Pré-filtragem no espaço-frequência para o sistema MC-CDMA

Doutoramento em Engenharia ElectrotécnicaO trabalho desta tese enquadra-se na área de comunicações móveis, mais especificamente em sistemas de portadora múltipla. O MC-CDMA, que combina a modulação OFDM com o espalhamento na frequência, é um dos candidatos mais promissores para a interface-ar dos futuros sistemas de comunicações móveis – 4G. O objectivo desta tese é propor e avaliar técnicas de pré-filtragem e de codificação, projectadas no espaço-frequência/tempo para o sistema MC-CDMA, no sentido descendente (Downlink). Inicialmente, são discutidos conceitos teóricos fundamentais para se compreender o mecanismo físico de propagação inerente às comunicações móveis, apresentando-se depois vários sistemas de portadora múltipla. É dada especial atenção ao sistema convencional MC-CDMA. Este sistema é importante porque serve de referência ao desempenho obtido com as técnicas de transmissão propostas nesta tese. Estas técnicas são projectadas tendo em conta as restrições em termos de complexidade do terminal móvel. Assim, a estação base é equipada com um agregado de antenas e o terminal móvel com uma antena, sendo neste último, apenas implementadas técnicas de detecção mono-utilizador. Assumindo, que a estação base conhece a resposta do canal antes da transmissão, são propostas diferentes estratégias de transmissão: os filtros são projectados no espaço-frequência para o sistema MC-CDMA combinados com ou sem equalização no terminal móvel; e o filtro é projectado apenas na frequência para o sistema MC-CDMA com codificação no espaço-frequência/tempo. O algoritmo é baseado na minimização da potência transmitida sujeita à total eliminação da interferência de acesso múltiplo e das distorções provocadas pelo canal rádio móvel. Todos os esquemas propostos são validados e comparados, através de simulações, em cenários típicos de interiores e exteriores. Como principal conclusão desta tese, destaca-se a significativa melhoria de desempenho obtido com estas técnicas, relativamente ao sistema convencional MC-CDMA. Além disso, este desempenho é conseguido com um terminal móvel de reduzida complexidade. Assim, estas técnicas permitem aumentar significativamente a capacidade do sistema e, simultaneamente, transferir grande parte da complexidade do terminal móvel para a estação base.The scope of this thesis targets multi-carrier modulation techniques for mobile radio communications system. MC-CDMA combining multi-carrier modulation and spreading in the frequency domain is widely viewed as a promising candidate for 4G air interfaces. The aim of this thesis is to propose and evaluate pre-filtering and coding techniques designed in space and frequency/time for the downlink MC-CDMA system. Initially, the fundamental propagation mechanisms inherent to mobile radio communications are discussed and then several multi-carrier schemes are presented. Furthermore, special attention is given to the conventional MCCDMA system, since it can be used as the reference benchmark performance for the advanced transmission techniques proposed in this thesis. These transmission schemes are designed taking into account the complexity constraints at the mobile terminal. Hence, the basestation is equipped with an antenna array and the mobile terminal comprises a single antenna and single user detection scheme. Based on the assumption that the basestation has prior channel knowledge, different transmission strategies are proposed: spacefrequency pre-filtering schemes combined with single user equalizers at the MT for the MC-CDMA system; frequency pre-filtering scheme for spacefrequency/ time coding MC-CDMA system. The algorithm is based on the minimization of the transmitted power subject to MAI and channel distortion elimination. The proposed pre-filtering schemes are assessed and compared through simulations in typical indoor and pedestrian scenarios. This work concluded that with the proposed pre-filtering schemes, we obtain a considerable performance improvement in typical indoor and outdoor scenarios with a low complexity mobile terminal design and allow a transfer of implementation complexity from the mobile to the basestation

Timing and Carrier Synchronization in Wireless Communication Systems: A Survey and Classification of Research in the Last 5 Years

