The use of multiple antennas at the transmitter and/or the receiver promises greatly increased capacity. This can be useful to meet the ever growing demand of wireless connectivity, provided we can find techniques to efficiently exploit the advantages of the Multiple-Input Multiple-Output (MIMO) system. This work explores the MIMO system in a flat quasi-static fading scenario. Such a channel occurs, for example, in packet data systems, where the channel fade is constant for the duration of a codeword and changes independently from one transmission to another. We first show why it is hard to compute the true constrained modulation outage capacity. As an alternative, we present achievable lower bounds to this capacity based on existing space-time codes. The bounds we compute are the fundamental limits to the performance of these space-time codes under maximum-likelihood decoding, optimal outer codes and asymptotically long lengths. These bounds also indicate that MIMO systems have different behavior under Gaussian signaling (unconstrained input) and under the finite alphabet setting. Our results naturally suggest the use of concatenated codes to approach near-capacity performance. However, we show that a system utilizing an iterative decoder has a fundamental limit – it cannot be universal and therefore it cannot perform arbitrarily close to its outage limit. Next, we propose two different transceiver structures that have good performance. The first structure is based on a novel BCJR-decision feedback decoder which results in performance within a dB of the outage limit. The second structure is based on recursive realizations of space-time trellis codes and uses iterative decoding at the receiver. This recursive structure has impressive performance even when the channel has time diversity. Thus, it forms the basis of a very flexible and robust MIMO transceiver structure.

Recent advancements in iterative processing have allowed communication systems to perform close to capacity limits withmanageable complexity.For manychannels such as the AWGN and flat fading channels, codes that perform only a fraction of a dB from the capacity have been designed in the literature. In this dissertation, we will focus on the design and analysis of near-capacity achieving codes for another important class of channels, namely inter-symbol interference (ISI)channels. We propose various coding schemes such as low-density parity-check (LDPC) codes, parallel and serial concatenations for ISI channels when there is no spectral shaping used at the transmitter. The design and analysis techniques use the idea of extrinsic information transfer (EXIT) function matching and provide insights into the performance of different codes and receiver structures. We then present a coding scheme which is the concatenation of an LDPC code with a spectral shaping block code designed to be matched to the channel??s spectrum. We will discuss how to design the shaping code and the outer LDPC code. We will show that spectral shaping matched codes can be used for the parallel concatenation to achieve near capacity performance. We will also discuss the capacity of multiple antenna ISI channels. We study the effects of transmitter and receiver diversities and noisy channel state information on channel capacity.

In recent years, space-time block coding (STBC) has emerged as an effective transmit-diversity technique to combat the detrimental effects of channel fading. In addition to STBC, high-order modulation schemes will be used in future wireless communication systems aiming to provide ubiquitous-broadband wireless access. Hence, advanced receiver schemes are necessary to achieve high performance. In this thesis, advanced and computationally-efficient receiver schemes are investigated and developed for single-carrier space-time (ST) block-coded transmissions over frequency-selective fading (FSF) channels. First, we develop an MMSE-based turbo equalization scheme for Alamouti ST block-coded systems. A semi-analytical method to estimate the bit error rate (BER) is devised. Our results show that the proposed turbo equalization scheme offers significant performance improvements over one-pass equalization. Second, we analyze the convergence behavior of the proposed turbo equalization scheme for Alamouti ST block-coded systems using the extrinsic information transfer (EXIT)-band chart technique. Third, burst-wise (BW)-STBC is applied for uplink transmission over FSF channels in block-spread-CDMA systems with multiuser interference-free reception. The performances of different decision feedback sequence estimation (DFSE) schemes are investigated. A new scheme combining frequency-domain (FD) linear equalization and modified unwhitened-DFSE is proposed. The proposed scheme is very promising as the error-floor behavior observed in the existing unwhitened DFSE schemes is eliminated. Fourth, we develop a FD-MMSE-based turbo equalization scheme for the downlink of ST block-coded CDMA systems. We adopt BW-STBC instead of Alamouti symbol-wise (SW)-STBC considered for WCDMA systems and demonstrate its superior performance in FSF channels. Block spreading is shown to be more desirable than conventional spreading to improve performance using turbo equalization. We also devise approximate implementations (AprxImpls) that offer better trade-offs between performance and complexity. Semi-analytical upper bounds on the BER are derived. Fifth, turbo multicode detection is investigated for ST block-coded downlink transmission in DS-CDMA systems. We propose symbol-by-symbol and chip-by-chip FD-MMSE-based multicode detectors. An iterative channel estimation scheme is also proposed. The proposed turbo multicode detection scheme offers significant performance improvements compared with non-iterative multicode detection. Finally, the impact of channel estimation errors on the performance of MMSE-based turbo equalization in ST block-coded CDMA systems is investigated.

