Orthogonal Frequency Division Multiplexing Symbol Duration Research Articles

Non pilot data-aided carrier and sampling frequency offsets estimation in fast time-varying channel

This paper considers the non pilot data-aided estimation of the carrier frequency offset (CFO) and sample frequency offset (SFO) of orthogonal frequency division multiplexing (OFDM) signals in fast time-varying channel. The main obstacle is the time-variant channel response, which deteriorates the estimation validity. A practical approach to mitigate this impact is to reduce the time consumption of one-shot estimation. In this way, we propose a method to reduce the time consumption to within one OFDM symbol duration. The maximum likelihood (ML) estimator is derived based on the observations of frequency domain constellations output of two FFTs on one symbol; its closed-form approximation is then derived to reduce the calculation burden. Remarkably, our method does not require any training symbol or pilot tone embedded in the signal spectrum, therefore achieves the highest spectral efficiency. Theoretical analysis and simulation results are employed to assess the performance of proposed method in comparison with existing alternatives.

Highly Accurate Technique for CO-OFDM Channel Estimation Technique Using Extreme Learning Machine (ELM)

In wireless systems, channel estimation is considered a problematic technology, due to the fact of the difference in time between wireless channels and the noise effect. Orthogonal frequency-division multiplexing (OFDM) is a promising candidate for future optical communications and has received wide concern. The article proposed a Coherent Optical (CO) orthogonal frequency division multiplexing (OFDM) scheme, which gives a scalable and flexible solution for increasing the transmission rate, being extremely robust to chromatic dispersion as well as polarization mode dispersion. Nevertheless, both coherent detection and OFDM are prone to phase noise due to the phase mismatch between the laser oscillators at the transmitter and receiver sides and the relatively long OFDM symbol duration compared to that of single carrier communications. An Extreme Learning Machine (ELM) with Pilot Assisted Equalization (PEM) is proposed for compensation of impairments caused by fibre nonlinearity in coherent optical communication systems. Channel estimation using ELM and the value of distortion is sent to the OSTBC receiving end based on the distortion information the data is decoded and pilot data is removed. FFT is applied to the data and QPSK demodulation is done in the data to get its original form. In addition, the article utilized a free-space optical communication system of multi-input multi-output orthogonal frequency division multiplexing (MIMO-OFDM) with a modified receiver structure. Simulation reveals that the proposed model exhibits significant BER (0.0112) performance and provides better spectral efficiency as compared with conventional systems and less computational complexity. This suggested that the proposed method shows better performance by using the CO-OFDM-FSO-MIMO-ELM-based channel estimation technique for high-speed data communication networks in real-time scenarios respectively.

Fast Channel Estimation for IRS-Assisted OFDM

In this letter, we study efficient channel estimation for an intelligent reflecting surface (IRS)-assisted orthogonal frequency division multiplexing (OFDM) system to achieve minimum training time. First, a fast channel estimation scheme with reduced OFDM symbol duration is proposed for arbitrary frequency-selective fading channels. Next, under the typical condition that the IRS-user channel is line-of-sight (LoS) dominant, another fast channel estimation scheme based on the novel concept of sampling-wise IRS reflection variation is proposed. Moreover, the pilot signal and IRS training reflection pattern are jointly optimized for both proposed schemes. Finally, the proposed schemes are compared in terms of training time and channel estimation performance via simulations, as well as against benchmark schemes.

Inner-symbol Interference (INSI) reduction in OFDM system using distinctive characteristics of the pilot symbols


 
 
 
 This research presents a wireless communication system using Quadrature Amplitude Modulation (16QAM) depending on using of the Orthogonal Frequency Division Multiplexing (OFDM). OFDM transmission system can effectively reduce inter symbol interference (ISI) caused by multipath fading, especially in the case of broadband data transmission. There are two kind of interference; inter symbol interference (ITSI) and inner symbol interference (INSI). ITSI is the interference caused by the delayed waves with larger than OFDM symbol duration, in order to avoid ITSI effectively, we insert guard interval every each OFDM symbol. On other hands, INSI is the interference caused by the delayed waves with less than OFDM symbol duration. To avoid INSI, this work proposes a new scheme in order to estimate the times of delayed waves by using distinctive characteristics of OFDM signal, which is inserted pilot signal periodically in frequency axis before IFFT at the transmitter. In this paper, we evaluate the estimation accuracy of the proposed estimation method and the BER performance of the proposed system under multipath fading environment by computer simulation with MATLAB.
 
 
 

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.

