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Abstract
In this paper, a multiplexing scheme, involving optical tandem single-sideband modulation, orthogonal optical single-sideband multiplexing, and digital signal processing (DSP)-assisted optical coherent detection, is proposed. Based on the design, a coherent radio-over-fiber (RoF) optical transmission system is established, which improves channel capacity and spectrum efficiency. Phase-modulated radio frequency signals with different bit rates are carried on an optical single-carrier, transmitted, received after 40 km transmission, and recovered by the DSP algorithm. System performances for all binary phase-shift keying channels are verified by simulation and experiment. Thus, the proposed multiplexing scheme is feasible to improve channel capacity and spectrum efficiency without increasing the system complexity. The work in this paper brings potential applications to coherent RoF systems and hybrid optical wireless communications for future broadband wireless access networks.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Radio-frequency (RF) spectrum resources are considerably valuable and comparatively limited for wireless networks and optical communication systems. In recent years, it has attracted significant attention to developing more advanced techniques that can use frequency resources sufficiently, such as digital subcarrier modulation (SCM), advanced modulation formats, the combination of various multi-dimensional multiplexing approaches, etc. [1–11]. Furthermore, for broadband wireless access networks, radio-over-fiber (RoF) systems based on microwave/millimeter-wave photonics can be combined with dense wavelength division multiplexing to increase the system capacity [12–21].
For improving spectral efficiency and reducing dispersion in the transmission system [22–25], the SCM schemes and other achieved advanced modulation using optical Mach–Zehnder modulators (MZMs) are demonstrated [26–35]. They have prompted the development of large bandwidth and high-speed transmission in data centers, backbone networks, and wireless access networks. Although intensity modulation direct detection (IM-DD) has been regarded as the most cost-effective solution for short-reach transmission, optical amplification and regeneration are required in IM-DD systems to ensure sufficient receiver sensitivity for the long-haul (≥80 km) and high-bit-rate (≥100 Gb/s) transmission [36–41]. In contrast, digital signal processing (DSP)-oriented optical coherent detection using a local oscillator (LO) outperforms direct detection solutions in terms of receiver sensitivity, transmission distance, and system capacity [42–51]. Nevertheless, except for a more complex optical front end, the DSP algorithm is used to compensate for frequency offset (FO), phase noise, and transmission impairments, which increases system complexity and power consumption [52–57]. Therefore, while guaranteeing system transmission performance, a comprehensive solution with improved spectral efficiency, reduced cost, and low power consumption is urgently required.
In this paper, a new multiplexing scheme is proposed to solve these problems by incorporating optical tandem single-sideband modulation (OTSSBM), high capacity optical orthogonal single-sideband multiplexing (OOSSBM), coherent detection with high sensitivity, and a DSP algorithm. For verifying the improved spectral efficiency and transmission capacity, a coherent RoF system is demonstrated by simulation and experiment. This paper is organized as follows. The scheme design and theoretical model are described in Section 2, and the system simulation is presented in Section 3. The experimental results and system performance are shown in Section 4, and the relevant DSP algorithm is expounded in Section 5. Finally, the conclusions are presented in Section 6.
2. Scheme design and theoretical model
2.1 Schematic diagram of designed coherent RoF system
Figure 1 illustrates the conceptual diagram of the proposed coherent RoF system employing a dual-electrode Mach-Zehnder modulator (DE-MZM). The developed system primarily includes optical tandem single-sideband modulation (OTSSBM), orthogonal optical single-sideband multiplexing (OOSSBM), coherent detection, and DSP algorithm. OTSSBM and OOSSBM are used to improve spectral efficiency and system performance. And the coherent detection and DSP algorithm are used for receiving and processing signals.