Timing and carrier synchronization is a fundamental requirement for any wireless communication system to work properly. Timing synchronization is the process by which a receiver node determines the correct instants of time at which to sample the incoming signal. Carrier synchronization is the process by which a receiver adapts the frequency and phase of its local carrier oscillator with those of the received signal. In this paper, we survey the literature over the last 5 years (2010–2014) and present a comprehensive literature review and classification of the recent research progress in achieving timing and carrier synchronization in single-input single-output (SISO), multiple-input multiple-output (MIMO), cooperative relaying, and multiuser/multicell interference networks. Considering both single-carrier and multi-carrier communication systems, we survey and categorize the timing and carrier synchronization techniques proposed for the different communication systems focusing on the system model assumptions for synchronization, the synchronization challenges, and the state-of-the-art synchronization solutions and their limitations. Finally, we envision some future research directions

Transmitter precoding for multi-antenna multi-user communications

Emerging wireless sensor networks and existing wireless cellular and ad hoc networks motivate the design of low-power receivers. Multi-user interference drastically reduces the energy efficiency of wireless multi-user communications by introducing errors in the bits being detected at the receiver. Interference rejection algorithms and multiple antenna techniques can significantly reduce the bit-error-rate at the receiver. Unfortunately, while interference rejection algorithms burden the receiver with heavy signal processing functionalities, thereby increasing the power consumption at the receiver, the small size of receivers, specifically in sensor networks and in downlink cellular communications, prohibits the use of multiple receive antennas. In a broadcast channel, where a central transmitter is transmitting independent streams to decentralized receivers, it is possible for the transmitter to have a priori knowledge of the interference. Multiple antennas can be used at the transmitter to enhance energy efficiency. In some systems, the transmitter has access to virtually an infinite source of power. A typical example would be the base station transmitter for the downlink of a cellular system. The power consumption at receivers can be reduced if some of the signal processing functionality of the receiver is moved to the transmitter.;In this thesis, we consider a wireless broadcast channel with a transmitter equipped with multiple antennas and having a priori knowledge of interference. Our objective is to minimize the receiver complexity by adding extra signal processing functions to the transmitter. We need to determine the optimal signal that should be transmitted so that interference is completely eliminated, and the benefits that can be obtained by using multiple transmit antennas can be maximized. We investigate the use of linear precoders, linear transformations made on the signal before transmission, for this purpose

Smart antennas: state of the art

Aim of this contribution is to illustrate the state of the art of smart antenna research from several perspectives. The bow is drawn from transmitter issues via channel measurements and modeling, receiver signal processing, network aspects, technological challenges towards first smart antenna applications and current status of standardization. Moreover, some future prospects of different disciplines in smart antenna research are given.Peer Reviewe

Implementação numa FPGA codificador/descodificador Alamouti para LTE

Mestrado em Engenharia Electrónica e TelecomunicaçõesMotivados por transmissões mais rápidas e mais fiáveis num canal sem fios, os sistemas da 4G devem proporcionar processamento de dados mais rápido a baixa complexidade, elevadas taxas de dados, assim como robustez na performance reduzindo também, a latência e os custos de operação. LTE apresenta, na sua camada física, tecnologias como OFDM e MIMO que prometem alcançar elevadas taxas de dados e aumentar a eficiência espectral. Especificamente a camada física do LTE emprega OFDMA para downlink e SC-FDMA para uplink. A tecnologia MIMO permite também melhorar significativamente o desempenho dos sistemas OFDM com as vantagens de multiplexação e diversidade espacial diminuindo o efeito de desvanecimento de multi-percurso no canal. Nesta dissertação são implementados um codificador e um descodificador com base no algoritimo de Alamouti num sistema MISO nomeadamente para serem incluídos num OFDM transceiver que segue as especificações da camada física do LTE. A codificação/descodificação de Alamouti realiza-se no espaço e frequência e os blocos foram projetados e simulados em Matlab através do ambiente Simulink com o auxílio dos blocos da Xilinx inseridos no seu software System Generator para DSP. Pode-se concluir que os blocos baseados no algoritmo de Alamouti foram implementados em hardware com sucesso.Motivated by faster transmissions and more reliable wireless channel, future 4G systems should provide faster data processing at low complexity, high data rates, as well as robustness in performance while also reducing the latency and operating costs. LTE presents in its physical layer technologies such as OFDM and MIMO that promise to achieve high data rates and increase spectral efficiency. Specifically the physical layer of LTE employs OFDMA on the downlink and SC-FDMA for uplink. MIMO technology also allows to significantly improve the performance of OFDM systems with the advantages of multiplexing and spatial diversity by decreasing the effect of multipath fading in the channel. In this thesis we implemented an encoder and a decoder based on an Alamouti algorithm in a MISO system namely to be added to an OFDM transceiver that follows closely the LTE physical layer specifications. Alamouti coding/decoding is performed in frequency and space and the blocks were projected and simulated in Matlab using Simulink environment through the Xilink's blocks in the System Generator for DSP. One can conclude that the blocks based on Alamouti algorithm were well-implemented

