Frequency-domain Channel Estimation Research Articles

Combined intra symbol frequency domain averaging channel estimation for polarization division multiplexed coherent optical OFDM/OQAM systems

Intrinsic imaginary interference (IMI) induced by chromatic dispersion (CD) and polarization mode dispersion (PMD) seriously degrades the performance of optical orthogonal frequency division multiplexing offset-quadrature amplitude modulation (O-OFDM/OQAM) systems. Therefore, effective channel estimations are required for such systems to maintain good transmission performance. In the previous publications, both the full loaded (FL) and the half loaded (HL) frequency-domain channel estimation methods for polarization-division-multiplexed (PDM) coherent optical OFDM/OQAM (CO-OFDM/OQAM) systems have been proposed. However, for these frequency-domain methods, the channel frequency-domain responses caused by the correlation of the amplified spontaneous emission (ASE) noise are not considered. In this paper, we systematically discuss the frequency-domain channel transmission model for PDM CO-OFDM/OQAM systems. With the analysis of the correlation of the ASE noise, we propose a combined intra symbol frequency-domain averaging (CISFDA) channel estimation method for PDM OFDM/OQAM systems. Compared with the full-loaded frequency-domain channel estimation (FL-FD) method, the CISFDA method promotes the system robustness against the ASE noise and fiber nonlinear effect significantly. The proposed method is validated by numerical Monte Carlo simulations of the PDM CO-OFDM/OQAM system. Our new algorithm outperforms the FL-FD method in the 36 Gb/s PDM OFDM CO-OFDM/OQAM system, and the transmission distance is improved by 500 km with the bit-error-rate (BER) target at 3.8×10-3.

An improved discrete Fourier transformation channel estimation algorithm with low complexity for orthogonal frequency division multiplexing‐based power line carrier communication systems

AbstractOrthogonal frequency division multiplexing (OFDM) technology has been increasingly applied to power line carrier communication (PLC). Discrete Fourier Transformation (DFT)‐based channel estimation algorithm is suitable for OFDM‐based PLC due to its low complexity. In view of the problem that the traditional DFT channel estimation does not consider the influence of noise inside cyclic prefix (CP), an improved DFT channel estimation based on signal‐to‐noise ratio (SNR) estimation is proposed. First, least‐square (LS) algorithm is performed and the frequency domain channel estimation is converted to the time domain through inverse DFT, and the average SNR of the system is estimated according to the pilot sequence. Second, the substitute SNR for each sample point inside CP is defined and used to filter impulse noise inside CP. Third, the average SNR is converted into the threshold of the useful signal energy inside CP, and the widespread background noise inside CP is filtered. The simulation results show that the proposed algorithm can obtain more accurate channel estimation than other DFT channel estimation algorithms because it can effectively filter out noise inside CP. In addition, compared with other similar algorithms, the proposed algorithm dose not result in a significant increase in complexity.

Common CP-OFDM Transceiver Design for Low-Complexity Frequency Domain Equalization

Common cyclic prefix-orthogonal frequency-division multiplexing (CCP-OFDM) groups multiple OFDM symbols and uses one CP to protect them in slow time-variant channels. However, due to this particular design of CCP-OFDM systems, complex time-domain estimation algorithms are used to estimate and equalize the channel, such as maximum likelihood (ML) estimators. In this letter, we propose a novel CCP-OFDM transceiver design enabling low-complexity frequency-domain channel estimation and equalization. Simulations under frequency selective channels have validated the effectiveness of the proposed scheme in terms of computational complexity, normalized mean squared error (NMSE), and bit error rate (BER).

