Optimal basis for banded channel equalizers in OFDM system over doubly selective channels (original) (raw)
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Equalization for OFDM over doubly selective channels
IEEE Transactions on Signal Processing, 2000
In this paper, we propose a time-domain as well as a frequency-domain per-tone equalization for orthogonal frequency-division multiplexing (OFDM) over doubly selective channels. We consider the most general case, where the channel delay spread is larger than the cyclic prefix (CP), which results in interblock interference (IBI). IBI in conjunction with the Doppler effect destroys the orthogonality between subcarriers and, hence, results in severe intercarrier interference (ICI). In this paper, we propose a time-varying finite-impulse-response (TV-FIR) time-domain equalizer (TEQ) to restore the orthogonality between subcarriers, and hence to eliminate ICI/IBI. Due to the fact that the TEQ optimizes the performance over all subcarriers in a joint fashion, it has a poor performance. An optimal frequency-domain per-tone equalizer (PTEQ) is then obtained by transferring the TEQ operation to the frequency domain. Through computer simulations, we demonstrate the performance of the proposed equalization techniques. Index Terms-Basis expansion model (BEM), doubly selective channels, orthogonal frequency-division multiplexing (OFDM), per-tone equalization (PTEQ), time-domain equalization (TEQ).
Low-complexity banded equalizers for OFDM systems in Doppler spread channels
EURASIP Journal on Applied Signal …, 2006
Recently, several approaches have been proposed for the equalization of orthogonal frequency-division multiplexing (OFDM) signals in challenging high-mobility scenarios. Among them, a minimum mean-squared error (MMSE) block linear equalizer (BLE), based on a band LDL factorization, is particularly attractive for its good tradeoff between performance and complexity. This paper extends this approach towards two directions. First, we boost the BER performance of the BLE by designing a receiver window specially tailored to the band LDL factorization. Second, we design an MMSE block decision-feedback equalizer (BDFE) that can be modified to support receiver windowing. All the proposed banded equalizers share a similar computational complexity, which is linear in the number of subcarriers. Simulation results show that the proposed receiver architectures are effective in reducing the BER performance degradation caused by the intercarrier interference (ICI) generated by time-varying channels. We also consider a basis expansion model (BEM) channel estimation approach, to establish its impact on the BER performance of the proposed banded equalizers.
Equalization of OFDM for doubly very selective channels
2010 IEEE 12th International Conference on Communication Technology, 2010
Ahstract-The performance reduction caused by inter-carrier interference (ICI) on orthogonal frequency division multiplexing (OFDM) systems over time-varying channels is relevant especially when large OFDM symbols are employed in order to achieve an higher throughput. In this paper we propose a design method of an equalization scheme using several full-length FFTs operating on non-overlapping windows of the received OFDM symbol. In particular the FFTs outputs are combined minimizing the mean square error (MSE) at the detection point. Subsequently we recall another technique to mitigate ICI, which still operates on sub blocks, but using smaller FFTs of size equal to the length of each sub-block. A comparison between these two schemes is performed in terms of computational complexity, which is an important issue in OFDM systems employing long symbols, and bit error rate, considering a possible extension of the new digital video broadcasting standard DVB-T2 to an hand-held version (DVB NGH).
Low-complexity pilot-aided channel estimation for OFDM systems over doubly-selective channels
2005
In this paper, we investigate channel estimation (CE) and data detection for OFDM systems over doublyselective channels. We derive an oversampling basis expansion model (BEM) for doubly-selective channels and its statistical properties. The time diversity in the Doppler-induced intercarrier-interference (ICI) and its relationship to the carrier frequency offset (CFO) induced ICI are illustrated using the BEM. We derive two low complexity linear minimum meansquare-error (LMMSE) channel estimators using the BEM. The sphere decoder (SD) is modified to equalize the ICI channel. A low-complexity iterative equalizer without matrix inversion is also proposed. Our proposed channel estimators have low complexity and achieve good performance. Furthermore, the low-complexity iterative equalizer performs close to SD.
Low complexity equalization for doubly selective channels modeled by a basis expansion
IEEE Transactions on Signal Processing, 2010
We propose a novel equalization method for doubly selective wireless channels, whose taps are represented by an arbitrary Basis Expansion Model (BEM). We view such a channel in the time domain as a sum of product-convolution operators created from the basis functions and the BEM coefficients. Equivalently, a frequency-domain channel can be represented as a sum of convolution-products. The product-convolution representation provides a low-complexity, memory efficient way to apply the channel matrix to a vector. We compute a regularized solution of a linear system involving the channel matrix by means of the GMRES and the LSQR algorithms, which utilize the product-convolution structure without ever explicitly creating the channel matrix. Our method applies to all cyclic-prefix transmissions. In an OFDM transmission with subcarriers, each iteration of GMRES or LSQR requires only flops and memory. Additionally, we further accelerate convergence of both GMRES and LSQR by using the single-tap equalizer as a preconditioner. We validate our method with numerical simulations of a WiMAX-like system (IEEE 802.16e) in channels with significant delay and Doppler spreads. The proposed equalizer achieves BERs comparable to those of MMSE equalization, and noticeably outperforms low-complexity equalizers using an approximation by a banded matrix in the frequency domain. With preconditioning, the lowest BERs are obtained within 3-16 iterations. Our approach does not use any statistical information about the wireless channel.
