal at λ3 − λRF (or λ3 λRF ) is sent to the receiver CO after fiber transmission. At the receiver CO, the transmitter data are recovered from the PM-QAM modulated optical baseband signal by homodyne coherent detection and baseband DSP [15]. The adopted local oscillator (LO) at the receiver CO has an operating wavelength of λ3 − λRF (or λ3 λRF ). Figures 1(a)–1(c) give the schematic optical spectra after heterodyne upconversion, optical OCS modulation, and TOF, respectively. For the PM-QAM signal, the fiber transmission from the transmitter CO to the transmitter BS and from the receiver BS to the receiver CO as well as the 2 × 2 MIMO wireless delivery from the transmitter BS to the receiver BS can all be considered based on a 2 × 2 MIMO model and denoted by a 2 × 2 Jones matrix. The multiplication of three 2 × 2 Jones matrices is still a 2 × 2 matrix. Thus, the classic constant modulus algorithm (CMA) can be used at the receiver CO to realize PM-QAM signal polarization de-multiplexing. Figure 2 shows the experimental setup for the opticalwireless-optical link at the Q-band, which can in order realize up to 40 Gb∕s PM-QAM signal transmission over 20 km SMF-28, 2 m 2 × 2 MIMO wireless link, and 20 km SMF-28. Four external cavity lasers (ECLs), with linewidth less than 100 kHz and output power of 14.5 dBm, are free-running and operate at different wavelengths. At the transmitter CO, the CW lightwave from ECL1 at 1549.39 nm is modulated by a 5–12.5 Gbaud electrical binary signal with the aid of an in-phase/quadrature (I/Q)
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Fig. 1. Principle of optical-wireless-optical link for PM-QAM signal transmission. Schematic optical spectra after (a) optical heterodyne upconversion, (b) optical carrier suppression (OCS) modulation, and (c) tunable optical filter (TOF).
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Fig. 2. Experimental setup. Optical spectra (0.1 nm resolution) after (a) polarization diversity at the transmitter BS, (b) PBC at the receiver BS, and (c) second 20 km SMF-28 transmission.
modulator. The electrical binary signal has a pseudorandom binary sequence (PRBS) length of 215 − 1 and is generated from a pulse pattern generator (PPG). For optical QAM modulation, the two parallel Mach–Zehnder modulators (MZMs) in the I/Q modulator are both biased at the null point and driven at the full swing. The phase difference between the upper and lower arms of the I/Q modulator is controlled at π∕2. Then, the generated optical QAM signal passes through an erbium-doped fiber amplifier (EDFA) and is polarization multiplexed by a polarization multiplexer. The polarization multiplexer includes a polarization-maintaining optical coupler (OC) to split the signal into two branches, an optical delay line (DL) to provide a 150-symbol delay, an optical attenuator to balance the power of two branches, and a polarization beam combiner (PBC) to recombine the signals. The generated PM-QAM modulated optical baseband signal is launched into 20 km SMF-28, which has 18 dB average fiber loss and 17 ps∕km∕nm chromatic dispersion (CD) at 1550 nm without optical dispersion compensation. The launched power into fiber is 0 dBm. At the transmitter BS, ECL2 at 1549.70 nm functioned as LO has 40 GHz frequency offset relative to ECL1. Two polarization beam splitters (PBSs) and two OCs are used to implement polarization diversity of the received signal and LO in the optical domain before heterodyne beating. Figure 2(a) shows the optical spectrum (0.1 nm resolution) for 50 Gb∕s bit rate after polarization diversity. 40 GHz frequency spacing exists between the signal and the LO. The power of the LO is 4 dB larger than that of
OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
the signal. It is worth noting that the X- or Y -polarization component after PBS in Fig. 2 does not mean that only X- or Y -polarization transmission signal exists at each output port of the PBS. In fact, each output port contains both X- and Y -polarization transmission signals. In this Letter, we define one output port of the PBS as the X-polarization component and the other as the Y -polarization for simplification. Two single-ended photodetectors (PDs), each with 70 GHz 3 dB bandwidth and 9 dBm input power, directly upconvert the PM-QAM modulated optical baseband signal into the PM-QAM modulated wireless mmwave signal at 40 GHz. The generated PM-QAM modulated wireless mm-wave signal is delivered over a 2 m 2 × 2 MIMO wireless link at the Q-band. Each pair of transmitter and receiver HAs has a 2 m wireless distance, the X- and Y -polarization wireless links are parallel, and two transmitter (receiver) HAs have a 10 cm wireless distance. Each HA has 25 dBi gain and a frequency range of 33–50 GHz, and is connected with an electrical amplifier (EA) with 17 GHz electrical bandwidth, 30 dB gain, and 20 dBm saturation output power. The 3 dB beam width at the input of each receiver HA is