Search

In order to improve the signal detection capability of orthogonal frequency-division multiplexing systems, a signal detection method based on an improved LSTM network for OFDM systems is proposed. The LSTM network is optimized by the Chameleon Swarm Algorithm (CLCSA) with the coupling variance and lens-imaging learning. The signal detection method based on the traditional LSTM network has the problem of a complex manual tuning process and insufficient stability. To solve the above problem, the improved Chameleon Swarm Algorithm is used to optimize the initial hyperparameters of the LSTM network and obtain the optimal hyperparameters. The optimal hyperparameters initialize the CLCSA-LSTM network model and the CLCSA-LSTM network model is trained. Finally, the trained CLCSA-LSTM network model is used for signal detection in the OFDM system. The simulation results show that the signal detection performance of the OFDM receiver has been significantly improved, and the dependence on CP and pilot overhead can be reduced. Under the same channel environment, the proposed method in this paper has better performance than other signal detection methods, and is close to the performance of the MMSE method, but it does not need prior statistical characteristics of the channel, so it is easy to implement.

Orthogonal frequency division multiplexing (OFDM) provides a promising modulation technique for underwater acoustic (UWA) communication systems. It is indispensable to obtain channel state information for channel estimation to handle the various channel distortions and interferences. However, the conventional channel estimation methods such as least square (LS), minimum mean square error (MMSE) and back propagation neural network (BPNN) cannot be directly applied to UWA-OFDM systems, since complicated multipath channels may cause a serious decline in performance estimation. To address the issue, two types of channel estimators based on deep neural networks (DNNs) are proposed with a novel training strategy in this paper. The proposed DNN models are trained with the received pilot symbols and the correct channel impulse responses in the training process, and then the estimated channel impulse responses are offered by the proposed DNN models in the working process. The experimental results demonstrate that the proposed methods outperform LS, BPNN algorithms and are comparable to the MMSE algorithm in respect to bit error rate and normalized mean square error. Meanwhile, there is no requirement of prior statistics information about channel autocorrelation matrix and noise variance for our proposals to estimate channels in UWA-OFDM systems, which is superior to the MMSE algorithm. Our proposed DNN models achieve better performance using 16QAM than 32QAM, 64QAM, furthermore, the specified DNN architectures help improve real-time performance by saving runtime and storage resources for online UWA communications.

Nonlinear behavior and power efficiency of the Power Amplifier (PA) contradictorily depend on the input signal amplitude distribution. The transmitted signal in multi-carrier modulation exhibits high Peak-to-Average Power Ratio (PAPR) and large bandwidths, leading to the degradation of the radio link and additional generation out-of-band interferences, which degrade the quality of the transmission. Practical solutions exist, like a power back-off, but with unacceptable efficiency performances of the transmitter. This paper deals with efficiency and linearity improvement using a new PAPR reduction method based on the combination of Discrete Cosine Transform and shaping technique. The main principle is to determine an optimal coding scheme according to a trade-off between coding complexity and performance benefits in the presence of PA nonlinearities. Simulation and experimental results in the context of OFDM signal and using a 20 W–3.7 GHz Radio-Frequency Power Amplifier show an improvement on PAPR reduction of about 3.25 dB. Also, the communication criteria like Bit Error Rate and Error Vector Magnitude are improved by about one decade and a half and $$8\%$$ 8 % , respectively.

