Seamless integration of 57.2-Gb/s signal wireline transmission and 100-GHz wireless delivery

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1 Seamless integration of 57.-Gb/s signal wireline transmission and -GHz wireless delivery Xinying Li, Jianjun Yu,,,* Ze Dong, 3,4 Zizheng Cao, 5 Nan Chi, Junwen Zhang, Yufeng Shao, and Li Tao Department of Communication Science and Engineering, Fudan University, Handan Road, Shanghai 433, China ZTE Corp. Beijing 876, China 3 ZTE USA, Morristown, NJ 796, USA 4 Georgia Institute of Technology, Atlanta, GA 333, USA 5 Hunan University, Changsha, Hunan, China * jianjun@fudan.edu.cn Abstract: We experimentally demonstrated the seamless integration of 57.-Gb/s signal wireline transmission and -GHz wireless delivery adopting polarization-division-multiplexing quadrature-phase-shift-keying (PDM-QPSK) modulation with 4-km single-mode fiber-8 (SMF-8) transmission and -m wireless delivery. The X- and Y-polarization components of optical PDM-QPSK baseband signal are simultaneously upconverted to GHz by optical polarization-diversity heterodyne beating, and then independently transmitted and received by two pairs of transmitter and receiver antennas, which make up a x multiple-input multiple-output (MIMO) wireless link based on microwave polarization multiplexing. At the wireless receiver, a two-stage down conversion is firstly done in analog domain based on balanced mixer and sinusoidal radio frequency (RF) signal, and then in digital domain based on digital signal processing (DSP). Polarization de-multiplexing is realized by constant modulus algorithm (CMA) based on DSP in heterodyne coherent detection. Our experimental results show that more taps are required for CMA when the X- and Y- polarization antennas have different wireless distance. Optical Society of America OCIS codes: (6.6) Fiber optics and optical communications; (6.84) Heterodyne; (6.565) Radio frequency photonics. References and links. J. Wells, Faster than fiber: the future of multi-gb/s wireless, IEEE Microw. Mag. (3), 4 (9).. J. Yu, G. K. Chang, Z. Jia, A. Chowdhury, M. F. Huang, H. C. Chien, Y. T. Hsueh, W. Jian, C. Liu, and Z. Dong, Cost-effective optical millimeter technologies and field demonstrations for very high throughput wireless-over-fiber access systems, J. Lightwave Technol. 8(6), (). 3. T. Nagatsuma, T. Takada, H.-J. Song, K. Ajito, N. Kukutsu, and Y. Kado, Millimeter- and THz-wave photonics towards -Gbit/s wireless transmission, IEEE Photonic Society s 3rd Annu. Meeting, Denver, CO, Paper WE4, Nov. 7,. 4. D. Zibar, R. Sambaraju, A. Caballero, J. Herrera, U. Westergren, A. Walber, J. B. Jensen, J. Marti, and I. Tafur Monroy, High-capacity wireless signal generation and demodulation in 75- to -GHz band employing alloptical OFDM, IEEE Photon. Technol. Lett. 3(), 8 8 (). 5. A. Kanno, K. Inagaki, I. Morohashi, T. Sakamoto, T. Kuri, I. Hosako, T. Kawanishi, Y. Yoshida, and K.-I. Kitayama, 4 Gb/s W-band (75 GHZ) 6-QAM radio-over-fiber signal generation and its wireless transmission, ECOC, Geneva, We..P., Sept.. 6. 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, Gbit/s hybrid optical fiber-wireless link in the W-band (75- GHz), Opt. Express 9(5), (). 7. C. W. Chow, F. M. Kuo, J. W. Shi, C. H. Yeh, Y. F. Wu, C. H. Wang, Y. T. Li, and C. L. Pan, GHz ultrawideband (UWB) fiber-to-the-antenna (FTTA) system for in-building and in-home networks, Opt. Express 8(), (). 8. A. Kanno, K. Inagaki, I. Morohashi, T. Kuri, I. Hosako, and T. Kawanishi, Frequency-stabilized W-band two tone optical signal generation for high-speed RoF and radio transmission, Proc. IEEE Photon. Conf. (IPC), Arlington, USA, TuJ4, Oct.. # $5. USD Received 3 Aug ; revised 9 Sep ; accepted 3 Sep ; published Oct (C) OSA October / Vol., No. / OPTICS EXPRESS 4364

