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Abstract
To meet the ultra-bandwidth high-capacity communication, improve spectral efficiency and reduce the complexity of system structure, we have proposed the independent triple-sideband signal transmission system based on photonics-aided terahertz-wave (THz-wave). In this paper, we demonstrate up to 16-Gbaud independent triple-sideband 16-ary quadrature amplitude modulation (16QAM) signal transmission over 20 km standard single mode fiber (SSMF) at 0.3 THz. At the transmitter, independent triple-sideband 16QAM signals are modulated by an in-phase/quadrature (I/Q) modulator. Carrying independent triple-sideband signals optical carrier coupled with another laser to generate independent triple-sideband terahertz optical signals with a carrier frequency interval of 0.3THz. While at the receiver side, enabled by a photodetector (PD) conversion, we successfully obtain independent triple-sideband terahertz signals with a frequency of 0.3THz. Then we employ a local oscillator (LO) to drive mixer to generate intermediate frequency (IF) signal, and only one ADC is used to sample independent triple-sideband signals, digital signal processing (DSP) is finally performed to obtain independent triple-sideband signals. In this scheme, independent triple-sideband 16QAM signals is delivered over 20 km SSMF under the bit error ratio (BER) of 7% hard-decision forward-error-correction (HD-FEC) threshold of 3.8 × 10−3. Our simulation results show that the independent triple-sideband signal can further improve THz system transmission capacity and spectral efficiency. Our simplified independent triple-sideband THz system has a simple structure, high spectral efficiency, and reduced bandwidth requirements for DAC and ADC, which is a promising solution for future high-speed optical communications.
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1. Introduction
In recent years, terahertz (THz) optical communication has attracted a great interest for its enormous potential in ultra-broadband communication of the fiber link. THz waves in frequency range from 0.1 THz to 10 THz [1], which provides a large transmission capacity comparable to the optical fiber communications counterpart [2,3]. Compared with the conventional radio frequency (RF) communication system, THz-wave has the large advantage in large available bandwidth and good directionality. Moreover, THz signal is less affected than infrared (IR) beams and also less subject to flicker effects caused by local refractive index changes [4]. Therefore, THz waves system can satisfy the demand of wide bandwidth and be regarded as a promising candidate for future fifth-generation (5 G) or even sixth-generation (6 G) network. Many researches and experimental demonstrations utilizing THz signal for high speed communication have been reported. THz signals can be generated by electric or photonic techniques. In these schemes based on electric technique, several design techniques have been proposed based on oscillators with frequency multipliers, harmonic oscillators and fundamental oscillators [5–12]. In such electric schemes, the transceiver is compact and simple, however, their modulation bandwidth and transmission rate are limited, and the mixers they used has a great intrinsic conversion loss, which results in a short transmission distance. Recently, methods of generating THz-wave frequencies based on photonics-aided technology can not only overcome the bandwidth limitation of electrical components, but also effectively promote the seamless integration of fiber and wireless networks, have been reported [13–18]. In these schemes, 2 optical carrier signals with different wavelength generated by 2 free-running lasers individually, and an optical coupler (OC) can be employed to realize the combination of the 2 optical signals, which is the wideband THz signal we desired. Song et al. presented a THz wave 24 Gbit/s wireless data transmission wireless link operating at 0.3THz by using a uni-travelling carrier photodiode (UTC-PD) and a schottky barrier diode (SBD) detector [19]. Based on photonics-aided wireless THz transmission system without THz amplifier, Li et al. demonstrated 124 Gbit/s signal transmission over 54 meters wireless distance and 44 Gbit/s signal transmission over 104 meters wireless distance [20]. However, it is difficult to realize simultaneous transmission of multiple signals, which result in limited spectral efficiency. Wang et al. experimentally demonstrate a 2 × 2 multiple-input multiple-output (MIMO) radio-over-fiber system at W-band and successfully realize a single carrier 16QAM transmission through 100 km standard single-mode fiber (SSMF) and 40 m W-band wireless link with a net bit rate of 51.2 Gbit/s [21]. Jia et al. successfully realized a photonic multi-channel THz wireless transmission system in the 350-475 GHz band. The employment of 6 THz carriers modulated with 10-Gbaud nyquist quadrature phase-shift keying (QPSK) baseband signal per carrier results in an overall capacity of up to 120 Gbit/s [22]. Li et al. experimentally demonstrate 132-Gb/s (12-Gbaud) photonics-aided single-carrier PDM-64QAM-PS 5.5 THz-wave signal transmission at 450 GHz over 20-km fiber-optics and 1.8-m wireless distance [23]. Cheng et al. propose a method of locally centralized photonics-aided coordinated multipoint (COMP) transmissions for mm-wave small cells [24]. Yu et al. experimentally demonstrated an ultra-broadband D-band (110∼170 GHz) wireless mm-wave signal delivery system adopting photonics-aided mm-wave generation and successfully realized PDM-QPSK wireless mm-wave signal delivery with the highest baud rate of 184 Gbit/s [25]. Although these schemes can support multiplexed signal transmission and improve spectral efficiency, however, their THz systems structure are complex and rely on high requirement modulator, which brings tremendous pressure to the cost of devices.
