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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.

In this work, an estimation algorithm based on the compressive sensing (CS) technique is proposed for an orthogonal frequency division multiplexing (OFDM) system applied to multi-path time-varying channels. A fitting polynomial is used to approximate each channel path, and full-band training symbols are adopted. The variation of the channel response can then be evaluated according to the observation on the inter-carrier interference (ICI) from the determinate signals. The CS-based technique is introduced to explore the sparsity of the double-selective channel and the usage of high-order fitting polynomials is allowed. The orthogonal matching pursuit (OMP) algorithm is designed to cooperate with the fitting polynomial model, and more accurate channel estimation results can be provided compared to those provided by conventional least-square (LS) algorithms. When multiple transmission antennas are required, the advantage of the proposed polynomial-fitting OMP (PF-OMP) algorithm is more obvious as more transmission antennas are used.

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 signal-to-noise ratio (SNR) gap approximation provides a closed-form expression for the SNR required for a coded modulation system to achieve a given target error performance for a given constellation size. This approximation has been widely used for resource allocation in the context of trellis-coded multicarrier systems (e.g., for digital subscriber line communication). In this contribution, we show that the SNR gap approximation does not accurately model the relation between constellation size and required SNR in low-density parity-check (LDPC) coded multicarrier systems. We solve this problem by using a simple modification of the SNR gap approximation instead, which fully retains the analytical convenience of the former approximation. The performance advantage resulting from the proposed modification is illustrated for single-user digital subscriber line transmission.

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.

In this paper, a diamond shape pilot arrangement for OFDM channel estimation is investigated. Such arrangement will decrease the number of pilots transmitted over the communication channel which in turn will increase data throughput while maintaining acceptable accuracy of the channel estimation, the adaptive antenna array (AAA) is combined with orthogonal frequency division multiplexing (OFDM) to combat the intersymbol interference (ISI) and the directional interferences. In this paper, The optimum beamformer weight set is obtained based on minimum bit error rate (MBER) criteria in diamond-type pilot-assisted in 3GPP long term evolution (LTE) OFDM systems under multipath fading channel. The simulation results show that the quadrature phase shift keying signaling based on MBER technique utilizes the antenna array elements more intelligently than the standard minimum mean square error (MMSE) technique.

In this paper, we consider an orthogonal frequency division multiplexing communication system that adopts frame-by-frame transmission in high-speed railway (HSR) scenario. Due to the increase in demand for the QoS sensitive services, an efficient QoS-aware transmission strategy that improves the system performance is required urgently. Many efforts have been devoted to addressing this problem with the assumption of block fading channel in a frame duration. However, due to the frequent channel quality variation in a frame duration and serious inter channel interference in HSR scenario, the throughput of the QoS sensitive services degrades severely. Hence, a robust cross-layer transmission strategy that combines adaptive modulation (AM) scheme with truncated automatic repeat request protocol is proposed. In this cross-layer formulation, the normalized average throughput is optimized subject to the average power and the packet loss rate (PLR) requirements. First, we derive the closed form average bit error rate that represents the PLR requirement at the physical layer. Second, we obtain the solution of robust AM scheme and power allocation policy in the case of continuous rate. Third, we present the adaptive bits and power allocation scheme in the case of discrete rate, which can be implemented in practice. Finally, the performance of the proposed transmission strategy is evaluated by extensive simulations. Comparing with the constant transmit power AM scheme, the throughput increases by 20%, which demonstrates that the proposed robust cross-layer design is suitable for the HSR communication systems.

International audience; Abstract|In OFDM systems, peak-to-average power ratio (PAPR) reduction of the signal is one of the main challenges that need to be overcome in order to use the transmitter in an efficient manner. As one of attractive techniques, interleaving can be used in PAPR reduction for multicarrier signals without spectrum distortion. In this paper, the authors propose to extend the possibilities of interleaving to improve PAPR reduction, to use a new coding of interleaver keys at the transmitter and a robust decoding procedure at the receiver. In order not to degrade the data rate, the use of null subcarriers to transmit side information to the receiver is proposed and evaluated.Simulation results in the context of the WLAN 802.11a standard in the presence of a nonlinear power amplifier model with memory, show a reduction of PAPR of approximately 5.2 dB, and an improvement of bit error rate and error vector magnitude of about 2 decades and 4% respectively, while respecting the spectral mask specication.

