The presence of dispersive fading and inter-carrier interference (ICI) constitute the major impediment to reliable communications in orthogonal frequency-division multiplexing (OFDM) systems. Recently iterative (``Turbo'') processing techniques, which have been successfully applied to many detection/decoding problems, have received considerable attention. In this thesis, we first aim on the design of iterative receiver for single antenna OFDM system with frequency offset and dispersive fading. Further work is then extended to space-time block coded (STBC) OFDM system. At last, the technique is applied to STBC-OFDM system through a newly built channel model, which is based on a physical description of the propagation environment. The performance of such systems are verified by computer simulations. The simulation results show that the iterative techniques work well in OFDM systems.

Digital wireless communications have considerably changed not only the way people communicate, but also the way research is conducted in the field of telecommunications. In fact, the nature of the wireless medium has created a number of new challenging and fascinating research topics. In particular, a prerequisite for achieving higher and higher transmission rates in wireless systems is to develop strategies for efficiently dealing with the frequency selectivity of the wireless channel. A good candidate is the so-called cyclic prefixed block transmission, and in particular its two most popular variants which are the cyclic prefixed single-carrier (CPSC) and orthogonal frequency division multiplexing (OFDM) modulations. In this context, this thesis aims at analyzing and optimizing the use of cyclic prefixed block transmission for wireless communications. Firstly, this thesis contributes to the current state-of-the-art on the performance comparison between CPSC and OFDM, focusing on the derivation of analytical results when possible. If the channel state information is not available at the transmitter side, CPSC with minimum mean square error linear receiver is shown to achieve the best trade-off between performance and complexity, as it exploits the multipath diversity under some realistic hypotheses. Secondly, this thesis aims at using a system-based or cross-layer criterion, called goodput, for allocating resource in a coded OFDM system. Interestingly, the well-known waterfilling solution, when adequately parametrized, is proved to be near-optimal from a goodput point of view. Finally, this thesis discusses the possibility of improving the performance of a CPSC system by exploiting the cyclic prefix for equalization purposes.

Satellite radio has attained great popularity because of its wide range of geographical coverage and high signal quality as compared to the terrestrial broadcasts. Most Satellite Digital Radio (SDR) based systems favor multi-carrier transmission schemes, especially, orthogonal frequency division multiplexing (OFDM) transmission because of high data transfer rate and spectral efficiency. It is a challenging task to find a suitable platform that supports fast data rates and superior processing capabilities required for the development and deployment of the new SDR standards. Field programmable gate array (FPGA) devices have the potential to become suitable development platform for such standards. Another challenging factor in SDR systems is the distortion of variable envelope signals used in OFDM transmission by the nonlinear RF power amplifiers (PA) used in the base station transmitters. An attractive option is to use a linearizer that would compensate for the nonlinear effects of the PA. In this research, an OFDM transmitter, according to European Telecommunications Standard Institute (ETSI) SDR Technical Specifications 2007-2008, was designed and implemented on a low-cost Xilinx FPGA platform. A weakly nonlinear PA, operating in the L-band SDR frequency (1.450-1.490GHz), was used for signal transmission. An FPGA-based, low-cost, adaptive linearizer was designed and implemented based on the digital predistortion (DPD) reference design from Xilinx, to correct the distortion effects of the PA on the transmitted signal.

