© 2011 IEEE. Reprinted, with permission, from: Chun-Ting Lin, Anthony Ng’oma, Wei-Yuan Lee, Chih-Yun Wang, Tsung-Hung Lu, Wen-Jr Jiang, Chun-Hung Ho, Chia-Chien Wei, Jyehong Chen, and Sien Chi; MIMO-Enhanced Radioover-Fiber System at 60 GHz 2011 European conference on optical communications, pp. We.10.P1.120; September 18-22, 2011 This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of Corning Incorporated’s products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to pubspermissions@ ieee.org. By choosing to view this document, you agree to all provisions of the copyright laws protecting it.
ECOC Technical Digest © 2011 OSA
MIMO-Enhanced Radio-over-Fiber System at 60 GHz Chun-Ting Lin1*, Anthony Ng’oma2, Wei-Yuan Lee3, Chih-Yun Wang1, Tsung-Hung Lu1, Wen-Jr Jiang3, Chun-Hung Ho1, Chia-Chien Wei4, Jyehong Chen3, and Sien Chi5 1. Institute of Photonic System, National Chiao Tung University, 301, Gaofa 3rd., Guiren Township, Tainan County 711, Taiwan. 2. Corning Incorporated, SP-DV-02-9, Corning, NY 14831, USA 3. Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University, Taiwan 4. Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, 1 University Rd., Puli, Nautou, Taiwan, 545 5. Department of Electrical Engineering, Yuan-Ze University, Taiwan, E-mail:
[email protected]*
Abstract: We employ 2x2 MIMO spatial multiplexing to double the spectral efficiency of a 60GHz RoF system. A 27.15-Gbps 16-QAM MIMO signal with 3m wireless transmission is experimentally demonstrated within 7-GHz license-free spectrum at 60 GHz. OCIS codes: (060.4510) Optical Communications ; (060.4080) Modulation
1. Introduction The rapid growth of data rates of new wireless applications led by interactive multimedia services is driving the need for multi-Gbps wireless communication technologies in the near future. The 7-GHz license-free spectrum at 60 GHz is the most promising solution to multi-Gbps wireless service [1-2]. However, 60-GHz and other millimeter-waves have very high propagation losses rendering them more suitable for short-range wireless links (~10m). Therefore, 60-GHz radio-over-fiber (RoF) technology is a promising candidate to provide broadband service, wide coverage, and mobility due to the unlimited bandwidth and low transmission loss of optical fiber. In this paper, we employ multiple-input multiple-output (MIMO) antenna technology in order to improve (double) the spectral efficiency of the 60 GHz RoF system. MIMO is a technology that is now widely used in many wireless systems, such as WLAN (IEEE 802.11n) and LTE. MIMO technology can significantly improve the capacity by spatial multiplexing to surpass the traditional Shannon’s limit. By increasing the number of antennas at the transmitter and receiver, MIMO can not only raise the data-rate but also increase the system reliability through spatial diversity [3-4]. Recently, studies into MIMO technology for 60-GHz RoF systems have been reported [5]. However, the reported data rate is less than 10 Gbps because of the limited spectral efficiency of the on-off keying (OOK) modulation format used. In this paper, we experimentally demonstrate 2x13.575-Gb/s 16-QAM MIMO signal transmission within 7-GHz license-free bandwidth at 60-GHz band. The power penalty is negligible after transmission over 25-km single mode fiber and 3m wireless distance.
Fig. 1. RoF with MIMO technology.
2. 2x2 MIMO technology with Frequency Domain Equalizer Figure 1 shows the fundamental concept of 2x2 MIMO RoF System. Two antennas, Tx1 and Tx2, transmit two different data streams to the receive antennas, Rx1 and Rx2 using the same frequency spectrum. The received signals will be the summation of these two transmitted signals with different channel coefficients due to the different transmission paths and they can be expressed as [ ]
[
][ ]
[
]
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where y and x are received and transmitted data, respectively, w is the noise, and h is the channel coefficient. If the channel coefficient is estimated using the appropriate training symbol, the transmitted data can be recovered by using the zero-forcing algorithm and written as ̅ [ ] [ ] [ ] ̅ Moreover, since the wideband 60 GHz RoF system has an uneven frequency response of up to 12 dB within 7 GHz license-free band [6], the channel response is compensated by using frequency domain equalizer (FDE). Figure 2 shows the block diagram of the 2x2 MIMO system with the FDE. The digital data stream after the encoder is mapped to different data symbols. The on-off training sequence is used to estimate the channel coefficients. Each block of the FDE consists of M+L symbols. Every block includes M data symbols and L symbols as a cyclic prefix (CP). The data streams are transmitted to the receiver through the two transmit antennas. After other two antennas receive the transmitted data, MIMO processing is employed to compensate for the uneven frequency response and to recover the data. MIMO processing includes FFT, FDE, zero-forcing algorithm, IFFT, and a symbol decoder.