International audience; Orthogonal Frequency Division Multiplexing (OFDM) has been shown to be an effective technique in wireless communications. It has been successfully used for Radio Frequency applications. However, reducing its main drawback known as PAPR (Peak-to-Average Power Ratio) is one of the main challenges to use efficiently the High Frequency Power Amplifier at the transmitter. As one of attractive technique, interleaving method can be used in PAPR reduction for multicarrier signals without spectrum distortion, but with additional Side-Information. In this paper, we propose to extend the possibilities of interleaving to improve PAPR reduction and make possible the downward compatibility by transmitting interleaver code via null-subcarriers. Simulation in the context of WLAN 802.11a standard in presence of a nonlinear Power Amplifier (PA) model with memory has been performed. Results show a PAPR reduction of about 5,2 dB and an improvement of about 1 decade in BER (Bit Error Rate) and 4% in EVM (Error Vector Magnitude), while respecting the spectral mask.

The data rates as well as quality of service (QoS) requirements for rich user experience in wireless communication services are continuously growing. While consuming a major portion of the energy needed by wireless devices, the wireless transceivers have a key role in guaranteeing the needed data rates with high bandwidth efficiency. The cost of wireless devices also heavily depends on the transmitter and receiver technologies. In this chapter, we concentrate on the problem of transmitting information sequences efficiently through a wireless channel and performing reception such that it can be implemented with state of the art signal processing tools. The operations of the wireless devices can be divided to RF and baseband (BB) processing. Our emphasis is to cover the BB part, including the coding, modulation, and waveform generation functions, which are mostly using the tools and techniques from digital signal processing. But we also look at the overall transceiver from the RF system point of view, covering issues like frequency translations and channelization filtering, as well as emerging techniques for mitigating the inevitable imperfections of the analog RF circuitry through advanced digital signal processing methods.

Improper signaling, where real and imaginary parts of the transmit signal are correlated and/or have unequal power, has received a lot of attention lately because it has been shown to increase achievable rates in many interference-limited communication systems. In this paper, we study whether improper signaling can also benefit an orthogonal frequency-division multiplexing (OFDM) underlay cognitive radio (UCR) system. We assume that the primary user (PU) transmits proper signals, while the secondary user (SU) is allowed to employ improper signaling. We consider two different rate constraints for the rate of the PU: i) the total rate of the PU, and ii) the rate of the PU in each subband. We propose an algorithm to implement improper signaling for each constraint. In both cases, we show that the benefits of improper signaling are relatively small and decrease rapidly with increasing number of subbands. This rather negative result shows that the use of improper signaling in interference scenarios needs to be justified on a case-by-case basis. The work of M. Soleymani, C. Lameiro and P. J. Schreier was supported by the German Research Foundation (DFG) under grants LA 4107/1-1 and SCHR 1384/8-1. The work of I. SantamarÍa has been partially supported by MINECO of Spain and AEI/FEDER funds of the E.U., under grant TEC2016-75067-C4-4-R (CARMEN).

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. © 2018, Institution of Russian Academy of Sciences. All rights reserved. This work was supported by grants the RFBR № 17-07-00886 and by Ural State Forest Engineering’s Center of Excellence in ”Quantum and Classical Information Technologies for Remote Sensing Systems”.

A phase noise (PN) mitigation scheme was proposed for orthogonal frequency division multiplexing (OFDM) in a previous work. The proposed scheme does not require detailed knowledge of PN statistics and can effectively compensate the PN with sufficient number of unknowns. In this paper, we analyze the performance of PN estimation/mitigation using the proposed scheme. It is shown that increasing the number of unknowns reduces the modeling error, yet increases the additive noise. Hence, increasing the number of unknowns increases the computational complexity and can even degrade the estimation performance. It is also shown that the PN spectral shape of the phase-locked-loop (PLL) based oscillator also affects the PN mitigation and that a larger PN may not necessarily degrade the performance of the OFDM system with PN mitigation. Simulations with realistic millimeter-wave (mmWave) PN and channel models are conducted to verify these findings.

