Mobile Orthogonal Frequency Division Multiplexing Research Articles

EM-EKF fast time-varying channel estimation based on superimposed pilot for high mobility OFDM systems

The scarcity of wireless spectrum resources and high mobility have brought serious challenges to orthogonal frequency division multiplexing (OFDM) communication systems. To improve spectral efficiency and combat fast time-varying channel fading, this paper studies the OFDM system with high mobility based on superimposed pilots, and adopts a basis expansion model (BEM) to describe fast time-varying channels. An iterative channel estimation algorithm combining expectation–maximization (EM) and extended Kalman filtering (EKF) is proposed, which is called EM-EKF in this paper. It uses EM to calculate the parameter matrix, measured values and the covariance of the process noise of the unknown first-order time-varying autoregressive (TVAR) model to obtain the optimal estimation. To further enhance the system’s robustness for fast time-varying channel fading, EKF and smoothing algorithms are jointly used for time-varying channel tracking. The results show that compared with the existing representative channel estimation algorithms, the iterative EM-EKF algorithm proposed in this paper not only solves the problem of unknown prior information, but also significantly outperforms other non-iterative algorithms, and the performance of the proposed algorithm is similar to that of the iterative EKF algorithm, which is more suitable for the actual OFDM communication systems with high mobility.

Link-Signature-Based Discriminatory Channel Estimation (LS-DCE) for Physical Layer Security in Stationary and Mobile OFDM Transceivers

In this work, we propose a physical layer security (PLS) scheme for orthogonal frequency-division multiplexing (OFDM) based transceivers, termed link-signature-based discriminatory channel estimation (LS-DCE). The proposed scheme exploits the space-time uniqueness of the channel responses (i.e. link-signatures) at different transceiver pairs to facilitate a reliable signal reception at legitimate receivers, while equivocating reception at non-legitimate adversaries. While most PLS studies focus on transmitted data to provide such an equivocation, LS-DCE focuses on channel estimations. Since the quality of estimates impacts the overall performance, LS-DCE facilitates distinctive detection error probabilities by enforcing a poor channel estimation at non-legitimate receivers as compared to legitimate ones. To validate the effectiveness of the proposed scheme, analytical, simulated, and experimental evaluations are performed. First, a theoretical analysis is established in terms of the probability of bit error (PBE) at the legitimate and non-legitimate receivers. Then, indoor/outdoor stochastic and deterministic channel models are adopted using correlated Nakagami-m simulation model and software-defined radio (SDR) testbed, respectively. With $-10\,{\rm dB}$ signal-to-noise ratio (SNR), it is found that a spatial correlation $\leq 0.1$ between the legitimate and non-legitimate channels results in an average PBE of 0.48, 0.48, and 0.49 for BPSK, QPSK, and 16-QAM, respectively, at the non-legitimate receiver; and 0.45, 0.46, and 0.48 with $\text{50}\,{\rm dB}$ SNR.

Performance evaluation of high mobility OFDM channel estimation techniques

In wireless communication, Orthogonal Frequency Division Multiplexing (OFDM) has been adopted due to its robustness to multipath fading and high data rate transmissions. At the other hand, the performance of OFDM systems severely degraded due to multi-path fading and Doppler frequency shifts in mobile systems, which causes inter-carrier-interference (ICI). Thus, Estimation of channel parameters is required at the receiver using a pre designed estimator where pilot tones are inserted in each OFDM symbol. In this paper, a random pilot data are generated and inserted in each OFDM symbol at equally spaced locations. The performance test of Least Square (LS) and Linear Minimum Mean Square (LMMSE) estimation methods are proposed with Discrete Fourier Transform (DFT) based on both LS and LMMSE, where different ITU channel models are considered in order to compare their performance for data transmission in high mobile systems with different Doppler frequencies exceeds 200 Hz and minimal number of pilots.