2 9. D. Zibar, R. Sambaraju, A. C. Jambrina, J. Herrera, and I. T. Monroy, Carrier recovery and equalization for photonic-wireless links with capacities up to 4 Gb/s in 75 GHz Band, Opt. Fiber Conf. (OFC ), Los Angeles, USA, OThJ4, Mar... T. Kuri, Y. Omiya, T. Kawanishi, S. Hara, and K. Kitayama, Optical transmitter and receiver of 4-GHz ultrawideband signal by direct photonic conversion techniques, IEEE Intl. Topic. Meeting Microw. Photon. (MWP6), Grenoble, France, W3 3, Oct. 6.. A. Kanno, K. Inagaki, I. Morohashi, T. Sakamoto, T. Kuri, I. Hosako, T. Kawanishi, Y. Yoshida, and K.-I. Kitayama, -Gb/s QPSK W-band (75GHz) wireless link in free space using radio-over-fiber technique, IEICE Electron. Express 8(8), 6 67 ().. J. Zhang, Z. Dong, J. Yu, N. Chi, L. Tao, X. Li, and Y. Shao, Simplified coherent receiver with heterodyne detection of eight-channel 5 Gb/s PDM-QPSK WDM signal after 4 km SMF-8 transmission, Opt. Lett. 37(9), ().. Introduction In order to realize the seamless integration of wireless and fiber-optic networks, 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 [ ]. Recently, the W-band (75GHz), with wider bandwidth and higher frequency, has attracted increasing interest as a candidate radio-frequency (RF) band to provide multi-gigabit wireless links for mobile data transmission. Meanwhile, the hybrid fiber-wireless link based on optically-modulated signal generation technique is expected to be suitable for W-band wireless transmissions as well as optical wireless links involving seamless integration of optical/wireless networks. Many fiber-wireless links in the W-band have been proposed and experimentally demonstrated in research community, and the bit rates of 4 and Gb/s have been attained adopting spectral efficient modulation format and digital coherent detection [4 6]. A -Gb/s fiber-wireless link in the W-band was experimentally demonstrated adopting polarization-division-multiplexing 6-ary quadrature-amplitudemodulation (PDM6QAM) with.-m wireless delivery, but without wireline fiber transmission [6]. Furthermore, in the scheme, the X- and Y-polarization components from two transmitter antennas were not received simultaneously at the wireless receiver because there is only one receiver antenna in the system, and the net data rate is smaller than Gb/s after removing the forward-error-correction (FEC) overhead as well. Because no polarizationdiversity heterodyne coherent receiver is employed, the receiver in the scheme is polarization sensitive and one polarization tracking system will be needed in the real system. In fact, one polarization controller was employed in [6] to simulate polarization tracking system. In this paper, we experimentally demonstrate the seamless integration of 57.-Gb/s signal wireline transmission and -GHz wireless delivery adopting polarization-divisionmultiplexing quadrature-phase-shift-keying (PDM-QPSK) modulation with 4-km singlemode fiber-8 (SMF-8) transmission and -m wireless delivery. The X- and Y-polarization components of the optical PDM-QPSK baseband signal are simultaneously up-converted to -GHz wireless carrier by optical polarization-diversity heterodyne beating, and then transmitted over a x multiple-input multiple-output (MIMO) wireless link. At the wireless receiver, a two-stage down conversion is firstly done in analog domain based on balanced mixer and sinusoidal RF signal, and then in digital domain based on digital signal processing (DSP). Polarization de-multiplexing is realized by constant modulus algorithm (CMA) based on DSP in heterodyne coherent detection. Our experimental results show that more taps are required for CMA when the X- and Y-polarization antennas have different wireless distance. The optimal CMA tap is longer than 3 when there is -cm difference on wireless distance between the X- and Y-polarization components. To our knowledge, the CMA tap for commercial G PDM-QPSK product is around 3, which means that more taps are required for this system if the X- and Y-polarization antennas have different distance.. Principle for the seamless integration of PDM signal wireline transmission and W- band wireless delivery over x MIMO wireless links Figure shows the architecture for the seamless integration of PDM signal wireline transmission and W-band wireless delivery over x MIMO wireless links, including central # $5. USD Received 3 Aug ; revised 9 Sep ; accepted 3 Sep ; published Oct (C) OSA October / Vol., No. / OPTICS EXPRESS 4365

