Semi-blind CFO estimation and ICA based equalization for wireless communication systems

In this thesis, a number of semi-blind structures are proposed for Orthogonal Frequency Division Multiplexing (OFDM) based wireless communication systems, with Carrier Frequency Offset (CFO) estimation and Independent Component Analysis (ICA) based equalization. In the first contribution, a semi-blind non-redundant single-user Multiple-Input Multiple-Output (MIMO) OFDM system is proposed, with a precoding aided CFO estimation approach and an ICA based equalization structure. A number of reference data sequences are carefully designed and selected from a pool of orthogonal sequences, killing two birds with one stone. On the one hand, the precoding based CFO estimation is performed by minimizing the sum cross-correlations between the CFO compensated signals and the rest of the orthogonal sequences in the pool. On the other hand, the same reference data sequences enable the elimination of permutation and quadrant ambiguities in the ICA equalized signals. Simulation results show that the proposed semi-blind MIMO OFDM system can achieve a Bit Error Rate (BER) performance close to the ideal case with perfect Channel State Information (CSI) and no CFO. In the second contribution, a low-complexity semi-blind structure, with a multi-CFO estimation method and an ICA based equalization scheme, is proposed for multiuser Coordinated Multi-Point (CoMP) OFDM systems. A short pilot is carefully designed offline for each user and has a two-fold advantage. On the one hand, using the pilot structure, a complex multi-dimensional search for multiple CFOs is divided into a number of low-complexity mono-dimensional searches. On the other hand, the cross-correlation between the transmitted and received pilots is explored to allow the simultaneous elimination of permutation and quadrant ambiguities in the ICA equalized signals. Simulation results show that the proposed semi-blind CoMP OFDM system can provide a BER performance close to the ideal case with perfect CSI and no CFO. In the third contribution, a semi-blind structure is proposed for Carrier Aggregation (CA) based CoMP Orthogonal Frequency Division Multiple Access (OFDMA) systems, with an ICA based joint Inter-Carrier Interference (ICI) mitigation and equalization scheme. The CFO-induced ICI is mitigated implicitly via ICA based equalization, without introducing feedback overhead for CFO correction. The permutation and quadrant ambiguities in the ICA equalized signals can be eliminated by a small number of pilots. Simulation results show that with a low training overhead, the proposed semi-blind equalization scheme can provide a BER performance close to the ideal case with perfect CSI and no CFO

Review of Recent Trends

This work was partially supported by the European Regional Development Fund (FEDER), through the Regional Operational Programme of Centre (CENTRO 2020) of the Portugal 2020 framework, through projects SOCA (CENTRO-01-0145-FEDER-000010) and ORCIP (CENTRO-01-0145-FEDER-022141). Fernando P. Guiomar acknowledges a fellowship from “la Caixa” Foundation (ID100010434), code LCF/BQ/PR20/11770015. Houda Harkat acknowledges the financial support of the Programmatic Financing of the CTS R&D Unit (UIDP/00066/2020).MIMO-OFDM is a key technology and a strong candidate for 5G telecommunication systems. In the literature, there is no convenient survey study that rounds up all the necessary points to be investigated concerning such systems. The current deeper review paper inspects and interprets the state of the art and addresses several research axes related to MIMO-OFDM systems. Two topics have received special attention: MIMO waveforms and MIMO-OFDM channel estimation. The existing MIMO hardware and software innovations, in addition to the MIMO-OFDM equalization techniques, are discussed concisely. In the literature, only a few authors have discussed the MIMO channel estimation and modeling problems for a variety of MIMO systems. However, to the best of our knowledge, there has been until now no review paper specifically discussing the recent works concerning channel estimation and the equalization process for MIMO-OFDM systems. Hence, the current work focuses on analyzing the recently used algorithms in the field, which could be a rich reference for researchers. Moreover, some research perspectives are identified.publishersversionpublishe

Development and verification of semi-blind receiver structures for broadband wireless communication systems