Aquesta tesi presenta l'aplicació d'una nova metodologia de prototipatge ràpid basada en plataformes per a subsistemes de comunicació, en concret, la part digital de la capa física del protocol sense fils HiperLAN/2.Les principals contribucions d'aquest treball de recerca són:- Formulació i validació d'una metodologia de disseny que transforma directament les especificacions a nivell de sistema a plataformes físiques, mantenint de forma intensiva la verificació heterogènia multinivell.- Exploració i implementació de noves arquitectures hardware per millorar algorismes complexos en la cadena HiperLAN/2, obtenint models sintetitzables per la capa física digital de l'emissor i receptor de l'HiperLAN/2.- Prototipatge de la part digital de la capa física de l'emissor HiperLAN/2 amb una tassa de transmissió de dades de 12MBits/s, en una plataforma adequada per a sistemes WLAN. El propòsit d'aquest prototipatge és demostrar l'alta productivitat de la metodologia que acaba implementant el disseny en plataformes físiques.Aquest treball s'estructura en quatre parts: conceptes fonamentals, metodologia de prototipatge ràpid, disseny de la capa física i prototipatge de l'emissor.Als conceptes fonamentals, es fa una introducció a la modulació OFDM explicant el model de senyal, propietats i avantatges d'aquesta modulació. A continuació, es presenta l'estàndard sense fils HiperLAN/2 que utilitza la modulació OFDM. En concret, es presenten els entorns de treball, la topologia, la capa física, la complexitat del model i tipus de ràfegues de l'estàndard HiperLAN/2. A la metodologia de prototipatge ràpid d'un sistema WLAN, es presenta la metodologia utilitzada pel prototipatge de l'emissor i receptor de la part digital de la capa física de l'HiperLAN/2. A continuació es fa una introducció al prototipatge ràpid, s'exposen els requeriments de les plataformes de prototipatge d'un sistema WLAN, es mostren l'estat de l'art d'entorns i plataformes per a sistemes WLAN i s'expliquen les arquitectures de les FPGAs de Virtex. A continuació, es presenta la metodologia WLAN amb els mètodes de sincronització i els fluxos de disseny que utilitza. En concret, es presenten dos fluxos de disseny diferents, un flux clàssic i l'altre més específic adaptats als sistemes WLAN, els quals ofereixen varies alternatives molt interessants de verificació heterogènia multinivell. Per acabar, es comenten els conceptes d'abstracció funcional, d'ortogonalitat i d'heterogeneïtat de la metodologia. Al disseny de la capa física de l'emissor i receptor es mostren en detall les seves estructures i els seus components computacionals. S'han proposat contribucions significatives pels mòduls de la IFFT/FFT, equalitzador de canal, sincronitzador, interleaver/deinterleaver i Viterbi. Els resultats de síntesis mostren la millora en comparació amb dissenys alternatius actuals. Els models obtinguts de l'emissor i del receptor mostren la seva funcionalitat i les seves prestacions a nivell de sistema. Al prototipatge de l'emissor, es descriuen el canvis aplicats al model de l'emissor per tal de fer el prototipatge en una plataforma específica. Al final, es presenten els resultats obtinguts a través de l'analitzador d'espectres i es fa una estimació del consum de potència de l'emissor que validen el mètode proposat. / This thesis presents the application of a new platform-based rapid prototyping methodology to the design of a communications subsystem, specifically the digital part of the physical layer of the HiperLAN/2 wireless protocol.The main contributions of this research work are: - Formulation and validation of a design methodology that directly transforms system level specifications to silicon platforms, while stressing heterogeneous multilevel verification.- Exploration and implementation of new hardware architectures to improve some complex algorithms in the HiperLAN/2 chain, obtaining synthesizable models for the digital part of the physical layer of HiperLAN/2 transmitter and receiver, which were validated in the hardware architecture. - Prototyping of the digital part of the physical layer of the HiperLAN/2 transmitter with a 12Mbits/s data transmission rate in a silicon platform, well-suited for WLAN systems. The aim of this prototype is to show the high productivity of the design methodology that ends into real platforms. This dissertation is structured in four parts: fundamental concepts, rapid prototyping methodology, design of the physical layer, and transmitter prototyping. The fundamental concepts part introduce OFDM modulation, and outlines the signal model as well as its advantages and disadvantages. Next, the standard wireless HiperLAN/2, that uses OFDM modulation, is presented. The working environment, network topology, physical layer, model complexity and burst types are discussed.The chapter on rapid prototyping methodology for a WLAN system describes the design methodology proposed to prototype the transmitter and receiver of the digital part of the physical layer of HiperLAN/2, starting from the WLAN system requirements that lead to the selection of specific prototyping platforms. Current HW implementation capabilities are presented through the architecture of Virtex FPGAs, used as core hardware device. Next, we detail the WLAN system design methodology with the design flow and synchronization methods used. Two design flows are presented, a more general flow and a specific one, adapted to WLAN systems. These design flows detail different powerful options of heterogeneous multilevel verification, that profit from modern concepts of functional abstraction, orthogonality and heterogeneity. The part about physical layer design of transmitter and receiver shows their structures and detail their computational components. Significant contributions have been proposed for IFFT/FFT, channel equalizer, synchronization, interleaver/deinterleaver and Viterbi modules. Synthesis results show their performance quality compared to current alternative designs. The overall results obtained from the transmitter and receiver models show also its global behavior and system level performance. Transmitter prototyping describes changes applied to the transmitter model in order to map it into the specific silicon platform. Out of this complete prototype we extracted data from the spectrum analyzer and of transmitter's power consumption that validate the proposed approach.

