The analog radio-over-fiber (A-RoF) technique can directly transmit radio-frequency (RF) signals between the baseband unit (BBU) and remote antenna unit (RAU) and offers the advantages of high spectral efficiency, ultralow latency, and a simple structure. In addition, millimeter-wave (mm-wave) mobile communication can utilize wide spectral resources to transmit high-rate signals. Therefore, the mm-wave over fiber based on the A-RoF technique and mm-wave mobile communication is considered the most potential solution for beyond-fifth generation (B5G) fronthaul. However, the A-RoF technique is sensitive to linear and nonlinear distortions and the generation of mm-wave signals requires high-bandwidth photonic and electronic devices. In our previous work, four-independent mm-wave signals were modulated on two orthogonal polarization states of a single wavelength based on a dual-polarization IQ modulator (DP-IQMZM) using the dual single-sideband (SSB) modulation and polarization division multiplexing (PDM) technique. Furthermore, a novel carrier polarization rotation module based on the self-polarization stabilization technique was proposed; thus, the four-independent mm-wave signals could be detected via self-coherent detection. Experimental results showed that the measured error vector magnitude (EVM) value of 800 MBaud 16-ary quadrature amplitude modulation (16-QAM) signals at 28 GHz over 50 km standard single-mode fiber (SSMF) transmission was 12.99% without digital signal processing (DSP). However, photonic frequency upconversion was not realized in our previous work. The bandwidth requirement of photonic and electronic components at the transmitter is high. In this study, we propose a scheme for upconverting four independent low-frequency RF signals to high-frequency mm-wave signals using photonic frequency upconversion. Moreover, no digital signal algorithm is used at the receiver, which is helpful for constructing a DSP-free RAU.
By paralleling one DP-IQMZM and one single-drive Mach-Zehnder modulator (MZM), which is used to generate second-order optical subcarriers, four low-frequency RF signals can be upconverted to high-frequency mm-wave signals using the self-heterodyne detection technique. In this way, the bandwidth requirement and sampling rate of photonic and electronic components at the transmitter can be reduced considerably. In addition, frequency offset compensation and carrier phase recovery are avoided. Furthermore, we analyze the causes of crosstalk between symmetric sidebands and propose a method for crosstalk elimination by accurately matching the phase and amplitude of the in-phase (I) and quadrature (Q) components of the employed DP-IQMZM.
The sideband suppression ratio can be increased from less than 20 dB to more than 30 dB using our proposed crosstalk elimination method between symmetric sidebands (Fig. 10). Moreover, the transmission performance of dual-SSB signals is very close to that of SSB signals [Fig. 11(a)], verifying the effectiveness of our proposed method. Experimental results show that four independent 1.6 GBaud 16-QAM mm-wave signals with a carrier frequency of 30 GHz could be generated at the receiver using four independent 1.6 GBaud 16-QAM signals with a carrier frequency of 10 GHz and a single-tone signal with a carrier frequency of 20 GHz at the transmitter. As shown in Fig. 12(b), the measured EVM value of 25.6 Gbit/s 16-QAM signals at 30 GHz over 50 km SSMF transmission are all below the threshold of 12.5% without the use of any DSP. Moreover, the minimum sampling rate of the DAC at the transmitter is 24 GSa/s [Fig. 13(a)].
A coherent radio-over-fiber transmission system based on the self-heterodyne detection technique is proposed, which can realize photonic frequency upconversion. In the proposed system, four low-frequency RF signals are upconverted to high-frequency mm-wave signals and no DSP algorithms are required in the RAU. The causes of crosstalk between symmetric sidebands are analyzed, and a crosstalk elimination method at the transmitter is proposed. Experimental results show that four independent 1.6 GBaud 16-QAM mm-wave signals with a carrier frequency of 30 GHz can be generated at the receiver using four independent 1.6 GBaud 16-QAM signals with a carrier frequency of 10 GHz and a single-tone signal with a carrier frequency of 20 GHz at the transmitter. Using the crosswalk elimination method between symmetric sidebands, the proposed system can support the transmission of four independent 1.6 GBaud 16-QAM mm-wave signals with a carrier frequency of 30 GHz over a 50 km SSMF without any DSP at the receiver. Moreover, the minimum sampling rate of the DAC at the transmitter is 24 GSa/s, effectively reducing the cost and complexity of B5G fronthaul systems. This research provides a potential solution for the mobile fronthaul network in the B5G mobile communication.
