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A hybrid lightwave transmission system for cable television (CATV)/millimeter-wave (MMW)/baseband (BB) signal transmission based on fiber-wired/fiber-wireless/fiber-visible laser light communication (VLLC) integrations is proposed and demonstrated. For down-link transmission, the light is intensity-modulated with 50-550 MHz CATV signal and optically promoted from 25 GHz radio frequency (RF) signal to 10 Gbps/50 GHz and 20 Gbps/100 GHz MMW data signals based on fiber-wired and fiber-wireless integrations. Good performances of carrier-to-noise ratio (CNR), composite second-order (CSO), composite triple-beat (CTB), and bit error rate (BER) are obtained over a 40-km single-mode fiber (SMF) and a 10-m RF wireless transport. For up-link transmission, the light is successfully intensity-remodulated with 5-Gbps BB data stream based on fiber-VLLC integration. Good BER performance is achieved over a 40-km SMF and a 10-m free-space VLLC transport. Such a hybrid CATV/MMW/BB lightwave transmission system is an attractive alternative, it gives the benefits of a communication link for broader bandwidth and higher transmission rate.
© 2015 Optical Society of America
1. Introduction
A hybrid lightwave transmission system is a promising candidate for broadband networks. A network which can support both wired and wireless communications concurrently has been necessary because of the increased demands for broad bandwidth and high-speed transmission rate [1–4]. Delivering hybrid cable television (CATV), millimeter-wave (MMW), and baseband (BB) signals over a lightwave transmission system has many advantages, like broad bandwidth, high-speed transmission rate, and wide service area. A hybrid lightwave transmission system that carries different optical wavelengths to deliver the CATV, MMW, and BB signals would be quite useful for a network to provide CATV, telecommunication, and data communication services. However, it is quite a challenge to deliver CATV, MMW, and BB signals simultaneously by using one optical fiber in an easy and cost-effective way. A single-wavelength system multiplexing different radio frequency (RF) signals could be a solution. Nevertheless, multiplexing these RF signals will generate distortions after beating among these hybrid RF signals. On the other hand, multiplexing these RF signals in the optical domain needs multiple distributed feedback (DFB) laser diodes (LDs) to support different services. These DFB LDs are wavelength-selected for each optical channel, and it will increase the complexity and sophistication of systems. Developing a configuration with potentially simple characteristic to ensure the implementation of a hybrid CATV/MMW/BB lightwave transmission system is essential. A hybrid lightwave transmission system for CATV/MMW/microwave (MW) signal transmission based on fiber-wireless convergence was demonstrated previously [5]. However, it has rooms for improvement. Such a full-duplex CATV/wireless-over-fiber lightwave transmission system is not flexible due to the same transmission rate for downstream MMW and upstream MW data signals. Moreover, for up-link transmission, the RF wireless subsystem can be replaced by the optical wireless subsystem. Thereby, the costly and sophisticated RF devices are not required. In this paper, a hybrid lightwave transmission system for CATV/MMW/BB signal transmission based on fiber-wired/fiber-wireless/fiber-visible laser light communication (VLLC) integrations is proposed and experimentally demonstrated. To the best of our knowledge, it is the first one that employs fiber-wired/fiber-wireless/fiber-VLLC integrations in a hybrid lightwave transmission system. This hybrid lightwave transmission system delivers downstream 550-MHz CATV/50-GHz MMW/100-GHz MMW signals and upstream 5-Gbps BB data stream. The transmission rates of downstream 50-GHz MMW data signal, 100-GHz MMW data signal, and upstream BB data stream are different between each other. It is practical and flexible for the real implementation of a hybrid lightwave transmission system. For down-link transmission, the light is intensity-modulated with 50-550 MHz CATV signal and optically promoted from 25-GHz RF signal to 10 Gbps/50 GHz and 20 Gbps/100 GHz MMW data signals based on fiber-wired and fiber-wireless integrations. For up-link transmission, the light is successfully intensity-remodulated with 5-Gbps BB data stream based on fiber-VLLC integration [6,7]. The parameters of carrier-to-noise ratio (CNR), composite second-order (CSO), and composite triple-beat (CTB) are used to evaluate the performances of downstream CATV signal. Meanwhile, the bit error rate (BER) parameter and eye diagram are used to evaluate the performances of 10 Gbps/50 GHz and 20 Gbps/100 GHz MMW downstream data signals, and 5-Gbps BB upstream data stream. A thorough investigation of a hybrid CATV/MMW/BB lightwave transmission system reveals that good performances of CNR, CSO, CTB, and BER are achieved over a 40-km single-mode fiber (SMF) and a 10-m RF wireless transport for down-link transmission; as well as over a 40-km SMF and a 10-m free-space VLLC transport for up-link transmission. Such a hybrid CATV/MMW/BB lightwave transmission system would be very attractive for providing broadband integrated services including CATV, telecommunication, and data communication services. It is shown to be a prominent one not only to present its advancement for the integration of fiber backbone and RF/optical wireless feeder networks, but also to reveal its advantage for broader bandwidth and higher transmission rate.