Modified frequency-domain channel-estimation based on the dual-dependent pilots for polarization-division-multiplexed coherent optical orthogonal frequency division multiplexing systems with offset-quadrature-amplitude modulation

In the past, several frequency-domain channel-estimation methods were proposed for coherent optical orthogonal frequency division multiplexing systems, which use offset-quadrature-amplitude modulation (CO-OFDM-OQAM). A common challenge for these methods is that the training sequences they use, even though they increase the magnitude of the pseudo-pilots, can make the peak-to-average power ratio (PAPR) of the signal worse. To address this problem, an innovative modified frequency-domain channel estimation method is designed and investigated for a polarization-division-multiplexed CO-OFDM-OQAM system that uses dual-dependent pilots. The operating principle of this new method is described in detail. Then, its PAPR performance and channel-estimation capability are examined using numerical simulations. The proposed method not only ensures a sufficiently large magnitude of the pseudo-pilot but also does not seriously degrade the PAPR of the signal. It is also found that the magnitude of its pseudo-pilot (2.3136) exceeds that of the interference approximation method with real pilots (1.1086), and it is only slightly worse than that of the enhanced IAM with complex pilots (2.6074). The PAPR of the new method (max 11.4 dB) is much lower than that of other frequently used methods. Finally, the channel-estimation capability of the proposed method is validated for both back-to-back and long-distance transmission scenarios. Furthermore, the BER performance of several different channel-estimation methods is also compared. The obtained results confirm the above observations.

A Low-Complexity Pilot-Based Frequency-Domain Channel Estimation for ICI Mitigation in OFDM Systems

A low-complexity pilot pattern and a frequency-domain channel estimation method for Inter-Carrier Interference (ICI) mitigation is proposed for Orthogonal Frequency Division Multiple Access (OFDM) systems. The proposed method exploits the band structure of the coupling matrix to perform an ICI-free channel estimation in the frequency domain. This ICI-free estimation relies on some conditions imposed over the pilot pattern that simplify the complexity of channel estimation significantly, since its complexity is the same as classical least squares (LS) channel estimation used in low mobility scenarios. Then, the ICI is removed by using a modified version of Minimum Mean Square Error (MMSE) equalization, which reduces the computational complexity considerably. This modified MMSE equalization relies on the sparse and banded structure of the coupling matrix and on a low complexity variant of the Cholesky decomposition, which is named LDLH factorization. It is shown that the proposed method greatly improves the Bit Error Rate (BER) in the high Signal-to-Noise Ratio (SNR) regime.

Joint fast time domain channel estimation with ICI cancellation for LTE-R systems

In this paper, we consider the high-speed railway (HSR) communication bases on the long-term evolution for railway (LTE-R) platform. Since the large Doppler spread is introduced due to the movement of the train, the orthogonality of subcarriers is destroyed resulting in the inter-carrier interference (ICI). Moreover, the critical speed of transceivers moving leads to a very fast-time varying channel within an OFDM symbol, hence the existing frequency domain channel estimation (FDCE) method cannot reliably estimate the channel state information (CSI). To better track the fast-time varying channel, we propose a new framework for LTE-R systems. First, we modify the WINNER II channel model and the D2a propagation scenario to approximate the multipath fading and high Doppler spread in high mobility scenarios. Second, we use a novel pilot structure-based time domain channel estimation (TDCE), helping to track the channel variations for each channel path separately. The CSI at data subcarriers is estimated by using different conventional interpolation methods to reconstruct the ICI channel matrix in the frequency domain. Different from the linear channel model, the channel time-variations of current OFDM data symbol are predicted from the third-order polynomial function, which is reconstructed base on CIR at the pilot position by using several constitutive OFDM symbols. The cubic Hermite function is chosen by considering its simplicity and efficiency. Lastly, we propose the deep neural network (DNN) based CE in LTE-R systems, which helps to further improve the accuracy of estimated channel information in the previous state, especially at low signal-to-noise (SNR) level. The simulation results show that our proposed CE method for each channel path agrees well with the theoretical derivation and the simulation. The system performance with the proposed framework is significantly improved in comparison with state-of-the-art methods.

