Channel Estimation For OFDM Systems Research Articles

Fast Time-Varying Channel Estimation for OFDM Systems With Narrowband Interference

The problem of channel estimation in OFDM with narrowband interference (NBI) has been only investigated for slow time-varying channels. For the first time, a novel estimation framework is proposed for fast time-varying channels. The noise variances and channel are jointly estimated via the maximum a posteriori (MAP) expectation maximization (EM) algorithm. By considering in the EM formulation the channel as the unwanted parameter and the noise variances as the parameters to be estimated, a closed-form analytical expression for the estimation of the noise variances is obtained; the channel being given by the Kalman smoother. Mean square error (MSE) and bit error rate (BER) simulations confirm the effectiveness and robustness of the approach against mobility and its adequacy for practical interference scenarios.

Estimation of Sparse Time Dispersive Channels in Pilot Aided OFDM Using Atomic Norm

We propose the use of the atomic norm minimization for the estimation of sparse time dispersive channels. The proposed estimation approach combines the atomic norm minimization, a super resolution method and the least squares (LS) method, and is intended for pilot aided channel estimation in OFDM systems. The combination of the atomic norm minimization and the super resolution method allows for gridless estimation of arbitrary delays of the individual paths in the channel impulse response, and the gains of those paths are then estimated using the LS method. To further improve the performance we also propose a version of the approach with reweighted atomic norm minimization. We compare the performance of the proposed approach with other grid based approaches for sparse channel estimation.

On the use of tracking loops for low-complexity multi-path channel estimation in OFDM systems

This paper treats pilot aided multi-path channel estimation with tracking loops for OFDM systems under slow to moderate fading conditions. Recent works have presented theoretical results for the tuning of second-order and third-order tracking loops in the particular context of Jakes׳s Doppler spectrum channel. The method for getting the loop coefficients resorted either to the use of a given constraint, which made the obtained coefficients sub-optimal, or was obtained in part by simulations. Here, we perform a global optimization of the coefficients without constraints to get the optimal coefficients, and analytical formulas are provided. One remarkable result of this optimization is that only the natural frequency depends on the transmission parameters, i.e., the channel Doppler spectrum, the power delay profile, and the noise variance. Consequently, only one parameter has to be tuned. Moreover, asymptotic performance is formulated in a more general way as a function of the 2rth moments of the Doppler spectrum (r is the loop order). Hence, all our derivations are usable for any Doppler spectrum and are not specific to Jakes׳s Doppler spectrum. A complete table sums up for the three orders of the theoretical results of the optimal coefficients together with the asymptotic performance. The performance is also compared with that of the asymptotic Kalman filter.

An eigen‐select denoising threshold for channel estimation in OFDM systems

SummaryIn reliable orthogonal frequency division multiplexing systems, channel estimator (CE) plays a crucial role. A pilot‐aided orthogonal frequency division multiplexing CE often employs denoising thresholds to identify significant and noisy channel impulse response taps. When the knowledge of channel statistics (KCS) is completely unavailable, the existing suboptimal thresholds require consistent estimation of one or more KCS parameters. In this paper, the effect of noise variance parameter estimation on CE's mean square error (MSE) performance is characterized analytically by computing threshold's tap‐detection probability. To mitigate MSE degradation due to KCS estimation, an eigen‐select threshold, which does not require any KCS estimation, is proposed. It utilizes eigen values of the auto‐covariance matrix formed by estimated channel impulse response. The proposed threshold is compared with existing state‐of‐art alternatives, for CE's MSE performance and system's BER performance, in correlated Rayleigh fading channel environments. Results reveal that, with the proposed threshold, performance plots converge to optimal solution more closely than the others. Copyright © 2015 John Wiley & Sons, Ltd.

다중 사용자 다이버시티를 이용한 OFDM 시스템의 성능 분석

Recently, multiuser diversity(MUD) technique is used to improve the performance of OFDM transmission systems. With STBC antenna diversity technique, the performance of OFDM systems can be also improved. Pilot symbol is used for the channel estimation in OFDM systems. However, imperfect channel estimates in this systems degrade the performance. The performance of MUD STBC OFDM systems using pilot symbol, gauged by the bit error rate, is analyzed considering the channel estimation error. Result shows that MUD can improve the required power by a factor of approximately 5 dB for users number 32.

Joint carrier frequency offset and channel estimation for OFDM systems with two receive antennas

SummaryCarrier frequency offset (CFO) in OFDM systems results in loss of channel orthogonality and hence degrades the system's performance. In this paper, we propose a new method for joint CFO/channel estimation for OFDM systems with two receive antennas. Our method avoids the complexity of full search methods in one or two dimensions. Using one training OFDM symbol and utilizing the knowledge of the structure of the inter‐channel interference that results from CFO, we develop a two‐stage estimation procedure. The first stage derives an initial CFO/channel estimate based on a one shot minimization step. The second stage refines this joint estimate by conducting a small CFO search in the vicinity of the initial estimate. This procedure provides CFO estimates over a full range of −N/2–N/2, (N: number of subcarriers), as well as the channel estimates. Computer simulations show an excellent performance that is very close to the Cramer–Rao lower bound and superior to some existing methods. The effect of antenna correlation on performance is also investigated through computer simulations, showing small performance degradation even at medium correlation coefficients (0.3–0.6). Copyright © 2015 John Wiley & Sons, Ltd.