about 10° × 10°. At the receiver BS, the CW lightwave from ECL3 at 1550.08 nm is first split by a polarization-maintaining OC into two branches. Each branch is modulated by the X- or Y -polarization component of the received wireless mm-wave signal with the aid of an intensity modulator (IM). Each IM has 3 dB bandwidth of ∼36 GHz, 2.8 V half-wave voltage, and 5 dB insertion loss. Each IM is DC-biased at the minimal output for OCS modulation when the wireless mm-wave signal is turned off. The driving voltage (peak-to-peak) on each IM is 1.7 V. The input optical power into each IM is 16 dBm. A PBC is used to recombine the two modulated branches. Figure 2(b) shows the optical spectrum (0.1 nm resolution) for 50 Gb∕s bit rate after the PBC. The generated optical OCS signal has an optical carrier at 1550.08 nm and two PM-QAM modulated sidebands separated by 40 GHz from the optical carrier. The relatively large power of the optical carrier is due to a limited extinction ratio and the limited driving voltage on the IM. An EDFA is used to amplify the power of the optical OCS signal. Then, a 0.3 nm TOF is used to suppress the upper sideband and the optical carrier as well as amplified spontaneous emission (ASE) noise. The generated optical baseband signal is sent into 20 km SMF-28. The launched power into fiber is 0 dBm. Figure 2(c) shows the optical spectrum (0.1 nm resolution) for 50 Gb∕s bit rate after 20 km SMF-28 transmission. At the receiver CO, ECL4 functioning as an LO has an operating wavelength identical to that of the optical baseband signal. A polarization-diversity 90° hybrid is used to realize polarization- and phase-diversity coherent detection of the LO and the received optical signal before balanced detection [15]. The analog-to-digital conversion is realized in the real-time digital oscilloscope (OSC) with 80 GSa∕s sampling rate and 20 GHz electrical bandwidth. The baseband DSP [15] is carried out after analog-todigital conversion. First, the clock is extracted using the “square and filter” method, and then the digital signal is resampled at twice the baud rate based on the recovered clock. Second, a T/2-spaced time-domain finite impulse response (FIR) filter is used for CD compensation, where the filter coefficients are calculated from the known fiber
CD transfer function using the frequency-domain truncation method. Third, two complex-valued, 39–59-tap, T/2spaced adaptive FIR filters, based on the classic CMA, are used to retrieve the modulus of the PM-QAM signal and realize polarization demultiplexing. The subsequent step is carrier recovery, which includes frequency-offset estimation (FOE) and carrier phase estimation (CPE). The former is based on the fast Fourier transform (FFT) method, while the latter is based on the fourth-power Viterbi–Viterbi algorithm. Finally, differential decoding is used to eliminate π∕2 phase ambiguity before bit-errorratio (BER) counting. In this experiment, the BER is counted over 10 × 106 bits (10 data sets; each set contains 106 bits). Figures 3(a) and 3(b) show photos of the 2 × 2 MIMO wireless link at the Q-band, the transmitter HA array, and the receiver HA array, respectively. Figure 4 shows the BER versus the OSNR for the 20 Gb∕s PM-QAM signal transmission over the opticalwireless-optical link. Here, without fiber transmission denotes the optical signal is transmitted back to back from the transmitter CO to the transmitter BS and from the receiver BS to the receiver CO, while without wireless delivery denotes the wireless mm-wave signal is sent directly from the transmitter BS to the receiver BS without wireless boosting. The adopted CMA-tap number is 39. To our knowledge, the tap number is about 13 for commercial 100G PM-QAM products. In our proposed scheme, there exist several occurrences of optical-toelectrical and electrical-to-optical conversion, during each time of which the PM-QAM signal is separated to go through different path lengths. Thus, a large path effect is introduced and a larger tap number is required. Moreover, the tap number will also be increased with the increase of the transmission bit rate. However, the required tap number will be reduced if we can match the length of the two different paths the PM-QAM signal is separated
Fig. 3. Photos for (a) 2 × 2 MIMO wireless link at the Q-band, (b) transmitter HA array, and (c) receiver HA array.
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Fig. 7. Received Y -polarization QAM constellations at the bit rate of 50 Gb∕s: (a) before clock extraction, (b) after clock extraction, (c) after CMA equalization, (d) after FOE, and (e) after CPE. Fig. 5. Received QAM constellations at the bit rate of 20 Gb∕s. X-polarization: (a) before clock extraction, (b) after clock extraction, (c) after CMA equalization, (d) after FOE, and (e) after CPE. Y -polarization: (f) before clock extraction, (g) after clock extraction, (h) after CMA equalization, (i) after FOE, and (j) after CPE.
