106 research outputs found

    Low-complexity iterative frequency domain decision feedback equalization

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    Single-carrier transmission with frequency domain equalization (SC-FDE) offers a viable design alternative to the classic orthogonal frequency division multiplexing technique. However, SC-FDE using a linear equalizer may suffer from serious performance deterioration for transmission over severely frequency-selective fading channels. An effective method of solving this problem is to introduce non-linear decision feedback equalization (DFE) to SC-FDE. In this contribution, a low complexity iterative decision feedback equalizer operating in the frequency domain of single-carrier systems is proposed. Based on the minimum mean square error criterion, a simplified parameter estimation method is introduced to calculate the coefficients of the feed-forward and feedback filters, which significantly reduces the implementation complexity of the equalizer. Simulation results show that the performance of the proposed simplified design is similar to the traditional iterative block DFE under various multipath fading channels but it imposes a much lower complexity than the latter

    Classical and Bayesian Linear Data Estimators for Unique Word OFDM

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    Unique word - orthogonal frequency division multiplexing (UW-OFDM) is a novel OFDM signaling concept, where the guard interval is built of a deterministic sequence - the so-called unique word - instead of the conventional random cyclic prefix. In contrast to previous attempts with deterministic sequences in the guard interval the addressed UW-OFDM signaling approach introduces correlations between the subcarrier symbols, which can be exploited by the receiver in order to improve the bit error ratio performance. In this paper we develop several linear data estimators specifically designed for UW-OFDM, some based on classical and some based on Bayesian estimation theory. Furthermore, we derive complexity optimized versions of these estimators, and we study their individual complex multiplication count in detail. Finally, we evaluate the estimators' performance for the additive white Gaussian noise channel as well as for selected indoor multipath channel scenarios.Comment: Preprint, 13 page

    An iterative frequency-domain decision feedback receiver for CDMA systems

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    Unless high-complexity receiver structures are employed, conventional DS-CDMA (direct sequence code division multiple access) schemes can have a poor performance in severe time-dispersive channels, since the orthogonality between the users is lost. In this paper, we propose an iterative frequency-domain decision feedback equalizer for the downlink transmission within DS-CDMA systems employing block-transmission techniques, with an appropriate cyclic extension appended to each block. Our performance results show that the proposed receiver structure has excellent performance, close to the single-user MFB (matched filter bound), even for severe time-dispersive scenarios and/or in the presence of strong interfering channels

    Técnicas de igualização adaptativas com estimativas imperfeitas do canal para os futuros sistemas 5G

