ADMM-based Detector for Large-scale MIMO Code-domain NOMA Systems

Large-scale multi-input multi-output (MIMO) code domain non-orthogonal multiple access (CD-NOMA) techniques are one of the potential candidates to address the next-generation wireless needs such as massive connectivity, and high reliability. This work focuses on two primary CD-NOMA techniques: sparse-code multiple access (SCMA) and dense-code multiple access (DCMA). One of the primary challenges in implementing MIMO-CD-NOMA systems is designing the optimal detector with affordable computation cost and complexity. This paper proposes an iterative linear detector based on the alternating direction method of multipliers (ADMM). First, the maximum likelihood (ML) detection problem is converted into a sharing optimization problem. The set constraint in the ML detection problem is relaxed into the box constraint sharing problem. An alternative variable is introduced via the penalty term, which compensates for the loss incurred by the constraint relaxation. The system models, i.e., the relation between the input signal and the received signal, are reformulated so that the proposed sharing optimization problem can be readily applied. The ADMM is a robust algorithm to solve the sharing problem in a distributed manner. The proposed detector leverages the distributive nature to reduce per-iteration cost and time. An ADMM-based linear detector is designed for three MIMO-CD-NOMA systems: single input multi output CD-NOMA (SIMO-CD-NOMA), spatial multiplexing CD-NOMA (SMX-CD-NOMA), and spatial modulated CD-NOMA (SM-CD-NOMA). The impact of various system parameters and ADMM parameters on computational complexity and symbol error rate (SER) has been thoroughly examined through extensive Monte Carlo simulations

A Tutorial on Decoding Techniques of Sparse Code Multiple Access

Sparse Code Multiple Access (SCMA) is a disruptive code-domain non-orthogonal multiple access (NOMA) scheme to enable future massive machine-type communication networks. As an evolved variant of code division multiple access (CDMA), multiple users in SCMA are separated by assigning distinctive sparse codebooks (CBs). Efficient multiuser detection is carried out at the receiver by employing the message passing algorithm (MPA) that exploits the sparsity of CBs to achieve error performance approaching to that of the maximum likelihood receiver. In spite of numerous research efforts in recent years, a comprehensive one-stop tutorial of SCMA covering the background, the basic principles, and new advances, is still missing, to the best of our knowledge. To fill this gap and to stimulate more forthcoming research, we provide a holistic introduction to the principles of SCMA encoding, CB design, and MPA based decoding in a self-contained manner. As an ambitious paper aiming to push the limits of SCMA, we present a survey of advanced decoding techniques with brief algorithmic descriptions as well as several promising directions

Multiple Access Techniques for Next Generation Wireless: Recent Advances and Future Perspectives

The advances in multiple access techniques has been one of the key drivers in moving from one cellular generation to another. Starting from the first generation, several multiple access techniques have been explored in different generations and various emerging multiplexing/multiple access techniques are being investigated for the next generation of cellular networks. In this context, this paper first provides a detailed review on the existing Space Division Multiple Access (SDMA) related works. Subsequently, it highlights the main features and the drawbacks of various existing and emerging multiplexing/multiple access techniques. Finally, we propose a novel concept of clustered orthogonal signature division multiple access for the next generation of cellular networks. The proposed concept envisions to employ joint antenna coding in order to enhance the orthogonality of SDMA beams with the objective of enhancing the spectral efficiency of future cellular networks

Deep Learning-based Auto-encoder for Time-offset Faster-than-Nyquist Downlink NOMA with Timing Errors and Imperfect CSI

We examine encoding and decoding of transmitted sequences for the downlink time-offset faster than Nyquist signaling non-orthogonal multiple access NOMA (T-NOMA) channel. We employ a previously proposed singular value decomposition (SVD)-based scheme as a benchmark. While this SVD scheme provides reliable communication, our findings reveal that it is not optimal in terms of bit error rate (BER). Additionally, the SVD is sensitive to timing offset errors, and its time complexity increases quadratically with the sequence length. We propose a convolutional neural network (CNN) auto-encoder (AE) for encoding and decoding with linear time complexity. We explain the design of the encoder and decoder architectures and the training criteria. By examining several variants of the CNN AE, we show that it can achieve an excellent trade-off between performance and complexity. The proposed CNN AE surpasses the SVD method by approximately 2 dB in a T-NOMA system with no timing offset errors or channel state information estimation errors. In the presence of channel state information (CSI) error variance of 1$\%$ and uniform timing error at $\pm$4\% of the symbol interval, the proposed CNN AE provides up to 10 dB SNR gain over the SVD method. We also propose a novel modified training objective function consisting of a linear combination of the traditionally used cross-entropy (CE) loss function and a closed-form expression for the bit error rate (BER) called the Q-loss function. Simulations show that the modified loss function achieves SNR gains of up to 1 dB over the CE loss function alone

