Browsing by Author "Yu, Min"

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APD-based Delay-Division-Multiplexing OFDM-PONs
(2018-01-01)
Hsu, Jhih-Hao
;
Yu, Min
;
Chia-Chien
;
Lin, Chi-Hsiang
;
Lin, Chun-Ting
Based on pre-allocated relative sampling delays among optical network users (ONUs) in an orthogonal frequency division multiplexing (OFDM) passive optical network (PON), the delay-division-multiplexing (DDM) technique employs signal pre-processing to enable demand data to be received by sub-Nyquist analog-to-digital sampling. This work employed an avalanche photodiode (APD) in a DDM-OFDM-PON to achieve higher loss budget, compared to a p-i-n photodiode (PIN). To successfully enable the pre-processing, however, the knowledge of channel response is required; thus, it is critical to estimate complete channel response in DDM-OFDM systems under the constraint of sub-Nyquist sampling. To avoid spectral overlapping in channel estimation, the subcarriers of training symbols in different frequency zones are allocated to different time slots. Unfortunately, this scheme may sustain inaccurate channel estimation due to different degrees of APD saturation for training subcarriers at different frequency zones, leading to significant penalty in performance. Thus, careful control of electrical driving power and optical received power for the training symbols will be important to reduce the impact of saturation on channel estimation, thereby improving transmission performance. We experimentally examined the impact of the accuracy of channel estimation in a 25-Gbps APD-based OFDM-PON. Compared to a PIN, we successfully demonstrated the DDM scheme based on an APD and 1/32 of Nyquist rate with 5-dB improvement in sensitivity after 25-km fiber transmission.
Employing Hybrid sub-Nyquist Sampling Rates to Support Heterogeneous Services of Varying Capacity in 25-Gbps DDM-OFDM-PON
(2018-04-01)
Hsu, Jhih-Hao
;
Yu, Min
;
Wei, Chia-Chien
;
Lin, Chi-Hsiang
;
Lin, Chun-Ting
Delay-division-multiplexing (DDM) involves the use of signal preprocessing for the detection of data via sub-Nyquist analog-to-digital sampling, based on preallocated relative sampling delays among optical network users (ONUs) in an orthogonal frequency-division multiplexing (OFDM) passive optical network (PON). If a DDM-PON consists of M virtual groups, the sampling rate at the receivers is only 1/M of the Nyquist rate, but the maximum capacity of each ONU is fixed at 1/M of the total capacity. In this study, we developed a novel DDM scheme that uses hybrid sub-Nyquist sampling rates to accommodate heterogeneous services of different capacities in an OFDM-PON. To equalize the transmission performances using hybrid sub-Nyquist sampling rates, the modification of preprocessing was proposed, and its necessity was demonstrated. Without loss of generality, we experimentally demonstrated a 25-Gbps DDM-OFDM-PON using hybrid sampling at 1/2, 1/8, and 1/32 (or 1/8, 1/16, and 1/32) of the Nyquist rate. Compared to a DDM-PON with a fixed sub-Nyquist rate, the received signals based on hybrid sub-Nyquist sampling rates show similar sensitivities. A loss budget of 26 dB was experimentally achieved using all sub-Nyquist sampling rates after employing the modified preprocessing.
Material and optical properties of trenched epitaxial lateral overgrowth of a-plane GaN
(2007)
Wang, Te-Chung
;
Lu, Tien-Chang
;
Ko, Tsung-Shine
;
Kuo, Hao-Chung
;
Chen, Hou-Guang
The crystal quality of a-plane GaN film was successfully improved by using trenched epitaxial lateral overgrowth (TELOG) of a-plane GaN. Not only the threading dislocation density but also the difference of anisotropic in-plane strain between orthogonal crystal axes can be mitigated by using TELOG. The low threading dislocation density investigated by transmission electron microscopy was estimated to be 3 x 10(7) cm(-2) on the N-face GaN wing. According the results of mu-PL and CL, the threading dislocations are the strongly non-radiative center in a-plane GaN film. Finally, we concluded that a narrower stripped GaN seeds and deeper stripped trenches etched into the surface of sapphire could derive a better quality a-plane GaN film.