A novel method on pilot selection for sparse channel estimation in OFDM systems

A novel method on pilot selection for sparse channel estimation in OFDM systems

Design and Analysis of an OFDM-based Differential Chaos Shift Keying Communication System

In this paper, a new non-coherent chaos-based communication scheme, named orthogonal frequency division multiplexing based different chaos shift keying (OFDM-DCSK), is presented inspired by multi-carrier direct sequence code- division multiple access (MC-DS-CDMA) system, multi-carrier DCSK (MC-DCSK) system, and channel estimation of OFDM system. In this scheme, all of the occupied subcarriers are grouped into several groups. In each group, one chaotic reference is transmitted over the central subcarrier, while multiple modulated data streams are transmitted over the remaining subcarriers. Therefore, this transmitter structure increases the spectral efficiency, compared with the conventional DCSK system. At the receiver side, no radio frequency delay circuits are needed to demodulate the received data, which makes the system easy to implement in practice. The bit error rate (BER) performance of the OFDM-DCSK system is derived based on Gaussian approximation under additive white Gaussian noise (AWGN) channel. Simulation results are given under AWGN and two-path Rayleigh flat fading channel, which confirm the effectiveness of this new design.

Joint Pilot Power & Pattern Design for Compressive OFDM Channel Estimation

This letter investigates the deterministic design of pilot power and pattern for sparse channel estimation in OFDM systems based on minimizing the coherence of the DFT sub-matrix. It has been suggested that the pilot pattern forming a cyclic difference set (CDS) or almost difference set (ADS) is optimal. So, we proposed a deterministic procedure that jointly optimized for pattern and power of pilots as a solution. First, pilot patterns forming CDS/ADS were gathered through a search. Then, the power was numerically allocated to the different pilots from all the patterns. Finally, the pilot pattern and power pair leading to minimum coherence was selected from the available pairs. Simulation results demonstrated that the proposed scheme outperformed the existing methods in terms of efficiency.

Adaptive Joint Semi-blind Estimation of CFO and Channel for OFDM Systems

The large number of subcarriers, used in OFDM systems, increases the sensivity to frequency offset which results from a Doppler shift, due to mobile movement, as well as from a mismatch between the carrier frequencies at the transmitter and receiver. The problem of channel estimation in a practical OFDM receiver that suffers from CFO has been addressed in this paper. We first derived the modified MUSIC based blind CFO estimator. Next, we propose new joint training-based and joint semi-blind CFO and channel estimation for OFDM systems. Whereas the training-based estimator only assumes knowledge of pilot tones, the semi-blind estimator assumes knowledge of the virtual subcarriers (VSCs). For both estimators we derive a maximum likelihood (ML) algorithm for block processing and LMS algorithm for adaptive processing. It is demonstrated that the CFO estimators are able to perform well with a very small number of OFDM blocks. The performance of our estimators was investigated by simulations.

An Improved DFT-Based Channel Estimation Algorithm for OFDM System on Time-Varying Multipath Fading Channels

An improved discrete Fourier transform DFT-based channel estimation for OFDM systems is proposed. Conventional DFT-based channel estimator improved its performance by suppressing noise, but it does not completely suppress noise. In order to overcome the disadvantage, this paper proposed a gate method without requiring any channel statistical information. This method comprehensively considersthe effects of the power of the strongest path and the noises. Computer simulation demonstrates the performance of the proposed algorithms in terms of bit error rate.

Compressive Sensing Based Channel Estimation for OFDM Systems Under Long Delay Channels

Time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) has advantages in spectral efficiency and synchronization. However, its iterative interference cancellation algorithm will suffer from performance loss especially under severely fading channels with long delays and has difficulty supporting high-order modulations like 256 QAM, which may not accommodate the emerging ultra-high definition television service. To solve this problem, a channel estimation method for OFDM under the framework of compressive sensing (CS) is proposed in this paper. Firstly, by exploiting the signal structure of recently proposed time-frequency training OFDM scheme, the auxiliary channel information is obtained. Secondly, we propose the auxiliary information based subspace pursuit (A-SP) algorithm to utilize a very small amount of frequency-domain pilots embedded in the OFDM block for the exact channel estimation. Moreover, the obtained auxiliary channel information is adopted to reduce the complexity of the classical SP algorithm. Simulation results demonstrate that the CS-based OFDM outperforms the conventional dual pseudo noise padded OFDM and CS-based TDS-OFDM schemes in both static and mobile environments, especially when the channel length is close to or even larger than the guard interval length, where the conventional schemes fail to work completely.