2.2 Optical tandem single-sideband modulation
Figure 2 shows the schematic diagram of the OTSSBM. Two modulated RF signals can be transmitted on upper and lower sidebands of the optical single-carrier, respectively. Compared with optical single-sideband, two sub-carriers with the RF signals are spaced twice apart from each other. Here, ${V_{r{f_2}}}(t )$ and ${V_{r{f_2}}}(t )$ are defined as two different RF signals, which can be given as follows:
Optical signal with amplitude ${E_{in}}$ and frequency ${f_c}$ is obtained from the continuous-wave (CW) laser and externally modulated by a pair of orthogonal subcarriers with microwave signal frequency ${f_{RF}}$, and voltage amplitudes ${V_{m1}}$ and ${V_{m2}}$. The orthogonal subcarriers, which are denoted as ${V_{m1}}\cos {\omega _{rf1}}t$ (B1) and ${V_{m2}}\sin {\omega _{rf2}}t$ (B2), are loaded to a 2-by-2 90° hybrid coupler. Accordingly, the two output signals of the 2-by-2 90° hybrid coupler combine the input electrical signals with different phase shifting, which are applied to the dual electrodes (B3 and B4) of the DE-MZM, respectively. As the voltage difference between direct current (DC) biases ${V_{DC1}}$ and ${V_{DC2}}$ is considered, the first arm of the DE-MZM can be biased at ${V_{DC1}}$, meanwhile the other arm is grounded. The output optical field of a CW laser is ${E_{in}}{e^{j{\omega _c}t}}$, where ${E_{in}}$ and ${\omega _c}$ are the optical field amplitude and angular frequency of the laser, respectively. The output optical field ${E_{out}}(t )$ (assuming 50/50 split ratio) for the DE-MZM is represented by
Here, ${E_{out}}(t )$ is derived using Eqs. (1)–(4). From Eq. (5), two different phase-modulated signals ${V_{rf1}}(t )$ and ${V_{rf2}}(t )$ are simultaneously carried on an optical single-carrier by using the DE-MZM. It indicates that the designed OTSSBM scheme can improve the channel capacity by multiplexing on the optical single-carrier, which is simple and efficient without increasing the system complexity.
When the modulation indices m1 and m2 of the DE-MZM are small (m1, m2<1), the induced nonlinear distortion can be neglected. The optical field ${{E_{in}}{e^{j{\omega_c}t}}}$ of a CW laser enters into the DE-MZM that is biased at the quadrature point, i.e. $|{{V_{DC - upper}} - {V_{DC - lower}}} |= {V_{DC1}}{{ - {V_{DC2}} = {V_\pi }} / 2}$. Therefore, its output optical field ${E_{out}}(t )$ can be expanded using the Jacobi-Anger expansion as
2.3 Orthogonal optical single-sideband multiplexing for coherent RoF system
As shown in Fig. 3, the OOSSBM is another multiplexing technique to improve spectral efficiency, consisting of a power combiner, a 1-by-2 90° hybrid coupler, and a DE-MZM. The input signal (B5) of the 1-by-2 90° hybrid coupler is the combination of the two RF signals via a power combiner. For OOSSBM, the frequencies of the two RF signals are identical, which means ${\omega _{rf1}} = {\omega _{rf2}} = {\omega _{rf}}$. The output electrical signals (B6 and B7) of the 1-by-2 90° hybrid coupler are a pair of orthogonally modulated subcarriers. Herein, the structure of the 1-by-2 90° hybrid coupler is different from that of the 2-by-2 90° hybrid coupler. One output signal (B7) has exactly the opposite amplitude as the input signal (B5), and the other output signal (B6) is the input signal (B5) multiplied by a phase shifting of $\textrm{ - }\pi $/2. The electrical signals at B5, B6, and B7 are described as
Here, φ1 and φ2 refer to the amount of phase shift introduced by the upper and lower arms of DE-MZM, respectively, which are determined by the bias voltages applied to the modulator. Equation (8) indicates that the two orthogonal multiplexing signals of OOSSBM are carried on the one sideband, i.e., the lower sideband of the optical carrier. It indicates that the OOSSBM can save the spectrum resource and improves the channel capacity. Similar to the OSSSBM, the output optical field in Eq. (8) can be further expanded to the Bessel function of the first kind using the Jacobi-Anger expansion as
3. Simulation results and discussion
For verifying the performance of the proposed scheme, a DSP-assisted coherent RoF system incorporating OTSSBM and OOSSBM is designed and numerically analyzed. In the system, the baseband signals with the phase-modulated format at bit-rates of 2 Gbps and 5 Gbps, respectively, are modulated by RF source. The frequency spacing between optical carrier (193.1 THz) and sub-carrier signals (193.09 THz and 193.11 THz) is 10 GHz. According to Eqs. (3) and (4), with the OTSSBM, B3(t) and B4(t) are obtained from B1(t) and B2(t). According to Eq. (7), with the OOSSBM, B6(t) and B7(t) are acquired from B5(t). The four BPSK channels (B3(t), B4(t), B6(t), and B7(t)) are multiplexed, and distributed around an optical single-carrier wavelength of 1552.5 nm (193.1 THz). For four multiplexing channels, after 80 km single-mode optical fiber (SMF) transmission, the simulation results are evaluated by eye diagrams, time-domain signal waveforms, and quality factor (Q-factor), respectively.