In wireless communications, the use of multiple antennas at both the transmitter and the receiver is a key technology to enable high data rate transmission without additional bandwidth or transmit power. Multiple-input multiple-output (MIMO) schemes are widely used in many wireless standards, allowing higher throughput using spatial multiplexing techniques. MIMO soft detection poses significant challenges to the MIMO receiver design as the detection complexity increases exponentially with the number of antennas. As the next generation wireless system is pushing for multi-Gbps data rate, there is a great need for high-throughput low-complexity soft-output MIMO detector. The brute-force implementation of the optimal MIMO detection algorithm would consume enormous power and is not feasible for the current technology. We propose a reduced-complexity soft-output MIMO detector architecture based on a trellis-search method. We convert the MIMO detection problem into a shortest path problem. We introduce a path reduction and a path extension algorithm to reduce the search complexity while still maintaining sufficient soft information values for the detection. We avoid the missing counter-hypothesis problem by keeping multiple paths during the trellis search process. The proposed trellis-search algorithm is a data-parallel algorithm and is very suitable for high speed VLSI implementation. Compared with the conventional tree-search based detectors, the proposed trellis-based detector has a significant improvement in terms of detection throughput and area efficiency. The proposed MIMO detector has great potential to be applied for the next generation Gbps wireless systems by achieving very high throughput and good error performance. The soft information generated by the MIMO detector will be processed by a channel decoder, e.g. a low-density parity-check (LDPC) decoder or a Turbo decoder, to recover the original information bits. Channel decoder is another very computational-intensive block in a MIMO receiver SoC (system-on-chip). We will present high-performance LDPC decoder architectures and Turbo decoder architectures to achieve 1+ Gbps data rate. Further, a configurable decoder architecture that can be dynamically reconfigured to support both LDPC codes and Turbo codes is developed to support multiple 3G/4G wireless standards. We will present ASIC and FPGA implementation results of various MIMO detectors, LDPC decoders, and Turbo decoders. We will discuss in details the computational complexity and the throughput performance of these detectors and decoders.

