Orthogonal Frequency Division Multiplexing Symbol Research Articles

A Novel Joint Method for Frequency Offset Estimation and Peak-to-Average Power Ratio Reduction in OFDM Systems Using Null Subcarriers

Orthogonal frequency division multiplexing (OFDM) is a spectral efficient multi-carrier modulation technique which offers many advantages in wireless as well as wired communication systems. Major issues of OFDM implementation are its high peak to average power ratio (PAPR) and extreme sensitivity to carrier frequency offset (CFO). In this paper, we propose a new scheme named optimum null swapping for combined PAPR reduction and CFO estimation in OFDM systems by using one training symbol (pilot) with some null subcarriers embedded in it. We exploit certain number of nulls in the pilot symbol for swapping with some data subcarriers of the data block in such a way as to reduce PAPR. By the proper allocation of null subcarriers based on some specific sequences at the transmitter, CFO is estimated at the receiver using the same pilot OFDM symbol. The computational complexity of the new objective function for the proposed joint optimum null swapping scheme is made significantly low by using the Tabu search algorithm. Moreover, the proposed method demands less side information to be send to the receiver for the reverse swapping of nulls in order to retrieve original data symbols. Simulation results show that the proposed joint scheme offers substantial reduction in PAPR apart from CFO estimation without any additional bandwidth overhead.

Extension of <inline-formula> <tex-math notation="TeX">$N$</tex-math></inline-formula>-Continuous OFDM Precoder Matrix for Improving Error Rate

The modulation technique of N-continuous orthogonal frequency division multiplexing (OFDM) has lower sidelobes than those of the original OFDM as a result of the continuous connection with higher-order derivatives between the OFDM symbols. Compared with the other sidelobe suppression schemes, for example, cancellation carrier insertion, and windowing, N-continuous OFDM does not require an extended guard interval nor the insertion of a wide guard band or cancellation carriers. However, N-continuous OFDM requires an iterative algorithm to remove the correction symbols from received symbols at the receiver and the large power of the correction symbols in the high-frequency area results in unavailable bandwidth. Here, we propose an extension of the N-continuous OFDM precoder matrix that flattens the bias power caused by the correction symbols in all frequency areas. Numerical experiments show that the proposed method improves the error rate performance without degrading the sidelobe suppression performance. The proposed method can also reduce the unavailable bandwidth.

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.

Multicarrier chaotic communications in multipath fading channels without channel estimation

A multi-carrier chaotic shift keying(MC-CSK) communication scheme with low probability of interception(LPI) is proposed in this article. We apply chaotic spreading sequences in the frequency domain, mapping a different chip of a chaotic sequence to an individual orthogonal frequency division multiplexing(OFDM) subcarrier. In each block size of $M$ OFDM symbols, we use one pilot OFDM symbol inserted time-spaced in all-frequency to transmit the reference chaotic signal and use the other M-1 OFDM symbols to transmit the information-bearing signals each spreaded by the reference chaotic signal. At the receiver, we construct a differential detector after DFT and recover the information bits from the correlations between the pilot OFDM symbol and the other M-1 OFDM symbols in each block size of M. Performance analysis and computer simulations show that the MC-CSK outperforms differential chaos shift keying(DCSK) in AWGN channels with high bandwidth efficiency for the block size of M=2 and that the MC-CSK exploits effectively the frequent diversity of the multipath channel.

Analysis of the Effect of Coherence Bandwidth on Leakage Suppression Methods for OFDM Channel Estimation

In this paper, we analyze the effect of the coherence bandwidth of wireless channels on leakage suppression methods for discrete Fourier transform (DFT)-based channel estimation in orthogonal frequency division multiplexing (OFDM) systems. Virtual carriers in an OFDM symbol cause orthogonality loss in DFT-based channel estimation, which is referred to as the leakage problem. In order to solve the leakage problem, optimal and suboptimal methods have already been proposed. However, according to our analysis, the performance of these methods highly depends on the coherence bandwidth of wireless channels. If some of the estimated channel frequency responses are placed outside the coherence bandwidth, a channel estimation error occurs and the entire performance worsens in spite of a high signal-to-noise ratio.

