Frequency Channel Estimation for Spectrally Efficient Frequency Division Multiplexing Systems

In spectrally efficient frequency division multiplexing (SEFDM) systems, the subcarrier spacing is compressed, below the orthogonality limit, by a factor (1-α), where α ≤ 1. In such systems, inter-carrier interference (ICI) is generated between the subcarriers due to the lack of orthogonality and the received pilot symbols are affected by ICI, thus preventing the receiver from correctly estimating the channel. Considering the deterministic nature of the induced ICI, in this paper a new SEFDM frequency-domain channel estimation method is proposed that decouples the ICI from the channel. The ICI is first found analytically at the receiver using an FFT. This information is then used to estimate the channel characteristics based on received pilot symbols. We show that for various values of α, the proposed method offers similar performance to time-domain methods, with reduced computational complexity, due to the use of an FFT instead of matrix inversion

A novel pilot expansion approach for MIMO channel estimation

A training-based MIMO channel estimation scheme is presented to operate in severe frequency and time selective fading channels. Besides the new pilot bits designed from the ‘Paley-Hadamard’ matrix to exploit its orthogonal and ‘Toeplitz-like’ structures and minimising its pilot length, a novel pilot expansion technique is proposed to estimate the length of the channel impulse response, by flexibly extending its pilot length as required in order to capture the number of multipath existed within the MIMO channel. The pilot expansion can also help to deduce the initial channel variation and its Doppler rate which can be subsequently applied for MIMO channel tracking using decision feedback Kalman filter during the data payload

Iterative (turbo processing) receiver design of OFDM systems in the presence of carrier frequency offset.

In this paper, based on the principle of turbo processing, we propose two iterative receiver schemes for carrier fre- quency offset (CFO) compensation in orthogonal frequency division multiplexing (OFDM) systems. Our CFO compensation designs, one in time domain and the other in frequency domain, are based on joint estimation of time-varying channel and CFO. In our schemes, the random CFO problem, a challenge for conventional pilot-aid methods, can be effectively solved using iter- ative (turbo processing) schemes. Furthermore, our comparative study shows that time domain compensation (TDC) is simpler to implement but frequency domain cancellation consisting of an iterative equalizer (FDC-IE) has better bit error rate (BER) performance

Robust Channel Estimation Methods for Spectrally Efficient FDM Systems

This paper proposes and explores a novel channel estimation scheme for non-orthogonal multi-carrier signals and systems; spectrally efficient frequency division multiplexing (SEFDM), in which higher spectral efficiency is achieved by violating the orthogonality of its subcarriers. The proposed scheme is distinguished by its simplicity, low computational complexity, high accuracy and performance independent of the number of subcarriers and compression factor. The presented results demonstrate the efficacy of the proposed scheme by comparing its complexity and performance to other estimation schemes

A Novel Pilot Expansion Approach for MIMO Channel Estimation

A training-based MIMO channel estimation scheme is presented to operate in severe frequency and time selective fading channels. Besides the new pilot bits designed from the ‘Paley-Hadamard’ matrix to exploit its orthogonal and ‘Toeplitz-like’ structures and minimising its pilot length, a novel pilot expansion technique is proposed to estimate the length of the channel impulse response, by flexibly extending its pilot length as required in order to capture the number of multipath existed within the MIMO channel. The pilot expansion can also help to deduce the initial channel variation and its Doppler rate which can be subsequently applied for MIMO channel tracking using decision feedback Kalman filter during the data payload.</p

Transmisión multiportadora sin prefijo cíclico para comunicaciones radio de alta capacidad