Multiple Access Techniques for Next Generation Wireless: Recent Advances and Future Perspectives

The advances in multiple access techniques has been one of the key drivers in moving from one cellular generation to another. Starting from the first generation, several multiple access techniques have been explored in different generations and various emerging multiplexing/multiple access techniques are being investigated for the next generation of cellular networks. In this context, this paper first provides a detailed review on the existing Space Division Multiple Access (SDMA) related works. Subsequently, it highlights the main features and the drawbacks of various existing and emerging multiplexing/multiple access techniques. Finally, we propose a novel concept of clustered orthogonal signature division multiple access for the next generation of cellular networks. The proposed concept envisions to employ joint antenna coding in order to enhance the orthogonality of SDMA beams with the objective of enhancing the spectral efficiency of future cellular networks

Cellular, Wide-Area, and Non-Terrestrial IoT: A Survey on 5G Advances and the Road Towards 6G

The next wave of wireless technologies is proliferating in connecting things among themselves as well as to humans. In the era of the Internet of things (IoT), billions of sensors, machines, vehicles, drones, and robots will be connected, making the world around us smarter. The IoT will encompass devices that must wirelessly communicate a diverse set of data gathered from the environment for myriad new applications. The ultimate goal is to extract insights from this data and develop solutions that improve quality of life and generate new revenue. Providing large-scale, long-lasting, reliable, and near real-time connectivity is the major challenge in enabling a smart connected world. This paper provides a comprehensive survey on existing and emerging communication solutions for serving IoT applications in the context of cellular, wide-area, as well as non-terrestrial networks. Specifically, wireless technology enhancements for providing IoT access in fifth-generation (5G) and beyond cellular networks, and communication networks over the unlicensed spectrum are presented. Aligned with the main key performance indicators of 5G and beyond 5G networks, we investigate solutions and standards that enable energy efficiency, reliability, low latency, and scalability (connection density) of current and future IoT networks. The solutions include grant-free access and channel coding for short-packet communications, non-orthogonal multiple access, and on-device intelligence. Further, a vision of new paradigm shifts in communication networks in the 2030s is provided, and the integration of the associated new technologies like artificial intelligence, non-terrestrial networks, and new spectra is elaborated. Finally, future research directions toward beyond 5G IoT networks are pointed out.Comment: Submitted for review to IEEE CS&

Rotation matrices for signal space diversity analysis and MC-CDMA versus SCMA performance and complexity comparison

