Optical direct-detection OFDM signal generation for radio-over-fiber link using frequency doubling scheme with carrier suppression

This investigation demonstrates the generation of OFDM-RoF signal using frequency doubling technique for the first time, to the author’s best knowledge. The 4-Gb/s OFDM signal using 16-QAM format modulated on each subcarrier at a center frequency of 19GHz is experimentally demonstrated. Benchmarked against the OOK format, the 16-QAM OFDM format has the higher spectral efficiency with a sensitivity penalty of less than 2.6 dB. After transmission over 50-km single mode fiber, the power penalties of RF OOK and OFDM signals are less than 0.5dB. ©2008 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.2360) Fiber optics links and subsystems; (060.4370). References and links 1. J. Lowery and J. Armstrong, “Orthogonal-frequency-division multiplexing for dispersion compensation of long-haul optical systems,” Opt. Express 14, 2079-2084 (2006). 2. B. Djordjevic and B. Vasic, “Orthogonal frequency division multiplexing for high-speed optical transmission,” Opt. Express 14, 3767-3775 (2006). 3. H. Bao and W. Shieh, “Transmission simulation of coherent optical OFDM signals in WDM systems,” Opt. Express 15, 4410-4418 (2007). 4. W. H. Chen, and W. I. Way, “Multichannel Single-Sideband SCM/DWDM Transmission System,” J. Lightwave Technol. 22, 1697-1693 (2004). 5. Wu and X. Zhang, “Impact of Nonlinear Distortion in Radio Over Fiber Systems with Single-Sideband and Tandem Single-Sideband Subcarrier Modulations,” J. Lightwave Technol. 24, 2076-2090 (2006). 6. J. Yu, Z. Jia, L. Yi, G. K. Chang, and T. Wang, “Optical millimeter wave generation or up-conversion using external modulators,” IEEE Photon. Technol. Lett. 18, 265–267 (2006). 7. J. J. O’Reilly, P. M. Lane, R. Heidemann, and R. Hofstetter, “Optical generation of very narrow linewidth millimeter wave signals,” Electron. Lett. 28, 2309–2311 (1992). 8. Lim, C. Lin, M. Attygalle, A. Nirmalathas, D. Novak, and R. Waterhouse, “Analysis of Optical Carrier-to-Sideband Ratio for Improving Transmission Performance in Fiber-Radio Links,” IEEE J. Lightwave Technol. 54, 2181-2187 (2006). 9. L. N. Langey, M. D. Elkin, C. Edge, M. J. Wale, U. Gliese, X. Huang, and A. J. Seeds, “Packaged semiconductor laser optical phase-locked loop (OPLL) for photonic generation, processing and transmission of microwave signals,” IEEE Trans. Microwave Theory Technol 47, 1257-1264 (1999). 10. S. Choi, Y. Shoji, and H. Ogawa, “Millimeter-wave fiber-fed wireless access system based on dense wavelength-division-multiplexing networks,” IEEE Trans. Microwave Theory Technol 56, 232-241 (2008). 11. T. Sakamoto, T. Kawanishi, and M. Izutsu, “Continuous-phase frequency-shift keying with external modulation,” IEEE J. Sel. Top. Quantum Electron. 12, 589–595 (2006). 12. W. Shieh, X. Yi, and Y. Tang, “Experimental Demonstration of Transmission of Coherent Optical OFDM Systems,” OFC/NFOEC 2007, OMP2, March, 2007. 13. V. J. Urick, J. X. Qiu, and F. Bucholtz, “Wide-band QAM-over-fiber using phase modulation and #90231 $15.00 USD Received 29 Nov 2007; revised 2 Apr 2008; accepted 6 Apr 2008; published 25 Apr 2008 (C) 2008 OSA 28 April 2008 / Vol. 16, No. 9 / OPTICS EXPRESS 6056 interferometric demodulation,” IEEE Photon. Technol. Lett., 16, 2374-2376 (2004). 14. M. KaveHrad and E. Savov, “Fiber-Optic Transmission of Microwave 64-QAM Signals,” IEEE J. Sel. Areas in Commun. 8, 1320-1326 (1990).