2. Experimental setup
The configuration of the proposed hybrid CATV/MMW/BB lightwave transmission systems based on fiber-wired/fiber-wireless/fiber-VLLC integrations is presented in Fig. 1. A broadband light source (BLS), comprising a DFB LD and a dual-arm Mach-Zehnder modulator (MZM), is employed at the transmission site to generate multiple equal-space optical sidebands. The dual-arm MZM are modulated with the CATV signal and the 25-GHz RF signal. For achieving modulation in intensity, the dual-arm MZM is operated at the quadrature point with a DC bias of -Vπ/2 and a peak-to-peak modulation of Vπ (Vπ is an operating voltage to obtain a phase shift of π). A total of 77 channels (CH2-78; 50-550 MHz) generated from a multiple signal generator are used to simulate the CATV signal. With a proper RF signal (6 dBm) to modulate the dual-arm MZM, multiple optical sidebands are created with a channel spacing of 25 GHz [inset (a) of Fig. 1]. Two optical interleavers (OILs) are deployed at the transmitting site to separate even and odd optical sidebands of optical signal. The OIL has two output ports; one output port provides the optical signal only with the even optical sidebands, and the other output port provides the optical signal only with the odd optical sidebands [8,9]. The zero optical sideband (central carrier) is utilized for CATV down-link transmission, the −1 and + 1 optical sidebands are utilized for 50-GHz MMW down-link transmission, the −2 and + 2 optical sidebands are utilized for 100-GHz MMW down-link transmission as well. And further, the + 2 optical sideband is utilized for 5-Gbps BB up-link transmission.
Fig. 1 The configuration of the proposed hybrid CATV/MMW/BB lightwave transmission systems based on fiber-wired/fiber-wireless/fiber-VLLC integrations.
The optical output of the dual-arm MZM is fed into the OIL1 to split the optical signal into even and odd sidebands. Following the OIL1 output with even sideband [inset (b) of Fig. 1], the optical signal passes through the OIL2. The OIL1 has an input channel spacing of 50-GHz and an output channel spacing of 100-GHz; whereas the OIL2 has an input channel spacing of 25-GHz and an output channel spacing of 50-GHz. Following the OIL2 output with even sideband [inset (c) of Fig. 1], the optical signal is intensity-modulated with the CATV signal. Furthermore, following the OIL2 output with odd sidebands [inset (d) of Fig. 1], two optical sidebands are separated by a spacing of 50 GHz. These two optical sidebands are intensity-modulated with a 10-Gbps data stream via an intensity modulator to generate a 10 Gbps/50 GHz MMW data signal [inset (e) of Fig. 1]. Meanwhile, following the OIL1 output with odd sidebands [inset (f) of Fig. 1], two optical sidebands are separated by a spacing of 100 GHz. The optical signal is split by a 1 × 2 optical splitter. One optical signal is selected by an optical circulator (OC) combined with an FBG1 (λc = 1539.76 nm), and then intensity-modulated with a 20-Gbps data stream via an intensity modulator. The other optical signal is selected by an OC combined with an FBG2 (λc = 1540.56 nm). These two optical signals are recombined by a 2 × 1 optical combiner to generate a 20 Gbps/100 GHz MMW data signal [inset (g) of Fig. 1]. All optical signals are then combined by a 3 × 1 optical combiner [inset (h) of Fig. 1] and amplified by an erbium-doped fiber amplifier (EDFA). The output power and noise figure of EDFA are 17 dBm and 4.5 dB at an input power of 0 dBm, respectively. The variable optical attenuator (VOA) is positioned after the EDFA, which results in less distortion as the optical power launched into the fiber is less. Over a 40-km SMF transport, the optical signal is fed into the OIL3 to separate the optical signal into even and odd sidebands.
