A Tutorial on Extremely Large-Scale MIMO for 6G: Fundamentals, Signal Processing, and Applications

Extremely large-scale multiple-input-multiple-output (XL-MIMO), which offers vast spatial degrees of freedom, has emerged as a potentially pivotal enabling technology for the sixth generation (6G) of wireless mobile networks. With its growing significance, both opportunities and challenges are concurrently manifesting. This paper presents a comprehensive survey of research on XL-MIMO wireless systems. In particular, we introduce four XL-MIMO hardware architectures: uniform linear array (ULA)-based XL-MIMO, uniform planar array (UPA)-based XL-MIMO utilizing either patch antennas or point antennas, and continuous aperture (CAP)-based XL-MIMO. We comprehensively analyze and discuss their characteristics and interrelationships. Following this, we examine exact and approximate near-field channel models for XL-MIMO. Given the distinct electromagnetic properties of near-field communications, we present a range of channel models to demonstrate the benefits of XL-MIMO. We further motivate and discuss low-complexity signal processing schemes to promote the practical implementation of XL-MIMO. Furthermore, we explore the interplay between XL-MIMO and other emergent 6G technologies. Finally, we outline several compelling research directions for future XL-MIMO wireless communication systems.Comment: 38 pages, 10 figure

Holographic MIMO Communications: Theoretical Foundations, Enabling Technologies, and Future Directions

Future wireless systems are envisioned to create an endogenously holography-capable, intelligent, and programmable radio propagation environment, that will offer unprecedented capabilities for high spectral and energy efficiency, low latency, and massive connectivity. A potential and promising technology for supporting the expected extreme requirements of the sixth-generation (6G) communication systems is the concept of the holographic multiple-input multiple-output (HMIMO), which will actualize holographic radios with reasonable power consumption and fabrication cost. The HMIMO is facilitated by ultra-thin, extremely large, and nearly continuous surfaces that incorporate reconfigurable and sub-wavelength-spaced antennas and/or metamaterials. Such surfaces comprising dense electromagnetic (EM) excited elements are capable of recording and manipulating impinging fields with utmost flexibility and precision, as well as with reduced cost and power consumption, thereby shaping arbitrary-intended EM waves with high energy efficiency. The powerful EM processing capability of HMIMO opens up the possibility of wireless communications of holographic imaging level, paving the way for signal processing techniques realized in the EM-domain, possibly in conjunction with their digital-domain counterparts. However, in spite of the significant potential, the studies on HMIMO communications are still at an initial stage, its fundamental limits remain to be unveiled, and a certain number of critical technical challenges need to be addressed. In this survey, we present a comprehensive overview of the latest advances in the HMIMO communications paradigm, with a special focus on their physical aspects, their theoretical foundations, as well as the enabling technologies for HMIMO systems. We also compare the HMIMO with existing multi-antenna technologies, especially the massive MIMO, present various...Comment: double column, 58 page

Learning Energy-Efficient Hardware Configurations for Massive MIMO Beamforming

Hybrid beamforming (HBF) and antenna selection are promising techniques for improving the energy efficiency~(EE) of massive multiple-input multiple-output~(mMIMO) systems. However, the transmitter architecture may contain several parameters that need to be optimized, such as the power allocated to the antennas and the connections between the antennas and the radio frequency chains. Therefore, finding the optimal transmitter architecture requires solving a non-convex mixed integer problem in a large search space. In this paper, we consider the problem of maximizing the EE of fully digital precoder~(FDP) and hybrid beamforming~(HBF) transmitters. First, we propose an energy model for different beamforming structures. Then, based on the proposed energy model, we develop an unsupervised deep learning method to maximize the EE by designing the transmitter configuration for FDP and HBF. The proposed deep neural networks can provide different trade-offs between spectral efficiency and energy consumption while adapting to different numbers of active users. Finally, to ensure that the proposed method can be implemented in practice, we investigate the ability of the model to be trained exclusively using imperfect channel state information~(CSI), both for the input to the deep learning model and for the calculation of the loss function. Simulation results show that the proposed solutions can outperform conventional methods in terms of EE while being trained with imperfect CSI. Furthermore, we show that the proposed solutions are less complex and more robust to noise than conventional methods.Comment: This preprint comprises 15 pages and features 15 figures. Copyright may be transferred without notic