Efficient co-channel interference suppression in MIMO-OFDM systems

In this paper, we investigate channel frequency response (CFR) matrix and interference-plus-noise covariance matrix (ICM) estimation in multiple-input multiple-output (MIMO) and orthogonal frequency division multiplexing (OFDM) systems to suppress co-channel interference at the receiver side. We employ least square criterion to perform the initial CFR estimation. Then we estimate the autocorrelation function of interference-plus-noise in the time domain, instead of estimating the ICM in the frequency domain directly. The autocorrelation function estimation has two steps. Firstly, we present the inherent relationship between the expectation of the sample autocorrelation function of the residual (SAFR) and the true autocorrelation function, which is actually a linear transformation. Based on this, we propose a compensating method. This compensation brings significant performance gains when the pilot OFDM symbol number is small. Secondly, since the compensated SAFR cannot be guaranteed to be an autocorrelation sequence (ACS), which will make the obtained ICM loss the positive semidefinite property, we utilize semidefinite programming (SDP) to find an ACS that is the nearest to the compensated SAFR. The SDP is solved in its dual form, which yields a significant reduced complexity. Finally the estimated ICM is reutilized to revise the CFR estimation. The estimated CFR and ICM show excellent interference suppression performances when being applied in an interference rejection combining receiver.

Composite Effect of E/O Nonlinearity and Optical Echo on EPWM-OFDM Transmission in Radio Over Fiber Channel

Radio over Fiber (RoF) is a promising technology for extending broadband wireless access to indoor and outdoor dead spots. However, commonly applied intensity-modulation / direct-detection RoF transmission presents some nonlinearity and produces echo distortion. Since multiple carrier modulation signals such as Orthogonal Frequency Division Multiplexing (OFDM) are widely employed in broadband wireless systems, the above issues become more serious. The OFDM signals have high Peak-to-Average Power Ratio, which makes them more sensitive to nonlinearity. Moreover, the large delay of optical echoes that are caused by the discontinuity of refractive index at both ends of Electrical/Optical (E/O) and Optical/Electrical converters exceeds the guard time of current OFDM symbols when the length of optical fiber is long. Since the nonlinearity and echo exist simultaneously in RoF channels, their composite effect is mathematically analyzed in this paper. A RoF-echo model is proposed for the modeling of the multiple light reflections and its validity is confirmed by experiments. Then, Envelop-Pulse-Width-Modulation- (EPWM-) RoF transmission is proposed for dealing with the nonlinearity and echo in RoF channel. The performance of the EPWM-RoF scheme for OFDM signal transmission is evaluated by simulations, which reveals that the scheme improves the transmission quality in Error Vector Magnitude under the composite environment of E/O nonlinearity and optical echo.

Fast Convergence on Blind and Semi-Blind Channel Estimation for MIMO–OFDM Systems

In this paper, blind and semi-blind subspace channel estimations with fast convergence rate are proposed for multiple-input multiple-output orthogonal frequency division multiplexing (OFDM) systems in the presence of virtual carriers. The subspace method has a main drawback of slow convergence rate when deriving the noise subspaces from the second-order statistics of received signals. This phenomenon is especially evident when the size of received signals is large. Inspired by this fact, we present a block matrix scheme (BMS) to generate a group of sub-vectors from each OFDM symbol when virtual carriers are used. The number of equivalent signals is increased and therefore, the convergence rate of channel estimation is enhanced. The semi-blind method is also investigated by incorporating subspace technology with least square scheme. The identifiability of the BMS-based channel estimation is analyzed to derive the applicable range of the BMS size. The computational complexity of the proposed channel estimation is calculated at the end. Computer simulations show that the proposed blind and semi-blind methods converge very well in channel estimation and equalization.