On Detection in MIMO-OFDM Systems Over Highly Mobile Wireless Channels

High mobility support of multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) systems plays a major role in wireless communication systems. Conventional detection approaches for MIMO-OFDM systems assume the channel is constant for at least one OFDM symbol duration. However, the time variation of the channel on account of mobility over an OFDM symbol period causes a loss of subchannel orthogonality which results in inter-carrier interference. In this paper, we propose the space alternating generalized expectation maximization (SAGE) based signal detection for MIMO-OFDM systems and compared by the zero forcing (ZF), minimum mean square error (MMSE), vertical Bell laboratories layered space-time (VBLAST) detection methods by considering complexity and performance tradeoffs. Simulation results show that the SAGE based detection has comparable performance with the VBLAST while needs lower computational complexity. It is also shown that both SAGE and VBLAST have a clear BER performance advantage over the MMSE and ZF detections. We also investigate the sensitivity of detection MIMO-OFDM signals to channel estimation errors by the application of a simple frequency domain based pilot based channel estimation. It is shown that the superiority of the SAGE scheme is also valid in the case of channel imperfections.

Joint Channel Estimation and Symbol Detection for OFDM Systems in Rapidly Time-Varying Sparse Multipath Channels

In this paper, we propose a space-alternating generalized expectation maximization (SAGE) based joint channel estimation and data detection algorithm in compressive sensing (CS) framework for orthogonal frequency-division multiplexing (OFDM) systems in rapidly time-varying sparse multipath channels. Using dynamic parametric channel model, the sparse multipath channel is parameterized by a small number of distinct paths, each represented by the path delays and path gains. In our model, we assume that the path gains rapidly vary within the OFDM symbol duration while the number of paths and path delays vary symbol by symbol. Since the convergency of the SAGE algorithm needs statistically independent parameter set of interest to be estimated, we specifically choose the discrete orthonormal Karhunen---Loeve basis expansion model (DKL-BEM) to provide statistically independent BEM coefficients within one OFDM symbol duration and use just a few significant BEM coefficients to represent the rapidly time-varying path gains. The resulting SAGE algorithm that also incorporates inter-channel interference cancellation updates the data sequences and the channel parameters serially. The computer simulations show that our proposed algorithm has better channel estimation and symbol error rate performance than that of the orthogonal matching pursuit algorithm that is commonly proposed in the CS literature.

Robust Decision Feedback Equalizer for OFDM System under Severe ISI Channel

Inter-symbol interference (ISI) problem is inevitable when the guard interval (GI) is shorter than the delay spread (DS) for an orthogonal frequency division multiplexing (OFDM) system. Iterative techniques have been proposed to overcome such a problem. However, most of existing algorithms are not efficient for an OFDM system with a small GI working under the channel with a large DS. Especially in the case of the DS spans a longer time than the half of the OFDM symbol duration. On the other hand, conventional algorithms, which can reduce the effects of the severe ISI, often employ several impractical assumptions to support the conclusions. In this paper, we present a robust decision feedback equalizer (DFE) for the OFDM system to overcome the severe ISI problem. The proposed DFE removes the ISI in a same manner as the residual inter-symbol interference cancellation (RISIC) algorithm. However, the inter-carrier interference (ICI) is reduced via cyclicity removal instead of the cyclicity restoration used in the conventional algorithms. The link-level simulation (LLS) results indicate that our proposed DFE scheme can dramatically improve the BER performance when the DS spans longer than the half of ODFM symbol duration.

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.

An Improved Channel Estimation Method in OFDM System

In this paper, we will focus on channel estimation (CE) in orthogonal frequency-division multiplexing (OFDM) systems. The time-varying (TV) channelsare modeled by a basis expansion model (BEM). Due to the time-variation, the channel matrix in the frequency domain is no longer diagonal, but approximately banded.We use a pilot-aided algorithm for estimation of rapidly varying wireless channels in OFDM systems. Theperforms is goodwhen the channels vary on the scale of a single OFDM symbol duration, which occurs in mobile communication scenarios such as WiMAX, WAVE, and DVB-T.We recover Fourier coefficients of the channel taps by the pilot information.We then estimate the BEM coefficients of the channel taps from their respective Fourier coefficients using a recently developed inverse reconstruction method.We compare some BEM models in inverse methodsto find out the best ones in certain conditions.

Signal detection through circular convolution reconstruction for OFDM system in fast varying channel

A signal detection algorithm is proposed for the orthogonal frequency division multiplexing (OFDM) system in the presence of fast time-varying channel. The channel is represented by a piece-wise linear variant model with normalized Doppler frequency of less than 0.2. The channel parameters are extracted through ISI/ICI (inter-symbol interference/inter-carrier interference) cancellation and circular convolution reconstruction. Meanwhile, an improved OFDM symbol detection algorithm is also proposed based on circular convolution reconstruction. The channel state information in the OFDM symbol duration can be obtained accurately from the adjacent two block pilots in a linear model. The simulation results show that the proposed method can not only track the channel variation, but also promise better performance gain in the OFDM symbol detection. Furthermore, the bit error ratio (BER) is close to the performance with the perfect channel state information.

