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Abstract
We propose and verify a direct detection (DD) system based on a single photodiode (PD) receiving the independent quadruple-single-sideband (quadruple-SSB) signal. At the transmitter side, an I/Q modulator is utilized to modulate the independent quadruple-SSB signal, the signal is received via one PD without optical bandpass filters (OBPFs). Then, the independent quadruple-SSB signal is separated into four sideband signals by subsequent digital signal processing (DSP). In the scheme of back-to-back (BTB), 1-km and 5-km standard single-mode fiber (SSMF) transmission, the four sideband signals are extensively studied and analyzed. The simulation results show that the bit error rate (BER) of 1Gbaud, 2Gbaud and 4Gbaud independent quadruple-SSB signal can reach the 7% hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10−3 when the received optical power (ROP) is −21, −20 and −17.2 dBm in 5-km SSMF transmission. Meanwhile, as the frequency interval gets wider, the crosstalk in the sideband signal reception can be mitigated and the BER decreases. This scheme for the first time demonstrates that the independent quadruple-SSB signal can further expand the system transmission capacity and enhance the spectrum efficiency. Our simplified independent quadruple-SSB signal direct detection system has a simple structure and high spectral efficiency, which will have a promising future in high-speed optical communication.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the remarkable explosion of wireless devices and bandwidth-consuming Internet applications, there is an increasingly challenging burden on ultra-high data rate wireless communications that must be addressed [1,2]. It is indispensable to construct high-speed, large-capacity and low-cost optical transmission networks [3]. A variety of advanced modulation formats have been proposed to improve the capacity of optical transmission systems, such as optical carrier suppression (OCS) modulation [4–7] technologies, independent-sideband (ISB) [8–12] and single-sideband (SSB) [13–16] modulations. Compared with the traditional double-sideband (DSB) [17] format, the SSB signal exhibits good tolerance to power fading caused by chromatic dispersion (CD) [18,19]. A bandwidth-economic coherent optical transmission technology using optical independent-sideband (O-ISB) modulation has been proposed and implemented, and the first experimental study on the bandwidth-economic beyond-100 G transmission by exploiting both O-ISB and software-defined optics (SDO) technologies has been conducted [8]. As discussed in [9], a novel and simple $2 \times 2$ multiple-input multiple-output (MIMO) optical-wireless integration system adopting O-ISB modulation enabled by an I/Q modulator and demonstrates the simultaneous generation and $2 \times 2$ MIMO wireless delivery of two independent 40 GHz QPSK wireless mm-wave signals. Compared with O-ISB, the transmission capacity of dual-SSB is twice that of O-ISB because the LSB and RSB carry independent data information. Recently, optical twin-SSB modulation has been used as an extension to optical SSB modulation [13–16]. Twin-SSB system inherits the advantages of optical SSB system while further improving the spectrum efficiency. However, there is a crosstalk between RSB signal and LSB signal caused by the limited sideband suppression ratio in transmitter side [14]. By properly adjusting polarization controller at the transmitter side, the crosstalk between the RSB signal and LSB signal of twin-SSB signal can be cancelled, which indicates the background noise of twin-SSB signal can be much more mitigated [15]. The asymmetric direct detection (ADD) of the twin-SSB signal is based on a simple receiver front-end composed of one optical filter and two PDs, and it exploits the photocurrent difference between a filtered and unfiltered signal pair to reconstruct and linearize the received twin-SSB signal with a high electrical spectral efficiency (ESE) [16]. Direct detection technology is related to the simple transmitter and receiver design, which requires less electrical processing and is free from laser phase noise [20]. The system based on DD is widely used in legacy optical networks of infrastructure because of its simple structure and low cost [21]. Therefore, it can become an excellent candidate for mid to long-haul transmission of optical communication systems. However, the systems of O-ISB and twin-SSB are complex and have much room for improvement in the transmission capacity of the systems.
To further enhance the spectrum efficiency and expand the system transmission capacity, we propose and validate a direct detection system based on a single PD receiving an independent quadruple-SSB signal. The independent quadruple-SSB signal carries four independent data information, so it has four times the transmission capability of O-ISB and twice that of optical dual-SSB, and the four sideband signals are received by only one PD, followed by extracting the four signals separately by de-mapping, without placing OBPFs at the receiver side to separate the independent quadruple-SSB signal, this system greatly simplifies the system structure to further reduce the implementation cost and improve the spectrum efficiency.