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    Wireless communication networks have been continuously experiencing an exponential growth since their inception. The overwhelming demand for high data rates, support of a large number of users while mitigating disruptive interference are the constant research focus and it has led to the creation of new technologies and efficient techniques. Orthogonal frequency division multiplexing (OFDM) is the most common example of a technology that has come to the fore in this past decade as it provided a simple and generally ideal platform for wireless data transmission. It’s drawback of a rather high peak-to-average power ratio (PAPR) and sensitivity to phase noise, which in turn led to the adoption of alternative techniques, such as the single carrier systems with frequency domain equalization (SC-FDE) or the multi carrier systems with code division multiple access (MC-CDMA), but the nonlinear Frequency Domain Equalizers (FDE) have been of special note due to their improved performance. From these, the Iterative Block Decision Feedback Equalizer (IB-DFE) has proven itself especially promising due to its compatibility with space diversity, MIMO systems and CDMA schemes. However, the IB-DFE requires the system to have constant knowledge of the communication channel properties, that is, to have constantly perfect Channel State Information (CSI), which is both unrealistic and impractical to implement. In this dissertation we shall design an altered IB-DFE receiver that is able to properly detect signals from SC-FDMA based transmitters, even with constantly erroneous channel states. The results shall demonstrate that the proposed equalization scheme is robust to imperfect CSI (I-CSI) situations, since its performance is constantly close to the perfect CSI case, within just a few iterations.Redes sem fios tĂȘm crescido de maneira contĂ­nua e exponencial desde a sua incepção. A tremenda exigĂȘncia para altas taxas de dados e o suporte para um elevado nĂșmero de utilizadores sem aumentar a interferĂȘncia disruptiva originada por estes sĂŁo alguns dos focos que levaram ao desenvolvimento de tĂ©cnicas de compensação e novas tecnologias. “Orthogonal frequency division multiplexing” (OFDM) Ă© um dos exemplos de tecnologias que se destacaram nesta Ășltima dĂ©cada, visto ter fornecido uma plataforma para transmissĂŁo de dados sem-fio eficaz e simples. O seu maior problema Ă© a alta “peak-to-average power ratio” (PAPR) e a sua sensibilidade a ruĂ­do de fase que deram motivo Ă  adoção de tĂ©cnicas alternativas, tais como os sistemas “single carrier” com “frequency domain equalization” (SC-FDE) ou os sistemas “multi-carrier” com “code division multiple access” (MC-CDMA), mas equalizadores nĂŁo lineares no domĂ­nio de frequĂȘncia tĂȘm sido alvo de especial atenção devido ao seu melhor desempenho. Destes, o “iterative block decision feedback equalizer” (IB-DFE) tem-se provado especialmente promissor devido Ă  sua compatibilidade com tĂ©cnicas de diversidade no espaço, sistemas MIMO e esquemas CDMA. No entanto, IB-DFE requer que o sistema tenha constante conhecimento das propriedades dos canais usados, ou seja, necessita de ter perfeito “channel state information” (CSI) constantemente, o que Ă© tanto irrealista como impossĂ­vel de implementar. Nesta dissertação iremos projetar um recetor IB-DFE alterado de forma a conseguir detetar sinais dum transmissor baseado em tecnologia SC-FDMA, mesmo com a informação de estado de canal errada. Os resultados irĂŁo entĂŁo demonstrar que o novo esquema de equalização proposto Ă© robusto para situaçÔes de CSI imperfeito (I-CSI), visto que o seu desempenho se mantĂ©m prĂłximo dos valores esperados para CSI perfeito, em apenas algumas iteraçÔes.Mestrado em Engenharia EletrĂłnica e TelecomunicaçÔe

    Transmission strategies for broadband wireless systems with MMSE turbo equalization