Orientadores: Celso de Almeida, Henry Ramiro Carvajal MoraTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de ComputaçãoResumo: A crescente necessidade por sistemas de comunicação sem fio que apresentem maior eficiência espectral e confiabilidade, tem norteado os estudos e as propostas tecnológicas futuras das comunicações. Temáticas como eficiência espectral, múltiplo acesso e algoritmos de detecção de sinais são muito importantes e fazem parte dos estudos propostos pelo 3GPP (Third Generation Partnership Project), para elaboração da quinta geração de sistemas móveis, 5G. Nesse sentido, a fim de colaborar com o futuro das comunicações, uma das vertentes desta tese apresenta contribuições na área de diversidade espacial de sinais (SSD - Signal Space Diversity), técnica essa que permite o aumento da ordem de diversidade do sistema, sem impactar na eficiência espectral do mesmo. A técnica SSD trabalha com constelações rotacionadas. Uma nova métrica de avaliação, denominada métrica K, será usada neste trabalho para a obtenção de parâmetros de rotação ótimos. Em uma das abordagens, serão propostas e avaliadas novas estruturas matriciais de rotação multidimensional. Uma comparação de desempenho, em termos da taxa de erro de bit (BER - Bit Error Rate) média destas matrizes propostas, com estruturas de rotação comumente encontradas na literatura será realizada. Em outra avaliação, por meio de simulação computacional, os parâmetros de rotação ótimos de matrizes de rotação ortogonais em cenários com um único usuário serão obtidos usando a métrica K. Os resultados mostram que os ângulos de rotação ótimos obtidos neste trabalho, apresentam melhor desempenho que os ângulos de rotação encontrados por meio da distância produto mínima (MPD - Minimum Distance Product), métrica essa comumente usada na literatura. Além disso, os resultados apontam que os ângulos de rotação ótimos variam de acordo com as configurações do sistema, sendo que, neste trabalho são avaliados diferentes ordens de modulação QAM, número de dimensões e quantidade de antenas usadas na recepção. Será também realizada uma análise em cenário multiusuário, com o objetivo de verificar o impacto no desempenho devido ao uso de diferentes parâmetros de rotação em cada um dos usuários. Expressões analíticas para auxiliar nessa avaliação serão apresentadas. Será verificado se os parâmetros de rotação encontrados para os cenários com um único usuário, podem ser utilizados por todos os usuários no cenário multiusuário, sem prejuízo de desempenho. Outra contribuição desta tese se refere à comparação realizada entre as técnicas de múltiplo acesso MC-CDMA (Multicarrier Code Division Multiple Access) e SCMA (Sparse Code Multiple Access), usando seus respectivos detectores multiusuários, MU-MLD (Multiuser Maximum Likelihood Detector) implementado via detecção esférica (SD - Sphere Detector) e MPA (Message Passing Algorithm). Nessa análise comparativa serão apresentadas curvas da BER média em função da razão sinal-ruído (SNR - Signal to Noise Ratio) em canais com desvanecimento. Esses resultados serão obtidos por meio de simulações de Monte Carlo. Serão também apresentados os resultados de complexidade dos detectores multiusuário, em termos do número de operações matemáticas, obtidos por meio de expressões analíticas e simulações. Nessas análises será considerado o enlace reverso de sistemas celulares sobrecarregados, ou seja, quando o número de usuários alocados no sistema é maior do que o número de recursos físicos disponíveis. Os resultados mostram que, em mesmas condições de eficiência espectral e sem o uso de códigos corretores de erros, a técnica MC-CDMA apresenta um melhor desempenho em relação à técnica SCMA e que o decodificador SD é menos complexo que o detector MPAAbstract: The growing necessity for wireless communication systems with greater spectral efficiency and reliability has been guiding the future communication technological proposals. Spectral efficiency, multiple access techniques and signal detection algorithms compose part of the 3GPP (Third Generation Partnership Project) research for the fifth generation of mobile communication systems, 5G. In order to cooperate with the communication development, this thesis presents contributions in the signal spatial diversity (SSD) area, technique that allows to increase the system diversity order without impact its spectral efficiency. The SSD technique uses rotated constellations. Thus, a new metric, named metric K, is used in this thesis in order to obtain the optimum rotation parameters for SSD systems. In addition, new multidimensional rotation matrices are proposed and evaluated in this work. A performance comparison, in terms of the mean bit error rate (BER), of these proposed rotation matrices with others rotation matrices commonly found in the literature is done. Employing the metric K, the optimum rotation parameters used in matrices are obtained by simulation, considering single user scenarios. The results show that the optimum rotation parameters obtained in this work present better performance compared to the rotation parameters obtained by the minimum product distance (MPD) metric, commonly used in the literature. Our analysis evidences that the optimum rotation parameters vary according to the system parameters, that is, the M-QAM modulation order, the number of dimensions and the number of antennas used at the receiver. Another analysis is done for a multiuser scenario. In this case, the performance impact due to the use of different rotation parameters for each user is evaluated. Matrix expressions to emulate the received signals in a multiuser scenario are also presented. Due to the high complexity to obtain the optimum rotation parameters for a multiuser scenario, we also evaluate if the single user optimum rotation parameters can be used by all the users in the multiuser scenario. Another thesis contribution is related to multicarrier code division multiple access (MC-CDMA) and sparse code multiple access (SCMA). Specifically, a performance comparison is carried out considering their respective multiuser detectors, that is the multiuser maximum likelihood detector (MU-MLD) implemented via sphere decoder (SD) for MC-CDMA and the message passing algorithm (MPA) for SCMA. The performance results are shown in terms of mean BER as a function of the signal to noise ratio (SNR) in fading channels, and they are obtained via Monte Carlo simulations. In addition, the multiuser detectors complexity, in terms of the mean number of mathematical operations, is obtained by simulation and also by analytics expressions. In these analysis, an overloaded uplink mobile system is considered, that is, the number of simultaneous active users is greater than the number of available physical resources. The results show that, in the same spectral efficiency conditions, the MC-CDMA technique presents better performance than SCMA technique and the SD multiuser decoder is less complex than the MPA detectorDoutoradoTelecomunicações e TelemáticaDoutora em Engenharia Elétric