2. Principle
2.1 Principle of 16QAM signal generation
Figure 1 explains the principle of 16QAM signal generation by independent quadruple-SSB signal. At the transmitter side, four sets of pseudo-random binary sequences (PRBSs) of quadruple-SSB signal, wherein PRBS1 and PRBS4 are modulated in the format of quadrature phase shift keying (QPSK), and PRBS2 and PRBS3 are modulated in staggered QPSK (S-QPSK). Then, each group data is up-sampled and root-raised cosine (RRC) shape respectively, and the four groups vector data generated are called left-sideband 1 (LSB1), left-sideband 2 (LSB2), right-sideband 1 (RSB1) and right-sideband 2 (RSB2) respectively. Subsequently, the baseband signals are converted into intermediate frequency (IF) signals through mixing with four complex sinusoidal forms: exp(-j2πfs1t), exp(-j2πfs2t), exp(j2πfs1t) and exp(j2πfs2t). In this paper, the vector signals at negative frequencies are defined as LSB1 signal and LSB2 signal, and the vector signals at positive frequencies are defined as RSB1 signal and RSB2 signal. It is worth noting that the four carriers may have either identical or different modulation formats, symbol rates and filter roll-off factors.
Fig. 1. Schematic diagram of synthesis signal generation for independent quadruple-SSB transmission. (i) Spectrum diagram of sum of sideband signals, (ii) Spectrum diagram of the real part of quadruple-SSB signal, (iii) Spectrum diagram of the imaginary part of quadruple-SSB signal, (iv) Spectrum diagram of quadruple-SSB signal after I/Q modulator, (v) Spectrum diagram of electrical signal received after PD. PRBS: pseudo-random binary sequence, RRC: root raised cosine, ECL: external cavity laser, MZM: Mach-Zehnder modulator, PM: phase modulator, PD: photodiode
Here, the carrier frequencies of the four signals are the same, the carrier signals of LSB1, LSB2, RSB1 and RSB2 can be expressed as:
where EL1(t), EL2(t), ER1(t) and ER2(t) represent LSB1, LSB2, RSB1, RSB2 signals respectively. After adding these four signals, an independent quadruple-SSB signal is obtained, as shown in Fig. 1(i). The synthesized signal is as follows:Further, take the real part and the imaginary part of the quadruple-SSB signal formed after superposition, in which the real part acts as the I-channel driving signal and the imaginary part acts as the Q-channel driving signal, and send them to the corresponding ports of I/Q modulator respectively. Figure 1(ii) and (iii) are the corresponding schematic diagram of the signal spectrum. The optical carrier emitted by the external cavity laser (ECL) with a center frequency of fc is modulated by an I/Q modulator to generate an optical signal. As shown in Fig. 1(iv), four independent vector signals of LSB1, LSB2, RSB1 and RSB2 are generated respectively during the output of the I/Q modulator, and the frequencies are fc - fs1, fc - fs2, fc + fs1, and fc + fs2 respectively. The optical signal generated by the I/Q modulator can be represented as:
2.2 Principle of separating signals
Equation (7) illustrates that the amplitude of the 16QAM constellation is the product of the amplitudes of the QPSK constellation and the S-QPSK constellation, and the phase of the 16QAM constellation is the sum of the phases of the QPSK and S-QPSK constellation. In short, the amplitude and phase of the 16 QAM constellation are combinations of the amplitudes and phases of the QPSK and S-QPSK constellations. The 16QAM signal synthesized by LSB1 and RSB1 is the same as the 16QAM signal synthesized by LSB2 and RSB2. Here, the modulation format is the same for the two left sidebands, and the same for the right sidebands. In general, the modulation formats of the four sideband signals can be different, resulting in different QAM signals.