Time Domain Pilot-Based Channel Estimation (TDPCE) Using Kalman Filtering for OFDM System

This work involves using the OFDM technique with the Time domain pilot-based channel estimation (TDPCE) to mitigate the effects of severe channel conditions such as fast and selective fading Raleigh channel with maximum Doppler shift equal to (120 Hz). The propose method has good robust performance, as compared to frequency domain channel estimation approaches.This method is based on comb pilot and Kalman filtering adaptive algorithm. Three digital modulation techniques including Quadrature Amplitude Modulation QAM 16, 32, and 64 were used in the OFDM transmitter with Rayleigh fading multipath channel. The OFDM performance was evaluated by measuring the bit error rate (BER) and the mean square deviation (MSD). Through simulation results, the (BER) has reached (0.0004, 0.004, and 0.01) when (Eb/No) is equal to (12 dB) using QAM 16, 32, and 64, respectively. Also, the convergence rate has almost reached (50, 50, and 125) iterations for the aforementioned QAM’s. These results can be taken into consideration as acceptable results in this paper compared to other results in the presence of severe conditions, where the value of maximum Doppler shift parameter of Rayleigh multipath fading channel is (120 Hz) or the user is available in a moving position.

Channel Estimation and Equalization for Alamouti SF-Coded OFDM-UWA Communications

In this paper, a new channel estimation and equalization algorithm for underwater acoustic (UWA) communications is presented. The proposed algorithm is developed to meet the requirements of underwater time-varying sparse channels that undergo Rayleigh fading. In addition, the algorithm takes into consideration a path-based channel model which describes each received path with significant power by an attenuation factor, a Doppler scale, and a delay. Transmit diversity enabled by Alamouti space-frequency block coding coupled with orthogonal frequency division multiplexing is employed in the form of two transmitters and multiple receivers. The proposed, non-data-aided, expectation-maximization (EM)-based maximum a posteriori probability sparse channel estimation first estimates the channel transfer functions from each transmit antenna to the receiver. Then, the estimation performance is greatly improved by taking into account the sparseness of the UWA channel and refining the estimation based on the sparse solution that best matches the frequency-domain channel estimates obtained during the first phase of the estimation process. Sparse channel path delays and Doppler scaling factors are estimated by a novel technique called delay focusing . After that, slow time-varying, complex-valued channel path gains are estimated using a basis expansion model based on the discrete Legendre polynomial expansion. Computer simulation results show that the resulting channel estimation algorithm can achieve excellent mean-square error and symbol error rate for both generated data and semi-experimental data taken at Sapanca Lake in Turkey and is capable of handling some mismatch due to different fading models.

Fast Block LMS Based Estimation of Angularly Sparse Channels for Single-Carrier Wideband Millimeter Wave Hybrid MIMO Systems

Adaptive block-based least-mean squares (BLMS)-based techniques are conceived for channel estimation in single-carrier (SC) wideband millimeter wave (mmWave) hybrid MIMO systems. In this context, a frequency-domain channel estimation model is developed for SC wideband systems, followed by a novel fast BLMS (FBLMS) technique, which has a significantly lower computational complexity than the existing channel estimation schemes designed for mmWave hybrid MIMO systems. The proposed FBLMS technique is also robust, since it does not require any second-order statistical information, such as the cross-covariance vector and covariance matrix. Next a beamspace domain representation of the mmWave MIMO channel is obtained, followed by the development of the sparse-FBLMS (SFBLMS) scheme for the estimation of the wideband mmWave MIMO channel that additionally exploits the angular-sparsity for improved channel estimation performance. Analytical expressions are derived for the mean squared estimation error (MSEE) and mean squared observation error (MSOE) of both the proposed FBLMS and SFBLMS techniques. Furthermore, a systematic procedure is developed for determining the beneficial range of the values of the regularization parameter, which ensures a high channel estimation accuracy of the SFBLMS over FBLMS. A hybrid precoder and combiner design is also proposed for SC wideband systems by employing the channel estimates obtained using the above techniques. Simulation results are presented to illustrate the performance of the proposed BLMS-based schemes in comparison to the existing schemes, which closely match the theoretical results derived.