Este trabajo fin de grado está orientado en una de las líneas de investigación seguidas en la actualidad para conseguir aumentar la capacidad de los sistemas de comunicaciones. Se ha centrado el trabajo en el marco de la modulación multiportadora Orthogonal Frequency Division Multiplexing (OFDM) por ser una de las técnicas más utilizadas en la actualidad en sistemas de banda ancha. El objetivo será aumentar la tasa de transmisión de un sistema de comunicaciones suprimiendo o reduciendo el prefijo cíclico, Cyclic Prefix (CP), perteneciente a OFDM. Se analizan y muestran las consecuencias en forma de interferencias que se tienen como resultado de no utilizar un CP mayor que la respuesta al impulso del canal. Las interferencias que se han encontrado son las denominadas como ISI (Inter-Symbol Interference) por no introducir un intervalo de guarda suficiente entre los símbolos OFDM e ICI (Inter-Carrier Interference) por la pérdida de ortogonalidad entre las subportadoras, provocando estas unos pésimos resultados en el sistema. Para poder combatir estos efectos se explicará y se implementará un algoritmo de cancelación de interferencias llamado “Cancelación residual de ISI” (RISIC). Este algoritmo consiste en realizar una combinación de cancelación de cola y una reconstrucción cíclica. Se ha analizado si los resultados obtenidos compensan el aumento de complejidad introducida en el sistema. Además, en este trabajo se investiga cómo esta técnica puede ser implementada en estándares actuales como Wimax (Worldwide Interoperability for Microwave Access) o LTE (Long Term Evolution), y así poder conseguir mayores velocidades de transmisión de datos.This final degree project deals with the research lines followed at present for increasing the capacity of communications systems. We have focused our work in multicarrier modulation Orthogonal Frequency Division Multiplexing (OFDM) for being one of the techniques used today in broadband systems. The goal that we want to achieve will be to increase the transmission rate of a communication system by eliminating or reducing the Cyclic Prefix (CP) pertaining to OFDM. Consequences are analyzed and displayed in the form of interference effects as result as not using a CP higher than the channel impulse response. The interferences that have been found are named ISI (Inter-Symbol Interference). This is because ISI doesn't introduce enough guard interval between OFDM symbols and ICI (Inter-Carrier Interference) for the loss of orthogonality between subcarriers and it provokes dismal system results. To combat these effects will be discussed and implemented an algorithm called interference cancellation "Residual ISI Cancellation" (RISIC). This algorithm is to perform a combination of tail cancellation cyclic reconstruction. We have analyzed if the results outweigh the increased complexity introduced into the system. In addition, this final degree project investigates how this technique can be implemented in existing standards such as WiMAX (Worldwide Interoperability for Microwave Access) or Long Term Evolution (LTE), so you can achieve higher data transmission speeds.Ingeniería de Sistemas de Comunicacione

