We propose a simple and reconfigured dispersion-tolerant single sideband (SSB) orthogonal frequency division multiplexing (OFDM) radio over fiber (RoF) system enabled by digital signal processing (DSP), one in-phase/quadrature (I/Q) modulator and direct-detection. The generated radio frequency (RF) is based on DSP and the frequency can be flexibly adjusted, which can be employed in the future software-defined radio access network (RAN). Based on our proposed system, we have experimentally demonstrated 16-ary quadrature amplitude modulation (16QAM) 21.87-Gb/s 21-GHz and 38-GHz SSB-OFDM RoF signal generation and transmission over 80-km single-mode fiber (SMF), respectively.
© 2016 Optical Society of America
The 5G era is coming. Software-defined radio access network (RAN) will be a key function of 5G . For air-interface waveform technology, orthogonal frequency division multiplexing (OFDM) is used in 4G. In the meantime, in order to meet the requirement of 5G, new waveform technology is intensively studied . Among these techniques, OFDM is still the foundation of them. OFDM provides major advantages in mitigating wireless channel impairments, while, in optical communication field, OFDM can tolerate various fiber dispersion . Besides, single-sideband (SSB) OFDM can overcome the walk-off or power fading effect resulting from the double-sideband (DSB) signal transmission in the fiber [4–6].
On the other hand, radio over fiber (RoF) technique can offer a strong and cost-effective solution on enhancing the system capacity and mobility of wireless links [7,8]. Therefore, the combination of SSB-OFDM and RoF system owns the aforementioned joint advantages. So far, some research works have been done on the SSB-OFDM RoF [9–15]. Ref. 9 has proposed the method of optical sideband filtering to generate the SSB-OFDM signal. Ref. 10 has demonstrated an approach of SSB modulation in optical heterodyning mm-wave RoF systems based on an injection-locked distributed feedback (DFB) laser. Refs. [13-14] has employed tandem single sideband (TSSB) modulation scheme. Ref.  has reviewed three SSB-OFDM RoF schemes based on optical frequency multiplication, include the frequency doubling scheme with TSSB modulation, the frequency sextupling scheme, and the frequency quintupling scheme using all-optical up-conversion. However, these work need the RF sinusoidal signal source to drive the optical modulator, or electrical RF mixer for up-conversion, or optical filter to filter one of the sidebands of DSB signal. Furthermore, these proposed systems cannot be flexible enough to reconfigure the RF.
In this paper, we propose a simple and flexible SSB-OFDM RoF system enabled by DSP, one in-phase/quadrature (I/Q) modulator and direct-detection. Compared with the previous research work [9–15], our system mainly has two advantages. Firstly, the system architecture of SSB-OFDM RoF signal generation is the simplest without RF source and optical filter, and only needs an I/Q modulator. Secondly, the generation and the down-conversion of RF are all based on DSP, which means that the RF of the system can be flexibly adjusted and software-defined. Based on our proposed system, we have experimentally demonstrated that 21.87-Gb/s 21-GHz and 38-GHz SSB-OFDM RoF signal generation and transmission over 80-km single-mode fiber (SMF), respectively, with bit-error rate (BER) less than the hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10−3.The sensitivity penalty caused by 80-km SMF is less than 1.5 dB. Besides, for the 38-GHz SSB-OFDM RoF signal, it has been transmitted over another span of 0.5-m wireless link with the penalty of 7-dB. We believe that the system has the potential application in the future 5G.
