Reduced complexity detection in MIMO systems with SC-FDE modulations and iterative DFE receivers

This paper considers a Multiple-Input Multiple-Output (MIMO) system with P transmitting and R receiving antennas and different overall noise characteristics on the different receiver antennas (e.g., due to nonlinear effects at the receiver side). Each communication link employs a Single-Carrier with Frequency-Domain Equalization (SC-FDE) modulation scheme, and the receiver is based on robust iterative frequency-domain multi-user detectors based on the Iterative Block Decision Feedback Equalization (IB-DFE) concept. We present low complexity efficient receivers that can employ low resolution Analog-to-Digital Converters (ADCs) and require the inversion of matrices with reduced dimension when the number of receive antennas is larger than the number of independent data streams. The advantages of the proposed techniques are particularly high for highly unbalanced MIMO systems, such as in the uplink of Base Station (BS) cooperation systems that aim for Single-Frequency Network (SFN) operation or massive MIMO systems with much more antennas at the receiver side.publishe

Simultaneous Wireless Information and Power Transfer in 5G communication

Green communication technology is expected to be widely adopted in future generation networks to improve energy efficiency and reliability of wireless communication network. Among the green communication technologies,simultaneous wireless information and power transfer (SWIPT) is adopted for its flexible energy harvesting technology through the radio frequency (RF) signa lthati sused for information transmission. Even though existing SWIPT techniques are flexible and adoptable for the wireless communication networks, the power and time resources of the signal need to be shared between infor- mation transmission and RF energy harvesting, and this compromises the quality of the signal. Therefore,SWIP Ttechniques need to be designed to allow an efficient resource allocation for communication and energy harvesting. The goal oft his thesisis to design SWIP Ttechniques that allow efficient,reliable and secure joint communications and power transference. A problem associated to SWIPT techniques combined with multi carrier signals is that the increased power requirements inherent to energy harvesting purposes can exacerbate nonlinear distortion effects at the transmitter. Therefore, we evaluate nonlinear distortion and present feasible solutions to mitigate the impact of nonlinear distortion effects on the performance.Another goal of the thesisis to take advantage of the energy harvesting signals in SWIP Ttechniques for channel estimation and security purposes.Theperformance of these SWIPT techniques is evaluated analytically, and those results are validated by simulations. It is shownthatthe proposed SWIPT schemes can have excellent performance, out performing conventional SWIPT schemes.Espera-se que aschamadas tecnologiasde green communications sejam amplamente ado- tadas em futuras redes de comunicação sem fios para melhorar a sua eficiência energética a fiabilidade.Entre estas,encontram-se as tecnologias SWIPT (Simultaneous Wireless Information and Power Transference), nas quais um sinal radio é usado para transferir simultaneamente potência e informações.Embora as técnicas SWIPT existentes sejam fle- xíveis e adequadas para as redes de comunicações sem fios, os recursos de energia e tempo do sinal precisam ser compartilhados entre a transmissão de informações e de energia, o que pode comprometer a qualidade do sinal. Deste modo,as técnicas SWIPT precisam ser projetadas para permitir uma alocação eficiente de recursos para comunicação e recolha de energia. O objetivo desta tese é desenvolver técnicas SWIPT que permitam transferência de energia e comunicações eficientes,fiáveis e seguras.Um problema associado às técnicas SWIPT combinadas com sinais multi-portadora são as dificuldades de amplificação ine- rentes à combinação de sinais de transmissão de energia com sinais de transferência de dados, que podem exacerbar os efeitos de distorção não-linear nos sinais transmitidos. Deste modo, um dos objectivos desta tese é avaliar o impacto da distorção não-linear em sinais SWIPT, e apresentar soluções viáveis para mitigar os efeitos da distorção não-linear no desempenho da transmissão de dados.Outro objetivo da tese é aproveitar as vantagens dos sinais de transferência de energia em técnicas SWIPT para efeitos de estimação de canal e segurança na comunicação.Os desempenhos dessas técnicas SWIPT são avaliados analiticamente,sendo os respectivos resultados validados por simulações.É mostrado que os esquemas SWIPT propostos podem ter excelente desempenho, superando esquemas SWIPT convencionais

MIMO signal processing in offset-QAM based filter bank multicarrier systems

Next-generation communication systems have to comply with very strict requirements for increased flexibility in heterogeneous environments, high spectral efficiency, and agility of carrier aggregation. This fact motivates research in advanced multicarrier modulation (MCM) schemes, such as filter bank-based multicarrier (FBMC) modulation. This paper focuses on the offset quadrature amplitude modulation (OQAM)-based FBMC variant, known as FBMC/OQAM, which presents outstanding spectral efficiency and confinement in a number of channels and applications. Its special nature, however, generates a number of new signal processing challenges that are not present in other MCM schemes, notably, in orthogonal-frequency-division multiplexing (OFDM). In multiple-input multiple-output (MIMO) architectures, which are expected to play a primary role in future communication systems, these challenges are intensified, creating new interesting research problems and calling for new ideas and methods that are adapted to the particularities of the MIMO-FBMC/OQAM system. The goal of this paper is to focus on these signal processing problems and provide a concise yet comprehensive overview of the recent advances in this area. Open problems and associated directions for future research are also discussed.Peer ReviewedPostprint (author's final draft