Signal Processing for Compressed Sensing Multiuser Detection

The era of human based communication was longly believed to be the main driver for the development of communication systems. Already nowadays we observe that other types of communication impact the discussions of how future communication system will look like. One emerging technology in this direction is machine to machine (M2M) communication. M2M addresses the communication between autonomous entities without human interaction in mind. A very challenging aspect is the fact that M2M strongly differ from what communication system were designed for. Compared to human based communication, M2M is often characterized by small and sporadic uplink transmissions with limited data-rate constraints. While current communication systems can cope with several 100 transmissions, M2M envisions a massive number of devices that simultaneously communicate to a central base-station. Therefore, future communication systems need to be equipped with novel technologies facilitating the aggregation of massive M2M. The key design challenge lies in the efficient design of medium access technologies that allows for efficient communication with small data packets. Further, novel physical layer aspects have to be considered in order to reliable detect the massive uplink communication. Within this thesis physical layer concepts are introduced for a novel medium access technology tailored to the demands of sporadic M2M. This concept combines advances from the field of sporadic signal processing and communications. The main idea is to exploit the sporadic structure of the M2M traffic to design physical layer algorithms utilizing this side information. This concept considers that the base-station has to jointly detect the activity and the data of the M2M nodes. The whole framework of joint activity and data detection in sporadic M2M is known as Compressed Sensing Multiuser Detection (CS-MUD). This thesis introduces new physical layer concepts for CS-MUD. One important aspect is the question of how the activity detection impacts the data detection. It is shown that activity errors have a fundamentally different impact on the underlying communication system than data errors have. To address this impact, this thesis introduces new algorithms that aim at controlling or even avoiding the activity errors in a system. It is shown that a separate activity and data detection is a possible approach to control activity errors in M2M. This becomes possible by considering the activity detection task in a Bayesian framework based on soft activity information. This concept allows maintaining a constant and predictable activity error rate in a system. Beyond separate activity and data detection, the joint activity and data detection problem is addressed. Here a novel detector based on message passing is introduced. The main driver for this concept is the extrinsic information exchange between different entities being part of a graphical representation of the whole estimation problem. It can be shown that this detector is superior to state-of-the-art concepts for CS-MUD. Besides analyzing the concepts introduced simulatively, this thesis also shows an implementation of CS-MUD on a hardware demonstrator platform using the algorithms developed within this thesis. This implementation validates that the advantages of CS-MUD via over-the-air transmissions and measurements under practical constraints

Avaliação da probabilidade de erro de bit e da eficiência espectral de sistemas celulares MC-CDMA que utilizam detecção multiusuário