Joint Channel Estimation and Precoding for Faster-Than-Nyquist Signaling

The performance of existing frequency-domain channel estimation and equalization algorithms for faster-than-Nyquist (FTN) signaling is seriously hampered with the noise enhancement phenomenon that results from their inverse or pseudo-inverse operations. Through simulations, we show that the linear precoding can effectively mitigate the above-mentioned noise enhancement phenomenon. At first, a low complexity precoding-based channel estimation (PCE) algorithm is proposed for FTN signaling over frequency-selective fading channels. In contrast with most existing frequency-domain channel estimation algorithms, the proposed PCE algorithm has much better mean square error performance, and the performance improvement enlarges with the increase of signal to noise ratio. Furthermore, a joint channel estimation and precoding (JCEP) algorithm is proposed to perform data detection for FTN signaling over frequency-selective fading channels. On the one hand, compared with the existing frequency-domain channel estimation and equalization algorithms, the JCEP algorithm greatly reduces the complexity of signal processing at receivers since it performs the linear precoding processing at transmitters. On the other hand, even with estimated channel state information (CSI), the proposed JCEP algorithm can approach the bit error rate (BER) performance of the Nyquist signaling for all the modulation types adopted in digital video broadcasting-satellite-second generation extension (DVB-S2X). More precisely, the BER performance degradation with perfect and estimated CSI is 0.06 dB and 0.07 dB respectively when the time acceleration parameter equals to 0.7 and the rolling factor is 0.45.

Frequency-domain averaging-based hybrid channel estimation for short-reach discrete multitone transmission systems

Channel estimation is a key digital signal processing algorithm in optical discrete multitone (DMT) transmission systems. The least-square (LS)-based channel estimation is a simple method, but it is very sensitive to various noises and other interference. The estimation accuracy can be improved using the LS-based intrasymbol frequency-domain averaging (ISFA) channel estimation method. However, there exists inaccurate channel estimation for the edge data subcarriers caused by chromatic dispersion and nonsymmetric averaging windows. We propose a hybrid channel estimation method with LS-based intersymbol frequency-domain averaging and modified ISFA to improve the estimation accuracy of edge data subcarriers and the stability of the bit error rate (BER) performance. The proposed hybrid channel estimation method is experimentally investigated in a short-reach 16-ary quadrature amplitude modulation-encoded DMT transmission system. After 20-km standard single-mode fiber transmission, with the help of the proposed hybrid channel estimation and at a BER of 3.8 × 10 − 3, the receiver sensitivity in terms of the received optical power can be improved by about 1.8 dB compared to that of the conventional ISFA. Furthermore, the proposed hybrid channel estimation has better BER stability.

Fine time synchronization and residual ISI influence in SC-FDE systems

We show a novel method for a fine time synchronization in a presence of residual intersymbol interferences (ISIs). The proposed method is compared with a (1) Cramér-Rao lower bound (CRLB) for data-aided timing estimators and with a (2) maximum likelihood (ML) estimator both in (a) ISI-free additive white Gaussian noise (AWGN) channel and (b) in a residual ISI channel. The effect of the residual ISI is introduced via a power-delay profile (PDP) with a delay spread spanning over several samples. We consider a single-carrier system with a frequency domain channel estimation. The proposed method almost reaches the performance of the ML estimator in AWGN, while in the presence of the residual ISI, the proposed method shows superior performance evaluated in terms of error vector magnitude improvement of up to 10%.