Novel DDM-OFDM-PON with Hybrid Sub-Nyquist Sampling Rates Featuring Heterogeneous ONUs with Different Capacities
(2018-01-01)
Hsu, Jhih-Hao
;
Yu, Min
;
Wei, Chia-Chien
;
Lin, Chi-Hsiang
;
Lin, Chun-Ting
We propose a delay-division-multiplexing-OFDM-PON (DDM-OFDM-PON) with hybrid sub-Nyquist sampling rates, wherein heterogeneous ONUs can request different capacities. 26-dB loss budget in 25-km 25-Gbps DDM-OFDM-PON was achieved using hybrid sampling at 1/32 / 1/16 / 1/8 or 1/32 / 1/8 / 1/2 Nyquist rates.
Novel Time Synchronization Scheme in Delay-Division-Multiplexing OFDM-PON Using Sub-Nyquist Sampling
(2019-01-01)
Yu, Min
;
Liu, Fumin
;
Chen, Wei-Lun
;
Lin, Chun-Ting
;
Zhou, Lei
We propose a novel time synchronization scheme in a DDM-OFDM-PON under the constraint of sub-Nyquist sampling. In a 25-Gb/s DDM-PON, the scheme can acquire precise delays, making the delay-related penalty in SNR <0.2 dB.
On channel estimation schemes for APD-based DDM-OFDM-PONs under sub-Nyquist sampling
(2018-09-03)
Hsu, Jhih-Hao
;
Yu, Min
;
Liu, Fumin
;
Lin, Chi-Hsiang
;
Lin, Chun-Ting
Implementing preprocessing in a delay-division multiplexing (DDM) orthogonal frequency-division multiplexing (OFDM) passive optical network (PON) requires a priori knowledge of channel responses, which need to be estimated under the constraint of sub-Nyquist analog-to-digital sampling. The localized approach allocates subcarriers in different frequency zones to training symbols in different time slots for channel estimation without spectral overlap. Unfortunately, the localized scheme is susceptible to inaccurate estimation when using an avalanche photodiode (APD), due to variations in APD saturation associated with different training symbols. Instead of localizing all subcarriers of a training symbol in a single frequency zone, we propose distributing training subcarriers through various frequency zones. This distributed scheme would prevent spectral overlap and also reduce the degree of variation in APD saturation, thereby improving the accuracy of channel estimation. Alternatively, we propose an orthogonal scheme in which each training symbol uses all of the subcarriers simultaneously. The orthogonality specified among consecutive training symbols should make it possible to estimate the channel response with low computational complexity. We conducted experiments to compare various schemes used for channel estimation in a 25-Gbps APD-based OFDM-PON. Our results revealed that the orthogonal scheme achieved the best results, and the localized scheme provided the worst channel estimates. We demonstrate the application of the orthogonal scheme in a penalty-free DDM system at 1/32 of the Nyquist rate, which provided a loss budget of 28 dB after fiber transmission over a distance of 25 km. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Trenched epitaxial lateral overgrowth of fast coalesced a-plane GaN with low dislocation density
(2006-12-18)
Wang, Te-Chung
;
Lu, Tien-Chang
;
Ko, Tsung-Shine
;
Kuo, Hao-Chung
;
Yu, Min
The crystal quality of a-plane GaN films was improved by using epitaxial lateral overgrowth on trenched a-plane GaN buffer layers. Not only the threading dislocation density but also the difference of anisotropic in-plane strain between orthogonal crystal axes can be mitigated by using trenched epitaxial lateral overgrowth (TELOG). The low threading dislocation density investigated by the cross-sectional transmission electron microscopy was estimated to be 3x10(7) cm(-2) on the N-face GaN wing. On the other hand, the Ga-face GaN wing with a faster lateral overgrowth rate could be influenced by the thin GaN layer grown on the bottom of the trenches, resulting in higher dislocation density generated. As a result, the authors concluded that a narrower stripped GaN seeds and deeper stripped trenches etched into the surface of sapphire could derive a better quality a-plane GaN film. Finally, they demonstrated the fast coalescence process of TELOG GaN films below 10 mu m thick. (c) 2006 American Institute of Physics.