Introduction
The combination of orthogonal frequency-division multiplexing (OFDM) and radio-over-fiber (RoF) systems (OFDM-RoF) has attracted considerable attention for future gigabit broadband wireless communication.The high peak to average power ratio (PAPR) and the nonlinear distortion of the optical transmitter are the main issues raised by OFDM and RoF systems [1][2][3][4][5], respectively.The optical radio frequency (RF) signal generation using an external Mach-Zehnder modulator (MZM) based on double-sideband (DSB), single-sideband (SSB), and double-sideband with optical carrier suppression (DSBCS) modulation schemes have been demonstrated [1,2,[4][5][6][7][8]. Since the optical RF signals are weakly modulated because of the narrow linear region of MZM, those that have undergone DSB and SSB modulation suffer from inferior sensitivities due to limited optical modulation index (OMI) [4][5][6]8].Hence, an optical filter is needed to improve the performance [8].Furthermore, the DSB signal experiences the problem of performance fading because of fiber dispersion [6].Among these modulation schemes, DSBCS modulation has been demonstrated to be effective in the millimeter-wave range with excellent spectral efficiency, a low bandwidth requirement for electrical components, and superior receiver sensitivity following transmission over a long distance [6].However, all of the proposed DSBCS schemes can only support on-off keying (OOK) format, and none can transmit vector modulation formats, such as phase shift keying (PSK), quadrature amplitude modulation (QAM), or OFDM signals, which are of utmost importance for wireless applications.
On the other hand, optical RF signal generations using remote heterodyne detection (RHD) have been also demonstrated [9][10].The advantage of RHD systems is that the vector signal can be modulated at baseband.Therefore, the bandwidth requirement of the transmitter is low.However, the drawback is that phase noise and wavelength stability of the lasers at both transmitter and receiver should be carefully controlled.
This study proposes a novel method for generating optical direct-detection OFDM-RoF signals using a new DSBCS modulation scheme that can carry vector signals.A frequency doubling scheme is employed to reduce the requirement of bandwidth of electronic components, which is an important issue at millimeter-wave RoF systems.Benchmarked against the OOK format, the 4-Gb/s 16-QAM OFDM format has the higher spectral efficiency with a sensitivity penalty of under 2.6-dB.-c).When both MZ-a and MZ-b are biased at the null point, the generated optical spectrum consists of an upper sideband (USB) and a lower sideband (LSB) with optical carrier suppression.Except with a phase difference of 90°, the RF signals sent into the MZ-a and MZ-b are exactly the same.When MZ-c is biased at the quadrature point, either USB or LSB will be eliminated at the output of the integrated MZM.

Concept of proposed system
The basic operation principle is as follow.When MZ-a and MZ-b are biased at the null point, only odd-order optical sidebands will be generated.To simplify the concept, only two first-order sidebands (USB and LSB) are shown in the optical spectrum insets of Fig. 1.Firstly, for the data path, the inset (i) shows the generated USB and LSB at the output of MZ-a.
Because a 90° phase shift is added at MZ-b, the generated USB and LSB need to rotate +90° and -90°, respectively, as shown in inset (ii).Biasing MZ-c at the quadrature point will add another +90° phase shift for all optical sidebands from MZ-b, as shown in inset (iv).After combination of insets (iii) and (iv), an optical single sideband (LSB) with carrier suppression is obtained as shown in inset (v).The operation principle of the sinusoidal signal path is similar and is shown in inset (a) to (d).Therefore, as both sinusoidal and data-modulated signals are simultaneously sent to integrated MZM, as shown in insets (3) and (4) of Fig. 1, the optical generated signal consisting of the two-tone lightwave, which can be converted into electrical RF signals by square-law photo-diode (PD) detection, can be produced at the output of the transmitter.
Since the RF signal can be modulated only at either the USB or the LSB, the proposed system can generate not only OOK signals but also PSK, QAM and OFDM signals.
Additionally, the relative intensity between USB and LSB can be easily tuned by adjusting the individual power of the electrical sinusoidal and data signals to optimize the performance of the optical RF signals.A frequency doubling approach is employed to reduce the cost of the electronic components, especially for the RF signal in the millimeter-wave range.Fig. 3(a).The OFDM signal is generated using a 4GHz arbitrary waveform generator (AWG) with eight carriers, and each subcarrier is encoded with 62.5MHz 16-QAM symbol, as shown in Fig. 4(a).The cyclic prefix is 1/32 symbol time.The OFDM signal is up-converted to 9.5GHz by a mixer, as shown in Fig. 4 AWG has only one output, the lower sideband and upper sideband of the electrical OFDM signal are not mutually independent.Notably, the constraint between the subcarriers' symbols has negligible effect on the system performance because every subcarrier is demodulated independently [12].Therefore, with such a setup, a 4-Gb/s RF OFDM signal that has 16 subcarriers and occupies a total bandwidth of 1GHz can be generated at a center frequency of 9.5GHz.The OOK baseband signal with a pseudo random binary sequence (PRBS) of word length of 2 31 -1 is generated by a pattern generator and then pass through a low-pass filter with a 3-dB bandwidth of 1 GHz.The OOK signal is up-converted to a center frequency of 9.5

