Fast Time-varying Fading Channels Research Articles

Iterative Joint Channel Estimation and Signal Detection for OFDM System in Double Selective Channels

Intercarrier interference caused by fast time-varying multipath fading channels degrades the system performance of high-mobility orthogonal frequency division multiplexing systems. This study consi...

Fast Time-Varying Multipath Channel Estimation and Signal Detection for of OFDM Systems with Interference Compensation

Orthogonal frequency division multiplexing (OFDM) is a widely adopted system for broadband wireless transmission, However, time-varying channels degrade the performance owing to the intercarrier interference (ICI). This study discusses the performance enhancement problems of channel estimation and signal detection in view of fast time-varying multipath fading channel characteristics of high mobility. A two-stage method is presented. In the coarse stage, initial channel estimation is performed using the signals at pilot subcarriers based on least-square (LS) estimate is performed. However, the initial estimate still needs to be improved because of the data-induced ICI. The coarse stage improves estimate accuracy by compensating the interferences. The refining stage, applies interference compensation and q-banded successive interference suppression (qSIS) methods to enhance the detection and re-estimate accuracy. Based on robustness, the detected data are called the “pseudo-pilots”, which together with the pilots are treated as a “visual-training” (VT) to improve channel re-estimation. Simulation results indicate that the proposed method is more robust than the traditional method in fast time-varying multipath fading channels with lower computational complexity. The proposed method is independent of the delay and fading properties of the channel.

A new inter‐carrier interference self‐cancellation mapping scheme over fast time‐varying fading channels

A new inter-carrier interference self-cancellation method with the data symbol mapping scheme of differential grouping weighted symmetry data-conjugate is studied and proposed to raise the bandwidth efficiency and further improve the system performance over fast time-varying fading channels. Comparing with the other existing self-cancellation methods, the proposed scheme can not only maintain the same bandwidth efficiency as the original orthogonal frequency division multiplexing system, but also show a better performance on carrier-to-interference ratio and bit error rate according to the final simulation results and performance analysis.

快时变衰落信道下OFDM系统的SAGE检测算法研究

OFDM技术可以有效地抑制多径效应的影响，从而被广泛地应用到无线通信系统中。快时变衰落信道下的OFDM系统将产生严重的子载波间干扰(ICI)，从而影响系统性能。本文对快时变OFDM系统，提出一种基于空间交替广义期望最大化算法(SAGE)的信号检测算法。理论分析的实验仿真结果表明，所提SAGE算法是有效的。 Orthogonal frequency division multiplexing (OFDM) technology can effectively suppress the im-pact of multipath effects, which is widely used in wireless communication systems. However, OFDM systems will lead to severe inter carrier interference (ICI) under fast time-varying fading channels, which will determine system performance. In this paper, an efficient data detection al-gorithm is proposed based on the space alternating generalized expectation maximization (SAGE) technique for OFDM systems under fast fading channels. Simulation results show that the pro-posed algorithm is effective for improving the performance of OFDM systems under fast time- varying fading channels.

Interference Control for Cognitive Network with High Mobility

Interference control (IC) between the secondary system and the primary system is an important issue for underlay cognitive radio network (CRN). The secondary system should limit the interference power to primary system by adjusting its transmission power. Many relevant works have been done based on the assumption of the quasistatic channel which is not suitable for the fast time-varying fading channel; the performance of IC in underlay CRN will become worse when the channel varies fast. This paper studies the IC issue in high mobility environment. By considering the channel state information (CSI) outdatedness, a short frame structure scheme and a mean interference power constraint scheme are proposed to reduce the influence of CSI outdatedness on IC performance. Furthermore, by considering the channel estimation error, a spherical error region model based robust IC scheme is designed as well. The proposed IC schemes of the secondary system are converted to the power allocation problems, and then they are formulated to optimization problem whose objects are to maximize the capacity of the secondary system with the interference constraints. The above optimization problems are solved by the water-filling style method. The simulation results show that the proposed IC schemes can effectively control the interference power to the primary system.