Submitted by Cristina Santos (cmaria@ua.pt) on 2018-10-11T11:03:54Z No. of bitstreams: 1 Documento.pdf: 3749092 bytes, checksum: ecaf0cce91d8b8817e531edf9531ab69 (MD5) Made available in DSpace on 2018-10-11T11:03:54Z (GMT). No. of bitstreams: 1 Documento.pdf: 3749092 bytes, checksum: ecaf0cce91d8b8817e531edf9531ab69 (MD5) Previous issue date: 2017-12-13 Mestrado em Engenharia Eletrónica e Telecomunicações

Since the appearance of mobile communications, the users of this technology have been growing exponentially every day. The escalating mobile traffic growth it has been imposed by the proliferation of smartphones and tablets. The increasing and more intensive use of wireless communications may lead to a future breaking point, where the traditional systems will fail to support the required capability, spectral and energy efficiency. On the other hand, to cover all this current need to have more and more data it is necessary to provide a new range of data rates around the gigabits per second. Today, almost all mobile communications systems use spectrum in the range of 300MHz – 3GHz. It is needed to start looking to the range of 3GHz – 300GHz spectrum for mobile broadband applications. Millimeter waves are one way to alleviate the spectrum gridlock at lower frequencies. MIMO based systems has been researched for the last 20 years and are now part of the current standards. However, to achieve more gains, a grander view of the MIMO concept envisions the use of a large scale of antennas at each base stations, a concept referred as massive MIMO. The symbiotic combination of these technologies and other ones will lead to the development of a new generation system known as the 5G. The knowledge of the channel state information at the transmitter is very important in real massive MIMO millimeter wave systems. In this dissertation a limited feedback strategy for a hybrid massive MIMO OFDM system is proposed, where only a part of the parameters associated to the link channel are quantized and fed back. The limited feedback strategy employs a uniform-based quantization for channel amplitudes, angle of departure and angle of arrival in time domain. After being fed back, this information is used to reconstruct the overall channel in frequency domain and the transmit antenna array, which are then used to compute the hybrid analog-digital precoders. Numerical results show that the proposed quantization strategy achieve a performance close to the one obtained with perfect full channel, with a low overhead and complexity Desde o aparecimento das comunicações móveis, os utilizadores desta tecnologia têm vindo a crescer exponencialmente todos os dias. A escalada do crescimento do tráfego móvel foi imposta, principalmente, pela proliferação de smartphones e tablets. O uso crescente e intensivo das comunicações sem fios pode levar no futuro a um ponto de rutura, onde os sistemas tradicionais não suportam a capacidade requerida, a eficiência espectral e eficiência enérgica. Por outro lado, para cobrir toda esta necessidade atual de ter mais e mais dados, é necessário fornecer taxas de transmissão mais elevadas, em torno dos gigabits por segundo. Hoje, quase todos os sistemas de comunicações móveis usam espectro na faixa de 300 MHz - 3GHz. É necessário começar a procurar a gama de espectro 3GHz - 300 GHz para aplicações de banda larga móvel. Aqui vamos apresentar as ondas milimétricas, sendo esta uma maneira de aliviar espectro em frequências mais baixas. Os sistemas baseados em MIMO foram alvo de pesquisa nos últimos 20 anos e agora fazem parte dos padrões atuais. No entanto, para obter mais ganhos, uma visão mais ampla do conceito MIMO prevê o uso de uma grande quantidade de antenas em cada estação base, um conceito referido como massive MIMO. A combinação simbiótica destas tecnologias levará ao desenvolvimento de um novo sistema de geração denominado 5G. O desenvolvimento de técnicas de conhecimento da informação do canal no transmissor é muito importante em sistemas massive MIMO millimeter wave reais. Nesta dissertação é proposta e avaliada uma estratégia de envio de informação de canal para o transmissor para sistemas massive MIMO OFDM híbrido, onde apenas uma parte dos parâmetros associados ao canal são quantificados e transmitidos para o transmissor. A estratégia de feedback proposta é baseada numa quantização uniforme das amplitudes de canal, ângulos de partida e de chegada, no domínio do tempo. Depois de serem enviadas, essas informações são usadas para reconstruir o canal geral no domínio da frequência e a matriz da antena de transmissão, que são então usadas para obter os precoders híbridos analógico-digitais. Os resultados numéricos mostram que a estratégia de quantificação proposta atinge um desempenho próximo ao obtido caso se conhecesse o canal perfeito no transmissor, com um baixo overhead e complexidade Mestrado em Engenharia Eletrónica e Telecomunicações

International audience; In this paper, we propose a new PAPR reduction method using interleaving technique without data rate loss. The main idea is to set up all possibility of interleaving to get the best reduction of PAPR and to transmit the resulting information via null-subcarriers (NS) available in OFDM (Orthogonal Frequency Division Multiplexing) standards. Thus, we develop a new coding of interleaver key based on mapping symbols at the transmitter and a robust decoding procedure at the receiver. Simulation results in the context of WLAN IEEE 802.11a standard show an improvement of PAPR reduction about 5,2 dB, with the same performance in BER (Bit Error Rate), while respecting the communication criteria (Data Rate and spectrum specification).