Massive MIMO for Internet of Things (IoT) Connectivity

Massive MIMO is considered to be one of the key technologies in the emerging 5G systems, but also a concept applicable to other wireless systems. Exploiting the large number of degrees of freedom (DoFs) of massive MIMO essential for achieving high spectral efficiency, high data rates and extreme spatial multiplexing of densely distributed users. On the one hand, the benefits of applying massive MIMO for broadband communication are well known and there has been a large body of research on designing communication schemes to support high rates. On the other hand, using massive MIMO for Internet-of-Things (IoT) is still a developing topic, as IoT connectivity has requirements and constraints that are significantly different from the broadband connections. In this paper we investigate the applicability of massive MIMO to IoT connectivity. Specifically, we treat the two generic types of IoT connections envisioned in 5G: massive machine-type communication (mMTC) and ultra-reliable low-latency communication (URLLC). This paper fills this important gap by identifying the opportunities and challenges in exploiting massive MIMO for IoT connectivity. We provide insights into the trade-offs that emerge when massive MIMO is applied to mMTC or URLLC and present a number of suitable communication schemes. The discussion continues to the questions of network slicing of the wireless resources and the use of massive MIMO to simultaneously support IoT connections with very heterogeneous requirements. The main conclusion is that massive MIMO can bring benefits to the scenarios with IoT connectivity, but it requires tight integration of the physical-layer techniques with the protocol design.Comment: Submitted for publicatio

Comparação do desempenho de arquiteturas híbridas para comunicações na banda das ondas milimétricas

Mestrado em Engenharia Electrónica e TelecomunicaçõesA proliferação massiva das comunicações sem os faz prever que o número de utilizadores aumente exponencialmente até 2020, o que tornar a necessário um suporte de tráfego milhares de vezes superior e com ligações na ordem dos Gigabit por segundo. Este incremento exigir a um aumento significativo da e ciência espectral e energética. Impõe-se portanto, uma mudança de paradigma dos sistemas de comunicação sem os convencionais, imposta pela introdução da 5a geração. Para o efeito, e necessário desenvolver novas e promissoras técnicas de transmissão, nomeadamente a utilização de ondas milimétricas em sistemas com um número massivo de antenas. No entanto, consideráveis desafios emergem ao adotar estas técnicas. Por um lado, este tipo de ondas sofre grandes dificuldades em termos de propagação. Por outro lado, a adoção de arquiteturas convencionais para sistemas com um número massivo de antenas e absolutamente inviável, devido ao custo e ao nível de complexidade inerentes. Isto acontece porque o processamento de sinal ao nível da camada f sica e maioritariamente feito em banda base, ou seja, no domínio digital requerendo uma cadeia RF por cada antena. Neste contexto as arquiteturas híbridas são uma proposta relativamente recente que visa simplificar a utilização de um grande número de antenas, dividindo o processamento entre os domínios analógico e digital. Para além disso, o número de cadeias RF necessárias e bastante inferior ao número total de antenas do sistema, contribuindo para obvias melhorias em termos de complexidade, custo e energia consumida. Nesta dissertação e implementada uma arquitetura híbrida para ondas milimétricas, onde cada cadeia RF está apenas conectada a um pequeno conjunto de antenas. E considerado um sistema contendo um transmissor e um recetor ambos equipados com um grande número de antenas e onde, o número de cadeias RF e bastante inferior ao número total de antenas. Pré-codificadores híbridos analógico/digital, recentemente propostos na literatura são utilizados e novos equalizadores híbridos analógico/digital são projetados. E feita uma avaliação de performance à arquitetura implementada e posteriormente comparada com uma outra arquitetura, onde todas as antenas estão conectadas a todas as cadeias RF.The expected massive proliferation of wireless systems points out an exponential increase in the number of users until 2020, which is needed to support up to one thousand times more tra c and connections in order of Gigabit per second. However, these goals require a signi cantly improvement in the spectral and energy e ciency. As a result, it is essential to make a paradigm shift in conventional wireless systems, imposed by the introduction of fth generation (5G). For this purpose, new and promising transmission techniques will be needed, namely the use of millimeter Waves (mmWave) in systems with a massive number of antenna elements. Nevertheless, considerable challenges emerge in the adoption of these techniques. On one hand, mmWave su er great di culties in terms of propagation. On the other hand, the using of conventional architectures for systems with a large number of antennas is absolutely impracticable because of the costs and the level of complexity. This happens because the signal processing in physical layer is mostly done in baseband, which means, that one RF chain for each antenna is required. In this context the hybrid architectures are a relatively recent proposal where the aim is to simplify the use of a large number of antenna elements, dividing the processing between the analog and digital domains. Moreover, the number of RF chains needed are much lower than the total number of antenna elements of the system, which contribute to obvious improvements in terms of complexity, costs and energy consumption. In this Dissertation a hybrid mmWave based architecture, where each RF chain is only connected to a small set of antennas, is implemented. It is considered a system comprising a transmitter and a receiver both equipped with a massive number of antennas and where the number of RF chains is much lower than the number of antennas. Hybrid analog/digital precoders recently proposed in the literature are used and a new hybrid analog/digital equalizer is designed. The implemented architecture is then evaluated and compared with other architecture, where all the antennas are connected to all RF chains

