Crescimento dos sistemas de comunicações móveis celulares e dos sistemas de rádio difusão de sinais de áudio e vídeo tem despertado grande interesse na pesquisa de novos métodos para a transmissão de sinais nestas redes. A necessidade de se transmitir dados em altas taxas, com significante eficiência no uso da largura de faixa de freqüências disponível, e por meio de um canal de propagação ruidoso e variante no tempo, constitui o principal problema para o desenvolvimento de novas técnicas de trasmissão de sinais. Dentro deste contexto, esta dissertação trata do uso dos conceitos de transmissão digital e filtragem adaptativa para a demodulação de sinais OFDM (Orthogonal Frequency Division Multiplexing). A equalização de sinais antes e após o estágio de DFT (Discrete Fourier Transform) no receptor e as técnicas de estimação de canal são o objeto principal de estudo deste trabalho. Os resultados dos experimentos são analisados em termos da taxa de erro de bit média obtida e da convergência dos algoritmos empregados nas etapas de equalização e estimação de canal no receptor.
Orthogonal Frequency Division Multiplexing (OFDM) system is one of the multicarrier techniques which is robust against Inter-symbol-Interference, multipath fading and very easy to apply in transmitters by using inverse fast Fourier transform IFFT and at the receivers by using fast Fourier transform FFT. In a communication system, channel estimation is very important issue for the data detection. In coherent detection, one of the popular techniques is to use pilot tones as a reference signal in OFDM symbols. In the comb-type pilot tones insertion, pilot tones are inserted into each OFDM symbols, but inserting a large number of pilot tones will lead to channel capacity reduction or bandwidth expansion [1-2]. In this work, to overcome this transmission loss, a modified least square (ModLS) algorithm for fast time varying wireless channel at comb-type pilot arrangement in QAM signals for OFDM system is proposed. The simulation results obtained from the proposed algorithm showed a good performance in noisy wireless channels. In addition, it has been compared with least square (LS) algorithm in different signal to noise ratios and different channel tabs.
In actual mobile radio systems, multipath conditions pose a problem, as the channel becomes frequency dependent. This point is especially critical in case of high frequency transmissions with very high data rate and high error performance, such as defined in the IEEE 802.15.3c which is an emerging 60-GHz standard supporting data rates of multi-giga bits per second (multi-Gbps) for short-range indoor applications. The deployment of such high speed wireless transmission has been very difficult throughout history mainly by two critical factors: the first one was the lack of wide enough spectrum and the second one is the high cost of high frequency circuits and devices. However, this trend is changing to the point that not too long ago, the substantial unlicensed spectrum became available at the millimeter-wave band of 60-GHz. Also, the advancement in technology drives the cost of 60-GHz circuits and devices much lower than in the past making possible its use for high definition audio and video wireless transmissions. In order to overcome the transmission channel issues, it is necessary to include a channel equalizer in the receiver, which must estimate the channel impulse response and make some operations to transform the frequency dependent channel to a flat channel. Nevertheless, the equalizer technology will depend on three different factors: first one the physical layer (PHY) technique under consideration for multi-Gbps Wireless Personal Area Network (WPAN) which basically could be orthogonal frequency division multiplexing (OFDM) or single-carrier frequency domain equalization (SC-FDE); second, the channel impulse response estimation carried out in order to determine the channel transfer function H(f), existing several methods to obtain an estimation; and third, the used equalization method and structure in order to reverse all distortions produced by the channel. This Master thesis has been carry out during an Erasmus program in the Technische Universitat of Braunchweig, Germany, and it is the first part of a whole European project for the study, analysis and deployment of the IEEE 802.15.3c standard for wireless communications with very high data rate and high error performance in the 60-GHz band. According to the instructions and requirements defined by professor Thomas Kürner which was in charge of this project, this thesis include: first, a theoretical study of all the different propagation effects which could affect a wireless communication channel in order to run not only the simulations presented in this thesis but also the future simulations; second, the development of a model in Matlab/Simulink that will be useful to carry out all the project simulations (taken into account the specifications collected in IEEE 802.15.3c standard); third, the results of the carried simulations for Single Carrier Channel estimation and equalization by using two different equalization methods in the frequency domain: Zero Forcing and Minimum Mean Square Error equalization. Taking this into account, the thesis is organized as follows. Section I is dedicated to the study of all the different propagation effects and problems which affects a wireless communication transmissions; In Section II, technologies and Physical Layer Modes are described attending the IEEE 802.15.3c in order to learn its different characteristics for the subsequent channel estimation; Section III is devoted to channel estimation and equalization methods description; estimation and equalization methods are selected in order to carry out the simulations in Section IV; finally, in Section V the developed system simulator as well as the obtained simulation results are presented after implementation of Zero Forcing and Minimum Mean Square Error equalization methods in Matlab/Simulink.