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.

Wireless communication networks have evolved continuously over the last four decades in order to meet the traffic and security requirements due to the ever-increasing amount of traffic. However this increase is projected to be massive for the fifth generation of wireless networks (5G), with a targeted capacity enhancement of 1000× w.r.t. 4G networks. This enhanced capacity is possible by a combination of major approaches (a) overhaul of some parts and (b) elimination of overhead and redundancies of the current 4G. In this work we focus on OFDM reference signal or pilot tones, which are used for channel estimation, link adaptation and other crucial functions in Long-Term Evolution (LTE). We investigate two aspects of pilot signals pertaining to its evolution - (a) impact of targeted interference on pilots and its mitigation and (b) adaptation of pilot patterns to match the channel conditions of the user. We develop theoretical models that accurately quantify the performance degradation at the user’s receiver in the presence of a multi-tone pilot jammer. We develop and evaluate mitigation algorithms to mitigate power constrained multi-tone pilot jammers in SISO- and full rank spatial multiplexing MIMO-OFDM systems. Our results show that the channel estimation performance can be restored even in the presence of a strong pilot jammer. We also show that full rank spatial multiplexing in the presence of a synchronized pilot jammer (transmitting on pilot locations only) is possible when the channel is flat between two pilot locations in either time or frequency. We also present experimental results of multi-tone broadcast pilot jamming (Jamming of Cell Specific Reference Signal) in the LTE downlink. Our results show that full-band jamming of pilots needs 5 dB less power than jamming the entire downlink signal, in order to cause Denial of Service (DoS) to the users. In addition to this, we have identified and demonstrated a previously unreported issue with LTE termed ‘Channel Quality Indicator (CQI) Spoofing’. In this scenario, the attacker tricks the user terminal into thinking that the channel quality is good, by transmitting interference transmission only on the data locations, while deliberately avoiding the pilots. This jamming strategy leverages the dependence of the adaptive modulation and coding (AMC) schemes on the CQI estimate in LTE. Lastly, we investigate the idea of pilot pattern adaptation for SISO- and spatial multiplexing MIMO-OFDM systems. We present a generic heuristic algorithm to predict the optimal pilot spacing and power in a nonstationary doubly selective channel (channel fading in both time and frequency). The algorithm fits estimated channel statistics to stored codebook channel profiles and uses it to maximize the upper bound on the constrained capacity. We demonstrate up to a 30% improvement in ergodic capacity using our algorithm and describe ways to minimize feedback requirements while adapting pilot patterns in multi-band carrier aggregation systems. We conclude this work by identifying scenarios where pilot adaptation can be implemented in current wireless networks and provide some guidelines to adapt pilots for 5G. Master of Science

The vast majority of existing secret key generation protocols exploit the inherent randomness of the wireless channel as a common source of randomness. However, independent noise added at the receivers of the legitimate nodes affects the reciprocity of the channel. In this paper, we propose a new simple technique to generate the secret key that mitigates the effect of noise. 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 study the properties of our generated SRP and derive a closed form expression for the probability mass function of the realizations of our SRP. We simulate an orthogonal frequency division multiplexing system and show that our proposed technique provides a drastic improvement in the key bit mismatch rate between the legitimate nodes when compared with 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. Moreover, we compute the conditional probabilities used to estimate the secret key capacity. Copyright 2016 John Wiley & Sons, Ltd. Copyright 2016 John Wiley & Sons, Ltd. Scopus

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.