Research indicates that multiprocessor-based architectures will provide a flexible alternative to hard-wired application-specific integrated circuits (ASICs) suitable to implement the multitude of wireless standards required by mobile devices, while meeting their strict area and power requirements. This shift in design philosophy has led to the software-defined radio (SDR) paradigm, where a significant portion of a wireless standard's physical layer is implemented in software, allowing multiple standards to share a common architecture. Turbo codes offer excellent error-correcting performance, however, turbo decoders are one of the most computationally complex baseband tasks of a wireless receiver. Next generation wireless standards such as Worldwide Interoperability for Microwave Access (WiMAX), support enhanced double-binary turbo codes, which offer even better performance than the original binary turbo codes, at the expense of additional complexity. Hence, the design of efficient double-binary turbo decoder software is required to support wireless standards in a SDR environment. This thesis describes the optimization, parallelization, and simulated performance of a software double-binary turbo decoder implementation supporting the WiMAX standard suitable for SDR. An adapted turbo decoder is implemented in the C language, and numerous software optimizations are applied to reduce its overall computationally complexity. Evaluation of the software optimizations demonstrated a combined improvement of at least 270% for serial execution, while maintaining good bit-error rate (BER) performance. Using a customized multiprocessor simulator, special instruction support is implemented to speed up commonly performed turbo decoder operations, and is shown to improve decoder performance by 29% to 40%. The development of a flexible parallel decoding algorithm is detailed, with multiprocessor simulations demonstrating a speedup of 10.8 using twelve processors, while maintaining good parallel efficiency (above 89%). A linear-log-MAP decoder implementation using four iterations was shown to have 90% greater throughput than a max-log-MAP decoder implementation using eight iterations, with comparable BER performance. Simulation also shows that multiprocessor cache effects do not have a significant impact on parallel execution times. An initial investigation into the use of vector processing to further enhance performance of the parallel decoder software reveals promising results. / Thesis (Master, Electrical & Computer Engineering) -- Queen's University, 2009-09-25 16:22:47.288

Turbo codes have been the subject of much research in recent years, producing results very close to the theoretical limit set by Shannon. The codes have been successfully implemented in satellite and video conferencing systems and provision has been made in 3rd generation mobile systems. These codes have not been used for short frame systems due to the delay at the decoder. In this thesis, comprehensive comparisons of the two common decoding algorithms are made, with reference to short frames. The effects of increasing memory size of component codes, frame sizes, utilising puncturing and errors in channel estimation are investigated over AWGN and Rayleigh fading channels. The decoder systems are compared for complexity as well as for equal numbers of iterations. Results show that less complex decoder strategies produce good results for voice quality bit error rates. Investigations are also made into the effects of errors in signal-to-noise ratio estimation at the SOVA turbo decoder, showing this decoding algorithm to be more resilient than Log-MAP decoders in published literature. The decoders are also tested over channels displaying inter-symbol interference. Channels include a time-invariant channel and three ETSI standard time-varying channels simulating indoor, pedestrian and vehicular situations, upgraded for more realistic Doppler effect. To combat these types of channels, a derivative of turbo codes, turbo equalisation is often used. To keep receiver delay to a minimum, decision feedback equalisation is used here. Results show that the combination can produce improvements in decoded results with increasing turbo iterations where ISI is low, but that iterative improvements do not occur under harsh circumstances. The combination produces much superior results compared with codes on their own under even the most extreme circumstances.

In recent years, space-time block coding (STBC) has emerged as an effective transmit-diversity technique to combat the detrimental effects of channel fading. In addition to STBC, high-order modulation schemes will be used in future wireless communication systems aiming to provide ubiquitous-broadband wireless access. Hence, advanced receiver schemes are necessary to achieve high performance. In this thesis, advanced and computationally-efficient receiver schemes are investigated and developed for single-carrier space-time (ST) block-coded transmissions over frequency-selective fading (FSF) channels. First, we develop an MMSE-based turbo equalization scheme for Alamouti ST block-coded systems. A semi-analytical method to estimate the bit error rate (BER) is devised. Our results show that the proposed turbo equalization scheme offers significant performance improvements over one-pass equalization. Second, we analyze the convergence behavior of the proposed turbo equalization scheme for Alamouti ST block-coded systems using the extrinsic information transfer (EXIT)-band chart technique. Third, burst-wise (BW)-STBC is applied for uplink transmission over FSF channels in block-spread-CDMA systems with multiuser interference-free reception. The performances of different decision feedback sequence estimation (DFSE) schemes are investigated. A new scheme combining frequency-domain (FD) linear equalization and modified unwhitened-DFSE is proposed. The proposed scheme is very promising as the error-floor behavior observed in the existing unwhitened DFSE schemes is eliminated. Fourth, we develop a FD-MMSE-based turbo equalization scheme for the downlink of ST block-coded CDMA systems. We adopt BW-STBC instead of Alamouti symbol-wise (SW)-STBC considered for WCDMA systems and demonstrate its superior performance in FSF channels. Block spreading is shown to be more desirable than conventional spreading to improve performance using turbo equalization. We also devise approximate implementations (AprxImpls) that offer better trade-offs between performance and complexity. Semi-analytical upper bounds on the BER are derived. Fifth, turbo multicode detection is investigated for ST block-coded downlink transmission in DS-CDMA systems. We propose symbol-by-symbol and chip-by-chip FD-MMSE-based multicode detectors. An iterative channel estimation scheme is also proposed. The proposed turbo multicode detection scheme offers significant performance improvements compared with non-iterative multicode detection. Finally, the impact of channel estimation errors on the performance of MMSE-based turbo equalization in ST block-coded CDMA systems is investigated.

