A hybrid CATV/16-QAM-OFDM visible laser light communication system

Following the development of fiber-to-the-home (FTTH) technology, wireless broadband in-house networks have attracted much attention to bridge the last connection among distributed fiber and home devices [1]. Relieving wire-line restraint, home users can access services anywhere and anytime they like. Nevertheless, to satisfy the rapid development of information technology, the ability of the wireless connection distance to meet the large-capacity demand is limited. Optical fiber provides an ideal way to distribute optical signals along with home devices to each corner. Single-mode fiber (SMF), in particular, has already established an undisputable position to deliver broadband integrated services to the curb, building or home [2, 3]. However, when SMF is deployed in-house, installation cost and convenience are undisputed bottlenecks that need to be overcome. Optical wireless communication systems, which use light propagating in free-space to transmit high-bandwidth signals and high-speed data, are therefore developed with high expectations to conquer the wireless connection distance issue. Visible laser light communication (VLLC) is an optical wireless communication medium using visible laser light within the wavelength range of 670–685 nm. VLLC has attracted a lot of attention as a promising candidate for in-house wireless communication because it has several advantages over traditional RF-based systems [4–6]. A vertical cavity surface emitting laser (VCSEL), with high modulation bandwidth characteristics, is shown to have a potential advantage in free-space VLLC systems [7]. In this work, a VLLC system employing a VCSEL and spatial light modulator (SLM) with hybrid CATV/16-QAM-OFDM modulating signals over a 5 m (4.5 m + 0.5 m) free-space link is proposed and demonstrated. Hybrid CATV/16-QAM-OFDM lightwave subcarrier transmission systems, including analog and digital signals, are attractive for network access, since they can deliver not only analog CATV signals but also digital 16-QAM-OFDM signals. CATV transport systems are deployed widely to provide broad bandwidth to subscribers. Compared with wireless TV systems, they can increase valued services to satisfy the requirements of subscribers [8]. OFDM is a method of digital modulation in which a signal is split into several narrowband channels on multiple carriers [9]. To adapt the SLM in a hybrid CATV/16-QAM-OFDM VLLC system, the transmitting light can be focused into a point to increase the free-space link. With the assistance of a push-pull scheme [10], low-noise amplifier (LNA) and equalizer at the receiving sites, good performances of composite second-order (CSO) and composite triple beat (CTB) are obtained, accompanied by an acceptable carrier-to-noise ratio (CNR) performance for the CATV signal, and a low bit error rate (BER) value and clear constellation map are achieved for the 16-QAM-OFDM signal. Such a hybrid CATV/16-QAM-OFDM VLLC system would be attractive for providing services including CATV, Internet and telecommunication services.

The configuration of our proposed VLLC systems employing VCSEL and SLM with hybrid CATV/16-QAM-OFDM modulating signals over a 5 m free-space link is shown in figure 1. A total of 77 carriers (CH2-78) generated from a multiple-signal generator (Matrix SX-16) are used to simulate the CATV signal. The 16-QAM-OFDM signal (10 Gbps/5 GHz) is generated offline by a MATLAB program and uploaded into an arbitrary waveform generator (AWG). Such a 16-QAM-OFDM signal is represented by 128 subcarriers, 512 FFT size, 10 G samples per second and 5 GHz intermediate frequency (IF), respectively. The CATV signal and the 16-QAM-OFDM one are combined using a 2  ×  1 RF combiner, with an effective bandwidth (BW) of 0.05–6 GHz. The VCSEL, with 3 dB modulation BW/wavelength range/color of 5.2 GHz/678‒680 nm/red, is directly modulated by the combined CATV/16-QAM-OFDM signals. After being emitted from the VCSEL, the light is diverged, launched into the convex lens, transmitted in the free space, fed into the SLM and focused on the high-BW photodiode (PD). The convex lens is employed to transfer the divergent beam into the parallel beam and the SLM is employed to focus the parallel beam into a point. The distance between the VCSEL and the SLM is 4.5 m and the distance between the SLM and the high-BW PD is 0.5 m. The high-BW PD has the detection wavelength range of 320–1 000 nm, with an active area diameter of around 0.04 mm and a responsivity of 0.43 mA mW⁻¹ (at 680 nm). After PD detection, the detected signal is split by a 1  ×  2 RF splitter, and goes through two separate RF filters. One of the signals is passed through a RF low-pass filter (LPF), with an effective BW of 0.05–1 GHz. Following the RF LPF, the signal is supplied into a push-pull scheme for distortion elimination and fed into an HP-8591 C CATV analyzer for CNR/CSO/CTB performances analysis. The other signal is passed through an RF high-pass filter (HPF), with an effective BW of 2–6 GHz. Following with the RF HPF, the received signal is amplified by an LNA with a low noise figure of around 3.8 dB and passed through an equalizer for data signal equalization. Finally, the 16-QAM-OFDM signal is analyzed by an OFDM analyzer, captured by a communication signal analyzer (CSA), and processed offline with a MATLAB program to evaluate the BER performance and the corresponding constellation map.

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By means of Fresnel lens function, the SLM can be operated as a dynamic convex lens by a GPIB controller [11]. The configuration of employing the SLM as a dynamic convex lens to increase the free-space link and coverage area is presented in figure 2. By employing the SLM as a dynamic convex lens, not only can the free-space link be increased, but also the mobility problem of the line-of-sight (LOS) system can be solved. The LOS system utilizes a narrow laser beam to get a high-speed data rate and a long free-space link. However, no mobility is provided in the LOS system. It means that as blocking occurs, a rapid performance degradation happens in the LOS system. The SLM employed as a dynamic convex lens can be added to the LOS system to solve the mobility problem. The SLM is worth employing due to the success of increasing the free-space link and coverage area.