In signal processing literature, noise's sources are often assumed to be Gaussian. However, in many fields the conventional Gaussian noise assumption is inadequate and can lead to the loss of resolution and/or accuracy. This is particularly the case of noise that exhibits impulsive nature. The latter is found in several areas, especially telecommunications. α-stable distributions are suitable for modeling this type of noise. In this context, the main focus of this thesis is to propose novel methods for the joint estimation of the state and the noise in impulsive environments. Inference is performed within a Bayesian framework using sequential Monte Carlo methods. First, this issue has been addressed within an OFDM transmission link assuming a symmetric α-stable model for channel distortions. For this purpose, a particle filter is proposed to include the joint estimation of the transmitted OFDM symbols and the noise parameters. Then, this problem has been tackled in the more general context of nonlinear dynamic systems. A flexible Bayesian nonparametric model based on Dirichlet Process Mixtures is introduced to model the α-stable noise. Moreover, sequential Monte Carlo filters based on efficient importance densities are implemented to perform the joint estimation of the state and the unknown measurement noise density; Dans un nombre croissant d'applications, les perturbations rencontrées s'éloignent fortement des modèles classiques qui les modélisent par une gaussienne ou un mélange de gaussiennes. C'est en particulier le cas des bruits impulsifs que nous rencontrons dans plusieurs domaines, notamment celui des télécommunications. Dans ce cas, une modélisation mieux adaptée peut reposer sur les distributions α-stables. C'est dans ce cadre que s'inscrit le travail de cette thèse dont l'objectif est de concevoir de nouvelles méthodes robustes pour l'estimation conjointe état-bruit dans des environnements impulsifs. L'inférence est réalisée dans un cadre bayésien en utilisant les méthodes de Monte Carlo séquentielles. Dans un premier temps, cette problématique a été abordée dans le contexte des systèmes de transmission OFDM en supposant que les distorsions du canal sont modélisées par des distributions α-stables symétriques. Un algorithme de Monte Carlo séquentiel a été proposé pour l'estimation conjointe des symboles OFDM émis et des paramètres du bruit α-stable. Ensuite, cette problématique a été abordée dans un cadre applicatif plus large, celui des systèmes non linéaires. Une approche bayésienne non paramétrique fondée sur la modélisation du bruit α-stable par des mélanges de processus de Dirichlet a été proposée. Des filtres particulaires basés sur des densités d'importance efficaces sont développés pour l'estimation conjointe du signal et des densités de probabilité des bruits

A multi-user diversity approach, called Opportunistic Interference Management (OIM), is considered in connection with Orthogonal Frequency Division Multiplexing (OFDM) systems. The OIM is applied to each consecutive group of OFDM sub-channels that are highly correlated. We use the OIM to transmit information in Qg consecutive sub-channels to d users in parallel. The expected parallel transmissions per group of sub-channels is then computed. The results show that the maximum expected parallel transmissions is achieved almost surely when enough mobile users exist. 2013 IEEE. Scopus 2-s2.0-84883699714

International audience; In this paper, we address the challenging problem of the OFDM reception in the presence of phase distortions. Phase noise and carrier frequency offset seriously degrade the performances of OFDM systems by destroying orthogonality of the subcarriers. Based on a Markov Chain Monte Carlo sampling mechanization, our approach consists in jointly estimating the phase noise, the frequency offset and the transmitted symbols. The proposed algorithm is implemented in the time domain in order to benefit from the redundancy information induced by the cyclic prefix and from the time correlation of the OFDM signal owing to the presence of virtual and/or pilot subcarriers. The algorithm's efficiency is enhanced by incorporating the Rao-Blackwellization technique as well as various sampling improvement strategies. Simulation results, provided in terms of bit error rate (BER) and mean square error (MSE), clearly illustrate the efficiency and the robustness of the proposed estimator.

[EN] A new transmission scheme for OFDM systems that uses the odd discrete Fourier transform to provide frequency diversity gain is proposed. Odd DFT allows the transmission of data subcarriers in frequencies that are centred between those employed by the traditional DFT. This fact is exploited to transmit data subcarriers on the group of frequencies that gives better performance using either traditional DFT or odd DFT. As an example, by using this approach a diversity gain up to 5.3 dB at a BER of 10-4 in a typical indoor channel model can be achieved. © 2011 The Institute of Electronics, Information and Communication Engineers., This work was supported by the Spanish Ministerio de Ciencia e Innovacion under grant TEC2008-06787.

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.

In this paper we propose a scheme with a dedicated relay node for an OFDM-based system. We consider an antenna array at the base station and a single antenna at both the user terminal and the relay node. The relay-assisted protocol considered is equalize-and-forward. The proposed scheme requires the computation of parameters which depend on unknown variables at the destination and we show that under some assumptions the scheme is suitable to provide all those requirements. Also, we show that since the pilots at the relay node are properly designed the TD-MMSE estimator can still provide the required channel estimate. The system performance is evaluated by considering a typical pedestrian scenario based on WiMAX specification. The results show that the degradation in the estimator performance is negligible. ©2010 IEEE. European project Enhanced Wireless Communication Systems Employing Cooperative Diversity – CODIV FP7/ICT/2007/215477 Portuguese Cooperative and Antenna Diversity for Broadband Wireless Networks – CADWIN PTDC/EEA–TEL/099241/2008 Portuguese Foundation for Science and Technology (FCT)

Recently there is a noticeable interest in investigation of the squared norm distribution of complex-valued Gaussian variables that have different variances. A typical example is consideration of the channel state information (CSI) in multi-antenna systems. A similar problem appears in communication systems interfered by Middleton Class A impulsive noise. In this paper we develop exact expressions for the squared norm distribution in the time and frequency domains in OFDM systems interfered by Class A impulsive noise. Accurate approximation by the chi-square distribution based on equating the first two moments has been also derived.

