In this paper, we proposed a system that realized simultaneously visible light positioning (VLP) and visible light communications (VLC) in the same band by using orthogonal frequency division multiple access (OFDMA). This system used the power of data sequence as the metric to estimate the transmission distances and consequently the receiver’s position, without deteriorating the VLC’s throughput. We theoretically analyzed the positioning errors in both overdetermined and determined VLP systems. In overdetermined VLP system, optimal selection of master light-emitting diode (LED) avoided the fluctuation of positioning error. Excluding the LED with longest transmission distance changed the overdetermined VLP system to determined VLP system, and reduced the positioning error. The power allocation scheme reduced further the positioning error in determined VLP system. This method was fast and effective. The well-matched theoretical and Monte-Carlo (MC) simulation results validated our proposed VLP based on VLC system using OFDMA.

In this paper, we investigate the physical layer security (PLS) for a multiple-input single-output (MISO) non-orthogonal multiple access (NOMA) visible light communication (VLC) system, whose information is intercepted by collusive eavesdroppers. For the case of known eavesdroppers' locations, a precoding scheme is proposed to maximize the secrecy capacity of the system by suppressing the rate of collusive eavesdroppers. Then we extend the scheme to the case of unknown eavesdroppers' locations. Specifically, the artificial noise is introduced in the precoding scheme to further suppress the average rate of eavesdroppers. To solve the non-convex optimization problem, we propose an iterative algorithm based on concave-convex procedure. Simulation results illustrate the outperformance of the proposed schemes in guaranteeing the PLS for the MISO NOMA VLC system under the interception of collusive eavesdroppers.

Flat optical frequency comb (OFC) can be generated by using single phase modulator (PM), where combined harmonics are applied to drive the PM. However, the number of comb lines and flatness of OFC are limited under this scheme. In this work, we use cascaded PMs with combined harmonics to generate flat OFC. The optimized phases and modulation indices in terms of amplitudes for each harmonic are obtained by solving optimization problem to minimize the fluctuation of OFC with a certain number of comb lines. Both simulation and experiment are carried out to evaluate the feasibility of the proposed scheme. Regarding the case of combined fundamental tone, third harmonic, and fifth harmonic, a 15-line OFC is generated with the flatness of 0.29 dB and 0.65 dB observed in simulation and experiment, respectively. Other feasible solutions to flat OFC with combined harmonics are also investigated.

We propose an absolute distance measurement method using alternately oscillating optoelectronic oscillators (OEOs) with high speed, high precision, and long range, and describe the dynamic characteristics of the measurement system. Measurement and reference OEOs are oscillated using a 2×2 optical switch, and rough and fine measurements are achieved by low- and high-order-mode oscillation. The distance is determined by the loop length difference between the two OEOs. OEO length control is not necessary, so the system is simple and the time per measurement is only 40 ms. The maximum measurement error is 3.4 µm with an emulated distance of 7.5 km, and the relative measurement accuracy reaches 4.5×10-10.

Non-orthogonal multiple access (NOMA) can increase the overall spectral efficiency of visible light communication (VLC) system. In this paper, we proposed a novel scheme to improve the bit error rate (BER) performance of a two-user NOMA VLC system by adjusting superposition constellation. The corresponding closed-form BER expressions are derived. Convex optimization is used to find the optimal parameters of the adjusted superposition constellation, where the overall BER is minimized. The BER performances are evaluated by theoretical analysis, Monte-Carlo (MC) simulation and experiment. The results illustrate that 8.9-dB and 8.1-dB signal-to-noise ratios (SNRs) are reduced to achieve BER of 10⁻³ for the two users, whose power allocation coefficients are 0.6 and 0.4, respectively.

Under the conventional scheme to generate an optical frequency comb (OFC) using an electro-optic modulator (EOM), the frequency interval of the OFC is determined via the frequency of the fundamental tone of the radio frequency (RF) driving signals. In this work, we use two harmonics without the fundamental tone to drive two EOMs, where the frequency interval of the generated flat OFC is the frequency of the fundamental tone. The orders of the two harmonics are coprime. Specifically, one harmonic drives the first branch of the dual-parallel Mach–Zehnder modulator (DPMZM) only, and the other harmonic drives the phase modulator (PM). The flatness of the OFC is achieved by adjusting the amplitude and phase of the RF driving harmonics as well as the bias of the EOM. Both a simulation and an experiment were carried out to verify the effectiveness of the proposed scheme. When the second harmonic drives the DPMZM and the third harmonic drives the PM, an 11-comb line OFC is generated, where the flatness of the OFC was 0.63 dB and 0.65 dB under the simulation and experiment, respectively. When the third harmonic drives the DPMZM and the second harmonic drives the PM, a 13-comb line OFC is generated, where the flatness of the OFC was 0.62 dB and 0.95 dB under the simulation and experiment, respectively. We also investigate the performance of the generated OFC when one harmonic drives two branches of the DPMZM and the other harmonic drives the PM. The comparison of the OFCs’ performance demonstrates the effectiveness of the proposed scheme.