Robust MC-CDMA Channel Tracking for Fast Time-Varying Multipath Fading Channel

An unscented Kalman filter (UKF)-based channel-tracking method is proposed for a fast time-varying multipath fading channel in a multicarrier code-division multiple-access (MC-CDMA) system. The mobile radio channel is modeled as an autoregressive (AR) random process. The parameters of the AR process and the channel gain are simultaneously estimated by the proposed method. One-step-ahead prediction can also be obtained during channel estimation. It is useful for the decision-directed channel-tracking design, particularly in the fast-fading channel. Meanwhile, the estimated parameters can enhance the minimum mean-square error (MMSE) equalizer for symbol detection. The simulation results show that the enhanced equalizer based on the proposed estimation algorithm performs much better than that based on the conventional channel estimators in symbol error rate.

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 Estimation for Broadband Wireless OFDM Systems Over Rician Fading Channels

In this paper, we present a new Doppler spread estimation algorithm for broadband wireless orthogonal frequency division multiplexing (OFDM) systems with fast time-varying and frequency-selective Rayleigh or Rician fading channels. The new algorithm is developed by analyzing the statistical properties of the power of the received OFDM signal in the time domain, thus it is not affected by the influence of frequency-domain inter-carrier interference (ICI) introduced by channel variation within one OFDM symbol. The operation of the algorithm doesn’t require the knowledge of fading channel coefficients, transmitted data, or signal-to-noise ratio (SNR) at the receiver. It is robust against additive noise, and can provide accurate Doppler spread estimation with SNR as low as 0 dB. Moreover, unlike existing algorithms, the proposed algorithm takes into account the inter-tap correlation of the discrete-time channel representation, as is the case in practical systems. Simulation results demonstrate that this new algorithm can accurately estimate a wide range of Doppler spread with low estimation latency and high computational efficiency.

A frequency domain equalization algorithm for fast time-varying fading channels

Conventional frequency domain equalization (FDE) schemes were originally devised for quasi-static channels. Thus, such equalization schemes could suffer from significant performance degradation in fast-fading channels. This paper proposes a frequency domain equalization algorithm to mitigate the effect of fast time-varying fading. First, a mathematical expression is derived to quantify the total interference resulting from the time variation of the channel. Then, the proposed approach attempts to eliminate the effect of time-variations of the channel. This cancellation allows efficient use of the classical FDE structures in fast time-varying fading environments, although they are built upon the quasi-static channel model. Simulation results of bit-error-rate performance are provided to demonstrate the effectiveness of the proposed algorithm.

A Full-Diversity Blind Channel Estimation and Equalization over Fast Time-Varying ISI Fading Channels

In this study, the authors propose a novel full-diversity combination algorithm to significantly improve the performance of the network Kalman-based blind equalizers. Based on the weighted Gaussian sum (WGS) technique and the network of extended Kalman filters (NEKF), the proposed full-diversity blind equalizer can employ the prediction errors of network of Kalman filters to achieve the maximum likelihood (ML) detection. In the first initial condition, the proposed full-diversity blind equalizer requires an initial training sequence in order to estimate the initial channel coefficients. For symbol detection, the proposed full-diversity blind equalizer demonstrates a significant improvement over the conventional WGS-IMM(Interacting Multiple Model) blind equalizer in the bit error rate (BER) performance. Besides, from the trade-off between performance and computational complexity, the proposed modified 2-Diversity blind equalizer is shown to be a best choice for the WGS-based blind equalizer.