According to the review above, traditional THz system can only simultaneously transmit one channel signal, the spectral efficiency is limited. To realize multiplexed signal transmission, they need to employ multiple modulators or wavelength-division multiplexing (WDM) system, which will lead to a complex system structure. To make up for the deficiencies of the above THz system. in this study, we have demonstrated a simplified THz system with a compact structure for transmitting multiplexed signals in one channel. An I/Q modulator is used to modulate independent triple-sideband signals, which are optically coupled to the local oscillator to generate a terahertz signal at a carrier frequency of 0.3THz. Without the WDM technology or multiple I/Q modulators, the system complexity can be significantly reduced, which has low requirements for ADC bandwidth. To the best of our knowledge, this is the first time to realize multiplexed THz signal generation and transmission with one channel. Based on the measured bit error rate (BER) and carrier-to-signal power ratio (CSPR) results, the simultaneous delivery of 16-Gbaud independent triple-sideband 16QAM THz signals at 0.3THz over 10 km, 20 km SSMF can be realized, respectively. Considering 7% hard-decision forward-error-correction (HD-FEC) overhead threshold of 3.8 × 10−3, the ideal BER less than the HD-FEC can be satisfied. The simulation results show the feasibility of our proposed scheme.
2. Principle
Photonics-aided generation technology of THz signals can effectively solve the bandwidth limitations of electrical devices, and has the flexibility to generate the desired frequency THz wave signals [26–28]. Figure 1 shows the basic principle of independent triple-sideband THz signal generation based on photonics-aided method. At first, two free-running lasers (Laser1 and Laser2), with a linewidth less than 100 kHz and maximal output power of 16 dBm, are adopted to generate continuous-wavelength (CW) light-wave at different frequencies of fc1 and fc2, respectively. The generating frequency spacing of two optical carrier fs (fs = fc1- fc2) is tunable. The CW light-wave from laser1 is modulated by independent triple-sideband signal at frequency fl, baseband and frequency fr, respectively. The independent triple-sideband signal is first generated by offline programming, and then used to drive the I/Q modulator, represented by ${S_{left}}(t)$, ${S_{base}}(t)$ and ${S_{right}}(t)$, respectively. After up-conversion, as shown in Fig. 1(i), the left sideband, baseband and right sideband signals can be expressed as
Fig. 1. The basic principle of photonics-aided THz-wave independent triple-sideband signal generation and transmission. TX: transmitter sideband. LO: local oscillator, OC: optical coupler, PD: photodetector.
Then the left sideband, baseband and right sideband signals are combined to obtain the independent triple-sideband signal, which can be expressed as
Consequently, the real part and imaginary part of the combing independent triple-sideband signal E(t) are uploaded into the two input ports of the IQ modulator. Next, the modulated CW light-wave from laser1 and the unmodulated one from laser2 are injected into optical coupler (OC). Here, the optical frequency of fc1 - fl, fc1, and fc1 + fr are used to carry left sideband, baseband, and right sideband signal, respectively, as is shown in Fig. 1(ii). After OC, the frequency spacing fs is our desired THz wave band. Then heterodyne beating in a single-ended photodetector (PD) uses optical-to-electrical conversion to get the independent triple-sideband electrical THz signal at carrier frequency of fs - fl, fs and fs + fr, respectively, as is shown in Fig. 1(iii).
After PD, the received electrical THz signal is firstly down-converted by a local oscillator (LO) with frequency at fLO [29]. Therefore, the generated independent triple-sideband intermediate frequency (IF) at fs - fl - fLo, fs-fLo. and fs + fr - fLo, respectively, as is shown in Fig. 1(iv). Finally, the received signal is performed digital signal processing (DSP).
3. Simulation setup and result discussion
3.1 Simulation setup
Figure 2 depicts the simulation setup of the demonstrated independent triple-sideband photonics-aided THz signals transmission system, which can realize up to 0.3THz 16-Gbaud independent triple-sideband 16QAM signal transmission over 20-km SSMF link. At the transmitter, external cavity laser1 (ECL1) operates at 191.1 THz with 16 dBm output power and generates a beam of CW light-wave to carry the independent triple-sideband 16QAM signal. The linewidth of the ECL1 is less than 100 kHz. The CW light wave is modulated by the electrical signal with an I/Q modulator, whose half wave voltage (Vπ) is 2.5 V. The I/Q modulator consists of two dual-drive Mach Zehnder modulators (DDMZMs), both biased at the null point and a phase modulator. The upper and lower branches of the I/Q modulator have a fixed phase difference of π/2. Independent triple-sideband 16QAM signal, used for the drive of the I/Q modulator, is generated by offline programming, as is shown in Fig. 2(a). Three sets of pseudo random binary sequence (PRBSs) with a pattern length of 214, respectively, are first mapped onto 16QAM modulation format, and then up-sampling is implemented. After passing a root raised cosine (RRC) filter, the independent triple-sideband signals are finally up-converted. Corresponding to 20 GHz carrier spacing, left sideband (LSB) is linearly converted located at a carrier frequency of -20 GHz, respectively, while right sideband (RSB) signal is linearly converted located at a carrier frequency of 20 GHz, respectively. The frequency of the CW light-wave generated from ECL2 is set as 191.4 THz so that the frequency spacing between ECL1 and ECL2 is 0.3 THz. Moreover, the output power of ECL2 is -7 dBm. An OC is employed to combine the two modulated and unmodulated light-waves.