The performance of digital communication systems, employed in cellular, broadcasting, and wireless access networks, can be significantly degraded by adverse channel conditions and interferences, which characterize wireless communications in urban environments. These interferences are typically modeled as non-Gaussian impulsive noises [1]–[3], such as the Middleton's Class-A noise that, together with the associated suppression techniques, has been widely investigated in the past (see [4]–[8] and references therein). Although multicarrier modulations, currently employed in most of the wireless communication systems, are inherently more resistant to impulsive noise (ImpN) than single carrier modulations, the counteraction of the performance degradation caused by ImpN is still a challenging research area for communication engineers [9]–[11]. Actually, countermeasures for Class-A noises can be easily generalized to any scenario characterized by multi- component Gaussian mixture noises [7], [8], and consequently also to alpha-stable impulsive noises, which can be modeled as a Gaussian mixture as well [12]– [14]. Assuming a Gaussian source impaired by memoryless ImpN, optimum system performance in terms of mean-squared error (MSE) and signal-to-noise power ratio (SNR) can be achieved by applying a Bayesian signal estimator [15]. Specifically, the optimal Bayesian estimator (OBE) for real-valued Gaussian mixture noise has been proposed in [8], and successively extended to complex signals in [9]. Although the OBE guarantees optimal MSE and SNR for uncorrelated ImpN, in some cases its implementation in practical receivers may be cumbersome or not attractive. For instance, if ImpN suppression is performed before A/D conversion, an analogic solution may be too complex, while if impulse noise suppression is performed after A/D conversion, the computational complexity and the system latency may be relatively high, especially for simple low-cost devices [8], [15]. Consequently, less complex solutions are typically based on signal thresholding, associated with blanking (nulling) [6], [8], [10], [16], clipping [7], [8], [15], or their combinations [5], [7], [11], [17]. Recently, an MMSE estimator, constrained to a given quantization resolution of the noisy observations [15], has been proved to converge to the MMSE optimal solution, e.g., the OBE, which conversely uses an infinite resolution of the observation. Although all the mentioned techniques can be used in any system impaired by additive ImpN, their application to OFDM, or any other multicarrier wireless system, is more challenging when the system is also affected by frequency-selective fading channels, as we will clarify.

In this paper, we develop the family of Golay–Rudin–Shapiro (GRS) m-complementary many-parameter sequences and many-parameter Golay transforms. The approach is based on a new generalized iteration generating construction, associated with n unitary many-parameter transforms and n arbitrary groups of given fixed order. We are going to use multi-parameter Golay transform in Intelligent-OFDM-TCS instead of discrete Fourier transform in order to find out optimal values of parameters optimized PARP, BER, SER, anti-eavesdropping and anti-jamming effects.

The paper deals with the problem of blind synchronization for OFDM/OQAM systems. Specifically, by exploiting the approximate conjugate-symmetry property of the beginning of a burst of OFDM/OQAM symbols, due to the presence of the time offset, a new procedure for blind symbol timing and CFO estimation is proposed. The performance of the derived blind estimators is analyzed by computer simulations; the results show that the proposed methods may provide acceptable performance for reasonable values of the signal-to-noise ratio.

Cyclostationary characteristics of communication signals can be exploited for performing various signal processing tasks. Receiver impairments that affect the cyclic statistics of signals may lead to a degradation in the performance of cyclostationarity-based signal processing algorithms. Inphase/quadrature (I/Q) imbalance, oscillator phase noise, and random sampling jitter can be counted amongst the typical receiver impairments encountered in wireless communication systems. In this work, we investigate the effects of these nonidealities on the second-order cyclic statistics of baseband communication signals. General results are derived for arbitrary complex-valued (and possibly improper) signals, and examples are provided for cases of practical interest. Publisher's Version

We present an exact analysis of the correlation coefficient, in the frequency domain, between the real and imaginary parts of the impulse noise squared norms for OFDM systems. We analyze a scenario where $M$ Gaussian randomly positioned impulses interfere the OFDM signal, with and without the presence of a background thermal noise. Variability of $M$ is taken into account through the Bernoulli-Gauss model. We demonstrate that rather high correlation values occur with not negligible probabilities under certain operation conditions. This has an impact on the design of impulse noise suppression techniques as well as on the error rate performance.

QC 20110517

Terrestrial or Handheld Digital Video Broadcasting (DVB-T/H) and Terrestrial Digital Multimedia Broadcasting (T-DMB) are two popular broadcasting standards that enable digital television transmissions to handheld receivers. This paper presents a comprehensive performance comparison between the physical layers of DVB-T/H and T-DMB when employed for mobile communications. By exploiting a recently proposed fast simulation model, we assess the BER of the two coded OFDM systems in several realistic scenarios, taking into account Rayleigh and Rice channels, different mobile speeds, inner and outer channel coding, channel estimation, and one or two receive antennas. Our comparison shows that the DVB-T/H physical layer performance highly depends on the delay spread of the channel, whereas T-DMB is less sensitive to the frequency selectivity of the channel. As a result, DVB-T/H yields better performance than T-DMB in typical Rayleigh channels with significant delay spread. On the contrary, at high SNR, T-DMB outperforms DVB-T/H in Rice channels with low delay spread. As a side result, we show the performance improvement of DVB-H produced by MPE-FEC at the data link layer.