IEEE Transactions on Consumer Electronics, 2004
Rapid variations of doubly-selective fading channels lead to the loss of orthogonality among subcarriers, resulting in inter-carrier interference (ICI) in orthogonal frequency division multiplexing (OFDM) systems. In this scenario, the one-tap frequency-domain equalizer is not applicable any more. Based on an ICI analysis in both time- and frequency-domain, we propose a novel zero-forcing equalizer (ZFE) for OFDM systems over doubly-selective channels using the existing frequency domain redundancy in many OFDM standards. To deal with the zero-forcing equalizer's disadvantage of noise amplification, which causes system performance degradation when there exist deep s in the channel's frequency response, we also provide the MMSE-based optimization of ZFE coefficients in the presence of channel noise. We show that ZFE matrix has a sparse structure, and thus a low computational complexity. Simulation results indicate that the proposed equalizer is capable of suppressing ICI effectively with a low complexity.
Low-Complexity Equalisation Methods for OFDM Systems in Doubly Selective Channels
2008
Time-selective channels in orthogonal frequency division multiplexing (OFDM) systems introduce intercarrier interference (ICI) which destroys the orthogonality among the subcarriers. Equalisation techniques are needed to mitigate ICI, requiring high computational complexity for large values of Doppler spread. In this paper, we propose low-complexity equalisation methods that result in 75% savings in computations over some of the conventional methods. The savings are achieved without any loss in performance. One of the methods is to apply the one-tap equaliser on selected subcarriers to detect the corresponding symbols. We develop a criterion to determine the eligible subcarriers.
Performance analysis of banded equalizers for OFDM systems in time-varying channels
2007 IEEE 8th Workshop on Signal Processing Advances in Wireless Communications, 2007
OFDM systems affected by severe time-varying channels greatly benefit from equalization schemes based on intercarrier interference (ICI) mitigation, which guarantee improved performance with respect to conventional one-tap equalizers. By exploiting a semianalytical approach, this paper assesses the BER performance of the so-called banded equalizers, i.e., those equalizers that take into account only the ICI produced by the closest subcarriers. Specifically, by exploiting the Gaussian approximation of the residual ICI at the equalizer output, we evaluate the BER of block linear equalizers designed under a minimum mean-squared error (MMSE) criterion. Simulation results show a very good agreement with the theoretical analysis. In addition, we derive a lower bound and an approximate upper bound for the BER of block decision-feedback equalizers.
Low-Complexity Equalization for TDS-OFDM Systems Over Doubly Selective Channels
IEEE Transactions on Broadcasting, 2005
Time variation of a multipath channel leads to interchannel interference (ICI) in orthogonal frequency-division multiplexing (OFDM) systems. It results in the performance degradation, therefore, limits the achievable throughput. Some methods have been proposed to suppress ICI, unfortunately, they are either computationally complex or at the price of spectral efficiency. In this paper, a low-complexity equalization method for time-domain synchronous OFDM (TDS-OFDM) systems is proposed under the assumption that the channel impulse response (CIR) varies in a linear fashion within a block period. The rationale behind our method is to use a finite power series expansion for the inverse of the equalization matrix. This method provides a desired tradeoff between the performance and the processing complexity. Theoretical analysis and simulation results demonstrate that the proposed method can effectively mitigate ICI caused by the channel variations with low computational complexity.
Wireless personal communications, 2010
In orthogonal frequency division multiplexing systems, significant inter-carrier interference (ICI) caused by doubly selective channels make challenge for reliable reception. In this paper, channel estimation and ICI cancellation are considered jointly. Relying on the basis expansion model (BEM) of time-varying channel, the linear system model of transceiver is established, and the corresponding joint optimization of the transmitted data and BEM coefficients is formulated. Due to the separability of the data and BEM coefficients, we use cyclic minimizing technique to perform channel estimation and equalization alternately. This yields a linear minimum mean square-error (LMMSE) channel estimator and a block MMSE equalizer respectively. The block MMSE equalizer has complexity O(N 3), where N is the number of data subcarriers. To reduce the complexity, instead of equalizing all the data simultaneously, we consider estimating each data symbol successively. This idea results in the per subcarrier interference canceller with lower complexity O(N 2). Finally, an iterative receiver consisting of the data-aided LMMSE channel estimator and the successive interference canceller is developed. Simulation results show the scheme is effective over the channel with relatively large Doppler spread.