Browsing by Author "Yang, Hua-Lung"

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A low-complexity adaptive antenna array code acquisition
(2008-05-01)
Yang, Hua-Lung
;
Wu, Wen-Rong
;
電信工程研究所
;
Institute of Communications Engineering
Conventionally, code acquisition with antenna array employs a correlator bank and the serial-search technique. Due to the inherent properties of correlators and serial search, mean acquisition time of the correlator-based approach is large, especially in strong interference environments. Recently, an adaptive-filtering approach has been applied to code acquisition. This method simultaneously performs beamforming and code-delay estimation with a spatial and a temporal filter. Its performance is significantly better than that of the correlator-based approach. However, its computational complexity will be high when the code-delay uncertainty is large. In this paper, we propose a low-complexity adaptive-filtering scheme to solve the problem. Incorporating a serial-search technique, we are able to significantly reduce the size of the temporal filter, so does the computational complexity. We also analyze the proposed algorithm and derive closed-form expressions for optimum solutions, mean-squared error, and mean acquisition time. Simulations show that while the proposed system somewhat compromises the performance, the computational complexity is much lower that of the original adaptive-filtering approach. (C) 2007 Elsevier B.V. All rights reserved.
A novel adaptive antenna array for DS/CDMA code acquisition
(2007-09-01)
Yang, Hua-Lung
;
Wu, Wen-Rong
;
電信工程研究所
;
Institute of Communications Engineering
Equipped with an adaptive beamformer, existing adaptive array code acquisition still relies on the correlator structure. Due to the inherent property of the associated serial-search scheme, its mean acquisition time is large, especially in strong interference environments. In this paper, we propose a novel adaptive filtering scheme to solve the problem.. The proposed scheme comprises two adaptive filters, an adaptive spatial and an adaptive temporal filter. With a specially designed structure, the spatial filter can act as a beamformer suppressing interference, while the temporal filter can act as a code-delay estimator. A mean squared error (MSE) criterion is proposed such that these filters can be simultaneously adjusted by a stochastic gradient descent method. The performance as well as the convergence behavior of the proposed algorithm are analyzed in detail. Closed-form expressions for optimum filter weights, optimum beamformer signal-to-interference-plus-noise ratio (SINR), steady-state MSE, and mean acquisition time are derived for the additive white Gaussian noise (AWGN) channel. Computer simulations show that the mean acquisition time of the proposed algorithm is much shorter than that of the correlator-based approach, and the derived theoretical expressions are accurate.
Low-complexity conjugate gradient algorithm for array code acquisition
(2007-05-01)
Yang, Hua-Lung
;
Wu, Wen-Rong
;
電信工程研究所
;
Institute of Communications Engineering
An adaptive array code acquisition for direct-sequence/ code-division multiple access (DS/CDMA) systems was recently proposed to enhance the performance of the conventional correlator-based method. The scheme consists of an adaptive spatial and an adaptive temporal filter, and can simultaneously perform beamforming and code-delay estimation. Unfortunately, the scheme uses a least-mean-square (LMS) adaptive algorithm, and its convergence is slow. Although the recursive-least-squares (RLS) algorithm can be applied, the computational complexity will greatly increase. In this paper, we solve the dilemma with a low-complexity Conjugate gradient (LCG) algorithm, which can be considered as a special case of a modified conjugate gradient (MCG) algorithm. Unlike the original conjugate gradient (CG) algorithm developed for adaptive applications, the proposed method, exploiting the special structure inherent in the input correlation matrix, requires a low computational-complexity. It can be shown that the computational complexity of the proposed method is on the same order of the LMS algorithm. However, the convergence rate is improved significantly. Simulation results show that the performance of adaptive array code acquisition with the proposed CG algorithm is comparable to that with the original CG algorithm.
Multirate adaptive filtering for low complexity DS/CDMA code acquisition
(2009-06-01)
Yang, Hua-Lung
;
Wu, Wen-Rong
;
電信工程研究所
;
Institute of Communications Engineering
Code acquisition in CDMA systems is conventionally conducted with a matched-filter based structure. However, the performance of this method degrades greatly while multiple access interference presents. Recently, an adaptive filtering scheme was proposed to solve this problem. It has been shown that the computational complexity of p this approach is proportional to the delay uncertainty and inversely proportional to the required acquisition time. When propagation delay is large and the required acquisition time is short, the computational complexity of the adaptive filtering approach will become high. In this paper, we propose a multirate adaptive code acquisition approach to alleviate this problem. The proposed scheme is comprised of several acquisition units operating in different processing rates. Thanks to the decimation property in multirate processing, the overall computational complexity can be greatly reduced. Theoretical analysis of adaptive filters and mean acquisition time is also provided. Experimental results show that while the proposed scheme can have comparable performance with respect to the original adaptive acquisition scheme, its computational complexity is much lower. (c) 2009 Elsevier B.V. All rights reserved.