3 office (CO) to generate optically-modulated PDM baseband signal, remote antenna units (RAUs) to up-convert the optical PDM baseband signal into the W-band, and end users to down-convert the received W-band PDM signal into the baseband. Fig.. The architecture for the seamless integration of PDM signal wireline transmission and W-band wireless delivery over x MIMO wireless links. Opt. Mod.: optical modulator, Pol. Mux: polarization multiplexer, SMF: single-mode fiber, OC: optical coupler, LO: local oscillator, PBS: polarization beam splitter, BPD: balanced photo detector, HA: horn antenna, Pow. Div.: power divider, CO: central office, RAU: remote antenna unit. At the optical baseband transmitter in the CO, the continuous wavelength (CW) lightwave from a laser is modulated by an optical modulator and then polarization-multiplexed by a polarization multiplexer to generate optical baseband signal. The optical modulator is driven by the transmitter data. After fiber-optic transmission, the baseband signal is respectively received by multiple RAUs. At the optical heterodyne up-converter in each RAU, there is a laser functioned as the local oscillator (LO), an optical 8 hybrid, two fast-response photo detectors (PDs) and two W-band transmitter horn antennas (HAs). Here, the LO is used as the carrier-frequency generating source. The frequency spacing between the LO and the laser in the CO is located in the W-band in order to generate the W-band central carrier frequency for the up-converted wireless signal. The optical 8 hybrid includes two polarization beam splitters (PBSs) and two optical couplers (OCs), and is used to implement polarization diversity of the received signal together with the LO in optical domain before heterodyne beating. Next, two fast-response PDs, functioned as two photo-mixers, directly up-convert the X- and Y-polarization components of the optical PDM signal into the W-band, respectively. It s worth noting that X- or Y-polarization component of the PDM signal after polarization diversity does not mean that only X- or Y-polarization signal exists at each output port of PBSs. In fact, each output port contains both X- and Y-polarization signals. In this paper, we define one output port of each PBS as X-polarization component and the other as Y- polarization for simplification. The central frequency of the X- and Y-polarization upconverted components should be equal to the frequency spacing between the LO and the laser in the CO. Then, the X- and Y-polarization components, at the same time, are independently sent into free space by two transmitter HAs, and then received by two corresponding receiver HAs at the end user, which makes up a x MIMO wireless link based on microwave polarization multiplexing. At each end user, there is a two-stage down conversion [6]. In the first stage, the X- and Y-polarization components are respectively down-converted to a lower intermediate frequency (IF) in analog domain based on balanced mixer and sinusoidal RF signal, and subsequently implemented analog-to-digital conversion in a digital storage # $5. USD Received 3 Aug ; revised 9 Sep ; accepted 3 Sep ; published Oct (C) OSA October / Vol., No. / OPTICS EXPRESS 4366

