Clipping noise mitigation for OFDM by decision-aided reconstruction

Clipping is often used to reduce the large peak-to-mean envelope power ratio of orthogonal frequency-division multiplexing (OFDM) signals. However, it introduces additional noise that degrades the system performance. A technique called decision-aided reconstruction (DAR) is proposed for mitigating the clipping noise. The performance of the proposed technique is evaluated for additive white Gaussian noise and static intersymbol interference channels. The effect of using a realistic channel estimate is also examined. Results show that the DAR technique can mitigate the clipping noise significantly for OFDM systems that have large block sizes     

基于ICI干扰消除的OFDM线性时变信道估计方法

多普勒频移破坏了OFDM子载波的正交性,导致系统产生子载波间干扰。针对此问题,文中提出了一种基于ICI干扰消除的时变信道估计方法。该方法利用前一符号的信道斜率,消除当前系统符号所受到的ICI干扰。在归一化多普勒频小于0.15时,所述算法的信道估计均方误差比传统算法更小,系统误码率性能更优秀。     

Joint channel and impulsive noise estimation method for OFDM systems

Aiming at the impulsive noise occurring in OFDM systems,an impulsive noise mitigation algorithm based on compressed sensing theory was proposed.The proposed algorithm firstly treated the channel impulse response and the impulsive noise as a joint sparse vector by exploiting the sparsity of both them.Then the sparse Bayesian learning framework was adopted to jointly estimate the channel impulse response,the impulsive noise and the data symbols,in which the data symbols were regarded as unknown parameters.Compared with the existing impulsive noise mitigation methods,the proposed algorithm not only utilized all subcarriers but also did not use any a priori information of the channel and impulsive noise.The simulation results show that the proposed algorithm achieves significant improvement on the channel estimation and bit error rate performance.     

Clipping noise cancellation in OFDM systems using oversampledsignal reconstruction

Clipping the OFDM signals in the digital part of the transmitter is one of the simplest methods to reduce the peak factor. However, it suffers from additional clipping distortion, peak regrowth after digital to analog conversion, and out-of-band radiation in the case of oversampled sequence clipping. We use oversampled sequence clipping to combat the effect of peak regrowth and propose a method to reconstruct the clipped samples and mitigate the clipping distortion in the presence of channel noise at the expense of bandwidth expansion. We show through extensive simulations that by slightly increasing the bandwidth of the system, we can significantly improve the performance while limiting the maximum amplitude of the analog signal     

Iterative receiver employing phase noise compensation and channel estimation for millimeter-wave OFDM systems

This paper proposes an iterative OFDM millimeterwave receiver employing low-complexity decision-directed phase noise compensation (DD-PNC) to alleviate degradation due to the phase noise. High bit-rate OFDM transceivers based on the single-chip Si RF-CMOS IC technology in the 60-GHz millimeter-wave band have been extensively studied for wireless personal area network (WPAN) systems, and the relatively large phase noise in the phase locked loop (PLL) synthesizer severely degrades transmission performance. The proposed OFDM receiver iterates DD-PNC and decision-directed channel estimation (DDCE) by exploiting the output of the channel decoder. DDPNC estimates the phase noise each sampling time by using the decoder output, and then it removes the estimate from a time-domain received signal. In addition, DDCE estimates a channel impulse response by using the compensated received signal. Computer simulations demonstrate that in the 64QAM modulation with the coding rate of 3/4, the proposed receiver with DD-PNC and DDCE can perfectly remove the phase noise of -85 dBc/Hz at 1 MHz offset, and that it can alleviate the degradation of the channel estimation due to the phase noise.     

Pilot-symbol-aided channel estimation for OFDM in wireless systems

In this paper, we investigate pilot-symbol-aided parameter estimation for orthogonal frequency division multiplexing (OFDM) systems. We first derive a minimum mean-square error (MMSE) pilot-symbol-aided parameter estimator. Then, we discuss a robust implementation of the pilot-symbol-aided estimator that is insensitive to channel statistics. From the simulation results, the required signal-to-noise ratios (SNRs) for a 10% word error rate (WER) are 6.8 dB and 7.3 dB for the typical urban (TU) channels with 40 Hz and 200 Hz Doppler frequencies, respectively, and they are 8 dB and 8.3 dB for the hilly-terrain (HT) channels with 40 Hz and 200 Hz Doppler frequencies, respectively. Compared with the decision-directed parameter estimator, the pilot-symbol-aided estimator is highly robust to Doppler frequency for dispersive fading channels with noise impairment even though it has some performance degradation for systems with lower Doppler frequencies     