The increasingly high demands for high data rate wireless communication services require spectrum- and energy-efficient solutions. In this thesis, a number of energy-efficient semi-blind receiver structures are proposed to perform Doppler spread estimation, channel estimation and equalisation for broadband wireless orthogonal frequency division multiplexing (OFDM) systems. A real-time wireless communication testbed is developed to verify the proposed semi-blind receiver structures. In the first contribution, a semi-blind Doppler spread estimation and Kalman filtering based channel estimation approach is proposed for wireless OFDM systems. A short sequence of reference data is carefully designed and applied as pilot symbols for Doppler spread estimation and channel estimation initialisation of the Kalman filter. Then the estimates of inter-carrier interference (ICI) caused by Doppler spread are gathered into the equivalent channel model and compensated for through channel equalisation, which dramatically reduces the computational complexity. The simulation results show that the proposed approach outperforms the conventional pilot aided Doppler spread and channel estimation schemes. In the second contribution, a semi-blind Doppler spread estimation and independent component analysis (ICA) based equalisation scheme aided by non-redundant precoding is proposed for wireless multiple-input multiple-output (MIMO) OFDM systems. A number of reference data sequences are selected from a pool of orthogonal sequences for two purposes. First, the reference data sequences are superimposed in the source data sequences through non-redundant linear precoding to enable the Doppler spread estimation by minimising the sum cross-correlation between the compensated signals and the rest of the orthogonal sequences in the pool. Second, the same reference data sequences are applied to eliminate the phase and permutation ambiguity in the ICA equalised signals. Simulation results show that the proposed semi-blind MIMO OFDM system can achieve a bit error rate (BER) performance which is close to the ideal case with perfect channel state information (CSI). In the third contribution, a real-time wireless communication testbed is developed with a vector signal generator, a vector signal analyser and a pair of antennas, to verify the effectiveness of the proposed receiver structures over the air in different environments such as Reverberation chamber and office area. Measurement results show a good match with simulation results. Also, a pilot is employed for three purposes at a semi-blind receiver: time synchronisation, Doppler spread estimation and Kalman filtering initialisation, which is an extension of the work in the first contribution

Advances in Multi-User Scheduling and Turbo Equalization for Wireless MIMO Systems