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to during each time of optical-to-electrical or electricalto-optical conversion. Neither 2 m wireless delivery nor 20 20 km SMF-28 transmission causes any OSNR penalty. Note that the potential effects in the RF domain, such as multipath, EA saturation, and weak power at the receiver HA, can cause OSNR penalty. However, no EA saturation effect exists in our experiment because all EAs work in a linear area. No multipath effect is observed due to high directionality of our HA system. Moreover, the multipath effect, even if exists, can be compensated by long-tap CMA [8]. We also adopt a powerful EA at the transmitter side to ensure the received wireless power is large enough. Figures 5(a)–5(e), respectively, show the X-polarization constellations for the 20 Gb∕s PM-QAM signal without wireless delivery and fiber transmission before clock extraction, after clock extraction, after CMA equalization, after FOE, and after CPE, while Figs. 5(f)–5(j) show the corresponding Y -polarization constellations. Figure 6 shows the BER versus the OSNR for the PM-QAM signal transmission over the optical-wirelessoptical link with 2 m wireless delivery and 20 20 km SMF-28 transmission at bit rates of 30, 40, and 50 Gb∕s, respectively. The CMA tap number is 51 for 30 and 40 Gb∕s bit rate and 59 for 50 Gb∕s bit rate. Larger OSNR is required at a higher bit rate at the same BER. Compared to the case of 30 Gb∕s bit rate, the OSNR penalty at the BER of 3.8 × 10−3 is 1.7 and 2.8 dB for the 40 and 50 Gb∕s bit rates, respectively. There exists an error floor at the BER of 1 × 10−3 for 50 Gb∕s bit rate, which is due to the bandwidth limitation of the optical and electrical components (including four EAs connected with each HA, two IMs, and the digital OSC) and limited OSNR (the maximal value is 21 dB). Figures 7(a)–7(e) show the Y -polarization
constellations for the 50 Gb∕s PM-QAM signal with 2 m wireless delivery and 20 20 km SMF-28 transmission before clock extraction, after clock extraction, after CMA equalization, after FOE, and after CPE, respectively. In conclusion, we propose and experimentally demonstrate an optical wireless integration system at the Qband, in which up to 40 Gb∕s PM-QAM signal can be first transmitted over 20 km SMF-28, then delivered over a 2 m 2 × 2 MIMO wireless link, and finally transmitted over another 20 km SMF-28. The PM-QAM modulated wireless mm-wave signal at 40 GHz is generated based on remote heterodyning and demodulated by the RF transparent photonic technique based on homodyne coherent detection and baseband DSP. We propose a new scheme and realize the conversion of the PM-QAM modulated wireless mm-wave signal to the optical signal as well as 20 km fiber transmission of the converted optical signal. References 1. NTT Group CSR Report 2011, http://www.ntt.co.jp/csr_e/ 2011report/. 2. A. Hirata, M. Harada, and T. Nagatsuma, J. Lightwave Technol. 21, 2145 (2003). 3. X. Pang, A. Caballero, A. Dogadaev, V. Arlunno, R. Borkowski, J. S. Pedersen, L. Deng, F. Karinou, F. Roubeau, D. Zibar, X. Yu, and I. T. Monroy, Opt. Express 19, 24944 (2011). 4. A. Kanno, T. Kuri, I. Hosako, T. Kawanishi, Y. Yasumura, Y. Yoshida, and K. Kitayama, in Proceedings of the European Conference and Exhibition on Optical Communication (2012), paper We.3.B.2. 5. X. Li, J. Yu, Z. Dong, J. Zhang, N. Chi, and J. Yu, Opt. Lett. 38, 742 (2013). 6. X. Li, Z. Dong, J. Yu, N. Chi, Y. Shao, and G. K. Chang, Opt. Lett. 37, 5106 (2012). 7. Z. Dong, J. Yu, X. Li, G. K. Chang, and Z. Cao, in Proceedings of the Optical Fiber Communication Conference (2013), paper OM3D.2. 8. X. Li, J. Yu, J. Zhang, Z. Dong, F. Li, and N. Chi, Opt. Express 21, 18812 (2013). 9. R. Sambaraju, D. Zibar, R. Alemany, A. Caballero, and J. Herrera, in Proceedings of the Optical Fiber Communication Conference (2010), paper OML1. 10. R. Sambaraju, D. Zibar, A. Caballero, I. T. Monroy, R. Alemany, and J. Herrera, Photonics Technol. Lett. 22, 1650 (2010). 11. X. Zhou and J. Yu, J. Lightwave Technol. 27, 3641 (2009). 12. J. Yu and X. Zhou, IEEE Commun. Mag. 48(3), S56 (2010). 13. J. Zhang, Z. Dong, J. Yu, N. Chi, L. Tao, X. Li, and Y. Shao, Opt. Lett. 37, 4050 (2012). 14. X. Li, Z. Dong, J. Yu, J. Yu, and N. Chi, Opt. Express 21, 8808 (2013). 15. J. Yu, X. Zhou, M. F. Huang, Y. Shao, D. Qian, T. Wang, M. Cvijetic, P. Magill, L. Nelson, M. Birk, S. Ten, H. B. Matthew, and S. K. Mishra, in Proceedings of the European Conference and Exhibition on Optical Communication (2008), paper PDP: Th. 3. E. 2.