Following the OIL3 output with even sideband [inset (i) of Fig. 1], the optical signal passes through the OIL4. The OIL3 has an input channel spacing of 50-GHz and an output channel spacing of 100-GHz; whereas the OIL4 has an input channel spacing of 25-GHz and an output channel spacing of 50-GHz. Following the OIL4 output with even sideband [inset (j) of Fig. 1], the optical signal is received by a CATV receiver, fed into a push-pull scheme for distortion suppression, and supplied to a spectrum analyzer for CNR, CSO, and CTB performance evaluation. Meanwhile, following the OIL4 output with odd sidebands [inset (k) of Fig. 1], the optical signal is separated by a 1 × 2 optical splitter. One of the outputs is reflected by an FBG3 (λc = 1540.36 nm) to form only one optical sideband, directly detected by a 10-GHz photodiode (PD), boosted by a 10-GHz low noise amplifier (LNA), and fed into a BER tester (BERT) for BER performance evaluation. The other output is detected by a 50-GHz PD, boosted by a 50-GHz power amplifier (PA), and wirelessly transmitted by a 50-GHz horn antenna (HA). Over a 10-m RF wireless transport, the 10 Gbps/50 GHz MMW data signal is received by a 50-GHz HA, boosted by a 50-GHz LNA, and down-converted by an envelope detector (ED) with a frequency range of 0.5 - 10 GHz. After ED detection, the 10-Gbps data stream is filtered by a 10-GHz low-pass filter (LPF) to remove the spurious signal. Eventually, the 10-Gbps data stream is supplied to a BERT for BER performance evaluation. Meanwhile, following the OIL3 output with odd sidebands [inset (l) of Fig. 1], the optical signal is split by a 1 × 3 optical splitter. One of the outputs is reflected by an FBG4 (λc = 1539.76 nm) to form only one optical sideband, directly detected by a 20-GHz PD, boosted by a 20-GHz LNA, and fed into a BERT for BER performance analysis. Another output is detected by a 100-GHz PD, boosted by a 100-GHz PA, and wirelessly transmitted by a 100-GHz HA. Over a 10-m RF wireless transport, the 20 Gbps/100 GHz MMW data signal is received by a 100-GHz HA, boosted by a 100-GHz LNA, and down-converted by an ED with a frequency range of 0.5 - 40 GHz. The antenna gains of 50-GHz and 100-GHz HAs are 20 dB and 25 dB, respectively. After ED detection, the 20 Gbps data stream is filtered by a 20-GHz LPF to remove the spurious signal. Finally, the 20-Gbps data stream is fed into a BERT for BER performance analysis.
The other output is reflected by an FBG5 (λc = 1540.56 nm) to form only one optical sideband, intensity-modulated with a 5-Gbps data stream via an intensity modulator, amplified by an EDFA, attenuated by a VOA, and transmitted by a 40-km SMF link for up-link transmission. Over a 40-km SMF link, the optical signal is detected by a 5-GHz PD, amplified by a 5-GHz LNA, and supplied to the vertical cavity surface emitting laser (VCSEL)-based VLLC subsystem. The VCSEL, with 3-dB modulation bandwidth/wavelength range/color of 5.2 GHz/680 nm/red, is directly modulated by the detected and amplified 5 Gbps data stream. After emitted from the VCSEL, the light is diverged, launched into the first convex lens, transmitted in the free space, and fed into the second convex lens. Over a 10-m free-space link, the visible light is detected by a 6.6-GHz PD with an active area diameter of about 0.04 mm and a responsivity of 0.43 mA/mW (at 680 nm), boosted by a 5-GHz LNA, and fed into a BERT for BER performance evaluation. Here, the optical wireless subsystem is deployed to substitute for the conventional RF wireless subsystem. Therefore, the costly and sophisticated RF devices are not involved in such up-link transmission subsystem.