In the simulation on a coherent RoF system, the light source used is a CW laser with a center wavelength of 1552.5 nm, linewidth of 100 kHz, and output power of 5 dBm. The DE-MZM has a switching voltage Vπ = 5.0 V and is biased at the quadrature point. The optical spectrum of the DE-MZM output is exhibited in Fig. 4. As can be seen from Fig. 4, the upper and lower sidebands locate on the two sides of the single optical carrier, and the power of the two sidebands and optical carrier are -24.8 dBm, -24.7 dBm and -20.3 dBm, respectively. Each subcarrier sideband are modulated by two BPSK RF signals at different data rates, and the four BPSK baseband signals are generated by bit pattern generators (BPG) with a pseudo-random bit sequence.
Furthermore, as shown in Figs. 5(a)–5(d), after 80 km SMF transmission, the time domain waveforms of baseband and received BPSK signals at the data rate of 2 Gbps and 5 Gbps are well-matched, respectively. The transmitted BPSK signals at 2 Gbps and 5 Gbps both have the amplitude values of 1, whose unit is normalized as arbitrary unit and dimensionless, and the demodulated and recovered BPSK signals at the corresponding data rates basically maintain the same signal amplitudes for different bit periods. Simulation results indicate that the received signals can accurately recover the original baseband signals for BPSK modulation format after the DSP assists, which shows that our designed system has small damage to transmitted signals and can recover the distorted baseband signals.
Fig. 5. Time-domain waveform of (a) baseband and (b) received BPSK signal at 2 Gbit/s; (c) baseband, and (d) received BPSK signal at 5 Gbit/s.
Figures 6(a)–6(d) show the eye diagrams of modulated and received BPSK signals at 2 Gbit/s and 5 Gbit/s in the coherent RoF system. For the BPSK modulation format, the opening of eye diagrams of the recovered signals in Figs. 6(b) and 6(d) are acceptable compared with that of the original baseband signals in Figs. 6(a) and 6(c), which confirms that the performance of our designed coherent RoF system is satisfactory.
Fig. 6. Eye diagrams of (a) baseband and (b) received BPSK signal at 2 Gbit/s, (c) baseband, and (d) received BPSK signal at 5 Gbit/s.
The structure of the coherent receiver in the RoF system is illustrated in Fig. 7, where the receiver consists of the 90° optical hybrids, polarization beam splitters (PBS), PIN photodiodes (PD), subtractors, and transimpedance amplifiers (TIA). After the coherent reception, the analog-to-digital converters (ADC) and offline DSP algorithm are used to sample, process, and recover the received electrical signals.
4. Experimental results and discussion
As depicted in Fig. 8, the experimental system is set up to verify the performance of the proposed scheme. A commercial Fujitsu DE-MZM with a 40-GHz bandwidth and an integrated coherent receiver with a 20-GHz bandwidth are used as the transmitter and receiver, respectively. An erbium-doped fiber amplifier (EDFA) is employed to amplify the phased-modulated optical signals. The narrowband optical bandpass filter (BPF) with tunable bandwidth of 0.27 nm and a considerably steep roll-off factor is used to filter the amplified spontaneous emission noise and undesired harmonic components. Two tunable CW lasers with linewidth < 100 kHz are used as the optical signal and local oscillator, respectively.
Fig. 8. Schematic diagram of the experimental system. BPG: bit pattern generator, BPF: optical bandpass filter, OS: optical signal, LO: local oscillator.