The emerging wireless communication systems, such as cellular communications systems and wireless net-works, are changing the life style nowadays dramatically. The prospect of modern wireless communication systems is very attractive by declaring the ability of ubiquitous access to information with high-quality and high-speed service. The Orthogonal Frequency Division Multiplexing (OFDM) technique is one promising candidate for the $4^{th}$ generation wireless systems, due to its merits of high flexibility and low equalization complexity for wideband wireless communication applications. To further enhance the communication system capacity and reliability, multiple antenna techniques can be integrated into OFDM systems. As a preliminary step, the physical characteristics of wideband radio channels and the channel modeling issue are addressed. The channel capacity with multiple antennas is presented by considering both cases of ideal and practical estimated channel state information (CSI) in the system. The fundamentals of the OFDM transmission technique are introduced. With the OFDM transmission structu-re the frequency-selective wideband radio channel is decomposed into a set of parallel subcarriers, and each subcarrier can be treated as a flat-fading narrowband channel. Several Multiple-Input-Multiple-Output (MIMO) technologies are discussed, in the scope of subcarrier-based MIMO encoding and decoding within the MIMO-OFDM transceiver structure, i.\ e.\ integrate the MIMO signal processing algorithms into a wideband OFDM system, where each OFDM subcarrier is regarded individually as a narrowband flat-fading subsystem with the Discrete Fourier Transform (DFT) and Inverse Discrete Fou-rier Transform (IDFT). The simulation results and analysis are presented under various radio channel condi-tions with ideal CSI. For practical reasons, channel estimation is necessary for coherent-detection MIMO-OFDM systems. The Pilot-based Channel Estimation (PBCE) schemes are implemented to evaluate the system performance with the realistically estimated CSI. With estimated CSI for MIMO encoding/decoding and data symbol detection, the system performance is reasonably degraded. The system performance results with estimated CSI are presented, and the impact of channel estimation errors is analyzed. In order to ensure a reliable and flexible data transmission in MIMO-OFDM systems, the indicator-based link adaptation procedure is employed to optimize the system throughput by selecting a proper Transmission Mode (TM) according to the instantaneous channel conditions. The Transmission Mode Selection (TMS) procedure with two-dimensional (2D) indicators is developed. A set of indicator candidates and their appro-ximation functions are proposed. With the indicator simulation results and the proposed TMS procedure for MIMO-OFDM systems, the average system throughput results are illustrated for both the conventional link adaptation method and the proposed 2D indicator-based TMS approach. Finally, the general system performance results of MIMO-OFDM systems are summarized. Arguments and suggestions are further made on how to design a MIMO-OFDM system in various wideband wireless com-munication applications.
Nowadays, the requirements in terms of communication are increasing exponentially and getting more diverse. The transmission data rates must be high while maintaining very good quality of service (QoS) despite the very hostile propagation channels. Generally, transmissions that are carried out on mobile radio channels are selective both in time and frequency. To overcome the channel selectivity and allows high transmission data rates, the Long Term Evolution (LTE) standard makes use of the Multiple Input Multiple Output-Orthogonal Frequency Division Multiplexing (MIMO-OFDM) technique. By implementing this technique in the context of mobile transmission, new approaches for time and frequency synchronization, equalization and channel estimation are needed. This book focuses on the study, modeling and development of efficient channel estimation techniques for broadband mobile radio channels and specifically for LTE systems. This book is interesting for both undergraduate and postgraduate students in the field of wireless communication systems. The professional engineers will find also this book useful in improving and designing new wireless communication systems.
Reduced-guard-interval (RGI) coherent optical (CO) orthogonal frequency-division multiplexing (OFDM) is a potential candidate for next generation 100G beyond optical transports, attributed to its advantages such as high spectral efficiency and high tolerance to optical channel impairments. First of all, we review the coherent optical systems with an emphasis on CO-OFDM systems as well as the optical channel impairments and the general digital signal processing techniques to combat them. This work focuses on the channel equalization and phase estimation of RGI CO-OFDM systems. We first propose a novel equalization scheme based on the equalization structure of RGI CO-OFDM to reduce the cyclic prefix overhead to zero. Then we show that intra-channel nonlinearities should be considered when designing the training symbols for channel estimation. Afterwards, we propose and analyze the phenomenon of dispersion-enhanced phase noise (DEPN) caused by the interaction ...
