Browsing by Author "Hsu, Chao-Yuan"

Now showing 1 - 5 of 5

A low-complexity ICI mitigation method for high-speed mobile OFDM systems
(2006)
Hsu, Chao-Yuan
;
Wu, Wen-Rong
;
電信工程研究所
;
Institute of Communications Engineering
Orthogonal frequency division multiplexing (OFDM) is a promising technique for frequency-selective fading channels. However, in high-speed mobile environments, the channel may be time-variant in one OFDM symbol. This will incur the intercarrier interference (ICI) degrading seriously the performance of an OFDM system. A simple remedy is to apply the zero-forcing (ZF) equalization method. Unfortunately, the direct ZF method requires an inverse of a huge matrix requiring a prohibitively high complexity. In this paper, we propose a low-complexity ZF method for ICI mitigation. The main idea is to explore the special structure of the ICI matrix and apply the Newton's iterative matrix inversion method. With our formulation, fast Fourier transforms (FFTs) can be used to reduce the complexity. The proposed method can reduce the complexity from the order of O(N(c)(3)) to that of O(N(c)log(2)N(c)) where N(c) is the number of subcarriers. Computer simulations show that the proposed method can cancel the ICI effectively and at the same time reduces the computational requirement dramatically.
A Low-Complexity Zero-forcing CFO Compensation Scheme for OFDMA Uplink Systems
(2008-10-01)
Hsu, Chao-Yuan
;
Wu, Wen-Rong
;
電信工程研究所
;
Institute of Communications Engineering
Similar to the conventional orthogonal frequency-division multiplexing (OFDM) system, an OFDM multiple access (OFDMA) system will have a carrier frequency offset (CFO) problem. Since CFOs of all users are different, CFO compensation in the OFDMA uplink system is much more involved. A simple, yet efficient, method is the zero-forcing (ZF) compensation method. However, it involves an inverse of an N x N CFO-induced ICI matrix, where N is the number of subcarriers. Thus, the complexity can become very high when N is large, a case commonly seen in OFDMA systems. In this work, we propose a low-complexity ZF method to overcome the problem. The main idea is to use Newton's method to solve matrix inversion iteratively. We explore the structure of the CFO-induced ICI matrix and develop a method that can implement Newton's method with fast Fourier transforms (FFTs). As a result, the required computational complexity is significantly reduced from O(N-3) to O(2N log(2) N). Simulations show that, with only three iterations, the proposed method can have similar performance to the direct ZF method.
ICI Mitigation for OFDMA Uplink Systems
(2008)
Hsu, Chao-Yuan
;
Wu, Wen-Rong
;
電信工程研究所
;
Institute of Communications Engineering
Orthogonal frequency-division multiplexing multiple access (OFDMA) has been considered as a promising multiple access technique in OFDM-based communication systems. In an ideal OFDMA system, no intercarrier interference (ICI) occurs. However, in high-speed mobile environments, the channel may be time-variant during one OFDMA symbol period. In such a condition, the mobility-induced ICI will degrade the system performance seriously. To remove the ICI effect, the zero-forcing (ZF) method is one of the simple techniques. Unfortunately, the direct ZF method needs to invert an N x N ICI matrix, requiring a prohibitively high complexity when N is large. In this paper, we first derive a structured OFDMA uplink signal model consisting of FFTs/IFFTs. Exploring this structure and using Newton's iteration for matrix inversion, we develop a low-complexity ZF method. With our formulation, fast Fourier transforms (FFTs) can be used to reduce the complexity. Thus, the proposed method can reduce the complexity from O(N(3)) to O(N log(2) N). Simulations show that the proposed method can have the similar performance to the direct ZF method while requires much lower complexity.
ICI mitigation method for high-speed mobile OFDM systems
(2008-05-01)
Wu, Wen-Rong
;
Hsu, Chao-Yuan
In high-speed mobile environments, the channel is time-varying within an OFDM symbol. This time-varying characteristic will destroy the orthogonality among subcarriers. Thus, the intercarrier interference (ICI) will occur and the system performance will be degraded. An ICI mitigation method for high-speed mobile OFDM systems is proposed, which explores the special structure of the ICI channel matrix and applies the Newton's iterative matrix inversion method. With our formulation, fast Fourier transform (FFT) can be used to reduce the computational complexity. The object of canceling the ICI can be accomplished without the need of any extra circuit.
Low-Complexity ICI Mitigation Methods for High-Mobility SISO/MIMO-OFDM Systems
(2009-07-01)
Hsu, Chao-Yuan
;
Wu, Wen-Rong
;
電信工程研究所
;
Institute of Communications Engineering
In orthogonal frequency-division multiplexing (OFDM) systems, it is generally assumed that the channel response is static in an OFDM symbol period. However, the assumption does not hold in high-mobility environments. As a result, intercarrier interference (ICI) is induced, and system performance is degraded. A simple remedy for this problem is the application of the zero-forcing (ZF) equalizer. Unfortunately, the direct ZF method requires the inversion of an N x N ICI matrix, where N is the number of subcarriers. When N is large, the computational complexity can become prohibitively high. In this paper, we first propose a low-complexity ZF method to solve the problem in single-input-single-output (SISO) OFDM systems. The main idea is to explore the special structure inherent in the ICI matrix and apply Newton's iteration for matrix inversion. With our formulation, fast Fourier transforms (FFTs) can be used in the iterative process, reducing the complexity from O(N(3)) to O( N log(2) N). Another feature of the proposed algorithm is that it can converge very fast, typically in one or two iterations. We also analyze the convergence behavior of the proposed method and derive the theoretical output signal-to-interference-plus-noise ratio (SINR). For a multiple-input-multiple-output (MIMO) OFDM system, the complexity of the ZF method becomes more intractable. We then extend the method proposed for SISO-OFDM systems to MIMO-OFDM systems. It can be shown that the computational complexity can be reduced even more significantly. Simulations show that the proposed methods perform almost as well as the direct ZF method, while the required computational complexity is reduced dramatically.