PAPR reduction in OFDM systems using peak insertion

A technique for Peak-to-Average Power Ratio (PAPR) reduction of Orthogonal Frequency Division Multiplexing (OFDM) signals by Peak Insertion (PI) is proposed in this paper. PI depends on the duality property of the DFT and PAPR duality of an impulse. A relatively high peak is inserted to the OFDM symbol in the frequency domain such that the PAPR of the transmitted time domain signal is reduced. PAPR reduction up to about 11dB at PAPR CCDF of 10−4 is achievable by PI, but at the expense of increased signal power, which can be reduced by scaling to the desired level without affecting its PAPR. Computer simulation tests show that a tradeoff can easily be made between BER and the PAPR of the transmitted signal to achieve a desired system performance. The PI technique does not require any sort of side information transmission, searching, optimization, and iterative or parallel application of IDFT. PI is faster, simpler, and capable of achieving greater PAPR reduction as compared with other similar techniques which result in transmitted power increase, and therefore, it seems to be suitable to be used for OFDM signal PAPR reduction.

ACO-OFDM-Specified Recoverable Upper Clipping With Efficient Detection for Optical Wireless Communications

The high peak-to-average-power ratio (PAPR) of orthogonal frequency-division multiplexing (OFDM) degrades the performance in optical wireless communication systems. This paper proposes a modified asymmetrically clipped optical OFDM (ACO-OFDM) with low PAPR via introducing a recoverable upper-clipping (RoC) procedure. Although some information is clipped by a predetermined peak threshold, the clipped error information is kept and repositioned in our proposed scheme, which is named RoC-ACO-OFDM, instead of simply being dropped in conventional schemes. The proposed method makes full use of the specific structure of ACO-OFDM signals in the time domain, where half of the positions are forced to zeros within an OFDM symbol. The zero-valued positions are utilized to carry the clipped error information. Moreover, we accordingly present an optimal maximum a posteriori (MAP) detection for the RoC-ACO-OFDM system. To facilitate the usage of RoC-ACO-OFDM in practical applications, an efficient detection method is further developed with near-optimal performance. Simulation results show that the proposed RoC-ACO-OFDM achieves a significant PAPR reduction, while maintaining a competitive bit-error rate performance compared with the conventional schemes.

A Blind Channel Estimation for 100+ Gb/s Optical IM-DD DMT Over 100-km SMF in 1550 nm

A blind and adaptive decision-feedback channel estimation method is proposed for optical intensity-modulated direct detection discrete multitone transmissions. Due to the channel state variation nature of the fiber, periodic channel estimation update is required. In previous works, periodic preambles are transmitted to acquire and track the channel state information at the receiver. In the proposed method, instead of using periodic preambles, the estimated channel is updated adaptively after decoding a sequence of orthogonal frequency-division multiplexing symbols using least-squares and minimum mean-square error criteria. The simulation results corroborate that the proposed method increases data-rate by 10%, and at the same time provides 0.24-dB Q-factor gains that can be converted into extra 2% data-rate increase, algorithm simplicity, or cost reduction.

Performance of Space–Time–Frequency Coding Over Indoor Power Line Channels

We present a multiple-input-multiple-output (MIMO) power line communication (PLC) system using coupled indoor differential mode ports, where 2 × 2 and 2 × 4 MIMO configurations are considered. In simulation, MIMO orthogonal frequency-division multiplexing (OFDM) PLC with the two OFDM symbol sequence reordering schemes, namely, Alamouti-scheme-based sequence reordering and circularly-shifted sequence reordering, is compared under the coupled channel conditions. For MIMO coding, we employ a maximum ratio combining scheme that is effective for both MIMO and single-input-single-output over indoor power line conditions. In simulation, we compare space-frequency coding (SFC), space-time coding (STC), and space-time-frequency coding (STFC) over statistical indoor power line channels, including fading, impulse noise, and crosstalk by coupling between ports. STFC and STC are more robust against crosstalk than SFC, and STFC, with some added complexity, outperforms the other coding schemes in terms of bit-error-rate performance. A least squares scheme for power line channel estimation is used, which is simple but has comparable performance to minimum mean square error. A decision-feedback-loop-based crosstalk cancellation scheme that effectively reduces the MIMO channel estimation error due to crosstalk is presented. Simulation results show that STFC is less sensitive to impulse noise than STC or SFC.