Nach einer Einleitung behandelt Teil 2 Mehrbenutzer-Scheduling für die Abwärtsstrecke von drahtlosen MIMO Systemen mit einer Sendestation und kanaladaptivem precoding: In jeder Zeit- oder Frequenzressource kann eine andere Nutzergruppe gleichzeitig bedient werden, räumlich getrennt durch unterschiedliche Antennengewichte. Nutzer mit korrelierten Kanälen sollten nicht gleichzeitig bedient werden, da dies die räumliche Trennbarkeit erschwert. Die Summenrate einer Nutzermenge hängt von den Antennengewichten ab, die wiederum von der Nutzerauswahl abhängen. Zur Entkopplung des Problems schlägt diese Arbeit Metriken vor basierend auf einer geschätzten Rate mit ZF precoding. Diese lässt sich mit Hilfe von wiederholten orthogonalen Projektionen abschätzen, wodurch die Berechnung von Antennengewichten beim Scheduling entfällt. Die Ratenschätzung kann basierend auf momentanen Kanalmessungen oder auf gemittelter Kanalkenntnis berechnet werden und es können Datenraten- und Fairness-Kriterien berücksichtig werden. Effiziente Suchalgorithmen werden vorgestellt, die die gesamte Systembandbreite auf einmal bearbeiten können und zur Komplexitätsreduktion die Lösung in Zeit- und Frequenz nachführen können. Teil 3 zeigt wie mehrere Sendestationen koordiniertes Scheduling und kooperative Signalverarbeitung einsetzen können. Mittels orthogonalen Projektionen ist es möglich, Inter-Site Interferenz zu schätzen, ohne Antennengewichte berechnen zu müssen. Durch ein Konzept virtueller Nutzer kann der obige Scheduling-Ansatz auf mehrere Sendestationen und sogar Relays mit SDMA erweitert werden. Auf den benötigten Signalisierungsaufwand wird kurz eingegangen und eine Methode zur Schätzung der Summenrate eines Systems ohne Koordination besprochen. Teil4 entwickelt Optimierungen für Turbo Entzerrer. Diese Nutzen Signalkorrelation als Quelle von Redundanz. Trotzdem kann eine Kombination mit MIMO precoding sinnvoll sein, da bei Annahme realistischer Fehler in der Kanalkenntnis am Sender keine optimale Interferenzunterdrückung möglich ist. Mit Hilfe von EXIT Charts wird eine neuartige Methode zur adaptiven Nutzung von a-priori-Information zwischen Iterationen entwickelt, die die Konvergenz verbessert. Dabei wird gezeigt, wie man semi-blinde Kanalschätzung im EXIT chart berücksichtigen kann. In Computersimulationen werden alle Verfahren basierend auf 4G-Systemparametern überprüft.After an introduction, part 2 of this thesis deals with downlink multi-user scheduling for wireless MIMO systems with one transmitting station performing channel adaptive precoding:Different user subsets can be served in each time or frequency resource by separating them in space with different antenna weight vectors. Users with correlated channel matrices should not be served jointly since correlation impairs the spatial separability.The resulting sum rate for each user subset depends on the precoding weights, which in turn depend on the user subset. This thesis manages to decouple this problem by proposing a scheduling metric based on the rate with ZF precoding such as BD, written with the help of orthogonal projection matrices. It allows estimating rates without computing any antenna weights by using a repeated projection approximation.This rate estimate allows considering user rate requirements and fairness criteria and can work with either instantaneous or long term averaged channel knowledge.Search algorithms are presented to efficiently solve user grouping or selection problems jointly for the entire system bandwidth while being able to track the solution in time and frequency for complexity reduction. Part 3 shows how multiple transmitting stations can benefit from cooperative scheduling or joint signal processing. An orthogonal projection based estimate of the inter-site interference power, again without computing any antenna weights, and a virtual user concept extends the scheduling approach to cooperative base stations and finally included SDMA half-duplex relays in the scheduling.Signalling overhead is discussed and a method to estimate the sum rate without coordination. Part 4 presents optimizations for Turbo Equalizers. There, correlation between user signals can be exploited as a source of redundancy. Nevertheless a combination with transmit precoding which aims at reducing correlation can be beneficial when the channel knowledge at the transmitter contains a realistic error, leading to increased correlation. A novel method for adaptive re-use of a-priori information between is developed to increase convergence by tracking the iterations online with EXIT charts.A method is proposed to model semi-blind channel estimation updates in an EXIT chart. Computer simulations with 4G system parameters illustrate the methods using realistic channel models.Im Buchhandel erhältlich: Advances in Multi-User Scheduling and Turbo Equalization for Wireless MIMO Systems / Fuchs-Lautensack,Martin Ilmenau: ISLE, 2009,116 S. ISBN 978-3-938843-43-

Blind detection of interfering cell data channel power level in 3GPP LTE/LTE-Advanced downlink

Nowadays wireless cellular networks can be seen as ubiquitous systems used by a majority of the world's population and their usage continues to grow in the future. Continuously higher data rates and shorter latencies are required due to the introduction of new mobile devices and services. In addition, mobile networks are more and more used as a primary connectivity solution in several places. Thus, new technologies are required to improve the capacity and latency of mobile networks. Long Term Evolution (LTE) and LTE-Advanced are technologies standardized by Third Generation Partnership Project (3GPP) with the potential to fulfill these requirements for future mobile networks. LTE/LTE-Advanced cellular networks are usually interference limited, because neighbouring cells use the same frequency band for data transmission. Because of this, several users may experience high interference levels and thus cannot achieve high data rates without proper counteractions. Consequently, advanced techniques to control, suppress or cancel the interference are of interest to be studied for LTE and LTE-Advanced by 3GPP. Network Assisted Interference Cancellation and Suppression (NAICS) techniques are currently been studied by 3GPP. One of the recent studies on NAICS is so called blind detection of interfering cell parameters in a user equipment for advanced non-linear receivers. Such receivers have the capability to suppress or cancel interference significantly but they require the knowledge of specific parameters of the interfering cell to perform efficiently. These parameters have to be either signaled by the network or blindly detected from the received signal in user equipment. The topic of this thesis is to study the feasibility of blind detection of interfering cell's data channel power level, which is crucial knowledge to non-linear receivers. The study is performed at radio link level by using numerical simulations, in which the transmitter and receiver processing are modeled in detail. In addition, also the effects caused by the radio channel to the transmitted signals are modeled. The performance of one non-linear receiver, namely Symbol Level Interference Cancellation (SLIC) receiver, with blind detection is compared to the performance of SLIC receiver which has the knowledge of all required parameters. While Linear Minimum Mean Squared Error - Interference Rejection Combiner (LMMSE-IRC) receiver operates as the baseline. From the simulation results it can be seen that the blind detector performs well. Consequently this blind detector can be one noteworthy option to avoid signaling of interfering cell's data channel power level