Control of water erosion of soil at mine sites requires an ability to predict the effects of different management practices on soil loss. Using soil loss models such as the Revised Universal Soil Loss Equation (RUSLE) requires calibration of the model for materials and situations that are not defined in the model’s handbook or software. The outer slopes of bauxite residue disposal areas are potentially highly erodible surfaces, and a recent evaluation of previous rehabilitation practices at Worsley Alumina, Collie, Western Australia, identified areas on the bauxite residue disposal areas where vegetation establishment and management of long term soil loss could be improved. Field experiments commencing in April 2000 at Worsley Alumina’s bauxite refinery, Collie, and laboratory tilting flume experiments run at the University of Queensland, were designed to quantify the effectiveness of different surface treatments on reducing short-term soil loss, and to model long-term erosion risks. Crushed ferricrete caprock – rock-pitch – and different types of mulches, seed mixes and fertiliser rates were applied to the compacted clay batter slopes used to contain bauxite residue, with runoff, soil loss and vegetation establishment monitored periodically over 27 months. Laboratory tilting flume results were related to the field data using the soil erosion models MINErosion, the Modified Universal Soil Loss Equation (MUSLE) and RUSLE to predict event-based and annual soil loss for different treatments. Turbo-mulch, a blocky, coarse pine bark, was the most effective surface treatment for promoting vegetation establishment, reducing rill formation and reducing soil loss, a result supported in both the field and laboratory results. Turbo-mulch and vegetation did not reduce runoff, but resulted in decreased soil loss. This shows the importance of protecting soil from raindrop impact and of the soil holding capacity of vegetation. Increased seed and fertiliser rates did not significantly affect native plant numbers or foliage cover on topsoil without turbo-mulch. Rock-pitch was found to be resistant to erosion and mass movement along a rock-pitch/compacted clay interface. Field erosion measurements ranged from 0.87 t/ha/yr for turbo-mulched treatments to 7.41 t/ha/yr for a treatment with a different seed mix, lacking turbo-mulch and lacking underlying rock-pitch. RUSLE soil loss predictions based on soil properties and soil loss estimates from the MINErosion model ranged from 0.27 to 60.0 t/ha/yr. RUSLE predictions based on tilting flume data ranged from 0.14 to 81.1 t/ha/yr. RUSLE overpredicted soil loss for treatments without turbo-mulch, and underestimated soil loss for turbo-mulched treatments, necessitating calibration based on the unique materials trialed in this study. The relative soil loss measured in the field was best represented by RUSLE predictions based on tilting flume data rather than the MINErosion model. MINErosion did not adequately describe the effect of bulk density and infiltration on soil loss of compacted/consolidated materials. MUSLE and RUSLE are adequate models for the Western Australian conditions of this study, but further research is required to calibrate the C factor for turbo-mulched surfaces and calibrate the P factor for rock-pitch.