Joint estimation of state and noise parameters in a linear dynamic system with impulsive measurement noise: Application to OFDM systems

Interference mitigation is one of the main challenges in wireless communication, especially in ad hoc networks. In such context, the Multiple Access Interference (MAI) is known to be of an impulsive nature. Therefore, the conventional Gaussian assumption is inadequate to model this type of interference. Nevertheless, it can be accurately modeled by stable distributions. In fact, it was shown in literature that the α-stable distribution is a useful tool to model impulsive data. In this paper, we tackle the problem of noise compensation in ad hoc networks. More precisely, this issue is addressed within an Orthogonal Frequency Division Multiplexing (OFDM) transmission link assuming a symmetric α-stable model for the signal distortion due to MAI. Based on Bayesian estimation, the proposed approach estimates the transmitted OFDM symbols in the time domain using the Sequential Monte Carlo (SMC) methods. Unlike existing schemes, we consider the more realistic case where the impulsive noise parameters are assumed to be unknown at the receiver. Consequently, our approach deals also with the difficult task of noise parameters estimation which can be very useful for other purposes such as target tracking in wireless sensor networks or channel estimation. Simulations results, provided in terms of Mean Square Error (MSE) and Bit Error Rate (BER), illustrate the efficiency and the robustness of this scheme.

Effective mirror-mapping-based intercarrier interference cancellation for OFDM underwater acoustic communications

Orthogonal frequency-division multiplexing (OFDM) techniques are promising for underwater acoustic (UWA) communications due to their robustness against large delay spread. However, OFDM systems in UWA channels suffer from intercarrier interference (ICI) caused by the Doppler effect. In this paper, we propose four low-complexity mirror-mapping-based ICI cancellation schemes in OFDM UWA communications without explicitly estimating ICI coefficients or carrier frequency offset (CFO), namely the mirror symbol repetition (MSR) scheme, the mirror conjugate symbol repetition (MCSR) scheme, the mirror conversion transmission (MCVT) scheme, and the mirror conjugate transmission (MCJT) scheme. We theoretically evaluate their performance by deriving their carrier-to-interference power ratio (CIR) expressions in multipath Rayleigh fading channels with CFO. The key idea of our proposed schemes is to employ data repetition within one or two consecutive OFDM symbols according to the mirror-mapping rules and apply the conjugate or conversion operations to remove ICI. Theoretical analyses and numerical results show that the proposed mirror-mapping-based schemes can effectively improve the CIR in comparison with the plain OFDM and the adjacent-mapping-based schemes. In addition, the effects of the channel length and the CFO deviation between two consecutive OFDM symbols are investigated. Finally, to demonstrate their benefits for OFDM UWA communications, these schemes have also been tested in a recent sea experiment conducted in Taiwan in May 2013. The decoding results validate that the proposed mirror-mapping-based schemes significantly improve the bit error rate (BER) performance of OFDM communications in UWA channels.

A New Approach to Enhance Performance of OFDM using BCH and Newly Designed Filter

Orthogonal frequency division multiplexing (OFDM) is a multicarrier communication technique and it has capability to rise above the frequency-selective fading problem, which is common in wireless broadband communications. This has made it popular since last few years. The OFDM system suffers from a number of drawbacks such as the high peak-toaverage power ratio (PAPR). Many PAPR reduction techniques used either destroy the system performance, which is the clipping technique or increases the overall complexity of the system, such as the selective mapping (SLM). In this paper a unique approach has been proposed to obtain a new code from the existing codes in order to reduce the bit error rate (BER), to mitigate the gap between the performance and Shannon limit while the bounded peak average power ratio (PAPR) of OFDM symbol is guaranteed. A new code based on the BCH with a Barthannwin Wave Filter has been proposed to achieve the desired results. General Terms BCH with Barthannwin Wave Filter