Joint channel and phase noise estimation in OFDM systems at very high speeds

In this paper, the problem of joint phase noise and channel estimation for OFDM systems over a fast time-varying frequency-selective channel is explored. Each channel tap time-variation within one OFDM symbol is approximated by a Basis Expansion Model (BEM). Joint estimation is performed on multiple OFDM symbols via the Extended Kalman Filtering in order to exploit the time-correlation of the parameters. The data symbols are estimated by means of an iterative pilot-based algorithm. It is shown that, with only 2 iterations, our algorithm outperforms the conventional one, and the performance approaches that of the ideal case for which the channel response and phase noise are known.     

一种改进的MIMO OFDM系统信道估计算法

本文对MIMO OFDM系统中基于训练序列的信道估计问题进行了研究，针对信道冲击响应的最大抽头数大于每个OFDM符号中导频数的情况，提出一种有效的结合前后若干训练序列进行信道估计的算法和结合方式。仿真结果表明，在基于无线局域网（WLAN）中打包传送的MIMO OFDM系统里，本文的方法比采用块状训练序列的估计算法有着更小的归一化均方误差。     

OFDM系统中基于导频的低秩信道估计方法

主要研究了正交频分复用(OFDM)系统中基于导频辅助的低秩信道估计方法,提出了一种秩的取值及自适应秩跟踪方法。低秩估计方法利用信道的频域(和/或时域)相关性以及奇异值分解技术。其中关键的秩估计也就是信号子空间的维数判定必须在计算复杂度和估计误差两方面折中考虑。仿真表明给出的秩估计方法能够比较准确地反映出信号子空间的大小,大大降低运算复杂度。在此基础上的低秩方法与线性最小均方误差(LMMSE)估计的性能几乎一样并且对子载波总数的大小和信道环境变化的敏感性相当小,通过分块估计还可以进一步降低运算量。     

Optimum pilot pattern for channel estimation in OFDM systems

The performance of channel estimation in orthogonal frequency division multiplexing (OFDM) systems significantly depends on the pilot signal, which is usually scattered in time and frequency domains. For a given pilot density, the authors optimally design the pilot pattern so as to minimize the mean squared error (MSE) of the channel estimate with the use of a general interpolator. The analytic results are verified by computer simulation.     

一种有效降低OFDM系统信道估计复杂度的方法

在应用于OFDM系统的信道估计方法中,维纳滤波是一种估计性能较好的算法,但是该方法计算复杂度大,不利于实现.本文基于块状导频方式,提出了选取相关子载波的相关系数法来简化维纳滤波的信道估计方法,该方法在估计性能损失不大的情况下,有效地降低了计算复杂度.文中在慢时变信道环境下对该方法进行了性能仿真,并对仿真结果进行了讨论.     

大气激光通信下的OFDM导频信道估计方法

针对大气激光通信下的OFDM(正交频分复用)系统,对几种基于导频的信道估计方法进行了比较分析,并给出了仿真结果。仿真结果表明,SVD-LMMSE(基于奇异值分解的线性最小均方误差)算法在提高信道估计精度的同时,可以降低系统计算的复杂度;增大CP(循环前缀)可以非常有效地提高系统性能。     

Joint data detection and channel estimation for OFDM systems

We develop new blind and semi-blind data detectors and channel estimators for orthogonal frequency-division multiplexing (OFDM) systems. Our data detectors require minimizing a complex, integer quadratic form in the data vector. The semi-blind detector uses both channel correlation and noise variance. The quadratic for the blind detector suffers from rank deficiency; for this, we give a low-complexity solution. Avoiding a computationally prohibitive exhaustive search, we solve our data detectors using sphere decoding (SD) and V-BLAST and provide simple adaptations of the SD algorithm. We consider how the blind detector performs under mismatch, generalize the basic data detectors to nonunitary constellations, and extend them to systems with pilots and virtual carriers. Simulations show that our data detectors perform well.     

