Browsing by Author "Lai, Wei-Chi"

Now showing 1 - 9 of 9

A 62.8 mW 4x4 MIMO-OFDM Modem with One-Symbol-Locked Timing Recovery, Frequency-Dependent I/Q Mismatch Estimation and Adaptive Equalization
(2009)
Sun, Ming-Fu
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Lin, You-Hsien
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Lai, Wei-Chi
;
Juan, Ta-Yang
;
Lee, Cheng-Yuan
A 4x4 multi-input multi-output (MIMO) orthogonal frequency-division multiplexing (OFDM) modem with one-symbol-locked timing recovery, anti-I/Q mismatch frequency recovery, frequency-dependent I/Q mismatch estimation and adaptive equalization is implemented in 0.13-mu m CMOS library. This chip occupies 4.6x4.6 mm(2) and consumes 62.8 mW at 1.2 V.
A Cost-Effective Preamble-Assisted Engine With Skew Calibrator for Frequency-Dependent I/Q Imbalance in 4 x 4 MIMO-OFDM Modem
(2013-08-01)
Lai, Wei-Chi
;
Liao, Yuan-Te
;
Hsu, Terng-Yin
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資訊工程學系
;
Department of Computer Science
Variations in I/Q gains, phases, and filters of the RF frontend, namely frequency-dependent I/Q imbalance (FDI), are an important factor in OFDM-based wireless access. To enable the proper function of a 4 x 4 MIMO-OFDM receiver this work proposes a low-complexity preamble-assisted solution using only one complex divider and one complex multiplier to handle significant FDI distortions. An all-digital multiphase and multi-rate clock generator (MPRCG) was built to support fast dynamic frequency scaling for FDI estimation and compensation and for efficient implementation. Based on the proposed MPRCG, a skew calibration was also realized to tune I/Q timing coherently via multiphase A/D clocking. Performance evaluation showed that the proposed approach incurs an SNR loss of 1.5 dB to maintain a packet-error rate of less than 10% under a 1 dB gain error, 15 degrees phase error and worse filter mismatch. Thus, this solution not only provides adequate performance, but also makes FDI estimation and compensation more cost-effective.
Cost-Efficient Frequency-Domain MIMO-OFDM Modem With an SIMD ALU-Based Architecture
(2015-12-01)
Yeh, Shao-Ying
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Liao, Yuan-Te
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Lai, Wei-Chi
;
Hsu, Terng-Yin
;
資訊工程學系
In this paper, a single instruction multiple data (SIMD) arithmetic logic unit (ALU)-based architecture is proposed to improve hardware efficiency in a 4 x 4 frequency-domain multiple input multiple output-orthogonal frequency division multiplexing modem based on a space-time block code (STBC). The majority of mathematic units in the proposed architecture are centralized so that any mathematic unit can be shared with any algorithm. Six advanced instructions are also defined in the ALU: 1) complex multiplication; 2) complex division; 3) correlation; 4) channel estimation; 5) 4 x 4 matrix inversion; and 6) STBC-based decoding. A scheduler is essential to handle all data paths and signaling flows smoothly with the use of an SIMD ALU. As a result, it is relatively easy to reconfigure the proposed design for different specifications. The very large scale integration implementation of this chip, using an in-house 65-nm CMOS process, consumes a total of 1.87 M gates and draws 33.7 mW at a supply voltage of 1 V.
Cross-Validation Estimation for Frequency-Dependent I/Q Imbalance in MIMO-OFDM Receivers
(2010-07-01)
Lai, Wei-Chi
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Juan, Ta-Yang
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Hsu, Terng-Yin
;
Chiou, Sheng-Lun
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資訊工程學系
The effects of variation in RF, such as I/Q imbalance and filter mismatch, are extremely important for OFDM wireless accesses. This work presents a low-computational estimation of I/Q imbalances with filter mismatches to improve performance in MIMO-OFDM receivers. For N x N MIMO-OFDM systems, the proposed cross-validation estimation is such that, only N + 1 preambles are required to extract the mismatches of filters, gains and phases. With the estimated parameters, frequency-domain filters are exploited to correct frequency-dependent I/Q imbalances. Through performance evaluation of a 2 x 2 MIMO-OFDM system, with ideal channel estimations this study incurs a SNR loss of 1-1.2 dB to maintain a 10% PER at 1-dB gain error, 10A degrees-phase error and the worst 180A degrees-filter mismatch. In addition, this algorithm is well-matched to IEEE 802.11n and new specifications discussed in IEEE 802.11 VHT study group.