I/Q imbalance mitigation for space-time block coded communication systems

Multiple-input multiple-output (MIMO) space-time block coded (STBC) wireless communication systems provide reliable data transmissions by exploiting the spatial diversity in fading channels. However, due to component imperfections, the in-phase/quadrature (I/Q) imbalance caused by the non-ideal matching between the relative amplitudes and phases of the I and Q branches always exists in the practical implementation of MIMO STBC communication systems. Such distortion results in a complex conjugate term of the intended signal in the time domain, hence a mirror-image term in the frequency domain, in the data structure. Consequently, I/Q imbalance increases the symbol error rate (SER) drastically in MIMO STBC or STBC MIMO orthogonal frequency division multiplexing (OFDM) communication systems, where both the signal and its complex conjugate are utilized for the information transmission, hence should be mitigated effectively. In this dissertation, the impact of I/Q imbalance in MIMO STBC systems over flat fading channels, the impact of I/Q imbalance in STBC MIMO-OFDM systems and in time- reversal STBC (TR-STBC) systems over frequency-selective fading channels are studied systematically. With regard to the MIMO STBC and the STBC MIMO-OFDM systems with I/Q imbalance, orthogonal space-time block codes (OSTBCs), quasi-orthogonal STBCs (QOSTBCs) and rotated QOSTBCs (RQOSTBCs) are studied, respectively. By exploiting the special structure of the received signal, low-complexity solutions are provided to mitigate the distortion induced by I/Q imbalance successfully. In addition, to mitigate I/Q imbalance while at the same time to exploit the multipath diversity for STBC OFDM systems over frequency-selective fading channels, a new encoding/decoing scheme for the grouped linear constellation precoded (GLCP) OFDM systems with I/Q imbalance is studied. In Chapter 1, the objectives of the research are elaborated. In Chapter 2, the various I/Q imbalance models are introduced, and the model used in this dissertation is established. In Chapter 3, the performance degradation caused by I/Q imbalance of the transceivers in MIMO STBC wireless communication systems over flat fading channels and the solutions are studied. A 2 Tx Alamouti system, a 4 Tx quasi-orthogonal STBC (QOSTBC) system, and a 4 Tx rotated QOSTBC (RQOSTBC) system with I/Q imbalance are studied in detail. By exploiting the special structure of the received signal, low-complexity solutions are proposed to mitigate I/Q imbalance successfully. Since STBCs are developed for frequency-flat fading channels, to achieve the spatial diversity in frequency-selective fading channels, MIMO-OFDM arrangements have been suggested, where STBCs are used across different antennas in conjunction with OFDM. In Chapter 4, the performance degradation caused by I/Q imbalance in STBC MIMO-OFDM wireless systems over frequency-selective fading channels and the solutions are studied. Similarly, a 2 Tx Alamouti system, a 4 Tx quasi-orthogonal STBC (QOSTBC) system, and a 4 Tx rotated QOSTBC (RQOSTBC) system with I/Q imbalance are studied in detail, and low-complexity solutions are proposed to mitigate the distortion effectively. However, OFDM systems suffer from the loss of the multipath diversity by converting frequency-selective fading channels into parallel frequency-flat fading subchannels. To exploit the multipath diversity and reduce the decoding complexity, GLCP OFDM systems with I/Q imbalance are studied. By judiciously assigning the mirror-subcarrier pair into one group, a new encoding/decoding scheme with a low-complexity is proposed to mitigate I/Q imbalance for GLCP OFDM systems in Chapter 5. Since OFDM communication systems have high peak-to-average power ratio (PAPR) problem and are sensitive to carrier frequency offset (CFO), to achieve both the spatial and multipath diversity, time-reversal STBC (TR-STBC) communication systems are introduced. In Chapter 6, the I/Q imbalance mitigating solutions in TR-STBC systems, both in the time domain and in the frequency domain, are studied