Orientador: Celso de AlmeidaTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de ComputaçãoResumo: Uma técnica que combina múltiplo acesso por divisão de código (CDMA) e multiplexação por divisão de frequências ortogonais (OFDM) foi proposta como uma opção para futuros padrões de comunicações móveis. Esta técnica é conhecida como múltiplo acesso por divisão de código com multiportadoras (MC-CDMA), a qual além de herdar as vantagens das técnicas CDMA e OFDM, também possui uma inerente diversidade em frequência. Apesar de ser uma técnica de múltiplo acesso, MC-CDMA foi tipicamente estudada usando detectores de um único usuário no receptor. Além disso, alguns trabalhos que têm estudado seu desempenho com detectores multi-usuário usam apenas simulações. Ademais, cenários de uma célula são tipicamente considerados, embora sistemas móveis operem em ambientes celulares. Esta dissertação visa complementar parcialmente as pesquisas prévias sobre MC-CDMA. Em geral, este trabalho aborda o desempenho do enlace reverso de sistemas MC-CDMA em termos da taxa de erro de bit (BER) e da eficiência espectral celular. Para isto, um sistema celular que usa os esquemas de reuso de frequências fracionário (FFR) e suave (SFR) é suposto. Entrelaçamento no domínio da frequência é usado no transmissor dos equipamentos dos usuários e, detecção multiusuário e um arranjo de antenas são considerados nos receptores das estações radio base. O transmissor dos equipamentos dos usuários também realiza controle de potência perfeito. Além disso, ruído aditivo Gaussiano branco, perda de percurso e desvanecimento lento e seletivo que segue a distribuição de Rayleigh são considerados no modelo do canal. As contribuições desta dissertação são resumidas a seguir. Expressões fechadas são obtidas para avaliar a BER média de um sistema celular que usa os detectores multiusuário: zero-forcing (ZF), minimum mean square error (MMSE) e maximum likelihood detector (MU-MLD). Adicionalmente, a técnica signal space diversity (SSD) é usada no sistema celular MC-CDMA. Para isto, o MU-MLD precisa ser empregado no receptor da estação radio base. Uma expressão precisa para avaliar a BER média neste cenário é também derivada. Ademais, uma análise assintótica das expressões da BER é feita para se obter mais informações sobre a ordem da diversidade e o comportamento do sistema no regime de alta relação sinal-ruído mais interferência. A complexidade computacional dos detectores multi-usuário também é obtida em termos do número de operações complexas realizadas durante o processo de detecção. Em particular, o MU-MLD é implementado através de um algoritmo de decodificação esférica (SD), a fim de reduzir sua complexidade. Algumas técnicas são fornecidas para reduzir ainda mais a complexidade da SD. Finalmente, uma expressão para avaliar a eficiência espectral celular média do sistema MC-CDMA nos cenários FFR e SFR é obtida. Esta análise é baseada em um algoritmo que calcula os raios de cobertura da célula para cada modulação usada no sistema, assumindo que modulação adaptativa é empregada. Para todos os cenários, modulações BPSK e M-QAM são consideradas. Simulações de Monte Carlo corroboram a precisão da análise matemática apresentadaAbstract: A hybrid technique combining code division multiple access (CDMA) and orthogonal frequency division multiplexing (OFDM) has been proposed as an option for future mobile communication standards. This technique is known as multicarrier code division multiple access (MC-CDMA), which, besides inheriting the advantages of CDMA and OFDM techniques, also possesses an inherent frequency diversity. Despite being a multiple access technique, MC-CDMA has been typically studied employing single-user detectors in the receiver. Moreover, some works that have studied their performance with multiuser detectors have done so far using only simulations. Furthermore, single cell scenarios are typically considered although mobile systems operate in cellular environments. This dissertation aims to partially complement previous research on MC-CDMA. In general, this work addresses the uplink performance of MC-CDMA systems in terms of the bit error rate (BER) and the cellular spectral efficiency. For this, a cellular system employing fractional frequency reuse (FFR) and soft frequency reuse (SFR) schemes is assumed. Frequency domain interleaving is performed in the transmitter into the user equipments and, multiuser detection and an antenna array are considered in the receivers at the base stations. The transmitter into the user equipments also performs perfect power control. Furthermore, additive white Gaussian noise, path-loss and slow frequency-selective Rayleigh fading are considered in the channel model. The contributions of this dissertation are summarized in the following. Closed-form expressions are derived to evaluate the mean BER of MC-CDMA cellular systems using the multiuser detectors: zero-forcing (ZF), minimum mean square error (MMSE) and maximum likelihood detector (MU-MLD). In addition, signal space diversity (SSD) is used in the MC-CDMA cellular system. For this, MU-MLD must be employed in the receiver at the base station. An accurate expression to evaluate the mean BER in this scenario is also derived. Moreover, an asymptotic analysis of the BER expressions is performed to obtain further insights of the diversity order and system behavior at the high signal-to-noise-plus-interference ratio regime. The computational complexity of the multiuser detectors is also obtained in terms of the number of complex operations performed during the detection process. In particular, MU-MLD is implemented via a sphere decoder (SD) algorithm in order to reduce its complexity. Some techniques are provided in order to further reduce the SD complexity. Finally, an expression to evaluate the mean cellular spectral efficiency of the MC-CDMA system in FFR and SFR scenarios is obtained. This analysis is based on an algorithm that calculates the cell coverage radius for each modulation used in the system, assuming that adaptive modulation is employed. For all analyzed scenarios, BPSK and M-QAM modulations are considered. Monte Carlo simulations corroborate the accuracy of the presented mathematical analysisDoutoradoTelecomunicações e TelemáticaDoutor em Engenharia ElétricaCAPE