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A schematic diagram of the push-pull scheme is illustrated in figure 3. Since the even-order harmonic distortions of systems can be eliminated by the push-pull scheme, the push-pull scheme output is given by

$$\begin{eqnarray}&&{{V}_{o}}={{a}_{1}}\cdot {{V}_{i}}+{{a}_{3}}\cdot V_{i}^{3}+{{a}_{5}}\cdot V_{i}^{5},\end{eqnarray} \tag{ 1 }$$

where V_o is the push-pull scheme output, V_i is the push-pull scheme input and a₁,a₃,a₅ are the amplitude coefficients (a₃ and a₅ are coefficients characterize nonlinearities). A VLLC CATV subcarrier transmission system with second-order and third-order non-linear distortions can be expressed as

$$\begin{eqnarray}&&{{P}_{o}}={{c}_{1}}\cdot {{P}_{i}}+{{c}_{2}}\cdot P_{i}^{2}+{{c}_{3}}\cdot P_{i}^{3},\end{eqnarray} \tag{ 2 }$$

where P_o is the system's output detected from PD, P_i is the system's input and c₁,c₂,c₃ are the amplitude coefficients (c₂ and c₃ are coefficients characterize nonlinearities). It is clear that P₀ is equal to V_i, substituting equation (2) into equation (1) and neglecting higher-order non-linear terms then yields

$$\begin{eqnarray}&&{{V}_{o}}=\left({{a}_{1}}\cdot {{c}_{1}}\right){{P}_{i}}+\left({{a}_{1}}\cdot {{c}_{2}}\right)P_{i}^{2}+\left({{a}_{1}}\cdot {{c}_{3}}+{{a}_{3}}\cdot c_{1}^{3}\right)P_{i}^{3}.\end{eqnarray} \tag{ 3 }$$

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Achieving linearity means to cancel out the non-linear terms and a push-pull scheme would have to cancel out the third-order nonlinear term. Setting the appropriate non-linear coefficient to cancel out the third-order nonlinear term:

$$\begin{eqnarray}&&{{a}_{1}}\cdot {{c}_{3}}=- {{a}_{3}}\cdot c_{1}^{3}.\end{eqnarray} \tag{ 4 }$$

Then the equation (3) can be modified as

$$\begin{eqnarray}&&{{V}_{o}}=\left({{a}_{1}}\cdot {{c}_{1}}\right){{P}_{i}}+\left({{a}_{1}}\cdot {{c}_{2}}\right)P_{i}^{2}.\end{eqnarray} \tag{ 5 }$$

It is obvious that from equation (5), third-order non-linear distortion can be eliminated by properly adjusting the nonlinear coefficient. A small third-order non-linear distortion is related to a high CTB value. The distortion performance of the directly modulated laser transmitter is limited by CTB, rather than CSO, yet the CTB is adjusted to obtain a higher value. And further, even-order non-linear distortions are related to CSO performance, yet the existence of only second-order non-linear distortion (fourth-order non-linear distortion is eliminated) leads to improvement of CSO performance.

Figure 4 shows the measured CNR/CSO/CTB values under NTSC channel number (CH2-78) over a 5 m free-space link, with and without a push-pull scheme, respectively. It is clear that the CNR value of a system with a push-pull scheme is degraded about 1 dB, compared with system without a push-pull scheme. Such CNR degradation is due to the insertion loss of the push-pull scheme. Higher CNR value can be acquired from a higher RF output level. However, the CNR value of a system with a push-pull scheme still meets the CATV CNR demand of the subscriber (≥ 43 dB). In contrast, the CSO and CTB values of systems with push-pull schemes are improved, especial CTB values. The CSO and CTB values of systems with push-pull schemes are higher than 53 and 54 dB, respectively. They satisfy the CATV CSO/CTB requirements of subscribers (≥ 53/53 dB). These improvement results can be attributed to the use of push-pull schemes to eliminate non-linear distortions.

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In parallel with verifying CATV performance, the measured BER curves of a 10 Gbps/5 GHz 16-QAM-OFDM data signal are presented in figure 5. At a free-space transmission distance of 5 m without employing an LNA and equalizer: with the CATV on, the BER is about 10⁻²; with the CATV off, the BER is about 10⁻³. At a free-space transmission distance of 5 m employing an LNA and equalizer: with the CATV on, the BER is around 10⁻⁴; with the CATV off, the BER is around 10⁻⁶. Such results mean that the large OFDM subcarrier of 5 GHz (modulated with 10 Gbps 16 QAM) is somewhat impacted by the low-frequency CATV modulation; interference is observed between the CATV signal and the 16-QAM-OFDM data signal. Both the LNA and equalizer play vital roles in error correction. They can further improve the error vector magnitude (EVM) and signal-to-noise ratio (SNR) of systems, leading to significant BER performance improvement. It also can be seen from figure 5 that a clear constellation map is obtained at a BER of 10⁻⁶ (with LNA and equalizer (CATV off)). Error-free transmission is obtained to demonstrate the possibility of establishing such a hybrid CATV/16-QAM-OFDM VLLC system.

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We propose and demonstrate a VLLC system employing VCSEL and SLM with hybrid CATV/16-QAM-OFDM modulating signals over a 5 m free-space link. With the assistance of a push-pull scheme, LNA and equalizer, good performances of CSO and CTB are obtained, accompanied by an acceptable CNR performance for a CATV signal, and a low BER value and clear constellation map are achieved for 16-QAM-OFDM signal. This demonstrates that such an optical free-space hybrid CATV/16-QAM-OFDM signal-transport system is very attractive and can accelerate VLLC deployment.