Large size FFTs over time‐varying channels

In orthogonal frequency division multiplexing (OFDM) signals, the usage of large size fast Fourier transforms (FFTs) reduces the guard interval percentage, and therefore increases the data throughput reducing the data overhead. In addition, for the same pilot pattern, the distance between adjacent pilots is smaller, which will improve the channel estimation. Nevertheless, up to now, they have not been considered for delivering mobile services as the inter-carrier interference (ICI) due to the Doppler effect is very critical. The main objective is to show that taking advantage of the latest improvements in error correction techniques, it is feasible to use large size FFTs for time-varying channels. Furthermore, there is also presented a theoretical estimation for quantifying the loss due to the ICI, and finally several simulation results that reinforce the idea that large OFDM symbols are suitable for mobile channels.

An Integer Time Delay Estimation Algorithm Based on Zadoff–Chu Sequence in OFDM Systems

This paper presents a multiple signal classification (MUSIC)-based symbol integer delay estimation algorithm for orthogonal frequency-division multiplexing (OFDM) systems using the Zadoff-Chu (ZC) sequence. By exploiting a property of the ZC sequence, this algorithm is derived from the maximum-likelihood (ML) integer delay estimation, which retains the ability of coping with an incomplete OFDM symbol with reduced complexity. It can be realized without information on channel statistic and noise. Furthermore, taking into account the inefficiency of the MUSIC-based algorithm in estimating the integer delay if the fractional delay is compensated on local ZC, we present a real-valued delay-tracking scheme that iteratively adjusts the sampling clock phase of an analog-to-digital converter (ADC). In particular, the fractional delay is tracked via the ML scheme, whereas the integer delay is estimated via the MUSIC-based algorithm. The numerical results finally confirm the efficiency and the reliability of the proposed MUSIC-based algorithm, even in the case of a small number of samples.

Performance Evaluation on the Basis of Bit Error Rate for Different Order of Modulation and Different Length of Subchannels in OFDM System

Today, we have required to accommodate a large number of users under a single base station. This can be possible only if we have some flexibility over the spectrum. Previously we have lots of multiplexing methods to accommodate large number of signals in time and frequency domain. But now we have required to accommodate a large number of users in the same bandwidth, without any fading over the received signal. So, orthogonality can be maintained over the frequency response. This technology is now more popular in the mobile communication domain, called Orthogonal Frequency Division Multiplexing (OFDM). Actually user data can be converted into the parallel form and then they are modulated using digital modulation techniques. Finally, they have followed by OFDM Modulator and cyclic prefix can be inserted into the OFDM symbols. Here, I have worked on the measurement of Bit error rate for different modulation techniques in OFDM technology. It has been considered that subchannel size is not constant. According to that I have concluded the overall idea regarding the performance under OFDM technology.

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.

An Improved Technique of Channel Estimation for OFDM in DVB-T2

Orthogonal Frequency Division Multiplexing (OFDM) with pilot assisted channel estimation is widely applied in wireless communication systems due to its high rate transmission capability and its robustness to multi-path fading and delay. In this paper an improved channel estimation method for OFDM in second generation Digital Video Broadcasting for Terrestrial (DVB-T2) is presented based on a pilot arrangement in time-varying frequency-selective fading channels. A new method which is based on an improved Domain Transform Least Square Estimation method (DTLSE) and a Two Dimensional Interpolation method (2-DI) have been proposed. The proposed methods give a good BER performance compared to other considered methods .The estimation matrix size for obtaining Channel Impulse Response (CIR) depends only on the length of the channel not on the number of pilot subcarriers nor the size of OFDM symbol, it does not require knowledge of channel statistics and works for any pilot pattern. The Minimum Mean Squared Error (MMSE) channel estimation was considered as the reference method that gives the best BER performance but suffers from a high computational load. The proposed method is compared with the two other methods: the improved DTLSE and MMSE. The proposed method proves to give a reduction in complexity compared to the MMSE technique with a comparable performance but better than the DTLSE one. General Terms Pilot-assisted channel estimation, wireless systems, DVB-T2.