Performance of multiantenna multicarrier direct-sequence code division multiple access using orthogonal variable spreading factor codes-assisted space–time spreading in time-selective fading channels

A downlink (base-to-mobile) multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) system employing multiple antennas at both the base station and each of the mobile terminals is investigated, when communicating over fast time-varying fading channels resulting in time-selective fading. In the considered multiantenna MC DS-CDMA, space–time spreading (STS) based on the family of orthogonal variable spreading factor (OVSF) codes is proposed in order to achieve the time diversity because of the time-selective fading, in addition to the transmit/receive diversity. In this contribution, the closed-form expressions for the single-user bit-error rate (BER) are derived, in order to gain insight into the achievable BER performance of the multiantenna MC DS-CDMA. The BER performance of the multiantenna MC DS-CDMA system is investigated, when communicating over correlated time-selective Rayleigh fading channels. The study and performance results show that the multiantenna MC DS-CDMA using OVSF codes-assisted STS constitutes a high-efficiency downlink space–time transmission scheme. It is capable of achieving the full diversity provided by the time-selective fading and multiple transmit/receive antennas. Furthermore, the proposed multiantenna MC DS-CDMA is capable of achieving a self-balance between the time diversity achieved and the multiuser interference suppression capability, when using low-complexity correlation detection.

Robust and Efficient Intercarrier Interference Mitigation for OFDM Systems in Time-Varying Fading Channels

Due to its spectral efficiency and robustness over multipath channels, orthogonal frequency-division multiplexing (OFDM) has served as one of the major modulation schemes for high-speed communication systems. In the future, wireless OFDM systems are expected to operate at high carrier frequencies, high speed, and high throughput for mobile reception, where fast time-varying fading channels are encountered. Channel variation destroys the orthogonality among the subcarriers and leads to intercarrier interference (ICI). ICI poses a significant limitation on wireless OFDM systems. The aim of this paper is to find an efficient method of providing reliable communications using OFDM in fast time-varying fading channels. It is observed that ICI power arises from a few adjacent subcarriers. This observation motivates us to design low-complexity -tap ICI equalizers. To employ these equalizers, channel state information is also required. In this paper, we also design a pilot-aided minimum mean square error (MMSE) channel estimation scheme for a time-varying wide-sense stationary uncorrelated scatters channel model. The MMSE channel estimator utilizes the statistical channel properties to achieve computational efficiency. Simulation results show that our proposed low-complexity ICI suppression scheme, which incorporates the -tap equalizer with the MMSE channel estimator, can significantly improve the performance of OFDM systems in fast time-varying fading channels.

Turbo equalization with nonlinear Kalman filtering for time-varying frequency-selective fading channels

In this paper, we present a low complexity turbo equalization receiver for data transmission over time-varying frequency-selective fading channels. In the receiver, an adaptive equalizer using nonlinear Kalman filters with delay is coupled with a soft-in soft-cut (SISO) decoder to perform the reception process iteratively. The proposed adaptive equalizer jointly optimizes the estimates for channel taps and data symbols in each iteration with the assistance of a priori information for the data symbols supplied by the SISO decoder. In this way, the correlation between the estimates of data symbols and channel taps is taken into account in the equalization process. This correlation is usually ignored in many other turbo equalizers with separated channel estimation and data equalization. The complexity of the proposed equalizer does not grow exponentially with the modulation constellation size, and is usually lower than that of many maximum a posteriori equalizers with joint channel estimation and data detection. Performance comparison with different types of turbo equalizers over different channel conditions by Monte Carlo simulations demonstrates the advantage of the proposed turbo equalizer for fast time-varying fading channels

Fast Time-Varying Dispersive Channel Estimation and Equalization for an 8-PSK Cellular System