광범위 고정형 무선 통신 시스템을 위한 상향 링크 초기 레인징 기법 설계

In this paper, an enhanced initial ranging algorithm for large-cell coverage fixed wireless communication system is proposed. In typical wireless communication system such as WiBro, because a round-trip delay between a transmitter and a receiver is within one OFDM (Orthogonal Frequency Division Multiplexing) symbol duration, a frequency-domain differential correlation method is generally used. However, the conventional method cannot be applied due to an increase of a maximum time delay in large-cell system. In case of an accumulative differential method, estimation errors can occur because of frequent sign transitions. In this paper, therefore, we propose an algorithm which can estimate a total timing offset in a ranging channel structure for 15 km cell. The proposed method can improve performance by sign comparison based sign error correction rule between the estimated values and using a weighting scheme based on channel correlation, the number of accumulations, and the noise reduction effect in normalization process. Also, it can estimate the integer timing offset of symbol duration by comparing peak-powers after compensating for the fractional timing offset of symbol duration.

Characterisation of impulse noise effects on space-time block-coded orthogonal frequency division multiplexing (OFDM) signal reception

This correspondence presents the characterisation of impulse noise effects on the space-time block-coded orthogonal-frequency-division-multiplexing (STBC-OFDM) symbol reception, in which the conventional OFDM system is combined with the antenna-diversity/spatial-diversity in order to achieve the high data transmission rates. The weak impulse noise may be combated by the longer OFDM symbol duration, in which the total energy of noise spreads among the simultaneously transmitted OFDM sub-carriers irrespective of the impulse noise energy distribution. Specifically, the focus of the presented work is on “noise bucket” effect, in which the STBC-OFDM system is considered to be working in the impulsive environment. It is demonstrated by simulation results that for the occurrence of a significant number of impulses per symbol duration, the noise distribution at the input of the receiver’s final decision device is approximately Gaussian. Therefore for simulation purpose, the impulsive environment may be generated by the mere incorporation of equivalent Gaussian noise. Besides, some simulation results are presented using 16-QAM / STBC-OFDM system to give some insight into the symbol error rate variations with respect to the different number of sub-carriers and characteristics of impulsive environment. Key words: Impulse noise, space-time block-coded (STBC), orthogonal-frequency-division-multiplexing, Gaussian distribution, i nverse discrete Fourier transform (IDFT), M-ary QAM.

Carrier Frequency Offset Estimation for OFDM Systems Over Mobile Radio Channels

In this paper, a new technique is proposed for blind estimation of carrier frequency offset (CFO) in wireless orthogonal frequency-division multiplexing (OFDM) systems with constant-modulus constellations. The proposed scheme is based on the assumption that the channel slowly changes in the time domain with respect to the OFDM symbol duration. As a consequence, the channel effect on a given subcarrier in two consecutive OFDM symbols is approximately the same. Based on this assumption, a cost function is derived such that the power difference between all subcarriers in two consecutive OFDM symbols is minimized. Using Monte Carlo simulation, we demonstrate that the proposed scheme has superior performance in both static and time-varying frequency-selective fading channels. The proposed system can rapidly and accurately estimate the CFO using only three trial values, given that the CFO is less than half of the subcarriers' frequency spacing.

Low-complexity joint data detection and channel equalisation for highly mobile orthogonal frequency division multiplexing systems

This study is concerned with the challenging and timely problem of channel equalisation and data detection for orthogonal frequency division multiplexing (OFDM) systems in the presence of frequency-selective and very rapidly time-varying channels. The algorithm is based on the space alternating generalised expectation-maximisation (SAGE) technique which is particularly well suited to multicarrier signal formats and can be easily extended to multi-input multi-output-OFDM systems. In fast fading channels, the orthogonality between subcarriers is destroyed by the time variation of a fading channel over an OFDM symbol duration which causes severe inter-carrier interference (ICI) and, in conventional frequency-domain approaches, results in an irreducible error floor. The proposed joint data detection and equalisation algorithm updates the data sequences in series leading to a receiver structure that also incorporates ICI cancellation, enabling the system to operate at high vehicle speeds. A computational complexity investigation as well as detailed computer simulations indicate that this algorithm has significant performance and complexity advantages over existing suboptimal detection and equalisation algorithms proposed earlier in the literature.