A comparative study of STBC transmissions at 2.4 GHz over indoor channels using a 2 × 2 MIMO testbed

In this paper we employ a 2×2 Multiple-Input Multiple-Output (MIMO) hardware platform to evaluate, in realistic indoor scenarios, the performance of different space-time block coded (STBC) transmissions at 2.4GHz. In particular, we focus on the Alamouti orthogonal scheme considering two types of channel state information (CSI) estimation: a conventional pilot-aided supervised technique and a recently proposed blind method based on second-order statistics (SOS). For comparison purposes, we also evaluate the performance of a Differential (non-coherent) space-time block coding (DSTBC). DSTBC schemes have the advantage of not requiring CSI estimation but they incur in a 3dB loss in performance. The hardware MIMO platform is based on high-performance signal acquisition and generation boards, each one equipped with a 1GB memory module that allows the transmission of extremely large data frames. Upconversion to RF is performed by two RF vector signal generators whereas downconversion is carried out with two custom circuits designed from commercial components. All the baseband signal processing is implemented off-line in MATLAB®, making the MIMO testbed very flexible and easily reconfigurable. Using this platform we compare the performance of the described methods in line-of-sight (LOS) and non-line-of-sight (NLOS) indoor scenarios.This work has been supported by Ministerio de Educación y Ciencia of Spain, Xunta de Galicia and FEDER funds of the European Union under grant numbers TEC2004-06451-C05-02, TEC2004-06451-C05-01, PGIDT05PXIC10502PN, and FPU grants AP2004-5127 and AP2006-2965

Design and Performance Analysis of the Dynamic Reduction of Intrinsic Interference Suppression and BER using QAM-based FBMC for MU-MIMO Communications

The present research work is focused on the study of co-channel interface with its minimization techniques without influencing its performance, in turn, which is desired to achieve the minimized complexity of Quadrature Amplitude Modulation (QAM)-based Filter Bank Multi-Carrier (FBMC) to minimize the interference and increase the spectral features with consideration of intrinsic features extractions for the ML (Maximum Likelihood) synthesis systems. The valid measures are given various concerns under consideration, to start with the consideration of the evaluation of the Cyclic Prefix Orthogonal Frequency Division Multiplexing (CP-OFD performance metrics along with the FBMC/QAM in signal transmission in a dedicated fading channel for the evaluation of the modulation order and BER as a required trade-off for quality assessments. From the results, it can be noted that the proposed FBMC QAM has performed better when compared with conventional FBMC systems. The present research also includes considering and calculating the efficiency of nonlinear channels with the Multi-User Multiple Input Multiple Output (MU-MIMO) and FBMC/QAM techniques. In continuation, the obtained results are dominating significantly to access the possible solution to meet the efficiency of the proposed system. In the next part of the research, it is considered with implementation of the sub-detector during the downlink of the system with the technique of threshold-driven strategy for better accuracy and minimization of the complexity in terms of ML detection in terms of order of its modulation. The calculations of the proposed technique with better BER are done on the recent MATLAB platform with its simulation demonstration for its detailed observation

