Proposed Channel Estimation Research Articles

MC‐CDMA MIMO systems with quasi‐orthogonal space–time block codes: Channel estimation and multiuser detection

SUMMARYThis paper investigates blind channel estimation and multiuser detection for quasi‐synchronous multi‐carrier code‐division multiple‐access (MC‐CDMA) multiple‐input multiple‐output (MIMO) systems with quasi‐orthogonal space–time block codes (QO‐STBC). Subspace‐based blind channel estimation is proposed by considering a QO‐STBC scheme that involves four transmit antennas and multiple receive antennas. Based on the first‐order perturbation theory, the mean square error of the channel estimation is derived. With the estimated channel coefficients, we employ minimum output energy and eigenspace receivers for symbol detection. Using the QO‐STBC coding property, the weight analyses are performed to reduce the computational complexity of the system. In addition, the forward–backward averaging technique is presented to enhance the performance of multiuser detection. Numerical simulations are given to demonstrate the superiority of the proposed channel estimation methods and symbol detection techniques. Copyright © 2011 John Wiley & Sons, Ltd.

Channel Estimation for OFDM System with Regularized Total Least-Squares Solution

Orthogonal frequency-division multiplexing (OFDM) combines the advantages of high performance and relatively low implementation complexity. However, for reliable coherent detection of the input signal, the OFDM receiver needs accurate channel information. The Doppler shift of fast-fading channels will generate inter-carrier interference (ICI) and, hence, degrade the performance of orthogonal frequency-division multiplexing (OFDM) systems. In this paper, we present regularized total least-squares (RTLS) scheme to eliminate the ICI and noise. A closed-form mathematical expression has been derived to express the channel estimation. It has been shown that the proposed channel estimation and data detect can effectively eliminate the ICI effect.

PCA Application in Channel Estimation in MIMO-OFDM System

Initial estimation is a considerable issue in channel estimation techniques, since all of the following processes depends on it, which in this paper its improvement is discussed. Least Square (LS) method is a common simple way to estimate a channel initially but its efficiency is not as significant as more complex approaches. It is possible to enhance channel estimation performance by using some methods such as principal component analysis (PCA), which is not prevalent in channel estimation, and its adaptation to channel information can be challenging. PCA method improves initial estimation performance by projecting data onto direction of eigenvectors by means of using simple algebra. In this paper, channel estimation is examined in Multiple Input Multiple Output-Orthogonal Frequency Division Multiplexing (MIMO-OFDM) system, with significant advantages such as an acceptable performance in frequency selective fading channel. Moreover the proposed channel estimation method manipulates the benefits of MIMO channel by using the information, gained by all channels to estimate the information of each receiver.

Low-Complexity Channel Estimation Using Short-Point FFT/IFFT for an Iterative Receiver in an LDPC-Coded OFDM System

A frequency-domain channel expanding technique (FD-CET) is proposed for channel estimation in an iterative receiver of a low-density parity-check coded orthogonal frequency division multiplexing system. The FD-CET can expand a subset of the frequency-domain channel gains to all of them in a short-point FFT/IFFT format. This technique is applied to estimate the channel gains utilizing the pilots and information from the decoder in an iterative manner. It is a novel low complexity channel estimation method. Numerical results demonstrate that the proposed channel estimator approaches a maximum-likelihood performance within only a few iterations under multi-path channels.

Estimation of highly selective channels for downlink LTE systemby a robust neural network

In this paper we propose a robust channel estimator for the downlink of a Long Term Evolution (LTE) system using a highly selective neural network. This method uses the information provided by the reference signals to estimate the total frequency response of the channel in two phases. In the first phase, the proposed method learns to adapt to the channel variations, and in the second phase it predicts the channel parameters. The performance of the estimation method in terms of complexity and quality is confirmed by theoretical analysis and simulations in an LTE/OFDMA (Orthogonal Frequency-Division Multiple Access) transmission system. The performance of the proposed channel estimator is compared with those of least-square decision feedback and modified Wiener methods. The simulation results show that the proposed estimator performs better than the above estimators and it is more robust at high-speed mobility.