Técnicas de gestão de feixe de onda para sistemas Massive MIMO nas redes 5G NR

The use of Millimeter wave (mmWave) spectrum frequencies is seen as a key enabler technology for the future wireless communication systems to overcome the bandwidth shortage of the sub 6GHz microwave spectrum band, enabling high speed data transmissions in the 5G/6G systems. Nevertheless, mmWave propagation characteristics are associated to significant free-path losses and many more attenuations that become even more harsher as the frequency increases, rendering the communication challenging at this frequencies. To overcome these distinct disadvantages, multiple antenna arrays are employed to allow beamforming techniques for the transmission of narrower concentrated beams in more precise directions and less interference levels between them, consequently improving the link budget. Thus, to constantly assure that the communication with each device is done using the beam pair that allows the best possible connectivity, a set of Beam Management control procedures is necessary to assure an efficient beamformed connection establishment and its continuous maintenance between the device and the network. This dissertation will address the description of the Initial Beam Establishment (IBE) BM procedure, focusing the selection of the most suitable transmit-receive beam pair available after completed beam sweeping techniques to measure the different power levels of the received signal. The main goal is to design a new 3GPP-standard compliant beam pair selection algorithm based on SSS angle estimation (BSAE), that makes use of multiple Synchronization Signal Blocks (SSBs) to maximize the Reference Signal Received Power (RSRP) value at the receiver, through the selected beam pair. This optimization is done using the Secondary Synchronization Signals (SSSs) present in each SSB to perform channel estimation in the digital domain (comprising the effects of the analog processing). Afterwards, the combination of those estimations were used to perform the equivalent channel propagation matrix estimation without the analog processing effects. Finally, through the channel propagation matrix, the angle that maximizes the RSRP was determined to compute the most suitable beam through the aggregated response vector. The obtained results show that the proposed algorithm achieves better performance levels compared to a conventional beam pair selection algorithm. Furthermore, a comparison with an optimal case is also done, i.e., the situation where the channel is known, and the optimal beam pair angle can be determined. Therefore, the similar performance results compared to the optimal case indicates that the proposed algorithm is interesting for practical 5G mmWave mMIMO implementations, according to 3GPP-compliant standards.O uso de frequências na banda das ondas milimétricas é visto como uma tecnologia chave para os futuros sistemas de comunicação móveis, tendo em vista a ultrapassar o problema da escassez de banda a sub-6 GHz, e por permitir as elevadas taxas de dados requeridas para sistemas 5G/6G. Contudo, a propagação deste tipo de ondas está associado a perdas acentuadas em espaço livre e várias atenuações que se tornam cada vez mais significativas com o aumento do valor da frequência, impondo obstáculos à comunicação. Para ultrapassar estas adversidades, agregados constituídos por múltiplos elementos de antena são implementados por forma a permitir técnicas de formação de feixe e possibilitar a transmissão de feixes mais estreitos e altamente direcionais, diminuindo os níveis de interferência e melhorando consequentemente o link budget. Deste modo, para assegurar constantemente que a comunicação efetuada em cada dispositivo ocorre utilizando o conjunto de feixes que proporciona o melhor nível de conectividade, é então necessário um conjunto de procedimentos de controlo de gestão de feixe, assegurando um estabelecimento eficiente da comunicação e a sua contínua manutenção entre um dispositivo e a rede. Esta dissertação descreve o procedimento de gestão de feixe conhecido como estabelecimento inicial de feixe, focando o processo de seleção do melhor par de feixe de transmissão-receção disponível após o uso de técnicas de varrimento de feixe por fim a efetuar medições dos diferentes níveis de potência do sinal recebido. O principal objetivo passa pela conceção de um novo algoritmo de estabelecimento de par de feixes baseado em estimações de ângulo (BSAE), que explora o uso de múltiplos SSBs definidos pelo 3GPP, por forma a maximizar o RSRP no recetor, através do feixe selecionado. Esta otimização é feita usando os sinais de sincronização secundários (SSSs) presentes em cada SSB para efetuar uma estimação de canal no domínio digital (que contém o efeito do processamento analógico). Depois, combinando essas estimações, foi feita uma estimação da matriz do canal de propagação, sem o efeito desse processamento analógico. Finalmente, através da matriz do canal de propagação, foi determinado o ângulo que maximiza o RSRP, e calculado o feixe através do vetor de resposta do agregado. Os resultados obtidos demonstram que o algoritmo proposto atinge melhor desempenho quando comparado com o algoritmo convencional de seleção de par de feixes. Foi feita ainda uma comparação com o caso ótimo, isto é, com o caso em que se conhece completamente o canal e se obtém um ângulo ótimo. Os resultados obtidos pelo algoritmo proposto foram muito próximos do caso ótimo, pelo que é bastante interessante para sistemas práticos 5G mmWave mMIMO, que estejam de acordo com o padrão 3GPP.Mestrado em Engenharia Eletrónica e Telecomunicaçõe

Un système de communication OFDM à faible probabilité d'interception

Les systèmes OFDM traditionnels présentent des propriétés cyclostationnaires qui peuvent être exploitées afin d’intercepter le signal sans fil transmis. Cependant, une telle possibilité est inacceptable dans les applications militaires où la sécurité est d’une importance extrême. Le préfixe cyclique est le paramètre OFDM le plus important qui génère des caractéristiques cyclostationnaires. Dans le but d’empêcher l’exploitation de ces caractéristiques, nous proposons une combinaison innovatrice de deux techniques qui permettent de réduire considérablement la cyclostationnarité et les propriétés spectrales de la forme d'onde OFDM, tout en conservant les avantages du préfixe cyclique. La taille du préfixe cyclique est variée pseudo-aléatoirement dans chaque symbole OFDM de sorte que les caractéristiques cyclostationnaires soient atténuées. Par ailleurs, une gigue de fréquence pseudo-aléatoire est introduite à la fréquence porteuse afin de masquer les raies spectrales correspondantes aux fréquences des sous-porteuses. Le système conçu génère ainsi une forme d'onde à faible probabilité d’interception. De plus, de bonnes performances sur les canaux à trajets multiples de type Rayleigh sont démontrées. Enfin, une preuve de concept d'un émetteur GNU Radio implémentant un préfixe cyclique de taille pseudo-aléatoire est exposée