Improving The Efficiency of Frequency Domain Channel Estimation in 16-QAM Polarization Division Multiplexing Coherent Optical OFDM Systems Using Complex Fixed-Point ICA

Improving The Efficiency of Frequency Domain Channel Estimation in 16-QAM Polarization Division Multiplexing Coherent Optical OFDM Systems Using Complex Fixed-Point ICA

A Time-Domain Approach to Channel Estimation and Equalization for the SC-FDM System

Similar to orthogonal frequency division multiplexing receiver, the frequency-domain (FD) channel estimation (CE) and equalization are indispensable parts in the coherent single carrier frequency division multiplexing (SC-FDM) receiver. When the channel varies slowly, the FD processing is cost effective. For the applications with high mobility, the traditional implementation of the SC-FDM receiver causes significant performance degradation. In this paper, we propose to estimate time-varying pieces of channel taps for pilot symbols based on basis expansion model (BEM), and subsequently to reconstruct time-domain (TD) channel response for data symbols by utilizing the Slepian sequences-based piece-wise interpolation. Furthermore, two simplified schemes, i.e., the Slepian sequences-based multiple-point interpolation and the segmented BEM, are developed to reduce the computational complexity of the TD-CE. The Cramer-Rao lower bound (CRLB) of channel impulse response (CIR) estimation is also analyzed. In the light of the sparsity of TD channel gain matrix, we design an iterative least-squares QR decomposition algorithm to equalize the SC-FDM symbols. In the simulated SC-FDM system, when considering the carrier frequency of 5.9 GHz and the velocity of about 510 km/h, we observe that the traditional FD methods cause the demodulation failure, while the proposed TD processing schemes achieve ideal error-probability performance and preserve a relatively low complexity.

Data Packet Structure and Modem Design for Dynamic Underwater Acoustic Channels

In underwater acoustic (UWA) communications, the propagated signal undergoes severe Doppler and multipath distortions. The Doppler estimation/compensation and channel estimation/equalization techniques are required to deal with these distortions and contribute significantly to the overall complexity of UWA modems. In this article, we propose a data packet structure for high data rate transmission in time-varying UWA channels with the channel dynamic modeled by velocity and acceleration between the transmitter and the receiver. The data packet consists of superimposed data and periodic pilot sequences. The superposition allows achievement of a high spectral efficiency for data transmission. The repeated pilot symbols allow the use of a low-complexity multibranch autocorrelation method for coarse estimation of Doppler parameters related to the velocity and acceleration. To refine these estimates, we further propose a low-complexity fine Doppler estimator based on dichotomous iterations. We also present a low complexity frequency domain channel estimator exploiting the channel sparsity. The proposed modem design has been evaluated using simulation and practical sea trials. The experiments demonstrate high detection performance of the proposed design, in particular in comparison with a more traditional design that ignores the acceleration between the transmitter and the receiver.

Semi-Blind Time-Domain Channel Estimation for Frequency-Selective Multiuser Massive MIMO Systems

The availability of accurate channel state information (CSI) is essential in multiuser massive multiple-input multiple-output (MIMO) systems. However, most published works focus on frequency-flat channel estimation which requires that the estimation must be repeated for every frequency slot, e.g., subcarrier, in a broadband system. Since the channels in the frequency-domain are the Fourier transform of a small set of time-domain channel coefficients, the time-domain channel estimation requires that fewer parameters be estimated. In this paper, we propose a semi-blind, time-domain, channel estimation technique for frequency-selective massive MIMO systems. Our solution depends on the subspace spanned by the signal eigenvectors of the received signal covariance matrix. Importantly, since the receiver samples at the symbol rate, time-domain-based estimation inherently has available enough samples for an accurate matrix estimate. To avoid asymptotic assumptions, we express each channel vector as a linear combination of the signal eigenvectors. We estimate the linear combination using a set of training symbols. Given the many samples of the received signal in the time-domain, we obtain a better channel estimate compared to conventional subcarrier-wise frequency-domain channel estimation. Unlike the previous published results, in this paper, we do not assume orthogonality of users' channels or knowledge of large-scale fading coefficients. In addition, our estimation procedure does not require orthogonality between the training symbols of the users in all cells.