Efficiency enhancement of InGaN/GaN multiple quantum well solar cells using CdS quantum dots and distributed Bragg reflectors
(2013)
Tsai, Yu-Lin
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Lin, Chien-Chung
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Han, Hau-Vei
;
Chen, Hsin-Chu
;
Chen, Kuo-Ju
In recent year, InGaN-based alloy was also considered for photovoltaic devices owing to the distinctive material properties which are benefit photovoltaic performance. However, the Indium tin oxide (ITO) layer on top, which plays a role of transparent conductive oxide (TCO), can absorb UV photons without generating photocurrent. Also, the thin absorber layer in the device, which is consequent result after compromising with limited crystal quality, has caused insufficient light absorption. In this report, we propose an approach for solving these problems. A hybrid design of InGaN/GaN multiple quantum wells (MQWs) solar cells combined with colloidal CdS quantum dots (QDs) and back side distributed Bragg reflectors (DBRs) has been demonstrated. CdS QDs provide down-conversion effect at UV regime to avoid absorption of ITO. Moreover, CdS QDs also exhibit anti-reflective feature. DBRs at the back side have effectively reflected the light back into the absorber layer. CdS QDs enhance the external quantum efficiency (EQE) for light with wavelength shorter than 400 nm, while DBRs provide a broad band enhancement in EQE, especially within the region of 400 nm similar to 430 nm in wavelength. CdS QDs effectively achieved a power conversion efficiency enhancement as high as 7.2% compared to the device without assistance of CdS QDs. With the participation of DBRs, the power conversion efficiency enhancement has been further boosted to 14%. We believe that the hybrid design of InGaN/GaN MQWs solar cells with QDs and DBRs can be a method for high efficiency InGaN/GaN MQWs solar cells.
Enhanced internal quantum efficiency in graphene/InGaN multiple-quantum-well hybrid structures
(2012-08-06)
Huang, Huei-Min
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Chang, Chiao-Yun
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Hsu, Yueh-Shan
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Lu, Tien-Chang
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Lan, Yu-Pin
The enhanced internal quantum efficiency of InGaN/GaN multiple-quantum-wells (MQWs) structure is demonstrated by paving the graphene layers on the MQWs surface. Compared to the conventional MQWs, the internal quantum efficiency of the graphene/InGaN MQWs hybrid structure exhibits a remarkable 2-fold increase. The high charge carrier density in graphene layer is accounted for the enhanced internal quantum efficiency. Moreover, the negligible photoluminescence emission peak shift with increasing the excitation power as well as the decrease of radiative recombination lifetime are attributed to the reduced quantum-confined Stark effect, which correlates to the screening of the polarization field in the c-plane nitride-based quantum well structure. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4745211]
Improvement of the Efficiency of InGaN-GaN Quantum-Well Light-Emitting Diodes Grown With a Pulsed-Trimethylindium Flow Process
(2009-04-01)
Hsueh, Tao-Hung
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Sheu, Jinn-Kong
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Lai, Wei-Chi
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Wang, Yi-Ting
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Kuo, Hao-Chung
This study demonstrated the enhancement of the light output power of InGaN-GaN multiple quantum-well light-emitting diodes (LEDs) that are grown with a pulsed-trimethylindium (pulsed-TMIn) How process by metal-organic vapor-phase epitaxy. At an injection current of 20 mA, the output power of the pulsed-TMIn treated LEDs was improved by 16% as compared to that of the conventional LEDs. In addition, a minor droop (versus injection current) in terms. of external quantum efficiency was also observed in the pulsed-TMIn treated LEDs as compared to conventional LEDs. This improvement could be attributed to the fact that the significant carrier localization effect in the pulsed-TMIn treated LEDs can lead to higher recombination efficiency. This contention is perhaps tentatively evidenced by the temperature-dependent photoluminescence results in which the activation energy of the pulsed-TMIn treated LEDs was increased by 21.8% as compared to that of conventional LEDs.
Improving efficiency of InGaN/GaN multiple quantum well solar cells using CdS quantum dots and distributed Bragg reflectors
(2013-10-01)
Tsai, Yu-Lin
;
Lin, Chien-Chung
;
Han, Hau-Vei
;
Chang, Chun-Kai
;
Chen, Hsin-Chu
This work demonstrates hybrid InGaN/GaN multiple quantum well (MQW) solar cells with enhanced power conversion efficiency using colloidal CdS quantum dots (QDs) and back-side distributed Bragg reflectors (DBRs). CdS QDs can absorb ultraviolet (UV) photons, which are strongly absorbed by indium tin oxide (ITO), and they emit photons with a longer wavelength. This process improves the collection of photon-generated carriers and is known as the luminescence down-shifting (LDS). Accordingly, CdS QDs can compensate for the poor utilization of UV photons in an ITO layer, enhancing the external quantum efficiency (EQE) in the UV range. The DBRs on the back of the solar cells can reflect photons of longer wavelengths back into the absorber layer, increasing the EQE (380-440 nm). The combination of CdS QDs and DBRs results in broadband EQE enhancement, and yields an overall power conversion efficiency that is 20.7% better than that of a reference device without CdS QDs and DBRs. (C) 2013 Elsevier B.V. All rights reserved.
Synchronous sampling and SNR-based gain control in DS/CDMA systems
(2007)
Lin, You-Hsien
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Lo, Shih-Lin
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Lai, Wei-Chi
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Juan, Ta-Yang
;
Hsu, Terng-Yin
Timing synchronization is an important component in a receiver designed to recover data form sampling digital waveform. A synchronous sampling recovery with dual correlator differential (DCD) based acquisition with effects of auto-gain controller (AGC) is proposed to achieve fast timing acquisition for direct sequence / code division multiple access (DS/CDMA) system over frequency-selective fading channel in this paper. It measures both received DCD difference power to determine the good sampling phase from an all-digital phase interpolator based multiphase generator. This solution can tolerate 50ppm system clock offset (SCO) under frequency-selective fading channel. Hence, we not only to determine the good sampling phase fast, but also to arrive stable variable gain amplifier (VGA) with AGC controller in DS/CDMA system approaches.