In this paper, a novel channel-estimation scheme for an 8-PSK enhanced data rates for GSM evolution (EDGE) system with fast time-varying and frequency-selective fading channels is presented. Via a mathematical derivation and simulation results, the channel impulse response (CIR) of the fast fading channel is modeled as a linear function of time during a radio burst in the EDGE system. Therefore, a least-squares-based method is proposed along with the modified burst structure for time-varying channel estimation. Given that the pilot-symbol blocks are located at the front and the end of the data block, the LS-based method is able to estimate the parameters of the time-varying CIR accurately using a linear interpolation. The proposed time-varying estimation algorithm does not cause an error floor that existed in the adaptive algorithms due to a nonideal channel tracking. Besides, the time-varying CIR in the EDGE system is not in its minimum-phase form, as is required for low-complexity reduced-state equalization methods. In order to maintain a good system performance, a Cholesky-decomposition method is introduced in front of the reduced-state equalizer to transform the time-varying CIR into its minimum-phase equivalent form. Via simulation results, it is shown that the proposed algorithm is very well suited for the time-varying channel estimation and equalization, and a good bit-error-rate performance is achieved even at high Doppler frequencies up to 300 Hz with a low complexity

Pilot Patterns for OFDM Communication Systems in a Fast Time-Varying Channel

In this letter, we focus on rearranged pilot patterns for channel estimation in a mobile communication system using Orthogonal Frequency Division Multiplexing (OFDM). The conventional pilot patterns for channel estimation in OFDM systems do not have robust characteristics in time-varying channels. In order to overcome this weakness of the conventional pilot patterns, we propose the pilot patterns with robust mobility for OFDM systems, which can achieve a good error performance in time-varying multi-path fading channels. Simulation results show that the bit error rate (BER) performances of the proposed pilot patterns are better than those of the conventional pilot patterns in fast time-varying fading channels under the same pilot density and data rate.

Adaptive MLSD receiver employing noise correlation

A per-survivor processing (PSP) maximum likelihood sequence detection (MLSD) receiver is developed for a fast time-varying frequency-selective Rayleigh fading channel with coloured additive noise, which follows an autoregressive (AR) model with unknown parameters. The correlation between noise samples is exploited to considerably enhance the performance of the communications. The maximum likelihood criterion is employed based on unknown noise parameters. This criterion has some desired properties, e.g. it has a unique joint minimum at the true values of the channel and the noise parameters. The new PSP–MLSD algorithm detects the input data and jointly estimates the noise and the channel parameters all together. The proposed structure can be viewed as a traditional PSP–MLSD receiver combined with an adaptive whitening filter. In a coloured noise environment, this scheme offers a faster tracking property, more accurate estimation of the channel and a substantially lower error probability compared with the traditional PSP–MLSD structure. The signal-to-noise ratio (SNR) improvement achieved by the proposed receiver, which can be called the noise whitening gain (NWG), is almost equal to the ratio of the energy of the additive noise to the energy of the unpredictable noise component. The square of the NWG gives also an accurate approximation for the bit error rate (BER) improvement ratio obtained by using the proposed algorithm compared with the traditional one.

A Multiple-Symbol Differential Detection Based on Channel Prediction for Fast Time-Varying Fading

This paper discusses a generalized concept of multiple-symbol differential detection (MDD) and analytically derives weight parameters based on channel prediction for MDD on fast time-varying channels. At first, this paper shows that adaptive maximum-likelihood sequence estimation employing per-survivor processing (PSP-MLSE) with a single channel tap is similar concept to MDD. Next, the weight parameters for MDD are derived according to the channel estimation of PSP-MLSE based on a high order channel prediction. Finally, computer simulation confirms that MDD with the analytically derived parameters mitigates floor of bit error rate (BER) on fast time-varying fading channels without channel state information.

Adaptive maximum-likelihood sequence estimation by means of combined equalization and decoding in fading environments

This paper proposes an adaptive maximum-likelihood sequence estimation (MLSE) by means of combined equalization and decoding, i.e., adaptive combined MLSE, which employs separate channel estimation for respective states in the Viterbi algorithm. First, an approximate metric including channel estimation is derived analytically for this proposed adaptive combined MLSE. Secondly, procedures to accomplish blind equalization are investigated for the proposed MLSE. Finally, its excellent BER performance on fast time-varying fading channels is confirmed by computer simulation, when the proposed MLSE operates as a blind equalizer. >