Figure 2(a) and Fig. 2(b) illustrate the relationship between the S-QPSK constellation diagram in LSB signal and the 16QAM superimposed by the QPSK constellation diagram in RSB π/4 phase shift keying. It can be observed from Fig. 2 that each constellation point of 16QAM can be modulated entirely by one point of S-QPSK signal and one point of QPSK signal. As shown in Fig. 2, the constellation point (2, 3) of 16QAM is composed of constellation point “2” of LSB signal and constellation point “3” of RSB signal together, which is the symbol “9” corresponding to 16QAM in Table 1. Each constellation point in the 16QAM formed by the superposition can be derived in the same way. The composition of the symbols corresponding to 16QAM demodulation is given in Table 1 (grey coding), and the symbol “9” for 16QAM can be specifically demodulated as “2” for S-QPSK and “3” for QPSK.
Fig. 2. Schematic diagram of 16QAM synthesis. (a) S-QPSK constellation diagram in LSB signal, (b) QPSK constellation diagram in RSB signal, (c) superimposed 16QAM signal.
Table 1. Symbol combinations for QPSK, S-QPSK, and 16QAM
3. Simulation setup
Figure 3 depicts the simulation setup diagram of receiving an independent quadruple-SSB signal based on a single PD. To validate the proposed synthesis scheme, at the transmitter side, an external cavity laser (ECL), operating at 193.1 THz and with an output power of −16 dBm, generates continuous-wavelength (CW) light-wave carrying the independent quadruple-SSB 16QAM signal, the line width of ECL1 is less than 100 kHz. The CW light-wave is modulated by an electrical signal with an I/Q modulator, whose half-wave voltage (Vπ) is 2.5 V. The I/Q modulator consists of two Mach-Zehnder modulators (MZMs) and a phase modulator, MZM-a and MZM-b were biased at the null point [22]. The phase difference between the upper and lower branches of the I/Q modulator is fixed as π/2. After being modulated by the I/Q modulator, the output optical signal is transmitted through the SSMF. At the receiver side, a variable optical attenuator (VOA) is used to adjust the value of the received optical power. Then, the optical signal enters PD for photoelectric conversion to obtain the electrical signal. The spectrum diagram of the electrical signal obtained after PD’s square law detection is shown in Fig. 3(ii).
Fig. 3. Simulation setup diagram of direct detection system based on single PD receiving independent quadruple-SSB signal. (i) Spectrum diagram of quadruple-SSB signal after I/Q modulator, (ii) Spectrum diagram of electrical signal received after PD; TX: transmitter sideband, OBPF: optical bandpass filter, SSMF: Standard Single-mode Fiber, VOA: variable optical attenuator, RX: receiver sideband.
After the electrical signals are detected at the received side, the offline digital signal processing (DSP) is carried out. First, the received radio-frequency (RF) signals are converted into the baseband signals by digital down-conversion. Then, the clock recovery algorithm is used to solve the problem of clock mismatch, followed by the cascaded multi-modulus algorithm (CMMA) [23] and blind phase search (BPS) [24] are applied to process the signals. Eventually, the four SSB signals are extracted and the BERs of LSB1, LSB2, RSB1 and RSB2 are calculated, respectively.
4. Simulation results
The measured output signal spectrum of the I/Q modulator is shown in Fig. 3(i), the frequency interval between LSB1 and RSB1 and the central carrier are both 11 GHz, and the frequency interval between LSB2 and RSB2 and the center carrier are both 20 GHz. After carrier suppression modulation, the central optical carrier has been compressed to a very low level. After SSMF transmission, the optical signals are detected through the PD, and the frequencies of the received signals will be located at 22 GHz and 40 GHz, the spectrum diagram of the signals are shown in Fig. 3(ii). Subsequently, the signals located at 22 GHz and 40 GHz are respectively down-converted to baseband to facilitate processing by DSP, which includes the clock recovery algorithm, CMMA and BPS algorithm, the constellation diagram corresponding to each digital signal processing process is shown in Fig. 4.
Fig. 4. (a) 22 GHz 4Gbaud 16QAM vector signal constellation under BTB transmission (b) 40 GHz 4Gbaud 16QAM vector signal constellation under BTB transmission. (i) and (v) are the constellations after PD, (ii) and (vi) are the constellations after re-timing, (iii) and (vii) are the constellations after CMMA, (iv) and (viii) are the constellations after BPS.