3. Experimental results and discussions
A schematic diagram of the push-pull scheme is presented in Fig. 2. The outputs of Q1 and Q2 are operated in antiphase. The two antiphase outputs are connected to a load (z) that causes the signal outputs to be added, but distortion due to nonlinearity in the outputs to be subtracted from each other. If the nonlinearity of both outputs is similar, then the distortion will be reduced dramatically. A symmetrical push–pull scheme will cancel the even-order harmonic distortions automatically [10]. The output of push-pull scheme can be written as:
where is the push-pull scheme output, is the push-pull scheme input; , , and are the amplitude coefficients (andare coefficients regarding nonlinearities). A CATV subcarrier transmission system with second-order and third-order nonlinear distortions is given by:where is the output of system detected from the PD, is the input of system, , , and are the amplitude coefficients (andare coefficients regarding nonlinearities). Clearly, is equal to ; substituting Eq. (2) into Eq. (1) leads to the following:Since the amplitude of the nonlinear term decreases with the increasing of the order number, yet the 4th - 15th orders of Pi are neglect to obtain the following equation:Achieving linearity means to delete the nonlinear terms, the following equation sets the suitable nonlinear coefficient to delete the third-order nonlinear term:Subsequently, the Eq. (4) can be derived as follows:As shown by Eq. (6), the third-order nonlinear distortion is deleted by proper moderation of the nonlinear coefficient. A small third-order nonlinear distortion is associated with a high CTB value. The distortion of the directly modulated laser transmitter is mainly limited by CTB. Thereby, the CTB is adjusted to get a higher value.The measured CNR/CSO/CTB values under NTSC channel number (CH2-78), with and without a push-pull scheme, are presented in Fig. 3. The CNR value of system with a push-pull scheme is deteriorated by around 1 dB compared with system without a push-pull scheme. This CNR deterioration is due to the insertion loss of the push-pull scheme. However, the CNR value of system with a push-pull scheme still satisfies the CATV CNR requirement at the optical node (≥50 dB). In contrast, the CSO and CTB values of system with a push-pull scheme are enhanced, particularly for the CTB value. The CSO and CTB values of system with a push-pull scheme are higher than 62 and 63 dB, respectively. They meet the CATV CSO/CTB demands at the optical node (≥60/60 dB). These improvement results can be attributed to the use of the push-pull scheme to delete the nonlinear distortions.
Fig. 3 The measured CNR/CSO/CTB values under NTSC channel number with and without a push-pull scheme.
The measured BER curves of 10 Gbps/50 GHz MMW signal for back-to-back (BTB), over a 40-km SMF transport (only one optical sideband), over a 40-km SMF transport (two optical sidebands), as well as over a 40-km SMF (two optical sidebands) and a 10-m RF wireless transport scenarios are presented in Fig. 4. At a BER of 10−9, there exists a power penalty of 2.9 dB between BTB and 40-km SMF transport (two optical sidebands) scenarios. Such a power penalty could be resulted from the fiber dispersion after 40-km transmission. And at a BER of 10−9, there exists a large power penalty of 4.5 dB between BTB and 40-km SMF (two optical sidebands) as well as 10-m RF wireless transport scenarios. Such a large power penalty could be attributed to the fiber dispersion after 40-km transmission and fading effect after 10-m transmission. Over a 40-km SMF transport, the RF power deterioration induced by fiber dispersion deteriorates the BER performance due to the nature characteristic of two optical sidebands. Over a 10-m RF wireless transport, the fading effect fluctuates the amplitude and phase of the received signal, which deteriorates the BER performance. Nonetheless, the power penalty is reduced to 0.6 dB when only optical sideband scenario is applied. This reduction is due to the cancellation of RF power degradation induced by fiber dispersion. In only one optical sideband scenario, however, the RF power degradation induced by fiber dispersion can be avoided [11,12]. Figure 4 also shows the eye diagrams of the 10 Gbps data channel, obtaining a 10-Gbps data stream from the direct detection of 10-GHz PD (40-km SMF; only one optical sideband) and 10 Gbps/50 GHz MMW data signal (40-km SMF and 10-m wireless), at a BER of 10−9. Clear eye diagrams are received by using LNA to boost the 10-Gbps data stream while increasing as little noise and distortion as possible and by using LPF to remove the spurious signal while decreasing as much noise and distortion as possible.
Fig. 4 The measured BER curves of 10 Gbps/50 GHz MMW signal for BTB, over a 40-km SMF transport, as well as over a 40-km SMF and a 10-m RF wireless transport scenarios.