The broadcast nature of wireless communications imposes the risk of information leakage to adversarial users or unauthorized receivers. Therefore, information security between intended users remains a challenging issue. Most of the current physical layer security techniques exploit channel randomness as a common source between two legitimate nodes to extract a secret key. In this paper, we propose a new simple technique to generate the secret key. Specifically, we exploit the estimated channel to generate a secondary random process (SRP) that is common between the two legitimate nodes. We compare the estimated channel gain and phase to a preset threshold. The moving differences between the locations at which the estimated channel gain and phase exceed the threshold are the realization of our SRP. We simulate an orthogonal frequency division multiplexing (OFDM) system and show that our proposed technique provides a drastic improvement in the key bit mismatch rate (BMR) between the legitimate nodes when compared to the techniques that exploit the estimated channel gain or phase directly. In addition to that, the secret key generated through our technique is longer than that generated by conventional techniques. 2015 IEEE. Qatar National Research Fund Scopus

This paper presents a theoretical characterization of nonlinear distortion effects in orthogonal frequency division multiplexing (OFDM) transmission systems. In the theoretical framework developed, it is shown that the effects on the decision variables of the in-band distortion introduced by a bandpass memoryless nonlinearity can be described by means of a complex gain and an additive Gaussian term with zero mean and suitable variance; analytical expressions for gain and variance are given. The conditions which allow this description are emphasized and discussed. As a consequence, a completely analytical procedure to evaluate error probability is also obtained and illustrated using OFDM/discrete multitone modulation (DMT) systems with rectangular pulse shaping; for the soft-envelope limiter nonlinearity, a closed form is derived. A comparison with simulation results is carried out to verify the accuracy of this method.

Pulse shaping is the one of the important an critical area for wireless communication. In this article we show the relation between pulse shaping for wide sense stationary uncorrelated scattering (WSSUS) channels and the notion of approximate eigenstructure for time-varying channels. Optimal link adaption to the scattering function of WSSUS channel is still an unsolved problem. In ulticarrier transmission such link adaption is performed by pulse shaping by properly adjusting transmitter and receiver filters. We consider pulse shaping for general signaling scheme called Weyl-Heisenberg signaling which includes Orthogonal Frequency Division Multiplexing (OFDM) and Offset Quadrature amplitude modulation (OQAM). We establish a general mathematical framework for joint transmitter and receiver pulse shape. Pulse design problem in the view of optimal averaged SINR is an interplay between localization and optimization strategies. The Localization problem can be expressed in terms of eigenvalues of approximate eigenstructure of Linear Time Varying (LTV) channel operators. In this thesis, we will show approximate eigenstructure and its relation with pulse design and several iterative algorithms for optimization.

The aim of this paper is to propose and assess the performance of cooperative schemes designed for the uplink OFDM based systems with mobile relays with an antenna array. We consider the use of an antenna array at the base station and a single antenna at the user terminal. We compare the situations at which the relay is equipped with one or two antennas. We assume that some of the user terminals deployed in a certain area could act as relaying-able terminals for the communication of other users. Two types of relay assisted protocols are considered: equalize-and-forward and decode-and-forward. The performance of the proposed relay-assisted schemes is evaluated under realistic scenarios, considering typical pedestrian scenarios based on WiMAX specifications. The proposed schemes are also compared against the OFDM based non-cooperative systems. Numerical results show that the availability of antenna arrays at the relays significantly improves the cooperative systems performance, which outperform the non-cooperative ones in all studied scenarios CODIV - FP7/ICT/2007/215477 PHOTON FPDC - EEA-TEL/72890/2006

In this paper we evaluate the coded and uncoded performance of a 2-antenna terminal relaying schem, for downlink OFDM based systems, using efficient space-frequency block coding protocols. We consider that the relay and the base station are equipped with 2 antennas and the user terminal has a single antenna. Two types of relay protocols are accounted for: Equalize-and-Forward and Decode-and-Forward. Signals expressions are derived and the bit error rates obtained for a pratical and preeminent scenario, with LTE parameters and using convolutional channel coding. The performance of the proposed relay-assited scheme is evaluated under realistic scenarios, considering typical pedestrian scenarios based on LTE specifications. Results are compared to those of other relay-assisted design approaches that have been recently proposed for the case of 1-antenna relay and the non-cooperative systems. For this comparison, we implement three propagation scenarios, with different SNR transmission conditions among links. Simulation results show that the availability of a bi-antenna array at the relay significantly improves the cooperative system performance, but improvements are highly dependent on the scenarios and relay protocol as well as of the cooperative second-hop quality. CODIV - FP7-ICT/2007-215477 AGILE - PTDC/EEA-TEL/71417/2006 FCT