We describe a new joint iterative channel estimation and equalization algorithm for joint channel estimation and data detection for orthogonal frequency division multiplexing (OFDM) systems in the presence of frequency selective and rapidly timevarying channels. The algorithm is based on the expectation maximization-maximum a posteriori (EM-MAP) technique which is very suitable for the multicarrier signal formats. The algorithm leads to a receiver structure that yields the equalized output using the channel estimates. The pilot symbols are employed to estimate the initial channel coefficients effectively and unknown data symbols are averaged out in the algorithm. The band-limited discrete cosine serial expansion of low dimensionality is employed to represent the time-varying fading channel. In this way the resulting reduced dimensional channel coefficients are estimated iteratively with tractable complexity. The extensive computer simulations show that the algorithm has excellent symbol error rate (SER) and mean square error (MSE) performances for very high mobility even during the initialization step. Copyright © Þenol et. al.

Positioning in urban or indoor environment is a hot topic, either due to regulations such as the E911 requiring US mobile telecommunication operators to be able to locate their subscribers in case of emergency, or due to the market development, with the extension of location - based services targeting the mass market concentrated in metropolitan areas. In urban or indoor areas, it is generally recognized that satellite - based positioning systems are not suitable (alone) to provide a continuous, reliable and accurate position to the user. Therefore, alternative positioning techniques may be useful to complement or replace satellite positioning in these environments. This PhD study ha s studied the possibility of using a mobile TV system based on the DVB - SH standard as system of opportunity for positioning. The advantage s of using a DVB - SH system for positioning are multiple. First, such system has a good availability in metropolitan areas, including indoor. Secondly, the emitters are synchronized and their density should be sufficient to track signals from several emitters simultaneously. This opens the possibility of using timing measurements from several emitters to find a position by trilateration . Also, the large bandwidth of the TV signal, required for the transmission of video content, should be beneficial for the accuracy of the timing measurements and for the robustness against multipath . Therefore, DVB - SH system seems to be an interesting candidate as system of opportunity for positioning. However, several challenges are to be solved for such a solution to be relevant. First, the signals propagate in the urban environment, which creates challenging conditions for positioning su ch as strong power fading, blockage of the desired line - of - sight signal or large echoes. Secondly, the DVB - SH standard uses an OFDM modulation, which has not been studied for positioning purpose. Therefore, techniques for fine tracking of the first receive d signal replica will have to be developed. Finally, a particularity of modern broadcast system is the use of a Single Frequency Network, in which all emitters send the same signal on the same carrier frequency. Therefore emitter identification in a Single Frequency will be another issue to be solved. This PhD study has proved the feasibility of positioning using DVB - SH signals. The main contributions of this work are the propositions of (1) an OFDM signal delay tracking method working in urban propagation channels, and (2) a modification to the network deployment permitting emitter identification and (3) a first assessment of the position accuracy using the proposed algorithms. These two methods have very low impact on the initial TV broadcasting service if the right set of signal parameters is chosen: no signal modification is required and the network deployment modification uses a feature already present in the DVB - SH standard. The positioning method was simulated using real urban propagation channel measurements. The obtained position has root mean square error of 4 0m. The main error contribution comes from tracking a non - line - of - sight signal. Further work would be required to deal with this issue, which would lower the position root mean square error to 7 m, which has been locally observed in the simulation for good tracking conditions, Le positionnement en environnement urbain est un domaine de recherche actif, de par la croissance des services grand public liées à la localisation, et à cause de réglementations émergentes liées aux situations d'urgence (E911). En environnement urbain ou à l'intérieur des bâtiments, il est communément admis que les systèmes de positionnement basés sur des satellites ne sont pas suffisants pour fournir une position précise, fiable et de manière continue. Des techniques de positionnement alternatives sont donc développées afin de remplacer ou compléter les systèmes de positionnement par satellite. Cette thèse étudie la possibilité de fournir un service de positionnement utilisant un futur système de diffusion de télévison vers les mobiles basé sur le standard DVB-SH. Le principal attrait de ce système pour du positionnement est sa bonne couverture en milieu urbain, ainsi que l'utilisation d'un réseau d'émetteurs synchronisés. Il est donc possible d'employer des mesures de temps d'arrivée des signaux afin de réaliser une triangulation pour calculer la position d'un récepteur. Afin de fournir ce service innovant, des techniques spécifiques d'estimation de pseudo-distance et d'identification d'émetteurs ont été développées dans le cadre de cette thèse. Le principal résultat de cette étude est d'avoir montré la faisabilité du positionnement utilisant un système DVB-SH, ne nécessitant que de légères modifications du système qui n'apportent aucune dégradation auservice de diffusion TV. De premières simulations ont montré une précision de positionnement autour de 40m en utilisant des mesures réalistes de canal de propagation urbain.

