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Abstract
The two sidebands of the independent dual-single-sideband (dual-SSB) signal can carry different information to achieve higher spectral efficiency. However, the two sidebands of the independent dual-SSB vector signal are received independently. Generally, the receiver divides the signal into two channels. For each channel, we use an optical bandpass filter (OBPF) to select the left sideband (LSB) or right sideband (RSB), respectively. Then a photodetector (PD) is used for photoelectric conversion, followed by subsequent digital signal processing (DSP). To reduce the complexity and cost of the receiver, we propose a new independent dual-SSB vector signal detection scheme based on a single PD combined with conventional DSP. An electric bandpass filter (EBPF) filters out high-frequency components after photoelectric conversion, and then the signal is quadrature demodulated and processed by the DSP algorithm. The LSB and RSB are quadrature phase-shift keying (QPSK) modulated with an initial phase difference of π/4. Simulation results show that the proposed scheme performs better bit error rate (BER). For back-to-back (BTB) transmission, the BER of 2-Gbaud independent dual-SSB vector signal (1-Gbuad RSB and 1-Gbaud LSB) can reach the hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10−3 when the input optical power into PD is −20 dBm. For 1-km and 2-km weak turbulence free-space optical (FSO) channel transmission, the BER of 2-Gbaud independent dual-SSB vector signal can reach the HD-FEC threshold when the input optical power into PD is −18.8 and −17 dBm, respectively. For 1-km weak turbulence FSO channel transmission, the BER of 4-, 8-, and 16-Gbuad independent dual-SSB vector signal can reach the HD-FEC threshold when the input optical power into PD is −17.8, −16, and −15 dBm, respectively.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the popularization and development of the Internet and the emergence of high-speed and high-quality data service applications such as cloud computing, Internet of Things, online video, and high-definition television, subscribers’ demand for high-speed wireless network bandwidth has grown exponentially [1–10]. Over the past three decades, wireless data transfer rates have tripled every 18 months [11]. The millimeter-wave (mm-wave) frequency band has sufficient bandwidth to support high-speed data transmission, which is one of the key technologies in radio over fiber (RoF) [12–14]. However, it is inconvenient or expensive to lay optical fibers in some cases. At this time, free-space optical (FSO) communication can be used as a substitute or backup for fiber-optic communication. FSO communication is an emerging wireless communication mode with laser and atmosphere as the information carrier and transmission channel [15]. FSO communication system can use the same working frequency as optical fiber communication. It has the advantages of high bandwidth, high speed, good anti-interference performance, no need for a frequency license, simple erection, and low cost [16]. It has good development potential and application prospects and can be used in emergency communications, mobile backhaul, and “last mile” communications [17]. Radio over FSO (RoFSO) technology combines the advantages of millimeter-wave and FSO [18–19] and can be used for mobile fronthaul. In the RoFSO system, the mm-wave signal is generated by a photoelectric combination and then sent into the FSO channel. For mm-wave generation, dual-sideband (DSB) modulation [20–21], single sideband (SSB) modulation [22–27] and optical carrier suppression (OCS) modulation can be used [28–30]. For DSB and OCS modulation, the left sideband (LSB) and right sideband (LSB) carry the same information with lower spectral efficiency. SSB modulation has higher spectral efficiency than DSB and OCS modulations. However, one of the disadvantages of SSB modulation is that it wastes half the bandwidth of the digital-to-analog converter (DAC) at the transmitter since it has only one sideband to carry information [31]. Independent dual-single-sideband (dual-SSB) modulation combines the characteristics of DSB and SSB, and the two sidebands carry different information [32–38]. In these schemes, the two sidebands of the dual-SSB vector signal are received independently at the receiver. Generally, the signal is divided into two channels at the receiver. For each channel, an optical bandpass filter (OBPF) is used to select the LSB/RSB, then a photodetector (PD) is used for photoelectric conversion, followed by subsequent DSP. The receiver needs two OBPFs and two PDs, which increases the complexity and cost of the system. In 2022, Zhou et al. proposed a new scheme for dual-SSB signal receiving [31]. Quadrature phase-shift keying (QPSK) modulation is used in the RSB of the transmitter, and geometric shaping 3PSK (GS-3PSK) modulation is used in LSB. DSP is used to separate the two sidebands after photoelectric conversion at the receiver. With appropriate geometric shaping, the constellation diagram after PD is a 12 quadrature amplitude modulation (QAM) distribution, corresponding to the 12 constellation points in the inner circle of the regular 16QAM signal. However, this scheme can only be applied to the modulation format given in this paper. We cannot extend it to the QPSK or higher-order modulation format for both sidebands.