Improved channel estimation with decision feedback for OFDM systems

An improved channel estimation strategy for a continuous orthogonal frequency division multiplexing transmission scheme is presented. Based on feedback from decoding decisions of forward error correction of the information data. No special redundancy such as pilot tones is required     

IMM-based Kalman filter for channel estimation in UWB OFDM systems

Ultra-wide band (UWB) communication is one of the most promising technologies for high-data rate wireless networks for short-range applications. This paper proposes a blind channel estimation method namely Interactive Multiple Model (IMM)-based Kalman algorithm for UWB OFDM systems. IMM-based Kalman filter is proposed to estimate frequency selective time-varying channel. In the proposed method, two Kalman filters are concurrently estimating channel parameters. The first Kalman filter, namely the Static Model Filter (SMF) gives an accurate result when the user is static while the second Kalman filter namely the Dynamic Model Filter (DMF) gives an accurate result when the receiver is in moving state. The static transition matrix in SMF is assumed as an Identity matrix where as in DMF, it is computed using Yule–Walker equations. The resultant filter estimate is computed as a weighted sum of individual filter estimates. The proposed method is compared with other existing channel estimation methods.     

Linear precoding assisted blind channel estimation for OFDM systems

We propose a novel precoding approach for single transmit/receiver antenna orthogonal frequency-division multiplexing (OFDM) systems that enables blind channel estimation. A nonredundant nonunitary linear precoder is applied on each pair of blocks before they enter the OFDM system. The structure induced to the transmitted blocks allows for blind channel estimation at the receiver based on simple cross-correlation operations. At the same time, the multipath diversity of the system is increased. An optimal precoding scheme is pursued that, for quadrature phase-shift keying signals results in minimum channel error and asymptotically minimum bit error rate. Analytic performance evaluation of the proposed approach, and Cramer-Rao bound for the proposed channel estimate, are presented.     

Finite-alphabet based channel estimation for OFDM and relatedmulticarrier systems

Novel blind channel estimators based on the finite alphabet property of information symbols are derived in this paper for OFDM and related multicarrier code-division multiple access (MC-CDMA) systems. The resulting algorithms are applicable not only to standard OFDM transmitters with cyclic prefix, but also to the zero padded OFDM transmissions that improve symbol recovery at the expense of altering the transmitter and complicating the equalizer. Based on FFT-processed received data, channel identifiability is guaranteed regardless of channel zero locations and various channel estimation algorithms become available by trading on the complexity for performance. Unlike existing blind channel estimators, the proposed alternatives require short data records especially for PSK transmissions. The inherent scalar ambiguity is easily resolved because it has unit amplitude and phase values drawn from a finite set. Decoupling channel from symbol estimation enables a phase-directed operation that improves upon decision-directed schemes that are known to suffer from error propagation. Practical issues are also addressed including the presence of frequency guard intervals, constellation and power loading, various frame designs, coded transmissions as well as semi-blind and online implementations for systems with training sequences. The algorithms are tested with simulations and also compared with existing alternatives in a realistic HIPERLAN/2 setting     

Totally blind APP channel estimation for mobile OFDM systems

A new two-dimensional blind channel estimation scheme for coherent detection of orthogonal frequency-division multiplexing (OFDM) signals in a mobile environment is presented. The channel estimation is based on the a posteriori probability (APP) calculation algorithm. The time-variant channel transfer function is completely recovered without phase ambiguity with no need for any pilot or reference symbols, thus maximizing the spectral efficiency of the underlying OFDM system. The phase ambiguity problem is solved by using a 4-QAM (quadrature amplitude modulation) scheme with asymmetrical arrangement. The results clearly indicate that totally blind channel estimation is possible for virtually any realistic time-variant mobile channel.     

Simplified channel estimation for OFDM systems with multipletransmit antennas

Multiple transmit-and-receive antennas can be used in orthogonal frequency division multiplexing (OFDM) systems to improve communication quality and capacity. In this paper, we present two techniques to improve the performance and reduce the complexity of channel parameter estimation: optimum training-sequence design and simplified channel estimation. The optimal training sequences not only simplify the initial channel estimation, but also attain the best estimation performance. The simplified channel estimation significantly reduces the complexity of the channel estimation at the expense of a negligible performance degradation. The effectiveness of the new techniques is demonstrated through the simulation of an OFDM system with two-transmit and two-receive antennas. The space-time coding with 240 information bits per codeword is used for transmit diversity. From the simulation, the required signal-to-noise ratio is only about 9 dB for a 10% word error rate for a channel with the typical urban- or hilly-terrain delay profile and a 40-Hz Doppler frequency