(b). Since the
GHz and the bandwidth of the up-converted OOK signal is around 2 GHz.
The RF OFDM/OOK signal at 9.5GHz is then split using a 90 o hybrid coupler to drive MZ-a and MZ-b. Figure 5(b) shows that the OFDM signal is modulated on the optical LSB.The same mechanism is used to split the 9.5GHz sinusoidal signal to drive MZ-a and MZ-b. Figure 5(a) reveals that a new optical carrier can be generated at the USB of the original carrier by 9.5GHz.Figure 5(c  digital signal processing (DSP) program is employed to demodulate the OFDM signal.The block diagram of the OFDM receiver is shown in Fig. 3(b).The demodulation process includes synchronization, Fast Fourier Transform (FFT), one-tap equalization, and QAM symbol decoding.The bit error rate (BER) performance is calculated from the measured error vector magnitude (EVM) [13][14].The 1.25-Gb/s OOK signal at 19GHz is also tested in the proposed architecture but is down-converted to baseband signal.Then, the OOK baseband signal is directly tested by the BER tester without the off-line DSP program.
Figures 6(a) and 6(b) show 16QAM constellation diagrams before and after the one-tap equalizer in back to back (BTB) case, respectively.The equalizer in OFDM transceiver is used to combat both frequency response of various microwave components and fiber dispersion.Figures 6(c) and 6(d) present the 16-QAM constellation diagrams after 25-km and 50-km SMF transmission, respectively.With the QAM's high bandwidth utilization efficiency, the 4-Gb/s data only occupies 1GHz bandwidth and fiber chromatic dispersion effect is negligible.Figure 7 shows the BER performance.For OFDM signals, the receiver sensitivity of 13.4dBm was achieved at a BER of 10 -9 in the BTB case.The penalty at a BER of 10 -9 is less than 0.2 dB following 25-km or 50-km SMF transmission.The performance of OOK signals is 2.6 dB better than that of OFDM signals at the cost of lower bandwidth efficiency.

Conclusions
It is of utmost important for the RoF system to support such vector signals that have been widely used in wireless communication.The OFDM format is particularly attractive due to its resistance to multi-path fading.The proposed architecture utilizes the carrier suppression technique to achieve frequency doubling and successfully transmits a high capacity 4-Gb/s OFDM signal over 50 km SMF with negligible sensitivity penalty.

Figure 1
Figure 1 schematically depicts the principle of the proposed optical direct-detection OFDM-RoF signal generation.An integrated x-cut LiNbO 3 MZM [11], comprising three single-electrode MZMs, is key to generating optical RF signals.Two sub-MZMs (MZ-a and MZ-b) are embedded in each arm of the main modulator (MZ-c).When both MZ-a and MZ-b are biased at the null point, the generated optical spectrum consists of an upper sideband (USB) and a lower sideband (LSB) with optical carrier suppression.Except with a phase difference of 90°, the RF signals sent into the MZ-a and MZ-b are exactly the same.When MZ-c is biased at the quadrature point, either USB or LSB will be eliminated at the output of the integrated MZM.The basic operation principle is as follow.When MZ-a and MZ-b are biased at the null point, only odd-order optical sidebands will be generated.To simplify the concept, only two first-order sidebands (USB and LSB) are shown in the optical spectrum insets of Fig.1.Firstly, for the data path, the inset (i) shows the generated USB and LSB at the output of MZ-a.

Figure 2 Fig. 1 .
Figure 2 depicts the experimental setup.Both RF OFDM and OOK signals are demonstrated in our proposed architecture.The block diagram of the typical OFDM transmitter is shown in