This paper proposes a new dual-SSB sideband signal detection scheme where both sidebands adopt QPSK modulation with an initial phase difference of π/4 at the transmitter. After the PD is used for direct detection at the receiver, an electric bandpass filter (EBPF) filters out the high-frequency 60 GHz mm-wave component. At the same time, the low-frequency 30 GHz mm-wave component is retained, then the signal is processed by quadrature demodulation and DSP. We demonstrate 1-Gbaud LSB QPSK and 1-Gbaud RSB QPSK signal transmission over 2-km weak turbulence FSO and 4-, 8-, and 16-Gbuad independent dual-SSB vector signal transmission over 1-km weak turbulence FSO channel. Simulation results show that the BER performance of the system is lower than the hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10−3.
2. Principle
2.1. Principle of independent dual-SSB vector signal generation
Figure 1 shows the schematic diagram of independent dual-SSB vector signal generation. Offline DSP generates the driving signal for the two MZM modulators. At the transmitter, two independent pseudo-random binary sequences (PRBSs), which are denoted by PRBS1 and PRBS2, are QPSK mapped and low-pass filtered to generate two independent signals represented by SL(t) and SR(t), respectively, where SL(t)=exp[jθ1(t)], SR(t)=exp[jθ2(t)], θ1(t) and θ2(t) are the phase information of the signal carried on the LSB and RSB, respectively.
Fig. 1. Scheme of independent dual-SSB vector signal generation. PRBS: pseudo-random binary sequence; ECL: external cavity laser; MZM: Mach–Zehnder modulator; PD: photodiode. (a) The schematic diagram of the LSB signal. (b) The schematic diagram of the RSB signal. (c) The schematic diagram of the modulated signal after passing through MZM-1. (d) The schematic diagram of the modulated signal after passing through MZM-2. (e) The schematic diagram of the modulated signal after coupling MZM-1 output and MZM-2 output. (f) The schematic diagram of the received signal after the PD.
After that, signal SL and SR are upconverted using complex sinusoidal radio-frequency (RF) sources with frequencies -fs and fs, respectively. The LSB and RSB signal can be expressed as follows:
The spectrograms of the LSB signal and the RSB signal are shown in Figs. 1(a) and 1(b), respectively. The real and imaginary parts of the LSB and RSB are loaded into a DAC. The real and imaginary parts of the LSB are used to drive the MZM-1 after passing through the DAC, while the real and imaginary parts of the RSB are used to drive MZM-2 after passing through the DAC. The continuous wave (CW) optical signal with the center frequency fc emitted from an external cavity laser (ECL) is divided into two paths with the same power. One is used as the optical input of MZM-1, and the other is used as the optical input of MZM-2. Both MZM-1 and MZM-2 work at the quadrature bias point. The outputs can be expressed as follows:
Figure 1(e) shows the output of the OC. Then, the signal is fed into the PD, where the left and right bands carrying the data and the center carrier will beat each other in the PD. The output of PD can be expressed as follows:
2.2. Principle of dual-SSB vector signal detection based on a single PD
An EBPF is used to filter out the spectral component with the central frequency of 2fs and then perform quadrature demodulation to obtain the in-phase and quadrature components, which can be expressed in the form of a complex number as follows:
where $S(t) = \exp [ - j{\pi / 4} - j{\theta _1}(t)]\textrm{ + }\exp [j{\pi / 4} + j{\theta _2}(t)]$.${{\sqrt 2 R{J_0}(k){J_1}(k)} / 4}$ is a constant and can be eliminated by normalization in the subsequent DSP, so we mainly consider the relationship between S(t) and the values of θ1(t) and θ2(t). Both the LSB signal and the RSB signal are QPSK modulated, where the phase θ1(t) value of the LSB signal is (π/4, 3π/4, 5π/4, 7π/4), the phase θ2(t) value of the LSB signal is (0, π/2, π, 3π/2). where A1 = 2sin(π/8), A2 = 2sin(3π/8), and Table 1 shows the values of S(t) corresponding to different values of θ1(t) and θ2(t). From the values of S(t) in the table, it can be found that its amplitude is (A1, A2). The phase is (π/8, 3π/8, 5π/8,7π/8, 9π/8, 11π/8, 13π/8, 15π/8). That is, the constellation distribution of signal S(t) is 16QAM, and each constellation point corresponds to a combination of θ1(t) and θ2(t). For example, a 16QAM constellation with amplitude A1 and phase 9π/8 at the receiver corresponds to the LSB signal with phase π/4 and RSB signal with phase π/2. Thus the information on the LSB and RSB can be recovered only by using a PD combined with conventional DSP.
Table 1. The relationship between S(t) and the value θ1(t) and θ2(t)
Figure 2 is consistent with the results in Table 1. Figures 2(a)–2(c) illustrate the phase and amplitude relationship between the 16QAM vector signal and LSB QPSK and RSB QPSK vector signals with different colors. In Fig. 2(a), purple, black, blue, and darkgray represent different phases of the LSB QPSK vector signal constellation. In Fig. 2(b), cyan, orange, green, and red represent different phases of the RSB QPSK vector signal constellation. In Fig. 2(c), each signal consists of two colors, which contain information with QPSK on the LSB and QPSK on the RSB. The radii of the two dotted circles in Fig. 2(c) are A1 and A2, respectively.
Fig. 2. Constellations of LSB QPSK, RSB QPSK, and 16QAM vector signals denoted by different colors. (a) LSB QPSK vector signal. (b) RSB QPSK vector signal. (c) 16QAM vector signal after the PD.
3. Simulation settings
Figure 3 shows the simulation setup of the independent dual-SSB detection scheme based on a single PD. At the transmitter, an ECL with a line width of 100 kHz and an output power of 14 dBm sends out CW optical signal with a center frequency of 193.1 THz, divided into two paths and sent to MZM-1 and MZM-2, respectively. Offline DSP is completed by MATLAB. The word lengths of PRBS1 and PRBS2 are both 215. The LSB signal is modulated by QPSK with phase (π/4, 3π/4, 5π/4, 7π/4), while the RSB signal is modulated by QPSK with phase (0, π/2, π, 3π/2), Figs. 3(a) and 3(b) are the corresponding constellation diagrams. The baud rate for both LSB and RSB signal are 1-Gbaud. The LSB signal is upconverted to −30 GHz, and the RSB signal is upconverted to 30 GHz
Fig. 3. Simulation setup of independent dual-SSB detection scheme based on a single PD. ECL: external cavity laser; MZM: Mach–Zehnder modulator; OBPF: optical bandpass filter; EBPF: electric bandpass filter; EA: Electrical Amplifier; PD: photodiode. (a) LSB signal constellation with QPSK modulation. (b) RSB signal constellation with QPSK modulation. (c) The output Spectrum of MZM-1. (d) The output spectrum of MZM-2. (e) The spectrum of the signal after passing through OBPF. (f) The spectrum of the output signal of the EA. (g) The spectrum of the output signal of the EBPF. (h) The constellation diagram after DSP (The input optical power of the PD is 0 dBm for back-to-back transmission)
After passing through the DAC, the up-converted LSB was divided into real and imaginary parts to drive MZM-1. Similarly, the upconverted RSB signal is divided into real and imaginary parts to drive MZM-2 after passing through DAC. Both MZM-1 and MZM-2 have a half-wave voltage of 4 V and work at the quadrature bias point. Figures 3(c) and 3(d) are the output spectra of MZM-1 and MZM-2, respectively. The outputs of the two MZMs are coupled together using OC and filtered using an OBPF with a center frequency of 193.1 THz and bandwidth of 72 GHz. Figure 3(e) shows the spectrogram of the output optical signal of the OBPF. The optical signal is amplified to 10 dBm and sent to the FSO channel. The parameters of the FSO channel are shown in Table 2.