2. Scheme of proposed SSB-OFDM ROF system
Figure 1 shows the scheme of our proposed SSB-OFDM RoF system. At the central office transmitter (COT), the digital SSB-OFDM RF signal is generated by DSP. Firstly, a pseudo-random binary sequence (PRBS) with a certain length is input to the discrete Fourier transform spread (DFT-S) OFDM modulation module. The DFT-S OFDM modulation module is responsible for the generation of SSB-OFDM baseband signal. Because high peak-to-average power ratio (PAPR) is the drawback of conventional OFDM, DFT-spread is employed to reduce the PAPR and resist high frequency power attenuation . Subsequently, the generated SSB-OFDM baseband signal is up-converted to radio frequency (RF) SSB-OFDM signal by mixing with a complex sinusoidal RF source at the frequency of -f1. Then, the real part and image part of the RF OFDM signal is respectively added with the real part and image part of another complex sinusoidal RF source at the frequency of f2, to generate the in-phase (I) and quadrature (Q) components of the signal. Next, the I/Q data is uploaded to a digital-to-analog converter (DAC). Then, the I and Q signal output from the DAC are used to drive the I and Q ports of an I/Q modulator, respectively. The I/Q modulator is operated at null point and each Mach-Zehnder modulator in the embedded modulator has one driving input port. Next, a continuous-wave (CW) lightwave at the frequency of fc is linearly modulated by the signal to generate SSB-OFDM RoF signal. And then, the generated optical signal is transmitted over a span of SMF. At the base station transmitter (BST), the SSB-OFDM RoF signal is converted into an electrical RF signal at the carrier frequency of (f1 + f2) by a single-ended photodiode (PD). After that, the adjustable RF SSB-OFDM signal is wireless transmitted via a pair of antennas.
At the receiver, the received signal is converted into RF SSB-OFDM digital signal by an analog-to-digital converter (ADC). Subsequently, the RF SSB-OFDM digital signal is processed by offline DSP. Firstly, the signal is down-converted to OFDM baseband signal by mixing with a complex sinusoidal RF source at the frequency of (f1 + f2). Then, the generated SSB-OFDM baseband signal is recovered to PRBS by the DFT-S OFDM demodulation module. Figures 1(i)-1(vi) show the schematic diagrams of the SSB-OFDM signal, RF carrier, the signal after SSB-OFDM adding RF carrier, SSB-OFDM RoF signal, SSB-OFDM RF signal after PD, and the baseband signal after down-conversion, respectively.
3. Experimental setup
Figure 2 shows the experimental setup of the proposed SSB-OFDM RoF system. The I/Q data of RF SSB-OFDM is generated by the off-line DSP as illustrated in the scheme. The inset in Fig. 2 also shows the photo of the experimental setup.
At the COT, a PRBS with the length of (231-1) is mapped into 16QAM. Then, the SSB-OFDM baseband signal is generated by DFT-S OFDM modulation module. Figure 3(a) shows the block diagram of the DFT-S OFDM modulation module. Here, M-point DFT is implemented. This DFT-spread step is used to reduce PAPR. The DFT size of M is mainly decided by the signal bandwidth, OFDM size and DAC sampling rate. The OFDM size of N is 1024. The cyclic prefix (CP) samples length is 16. And for every 100 OFDM symbols, one training symbol is inserted for realizing the time synchronization and channel estimation.
Considering the future 5G operating frequency and the limitation of available devices, especially the adopted power amplifiers (PAs) working in K band (17-27 GHz) and Q band (36-41 GHz), we assign the RF to 21 GHz at K band and 38 GHz at Q band, respectively. For 21-GHz RF, f1 and f2 are assigned to 9.5 and 11.5 GHz, respectively. For 38-GHz RF, f1 and f2 are assigned to 15 and 23 GHz, respectively. Figures 4(a)-4(b) show the calculated spectrum of transmitted signal with the 21-GHz and 38-GHz frequency spacing, respectively.