The measured BER curves of 20 Gbps/100 GHz MMW signal for BTB, over a 40-km SMF transport (only one optical sideband), over a 40-km SMF transport (two optical sidebands), as well as over a 40-km SMF (two optical sidebands) and a 10-m RF wireless transport scenarios are shown in Fig. 5. At a BER of 10−9, there exists a power penalty of 3.8 dB between BTB and 40-km SMF transport (two optical sidebands) scenarios. And at a BER of 10−9, a large power penalty of 5.3 dB is presented between BTB and 40 km SMF (two optical sidebands) as well as 10 m RF wireless transport scenarios. When only optical sideband scenario is applied, nevertheless, the power penalty is reduced to 1.1 dB. In two optical sidebands intensity modulation, if 100-GHz signal fading due to 40 km fiber dispersion happens, power penalty should be changed much more. However, given that the SMF length is only 40 km, the BER performance deterioration due to fiber dispersion is restricted. Figure 5 also shows the eye diagrams of the 20-Gbps data channel, obtaining a 20-Gbps data stream from the direct detection of 20-GHz PD (40-km SMF; only one optical sideband) and 20 Gbps/100 GHz MMW data signal (40-km SMF and 10-m wireless), at a BER of 10−9. The amplitude and phase fluctuations in the signal are somewhat observed wherein obtaining 20-Gbps data stream from the 20 Gbps/100 GHz MMW data signal. This eye diagram deterioration is resulted from the fiber dispersion and fading effects.
Fig. 5 The measured BER curves of 20 Gbps/100 GHz MMW signal for BTB, over a 40-km SMF transport, as well as over a 40-km SMF and a 10-m RF wireless transport scenarios.
For up-link transmission, the measured BER curves of 5-Gbps data stream for BTB, over a 40-km SMF transport (only one optical sideband), as well as over a 40-km SMF (only one optical sideband) and a 10-m free-space transport scenarios are shown in Fig. 6. At a BER of 10−9, a power penalty of 2.8 dB is existed between BTB and 40 km SMF transport scenarios. However, a larger power penalty of 4.2 dB is existed between BTB and 40 km SMF as well as 10 m free-space transport scenarios, due to further transmission over a 10-m free-space link. Further transmission over a 10-m free-space link leads to lower received optical power, and finally results in lower signal-to-noise ratio and worse BER performance. Figure 6 also shows the eye diagrams of the 5-Gbps data channel, obtaining a 5-Gbps data stream from the direct detection of 5-GHz PD (40-km SMF) and 6.6-GHz PD (40-km SMF and 10-m free-space), at a BER of 10−9. Clear eye diagrams are received by employing LNA to amplify the 5-Gbps data stream while adding as little noise and distortion as possible.
Fig. 6 The measured BER curves of 5 Gbps data stream for BTB, over a 40-km SMF transport, as well as over a 40-km SMF and a 10-m free-space transport scenarios.
To show a direct association with the BER performance and the RF/optical wireless distance, we measure and list the power penalties (between BTB and 40-km SMF as well as N-m RF/optical wireless transport scenarios; N ≤ 10) of 10 Gbps/50 GHz MMW signal, 20 Gbps/100 GHz MMW signal, and 5-Gbps data stream at different RF/optical wireless distances (up to 10 m) in Table 1. Table 1 clearly shows that as the wireless distance increases the power penalty value increases as well. Longer RF/optical wireless distance leads to larger fading effect (RF wireless) and lower received optical power (optical wireless), and results in the degradation of BER performance.
Table 1. The power penalties at different RF/optical wireless distances (up to 10 m).
4. Conclusions
A hybrid CATV/MMW/BB lightwave transmission system based on fiber-wired, fiber-wireless, and fiber-VLLC integrations is proposed and experimentally demonstrated. As far as we know, it is the first time to employ fiber-wired, fiber-wireless, and fiber-VLLC convergences in a hybrid CATV/MMW/BB lightwave transmission system. Through a thorough investigation, good performances of CNR, CSO, CTB and BER are achieved over a 40-km SMF and a 10-m RF wireless transport for down-link transmission; as well as over a 40-km SMF and a 10-m free-space transport for up-link transmission. Such a hybrid CATV/MMW/BB lightwave transmission system is attractive not only to demonstrate its enhancement in the integration of fiber backbone and wireless feeder networks, but also to provide the advantage of a communication channel for broader bandwidth and higher transmission rate.
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