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    This monograph details efficient transmission strategies for single-carrier wireless broadband communication systems employing iterative (turbo) equalization. In particular, the first part focuses on the design and analysis of low complexity and robust MMSE-based turbo equalizers operating in the frequency domain. Accordingly, several novel receiver schemes are presented which improve the convergence properties and error performance over the existing turbo equalizers. The second part discusses concepts and algorithms that aim to increase the power and spectral efficiency of the communication system by efficiently exploiting the available resources at the transmitter side based upon the channel conditions. The challenging issue encountered in this context is how the transmission rate and power can be optimized, while a specific convergence constraint of the turbo equalizer is guaranteed.Die vorliegende Arbeit beschĂ€ftigt sich mit dem Entwurf und der Analyse von effizienten Übertragungs-konzepten fĂŒr drahtlose, breitbandige EintrĂ€ger-Kommunikationssysteme mit iterativer (Turbo-) Entzerrung und Kanaldekodierung. Dies beinhaltet einerseits die Entwicklung von empfĂ€ngerseitigen Frequenzbereichs-entzerrern mit geringer KomplexitĂ€t basierend auf dem Prinzip der Soft Interference Cancellation Minimum-Mean Squared-Error (SC-MMSE) Filterung und andererseits den Entwurf von senderseitigen Algorithmen, die durch Ausnutzung von Kanalzustandsinformationen die Bandbreiten- und Leistungseffizienz in Ein- und Mehrnutzersystemen mit Mehrfachantennen (sog. Multiple-Input Multiple-Output (MIMO)) verbessern. Im ersten Teil dieser Arbeit wird ein allgemeiner Ansatz fĂŒr Verfahren zur Turbo-Entzerrung nach dem Prinzip der linearen MMSE-SchĂ€tzung, der nichtlinearen MMSE-SchĂ€tzung sowie der kombinierten MMSE- und Maximum-a-Posteriori (MAP)-SchĂ€tzung vorgestellt. In diesem Zusammenhang werden zwei neue EmpfĂ€ngerkonzepte, die eine Steigerung der LeistungsfĂ€higkeit und Verbesserung der Konvergenz in Bezug auf existierende SC-MMSE Turbo-Entzerrer in verschiedenen Kanalumgebungen erzielen, eingefĂŒhrt. Der erste EmpfĂ€nger - PDA SC-MMSE - stellt eine Kombination aus dem Probabilistic-Data-Association (PDA) Ansatz und dem bekannten SC-MMSE Entzerrer dar. Im Gegensatz zum SC-MMSE nutzt der PDA SC-MMSE eine interne EntscheidungsrĂŒckfĂŒhrung, so dass zur UnterdrĂŒckung von Interferenzen neben den a priori Informationen der Kanaldekodierung auch weiche Entscheidungen der vorherigen Detektions-schritte berĂŒcksichtigt werden. Durch die zusĂ€tzlich interne EntscheidungsrĂŒckfĂŒhrung erzielt der PDA SC-MMSE einen wesentlichen Gewinn an Performance in rĂ€umlich unkorrelierten MIMO-KanĂ€len gegenĂŒber dem SC-MMSE, ohne dabei die KomplexitĂ€t des Entzerrers wesentlich zu erhöhen. Der zweite EmpfĂ€nger - hybrid SC-MMSE - bildet eine VerknĂŒpfung von gruppenbasierter SC-MMSE Frequenzbereichsfilterung und MAP-Detektion. Dieser EmpfĂ€nger besitzt eine skalierbare BerechnungskomplexitĂ€t und weist eine hohe Robustheit gegenĂŒber rĂ€umlichen Korrelationen in MIMO-KanĂ€len auf. Die numerischen Ergebnisse von Simulationen basierend auf Messungen mit einem Channel-Sounder in MehrnutzerkanĂ€len mit starken rĂ€umlichen Korrelationen zeigen eindrucksvoll die Überlegenheit des hybriden SC-MMSE-Ansatzes gegenĂŒber dem konventionellen SC-MMSE-basiertem EmpfĂ€nger. Im zweiten Teil wird der Einfluss von System- und Kanalmodellparametern auf die Konvergenzeigenschaften der vorgestellten iterativen EmpfĂ€nger mit Hilfe sogenannter Korrelationsdiagramme untersucht. Durch semi-analytische Berechnungen der Entzerrer- und Kanaldecoder-Korrelationsfunktionen wird eine einfache Berechnungsvorschrift zur Vorhersage der Bitfehlerwahrscheinlichkeit von SC-MMSE und PDA SC-MMSE Turbo Entzerrern fĂŒr MIMO-FadingkanĂ€le entwickelt. Des Weiteren werden zwei Fehlerschranken fĂŒr die Ausfallwahrscheinlichkeit der EmpfĂ€nger vorgestellt. Die semi-analytische Methode und die abgeleiteten Fehlerschranken ermöglichen eine aufwandsgeringe AbschĂ€tzung sowie Optimierung der LeistungsfĂ€higkeit des iterativen Systems. Im dritten und abschließenden Teil werden Strategien zur Raten- und Leistungszuweisung in Kommunikationssystemen mit konventionellen iterativen SC-MMSE EmpfĂ€ngern untersucht. ZunĂ€chst wird das Problem der Maximierung der instantanen Summendatenrate unter der BerĂŒcksichtigung der Konvergenz des iterativen EmpfĂ€ngers fĂŒr einen Zweinutzerkanal mit fester Leistungsallokation betrachtet. Mit Hilfe des FlĂ€chentheorems von Extrinsic-Information-Transfer (EXIT)-Funktionen wird eine obere Schranke fĂŒr die erreichbare Ratenregion hergeleitet. Auf Grundlage dieser Schranke wird ein einfacher Algorithmus entwickelt, der fĂŒr jeden Nutzer aus einer Menge von vorgegebenen Kanalcodes mit verschiedenen Codierraten denjenigen auswĂ€hlt, der den instantanen Datendurchsatz des Mehrnutzersystems verbessert. Neben der instantanen Ratenzuweisung wird auch ein ausfallbasierter Ansatz zur Ratenzuweisung entwickelt. Hierbei erfolgt die Auswahl der Kanalcodes fĂŒr die Nutzer unter BerĂŒcksichtigung der Einhaltung einer bestimmten Ausfallwahrscheinlichkeit (outage probability) des iterativen EmpfĂ€ngers. Des Weiteren wird ein neues Entwurfskriterium fĂŒr irregulĂ€re Faltungscodes hergeleitet, das die Ausfallwahrscheinlichkeit von Turbo SC-MMSE Systemen verringert und somit die ZuverlĂ€ssigkeit der DatenĂŒbertragung erhöht. Eine Reihe von Simulationsergebnissen von KapazitĂ€ts- und Durchsatzberechnungen werden vorgestellt, die die Wirksamkeit der vorgeschlagenen Algorithmen und Optimierungsverfahren in MehrnutzerkanĂ€len belegen. Abschließend werden außerdem verschiedene Maßnahmen zur Minimierung der Sendeleistung in Einnutzersystemen mit senderseitiger Singular-Value-Decomposition (SVD)-basierter Vorcodierung untersucht. Es wird gezeigt, dass eine Methode, welche die Leistungspegel des Senders hinsichtlich der Bitfehlerrate des iterativen EmpfĂ€ngers optimiert, den konventionellen Verfahren zur Leistungszuweisung ĂŒberlegen ist