Due to the limitation of experimental conditions, OTSSBM and OOSSBM are implemented by discrete components for a coherent RoF system. For OTSSBM and OOSSBM in Fig. 8, two discrete double-balanced multipliers with extra RF phase shifters are used to acquire the ideal electrical signals (B1, B2; B5). The 2-by-2 and 1-by-2 90° hybrid couplers are utilized for OTSSBM and OOSSBM multiplexing of two channels, respectively. Furthermore, two BPSK signals at data rates of 2 Gbps and 5 Gbps are generated by the BPG and modulated by the RF carrier at 10 GHz. The phase-modulated signals are loaded into each input port (B1, B2) of the 2-by-2 90° hybrid coupler for OTSSBM multiplexing. The orthogonally multiplexed BPSK signals are applied to one input port (B5) of a 1-by-2 90° hybrid coupler that generates the OOSSBM. At the coherent receiver, the real-time oscilloscope (RTO) with a rate of 80 GSa/s and a 30-GHz bandwidth samples 1 million data points. The offline DSP algorithm is adopted to separate the multiplexed channels in both cases.
Figures 9(a) and 9(b) show the electrical spectra measured at B1 and B2 when employing the RF signals at data rates of 2 Gbps and 5 Gbps, respectively. The peak-to-peak voltage of the BPSK signals is chosen as 450 mV while keeping the same input power of the multiplier. The two multipliers have different losses and isolation degrees, which result in inconsistent performance between the channels. For OTSSBM, the RF signals at 2 Gbps and 5 Gbps are modulated at the lower and upper sides of the optical single-carrier (${\lambda _c}$, 1552.5 nm) with a 10 GHz frequency offset. For OTSSBM and OOSSBM, Figs. 9(c) and 9(d) show the optical spectra measured at A. The inset in Fig. 9(d) shows the passively combined electrical signal measured at B5. Figures 9(a) and 9(b) indicate that the RF spectrum of up-converted BPSK signal at 5 Gbps has a broader main lobe than that of up-converted BPSK signal at 2 Gbps. As is shown in Fig. 9(c), the different RF signals from Figs. 9(a) and 9(b) are modulated to the lower and upper sidebands of optical single-carrier, respectively. Figure 9(d) indicates that an orthogonally multiplexed signal combined by the RF signals in Figs. 9(a) and 9(b) is modulated to the lower sideband of the optical carrier. The optical spectra both in Figs. 9(c) and 9(d) show that the spectrum and bandwidth resources are utilized effectively by our designed scheme.
Fig. 9. RF spectra of up-converted BPSK signals at (a) 2 Gbps and (b) 5 Gbps, and optical spectra measured for (c) OTSSBM and (d) OOSSBM.
As depicted in Fig. 10(a), the measured and optimized Q-factors of single-channel always outperform those of dual-channel for OTSSBM. Herein, the measured difference of the Q-factors is introduced by the interplay of the electrically combined channels. The Q-factors of BPSK signals at 2 Gbps are always superior to those of BPSK signals at 5 Gbps due to the non-ideal characteristics of the discrete components. However, the Q-factors higher than six are still achieved reliably for all channels. Figure 10(b) illustrates the comparison of measured Q-factors for OTSSBM and OOSSBM under the same electrical conditions, i.e., bias voltage at 0.5 V (0.5 Vbias), and peak-to-peak voltage at 450 mV (450 mVpp). The channels of OOSSBM have lower Q-factors than that of OTSSBM. The main reason is that the passively combined orthogonal signals have lower average power applied to the optical modulator than the OTSSBM loaded signals. It is also verified by the results that the optical carrier-to-sideband power ratio drops 3 dB for the OOSSBM in Fig. 9(d) compared to the OTSSBM in Fig. 9(c). The degradation is compensated for by adjusting the modulator bias and using stronger loaded signal power, i.e., 0.7 Vbias and 650 mVpp for the signal at 2 Gbps/0.7 Vbias and 600 mVpp for the signal at 5 Gbps, as depicted in Fig. 10(c). Under this condition, the Q-factors of OTSSBM are hardly degraded even if the error characteristic occurs to deteriorate. For the BPSK signals at 2 Gbps and 5 Gbps, Q-factors are measured and compared under different conditions, including single-channel and dual-channel for OTSSBM, the bias voltage of DE-MZM at 0.5 V and 0.7 V for OOSSBM, and back-to-back (BtB) and after 40km transmission for both OTSSBM and OOSSBM. The experimental results show that when all other conditions are the same, the Q-factors of OTSSBM, 0.7 Vbias for OOSSBM, BPSK signal at 2 Gbps, single-channel, and BtB are always better than those of OOSSBM, 0.5 Vbias for OOSSBM, BPSK signal at 5 Gbps, dual-channel, and after transmission, respectively.