Fig. 2. Simulation setup of the demonstrated independent triple-sideband THz signals transmission system. ECL: external cavity laser, VOA: variable optical attenuator, RRC: root raised cosine, LO: local oscillator, EA: electrical amplifier.
Figure 3(a) shows the optical spectrum of independent triple-sideband signal after OC. Then, a variable optical attenuator (VOA) is used to adjust the input optical power into PD. After heterodyne beating, the desired THz signal at 0.3THz can be obtained, as is shown in Fig. 3(b). Consequently, the THz signal is firstly down-converted to an IF signal by a mixer, which is driven by a microwave LO at 260 GHz. Therefore, the center frequency of the IF signal is 40 GHz. As the photocurrent is small, we use an electrical amplifier (EA) with 30 dB gain to amplify the electric signals. The IF signal finally is performed by a DSP. Particularly, for independent triple-sideband signal modulation, the detailed offline DSP includes down-sampling, IF down conversion, clock recovery, cascaded multi-modulus algorithm (CMMA) equalization, carrier recovery, decoding, and BER calculation, just as shown in Fig. 4.
Fig. 4. Receiver-based offline DSP for independent triple-sideband IF signal as well as recovered constellations, Origin, after CMMA, and after carrier recovery, respectively.
3.2 Result and discussion
It is well known that high CSPR can effectively mitigate the crosstalk caused by modulated signal. The third-order harmonics will increase with the increasing of CSPR. In addition, the optical signal-to-noise ratio (OSNR) of the signal will be decreased as the CSPR increasing. Therefore, the optimal CSPR is critical. In order for this system to work optimally, the essential parameter CSPR must firstly be measured.
Figure 5 gives the measured 16-Gbaud right sideband (RSB) signal BER versus the CSPR at received optical power of -10 dBm. CSPR values from 14 dB to 27 dB are controlled by changing optical carrier power or the bias point of the IQ modulator and fixing the signal power loading into the IQ modulator. In the range of 14 ∼ 21 dB, the BER start to decrease as the CSPR increasing. However, when the CSPR is greater than 21 dB, the BER turns to deteriorate. Therefore, the optimal CSPR is 21 dB. A similar conclusion can be found from the constellation inserted in Fig. 5 when the CSPR is too low, the crosstalk caused by the beating term of the modulated signal is not negligible, which will result in the constellation not being separated as shown in Fig. 5(i). At a higher CSPR, the signal-to-noise ratio of the signal is too low, which will also result in the constellation not being separated as shown in Fig. 5(iii).
Fig. 5. The measured RSB signal BER versus CSPR for 16-Gbaud 16QAM signals. Constellations with different CSPR: inset (i): 14 dB, inset (ii):21 dB, inset (iii): 27 dB.
In the remainder demonstration, the CSPR value are all set at 21 dB. Figure 6 shows the independent triple-sideband signal BER versus received optical power (ROP) for 16-Gbaud 16QAM signals at 0.3THz after 10-km SSMF transmission cases. In 10-km SSMF case, the required ROP is about -14 dBm for the THz system at the HD-FEC threshold. Left sideband (LSB) and right sideband (RSB) signal have similar BER trends.
Fig. 6. The measured BER results versus received optical power for 16-Gbaud independent triple-sideband 16QAM signals at 0.3THz after 10-km SSMF transmission cases.
Figure 7 gives the independent triple-sideband signal BER versus received optical power for 16-Gbaud 16QAM signals at 0.3THz after 20-km SSMF transmission cases. The measured results show that the independent triple-sideband signal BER can be below 3.8 × 10−3 when the received optical power is over -13 dBm. It can be observed that the BER performance trends of independent triple-sideband signal also complies with Fig. 6.
Fig. 7. The measured BER results versus received optical power for 16-Gbaud independent triple-sideband 16QAM signals at 0.3THz after 20-km SSMF transmission cases.
4. Conclusion
In this paper, we proposed an independent triple-sideband THz communication system enabled by photonics-aided THz wave generation technique. The system can successfully realize independent triple-sideband 16-Gbaud 16QAM signal SSMF transmission over 20 km with THz carrier frequency up to 0.3THz, the BER is below the 7% HD-FEC threshold of 3.8 × 10−3. To the best of our knowledge, it is the first time that simultaneous independent triple-sideband THz signal transmission is realized based on one I/Q modulator. Owing to the simple system structure, high spectral efficiency, and low bandwidth device requirements, our work can promote the application of THz-wave communication, which has huge market prospects and application potential.
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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