Equalization methods in OFDM and FMT systems for broadband wireless communications (original) (raw)
Related papers
IEEE Transactions on Signal Processing, 2005
To ease equalization in a multicarrier system, a cyclic prefix (CP) is typically inserted between successive symbols. When the channel order exceeds the CP length, equalization can be accomplished via a time-domain equalizer (TEQ), which is a finite impulse response (FIR) filter. The TEQ is placed in cascade with the channel to produce an effective shortened impulse response. Alternatively, a bank of equalizers can remove the interference tone-by-tone. This paper presents a unified treatment of equalizer designs for multicarrier receivers, with an emphasis on discrete multitone systems. It is shown that almost all equalizer designs share a common mathematical framework based on the maximization of a product of generalized Rayleigh quotients. This framework is used to give an overview of existing designs (including an extensive literature survey), to apply a unified notation, and to present various common strategies to obtain a solution. Moreover, the unification emphasizes the differences between the methods, enabling a comparison of their advantages and disadvantages. In addition, 16 different equalizer structures and design procedures are compared in terms of computational complexity and achievable bit rate using synthetic and measured data.
Receiver architectures for FMT broadband wireless systems
IEEE VTS 53rd Vehicular Technology Conference, Spring 2001. Proceedings (Cat. No.01CH37202), 2001
Orthogonal Frequency Division Multiplexing (OFDM) and in particular Discrete Multitone (DMT) modulation has been proposed for the physical layer of wideband wireless local area networks. As an alternative, Filtered Multitone (FMT) modulation can be also considered, since it exhibits significantly lower spectral overlapping between adjacent subchannels, so providing higher transmission efficiency than DMT.
Analysis of Per-Channel Equalized Filtered Multitone Modulations Over Time-Varying Fading Channels
2006 IEEE 17th International Symposium on Personal, Indoor and Mobile Radio Communications, 2006
We consider the performance of an FMT system with perchannel equalization over frequency-selective time-varying fading channels. Due to the distortion caused by frequency and time selectivity of the fading channel, both ICI and ISI exist in an FMT system and cause performance degradation. By using a per-channel equalizer at the FMT receiver with sufficiently large number of equalizer taps, the ISI can be mitigated significantly, but the ICI still exists. In this paper, the effects of the interference caused by the time-frequency dispersive channel is quantified by analyzing the average system carrier to interference (C/I) ratio of the FMT system under different fading conditions. A closed-form expression for the C/I ratio and its upper bound are provided for the FMT system, which lead to a better understanding of the trade-off between spectral efficiency and system performance degradation. Moreover, comparisons between FMT and OFDM systems under the same channel conditions and spectral efficiency are also provided. Numerical and simulation results of the system C/I ratio further confirm and support the obtained analytical results.
IEEE Transactions on Signal Processing, 2000
Several high-speed communication standards modulate encoded data on multiple carrier frequencies using the inverse Fourier transform (FFT). The real part of the quantized inverse FFT samples form a symbol. The symbol is periodically extended by prepending a copy of its last few samples, a.k.a. a cyclic prefix. When the cyclic prefix is longer than the channel order, amplitude and phase distortion can be equalized entirely in the frequency domain. In the receiver, prior to the FFT, a timedomain equalizer, in the form of a finite impulse response filter, shortens the effective channel impulse response. Alternately, a bank of equalizers tuned to each carrier frequency can be used. In earlier work, we unified optimal multicarrier equalizer design algorithms as a product of generalized Rayleigh quotients. In this paper, we convert the unified theoretical framework into a framework for fast design algorithms. The relevant literature is reviewed and classified according to this framework. We analyze the achieved bit rate vs. implementation complexity (in terms of multiply-and-accumulate operations) tradeoffs in the original and fast design algorithms. The comparison includes multiple implementations of each of 16 different equalizer structures and design algorithms using synthetic and measured discrete multitone modulated data.
Tracking behavior of adaptive equalizers in filtered multitone communication systems
2009 Conference Record of the Forty-Third Asilomar Conference on Signals, Systems and Computers, 2009
Although orthogonal frequency division multiplexing (OFDM) communication systems that uses inverse discrete Fourier (IDFT) for multicarrier modulation and discrete Fourier transform (DFT) for demodulation are dominantly adopted in the current broadband communication standards, there have been some reports in recent years that discuss the shortcomings of OFDM in highly mobile and/or multiple access environments. To resolve these problems, a number of authors have proposed a shift from OFDM to filterbank-based multicarrier (FBMC) techniques. In a recent work, we presented a thorough study of filtered multitone (FMT), a class of FBMC systems, in time-varying frequency selective channels. We derived close-form equations for the optimum parameters of per-tone fractionally spaced equalizers and also signal to interference plus noise ratio (SINR) and used these to evaluate FMT in typical wireless mobile environments. These results provide an upper limit to the performance of this class of FBMC systems. In this paper, we study the performance of an adaptive channel estimation algorithms that attempts to reach this upper limit. To further improve the channel estimate, the proposed method uses a minimum mean-square error (MMSE) channel estimator and considers the fact that the channel impulse response is limited in time.