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.

This paper is concerned with the challenging and timely problem of joint channel estimation and data detection for orthogonal frequency division multiplexing (OFDM) systems in the presence of frequency selective and very rapidly time varying channels. The detection and estimation algorithm is based on the space alternating generalized expectation maximization (SAGE) technique which is particularly well-suited to multicarrier signal formats. In order to reduce the computational complexity of the algorithm, we apply the cosine orthogonal basis functions to describe the time-varying channel. It is shown that, depending on the normalized Doppler frequency, only a small number of expansion coefficients is sufficient to approximate the channel perfectly and there is no need to know the correlation function of the input signal. The proposed SAGE joint detection algorithm updates the data sequences in serial and the channel parameters are updated in parallel, leading to a receiver structure that also incorporates a partial interference cancelation of the interchannel interference. Computer simulations show that the cosine transformation represents the time-varying channel very effectively and the proposed algorithm has excellent symbol error rate and channel estimation performance even with a very small number of channel expansion coefficients employed in the algorithm, resulting in reduction of the computational complexity substantially.

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.

Digital signal processing (DSP) techniques have played an important role in channel equalization and estimation in communication systems. While channel equalization and estimation are usually done by pilot-assisted methods in most systems, algorithms for blind channel estimation have also been largely studied due to high bandwidth efficiency. However, up to date, most blind methods possess disadvantages such as slow convergence speed, high complexity, poor performance, etc., compared to pilot-assisted methods. These drawbacks have made many consider blind methods as inapplicable in modern communication systems which feature fast-varying channels. In this thesis, we consider the blind channel estimation problem in block transmission systems with linear redundant precoding (LRP) which have been widely adopted in modern communication systems in recent years. The main contribution of this thesis is to considerably reduce the amount of received data required for blind estimation and suggest blind methods which are applicable even in fast-varying environments (e.g., in wireless channels). New algorithms are proposed, performance analysis derived, and theoretical issues studied. The first part of the thesis focuses on new algorithms for blind channel estimation and blind block synchronization in LRP systems. Two major types of linear redundant precoding, namely zero-padding (ZP) and cyclic prefixing (CP), are considered in this thesis. We first propose a generalized, subspace-based algorithm for blind channel estimation in ZP systems of which two previously reported algorithms are special cases. The generalization uses an integer parameter called {it repetition index} which represents the number of repeated uses of each received block. The number of received blocks required for subspace-based blind estimation is roughly inversely proportional to the repetition index. By choosing a larger repetition index, the amount of received data can be significantly reduced. The concept of repetition index is also applied in blind channel estimation in CP systems, which are more widely used than ZP systems in many current communication standards such as orthogonal frequency division multiplexing (OFDM) systems. The use of repetition index in CP systems is much less obvious and conceptually more complicated than in ZP systems. By choosing a repetition index larger than unity, the number of received blocks needed for blind estimation is significantly reduced compared to all previously reported methods. Theoretically, the proposed method can perform blind estimation using only three received blocks in absence of noise. In practice, the number of received blocks needed to yield a satisfactory bit error rate performance is usually on the order of half the block size. The proposed algorithm can be directly applied in OFDM systems without any modification of transmitter structure. A semiblind algorithm for channel estimation in OFDM systems is also proposed based on the extension of the blind algorithm. Another important problem, namely the blind block synchronization, is also studied. Most existing blind estimation methods in LRP systems assume the block boundaries of the received streams are perfectly known to the receiver, but this assumption is usually not true in practice since no extra known samples are transmitted. Two algorithms for blind block synchronization are proposed for ZP and CP systems, respectively. In particular, the block synchronization problem in CP systems is a broader version of the timing synchronization problem in the OFDM systems. The proposed algorithms exploit the concept of repetition index and both theoretical and simulation results suggest their advantages over all previously reported algorithms, especially when the amount of received data is limited. The second part of the thesis deals with theoretical issues related to blind channel estimation. Performance analysis of the generalized blind channel estimation algorithm in ZP systems is first given and shows that the system performance in terms of channel estimation mean square error (MSE) is very close to the Cramer-Rao bound (CRB), even when only two received blocks are available. Another important theoretical problem, namely the signal richness preservation problem, is also studied. Signal richness is an essential property for input signals in subspace-based blind channel estimation algorithms studied in this thesis. This property, however, may be altered by a linear precoder. Necessary and sufficient conditions for a linear precoder to preserve signal richness are explored. Several relevant interesting mathematical problems are also studied.