Efficient Signal Detection Methods for High Mobility OFDM System with Transmit Diversity

In this paper, we propose two efficient signal detection methods for high mobility space–time block code based orthogonal frequency division multiplexing (OFDM). The first proposed method is based on order block decision feedback equalizer (OBDFE) which is the column norm ordering version of the conventional BDFE method. To improve the system performance further, we have proposed a hybrid OBDFE–MPD method by cascading the OBDFE method with a multistage parallel decoder (MPD). The advantage of the OBDFE–MPD method is that it does not need additional complexity as most of the computations are performed in the initial stage by the OBDFE method. These two proposed methods are compared with minimum mean square error (MMSE), block linear minimum mean square error (BLMMSE), block decision feedback equalizer (BDFE), vertical Bell laboratories layered space–time (VBLAST) and space alternating generalized expectation maximization (SAGE) in terms of performance and computational complexity. From the simulation results and computational complexity, it is shown that both the proposed OBDFE and OBDFE–MPD methods provide a great trade-off between performance and computational complexity for high mobility OFDM system with transmit diversity.

Parallel Iterative Inter-carrier Interference Cancellation in Underwater Acoustic Orthogonal Frequency Division Multiplexing

In underwater acoustic orthogonal frequency division multiplexing (OFDM) communication system, channel variations would introduce inter-carrier interference (ICI). This gets more severe as mobile speed, carrier frequency or OFDM symbol duration increases. In this paper, we propose a kind of parallel iterative ICI cancellation algorithm to combat that problem. The proposed method extracts channel variations from the adjacent OFDM symbols, and constantly improves the estimation accuracy of ICI through iteration. Finally, simulation and trial in real lake channel verify the availability and stability of the algorithm in the underwater acoustic time varying channel. In addition, this algorithm avoids the matrix inversion, reducing the complexity of the algorithm, and has the obvious advantage in accuracy and speed.

Partial Update Sequential Probabilistic Integer Least Square Inter Carrier Interference Mitigation Algorithm in Mobile OFDM System

The performance of mobile orthogonal frequency division multiplexing (OFDM) system is affected by the intercarrier interference (ICI). A recently proposed sequential probabilistic integer least squares (SPILS) algorithm remarkably improves the performance of ICI mitigation. The corresponding low complexity Quick SPILS (QSPILS) algorithm is also proposed to reduce the complexity of SPILS algorithm but at the cost of rapidly reducing the performance. In this letter, we propose a new low complexity SPILS algorithm by using the banded property of the frequency domain channel matrix. The proposed algorithm achieves the performance much closer to that of the SPILS and at the same time requires the computation complexity on the same level of that of QSPILS algorithm.

Energy Efficient Doppler Assisted Channel Estimation for Highly Mobile OFDM Systems

The complexity of channel estimation algorithms is of critical importance in high mobility orthogonal frequency division multiplexing (OFDM) systems. To reduce the complexity of the algorithm, the existing channel estimation algorithms take advantage of the correlations between the channel coefficients. However, they do not take into account the variations of the correlations with the delay tap. In this paper, we consider the change in correlation between channel coefficients for different delay taps. By considering these variations, the number of computations needed for channel estimation can be decreased. This results in a reduction in the circuit energy. The simulation results show that the proposed scheme consumes less total energy per bit for transmission distances less than about 170 m when the mobile speed is 324 Kmph.

On the Banded Approximation of the Channel Matrix for Mobile OFDM Systems

In mobile orthogonal frequency-division multiplexing (OFDM) systems, a frequency-domain channel matrix represents the same-carrier channel frequency response in the diagonal and intercarrier interference (ICI) between the subcarriers in the off-diagonals, respectively. A variety of the banded equalizers manipulated the banded approximation of the channel matrix to be exploited by the low-complexity equalizations. In this paper, we derive a simple and tight lower bound on the variance of the individual coefficients in the channel matrix for insights into the banded approximations. We obtain the errors introduced with the banded approximation and the ICI-mitigation gains of the banded equalizers in simple closed forms. The derivations of the banded approximation errors (BAEs) are beneficially applicable to the equalizers that perform the minimum mean square error (MMSE) estimation with the banded channel matrix. Simulations show that both the block MMSE banded equalizers and the block turbo MMSE banded equalizers significantly reduce the error floors by considering the BAEs.