Analysis and Compensation of DC Offset in OFDM Systems Over Frequency-Selective Rayleigh Fading Channels

Direct-conversion receivers have become popular in radio-frequency (RF) circuit design for their advantages of low cost, low power consumption, and fewer chip components over other architectures such as heterodyne receivers. However, the direct-conversion receiver architecture often suffers from direct current (dc) offset, which is a consequence of the imperfect direct-conversion process. In this paper, we study the effects of dc offset on the symbol error rate (SER) performance of orthogonal frequency-division multiplexing (OFDM) systems in multipath Rayleigh fading channels. Since OFDM system performance is sensitive to carrier frequency offset (CFO), the CFO must be estimated and compensated at the receiver. Due to CFO compensation, the dc offset caused by direct-conversion receivers and/or mixed-signal circuits no longer only affects the dc subcarrier and is spread over all subcarriers. By deriving the analytical SER formulas for OFDM systems with various modulation formats, the dependency of SER on dc offset and CFO is clearly quantified. These SER formulas can help system designers determine suitable specifications of RF components and understand whether digital DC offset compensation is necessary or not. Finally, we propose and analyze a simple DC offset estimation and cancellation scheme under the assumption that DC offset holds constant in one OFDM symbol duration. Numerical results demonstrate the effectiveness of the proposed DC offset-cancellation scheme.

ICI mitigation for pilot-aided OFDM mobile systems

Orthogonal frequency-division multiplexing (OFDM) is robust against frequency selective fading due to the increase of the symbol duration. However, for mobile applications channel time-variations in one OFDM symbol introduce intercarrier-interference (ICI) which degrades the performance. This becomes more severe as mobile speed, carrier frequency or OFDM symbol duration increases. As delay spread increases, symbol duration should also increase in order to maintain a near-constant channel in every frequency subband. Also, due to the high demand for bandwidth, there is a trend toward higher carrier frequencies. Therefore, to have an acceptable reception quality for the applications that experience high delay and Doppler spread, there is a need for ICI mitigation within one OFDM symbol. We introduce two new methods to mitigate ICI in an OFDM system with coherent channel estimation. Both methods use a piece-wise linear model to approximate channel time-variations. The first method extracts channel time-variations information from the cyclic prefix. The second method estimates these variations using the next symbol. We find a closed-form expression for the improvement in average signal-to-interference ratio (SIR) when our mitigation methods are applied for a narrowband time-variant channel. Finally, our simulation results show how these methods would improve the performance in a highly time-variant environment with high delay spread.

Impulsive noise suppression in OFDM based communication systems

Orthogonal frequency division multiplexing (OFDM) is a technique used for terrestrial digital video broadcasting (DVB-T) and many other modern applications. The longer OFDM symbol duration provides an advantage in a presence of weak impulsive noise, because impulsive noise energy is spread among simultaneously transmitted OFDM sub-carriers. However, it has been recently recognized that this advantage turns into a disadvantage if the impulsive noise energy exceeds certain threshold. In this paper the algorithm for impulsive noise suppression in OFDM receivers is proposed and investigated. Whereas traditional methods for impulsive noise suppression are implemented in a time domain before OFDM demodulation, proposed algorithm compensates impulsive noise in a frequency domain after OFDM demodulation and channel equalization. The method is applied to DVB-T and its performance is studied by means of simulation.

A sequential Monte Carlo blind receiver for OFDM systems in frequency-selective fading channels

We consider the application of the sequential Monte Carlo (SMC) methodology to the problem of blind symbol detection in a wireless orthogonal frequency-division multiplexing (OFDM) system over a frequency-selective fading channel. Bayesian inference of the unknown data symbols in the presence of an unknown multipath fading channel is made only from the observations over one OFDM symbol duration. A novel blind SMC detector built on the techniques of importance sampling and resampling is developed for differentially encoded OFDM systems. The performance of different schemes of delayed-weight estimation methods is studied. Furthermore, being soft-input and soft-output in nature, the proposed SMC detector is employed as the first-stage demodulator in a turbo receiver for a coded OFDM system. Such a turbo receiver successively improves the receiver performance by iteratively exchanging the so-called extrinsic information with the maximum a posteriori (MAP) outer channel decoder. Finally, the performance of the proposed sequential Monte Carlo receiver is demonstrated through computer simulations.

Bounds on the interchannel interference of OFDM in time-varying impairments

In this article, tight and universal bounds have been derived for the interchannel interference (ICI) of an orthogonal frequency-division multiplexing (OFDM) signal resulting from Doppler spread. The universal bound depends only on the product of the maximum Doppler frequency and the OFDM symbol duration. The tight bound also depends on the variance of the Doppler spectrum. Compared with the exact ICI expressions derived by other researchers, these bounds are easier to evaluate and can provide useful insight.