This dissertation deals with the channel estimation techniques for orthogonal frequency division multiplexing (OFDM) systems such as in IEEE 802.11. Although there has been a great amount of research in this area, characterization of typical wireless indoor environments and design of channel estimation schemes that are both robust and practical for such channel conditions have not been thoroughly investigated. It is well known that the minimum mean-square-error (MMSE) estimator provides the best mean-square-error (MSE) performance given a priori knowledge of channel statistics and operating signal-to-noise ratio (SNR). However, the channel statistics are usually unknown and the MMSE estimator has too much computational complexity to be realized in practical systems. In this work, we propose two simple channel estimation techniques: one that is based on modifying the channel correlation matrix from the MMSE estimator and the other one with averaging window based on the LS estimates. We also study the characteristics of several realistic indoor channel models that are of potential use for wireless local area networks (LANs). The first method, namely MMSE-exponential-Rhh, does not depend heavily on the channel statistics and yet offer performance improvement compared to that of the LS estimator. The simulation results also show that the second method, namely averaging window (AW) estimator, provides the best performance at moderate SNR range.

Our main objective in this thesis is to study the effect of narrow band interference on OFDM systems operating in the 2.4 Ghz ISM band and identify ways to improve upon existing techniques to deal with them. We first consider how narrow band signals interfere with OFDM systems. Various noise variance estimation and signal to noise ratio estimation techniques for OFDM systems are then discussed. We also study the conventional Viterbi Algorithm that is used in OFDM wireless systems and the proposed modifications to it in the literature. Our main contribution is a detailed experimental analysis of a modified Viterbi Algorithm that outperforms the conventional one in the presence of narrow band interference. Interference samples captured using a wireless hardware platform were used in simulation to test this modified algorithm. From our analysis we realize that in the presence of narrow band frequency selective interference (such as Bluetooth), the conventional Viterbi Algorithm can be modified to improve the performance of OFDM systems.

Fiber-optic networks are an integral part of todays digital communication system. In these networks, distances of typically 400 km to 6000 km are linked together, and information is transfered at extremely high data rates. As the demands for capacity increases, finding new methods for cost effective long-haul transmission systems that can be used to increase the capacity becomes of high interest. In this work Orthogonal Frequency Division Multiplexing (OFDM), which is a standard digital modulation format in many wireless communication systems, for instance the IEEE 802.11n, is adapted to the optical domain and used for data transmission. The advantage of OFDM in the optical domain is that it transforms a high data rate stream into many simultaneously low bit rate streams that are efficiently frequency multiplexed. By doing so high spectral efficiency is achieved and many of the impairments encountered in high data rate transmissions are avoided. The disadvantage is however, that OFDM has inherently a high peak-to-average power ratio. As a result, OFDM suffers from nonlinearities occurring along the transmission line. The low nonlinear tolerance of OFDM in fiber optic applications restricts the feasible transmission distance. The goal of this work is to assess the suitability of OFDM in fiber-optic communications