Authentication scheme for mobile OFDM based on security information technology of physical layer over time-variant and multipath fading channels

This work investigates security information technologies for wireless mobile environment. Traditionally, security is viewed as an independent feature addressed above the physical layer (PHY). All widely used cryptographic protocols are established assuming the PHY layer is merely used to provide an error-free link. However, with the emergence of ad hoc and decentralized networks, higher-layer security techniques are complex and hard to be implemented. To complement and enhance traditional security mechanism, we study a novel authentication scheme for mobile orthogonal frequency division multiplexing (OFDM) based on the channel power-delay profile (PDP),1The PDP is interchangeable with the channel-tap power in this study.1 considering channel time and frequency selectivities. More specifically, the PHY layer, employing the properties of OFDM signals over time-variant and multipath fading channels, is aimed to assist the overall authentication process. To identify different transmitters (TXs), we employ the hypothesis test based on their PDPs to distinguish different transmission terminals. It is advantageous to use the PDP as the wireless signature, because it depends on the surrounding environment and is therefore considered to be very hard to mimic. The proposed PDP estimation is obtained based on the redundancy of cyclic prefix (CP), which is a common feature for almost all OFDM systems. The performance is analyzed, and simulations confirm the advantages of the proposed method.

Improvement in Channel Estimation and Error Rate Performance in Mobile Cooperative OFDM Systems with Intercarrier Interference

In mobile cooperative orthogonal frequency division multiplexing (OFDM) systems with amplify-and-forward (AF) relays, in which the source and relay terminals are moving, Doppler frequency might become very large due to the formation of double-hop channels, possibly causing very large intercarrier interference (ICI). Therefore, ICI mitigation technique capable of reducing the impact of high Doppler frequency is required. This paper reports the study of ICI mitigation technique that uses frequency domain equalizer (FDE) with improvements in three aspects, namely, better channel estimation using cubic-spline approximation, use of selection combining to utilize cooperative diversity in order to suppress ICI, and simple error correcting code with double interleaving to suppress errors due to residual ICI. By using the proposed ICI mitigation technique, error rate performance of mobile cooperative OFDM systems can be improved significantly. Even at high Doppler frequencies, error floors due to residual ICI can be decreased by up to two decades.

A Low-Complexity Robust OFDM Receiver for Fast Fading Channels

Orthogonal frequency division multiplexing (OFDM) receivers suffer significant performance degradation in time-varying fast fading channels because of the inter-carrier interference (ICI) induced by the Doppler effect. We previously proposed a novel iterative decision-directed channel estimation and ICI cancellation technique which can provide robust performance for mobile OFDM receivers moving at fast vehicle speed. The nutshell of the proposed technique is a channel estimator which is capable of estimating the ICI gains on each off-diagonal vector of the channel frequency response matrix of a doubly-selective fading channel, which corresponds to the ICI induced by the neighbor subcarrier symbol to current subcarrier. In this paper, we generalize the channel estimator and provide theoretical analysis on the achievable performance. As an application to digital video broadcasting terrestrial system, we provide intensive simulation results and detailed comparison on implementation complexity and performance with existing techniques to showcase the significance of the proposed technique in practical applications.

Two compressive sensing‐based estimation schemes designed for rapidly time‐varying channels in orthogonal frequency division multiplexing systems

The problem of estimating rapidly time-varying channels is considered to be one of the key challenges in high mobility orthogonal frequency division multiplexing (OFDM) systems. In such scenarios, fast time variation within OFDM symbol duration requires overloaded measurements for estimation. By exploiting the inherent sparsity of wireless channels, the authors cast the channel estimation as a compressive sensing (CS) problem to reduce the required pilot symbols. Different from the existing CS-based estimators which mainly focus on the diagonal matrix model and treat the inter-carrier interference as additive noise, the proposed methods are designed for the non-diagonal matrix, a more precise representation of fast fading channels. To handle this more complex channel model, an iterative estimation scheme is presented, which adopts the recently introduced modified-CS algorithm. In addition, a more simplified scheme is also designed by utilising a reasonable approximation of the system model. Compared to the former method, it has a reduced computational complexity with limited performance degradation. The simulation results demonstrate that the two proposed CS-based methods are robust to large Doppler spreading and have better performance than conventional CS-based estimators for fast time-varying channels in OFDM systems.