Οι τηλεπικοινωνίες με τη χρήση δορυφορικών καναλιών αναμένεται να διαδραματίσουν σημαντικό ρόλο στα μελλοντικά συστήματα επικοινωνιών τρίτης και τέταρτης γενιάς (3G και 4G). Ωστόσο ο δρόμος για την ευρεία χρησιμοποίηση δορυφορικών συστημάτων θα πρέπει να περάσει μέσα από τη μελέτη και αντιμετώπιση ορισμένων προβλημάτων και περιορισμών που εμφανίζονται σε αυτά. Ένα τέτοιο βασικό πρόβλημα είναι η μη γραμμική σχέση εισόδου – εξόδου του δορυφορικού καναλιού, που κατά κύριο λόγο οφείλεται στη χρήση ενισχυτών χαμηλής κατανάλωσης ισχύος στο δορυφόρο. Επιπλέον, η ανάγκη για μετάδοση με υψηλούς ρυθμούς θέτει περαιτέρω περιορισμούς στην αντιμετώπιση των παραμορφώσεων που υπεισέρχονται στο εκπεμπόμενο σήμα κατά τη μετάδοσή του στον ασύρματο τηλεπικοινωνιακό δίαυλο. Για να γίνει λοιπόν δυνατή η αποτελεσματική αντιμετώπιση των παραμορφώσεων αυτών θα πρέπει να χρησιμοποιηθούν αποτελεσματικές τεχνικές ισοστάθμισης και αποκωδικοποίησης στο δέκτη. Αρχικά αυτά τα δυο στάδια στο δέκτη λειτουργούσαν ανεξάρτητα και δεν υπήρχε ανταλλαγή πληροφορίας μεταξύ τους για να επιτευχθεί καλύτερη απόδοση. Στη συνέχεια για να γίνει πραγματικότητα αυτή η βελτίωση της απόδοσης δημιουργήθηκε η Turbo ισοστάθμιση στην οποία υπάρχει επαναληπτική αλληλεπίδραση των βαθμίδων του ισοσταθμιστή και του αποκωδικοποιητή στο δέκτη του συστήματος. Η αλληλεπίδραση αυτή επιτυγχάνεται με την ανταλλαγή soft πληροφορίας. Το ερώτημα που δημιουργείται αφού έχουμε καταλήξει στην δομή του δέκτη είναι με πιο τρόπο θα υλοποιηθούν ο ισοσταθμιστής και ο αποκωδικοποιητής. Η βέλτιστη λύση θα ήταν να χρησιμοποιηθεί για τα δυο αυτά στάδια του δέκτη ο αλγόριθμος του MAP (Maximum A-posteriori Probability). Λόγω όμως της σημαντικής υπολογιστικής πολυπλοκότητας (και σαν συνέπεια και του κόστους σε χρόνο) αυτής της μεθόδου είμαστε αναγκασμένοι να καταφύγουμε σε εναλλακτικές λύσεις. Η λύση λοιπόν την οποία αρχικά μελετούμε είναι αυτή της χρήσης ενός SIC (Soft Interference Canceller) στο στάδιο του ισοσταθμιστή ο οποίος προσπαθεί να ελαχιστοποιήσει το μέσο τετραγωνικό σφάλμα (MMSE) μεταξύ των δεδομένων που μεταδόθηκαν και των εκτιμήσεων του canceller χρησιμοποιώντας γραμμικά φίλτρα προκειμένου να προβεί στην εκτίμηση των τελευταίων. Παρόλα αυτά λόγω των πολύ ισχυρών παραμορφώσεων που εισάγονται από το δορυφορικό μας κανάλι η μέθοδος δεν είναι τόσο αποτελεσματική όσο θα θέλαμε. Γι’ αυτό το λόγο καταφεύγουμε στην υλοποίηση ενός δεύτερου SIC παρόμοιας φιλοσοφίας λειτουργίας με τον προηγούμενο, ο οποίος όμως μπορεί να αντιμετωπίσει περισσότερο αποτελεσματικά τις μη γραμμικότητες (NonLinearities) του δορυφορικού καναλιού. Όλες οι παραπάνω τεχνικές οι οποίες αναφέρθηκαν έχουν ως στόχο να μπορέσουν να κάνουν δυνατή, όσο το δυνατόν πιο αξιόπιστα, τη μετάδοση πληροφορίας μέσω των μη γραμμικών δορυφορικών καναλιών. Για να μπορέσουμε όμως να διερευνήσουμε το κατά πόσο είναι τελικά αυτό εφικτό θα πρέπει να μοντελοποιήσουμε με κάποιο τρόπο αυτά τα κανάλια. Βέβαια επειδή η συμπεριφορά τους διαφέρει δραματικά σε σχέση με τα συνηθισμένα κανάλια που συναντάμε χρησιμοποιούμε για την προσομοίωση τους τις σειρές Volterra. Τέλος μέσω κατάλληλων αριθμητικών προσομοιώσεων οι τεχνικές που αναφέρθηκαν παραπάνω αξιολογούνται και συγκρίνονται με βάση κριτήρια επίδοσης, όπως η πιθανότητα σφάλματος ανίχνευσης. / -