6G Wireless Communications in 7-24 GHz Band: Opportunities, Techniques, and Challenges

The sixth generation (6G) wireless communication nowadays is seeking a new spectrum to inherit the pros and discard the cons of sub-6 GHz, millimeter-wave (mmWave), and sub-terahertz (THz) bands. To this end, an upper mid-band, Frequency Range (FR) spanning from 7 GHz to 24 GHz, also known as FR3, has emerged as a focal point in 6G communications. Thus, as an inexorable prerequisite, a comprehensive investigation encompassing spectrum utilization and channel modeling is the first step to exploit potential applications and future prospects of using this FR in the 6G ecosystem. In this article, we provide FR3 deployment insights into emerging technologies including non-terrestrial network (NTN), massive multi-input multi-output (mMIMO), reconfigurable intelligent surface (RIS), and joint communications and sensing (JCAS). Furthermore, leveraging ray-tracing simulations, our investigation unveils the channel characteristics in FR3 are close to those in the sub-6 GHz band. The analysis of RIS-aided communication shows a higher spectral efficiency achieved in FR3 compared to other FRs when using the same RIS size. Finally, challenges and promising directions are discussed for FR3-based communication systems.Comment: 7 pages, 5 figures, 1 tabl

Theoretical analysis of nonlinear amplification effects in massive MIMO systems

To fulfill 5th Generation (5G) communication capacity demands, the use of a large number of antennas has been widely investigated, and the array gain and spatial multiplexing that are offered by massive multiple input multiple output (mMIMO) have been used to improve the capacity. Fully digital architectures are not feasible for a large number of antennas, and hybrid analog/digital systems have emerged as options to retain a high number of antennas without as many radio frequency (RF) chains. However, these systems have, as consequences, non-avoidable nonlinear effects due to power amplifiers functioning in nonlinear regions. The strong nonlinear effects throughout the transmission chain will have a negative impact on the overall system’s performance. Being able to access this impact is very important. For this purpose, we propose analytical and semi-analytical tools that allow for the evaluation of the nonlinear effects of a hybrid analog/digital orthogonal frequency-division multiplexing (OFDM) system. The proposed analysis starts with the characterization of the power amplifier’s (PA) nonlinear response. This response is then used to derive a semi-analytic bit error rate expression. The theoretical tools are validated by using numerical results from two different cases: in the first one, the nonlinear PA response is assumed to follow an analytical model found in the literature and, in the second, the used nonlinear polynomial model mimics the response of a real amplifier. Using these two scenarios, the proposed tools are shown to be accurate making it possible to predict the nonlinearities’ penalties in hybrid analog/digital OFDM systems and/or to assess the optimal operation point for a specific nonlinear amplifier.publishe