On approaching to generic channel equalization techniques for OFDM based systems in time variant channels (original) (raw)
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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.
Indonesian Journal of Electrical Engineering and Computer Science
The nature of future wireless applications requires high data rates and for this OFDM technique is used. OFDM stands for orthogonal frequency division multiplexing and is a type of multi-carrier transmission where all the subcarriers are orthogonal to each other. At high data rates, the channel distortion to the data is very important and it is somewhat impossible to recover the transmitted data with a simple receiver. So a complex receiver structure is needed which uses computationally expensive equalization and channel estimation algorithms to estimate the channel. These estimations can be used within the received data to recover the originally transmitted data. OFDM can simplify the equalization problem by changing the frequency-selective channel into a flat channel. The radio channels in mobile radio systems are usually multipath fading channels that results in intersymbol interference (ISI) in the received signal. To remove ISI from the signal, many kind of equalizers can be used. The need for equalizers arises from the fact that the channel has amplitude and phase dispersion which results in the interference of the transmitted signals with one another which is known as ISI .So, to solve this problem equalizers are designed. Equalizer is intend to work in such a way that Bit Error Rate (BER) should be low and Signal-to-Noise Ratio (SNR) should be high. An equalizer within a receiver compensates for the average range of expected channel amplitude and delay characteristics. This paper deals with the various equalization techniques (LMS, RLS and CMA) used for OFDM system .A comparative analysis of different equalization technique in terms of BER is done using MATLAB Simulink.
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).
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.
An Overview On Equalization Techniques for MIMO-OFDM Systems
2008
In this paper, we overview the fundamental techniques of MIMO OFDM equalization in channels where the maximum delay exceeds the length of the Guard Interval. The paper is divided to three main parts explaining frequency domain, time domain and turbo equalization, respectively. In frequency domain, per-tone equalization was chosen to be explained. Swapping the filtering operations of MIMO channel and sliding FFT, one can use the same equalization method of MIMO SC for each tone of MIMO OFDM. In time domain, we discuss techniques in which second order statistics of received signal is used. It is proven that this method is much more efficient than other time domain techniques such as channel shortening. In last chapter we will explain using of Turbo equalization in MIMO OFDM systems which is an iterative equalization and decoding technique for suppressing ISI.
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.
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
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).
Per-tone equalization for mimo ofdm systems
IEEE Transactions on Signal Processing, 2003
This paper focuses on multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems with channel order larger than the cyclic prefix (CP) length. Writing the demodulating fast Fourier transform (FFT) as a sliding FFT followed by a downsampling operation, we show in this paper that by swapping the filtering operations of the MIMO channel and the sliding FFT, the data model for the temporally smoothened received signal of each individual tone of the MIMO OFDM system is very similar to the data model for the temporally smoothened received signal of a MIMO single-carrier (SC) system. As a result, to recover the data symbol vectors, the conventional equalization approach for MIMO SC systems can be applied to each individual tone of the MIMO OFDM system. This so-called per-tone equalization (PTEQ) approach for MIMO OFDM systems is an attractive alternative to the recently developed time-domain equalization (TEQ) approach for MIMO OFDM systems. In the second part of this paper, we focus on direct per-tone equalizer design and adapt an existing semi-blind equalizer design method for space-time block coding (STBC) SC systems to the corresponding semi-blind per-tone equalizer design method for STBC OFDM systems.