Subsequently, digital to analog conversion is implemented by the DAC with 92-GSa/s sample rate and 20-GHz bandwidth. Then, the I and Q component outputs from the DAC are respectively boost by a pair of electrical amplifiers (EAs), with 32-GHz 3-dB bandwidth. And then, the amplified I/Q signal is used to drive an I/Q modulator with 30-GHz bandwidth and less than 7.5-dB insertion loss. Meanwhile, the CW lightwave at 1544.67 nm from an external cavity laser (ECL) is fed into the I/Q modulator to generate RoF signal. Then the generated signal will pass through an optional erbium-doped fiber amplifier (EDFA) if the following fiber length is longer than 50-km. After that, the signal is transmitted over a span of SMF-28. Figures 5(a) and 5(b) show the optical spectrum of the 21-GHz and 38-GHz SSB-OFDM RoF signal, respectively. At the BST, the received SSB-OFDM RoF signal is boost by an EDFA. A variable optical attenuator (VOA) is placed before the PD in order to adjust the input optical power. Then, the signal is converted into RF electrical signal after 50-GHz PD. And then, the generated signal is boost by a K band or Q band PA corresponding to the adopted RF. The K band and Q band PA have 31-dB and 32-dB gain, respectively. For 38-GHz RF, the amplified signal is further transmitted by a pair of Q band horn antennas. The wireless distance is 0.5 m.
4. Experimental results and discussions
Figure 6 shows the BER versus the input power into PD for the 21-GHz RF signal. Compared with 4-GHz bandwidth back-to-back (BTB) transmission, 6-GHz bandwidth BTB transmission causes 0.5-dB penalty. For 6-GHz bandwidth SSB-OFDM signal, the performance after transmission over 50-km SMF causes no penalty. For 80-km SMF transmission, the penalty is about 1.5 dB because of the fiber dispersion effect and the increased accumulated amplified-spontaneous-emission (ASE) noise caused by inserting another EDFA. Considering the 6-GHz signal bandwidth, counting the training sequence 1/101, CP 1/65 and 7% FEC overhead, the net bit rate is 6 × 4 × (100/101) × (64/65)/1.07 = 21.87 Gb/s, with the BER less than the HD-FEC threshold of 3.8 × 10−3.
Figure 7 (a) shows the received spectrum of 38-GHz RF SSB-OFDM signal with 6-GHz bandwidth. The spectrum is not flat within the 6-GHz bandwidth, mainly due to the uneven frequency response of the adopted Q band PA. Figures 7(b)-7(c) show the constellations of 38-GHz RF SSB-OFDM signal with 6-GHz bandwidth for two cases. One is BTB transmission with the power into PD of −7.4-dBm and the BER of 6.4 × 10−4. The other is 80-km SMF and 0.5-m wireless transmission with the power into PD of −0.6-dBm and the BER of 3.0 × 10−3. Figure 7 (d) gives the measured BER versus the input power into the PD for the 38-GHz RF signal. Compared with 4-GHz bandwidth back-to-back (BTB) transmission, 6-GHz bandwidth BTB transmission causes 1-dB penalty. For the 6-GHz bandwidth SSB-OFDM, 50 and 80-km fiber transmission cause about 1-dB and 1.5-dB penalty, respectively. For the same length of fiber, the performance of 38-GHz RF signal is worse than that of 21-GHz RF signal, because the increasing RF causes more serious fiber dispersion and the devices have different performance in different frequency bands. Moreover, the 0.5-m wireless transmission causes 7-dB penalty due to the wireless path loss. For the fiber and wireless transmission, the penalty caused by the fiber length within 80-km almost can be ignored due to the increasing power into PD for overcoming the effect of the wireless link.
We propose a simple and flexible SSB-OFDM RoF scheme. The scheme mainly has two advantages. Firstly, the system architecture is simple without RF signal source to drive the optical modulator, and it only needs an I/Q modulator. Secondly, the DSP-based RF generation and down-conversion to baseband, ensure that the RF in the system can be flexibly reconfigured. Based on the scheme, we have experimentally demonstrated 21.87-Gb/s 21-GHz and 38-GHz SSB-OFDM RoF signal generation, transmission, and reception, respectively, with the BER less than the HD-FEC threshold of 3.8 × 10−3. The sensitivity penalty caused by the 80-km SMF is less than 1.5 dB. Furthermore, the 38-GHz SSB-OFDM signal has been transmitted over another span of 0.5-m wireless link with the sensitivity penalty of 7-dB. As a result, it can be a promising candidate for the future software-defined RAN application.
National Natural Science Foundation of China (NSFC) (No. 61527801, No. 61325002).
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