    Low-complexity iterative receiver algorithms for multiple-input multiple-output underwater wireless communications

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    This dissertation proposes three low-complexity iterative receiver algorithms for multiple-input multiple-output (MIMO) underwater acoustic (UWA) communications. First is a bidirectional soft-decision feedback Turbo equalizer (Bi-SDFE) which harvests the time-reverse diversity in severe multipath MIMO channels. The Bi-SDFE outperforms the original soft-decision feedback Turbo equalizer (SDFE) while keeping its total computational complexity similar to that of the SDFE. Second, this dissertation proposes an efficient direct adaptation Turbo equalizer for MIMO UWA communications. Benefiting from the usage of soft-decision reference symbols for parameter adaptation as well as the iterative processing inside the adaptive equalizer, the proposed algorithm is efficient in four aspects: robust performance in tough channels, high spectral efficiency with short training overhead, time efficient with fast convergence and low complexity in hardware implementation. Third, a frequency-domain soft-decision block iterative equalizer combined with iterative channel estimation is proposed for the uncoded single carrier MIMO systems with high data efficiency. All the three new algorithms are evaluated by data recorded in real world ocean experiment or pool experiment. Finally, this dissertation also compares several Turbo equalizers in single-input single-output (SISO) UWA channels. Experimental results show that the channel estimation based Turbo equalizers are robust in SISO underwater transmission under harsh channel conditions --Abstract, page iv

    On Linear Frequency Domain Turbo-Equalization of Non Linear Volterra Channels

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    International audienceThis article deals with iterative Frequency Domain Equalization (FDE) for Single Carrier (SC) transmissions over Volterra non linear satellite channels. SC-FDE has gained much importance in recent research for its efficient implementation at the receiver and its interesting low Peak to Average Power Ratio (PAPR) at the transmitter. However, nearly saturated power amplifiers on board satellites generate linear and non linear Inter Symbol Interference (ISI) at the receiver. It is thus interesting to investigate the implementation of SC-FDE for non linear channels. To do so, a frequency domain equivalent satellite channel is derived based on the time domain Volterra series representation of the non linear channel. Then a Minimum Mean Square Error (MMSE)-based iterative frequency domain equalizer is designed. It is shown that the proposed equalizer consists of a Soft Interference Canceller (SIC) which subtracts both the linear and non-linear soft frequency symbols. The equalizer performance is then compared to the equivalent time domain implementation. Results show that a channel-memory independent efficient implementation is achieved at the price of a negligible spectral efficiency loss due to cyclic prefix insertion

