Zizheng Cao

WDM-RoF-PON Architecture for Flexible Wireless and Wire-Line Layout

We experimentally demonstrate what we believe to be a novel wavelength division multiplexing-passive optical networking (WDM-PON) architecture compatible with 60 GHz radio-over-fiber systems to simultaneously provide wireless or wired services. We place each wireless/wire-line channel in one WDM slot to carry a 2.5 Gb/s orthogonal frequency division multiplexing (OFDM) signal and reuse downstream wavelengths of the wireless and wire-line channels for uplink by an on-off-keying modulation format signal. For the independent design for wireless and wire-line channels, the layouts of wireless or wire-line channels are flexible for different applications.

Direct-Detection Optical OFDM Transmission System Without Frequency Guard Band

Interleaver and turbo code techniques are used in high spectral-efficient direct-detection optical orthogonal frequency-division-multiplexing (OFDM) transmission systems allowing significant improvement of optical receiver sensitivity by mitigating the unbalanced noise distribution in subcarrier signals. The optical signal-to-noise ratio of 5.18 Gb/s, 7.77-Gb/s coded OFDM (5.18 Gb/s after decoding), and 7.77-Gb/s coded OFDM with interleaver (5.18 Gb/s after decoding) over 200-km single-mode fiber at a bit-error rate of 10-4 are 23.7, 18.0, and 17.1 dB, respectively.

Reduction of Intersubcarrier Interference and Frequency-Selective Fading in OFDM-ROF Systems

This paper theoretically investigates the signal degradation induced by intersubcarrier interference and frequency-selective fading in orthogonal frequency-division-multiplexing radio-over-fiber systems. Turbo codes and bit interleaver technologies are proposed to effectively mitigate unbalanced error distribution because of intersubcarrier interference and frequency-selective fading. Experimental results show that the receiver sensitivities are improved as far as 5 dB after 50-km SMF-28 transmission for 6.25-Gb/s wireless signals on optical generated 60-GHz carrier.

Direct-Detection Optical OFDM Transmission System With Pre-Emphasis Technique

We investigated the effect of subcarriers-to-subcarriers mixing interference and frequency fading in direct-detection (DD) optical orthogonal frequency-division multiplexing (OFDM) system. We also proposed and experimentally demonstrated a novel DD OFDM transmission scheme with pre-emphasis technique. By pre-emphasizing the power of the OFDM subcarriers at the high frequency, the received sensitivity at our experiment was improved about 2 dB at a bit error rate of 1 × 10-4 after 100-km SMF-28 transmission compared to the regular one without this technique.

Fiber-Wireless Transmission System of PDM-MIMO-OFDM at 100 GHz Frequency

We experimentally demonstrated a seamlessly integrated fiber-wireless system that delivers 30.67-Gb/s polarization division multiplexing-multiple input multiple output-orthogonal frequency division multiplexing (PDM-MIMO-OFDM) signal through 40-km fiber and 5-m wireless transmission over free-space at 100 GHz adopting heterodyne coherent detection. De-multiplexing is realized by channel estimation based on a pair of time-interleaved training sequences (TSs). The bit-error ratio (BER) for the 30.67-Gb/s PDM-MIMO-OFDM signal is less than the pre-forward-error-correction (pre-FEC) threshold of 3.8 × 10-3 when the optical signal-to-noise ratio (OSNR) is larger than 19.3 dB after both 40-km single-mode fiber-28 (SMF-28) transmission and 5-m wireless delivery at 100 GHz. We also find that the BER performance is sensitive to the multipath effects induced by the transmission distance difference in the wireless link.

Reversely Modulated Optical Single Sideband Scheme and Its Application in a 60-GHz Full Duplex ROF System

The reversely modulated optical single sideband scheme (IM-OSSB) based on a parallel Mach-Zehnder modulator (P-MZM) is proposed. In this P-MZM, one sub-MZM is employed for data modulation and the other is used for optical millimeter wave (mm-wave) generation. Due to the individual modulation, this scheme is data-format-transparent and can be used to generate a high performance optical mm-wave signal at the optimized direct current (dc) bias. There is electrical mixer free at the transmitter and thus the bandwidth limitation, nonlinearity and conversion loss from the electrical mixer are released in this scheme. Moreover, the modulation power efficiency of IM-OSSB can be improved by adjusting the dc bias in the modulator. Based on IM-OSSB, we demonstrate a 60-GHz full duplex radio-over-fiber system. The experimental results show that the power penalty of 2.9-Gb/s on-off keying data carried by 58-GHz mm-wave after transmission over 50-km SMF-28 is negligible.

Hadamard transform combined with companding transform technique for PAPR reduction in an optical direct-detection OFDM system

In this paper, we propose a novel method to reduce the high peak-to-average power ratio (PAPR) of the optical orthogonal frequency division multiplexing (OFDM) signal. The method is based on the technique of the Hadamard transform combined with a companding transform. We experimentally demonstrate that, in an optical intensity-modulation direct-detection OFDM transmission system, the hybrid method has better performance in reducing the PAPR compared with the case of applying only the Hadamard transform or only a companding transform.

SSBI mitigation at 60GHz OFDM-ROF system based on optimization of training sequence

We have theoretically and experimentally investigated the effect of the interference between subcarrier-signal beat interference (SSBI) in 60 GHz orthogonal frequency division multiplexing - radio-over-fiber (OFDM-ROF) system. In order to reduce the influence of SSBI, we compared four kinds of OFDM frames with different training sequences as all-real, all-complex, complex-zero and real-zero training. The experimental results show the power penalty of all-real, all-complex, complex-zero and real-zero training is 2.5, 5.5, 4 and 1dB at BER of 1x10(-3) after 20 km standard single mode fiber (SMF) transmission, respectively. The real-zero training OFDM frame with interleave structure and lower modulation order signal suffered from the least SSBI shows the best performance.

Preliminary results of ELMy H-mode experiments on the HL-2A tokamak

Typical ELMy H-mode discharges have been achieved on the HL-2A tokamak with combined auxiliary heating of NBI and ECRH. The minimum power required is about 1.1 MW at a density of 1.6 × 1019 m−3 and increases with a decrease in density, almost independent of the launching order of the ECRH and NBI heating. The energy loss by each edge localized mode (ELM) burst is estimated to be lower than 3% of the total stored energy. At a frequency of typically 400 Hz, the energy confinement time is only marginally reduced by the ELMs. The supersonic molecular beam injection fuelling is found to be beneficial for triggering an L–H transition due to less induced recycling and higher fuelling efficiency. The dwell time of the L–H transition is 20–200 ms, and tends to decrease as the power increases. The delay time of the H–L transition is 10–30 ms for most discharges and is comparable to the energy confinement time. The ELMs with a period of 1–3 ms are sustained for more than ten times the energy confinement time with enhanced confinement factor H89 > 1.5, which tends to decrease with the total heating power. The confinement time in the H-mode discharges increases with plasma current approximately linearly.

Reducing the peak-to-average power ratio with companding transform coding in 60 GHz OFDM-ROF systems

To reduce the nonlinear distortion in electrical devices, optical modulators, and transmission fiber, we adopt a companding transform coding technique to reduce the peak-to-average power ratio of the orthogonal frequency division multiplexing (OFDM) signal in an OFDM radio-over-fiber system. By adopting this technique, our experimental results show that the receiver sensitivity at a bit error ratio of 1 × 10-3 for a 2.98 Gb/s OFDM signal carried on a 60 GHz optical millimeter-wave after 50 km of standard single-mode fiber transmission is improved by about 0.6, 2.2, and 3.8 dB at launch powers of 0, 6, and 12 dBm, respectively.