The aim of this paper is to adapt and evaluate the performance of two classical pilot-aided channel estimation schemes designed for OFDM systems: least square and minimum mean square error, in a relay-assisted scenario. We consider the Amplify-and-Forward, which is the simplest relay protocol for cooperative systems. Also, we consider the use of a single antenna at all the terminals: base station, user terminal and relay node. In our scenario, it is assumed that some of the user terminals deployed in a certain area could act as relaying-able terminals for the communications of other users. In this communication we show that the classical channel estimation techniques designed for conventional wireless communications can be extended for relay-assisted based systems under some modifications. © 2009 IEEE. European CODIV project (ref. FP7-ICT-2007-215477) Portuguese Foundation for Science and Technology (FCT)

This contribution deals with phase noise estimation from pilot symbols. The phase noise process is approximated by an expansion of discrete cosine transform (DCT) basis functions containing only a few terms. We propose a feedforward algorithm that estimates the DCT coefficients without requiring detailed knowledge about the phase noise statistics. We demonstrate that the resulting (linearized) mean-square phase estimation error consists of two contributions: a contribution from the additive noise, that equals the Cramer-Rao lower bound, and a noise independent contribution, that results front the phase noise modeling error. We investigate the effect of the symbol sequence length, the pilot symbol positions, the number of pilot symbols, and the number of estimated DCT coefficients it the estimation accuracy and on the corresponding bit error rate (PER). We propose a pilot symbol configuration allowing to estimate any number of DCT coefficients not exceeding the number of pilot Symbols, providing a considerable Performance improvement as compared to other pilot symbol configurations. For large block sizes, the DCT-based estimation algorithm substantially outperforms algorithms that estimate only the time-average or the linear trend of the carrier phase. Copyright (C) 2009 J. Bhatti and M. Moeneclaey.

In this paper a theoretical analysis of the iterative signal reconstruction algorithm for impulsive noise mitigation in orthogonal frequency-division multiplexing (OFDM) systems is developed. The following main results are developed: first, analytical model for the total noise in the frequency domain, and second the model for the total noise probability density function (pdf) in the frequency domain, both defined for each step of the iterative reconstruction process. Finally, based on the pdf of the total noise, explicit expressions for BER in kth iteration are defined as well. The main intention of the paper is to present the approach to theoretical analysis of the iterative impulsive noise mitigation algorithm that has not yet been appeared in the literature, because the theoretical analysis of the noise pdf during iterations has been considered as too complex a problem. Analyses and analytical results presented in the paper are given for scenario with a fixed number of noise impulses per frame. However, this is not a handicap of the proposed approach, since all presented models can be used as building blocks for scenarios with other impulsive noise distributions including Bernoulli-Gaussian and Middleton's Class A.

The paper investigates different diversity concepts for OFDM systems. These are space frequency block codes (SFBCs) and code division multiplexing (CDM). The performances of both, and of combinations of both, are compared for coded OFDM systems in a Rayleigh fading channel. The applied SFBCs and CDM have in common that they have no rate loss and thus do not decrease the bandwidth efficiency of the system. While the SFBCs exploit space and frequency diversity, CDM exploits frequency and time diversity in OFDM systems. Additionally, the effects due to diversity gains by channel coding are taken into account in the investigations.

Orthogonal frequency division multipledng (OFDM) is a multiplexing scheme which significantly simplifies the implementation of broadband wireless communications systems. The standards for digital broadcasting (DAB and DVB-T), wireless LANs (IEEE 802.11a and HIPERLAN/2) and fixed wireless access (IEEE 802.16a and HIPERMAN) are examples for the successful application of OFDM for wireless communications with high data rates [1]. Moreover, current trends in multiple access technologies are in the application of multi-carrier CDMA (MC-CDMA), multi-carrier DS-CDMA (MC-DS-CDMA) and spread spectrum multi-carrier multiple access (SS-MC-MA), which are access schemes based on combinations of OFDM with code division multiplexing [1–5].