Table 2. The FSO channel parameter setting
At the receiver, the output signal of the PD with 3 dB bandwidth of 90 GHz is amplified by an electrical amplifier (EA) with 10∼70-GHz working range and 30 dB gain. Figure 3(f) is the frequency spectrum of the output signal of the EA. It can be seen that it includes two sidebands with center frequencies of 30 GHz and 60 GHz. An EBPF with a center frequency of 30 GHz and a bandwidth of 10 GHz filters out the high-frequency components at 60 GHz, and quadrature demodulation is performed. Figure 3(g) is the output spectrum of the EBPF. Then use the offline DSP algorithm to recover the data on LSB and RSB, including normalization, resampling, timing recovery, cascaded multi-modulus algorithm (CMMA) and blind phase search (BPS), decision, LSB and RSB separation, QPSK demodulation, and BER statistics [39–41]. Figure 3(h) shows the signal constellation after BPS in DSP when the input optical power of PD is 0 dBm for back-to-back (BTB) transmission.
4. Results analysis
As shown in Fig. 4, we simulated the BTB transmission of 1-Gbaud LSB -30 GHz QPSK and 1-Gbaud RSB 30 GHz QPSK modulated signal and 1-km 2-km, and 3-km FSO channel transmission, where the parameter settings of the FSO channel are shown in Table 2.
The dotted line in Fig. 4 shows the measured difference between the BER of RSB and LSB signal versus the input optical power into PD for BTB transmission. We can find that the BER difference between the two sidebands is slight because the signal on both sidebands uses the same modulation format. Therefore, we calculate the total BER for the LSB and RSB signal in subsequent simulations, which obtained by comparing the bit sequence at the transmitter with the recovered bit sequence at the receiver. The solid line in Fig. 4 shows the measured total BER of RSB and LSB signal versus the input optical power into PD for BTB transmission. The inset in Fig. 4 is the constellation diagram after the DSP at the receiver when the PD input optical power is −19 dBm, the corresponding BER is 1.2 × 10−3, which is lower than the HD-FEC threshold of 3.8 × 10−3.
Figure 5 gives the measured total BER of RSB and LSB signal versus the input optical power into PD for 1-km FSO channel transmission for different turbulence cases: weak turbulence (WT), (2) medium turbulence (MT), (3) strong turbulence (ST). In addition, we also put the BER curve of BTB transmission here as a reference. The BERs of the ‘BTB’ and ‘WT’ cases can reach below the HD-FEC threshold of 3.8 × 10−3, but the ‘MT’ and ‘ST’ cases cannot. Compared to the ‘BTB’ case, about a 1.2 dB power penalty exists for the ‘WT’ case at the HD-FEC threshold of 3.8 × 10−3. Figure 5 also gives the measured BER of traditional 30 GHz SSB 16QAM signal versus the input optical power into PD for 1-km FSO channel transmission for different turbulence cases. We can find that the performance of dual-SSB QPSK signal is better than traditional SSB 16QAM signal in any case. It may be because the dual SSB signal occupies a wider frequency range. The frequency interval between the LSB and RSB of the dual-SSB QPSK signal is 60 GHz, while the frequency interval between the two sidebands of the traditional SSB 16QAM signal is 30 GHz.