The Receiving Method Based on Signal Separation by Spread Spectrum in the Cooperative System with Step Direct Channel

In high-speed mobile Orthogonal Frequency Division Multiplexing (OFDM) cooperative system, direct channel signal presents a step change due to carriage loss, which interferes with signal reception. In the proposed system, spread spectrum is adopted at relay to separate the receiving mixed signals. Subsequently, the fading coefficient threshold of direct channel is obtained by simulating outage probability of direct channel. Finally, separated direct channel signal is selectively received based on the ratio relationship between its current fading coefficient and the obtained threshold. The proposed receiving strategy reduces Bit Error Rate compared with other receiving methods. Moreover, throughput of the proposed system increases when the fading coefficient is higher than threshold.

Successive interference cancelation and MAP decoding for mobile MIMO OFDM systems and their convergence behavior

Turbo equalization schemes based on minimum mean square error criteria available in the literature for multiple-input multiple-output (MIMO) systems are computationally expensive, as they require a relatively large matrix inversion. In this article, we propose a suboptimal, successive interference cancelation (SIC)-based maximum a posteriori (MAP) decoding in doubly dispersive channels for orthogonal frequency division multiplexing (OFDM) MIMO systems (SIC-MAP-MIMO). SIC-MAP-MIMO leverages on the soft feedback symbol estimate to remove the intercarrier interference and coantenna interference from the received data thus making the subsequent MAP decoding simple. Extrinsic information transfer chart analysis supplemented with numerical simulation results show that SIC-MAP-MIMO achieves comparable BER performance to similar equalization schemes but with significant computational savings.

Second-Order Cyclostationarity of Mobile WiMAX and LTE OFDM Signals and Application to Spectrum Awareness in Cognitive Radio Systems

Spectrum sensing and awareness are challenging requirements in cognitive radio (CR). To adequately adapt to the changing radio environment, it is necessary for the CR to detect the presence and classify the on-the-air signals. The wireless industry has shown great interest in orthogonal frequency division multiplexing (OFDM) technology. Hence, classification of OFDM signals has been intensively researched recently. Generic signals have been mainly considered, and there is a need to investigate OFDM standard signals, and their specific discriminating features for classification. In this paper, realistic and comprehensive mathematical models of the OFDM-based mobile Worldwide Interoperability for Microwave Access (WiMAX) and third-Generation Partnership Project Long Term Evolution (3GPP LTE) signals are developed, and their second-order cyclostationarity is studied. Closed-from expressions for the cyclic autocorrelation function (CAF) and cycle frequencies (CFs) of both signal types are derived, based on which an algorithm is proposed for their classification. The proposed algorithm does not require carrier, waveform, and symbol timing recovery, and is immune to phase, frequency, and timing offsets. The classification performance of the algorithm is investigated versus signal-to-noise ratio (SNR), for diverse observation intervals and channel conditions. In addition, the computational complexity is explored versus the signal type. Simulation results show the efficiency of the algorithm is terms of classification performance, and the complexity study proves the real time applicability of the algorithm.

Improved ICI Mitigation Scheme over Time-varying Channels for High-Mobility OFDM Systems

Based on the space alternating generalized expectation maximization (SAGE) technique, and the ordered serial interference cancellation (OSIC) algorithm, an improved low-complexity inter-carrier interference (ICI) mitigation scheme, named SAGE-OSIC, is proposed for high mobile orthogonal frequency division multiplexing (OFDM) systems, where the subcarriers are updated in series, according to the order of the decreasing signal to interference plus noise ratio (SINR) or signal to noise ratio (SNR). The computational complexity is investigated, and simulation studies are concluded over multipath fading and rapidly time-varying channels. It is indicated that the proposed SAGE-OSIC scheme significantly outperforms the existing low-complexity ICI mitigation methods based on SAGE, without any increase in the computational complexity, especially for the large Doppler shifts in highly mobile environments. Furthermore, it can also be concluded that the performance of the proposed SAGE-OSIC approaches that of the vertical Bell laboratory layered space-time (VBLAST) scheme, which demands very high computation.