4 oscilloscope (OSC). Then, the second-stage down conversion and final data recovery is realized with DSP in digital domain. 3. Experimental setup Figure shows the experimental setup for the seamless integration of 57.-Gb/s signal wireline transmission and -GHz wireless delivery adopting PDM-QPSK modulation with 4-km SMF-8 transmission and -m wireless delivery. At the optical baseband transmitter, there is an external cavity laser (ECL) with linewidth less than khz and maximal output power of 4.5dBm. The CW lightwave at 558.5nm from ECL is modulated by inphase/quadrature (I/Q) modulator. I/Q modulator is driven by a 4.3-Gbaud electrical binary signal, which, with a pseudo-random binary sequence (PRBS) length of 5, is generated from a pulse pattern generator (PPG). For optical QPSK generation, the two parallel Mach- Zehnder modulators (MZMs) in I/Q modulator are both biased at the null point and driven at the full swing to achieve zero-chirp - and π-phase modulation. The phase difference between the upper and the lower branches of I/Q modulator is controlled at π/. The subsequent polarization multiplexing is realized by polarization multiplexer, comprising a PBS to halve the signal into two branches, an optical delay line (DL) to provide a 5-symbol delay, an optical attenuator to balance the power of two branches and a polarization beam combiner (PBC) to recombine the signal. The generated signal is launched into the straight line of five spans (the maximal distance) of 8-km SMF-8. Each span has 8-dB average loss and 7- ps/km/nm chromatic dispersion (CD) at 55nm without optical dispersion compensation. Erbium-doped fiber amplifier (EDFA) is used to compensate the loss of each span. The total launched power (after EDFA) into each span is dbm. Optical power (dbm).nm Wavelength (nm) Fig.. Experimental setup for the seamless integration of 57.-Gb/s signal wireline transmission and -GHz wireless delivery. Inset (a) shows the X-polarization optical spectrum (.-nm resolution) after polarization-diversity splitting. EDFA: Erbium-doped fiber amplifier, SMF: single-mode fiber. At the optical up-converter, an ECL with linewidth less than khz is used as the LO at 557.7nm, which has -GHz frequency offset (IF = GHz) relative to the received signal. The principle of the up-converter is introduced in detail in Section. Each -GHz PD has 75-GHz 3-dB bandwidth and 7.5-dBm input power. The X- and Y-polarization upconverted components on -GHz wireless carrier independently pass through two -GHz narrowband electrical amplifiers (EAs) with 3-dB gain, and then, are simultaneously sent into a x MIMO wireless air link based on microwave polarization multiplexing. Each pair of transmitter and receiver HAs (transmitter HA and receiver HA as well as transmitter # $5. USD Received 3 Aug ; revised 9 Sep ; accepted 3 Sep ; published Oct (C) OSA October / Vol., No. / OPTICS EXPRESS 4367

5 HA and receiver HA) has a.5~.5-m wireless distance, the X- and Y- polarization wireless links are parallel and two transmitter (receiver) HAs have a 4-cm distance. Each HA has a 5-dBi gain. Inset (a) shows the X-polarization optical spectrum after polarizationdiversity splitting. Here, ch denotes the LO, while ch the received signal. The frequency spacing and power difference between ch and ch is GHz and db, respectively. At the wireless receiver, two-stage down conversion is implemented for the X- and Y- polarization received components. A -GHz sinusoidal RF signal firstly passes through an active frequency doubler (x) and an EA in serial, and is then halved into two branches by a power divider. Next, each branch passes through a passive frequency tripler (x3) and an EA. As a result of this cascaded frequency doubling, an equivalent 7-GHz RF signal is provided for the corresponding balanced mixer. Therefore, the X- and Y-polarization components centered on 8GHz (IF = 8GHz) are obtained after first-stage down conversion, as shown in Fig. 3(a). Each band-pass low-noise amplifier (LNA) after the mixer is centered on GHz and has a 5-dB noise figure. The analog-to-digital conversion is realized in the real-time OSC with -GSa/s sampling rate and 45-GHz electrical bandwidth x Fig. 3. (a) Electrical spectrum after first-stage down conversion; (b) DSP. Figure 3(b) shows the detailed DSP after analog-to-digital conversion. Firstly, the received signals are down-converted to the baseband by multiplying synchronous cosine and sine