60-GHz optical / wireless MIMO system integrated with optical subcarrier multiplexing and 2 x 2 wireless communication

This paper proposes a 2x2 MIMO OFDM Radio-over-Fiber scheme based on optical subcarrier multiplexing and 60-GHz MIMO wireless transmission. We also schematically investigated the principle of optical subcarrier multiplexing, which is based on a dual-parallel MachZehnder modulator (DP-MZM). In our simulation result, combining two MIMO OFDM signals to drive DP-MZM gives rise to the PAPR augmentation of less than 0.4 dB, which mitigates nonlinear distortion. Moreover, we applied a Levin-Campello bit-loading algorithm to compensate for the uneven frequency responses in the V-band. The resulting system achieves OFDM signal rates of 61.5-Gbits/s with BER of 3 10 over 25-km SMF transmission followed by 3-m wireless transmission. ©2015 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.4230) Multiplexing; (060.4510) Optical communications. References and links 1. J. Wells, “Faster than fiber: the future of multi-Gb/s wireless,” IEEE Microwave Mag. 10(3), 104–112 (2009). 2. X. D. Pang, Y. Zhao, L. Deng, M. B. Othman, X. B. Yu, J. B. Jensen, D. Zibar, and I. T. Monroy, “Seamless Translation of Optical Fiber PolMux-OFDM into a 2×2 MIMO Wireless Transmission Enabled by Digital Training-Based Fiber-Wireless Channel Estimation,” in Proceedings of the IEEE Communications and Photomics Conference and Exhibition, ACP, Asia, INSPEC Accession Number. 12784470, (2011), pp. 1–6. 3. A. Kanno, T. Kuri, I. Hosako, T. Kawanishi, Y. Yoshida, Y. Yasummura, and K. Kitatama, “Coherent Optical and Radio Seamless Transmission Based on DPS-Aided Radio-over-Fiber Technology,” Opt. Fiber Conf., Los Angeles, USA, Plenary Talk (2013). 4. L. Tao, Z. Dong, J. Yu, N. Chi, J. Zhang, X. Li, Y. Shao, and G.-K. Chang, “Experimental Demonstration of 48Gb/s PDM-QPSK Radio-Over-Fiber System Over 40-GHz mm-Wave MIMO Wireless Transmission,” IEEE Photon. Technol. Lett. 24(24), 2276–2279 (2012). 5. C. T. Lin, Y. M. Lin, J. Y. Chen, S. P. Dai, P. T. Shih, P.-C. Peng, and S. Chi, “Optical direct-detection OFDM signal generation for radio-over-fiber link using frequency doubling scheme with carrier suppression,” Opt. Express 16(9), 6056–6063 (2008). 6. H. T. Huang, P. T. Shih, C. T. Lin, Y. H. Cheng, W. L. Liang, C. H. Ho, C. C. Wei, and A. Ng’oma, “2 x 2 MIMO OFDM-RoF System Employing LMS-Based Equalizer With I/Q Imbalance Compensation at 60 GHz,” IEEE Photon. J. 6(3), 7200307 (2014). 7. S. H. Han and J. H. Lee, “An Overview of Peak-to-Average Power Ratio Reduction Techniques for Multicarrier Transmission,” IEEE Wirel. Commun. 12(2), 56–65 (2005). 8. J. Campello, “Optimal Discrete Bit Loading for Multicarrier Modulation Systems,” International Symposium on Information Theory (ISIT), 193 (1998).


Introduction
Increasing demand for high-definition video and interactive on-line services has made it necessary to provide broadband wireless at the multi-Gbits/s rate.Greater wireless capacity can be achieved by increasing the carrier frequency through the expansion of bandwidth and the introduction of a modulation format with higher spectral efficiency.The license-free Vband (57-64 GHz) can provide considerably more bandwidth than WLAN (2.4 GHz and 5 GHz [1].However, V-band wireless transmission is limited by high propagation loss in air.The need to remedy this situation led to the development of radio-over-fiber (RoF) technology, combining the advantages of fiber and wireless as a means of expanding the coverage of millimeter-wave (MMW) wireless signals.A higher modulation format can also improve spectral efficiency, such that data rates can be increased even within the same useable bandwidth.Moreover, multi-input multi-output (MIMO) technology can improve spectral efficiency by transmitting multiple signals within the same bandwidth.This has led to the widespread adoption of modulation formats with high spectral efficiency and MIMO technology in communication systems.Recently, several systems combining polarization division multiplexing (PDM) and MIMO with coherent heterodyne have been proposed [2][3][4].In [2], a direct-detection (DD) RoF system with PDM was presented; however, polarizing variations of the optical carrier can induce signal fading after the polarization beam splitter (PBS) and photodetector (PD).This necessitates controlling the state of polarization (SoP) using a polarization controller within the DD-PDM RoF system prior to PBS.In [3,4], a coherent-detection (CO) PDM RoF system was proposed to solve issues related to the SoP in DD RoF systems.Nonetheless, a narrow bandwidth laser and additional digital signal process (DSP) algorithms are required to avoid or compensate phase noise.
This paper proposes a V-band 2x2 MIMO OFDM RoF system employing optical subcarrier multiplexing (SCM).This paper systematically outlines the concept on which the proposed system is based and evaluates wireless performance at 60 GHz.Using bit-loading technology, the proposed system achieved OFDM signals as high as 61.5-Gbits/s with a bit error rate (BER) below the forward error correction (FEC) threshold of 3 10 − over 25-km single-mode fiber (SMF) followed by 3-m wireless transmission.Following the transfer from serial to parallel and discrete Fourier transform (DFT), a frequency domain equalizer LMS-based equalizer with IQ imbalance compensation is used to represent two MIMO signals [6].The QAM signals are de-mapped to binary data streams and the BERs are measured according to error count, as shown in the inset (ii) of Fig. 3.