Noniterative Joint Frequency Offset and Channel Estimation for MIMO OFDM Systems Using Cascaded Orthogonal Pilots

In this paper, we reconsider the issues of frequency offset and channel estimation in multiple-input-multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) systems. We first design “cascaded orthogonal” pilot symbols and adopt the properties of Fourier transform and Hadamard product to deliver a closed-form estimate of frequency offset; meanwhile, another closed-form estimate of channel under minimum mean square error (MSE) criterion is presented. A tight lower bound for the MSE of the channel is also derived. The proposed channel estimator is robust to the residual frequency offset, whereas the frequency offset estimator is nearly independent of the unknown channel vector and is also robust to channel variations. Simulation results show that the proposed method greatly reduces the MSE of the channel in the moderate-to-high value of signal-to-noise ratio (SNR), even at a large normalized (by subcarrier spacing) frequency offset |ν| = 0.5. Compared with the optimal pilot design without frequency offset, our solution performs only slightly worse for low values of frequency offset (|ν| ≤ 0.01), but the difference is insignificant. The computational complexity of the proposed scheme is also much lower than that of the conventional iterative means.

Appropriate Algorithms for Estimating Frequency-Selective Rician Fading MIMO Channels and Channel Rice Factor: Substantial Benefits of Rician Model and Estimator Tradeoffs

The training-based channel estimation (TBCE) scheme in multiple-input multiple-output (MIMO) frequency-selective Rician fading channels is investigated. We propose the new technique of shifted scaled least squares (SSLS) and the minimum mean square error (MMSE) estimator that are suitable to estimate the above-mentioned channel model. Analytical results show that the proposed estimators achieve much better minimum possible Bayesian Cramér-Rao lower bounds (CRLBs) in the frequency-selective Rician MIMO channels compared with those of Rayleigh one. It is seen that the SSLS channel estimator requires less knowledge about the channel and/or has better performance than the conventional least squares (LS) and MMSE estimators. Simulation results confirm the superiority of the proposed channel estimators. Finally, to estimate the channel Rice factor, an algorithm is proposed, and its efficiency is verified using the result in the SSLS and MMSE channel estimators.

Simulation Performance of WAVE System with Combined DD-CE and LMMSE Smoothing Scheme in Small-Scale Fading Models

This paper investigates the performance of IEEE 802.11p wireless access in vehicular environments (WAVE) system in small-scale fading models reported by Georgia Institute of Technology (Georgia Tech). We redesign the small-scale fading models to be applied to the computer simulation and develop the IEEE 802.11p WAVE physical layer simulator to provide the bit error rate and packet error rate performances. Moreover, a new channel estimator using decision directed channel estimation and linear minimum mean square error smoothing is proposed in order to improve the performance of the conventional least square channel estimator using two identical long training symbols. The simulation results are satisfactorily coincident with the scenarios of Georgia Tech report, and the proposed channel estimator significantly outperforms the conventional channel estimator.

A Robust Channel Estimator for High-Mobility STBC-OFDM Systems

In this paper, a robust channel estimator for high- mobility space-time block code-orthogonal frequency division multiplexing (STBC-OFDM) systems is proposed and applied in IEEE 802.16e systems. A high-performance two-stage channel estimation method is adopted. The proposed architecture reduces computational complexity effectively and improves 85.2% of the hardware implementation. The performances of the proposed design have been demonstrated through the simulation of an STBC-OFDM system with two transmit antennas and one receive antenna. At the vehicle speed of 120 and 240 km/hr for quadrature phase shift keying (QPSK) modulation, the proposed design can achieve the bit-error rate (BER) of about 10-4 and 10-3 without using channel coding. Moreover, it has significant performance improvement as compared with interpolation-based channel estimation methods. The proposed channel estimator implemented in 90 nm CMOS technology can support up to 29.03 Mbps (uncoded) downlink data transmission. The design only requires 859.6 K gates and dissipates 43.71 mW at 83.3 MHz operating frequency with 1 V power supply.

Channel Estimation and Data Detection for OFDM Systems Over Fast-Fading and Dispersive Channels

The Doppler shift of fast-fading channels will generate intercarrier interference (ICI) and, hence, degrade the performance of orthogonal frequency-division multiplexing (OFDM) systems. In this paper, a novel channel-estimation and data-detection method for OFDM systems over the fast-fading dispersive channels has been proposed and investigated. A new pilot pattern is also proposed, and it is composed of a comb-type pilot pattern and a grouped pilot pattern. The required number of pilot subcarriers is, therefore, significantly reduced compared with that of using the grouped pilot pattern only. We present the MSE analysis of the proposed channel estimation based on the proposed pilot pattern and thoroughly investigate the MSE performance of channel estimation by a numerical method. A closed-form mathematical expression has been derived to express the MSE of the proposed channel estimation as a function of the Doppler shift, the number of resolvable paths of the channel, the polynomial order, and so on. Both computer simulation and numerical results show that the proposed channel estimation and data detection based on the proposed pilot pattern have effectively eliminated the ICI in terms of the MSE of the channel estimation and bit-error-rate (BER) performance, whereas the required pilot number per OFDM symbol is reduced by about 92.3% in comparison with that of using the grouped and equispaced pilot pattern.