Equalization of MIMO Channels in LTE-Advanced

LTE-Advanced is one of the most evolving and competing standards that target the high speed 4G wireless communications. In order to meet the target of this new cellular technology developed under auspices of the 3GPP standardization bodies, it is necessary to ensure that this technology is able to provide the headline requirements recommended for the terrestrial components of the IMT-Advanced radio interface for 4G broadband mobile communications. One of the key radio technologies that will enable LTE-Advanced to achieve the high data throughput rates is the use of MIMO antennas that play an important role as the conventional communications like using more bandwidths and higher modulation types are limited. Together with this are the downlink OFDMA and the uplink SC-FDMA techniques that are employed to improve the system architecture burdened with the data rates rising pretty well above what was previously in use. The combination of these technologies will help LTE-Advanced keep pace with other wireless technologies that may be competing to offer very high data rates and high level of mobility. But achieving the high data rate up to 1 Gbits/s in 4G mobile networks over wide frequency bandwidths and recovering the original information without being corrupted and downgraded has been a daunting task for engineers. Thus, this paper will briefly discuss the performances of MIMO equalization techniques such as MMSE, ZF and ZF-SIC equalizers in a Rayleigh multichannel fading.fi=Opinnäytetyö kokotekstinä PDF-muodossa.|en=Thesis fulltext in PDF format.|sv=Lärdomsprov tillgängligt som fulltext i PDF-format

Implementation of Time Reversal Technique for Energy Efficient Wireless Communication

From the past few years, the mobile industry has developed quickly, providing network coverage to more than 4 billion users. So more network infrastructure and maintenance of network is required which increases the energy cost and energy consumption.. So “Green” wireless communication is taking much effort for improving energy efficiency and to reduce environment impact. A new energy efficient Time Reversal Technique is proposed. Time reversal is a pre-filtering/focusing signal processing technique for green wireless communication. The temporal effect and spatial focusing effects reduce the interference and receiver complexity for TR wireless communication. In multi path environment it adds all the reflected signals constructively and increase the signal strength at the receiver. It require low transmit power than the direct transmission of signal. Therefore the focusing property of TR simplify receiver design. The theoretical and simulation analysis show reduction in transmit power and improved BER. In this thesis work, time reversal technique and its focusing properties are studied in detail. The temporal and spatial focusing effects of TR technique are simulated in MATLAB. TR technique is implemented for single user and multi user case. A new wireless channel access method Time Reversal Division Multiple Access also implemented for multi user downlink link system

Scaling up MIMO: Opportunities and Challenges with Very Large Arrays

This paper surveys recent advances in the area of very large MIMO systems. With very large MIMO, we think of systems that use antenna arrays with an order of magnitude more elements than in systems being built today, say a hundred antennas or more. Very large MIMO entails an unprecedented number of antennas simultaneously serving a much smaller number of terminals. The disparity in number emerges as a desirable operating condition and a practical one as well. The number of terminals that can be simultaneously served is limited, not by the number of antennas, but rather by our inability to acquire channel-state information for an unlimited number of terminals. Larger numbers of terminals can always be accommodated by combining very large MIMO technology with conventional time- and frequency-division multiplexing via OFDM. Very large MIMO arrays is a new research field both in communication theory, propagation, and electronics and represents a paradigm shift in the way of thinking both with regards to theory, systems and implementation. The ultimate vision of very large MIMO systems is that the antenna array would consist of small active antenna units, plugged into an (optical) fieldbus.Comment: Accepted for publication in the IEEE Signal Processing Magazine, October 201

Interference mitigation using group decoding in multiantenna systems

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