In this paper we propose a novel scheme for adaptive pilot symbol aided channel estimation. The optimum Wiener filter for pilot symbol aided channel estimation requires the perfect knowledge of the channel correlation function. Since in practice the channel correlation function is unknown, a robust Wiener filter with model mismatch is employed for pilot symbol aided channel estimation [1]. Due to the model mismatch, the robust Wiener filter can perform significantly worse for certain channel models than the optimum Wiener filter. Therefore, we propose an adaptive filter in the frequency domain to reduce the performance loss with a moderate increase in complexity. The received pilot symbols are first interpolated in frequency and time direction. Then, a block frequency normalized least-mean-square algorithm adapts the filter coefficients without making any assumptions about the channel and noise statistics.

In this paper, we investigate different antenna diversity concepts, which can be easily applied to orthogonal frequency division multiplexing (OFDM) systems. The focus is on standard compatibility, i.e. this schemes can be implemented to already existing OFDM systems without changing the standards. The introduced diversity techniques are applied exemplarily to the DVB-T system. Bit error performance investigations were done by simulation for different DVB-T and diversity parameter sets.

In OFDM transmission schemes, phase-noise from oscillator instabilities in the receiver is a potentially serious problem, especially when bandwidth efficient, high order signal constellations are employed. The paper analyses the two effects of phase-noise: inter-carrier interference (ICI) and a phase error common to all OFDM sub-carriers. Through numerical integration, the ICI power can be evaluated and is shown as a function of the number of OFDM sub-carriers and various parameters of the phase-noise model. Increasing the number of sub-carriers causes an increase in the ICI power, which our analysis indeed shows to become a potential problem, since it can lead to a BER floor. The analysis allows the design of low-cost tuners through specifying the required phase-noise characteristics. A similar technique is applied to calculate the variance of the common phase error. After showing that the common phase error is essentially uncorrelated from symbol to symbol, we propose a simple feed-forward correction technique based on pilot cells, that dramatically reduces the degradation due to phase-noise. This is confirmed by BER simulations of a coded OFDM scheme (proposed for terrestrial transmission of digital television) with 64 QAM.

This paper proposes a computationally efficient, linear minimum mean square error (MMSE) channel estimation algorithm based on KL series expansion for OFDM systems. Based on such expansion, no matrix inversion is required in the proposed MMSE estimator. Moreover, truncation in the linear expansion of channel is achieved by exploiting the optimal truncation property of the KL expansion resulting in a smaller computational load on the estimation algorithm. The performance of the proposed approach is studied through analytical and experimental results. We provide performance analysis results studying the influence of the effect of SNR and correlation mismatch on the estimator performance. Simulation results confirm our theoretical results and illustrate that the proposed algorithm is capable of tracking fast fading and improving performance. Publisher's Version

IEEE Communications Society;ICC Globecom GLOBECOM'04 - IEEE Global Telecommunications Conference -- 29 November 2004 through 3 December 2004 -- Dallas, TX -- 64670 This paper proposes a computationally efficient, pilot-aided minimum mean square error (MMSE) channel estimation algorithm for OFDM systems. The proposed approach employs a convenient representation of the discrete multipath fading channel based on the Karhunen-Loeve (KL) orthogonal expansion and estimates uncorrelated series expansion coefficients. Moreover, optimal rank reduction is achieved in the proposed approach by exploiting the optimal truncation property of the KL expansion resulting in a smaller computational load on the estimation algorithm. The performance of the proposed approach is studied through analytical and experimental results. We first consider the stochastic Cramer-Rao bound and derive the closed-form expression for the random KL coefficients. We then exploit the performance of the MMSE channel estimator based on the evaluation of minimum Bayesian MSE. IEEE; Strateg Alliance Metroplex Technol; Freescale; IDT; Nokia; Univ Texas Austin, ECE; Wireless Valley Commun Inc; ALCATEL; ALTERA; Raytheon; SMU Eng; UTA Eng; Univ Texas Dallas, Erik Jonsson Sch Engn & Comp Sci; Axes Technol Publisher's Version