A Low-Complexity and Efficient Channel Estimator for Multiband OFDM-UWB Systems

This paper proposes an efficient channel-estimation scheme for multiband (MB) orthogonal frequency-division multiplexing (OFDM)-based ultrawideband (UWB) communication systems and, more specifically, for practical implementation of low-cost, high-speed, UWB-based wireless universal serial bus (USB) devices. The proposed channel estimator consists of two stages. The first stage employs a simple least-squares (LS) method together with a frequency-domain smoothing operation that estimates the channel using the available training sequence. The second stage uses this channel estimate to detect the frame header and then refines the channel estimate by using a decision-directed technique. The mean-squared error performance and computational complexity of the proposed scheme are analyzed. Numerical examples show that the proposed scheme substantially outperforms the conventional LS approach and that it performs comparably to the maximum-likelihood estimator, under various highly noisy multipath channel conditions.

Channel estimation in bit-interleaved coded modulation with iterative decoding

This study improves the fading channel estimation in bit-interleaved coded modulation systems with iterative decoding (BICM-ID) and signal space diversity (SSD) by embedding a training sequence. Existing training schemes work well at high signal-to-noise ratio (SNR) or slowly time-varying channels whereas the applications of BICM-ID are beneficial at low SNR and fast time-varying fading channels. Motivated by the power/bandwidth efficiency of the SSD technique and the fact that superimposed training outperforms pilot symbol-assisted modulation (PSAM) training over relatively fast time-varying channels, a new superimposed training sequence is explored. The proposed training sequence inserts pilot bits into the coded bits prior constellation mapping and signal rotation. This becomes a superimposed training sequence in the rotated symbols and helps the estimator to track fast variation of the channel gains. A soft iterative channel estimator is developed to work with the superimposed training sequence. The performance of the proposed scheme, namely SSD-pilot, is shown to be superior to PSAM scheme. To gauge the performance improvement achieved with the proposed channel estimation, an analytical bound on the asymptotic bit error probability for BICM-ID using SSD over correlated fading channels is provided. The Cramer-Rao bound on the mean-square error of the channel estimator is also derived to evaluate the performance of the iterative channel estimator.

Doppler Spread Mitigation Using Harmonic Transform for Wireless OFDM Systems in Mobile Communications

In wireless OFDM systems, the system performance is suffered from frequency offset and symbol timing offset due to the Doppler effect. Using the discrete Fourier transform (DFT) and inverse discrete Fourier transform (IDFT) for traditional signal transformation from the time-domain into frequency-domain, and vice versa, the system performance may be severely degraded. To make the OFDM system that can tolerate the above problems, we have considered that the harmonic transform can be applicable to the traditional signal transformation, thereby improving the system performance. In this paper, we combine the good characteristics of harmonic transform and instantaneous frequency to be a novel transformation for wireless OFDM systems. We propose a modified discrete harmonic transform (MDHT) which can be performed adaptively. Our proposed scheme called the modified discrete harmonic transform OFDM (MDHT-OFDM scheme). We derive the equations of the novel discrete harmonic transform which are suitable for wireless OFDM systems and the novel channel estimation cooperated with the novel transformation. The proposed channel estimation is performed in both time-domain and frequency-domain. The performance of a MDHT-OFDM scheme is evaluated by means of a simulation. We compare the performance of a MDHT-OFDM scheme with one of the conventional DFT-OFDM scheme in the term of symbol error rate (SER). MDHT-OFDM scheme can achieve better performance than that of the conventional DFT-OFDM scheme in mitigating the Doppler spread.