A threshold-based procedure to estimate sparse channels in an orthogonal frequency division multiplexing (OFDM) system is proposed. An optimal threshold is derived by maximising the probability of correct detection between significant and zero-valued taps estimated by the least squares (LS) estimator. Improved LS estimates are obtained by pruning the LS estimates with the statistically derived threshold. � 2008 The Institution of Engineering and Technology.

Cyclostationarity is an inherent characteristic of many man-made communication signals, which, if properly recognized, can be exploited for performing various signal-processing tasks. Determining the cyclostationary characteristics of a signal of interest is the first step in the design of signal processing systems exploiting this cyclostationary behaviour. This paper investigates the cyclostationary statistics of various signalling schemes employed in ultra-wideband (UWB) communication systems. Analytical expressions are derived for the cyclic autocorrelation and spectral correlation density functions in the presence of random timing and frequency jitter, which are characterized by discrete-time stationary random processes with known distribution functions. Publisher's Version Q2 WOS:000253788900002

We consider the design of turbo receiver structures for space-frequency block coded orthogonal frequency division multiplexing (SFBC-OFDM) systems in the presence of unknown frequency and time selective fading channels. The turbo receiver structures for SFBC-OFDM systems under consideration consists of an iterative MAP expectation/maximization (EM) channel estimation algorithm, soft MMSE-SFBC decoder and a soft MAP outer-channel-code decoder. MAP-EM employs iterative channel estimation and it improves receiver performance by re-estimating the channel after each decoder iteration. Moreover, the MAP-EM approach considers the channel variations as random processes and applies the Karhunen-Loeve (KL) orthogonal series expansion. The optimal truncation property of the KL expansion can reduce computational load on the iterative estimation approach. The performance of the proposed approaches are studied in terms of mean square error and bit-error rate. Through computer simulations, the effect of a pilot spacing on the channel estimator performance and sensitivity of turbo receiver structures on channel estimation error are studied. Simulation results illustrate that receivers with turbo coding are very sensitive to channel estimation errors compared to receivers with convolutional codes. Moreover, superiority of the turbo coded SFBC-OFDM systems over the turbo coded STBC-OFDM systems is observed especially for high Doppler frequencies.

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.

This work was supported in part by The Research Fund of The University of Istanbul, Project numbers: UDP-45/24072002, 1680/15082001and The Scientific and Technical Council of Turkey (TUBITAK) Project number: 100EE006. This paper proposes a new joint channel tracking and symbol detection scheme for pilot symbol-assisted OFDM systems in multipath fading. The proposed scheme uses Kalman filters for both channel tracking and subsequent equalization which are combined in the coupled estimator structure. Modelling the multipath fading channel as random processes to describe channel variations in a general AR framework lends itself to a state-space representation that enables the application of Kalman filtering for the tracking of channel variations. However, the proposed tracking algorithm requires knowledge of the transmitted symbols. This implies that an iterative method should be sought to obtain alternatively either channel or transmitted symbols. To compose the coupled estimator structure, a linear Kalman filter equalizer with the corresponding state-space model is therefore proposed for the detection of transmitted symbols. With the proposed Kalman filters, iterative structure is utilized to decode transmitted symbols and subsequently to track channel parameters. Finally, the performance of the proposed method is investigated through the experimental results. Scientific and Technical Council of Turkey TUBITAK University of Istanbul Publisher's Version Q2 WOS:000186027100003