    Design and performance evaluation of turbo FDE receivers

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    Dissertação apresentada na Faculdade de CiĂȘncias e Tecnologia da Universidade Nova de Lisboa para obtenção do Grau de Mestre em Engenharia ElectrotĂ©cnica e de ComputadoresIn recent years, block transmission techniques were proposed and developed for broadband wireless communication systems, which have to deal with strongly frequency-selective fading channels. Techniques like Orthogonal Frequency-Division Multiplexing (OFDM)and Single Carrier with Frequency Domain Equalization (SC-FDE) are able to provide high bit rates despite the channel adversities. In this thesis we concentrate on the study of single carrier block transmission techniques considering receiver structures suitable to scenarios with strongly time-dispersive channels. CP-assisted (Cycle Pre x) block transmission techniques are employed to cope with frequency selective channels, allowing cost-e ective implementations through FFT-based (Fast Fourier Transform) signal processing. It is investigated the impact of the number of multipath components as well as the diversity order on the asymptotic performance of SC-FDE schemes. We also propose a receiver structure able to perform a joint detection and channel estimation method, in which it is possible to combine the channel estimates, based on training sequences, with decision-directed channel estimates. A study about the impact of the correlation factor estimation in the performance of Iterative Block-Decision Feedback Equalizer (IB-DFE) receivers is also presented

    A spectrally efficient transmission scheme for signals with large bandwidth

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    In this paper we consider single-carrier with frequency-domain equalization (SC-FDE) schemes where the transmission bandwidth is above the symbol rate. To allow high spectral efficiencies, several channels share the same bandwidth. Since the co-channel interference (CCI) levels can be very high, we propose iterative FDE receivers where we jointly detect all users sharing the same channel. Our performance results show that we can have excellent performances, even with several users sharing the same channel. In fact, we can have the maximum theoretical spectral efficiency even with signals that have bandwidth substantially above the symbol rate

    IB-DFE receivers with space diversity for CP-assisted DS-CDMA and MC-CDMA systems

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    Multi-Carrier Code Division Multiple Access (MC-CDMA), currently regarded as a promising multiple access scheme for broadband communications, is known to combine the advantages of an Orthogonal Frequency Division Multiplexing (OFDM)-based, Cyclic Prefix (CP)-assisted block transmission with those of CDMA systems. Recently, it was recognised that DS-CDMA (Direct Sequence) implementations can also take advantage of the benefits of the CP-assisted block transmission approach, therefore enabling an efficient use of Fast Fourier Transform (FFT)-based, chip level Frequency-Domain Equalisation (FDE) techniques. When employing a linear FDE with both MC-CDMA and DS-CDMA, the FDE coefficients can be optimised under the Minimum Mean Squared Error (MMSE) criterion, so as to avoid significant noise enhancement. The residual interference levels can be very high, especially for fully loaded scenarios, since the FDE/MMSE does not perform a perfect channel inversion. This paper deals with CP-assisted DS-CDMA systems and MC-CDMA systems with frequency-domain spreading. We consider the use of Iterative Block Decision Feedback Equalisation (IB-DFE) FDE techniques as an alternative to conventional, linear FDE techniques, and derive the appropriate IB-DFE parameters in a receiver diversity context. Our performance results show that IB-DFE techniques with moderate complexity allow significant performance gains in both systems, with bit error rate (BER) that can be close to the single-code matched filter bound (MFB) (especially for the CP-assisted DS-CDMA alternative), even with full code usage. Copyright © 2007 John Wiley & Sons. Ltd
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