Technique de reconfiguration d'antennes par décomposition des diagrammes de rayonnement

RÉSUMÉ L'expansion des télécommunications entraîne une demande de plus en plus forte en débit et en qualité de service. Ceci implique une constante évolution des systèmes sans-fil utilisés. Le déploiement des systèmes à multiples antennes (MIMO) est une solution très intéressante pouvant augmenter la fiabilité du canal et la quantité d'information transmise. Cependant, de nouveaux problèmes tels que la corrélation des canaux apparaissent. Parallèlement à cette technologie, les antennes reconfigurables sont utilisées depuis bien longtemps dans divers domaines comme celui des radars. Leur capacité à s'adapter à leur environnent n'a pas échappé aux chercheurs. En combinant ce type d'antennes dans des systèmes MIMO, diverses études indiquent une possible diminution de la corrélation des canaux par modification du diagramme de rayonnement de l'antenne par exemple. Tout ceci est très intéressant, encore faut-il une méthode de reconfiguration capable de s'exécuter rapidement. Une technique assurant la meilleure configuration consiste à estimer le canal de transmission pour toutes les configurations possibles de l'antenne. Cependant, toutes ces estimations font perdre un temps précieux et affectent la capacité du canal. Une manière plus élaborée de configurer une antenne est de déterminer la position des sources par des méthodes comme MUSIC ou ESPRIT. Malheureusement, le nombre de sources pouvant être trouvées est limité et la complexité est assez importante. On se propose ici de mettre au point une technique dont le but principal est de reconfigurer une antenne en diminuant le plus possible le nombre d'estimations du canal. Les canaux non-estimés vont devoir être reconstruits par un certain calcul détaillé dans notre étude. Brièvement, ce calcul se base sur une décomposition mathématique du diagramme de rayonnement de l'antenne en fonction de coefficients dépendants uniquement de la configuration de l'antenne et de diagrammes de rayonnement particuliers de l'antenne (configurations fixées). Cette technique doit bien entendu être utilisable dans un système MIMO. Les antennes étudiées sont des réseaux d'antennes linéaires uniformes (ULA) et des antennes à perte formées à partir de méta-matériaux (CRLH).Cette étude va nous mener à observer le comportement de notre technique face à des erreurs d'estimation des canaux mesurés et des erreurs sur la reconstruction des canaux non-mesurés. Chacune de ces erreurs a ses propres caractéristiques mais au final elles dépendent toutes deux des configurations des canaux mesurés. On explique qu'un moyen simple pour parer à ces deux erreurs consiste à mesurer beaucoup de canaux ce qui va à l'encontre du but fixé par notre technique. ----------ABSTRACT The demand for faster data rates and better quality of services constantly increases with the expansion of wireless telecommunications. Employing Multiple Input Multiple Output (MIMO) systems is a very interesting solution to improve reliability of channel and enhance data transmission. Unfortunately, new problems such as channel correlation appear. Next to these systems, reconfigurable antennas have been used for a long time in different fields like radars. Their capability to adapt to their environment has generated significant interest in adding new degrees of freedom. By combining this kind of antenna with a MIMO system, different studies show some enhancements in adapting dynamically their radiation pattern for example. To dynamically adapt antenna, a fast and reliable reconfigurable method has to be used. In order to find the optimal antenna configuration we can proceed to a complete scanning of available configurations, which is time consuming and decreases the achievable data rate. We can also use direction of arrival estimation using the MUSIC algorithm. However, in a rich multipath environment, the direction of arrival is hard to estimate accurately with the MUSIC algorithm. Other algorithms, such as ESPRIT, could be used to estimate the channel parameters. However, they are computationally intensive, especially in a MIMO environment. Furthermore the number of direction of arrival which can be found is limited by the number of receiver. To save time we describe here a technique using estimations of the channel for a low number of configurations of the antenna