El creciente avance de las telecomunicaciones durante estos últimos años, ha despertado el interés de muchos investigadores en conseguir más y mejores técnicas que permitan establecer comunicaciones inalámbricas robustas y que a su vez, soporten grandes flujos de información a alta velocidad. No obstante, la utilización de un canal inalámbrico para el envío y recepción de señales, introduce efectos indeseados tales como interferencia y ruido, debido a que entre el emisor y el receptor se interponen diferentes agentes que se comportan de aletoriamente. Es así como se han desarrollado métodos que han despertado gran interés entre los investigadores, debido a la obtención de resultados sorprendentes. Entre estos métodos destacan el algoritmo MAP y los códigos LDPC. El primero puede ser utilizado tanto en el aspecto de la ecualización de señales o en la decodificación de señales, mientras que el segundo se utiliza específicamente para codificar y decodificar señales. De este modo, el objetivo principal de esta memoria es unir estos dos algoritmos para que trabajen de forma iterativa, utilizando la información proporcionada por el ecualizador para mejorar el proceso de decodificación y a su vez, realizar el proceso de ecualización utilizando la información del decodificador. A este sistema retroalimentado se le llama Esquema Turbo. Además, se plantea el algoritmo para lenguajes no binarios. Para realizar este trabajo, primeramente se hizo un desglose de un sistema de comunicaciones actual, reconociendo cada uno de los bloques componentes, analizando su funcionamiento teórico, introduciendo modificaciones según corresponda y a partir de este análisis obtiene un diagrama de bloques que resume el funcionamiento general del algoritmo propuesto. Seguidamente se realizan pruebas a pequeña escala, utilizando un modelo de canal TDL Gaussiano de tres derivaciones, capaz de emular el comportamiento de una señal bajo interferencia y ruido, con el objetivo de demostrar el correcto funcionamiento del algoritmo y su convergencia, para posteriormente someter el esquema a transmisiones de grandes bloques de información, midiendo su desempeño bajo distintos escenarios y estudiando la tasa de símbolos errados (SER) variando la razón señal ruido (SNR). Los resultados obtenidos de esta investigación dicen que este algoritmo es capaz de lograr una corrección completa en señales pequeñas, incluso bajo condiciones en que de SNR se reduce hasta 5 dB para 7 iteraciones. Sin embargo, al momento de enviar bloques de información de tamaño mucho mayor (105 símbolos), el algoritmo presenta un piso de SNR de 10 dB, ya que al aumentarlo por sobre ese valor no se lograba mejoramiento de decodificación, debido a que se utilizó un esquema de codificación de bloques con matrices pequeñas. Esto se explica justamente porque la elaboración de matrices de codificación para códigos LDPC no binarios, para bloques grandes de información, es un tema de investigación que no se encuentra resuelto hasta la fecha.