Forward Collision Vehicular Radar With IEEE 802.11: Feasibility Demonstration Through Measurements

Increasing safety and automation in transportation systems has led to the proliferation of radar and IEEE 802.11pbased dedicated short-range communication (DSRC) in vehicles. However, current implementations of vehicular radar devices are expensive, use a substantial amount of bandwidth, and are susceptible to multiple security risks. In this paper, we use the IEEE 802.11 orthogonal frequency-division multiplexing communications waveform, as found in IEEE 802.11a/g/p, to perform radar functions. In this paper, we present an approach that determines the mean-normalized channel energy from frequencydomain channel estimates and models it as a direct sinusoidal function of target range, enabling closest target range estimation. In addition, we propose an alternative to vehicular forward collision detection by extending IEEE 802.11 DSRC and WiFi technology to radar, extending the foundation ofjoint communications and radar frameworks. Furthermore, we perform an experimental demonstration near DSRC spectrum using IEEE 802.11 standard compliant software defined radios with potentially minimal modification through algorithm processing on frequency-domain channel estimates. The results of this paper show that our solution delivers sufficient accuracy and reliability for vehicular RADAR if we use the largest bandwidth available to IEEE 802.11p (20 MHz). This indicates significant potential for industrial devices with joint vehicular communications and radar capabilities.

Improved iterative FD‐CE and turbo equalisation for doubly‐selective high‐frequency channel

In this study, the authors investigate joint iterative frequency-domain channel estimation (FD-CE) and turbo equalisation for doubly selective high-frequency channel. The performance of time and FD turbo equalisers (TD/FD-TE) by applying them to Appendix C of military standard (MIL-STD)-188-110C is compared. Compared to TD-TE, FD-TE has reduced computational complexity; however, its performance degrades for time-varying channels. To improve the performance of FD-TE, we propose a new iterative CE scheme which can be used jointly with FD-TE. We employ low-complexity FD least mean-square algorithm in iterative process. By the use of soft information fed back from the decoder from the previous iteration, we improve the quality of the channel estimate over iterations. Simulation results show that at bit error rate (BER) of 1×10 -5 the proposed algorithm improves the performance of FD-TE ~2.5 and 3.5 dB for 8 phase shift keying (PSK) and 16 quadrature-amplitude modulation (QAM), respectively. Also, we study the impact of CE error on the performance of FD-TE. For 16QAM when SNR values are more than 20 dB, there is a gain between 1 and 3 dB when we consider the variance of CE error.

Efficient frequency‐domain channel equalisation methods for OFDM/OQAM‐PON with intensity modulation and direct detection

In this study, the authors present efficient frequency-domain channel estimation (CE) methods based on the adjacent subcarriers frequency-domain averaging (ASFA) and minimum mean squared error (MMSE) schemes for the orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) passive optical network (PON) system. The complete channel transmission response for OFDM/OQAM-PON with the chromatic dispersion induced intrinsic imaginary interference effect is systemically derived. Compared with the conventional interference approximation CE method, ASFA and MMSE offer improved receiver sensitivity.

Efficient frequency‐domain channel equalisation methods for OFDM visible light communications

The authors present efficient frequency-domain channel estimation methods based on the intra-symbol frequency-domain averaging (ISFA), minimum mean squared error (MMSE) and weighted inter-frame averaging (WIFA) schemes for the orthogonal frequency division multiplexing (OFDM) visible light communications (VLC) system. OFDM-VLC with quadrature phase shift keying, 16- and 64-quadrature amplitude modulation mapping is experimentally demonstrated. Compared with the conventional least square channel estimation method, ISFA, MMSE and WIFA offer improved performance with MMSE offering the best performance in terms of the error vector magnitude but at the cost of high complexity. The authors show that the WIFA can improve the estimation accuracy of time-varying VLC optical channel.