Design and Performance Evaluation of Filtered Multitone (FMT) in Doubly Dispersive Channels
2011 IEEE International Conference on Communications (ICC), 2011
Orthogonal frequency division multiplexing (OFDM) is the most popular scheme for broadband communications. However, it has been shown that OFDM has disadvantages in time-varying channels. Filter bank multicarrier (FBMC) is an alternative multicarrier modulation method that can be designed for robust performance in doubly (time and frequency) dispersive channels. The emphasis of this paper is on a class of FBMC systems that are called filtered multitone (FMT). We note that although unlike the more popular FBMC systems that use offset quadrature amplitude modulation (OQAM) or cosine modulated techniques to achieve full bandwidth efficiency, FMT does not have full bandwidth efficiency, its possible use in multiple-input multiple-output (MIMO) channels still makes it an attractive method. In a recent work, we have noted that the FBMC systems that are based on OQAM/cosine modulated techniques are very restrictive in MIMO channels and introduced a class of FMT systems with robust performance in doubly dispersive channels. This paper extends our previous results and shows that further improvement can be obtained by organizing the data symbols in a hexagonal lattice structure, as compared to the more popular rectangular structure. In addition, results that compare our proposed FMT systems with the conventional FMT and OFDM are presented and we show that a substantial gain in performance is achieved.
Equalization for Discrete Multitone Transceivers
2001
ABSTRACT G.DMT and G.lite Asymmetric Digital Subscriber Line (ADSL) modems and some Very-high-speed Digital Subscriber Line (VDSL) modems rely on discrete multitone modulation (DMT). In an ADSL discrete multitone receiver, a time-domain equalizer (TEQ) reduces intersymbol interference (ISI) by shortening the effective duration of the channel impulse response. Previous TEQ design methods such as minimum mean-squared error (MMSE), maximum shortening signal-to-noise ratio (MSSNR), and maximum geometric signal-to-noise ratio (MGSNR) do not directly optimize channel capacity. In this dissertation, I develop a TEQ design method to optimize channel capacity at the output of the TEQ. First, I partition an equalized multicarrier channel into its equivalent signal, noise, and ISI paths to develop a new subchannel signal-to-noise (SNR) definition. Using the new subchannel SNR definition, I derive a nonlinear function of TEQ taps that measures channel capacity. Based on the nonlinear function, I propose the optimal maximum-channel-capacity (MCC) TEQ that achieves 99.87% of the matched filter bound on ADSL channel capacity with a 17-tap equalizer. I also derive a computationally-efficient, near-optimal minimum-ISI (min-ISI) method that generalizes the MSSNR method by weighting the ISI in the frequency domain. The frequency domain weighting increases computational complexity for higher bit rate. Based on simulations using eight different carrier-serving-area ADSL channels, (1) the proposed methods yield higher bit rates than the MMSE, MSSNR, and MGSNR methods
Fast Time-Domain Equalization For Discrete Multitone Modulation Systems
2000
In discrete multitone receivers, a time-domain equalizer TEQ shortens the e ective c hannel impulse response. The e ective c hannel impulse response is ideally nonzero inside a short window and zero elsewhere. The samples outside of the window cause intersymbol interference. The ratio between the samples inside and outside of this window is called the shortening signalto-noise SSNR. In this paper, we develop a suboptimal method for maximum SSNR TEQ design that requires two orders of magnitude fewer computations than the original maximum SSNR method. We reduce computation by using a proposed heuristic to estimate the optimal delay of the window and a proposed divideand-conquer method to compute the TEQ taps. For typical ADSL channels, the tradeo for the reduction in computation is roughly 4 dB SSNR for two-tap TEQs and less than 1 dB for 17-tap TEQs.
Channel Equalization in Filter Bank Based Multicarrier Modulation for Wireless Communications
EURASIP Journal on Advances in Signal Processing, 2007
Channel equalization in filter bank based multicarrier (FBMC) modulation is addressed. We utilize an efficient oversampled filter bank concept with 2x-oversampled subcarrier signals that can be equalized independently of each other. Due to Nyquist pulse shaping, consecutive symbol waveforms overlap in time, which calls for special means for equalization. Two alternative linear low-complexity subcarrier equalizer structures are developed together with straightforward channel estimation-based methods to calculate the equalizer coefficients using pointwise equalization within each subband (in a frequency-sampled manner). A novel structure, consisting of a linear-phase FIR amplitude equalizer and an allpass filter as phase equalizer, is found to provide enhanced robustness to timing estimation errors. This allows the receiver to be operated without time synchronization before the filter bank. The coded error-rate performance of FBMC with the studied equalization scheme is compared to a cyclic prefix OFDM reference in wireless mobile channel conditions, taking into account issues like spectral regrowth with practical nonlinear transmitters and sensitivity to frequency offsets. It is further emphasized that FBMC provides flexible means for high-quality frequency selective filtering in the receiver to suppress strong interfering spectral components within or close to the used frequency band.
LMS-based Equalization in Filter Bank Multicarrier Wireless Communication Systems
Multicarrier systems give many attractive characteristics for high data rates wireless communications. Filter bank multicarrier systems (FBMC) provide some advantages more than the traditional orthogonal frequency division multiplexing (OFDM) with cyclic prefix (CP). The most obvious difference between the two techniques is in frequency selectivity. In this paper, we will design adaptive channel equalization algorithms for FBMC system with offset QAM modulation(OQAM). Our proposed algorithms will be optimized based on well-known cost functions, which is the mean-squared error (MSE) criterion and it is based on least-mean-square (LMS) algorithm. Thus we will have an adaptive equalizer with lower complexity because of applying the equalizer to each sub-carrier. We have used in our simulation practical channel information based on the International Telecommunications Union ITU Standards. In this paper, we aim to optimize and evaluate the convergence characteristic curves of LMS equalization algorithm per-subcarrier.