Linear Transmit-Receive Strategies for Multi-user MIMO Wireless Communications

Die Notwendigkeit zur Unterdrueckung von Interferenzen auf der einen Seite und zur Ausnutzung der durch Mehrfachzugriffsverfahren erzielbaren Gewinne auf der anderen Seite rueckte die raeumlichen Mehrfachzugriffsverfahren (Space Division Multiple Access, SDMA) in den Fokus der Forschung. Ein Vertreter der raeumlichen Mehrfachzugriffsverfahren, die lineare Vorkodierung, fand aufgrund steigender Anzahl an Nutzern und Antennen in heutigen und zukuenftigen Mobilkommunikationssystemen besondere Beachtung, da diese Verfahren das Design von Algorithmen zur Vorcodierung vereinfachen. Aus diesem Grund leistet diese Dissertation einen Beitrag zur Entwicklung linearer Sende- und Empfangstechniken fuer MIMO-Technologie mit mehreren Nutzern. Zunaechst stellen wir ein Framework zur Approximation des Datendurchsatzes in Broadcast-MIMO-Kanaelen mit mehreren Nutzern vor. In diesem Framework nehmen wir das lineare Vorkodierverfahren regularisierte Blockdiagonalisierung (RBD) an. Durch den Vergleich von Dirty Paper Coding (DPC) und linearen Vorkodieralgorithmen (z.B. Zero Forcing (ZF) und Blockdiagonalisierung (BD)) ist es uns moeglich, untere und obere Schranken fuer den Unterschied bezueglich Datenraten und bezueglich Leistung zwischen beiden anzugeben. Im Weiteren entwickeln wir einen Algorithmus fuer koordiniertes Beamforming (Coordinated Beamforming, CBF), dessen Loesung sich in geschlossener Form angeben laesst. Dieser CBF-Algorithmus basiert auf der SeDJoCo-Transformation und loest bisher vorhandene Probleme im Bereich CBF. Im Anschluss schlagen wir einen iterativen CBF-Algorithmus namens FlexCoBF (flexible coordinated beamforming) fuer MIMO-Broadcast-Kanaele mit mehreren Nutzern vor. Im Vergleich mit bis dato existierenden iterativen CBF-Algorithmen kann als vielversprechendster Vorteil die freie Wahl der linearen Sende- und Empfangsstrategie herausgestellt werden. Das heisst, jede existierende Methode der linearen Vorkodierung kann als Sendestrategie genutzt werden, waehrend die Strategie zum Empfangsbeamforming frei aus MRC oder MMSE gewaehlt werden darf. Im Hinblick auf Szenarien, in denen Mobilfunkzellen in Clustern zusammengefasst sind, erweitern wir FlexCoBF noch weiter. Hier wurde das Konzept der koordinierten Mehrpunktverbindung (Coordinated Multipoint (CoMP) transmission) integriert. Zuletzt stellen wir drei Moeglichkeiten vor, Kanalzustandsinformationen (Channel State Information, CSI) unter verschiedenen Kanalumstaenden zu erlangen. Die Qualitaet der Kanalzustandsinformationen hat einen starken Einfluss auf die Guete des Uebertragungssystems. Die durch unsere neuen Algorithmen erzielten Verbesserungen haben wir mittels numerischer Simulationen von Summenraten und Bitfehlerraten belegt.In order to combat interference and exploit large multiplexing gains of the multi-antenna systems, a particular interest in spatial division multiple access (SDMA) techniques has emerged. Linear precoding techniques, as one of the SDMA strategies, have obtained more attention due to the fact that an increasing number of users and antennas involved into the existing and future mobile communication systems requires a simplification of the precoding design. Therefore, this thesis contributes to the design of linear transmit and receive strategies for multi-user MIMO broadcast channels in a single cell and clustered multiple cells. First, we present a throughput approximation framework for multi-user MIMO broadcast channels employing regularized block diagonalization (RBD) linear precoding. Comparing dirty paper coding (DPC) and linear precoding algorithms (e.g., zero forcing (ZF) and block diagonalization (BD)), we further quantify lower and upper bounds of the rate and power offset between them as a function of the system parameters such as the number of users and antennas. Next, we develop a novel closed-form coordinated beamforming (CBF) algorithm (i.e., SeDJoCo based closed-form CBF) to solve the existing open problem of CBF. Our new algorithm can support a MIMO system with an arbitrary number of users and transmit antennas. Moreover, the application of our new algorithm is not only for CBF, but also for blind source separation (BSS), since the same mathematical model has been used in BSS application.Then, we further propose a new iterative CBF algorithm (i.e., flexible coordinated beamforming (FlexCoBF)) for multi-user MIMO broadcast channels. Compared to the existing iterative CBF algorithms, the most promising advantage of our new algorithm is that it provides freedom in the choice of the linear transmit and receive beamforming strategies, i.e., any existing linear precoding method can be chosen as the transmit strategy and the receive beamforming strategy can be flexibly chosen from MRC or MMSE receivers. Considering clustered multiple cell scenarios, we extend the FlexCoBF algorithm further and introduce the concept of the coordinated multipoint (CoMP) transmission. Finally, we present three strategies for channel state information (CSI) acquisition regarding various channel conditions and channel estimation strategies. The CSI knowledge is required at the base station in order to implement SDMA techniques. The quality of the obtained CSI heavily affects the system performance. The performance enhancement achieved by our new strategies has been demonstrated by numerical simulation results in terms of the system sum rate and the bit error rate