Fig. 2. Block diagram of 2x2 MIMO system with FDE
Fig. 3. Experimental setup of the proposed system.
3.
Experimental Setup and Results
Figure 3 schematically depicts the experimental setup of the proposed 60-GHz RoF system employing 2x2 MIMO system. A simple optical transmitter using only one single-electrode MZM is utilized [6]. To realize optical direct-detection vector signals, the driving RF signal consists of the vector signal at 22GHz and the sinusoidal signal at 38.5GHz. The single-electrode MZM is biased at the null point to suppress the undesired optical carrier. Hence,
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the generated optical signal consists of two data-modulated sidebands and two pilot tones as shown in Fig. 3(a). To overcome fading issue, 33/66 optical interleaver is utilized to remove one data-modulated sideband and one pilot tone. The vector signals are generated by an arbitrary waveform generator (AWG) using a Matlab ® program and up-converted to 22 GHz. One block of data comprises 256 symbols of vector signals and 8 symbols of CP length. 16-QAM signal with a symbol rate of 7Gbaud is utilized. Therefore, an optical 13.575-Gb/s 16-QAM signal that occupies a total bandwidth of 7 GHz can be achieved. After square-law PD detection, an electrical 13.575-Gb/s 16-QAM signal is generated. The second optical signal is transmitted a further 1.5km before detection, resulting in the generation of a second uncorrelated 13.575-Gb/s 16-QAM signal. Consequently, these two independent data streams are transmitted simultaneously to realize a 2x2 MIMO wireless system. The wireless transmission distance was 3m. In practice, two independent RoF signals could be simultaneously transmitted over the same fiber by using the polarization-division-multiplexing (PMD) or wavelength-division-multiplexing (WDM) schemes. Two pairs of the mixed signals at 60.5 GHz were down-converted to 5.5 GHz as shown in inset (c) of Fig. 3 and captured by a real time scope with a 50-GHz sample rate and a 3-dB bandwidth of 12.5 GHz. An off-line DSP program was employed to demodulate the MIMO signals. The overhead of the signal is 8/264, and the total data rate with 2x2 MIMO technology is 27.15 Gb/s. The MIMO demodulation process includes synchronization, FDE, zero-forcing algorithm, and QAM symbol decoding. The bit error rate (BER) performance is calculated from the measured error vector magnitude (EVM). Figure 4 shows clear 16-QAM constellation diagrams for the Tx1, Tx2 and Tx1+Tx2 MIMO transmission in back-to-back (BTB) and following single-mode fiber transmission cases. Due to the frequency response of various millimeter-wave components at 60 GHz and fiber dispersion, single-carrier modulated signals suffer serious ISI, which severely degrades system performance. We use the FDE to equalize the signal in frequency domain to successfully deal with ISI issues and also separate the MIMO signals. Figure 5 is the BER curves of 16-QAM MIMO signals in BTB and after transmission over 25-km single-mode fiber. The power penalty after transmission over 25-km single-mode fiber is negligible. Notably, the BER can be lower than FEC limit for both cases of back-to-back and 25-km single-mode fiber transmission, and the total data rate of the 2x2 MIMO signal was 27.15Gbps.
Fig. 4. Constellations (-3dBm) of MIMO 16-QAM signals.
Fig. 5. BER curve of 27.15Gbps MIMO signals.
3. Conclusion We experimentally demonstrate the efficacy of 2x2 MIMO techniques for wireless data capacity improvement at 60 GHz with a 27.15-Gbps wireless signal transmission using 7-GHz license-free spectrum at 60GHz and single-carrier data modulation. Transmission over 25-km standard single-mode fiber and 3m wireless distance were achieved with negligible penalty. 4.
References
[1] M. Weiß et al., Proc. ECOC’08, Tu.3.F.6 (2008). [2] P. Smulders, IEEE Comm. Magn. 40, 140-146 (2002). [3] K. Zhu et al., Proc. OFC’11, OTuO2 (2011). [4] A. Caballero et al., Proc. OFC’11, OWT8 (2011) [5] Shu-Hao Fan, ECOC, Th.9.B.1 (2010). [6] W. J. Jiang et al., IEEE/OSA JLT 28, 2238-2246 (2010).
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