Fig. 10. Q-factors of (a) OTSSBM (single channel and dual channel), (b) comparison of OTSSBM and OOSSBM, and (c) OOSSBM for different Vbias and Vpp.
5. Digital signal processing (DSP) algorithm
For demodulating signals and separating the different channels at the receiving end, the DSP algorithm is used due to its ability to increase spectral efficiency and mitigate the impairments. As depicted in Fig. 11, a DSP algorithm flowchart is developed to separate channels and evaluate system performance. Sampled data from the RTO is first transformed to the frequency domain using fast Fourier transform (FFT). Being considered the channel distribution in the optical spectrum, each channel is located at $\pm$10 GHz in turn. One of the channels is shifted to the baseband by either positive or negative 10 GHz for further processing. A digital lowpass filter with appropriate bandwidth is employed. The signals are then shaped to have one sample/symbol for each channel. The FO between the optical signal and LO laser is estimated and compensated for by the FFT algorithm. The BPSK signals are detected after the Viterbi-Viterbi phase recovery algorithm with the phase tracking and correction for cycle-slips. The Q-factor is measured after acquiring the demodulated signal.
For the case of OOSSBM multiplexed signals, the DSP algorithm flowchart is nearly the same aside from the stage after the FO estimation. For OTSSBM and OOSSBM, the phase recovery algorithm is used to improve the recovery results, and fourth-power is applied, and an additional channel identification stage is inserted. Although the recovered data has an intermediate state between symbols due to different baud rates and relevant re-timing, the Viterbi-Viterbi algorithm with moving-average works effectively. After phase recovery, the processed data is then separated into the real and imaginary parts for further channel identification. Each part undergoes independent cross-correlation checks with data from two reference channels. By means of the cross-correlation peak strength, either the real or imaginary parts of the data are selected for demodulation and subsequent Q-factor measurement. As illustrated in Fig. 10, the DSP algorithm, which solves the problems of the phase recovery, frequency offset estimation, and phase noise cancellation, processes and recovers the received signals. These results confirm that our designed DSP algorithm works effectively.
6. Conclusions
We propose a novel multiplexing scheme for the DSP-assisted coherent RoF system. The RF BPSK signals at 2 Gbps and 5 Gbps, respectively, are transmitted. And they are modulated on lower and upper sidebands of optical single-carrier using OTSSBM. Moreover, the BPSK signals at 2 Gbps and 5 Gbps are orthogonally multiplexed on one sideband for OOSSBM, which confirms the feasibility of multiplexing multi-channels on optical single-carrier. The DSP algorithm is developed and applied in coherent detection, without extra electrical and optical hardware for channel separation. The performance of the proposed coherent RoF system is evaluated by Q-factor. The Q-factors higher than six are achieved for the multiplexed channel cases of OTSSBM and OOSSBM after 40 km transmission. The research results improve the channel capacity and spectrum efficiency of the optical single-carrier systems. While using polarization multiplexing, the channel capacity can be further increased. This work will benefit the future development of coherent RoF systems and hybrid optical wireless communications.
Funding
National Natural Science Foundation of China (61690195, 61835002, 61875238); State Key Laboratory of Information Photonics and Optical Communications (IPOC2018ZT09, IPOC2020ZT06).
Acknowledgments
G. Chen would like to express his deep gratitude for the financial support from China Scholarship Council postgraduate program. Guang Chen is considerably grateful to Dr. Chulsoo Park’s experimental guidance and help. Moreover, the authors would also like to thank Prof. Xiaoli Yin at BUPT for helpful discussions and the reviewers for valuable comments and suggestions.
Disclosures
The authors declare no conflicts of interest.
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