In a traditional orthogonal frequency division multiplexing (OFDM) wireless communication system, a guard interval using cyclic prefix is injected to avoid an inter-symbol interference (ISI) and an inter-carrier interference (ICI). This approach has been proven to work well, however, only with time-variant channels wherein a high speed mobility element is not considered. Furthermore, a transmission in the OFDM system is considered a low efficiency due to the use of a large percentage of pilot data for channel estimation. In this research, an enhanced channel equalization in a multi-path propagation environment at a high frequency mobile network technologies is proposed. The multi-path propagation resulting from the variety of signal paths that may exist between the transmitter and receiver can give a rise to an interference in a variety of ways including distortion of the signal, loss of data and fading. These paths are created as a result of reflections from buildings, mountains, or other reflective surfaces. Having a high speed mobility system with high frequency mobile network technologies such as but not limited to 4G network lead to a frequency selective channel in the frequency domain and Inter-symbol Interference in the time domain. Furthermore, this phenomenon, i.e. high speed mobility causes a Doppler shift in which results into a time variant and frequency selective channel. Thus, we present a wireless communication channel scenario that is characterized and modeled by a high relative velocity between transmitter and receiver, and fast changes of environment conditions for wave propagation. Furthermore, we present a method for enhancing the equalization of such dynamic channel. Thus a dynamic channel is developed utilizing Jakes and auto-regressive models (AR). More specifically, the enhanced equalization method we are proposing is a combination of a multi-stage time and frequency domain equalizer with a feed-forward loop. Inserting pilot data at the transmitter and using said pilot data to estimate the channel in time domain. The addition of the feed-forward is utilized to capture a part of the received signal, then extracting the channel information while the signal is being delayed until the information is obtained. The estimated channel is forwarded to the MMSE equalizer. The first stage of the multi-stage equalizer is using pilot data as measurement data in the frequency domain. The second stage is to convert the measurement data to the time domain. In the third stage, we estimate the channel using the pilot data and use it as measurement data in the time domain, i.e. input to a Kalman filter. In OFDM system, the consecutive blocks for the channel coefficients for each tap in time domain are not totally independent, but partially correlated. Such correlation can improve the channel estimation when taken into account. Using the Kalman filter, we can extract this information from previous blocks and use them for the newly arrived one, and perform a better channel estimation. In the fourth stage, we use the output of the Kalman filter as a feed-forward to the frequency equalizer for each subcarrier in which the data is converted from time domain to frequency domain. The underlying wok (sic) presents a unified approach to the equalization approach that employs both time and frequency domains data to enhance the equalization scheme. In order to reduce the complexity of the system model, we utilize the autocorrelation and Doppler frequency to dynamically select the previous OFDM symbols that will be stored in the memory. In addition to deriving earlier results in a unified manner, the approach presented also leads to an enhanced performance results without imposing any restrictions or limitations on the OFDM system such as increasing the number of pilots or cyclic prefix.
Orthogonal frequency-division multiplexing (OFDM) access schemes are becoming more prevalent among cellular and wireless broadband systems, accelerating the need for smaller, more energy efficient receiver solutions. Up to now the majority of OFDM texts have dealt with signal processing aspects. To address the current gap in OFDM integrated circuit (IC) instruction, Chiueh and Tsai have produced this timely text on baseband design. OFDM Baseband Receiver Design for Wireless Communications covers the gamut of OFDM technology, from theories and algorithms to architectures and circuits. Chiueh and Tsai give a concise yet comprehensive look at digital communications fundamentals before explaining modulation and signal processing algorithms in OFDM receivers. Moreover, the authors give detailed treatment of hardware issues -- from design methodology to physical IC implementation. Closes the gap between OFDM theory and implementation Enables the reader to transfer communication receiver concepts into hardware design wireless receivers with acceptable implementation loss achieve low-power designs Contains numerous figures to illustrate techniques Features concrete design examples of MC-CDMA systems and cognitive radio applications Presents theoretical discussions that focus on concepts rather than mathematical derivation Provides a much-needed single source of material from numerous papers Based on course materials for a class in digital communication IC design, this book is ideal for advanced undergraduate or post-graduate students from either VLSI design or signal processing backgrounds. New and experienced engineers in industry working on algorithms or hardware for wireless communications devices will also find this book to be a key reference.