Approaching universal frequency reuse through base station cooperation

Base Station (BS) architectures are a promising cellular wireless solution to mitigate the interference issues and to avoid the high frequency reuse factors implemented in conventional systems. Combined with block transmission techniques, such as Orthogonal Frequency-Division Multiplexing (OFDM) for the downlink and Single-Carrier with Frequency-Domain Equalization (SC-FDE) for the uplink, these systems provide a significant performance improvement to the overall system. Block transmission techniques are suitable for broadband wireless communication systems, which have to deal with strongly frequency-selective fading channels and are able to provide high bit rates despite the channel adversities. In BS cooperation schemes users in adjacent cells share the same physical channel and the signals received by each BS are sent to a Central Processing Unit (CPU) that combines the different signals and performs the user detections and/or separation, which can be regarded as a Multi-User Detection (MUD) technique. The work presented in this thesis is focused on the study of uplink transmissions in BS cooperations systems, considering single carrier block transmission schemes and iterative receivers based on the Iterative-Block Decision Feedback Equalization (IB-DFE) concept, which combined with the employment of Cyclic Prefix (CP)-assisted block transmission techniques are appropriate to scenarios with strongly time-dispersive channels. Furthermore, the impact of the sampling and quantization applied to the received signals from each Mobile Terminal (MT) to the corresponding BS is studied, with the achievement of the spectral characterization of the quantization noise. This thesis also provides a conventional analytical model for the BER (Bit Error Rate) performance complemented with an approach to improve its results. Finally, this thesis addresses the contextualization of BS cooperation schemes in clustered C-RAN (Centralized-Radio Access Network)-type solutions.As arquitecturas BS cooperation são uma solução promissora de redes celulares sem fios para atenuar o problema da interferência e evitar os factores de reuso elevados, que se encontram implementados nos sistemas convencionais. Combinadas com técnicas de transmissão por blocos, como o OFDM para o downlink e o SC-FDE no uplink, estes sistemas fornecem uma melhoria significativa no desempenho geral do sistema. Técnicas de transmissão por blocos são adequadas para sistemas de comunicações de banda larga sem fios, que têm que lidar com canais que possuem um forte desvanescimento selectivo na frequência e são capazes de fornecer ligações com taxas de transmissão altas apesar das adversidades do canal. Em esquemas BS cooperation os terminais móveis situados em células adjacentes partilham o mesmo canal físico e os sinais recebidos em cada estação de base são enviados para uma Unidade Central de Processamento (CPU) que combina os diferentes sinais recebidos associados a um dado utilizador e realiza a detecção e/ou separação do mesmo, sendo esta considerada uma técnica de Detecção Multi-Utilizador (MUD). O trabalho apresentado nesta tese concentra o seu estudo no uplink de transmissões em sistemas BS cooperation, considerando transmissões em bloco de esquemas monoportadoras e receptores iterativos baseados no conceito B-DFE, em que quando combinados com a implementação de técnicas de transmissao por blocos assistidas por prefixos cíclicos (CP) são apropriados a cenários com canais fortemente dispersivos no tempo. Além disso, é estudado o impacto do processo de amostragem e quantização aplicados aos sinais recebidos de cada terminal móvel para a estação de base, com a obtenção da caracterização espectral do ruído de quantização. Esta tese também fornece um modelo analítico convencional para a computação do desempenho da taxa de erros de bit (BER), com um método melhorado para o mesmo. Por último, esta tese visa a contextualização dos sistemas BS cooperation em soluções do tipo C-RAN

Performance Evaluation of Low Complexity Massive MIMO Techniques for SC-FDE Schemes

Massive-MIMO technology has emerged as a means to achieve 5G's ambitious goals; mainly to obtain higher capacities and excellent performances without requiring the use of more spectrum. In this thesis, focused on the uplink direction, we make a study of performance of low complexity equalization techniques as well as we also approach the impact of the non-linear elements located on the receivers of a system of this type. For that purpose, we consider a multi-user uplink scenario through the Single Carrier with Frequency Domain Equalization (SC-FDE) scheme. This seems to be the most appropriate due to the low energy consumption that it implies, as well as being less favorable to the detrimental effects of high envelope fluctuations, that is, by have a low Peak to Average Power Ratio (PAPR) comparing to other similar modulations, such as the Orthogonal Frequency Division Multiplexing (OFDM). Due to the greater number of antennas and consequent implementation complexity, the equalization processes for Massive- MIMO schemes are aspects that should be simplified, that is, they should avoid the inversion of matrices, contrary to common 4G, with the Zero Forcing (ZF) and Minimum Mean Square Error (MMSE) techniques. To this end, we use low-complexity techniques, such as the Equal Gain Combining (EGC) and the Maximum Ratio Combining (MRC). Since these algorithms are not sufficiently capable of removing the entire Inter-Symbol Interference (ISI) and Inter-User Interference (IUI), we combine them with iterative techniques, namely with the Iterative Block with Decision Feedback Equalizer (IB-DFE) to completely remove the residual ISI and IUI. We also take into account the hardware used in the receivers, since the effects of non-linear distortion can impact negatively the performance of the system. It is expected a strong performance degradation associated to the high quantization noise levels when implementing low-resolution Analog to Digital Converters (ADCs). However, despite these elements with these configurations become harmful to the performance of the majority of the systems, they are considered a desirable solution for Massive-MIMO scenarios, because they make their implementation cheaper and more energy efficient. In this way, we made a study of the impact in the performance by the low-resolution ADCs. In this thesis we suggest that it is possible to bypass these negative effects by implementing a number of receiving antennas far superior to the number of transmitting antennas