The OFDM technology has been extensively used in many radio communicationtechnologies. For example, OFDM is the core technology applied in WiFi, WiMAXand LTE. Its main advantages include high bandwidth utilization, strong noise im-munity and the capability to resist frequency selective fading. However, OFDMtechnology is not only applied in the field of radio communication, but has alsobeen developed greatly in acoustic communication, namely the so called acousticOFDM. Thanks to the acoustic OFDM technology, the information can be em-bedded in audio and then transmitted so that the receiver can obtain the requiredinformation through certain demodulation mechanisms without severely affectingthe audio quality.This thesis mainly discusses how to embed and transmit information in audioby making use of acoustic OFDM. Based on the theoretical systematic structure, italso designs a simulation system and a measurement system respectively. In thesetwo systems, channel coding, manners of modulation and demodulation, timingsynchronization and parameters of the functional components are configured in themost reasonable way in order to achieve relatively strong stability and robustnessof the system. Moreover, power control and the compatibility between audio andOFDM signals are also explained and analyzed in this thesis.Based on the experimental results, the author analyzes the performance of thesystem and the factors that affect the performance of the system, such as the typeof audio, distance between transmitter and receiver, audio output level and so on.According to this analysis, it is proved that the simulation system can work steadilyin any audio of wav format and transmit information correctly. However, dueto the hardware limitations of the receiver and sender devices, the measurementsystem is unstable to a certain degree. Finally, this thesis draws conclusions of theresearch results and points out unsolved problems in the experiments. Eventually,some expectations for this research orientation are stated and relevant suggestionsare proposed.

This paper presents a reconfigurable FFT architecture for variable length andmultistreaming WiMax wireless standard. The architecture processes 1 streamof 2048-pt FFT, up to 2 streams of 1024-pt FFT or up to 4 streams of 512-ptFFT. The architecture consists of 11 SDF pipelined stages and radix-2 butterflyis calculated in each stage. The sampling frequency of the system is varied inaccordance with FFT length. The wordlength and buffer length in each stage isconfigurable depending on the FFT length. Latch-free clock gating technique isused to reduce power consumption.The architecture is synthesized for Virtex-6 XCVLX760 FPGA. Experimentalresults show that the architecture achieves the throughput as required by theWiMax standard and the design has additional features compared to the previousapproaches. The design used 1% of the total available FPGA resources andmaximum clock frequency of 313.67 MHz was achieved.

OFDM is a fast growing technology in the area of wireless communication due to its numerous advantages and applications. The current and future technologies in the area of wireless communications like WiMAX, WiFi, LTE, MBWA and DVB-T uses the OFDM signals. The OFDM technology is applicable to the radio communication as well as the acoustic communication. Though the licensed spectrum is intended to be used only by the spectrum owners, Cognitive radio is a concept of reusing this licensed spectrum in an unlicensed manner. Cognitive radio is motivated by the measurements of spectrum utilization . Cognitive radio must be able to detect very weak primary users signal and to keep the interference level at a maximum acceptable level. Hence spectrum sensing is an essential part of the cognitive radio. Spectrum is a scarce resource and spectrum sensing is the process of identifying the unused spectrum, without causing any harm to the existing primary user’s signal. The unused spectrum is referred to as spectrum hole or white space and this spectrum hole could be reused by the cognitive radio. This thesis work focuses on implementing primary acoustic transmitter to transmit the OFDM signals from a computer through loudspeaker and receive the signals through a microphone. Then by applying different detection methods on the received OFDM signal for detection of the spectrum hole, the performance of these detection methods is compared here. The commonly used detection methods are power spectrum estimation, energy detection and second–order statistics (GLRT approach, Autocorrelation Function (ACF) detection and cyclostationary feature detection ). The detector based on GLRT approach exploits the structure of the OFDM signal by using the second order statistics of the received data. The thesis mainly focuses on GLRT approach and ACF detectors and compare their performance.