Performance of Gaussian and hyperbolic scattering channels in mobile OFDM systems

This paper estimates the inter-carrier-interference power P ICI of Gaussian and hyperbolic scattering channels in a mobile orthogonal frequency division multiplexing (OFDM) system by using their angle-of-arrival (AoA) probability density functions (pdf). The P ICI difference of the two channels is estimated and plotted as functions of ( τ , ζ ) , ( τ , ϵ ) and ( τ , f d T s ) , where τ is the ratio of the coverage distance D and effective radius R eff , ζ frequency tracking, f d maximum Doppler spread, T s symbol duration and ϵ travelling direction of a user equipment. Experimental P ICI difference for ϵ = 0 , ζ = 0 and f d T s = 0.05 is obtained and plotted with the simulated results for comparison purposes. The ICI power is also plotted against ( f d T s , ϵ ) , ( f d T s , ζ ) , and ( ζ , ϵ ) for specific values of τ . Detailed analyses on the performance of the hyperbolic and Gaussian AoA pdfs are given. Relative merits of each pdf are discussed. Closed-form bit error rate (BER) expressions of OFDM communication systems in an added white Gaussian noise (AWGN) channel and Rayleigh fading environment using bipolar phase shift keying (BPSK), OFDM-BPSK, are derived as functions of P ICI and phase noise to validate the effectiveness of the approximate Gaussian and hyperbolic AoA pdfs. Future work is also outlined.

Optimum superimposed training for mobile OFDM systems

Superimposed training (SIT) design for estimating of time-varying multipath channels is investigated for mobile orthogonal frequency division multiplexing (OFDM) systems. The design of optimum SIT consists of two parts: The optimal SIT sequence is derived by minimizing the channel estimates' mean square error (MSE); the optimal power allocation between training and information data is developed by maximizing the averaged signal to interference plus noise ratio (SINR) under the condition of equal powered paths. The theoretical analysis is verified by simulations. For the metric of the averaged SINR against signal to noise ratio (SNR), the theoretical result matches the simulation result perfectly. In contrast to an interpolated frequency-multiplexing training (FMT) scheme or an SIT scheme with random pilot sequence, the SIT scheme with proposed optimal sequence achieves higher SINR. The analytical solution of the optimal power allocation is demonstrated by the simulation as well.

ICI reduction scheme for OFDM system with phase noise over time‐varying channels

AbstractFor mobile orthogonal frequency division multiplexing (OFDM) systems, time‐varying channels and random phase noise introduced by the oscillator result in severe intercarrier interference (ICI), respectively, and degrade the overall OFDM system performance. However, the existing ICI reduction methods only aim at a single interference source, i.e. either time‐varying channels or phase noise. Therefore, these methods are not suitable for the actual situation. In this paper, we analyze the spectral property of the transfer function composed of time‐varying channels and phase noise, and propose that the transfer function can be approximated by a finite parameter complex exponential basis expansion model (CE‐BEM). Then, a pilot‐aided minimum mean square error estimation is adopted to estimate the CE‐BEM coefficients in order to reconstruct the transfer function and reduce ICI. Finally, our simulation results show how the proposed scheme would improve the system performance in a time‐varying environment with phase noise. Copyright © 2008 John Wiley & Sons, Ltd.

Coherent and Differential ICI Cancellation for Mobile OFDM with Application to DVB-H

We develop a reduced-complexity hybrid frequency/time-domain orthogonal frequency division multiplexing (OFDM) channel estimation algorithm for high-mobility scenarios where the channel varies significantly within each OFDM block, resulting in severe intercarrier interference (ICI). The algorithm exploits the banded and symmetric structure of the channel matrix in the frequency domain and its sparse structure in the time-domain to achieve significant complexity reductions, which we quantify for the Digital Video Broadcasting-Handheld (DVB-H) system. Furthermore, we compare coherent and differential mobile OFDM detection for DVB-H.