DFT-based Cluster Analysis Channel Estimation Algorithm for OFDM Systems on Multipath Channels

In this paper, a enhanced DFT-based channel estimation for OFDM systems is proposed. Conventional DFT-based channel estimations improve the performance by suppressing time domain noise beyond the length of the cyclic prefix (CP), so it does not completely suppress the noise. However, they potentially require information on channel impulse responses and may also result in mean-square error (MSE) floor due to incorrect channel information such as channel delay spread. In order to overcome the disadvantage, our proposed channel estimation can improve the performance by deciding significant channel taps adaptively. Significant channel taps are detected on the basis of cluster discriminant analysis. Simulation results demonstrate that the proposed algorithm outperforms the conventional DFT-based estimation in terms of BER and MSE performance.

Complex Amplitudes Tracking Loop for multipath channel estimation in OFDM systems under slow to moderate fading

This paper deals with multipath channel estimation for Orthogonal Frequency-Division Multiplexing systems under slow to moderate fading conditions. Most of the conventional methods exploit only the frequency-domain correlation by estimating the channel at pilot frequencies, and then interpolating the channel frequency response. More advanced algorithms exploit in addition the time-domain correlation, by employing Kalman filters based on the approximation of the time-varying channel. Adopting a parametric approach and assuming a primary acquisition of the path delays, channel estimators have to track the complex amplitudes of the paths. In this perspective, we propose a less complex algorithm than the Kalman methods, inspired by second-order Phase-Locked Loops. An error signal is created from the pilot-aided Least-Squares estimates of the complex amplitudes, and is integrated by the loop to carry out the final estimates. We derive closed-form expressions of the mean squared error of the algorithm and of the optimal loop coefficients versus the channel state, assuming a Rayleigh channel with Jakes' Doppler spectrum. The efficiency of our reduced complexity algorithm is demonstrated, with an asymptotic mean squared error lower than the first-order auto-regressive Kalman filters reported in the literature, and almost the same as a second-order Kalman-based algorithm.

Time domain leakage in DFT‐based channel estimation for OFDM systems with guard bands

SUMMARYIn this paper, a mathematical analysis of the time domain leakage problem in the discrete Fourier transform (DFT) based channel estimation technique is presented. The time domain leakage is because of the absence of pilots in the guard band. Several solutions to this problem were previously proposed based on reducing the leakage in the frequency domain. These solutions significantly increase the receiver complexity. In this paper, the root cause of the leakage problem is linked to the time domain and a modification is proposed carrying slight additional complexity over the conventional DFT‐based estimator. Performance evaluation is assessed in terms of the bit error rate and the mean square error. The proposed modification significantly decreases the error floor of the conventional DFT‐based channel estimation technique. In addition, its performance is comparable to other more complex techniques recently proposed.Copyright © 2013 John Wiley & Sons, Ltd.

An Improved Signal Detection Technique for OFDM System

In this paper a signal detection technique based on pilots which are transmitted for channel estimation in OFDM system is proposed in AWGN channel. We analyse the algorithm based on pilots and derive an improved signal detection technique. The performance is compared in terms of detection probability and ROC curves are given. The simulation results show that the improved detection technique whose computational complexity is not high can increase the precision of the detection probability at low SNR.

컨볼루션 코딩 OFDM 시스템의 성능 분석

OFDM technique uses multiple sub-carriers for the data transmission. Therefore, bit error rate increases because of inter-carrier interference caused by nonlinear high power amplifier and carrier frequency offset. Wireless OFDM transmission over multi path fading channels is characterized by small transmission gain in multiple sub-carrier frequency interval. Therefore bit error rate increases because of burst errors. Inter-leaver and convolution error control coding are effective for the reduction of this burst error. Pilot symbol is used for the channel estimation in OFDM systems. However, imperfect channel estimates in this systems degrade the performance. The performance of this convolution coding OFDM systems using inter-leaver, gauged by the bit error rate, is analyzed considering the nonlinear high power amplifier, carrier frequency offset and channel estimation error.

Channel Estimation of OFDM systems in Slow and Fast Fading Channels

Channel Estimation of OFDM systems in Slow and Fast Fading Channels

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.

A Study of Deterministic Pilot Allocation for Sparse Channel Estimation in OFDM Systems

In this letter, we investigate the deterministic pilot allocation for sparse channel estimation in OFDM systems. Based on the rule of minimizing the coherence of the DFT submatrix, we derive that the pilot design according to the cyclic different set (CDS) is optimal. However, the CDS only exists for some specific number of OFDM subcarriers. For those cases where the CDS is unavailable, we propose a scheme using discrete stochastic approximation to obtain a near-optimal pilot pattern. Simulation results demonstrate that our scheme is much faster convergent and more efficient than the exhaustive search; and it has been shown that substantial improvement for channel estimation can be achieved.