Pilot-Aided Channel Estimation for WiMAX 802.16e Downlink Partial Usage of Subchannel System Using Least Squares Line Fitting

This paper presents a pilot-aided channel estimation method which is particularly suitable for mobile WiMAX 802.16e Downlink Partial Usage of Subchannel mode. Based on this mode, several commonly used channel estimation methods are studied and the method of least squares line fitting is proposed. As data of users are distributed onto permuted clusters of subcarriers in the transmitted OFDMA symbol, the proposed channel estimation method utilizes these advantages to provide better performance than conventional approaches while offering remarkably low complexity in practical implementation. Simulation results with different ITU-channels for mobile environments show that depending on situations, enhancement of 5dB or more in term of SNR can be achieved.

Interference Rejection for Parametric Channel Estimation in Reuse-1 Cellular OFDM Systems

This paper proposes a joint channel-estimation method for the desired and interfering channels for cell edge users in reuse-1 orthogonal frequency-division multiplexing (OFDM) cellular systems. The following assumptions are made in proposing the algorithm: 1) the desired and interferer's channel multipath delays do not overlap; 2) the same pilot sequence is sent from the desired and interfering base stations (BSs); and 3) reuse-3 preamble symbols, as in the preamble structure in IEEE 802.16d/e (Wimax) systems, are used to obtain initial channel estimates without interference. If we make these assumptions, then it is possible to estimate the desired and interfering channels, even with reuse-1 pilots. The proposed pilot-based channel estimation technique exploits the delay subspace structure to reduce the impact of cochannel interference (CCI) on channel estimation. Delay subspace refers to the set of basis vectors spanning the frequency response of the desired and interfering multipath channels. This enables us to jointly estimate and track the desired and interfering channels with a lower mean-squared error (MSE), when compared with the conventional modified least-squares (mLS) technique, which ignores the structure of interference. The significance of accurate channel estimates in symbol detection schemes is demonstrated for systems employing two or more receiver antennas. The proposed channel estimator significantly improves the performance of symbol-detection schemes based on either interference nulling combiner (INC) or minimum MSE diversity combiner (MMSE-DC), when compared with detection schemes using mLS-based channel estimates. Analytical expressions are derived for the MSE of the estimated multipath delays. Simulation results are also provided to show the improved performance for the proposed channel-estimation method compared with the mLS-based channel estimation method, when used in conjunction with INC and MMSE-DC detectors.

A joint channel estimation and data detection receiver for multiuser MIMO IFDMA systems

In this paper, we are interested in the problem of joint channel estimation and data detection for multi-input multi-output (MIMO) interleaved frequency division multiple access (IFDMA) systems. Although IFDMA is free from the multiple access interference (MAI), it suffers from intersymbol interference (ISI). MIMO-IFDMA system suffers from both ISI and multi-stream interference (MSI). The block iterative generalized decision feedback equalizer (BI-GDFE) is an iterative and effective interference cancelation scheme which could provide near maximum likelihood (ML) performance with very low complexity. However, BI-GDFE needs the channel state information (CSI). In this paper, we utilize the soft estimates of the transmitted symbols provided by the BI-GDFE to estimate the channel via an Expectation Maximization (EM)-based algorithm. By doing so, a joint channel estimation and data detection receiver is developed. To evaluate the performance of the proposed channel estimation algorithm, we derive the Cramér-Rao Lower Bound (CRLB). Computer simulations show that the bit error rate (BER) performance of the proposed joint channel estimation and signal detection receiver can reach the performance of the BI-GDFE with perfect CSI.