In the simulation experiment, one carrier frequency is set at 11 GHz and the other at 20 GHz, and we verify that the signals of LSB1, LSB2, RSB1 and RSB2 are carried out at the baud rates of 1Gbaud, 2Gbaud and 4Gbaud respectively, the performance of independent quadruple-SSB system in back-to-back (BTB), 1-km, 2-km, and 5-km SSMF transmission. Figure 5(a)-(c) show the BER of the LSB1, LSB2, RSB1 and RSB2 signals versus ROP in BTB transmission at baud rates of 1Gaud, 2Gbaud and 4Gbaud, respectively. Obviously, in BTB transmission, when the baud rate of quadruple-SSB signal is 1Gbaud, 2Gbaud or 4Gbaud, and the corresponding ROP is larger than −22, −21 and −18.4 dBm, respectively. The BER of four independent SSB signals (LSB1, LSB2, RSB1 and RSB2) are below the $7\%$ HD-FEC threshold of $3.8 \times {10^{ - 3}}$.
Fig. 5. Measured BER versus received optical power with different baud rates in BTB transmission. (a) 1Gbaud, (b) 2Gbaud, (c) 4Gbaud.
Figure 6(a)-(c) depict the BER of the LSB1, LSB2, RSB1 and RSB2 signals versus ROP in 1-km SSMF transmission at baud rates of 1Gaud, 2Gbaud and 4Gbaud., respectively. It can be perceived that in 1-km SSMF transmission, when the ROP is larger than −22, −20.8 and −18 dBm, respectively, the BER of 1Gbaud, 2Gbaud and 4Gbaud independent quadruple-SSB signal can all reach the 7% HD-FEC threshold of 3.8 × 10−3.
Fig. 6. Measured BER versus received optical power with different baud rates in 1-km SSMF transmission. (a) 1Gbaud, (b) 2Gbaud, (c) 4Gbaud.
Figure 7(a)-(c) show the BER of the LSB1, LSB2, RSB1 and RSB2 signals versus ROP in 5-km SSMF transmission at baud rates of 1Gaud, 2Gbaud and 4Gbaud. It can be observed that in 5-km SSMF transmission, when the corresponding ROP is larger than −21, −20 and −17.2 dBm, respectively, the BER of 1Gbaud, 2Gbaud or 4Gbaud independent quadruple-SSB signal (LSB1, LSB2, RSB1 and RSB2) are below the 7% HD-FEC threshold of 3.8 × 10−3. It can be concluded that with the increase of signal transmission rate, limited bandwidth resources will lead to the deterioration of system performance.
Fig. 7. Measured BER versus received optical power with different baud rates in 5-km SSMF transmission. (a) 1Gbaud, (b) 2Gbaud, (c) 4Gbaud.
Figure 8 depicts the independent quadruple-SSB signal with a baud rate of 4Gbaud and the influence of carrier frequency variation on the BER in BTB transmission. As the frequency interval becomes narrower, the crosstalk in the sideband signal reception becomes more severe and the BER of the four sideband signals all increase. When the ROP is −17 dBm and the frequency interval is within the range shown in Fig. 8, the BER of the four sideband signals can still be below the 7% HD-FEC threshold of 3.8 × 10−3.
5. Conclusion
In this paper, a direct detection system receiving the independent quadruple-SSB signal based on a single PD is proposed. This independent quadruple-SSB system is realized based on only one I/Q modulator, and a de-mapping algorithm after DSP is used to extract four independent SSB signals instead of OBPFs to extract them. It can save two pairs of OBPFs and three PDs, simplify the system structure and improve the system transmission performance. The simulation results demonstrate that the system can successfully realize independent quadruple-SSB 4Gbaud 16QAM signal SSMF transmission over 5-km with the BER can reach the 7% HD-FEC threshold of 3.8 × 10−3. Meanwhile, the BER of this system decreases when the frequency interval between SSBs gets larger. Apart from reducing the cost and complexity of the system, we maintain that the new independent quadruple-SSB system can effectively increase the transmission capacity and improve the spectrum efficiency of the system.
Funding
National Key Research and Development Program of China (2018YFB1801503); National Natural Science Foundation of China (61675048, 62141503); The project of Hunan Provincial Department of Education (21B0514); Key Laboratory of Electromagnetic Wave Information Science (EMW201911).
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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