Figure 6 shows the received constellations of 30 GHz 2-Gbaud 16QAM vector signal under 1-km weak turbulence FSO transmission, the input optical power into PD is −19 dBm. Figures 6(a)–6(d) show the constellation after PD, after timing recovery, after CMMA, and after BPS respectively. The LSB and RSB signals can then be separated from the received 16QAM signal by a traditional DSP algorithm.
Fig. 6. Constellations of 30 GHz 2-Gbaud 16QAM vector signal for 1-km FSO transmission under weak turbulence. (a) After the PD. (b) After timing recovery. (d) After the CMMA. (d) After BPS.
Figure 7 gives the measured total BER of RSB and LSB signal versus the input optical power into PD for 2-km FSO channel transmission for different turbulence cases. In addition, we also put the BER curve of BTB transmission here as a reference. The BERs of the ‘BTB’ and ‘WT’ cases can reach below the HD-FEC threshold of 3.8 × 10−3, but the ‘MT’ and ‘ST’ cases cannot. Compared to the ‘BTB’ case, about a 3 dB power penalty for the ‘WT’ case at the HD-FEC threshold of 3.8 × 10−3. In addition, the performance of the ‘MT’ case and the ‘ST’ case are close. It may be due to the influence of transmission distance. The transmission of 2km is too far under ‘MT’ case and the performance is very poor.
Figure 8 gives the measured total BER of RSB and LSB signal versus the input optical power into PD for 3-km FSO channel transmission for different turbulence cases. In addition, we also put the BER curve of BTB transmission here as a reference. The BER cannot reach below the HD-FEC threshold of 3.8 × 10−3 after 3 km FSO channel transmission. Compared with Fig. 6, it can be found that the BER performance of the 3-km weak turbulent FSO channel transmission is significantly reduced compared to the 2-km weak turbulence FSO channel transmission. Increasing the input optical power of the FSO channel may improve the BER performance.
In the case of 1-km weak turbulence FSO transmission, Fig. 9 shows the measured total BER of RSB and LSB signal versus the input optical power into PD for 2-Gbuad, 4-Gbaud, 8-Gbuad, and 16-Gbuad independent dual-SSB vector signal, respectively. For all the different baud rates, the BER can reach below the HD-FEC threshold of 3.8 × 10−3, and the corresponding input optical power into PD is −18.8, −17.8, −16, and −15 dBm, respectively. The BER performance of the 8 Gbaud and 16 Gbaud signals in Fig. 9 is close. It may be due to the insufficient bandwidth of the OBPF. The bandwidth of the OBPF filter is 72 GHz, which is not very sufficient for the 16 Gbaud dual-SSB signal with a frequency interval of 60 GHz between the LSB and RSB.
Fig. 9. BER versus input optical power into PD with different baud rates for 1-km weak turbulence FSO channel transmission.
5. Conclusion
The receiver of the traditional independent dual-SSB signal needs two OBPFs and two PDs, which will increase the complexity and cost of the system. Therefore, we propose an independent dual-SSB signal demodulation scheme based on a single PD combined with DSP, and it does not require an OBPF. Our simulation results demonstrate that the BER of independent −30 GHz 1-Gbaud LSB QPSK and 30 GHz 1-Gbaud RSB QPSK signal can be less than the HD-FEC threshold of 3.8 × 10−3 after 1-km or 2-km weak turbulence FSO channel transmission, and the BER of −30 GHz 8-Gbaud LSB QPSK and 30 GHz 8-Gbaud RSB QPSK signal can less than the HD-FEC threshold of 3.8 × 10−3 after 1-km weak turbulence FSO channel transmission. We believe that the proposed new independent dual-SSB system can reduce the complexity and cost of the system and can transmit longer distances when the input optical power of the FSO channel is increased.
Funding
National Natural Science Foundation of China (61775033, 62025105); Chongqing Municipal Education Commission (CXQT21019, KJQN202100616); Chongqing University of Posts and Telecommunications (A2016-94).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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