퍼지 추론법을 적용한 OFDM 시스템의 LS(Least Square) 채널추정 기법

본 논문에서는 최근 여러 분야에서 불확실성에 대한 예측을 위해 사용되는 Fuzzy 추론법을 OFDM(Othgonal Frequency Division Multiplexing)의 채널추정 방식에 적용함으로써 향상된 성능과 낮은 복잡도를 갖는 새로운 채널추정 방식을 제안하였다. 제안된 방식은 LS(Least Square) 채널추정 이전에 Pilot을 이용하여 Fuzzy추론법에 의하여 채널의 통계적 특성을 계산하고 이에 대한 보간을 해 줌으로써 채널추정 성능을 향상시키는 방식이다. QPSK(Quadrature Phase Shift Keying)를 적용한 시뮬레이션 결과 제안된 채널추정 방식은 MSE(Mean Square Error)가 <TEX>$10^{-3}$</TEX>인 지점에서 MMSE(Mimimum Mean Square Error) 채널추정 방식보다는 약 1.3 dB 정도 성능이 열화되는 것으로 나타났지만 LS 채널 추정 방식과 비교하면 약 5.5 dB 정도의 성능 이득이 있는 것으로 분석되었으며, SER(Symbol Error Rate)은 SNR(Signal to Noise Ratio)이 20 dB인 지점에서 MMSE 채널추정 방식과 제안된 채널추정방식이 각각 <TEX>$10^{-1.96}$</TEX>, <TEX>$10^{-1.93}$</TEX> 정도로 유사한 성능을 보이는 것으로 분석되었고, LS 채널추정 방식 보다 약 <TEX>$10^{-0.35}$</TEX> 정도 제안된 방식의 성능이 향상된 것으로 분석되었다. In this paper, the new channel estimation was proposed that have the low complexity and high performance using Fuzzy inference method uses recently from various field for estimation about uncertainty in channel estimation of OFDM. Proposed method is channel estimation performance improve, calculation and interpolation for statistics character of channel using the pilot before LS channel estimation by Fuzzy inference method. Simulation result in QPSK proposed channel estimation method shows the enhancement of 5.5dB compared to the LS channel estimation and the deterioration of 1.3dB compared to the MMSE channel estimation in mean square error point <TEX>$10^{-3}$</TEX>. symbol error rate shows similarity performance the MMSE <TEX>$10^{-1.96}$</TEX>, proposed channel estimation <TEX>$10^{-1.93}$</TEX> and enhancement of <TEX>$10^{-0.35}$</TEX> compared to the LS channel estimation in signal to noise ratio point 20dB.

A Fast LMMSE Channel Estimation Method for OFDM Systems

A fast linear minimum mean square error (LMMSE) channel estimation method has been proposed for Orthogonal Frequency Division Multiplexing (OFDM) systems. In comparison with the conventional LMMSE channel estimation, the proposed channel estimation method does not require the statistic knowledge of the channel in advance and avoids the inverse operation of a large dimension matrix by using the fast Fourier transform (FFT) operation. Therefore, the computational complexity can be reduced significantly. The normalized mean square errors (NMSEs) of the proposed method and the conventional LMMSE estimation have been derived. Numerical results show that the NMSE of the proposed method is very close to that of the conventional LMMSE method, which is also verified by computer simulation. In addition, computer simulation shows that the performance of the proposed method is almost the same with that of the conventional LMMSE method in terms of bit error rate (BER).

Robust low-complexity joint space–time–frequency channel estimation with transmit diversity in mobile wireless channels

In this paper, a robust low-complexity joint space–time–frequency channel estimation with transmit diversity in mobile wireless channels is proposed. Based on the orthogonal frequency division multiplexing (OFDM), the previously known training sequences are firstly blocked, space–frequency complex orthogonally encoded, up-converted and transmitted with diversity in the transmitter. In the receive end, each subchannel frequency response is estimated directly through the inverse pilot space–frequency complex orthogonally decoding. Wiener filtering and interpolating with the time- and frequency-domain correlations of the frequency response of the mobile wireless channels, low-complexity joint space–time–frequency channel estimator is finally constructed. In our proposed scheme, the high-level large-scale matrix inverse operations in traditional multiple input multiple output (MIMO) channel estimations are avoided. Separate and independent channel estimation corresponding to each pair of transmit and receive antennas in the MIMO space–frequency block coding system is achieved with transmit diversity, which can be used to significantly reduce the computation complexity with no degradation in system performance. Moreover, to successfully decoding the known pilot subblock coding matrix, it is only required that the fading channel keep approximately constant in each space–frequency coding subblock during one OFDM symbol block period. Making full use of the best time- and frequency-domain filtering, it makes our proposed scheme even more robust and accurate to estimate and track the channels with rapid time-varying dispersive fading. Numerical results are provided to validate the performance of our proposed channel estimators with diversity in mobile wireless channels.

Channel estimation for OFDM systems in non-sample-spaced multipath channels

A discrete Fourier transform (DFT)-based channel estimator for orthogonal frequency division multiplexing (OFDM) systems is proposed. In multipath channels with non-sample-spaced time delays, the conventional DFT approach will cause power leakage and result in an error floor. Proposed is a novel symmetric extension DFT method to reduce the leakage power efficiently. Numerical results show that the proposed channel estimator can eliminate the error floor substantially.