Browsing by Author "Liu, Jen-Yang"

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Digital predistortion for concurrent dual-band transmitter using a 2D-LUT based Hammerstein architecture
(2017-01)
Cheng, Sheng-Lung
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Wu, Wen-Rong
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Peng, Chun-Hsien
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Liu, Jen-Yang
;
Hsu, Chen-Jui
Digital predistortion has been proved to be an effective remedy for the nonlinear distortion in radio-frequency power amplifier. Unfortunately, conventional digital predistorters (DPDs) developed for single-band transmitters can not be directly applied to concurrent dual-band. To solve the problem, DPDs with two dimensional memory polynomials were then proposed. Although they can provide good performance, its computational complexity is high for the estimation of polynomial coefficients. Look-up-table (LUT) based DPDs, developed for single-band transmitters, can effectively solve the problem. However, there are limited studies about LUT based DPDs for concurrent dual-band transmitters. In this paper, we propose a two-dimensional LUT based Hammerstein DPD to solve the problem. Analyzing the signals on both bands, we propose a new adaptive algorithm with the gradient-descent and recursive-least-squares (RLS) methods to train the DPD iteratively. With the proposed method, the computational complexity in the identification of the DPDs can be significantly reduced. Finally, simulations and experiments are conducted to demonstrate that the performance of the proposed DPD is comparable to that of the polynomial based DPD.
Noise-Enhanced Blind Multiple Error Rate Estimators in Wireless Relay Networks
(2012-03-01)
Liu, Jen-Yang
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Su, Yu T.
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電機工程學系
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Department of Electrical and Computer Engineering
Data detection or fusion based on output from multiple wireless links often requires channel state information (CSI) about the links' error rate (ER) performance. We consider the scenario that these links include direct source-destination (SD) links and two-hop links that require an intermediate decode-and-forward (DF) node to relay the source signal. Conventional destination-based estimators suffer from slow convergence and are incapable of simultaneously blind estimating all ERs, including, in particular, those of the source-relay (SR) links. They may also require various degrees of CSI about the ERs of the SD and relay-destination (RD) links to remove the ambiguity arising from the insufficient number of links in the network and from that due to the symmetric nature of a cascaded source-relay-destination link's ER as a function of its component SR and RD links' ERs. We propose novel Monte-Carlo-based estimators that overcome all these shortcomings. The estimation process involves injecting noise into the samples received by the destination node to create virtual links and alter link output statistics. We show that the latter scheme exhibits a stochastic resonance effect, i.e., its mean squared estimation error (MSEE) performance is enhanced by injecting proper noise, and there exists an optimal injected noise power level that achieves the maximum improvement. The stochastic resonance effects are analyzed, and numerical examples are provided to display our estimators' MSEE behaviors, as well as to show that the ER performance of the optimal detector using the proposed estimators is almost as good as that with perfect ER information.
Performance analysis of transform domain communication systems in the presence of spectral mismatches
(2007)
Liu, Jen-Yang
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Su, Yu T.
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電信工程研究所
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Institute of Communications Engineering
Recently, a transform domain communication system (TDCS) that based on spectrum-dependent complex spreading sequence (SS) has been proposed for low probability of interception (M) and cognitive radio applications. This technique requires both sides of a wireless link to estimate current spectrum condition and select unused spectrum holes for generating a multi-carrier based spreading sequence. This paper analyzes the effect of discrete spectrum estimation errors on one or both sides of such a TDCS link. The discrete spectrum estimation is carried out in a perchannel detection manner, Le., both sides independently determine which subcarriers (channels) within the SS band are available. Our analysis indicates that an irreducible error floor arises due to spectrum estimation errors. The bit error probability performance improves and the error floor is reduced when the probability of single subcarrier detection error p decreases and/or the number of subcarriers increases. We also find that the system performance is more sensitive to the receiver's than to the transmitter's spectrum estimation error.