functions, which are generated from a digital LO for down conversion []. Secondly, a T/- 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. Fourthly, two complex-valued, 3~33-tap, T/-spaced adaptive FIR filters, based on the classic CMA, are used to retrieve the modulus of the PDM- QPSK signal and realize polarization de-multiplexing. The subsequent step is carrier recovery, which includes frequency-offset estimation and carrier-phase estimation (CPE). The former is based on fast Fourier transform (FFT) method while the latter fourth-power Viterbi- Viterbi algorithm. Finally, differential decoding is used to eliminate the π/ phase ambiguity before bit-error-ratio (BER) counting. In this experiment, the BER is counted over 6 bits ( data sets, and each data set contains 6 bits). 4. Experimental results and discussions Figure 4 gives the BER versus optical signal-to-noise ratio (OSNR) at.-nm noise level after -m wireless delivery. The launched power into fiber is dbm. Here, back-to-back (BTB) denotes no fiber transmission. Compared to the BTB case, there is almost no OSNR penalty after 4-km SMF-8 transmission. Inset (a) and (b) show the X- and Y-polarization constellations after CPE over 4-km SMF-8 transmission, respectively. Figure 5 gives the X-polarization constellations after CMA and further CPE in the case of 3- and 3-tap CMA length, respectively. The Y-polarization constellations show the similar performance. There is 5-cm difference on wireless distance between the X- and Y-polarization components. It s worth noting that transmitter HA, receiver HA and receiver HA are all fixed, while the distance between transmitter HA and receiver HA is changed by moving transmitter HA. We can see that the constellations for the 3-tap CMA length are much clearer than those for the 3-tap CMA length. Figure 6 gives the Y-polarization constellations after CPE for 3-, 3- and 33-tap CMA length, respectively. There is -cm difference on wireless distance # $5. USD Received 3 Aug ; revised 9 Sep ; accepted 3 Sep ; published Oct (C) OSA October / Vol., No. / OPTICS EXPRESS 4368

6 between the X- and Y-polarization components. Similarly, the constellation becomes clearer as the length of CMA tap increases. Furthermore, more CMA taps will be required for the larger difference on wireless distance between the X- and Y-polarization components. The Xpolarization constellations after CPE show the similar performance. To our knowledge, the CMA tap for commercial G PDM-QPSK product is around 3, which means that more taps are required for this system if X- and Y-polarization antennas have different distance.. Back to back After 4km transmission X-pol: after Carrier Phase Estimation BER (a).5.5 (b) E-3 E OSNR (db) 6 8 Fig. 4. BER versus OSNR after -m wireless delivery. Inset (a) and (b) show the X- and Ypolarization constellations after CPE over 4-km SMF-8 transmission, respectively. X-pol: after CMA X-pol: after Carrier Phase Estimation X-pol: after CMA X-pol: after Carrier Phase Estimation Fig. 5. X-polarization constellations. (a) 3 tap, after CMA; (b) 3 tap, after CPE; (c) 3 tap, after CMA; (d) 3 tap, after CPE Fig. 6. Y-polarization constellations after CPE for 3-, 3- and 33-tap CMA length. 5. Conclusion We first experimentally demonstrated a fiber-wireless system that is capable to deliver 57.Gb/s PDM-QPSK signal over 4-km SMF-8 and -m wireless delivery at GHz over a x MIMO wireless link based on microwave polarization multiplexing, adopting two-stage down conversion both in analog and digital domains. The optimal CMA tap is longer than 3 when there is -cm difference on wireless distance between the X- and Y-polarization components. To our knowledge, the CMA tap for commercial G PDM-QPSK product is around 3, which means that more taps are required for this system if X- and Y-polarization antennas have different distance. # $5. USD (C) OSA Received 3 Aug ; revised 9 Sep ; accepted 3 Sep ; published Oct October / Vol., No. / OPTICS EXPRESS 4369