We investigate the transmission performance of 28-Gb/s nonreturn-to-zero on-off keying (NRZ-OOK), duobinary, and four-level pulse amplitude modulation signals over different fiber lengths without any inline amplification or pre/ post-processing digital signal processing technique by utilizing an O-band 18G-class directly modulated laser (DML). By experiment, it is verified that negligible sensitivity penalty is introduced by DML chirp effect after fiber transmission, which is due to the extremely low dispersion of O-band wavelengths. Meanwhile, the results indicate that NRZ-OOK outperforms the other two formats in receiver sensitivity, and for 20-km transmission, a power budget of 25.5 dB is achieved without pre-amplification. According to the experimental results, O-band DML-based NRZ-OOK transmission is a cost-effective solution for future 100G-EPON.

This paper firstly presents the channel modeling for full duplex (FDX) modulating retro-reflector (MRR) free-space optical (FSO) systems under weak-to-strong turbulence conditions. The uplink channel in the MRR FSO communication is a conventional FSO channel that is typically modeled by a Gamma-Gamma (ΓΓ) distribution. The downlink channel is jointly affected by the turbulence-induced fading in two opposite passes as well as the reflection effect of the MRR, and its normalized channel fading can be described as the product of two correlated ΓΓ random variables (RVs). The channel model of the downlink is approximated by a single α-μ distribution via moment matching method. Then, we propose an effective signal modulation and detection scheme to achieve the FDX communication. The bit error rates (BERs) of the MRR FSO systems are calculated by the analytical formulas that are derived based on the channel models of the two links, and also evaluated by the Monte-Carlo simulations to validate the derived formulas. The effect of system configurations and propagation conditions on the system performance is investigated, and the optimal designs to make a trade-off between the uplink and downlink performance are discussed.

Filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) is a promising candidate for 5G mobile fronthaul system. Due to the inherent imaginary interference of FBMC/OQAM signals, an accurate channel estimation is particularly indispensable. In this paper, an efficient method is proposed based on complex-valued neural networks (CVNN) for an accurate channel estimation of FBMC/OQAM signals with low computational complexity and pilot overhead. Here, the channel frequency response (CFR) is first calculated for training the CVNN. Next, the CFR estimated from the extracted pilots is exploited as the input of the trained CVNN to yield the CFR of data symbols. We experimentally demonstrate a 12.5-GBd intensity modulation direct detection (IM/DD) FBMC/OQAM transmission system over 30-km and 50-km standard single mode fibers (SSMF). The experimental results show that the proposed method achieves 3-dB and 1-dB receiver sensitivity improvement at the bit error ratio (BER) of 3.8×10 ⁻³ with only 5% pilot overhead respectively, compared to the conventional least square (LS) and linear minimum mean error (LMMSE) methods. When the pilot overhead decreases from 10% to 1%, its BER performance is always better than LS and LMMSE. For the computational complexity, its complexity is the same order of magnitude as LS and lower than LMMSE. On the other hand, the CVNN requires less hidden neurons to keep the similar BER performance as real-valued neural networks (RVNN). Meanwhile, it has excellent resilience to fiber chromatic dispersion over 30-km and 50-km SSMF.

In this paper, we propose a dynamic key technique based on Cellular Neural Network (CNN) for security improvement in the orthogonal frequency division multiplexing passive optical network (OFDM-PON). To enhance the encryption scheme security, a six-dimensional CNN hyperchaotic system is employed to encrypt the data. And, the keys are divided into the dynamic and static. The dynamic key is randomly extracted from a key set by incorporating the random feature of the input data. Then, the chaotic sequence generated by the dynamic key is served as the synchronous sequence for encryption. Moreover, the chaotic sequences generated by the static keys are used to resist the chosen-plaintext attacks (CPAs) and scramble the phase of QAM symbols on the frequency domain. With these processing techniques, the multi-fold data encryption can create a key space of ~10 ³¹⁵ to protect against the exhaustive trial. The transmission of 10-Gb/s encrypted 16-QAM-based OFDM signal is demonstrated over 20-km single mode fiber (SMF) by experiment. The results show that our proposed scheme can provide excellent confidentiality of data transmission against the CPAs and brute-force attack.

A bandwidth-efficient and low-cost spectral-null-filling scheme simultaneously delivering a 56-Gbps 4-level pulse amplitude modulation (PAM-4) digital signal and a 10×400-MHz analog radio frequency (RF) signal on a single wavelength is demonstrated for high-capacity wired and wireless optical access networks. The data rate of the PAM-4 signal is consistent with the next-generation passive optical network (PON), and the PAM-4 signal inherently provides a spectral null at 28 GHz, which can be filled with the RF signal exactly at the 5G-specified millimeter wave (mmW) band. Since pulse shaping is not essential in this hybrid transmission system, a low-cost 2-bit digital-to-analog converter (DAC) is adopted for the PAM-4 signal generation. Volterra nonlinear equalizer is adopted to effectively eliminate the analog-to-digital crosstalk without reserving a frequency gap in between. The digital-analog hybrid signal is simultaneously transmitted over a 25-km standard single-mode fiber in telecom C-band and the impact of chromatic dispersion on the number of supported mmW bands is analyzed. In addition, three types of hybrid transmitters are experimentally demonstrated, among which the first-time proposed IQ-modulator-based hybrid transmitter shows the best performance.