Equalization for OFDM over doubly selective channels (original) (raw)
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Time-domain and frequency-domain per-tone equalization for OFDM over doubly selective channels
Signal Processing, 2004
In this paper, we propose new time-and frequency-domain per-tone equalization techniques for orthogonal frequency division multiplexing (OFDM) transmission over time-and frequency-selective channels. We present one mixed time-and frequency-domain equalizer (MTFEQ) and one frequency-domain per-tone equalizer. The MTFEQ consists of a one-tap time-varying (TV) time-domain equalizer (TEQ), which converts the doubly selective channel into a purely frequency-selective channel, followed by a one-tap frequency-domain equalizer (FEQ), which then equalizes the resulting frequency-selective channel in the frequency-domain. The frequency-domain per-tone equalizer (PTEQ) is then obtained by transferring the TEQ operation to the frequency-domain. While the one-tap TEQ of the MTFEQ optimizes the performance on all subcarriers in a joint fashion, the PTEQ optimizes the performance on each subcarrier separately. This results into a significant performance improvement of the PTEQ over the MTFEQ, at the cost of a slight increase in complexity. Through computer simulations we show that the MTFEQ suffers from an early and high error floor, while the PTEQ outperforms the MMSE equalizer for OFDM over purely frequency-selective channels, it
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.
Equalisation of SIMO-OFDM systems with insufficient cyclic prefix in doubly selective channels
Iet Communications, 2009
Time variations of a doubly selective wireless channel and insufficient cyclic prefix (CP) length of an orthogonal frequency division multiplexing (OFDM) transmission system cause intercarrier interference (ICI) and interblock interference (IBI) as significant limitations. This paper investigates the problem of joint ICI and IBI mitigation in single-input multiple-output OFDM (SIMO-OFDM) systems. It is assumed, unlike most existing literature, that the channel delay spread is larger than the CP, and also the channel varies on each OFDM block. First, doubly selective channel is modelled using basis expansion model (BEM) and a closed-form expression for signal-to-interference-plus-noise ratio (SINR) is derived. Then, a time-domain equaliser is developed, which maximises the SINR for all subcarriers. Moreover, a frequency-domain equalisation approach is proposed which is based on the MSE minimisation per tone. A low-complexity implementation of the pertone equaliser is also derived. An important feature of the proposed equalisers is that no bandwidth expansion or redundancy insertion is required except for the CP. Finally, complexity comparison and simulation results over Rayleigh fading channel are provided to illustrate the effectiveness of the proposed approaches. Since both equalisers are designed in the frequency domain, they provide significant interference cancellation. † c , L and L 0 = 0: the proposed FEQ reduces to the PTEQ proposed for discrete multitone transmission (DMT)-based systems in .
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 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.
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.
New equalization approach for OFDM over dispersive and rapidly time varying channel
The paper proposes and analyses a new receiver structure to mitigate the effect of Doppler on the reception of OFDM signals. A Discrete-Frequency channel representation is developed for the link between the input of the transmit I-FFT and the output of the receive FFT. It is based on a Taylor expansion of the time variations of the received subcarrier amplitudes. The model realistically addresses the correlation of fading at neighboring subcarriers. We study a new type of receiver which estimates not only amplitudes but also derivatives of subcarriers amplitudes. An adaptive MMSE filter is proposed to cancel the Intercarrier Interference (ICI) resulting from Doppler. This results in a substantial improvement of the link performance.
2003
In this paper, we address the problem of OFDM transmission over a time-varying, frequency-selective channel with high Doppler spread. This creates situations where the channel significantly evolves over the time span of one OFDM symbol. We analyze the CP-OFDM transmission mechanism, and the impairments due to the channel variations, using a decomposition of these variations over a base of sinusoid functions sampling the Doppler spectrum at subcarrier frequencies. On the one hand, this leads to a fairly parsimonious parameterization of the time-varying channel impulse response. On the other hand we show that, considering a whole CP-OFDM symbol, this leads to a duality between equalization of the delay spread of a time-varying channel in time domain, and the equalization of the Doppler spread of a frequency selective channel in frequency domain. Using this duality, we show that equalization in the frequency domain can benefit from all known time domain equalization methods.
Simple equalization of time-varying channels for OFDM
Communications Letters, IEEE, 2005
We present a block minimum mean-squared error (MMSE) equalizer for orthogonal frequency-division multiplexing (OFDM) systems over time-varying multipath channels. The equalization algorithm exploits the band structure of the frequency-domain channel matrix by means of a band LDL H factorization. The complexity of the proposed algorithm is linear in the number of subcarriers and turns out to be smaller with respect to a serial MMSE equalizer characterized by a similar performance.
Optimal basis for banded channel equalizers in OFDM system over doubly selective channels
2010 IEEE International Conference on Acoustics, Speech and Signal Processing, 2010
We prove that discrete Fourier basis channel model is optimal for least squares estimation of channel coefficients used for low-complexity equalization of OFDM transmission over doubly selective channels. We show that regardless of channel statistics, the channel model order is determined by the number of coefficients used in the equalization. Our theoretical findings are numerically validated by using linear zero forcing equalizers and non-linear maximum likelihood equalizer with pilot-aided channel estimation.