Experimental results
One of important parameters in OFDM RoF systems is the peak-to-average power ratio (PAPR), which is largely dominated by the number of OFDM subcarriers.The driving signals of MZM-A and MZM-B comprise two OFDM signals (Signal-1 and Signal-2), which can lead to an increase in the PAPR, resulting in nonlinear distortion.Figure 4 presents the simulation results of the complementary cumulative distribution function (CCDF).The CCDF of the PAPR denotes the probability that the PAPR of a data block will exceed a given threshold.With OFDM signals performing IDFT, PAPR is calculated according to the maximum peak power of one data block in the time domain to the average power of one data block [7].Simulation parameters include a FFT length of 512, a CP of 1/32 symbol time, 298 subcarriers, 1000 blocks, an AWG sampling rate of 12 GSample/s, center frequency of 21.5 GHz, and channel sampling rate of 120 GSample/s.After combining the two signals, no discernable increase in PAPR was observed for a CCDF probability of 0.1.A probability of 0.01 resulted in a PAPR increase of less than 0.4 dB.In V-band communication, the frequency response of components is susceptible to serious fluctuations.The Levin-Campello bit-loading algorithm is employed to deal with the uneven frequency response of the V-band in order to increase the data rate by obtaining weighting factors and bit locations corresponding to OFDM subcarriers at a target BER of 3 10 − [8].Inset (i) of Fig. 6 presents the bit location of each OFDM subcarrier after assigning formats of 8-QAM, 16-QAM, 32-QAM, and 64-QAM.Following the estimation of weighting factor and bit location, an OFDM signal with bit loading is generated using the offline DSP program in the inset (i) of Fig. 3. Using the bit-loading algorithm with a PD received power of −2 dBm, the total data rate of 2x2 OFDM MIMO can be increased from 55.875 Gbits/s to 61.5 Gbits/s following 25-km SMF transmission.The SNRs of two MIMO OFDM signals with bit-loading are presented in the inset (ii) of Fig. 6.The constellations based on modulation with different QAM formats can be clearly differentiated, as shown in Fig. 7.

Conclusion
This paper presents a 2x2 MIMO SCM RoF system based on DP-MZM for operation at 60 GHz.Experimental results show that the 2x2 MIMO 16-QAM OFDM signals are able to achieve a BER of 3 10 − for BTB and 25-km SMF transmission.Using the bit-loading algorithm to compensate for the uneven wireless channel response at 60 GHz, the data rate can be increased from 55.875 Gbits/s to 61.5 Gbits/s.

Figure 1
Figure 1 illustrates the principle underlying the proposed 2x2 MIMO SCM RoF system.A dual-parallel Mach-Zehnder modulator (DP-MZM) comprising three single-electrode sub-MZMs plays a key role in generating optical 2x2 MIMO signals [5].The electrical driving DP-MZM signals comprise two OFDM signals at the center frequency of 21.5 GHz and an electrical 39-GHz carrier.For the electrical 39-GHz carrier signal, when two sub-MZMs

Figure 2
Figure 2 presents the experimental setup.The 2x2 MIMO driving signals comprise two 21.5-GHzOFDM signals and one 39-GHz sinusoidal signal.The electrical baseband OFDM signals are generated using an arbitrary waveform generator (AWG) at a sampling rate of 12

Fig. 2 .
Fig. 2. Experimental setup of 2x2 MIMO SCM RoF system at 60GHz.Optical spectrum (i), and (ii) are obtained at the outputs of interleaver.GSample/s and then up-converted to 21.5 GHz via two different electrical mixers, as shown in insets (a) and (b) of Fig. 1.Other parameters include an FFT length of 512, a cyclic prefix (CP) of 1/32 symbol time, and 298 subcarriers.After passing through the DP-MZM and erbium-doped fiber amplifier (EDFA), the optical signals are launched into a 25-km SMF, whereupon optical MIMO signals are divided using a 33/66 optical interleaver (Optoplex Corporation) to be detected by high-speed PDs for wireless transmission.Insets (e) and (f) in Fig. 1 present the electrical frequency spectrum.Two pairs of rectangular horn antennas with

Figure 3
Figure 3 presents the offline Matlab ® DSP program.At the transmitter, two quadrature amplitude modulation (QAM) OFDM signals are mapped through two independent binary data streams based on Gray code.After the OFDM signals are transferred to the time domain via inverse discrete Fourier transform (IDFT) and a CP is added, digital OFDM signals are sent to AWG for analog to digital conversion (ADC), as shown in the inset (i) of Fig. 3.After receiving signals down-converted to baseband, they are synchronized and removed CP.Following the transfer from serial to parallel and discrete Fourier transform (DFT), a frequency domain equalizer LMS-based equalizer with IQ imbalance compensation is used to

Figure 5
presents the BER curves for 55.875-Gbits/s 16-QAM OFDM signals with 3-m wireless transmission.In the back-to-back (BTB) case, OFDM signals are shown to meet the FEC BER limit of 3 10 − .Following SMF transmission over a distance of 25 km, the MIMO OFDM signals maintained a BER of 3 10 − with no power penalty.The constellations for the BTB and 25-km transmission cases with PD received power of −2 dBm, which is the optimal situation, are also presented in the Fig. 5. Constellations of Signal-1 and Signal-2 can be clearly differentiated in both case of BTB and 25-km transmission.