Qianwu Zhang

Cost-effective sampling clock synchronisation scheme with its implementation in optical orthogonal frequency division multiplexing systems

A cost-effective scheme for sampling clock synchronisation based on the interpolation algorithm is proposed and implemented via field programmable gate array (FPGA). This scheme can provide a simpler and more economical achievement than the schemes that utilise expensive external clock recovery circuits. Low system complexity is another advantage of the proposed scheme due to the utilisation of the results of the frame synchronisation. Besides, a novel fractional sampling interval calculating algorithm which aims to calculate the fractional sampling time offset in a sampling interval is also proposed and this simpler operation algorithm based on least squares method is proved to fully satisfy the multi-subcarrier characteristic of orthogonal frequency division multiplexing (OFDM) symbol. The design and experimental results of a 2.5 Gb/s FPGA-based optical OFDM receiver by using the scheme that was proposed earlier are presented here. A major improvement in this scheme for sampling clock synchronisation is demonstrated to be suitable for practical application.

Low-complexity joint symbol synchronization and sampling frequency offset estimation scheme for optical IMDD OFDM systems.

A low-complexity joint symbol synchronization and SFO estimation scheme for asynchronous optical IMDD OFDM systems based on only one training symbol is proposed. Numerical simulations and experimental demonstrations are also under taken to evaluate the performance of the mentioned scheme. The experimental results show that robust and precise symbol synchronization and the SFO estimation can be achieved simultaneously at received optical power as low as -20dBm in asynchronous OOFDM systems. SFO estimation accuracy in MSE can be lower than 1 × 10-11 under SFO range from -60ppm to 60ppm after 25km SSMF transmission. Optimal System performance can be maintained until cumulate number of employed frames for calculation is less than 50 under above-mentioned conditions. Meanwhile, the proposed joint scheme has a low level of operation complexity comparing with existing methods, when the symbol synchronization and SFO estimation are considered together. Above-mentioned results can give an important reference in practical system designs.

An Effective Sampling Frequency Offset Compensation Method for OFDMA-PON

A sampling frequency offset compensation method based on frequency domain correlation of long training symbols for OFDMA-PON is experimentally demonstrated which shows 100ppm of sampling frequency offset compensation capability under ROP of -4dBm.

A Symbol Synchronization Method Based on Gold Sequences for Real-time Upstream OFDMA-PON

A symbol synchronization method based on single carrier modulated Gold sequences for upstream OFDMA-PON is demonstrated which shows that precise synchronization can be realized and hardware logic resources requirements can be effectively released.

Dual-drive Mach–Zehnder modulator-based reconfigurable and transparent spectral conversion for dense wavelength division multiplexing transmissions

Abstract. A digital signal process enabled dual-drive Mach–Zehnder modulator (DD-MZM)-based spectral converter is proposed and extensively investigated to realize dynamically reconfigurable and high transparent spectral conversion. As another important innovation point of the paper, to optimize the converter performance, the optimum operation conditions of the proposed converter are deduced, statistically simulated, and experimentally verified. The optimum conditions supported-converter performances are verified by detail numerical simulations and experiments in intensity-modulation and direct-detection-based network in terms of frequency detuning range-dependent conversion efficiency, strict operation transparency for user signal characteristics, impact of parasitic components on the conversion performance, as well as the converted component waveform are almost nondistortion. It is also found that the converter has the high robustness to the input signal power, optical signal-to-noise ratio variations, extinction ratio, and driving signal frequency.

A Single Pilot Subcarrier-Based Sampling Frequency Offset Estimation and Compensation Algorithm for Optical IMDD OFDM Systems

A sampling frequency offset (SFO) estimation and compensation scheme for asynchronous optical intensity-modulation and direct-detection orthogonal frequency division multiplexing (OFDM) systems by using one single pilot subcarrier is proposed. Experimental demonstrations are also under taken to evaluate the performance of the mentioned scheme, and results show that precise SFO estimation with an accuracy of <; ±0.2 ppm can be achieved under the SFO range from -1000 to 1000 ppm after 25 km standard single mode fiber (SSMF) transmission. Meanwhile, an SFO compensation method by extending the channel response with the phase shift correction term is also presented; more than 1000 and 400 ppm SFO effects can be compensated by using the phase shift correction term and its approximation, respectively.

Retrieving quasi-phase-matching structure with discrete layer-peeling method

An approach to reconstruct a quasi-phase-matching grating by using a discrete layer-peeling algorithm is presented. Experimentally measured output spectra of Šolc-type filters, based on uniform and chirped QPM structures, are used in the discrete layer-peeling algorithm. The reconstructed QPM structures are in agreement with the exact structures used in the experiment and the method is verified to be accurate and efficient in quality inspection on quasi-phase-matching grating.

Interconnecting data based on vortex beams by adjusting the ellipticity of a ring-core fiber

In order to exchange data in a space-division multiplexing (SDM) system, a novel vortex-beam-based data interconnection concept, which is achieved by adjusting the ellipticity of a ring-core fiber, is proposed. A new ring-core fiber is also designed and fabricated for exchanging and propagating the data carried by first- or second-order vortex (orbital angular momentum) beams. The proposed scheme is not only analyzed and simulated in principle, but is also verified through experiments. The numerical results demonstrate that the vortex beams can be exchanged by appropriately adjusting the phase difference (with respect to the ellipticity of a ring-core fiber) between the even and odd vector modes. A new experimental platform is designed and established for the sake of investigating the feasibility of the proposed scheme. The experimental results are consistent with the results of the simulation, and demonstrate that the data carried by the first- or second-order vortex beams can be successfully switched with acceptable bit error rates (BERs) between the first-order vortex beams (L=1 or -1) or between the second-order vortex beams (L=2 or -2, left or right circular polarization), respectively. The measured BERs and constellation diagrams of 16-QAM are employed to evaluate the data exchange performance with respect to different cases (i.e., data exchange once or twice, and data exchange with or without crosstalk). The measured BERs and constellation diagrams also demonstrate that the performance degrades with increase in topological charge or crosstalk. The proposed scheme is flexible, simple, and reliable for data exchange in a SDM system.

Projection histogram assisted common phase estimation algorithm in coherent optical OFDM system

We propose a novel common phase error estimation method based on projection histogram (PH). Experimental results show that the proposed method can effectively compensate the common phase error. Compared to BPS, similar performances are obtained with much lower complexity.

Investigation on adaptive equalization techniques for 10G-glass optics based 100G-PON system

Requirements on adaptive equalization of 25Gb/s NRZ transmission system for 10G-class optics based NG-EPON are analysed with FFE, DFE and MLSE. Experimental results show that combination of FFE and DFE can achieve similar performance in B2B case and about 1.2 dB decline for 25-km transmission case compared to MLSE.