Key Enabling Technologies for 5G: Millimeter-Wave and Massive MIMO

[EN] 5G wireless networks are expected to operate with orders of magnitude higher performance than the current 4G deployments. The demand for 5000 times higher data rates leads to the necessity of finding new techniques to increase spectral efficiency and of exploring new frequency bands above 6 GHz. It has been proved that from UHF up to C band, a significant increase in system spectral efficiency can be reached through various techniques, such as Coordinated Multi-Point (CoMP), Massive Multiple-Input-Multiple-Output (MIMO), and interference management and cancellation; still, the resulting performance will not cope with the full expectations of IMT-2020 and 5G-PPP requirements for 5G networks, mainly in terms of offering 10 Gbps peak data rates with connection densities of 100 k¿1 M devices/km2. To overcome this limitation, the future architecture of such 5G networks is being defined to be deployed on small cells and to use higher frequency bands, such as super high frequency (SHF, 3¿30 GHz) or extremely high frequency (EHF, 30¿300 GHz), also referred as to centimeter and millimeter wave bands, respectively.Cardona Marcet, N.; Correia, LM.; Calabuig Soler, D. (2017). Key Enabling Technologies for 5G: Millimeter-Wave and Massive MIMO. International Journal of Wireless Information Networks. 24(3):201-203. doi:10.1007/s10776-017-0366-zS20120324

Esquemas de pré-codificação e equalização para arquiteturas híbridas sub-conectadas na banda de ondas milimétricas

In the last years, the demand for high data rates increased substantially and the mobile communications are currently a necessity for our society. Thus, the number of users to access interactive services and applications has increased. The next generation of wireless communications (5G) is expected to be released in 2020 and it is projected to provide extremely high data rates for the users. The millimeter wave communications band and the massive MIMO are two promising keys technologies to achieve the multi Gbps for the future generations of mobile communications, in particular the 5G. The conjugation of these two technologies, allows packing a large number of antennas in the same volume than in the current frequencies and increase the spectral efficiency. However, when we have a large number of antennas, it is not reasonable to have a fully digital architecture due to the hardware constrains. On the other hand, it is not feasible to have a system that works only in the analog domain by employing a full analog beamforming since the performance is poor. Therefore, it is required a design of hybrid analog/digital architectures to reduce the complexity and achieve a good performance. Fully connected and sub-connected schemes are two examples of hybrid architectures. In the fully connected one, all RF chain connect to all antenna elements while in the sub-connected architecture, each RF chain is connected to a group of antennas. Consequently, the sub-connected architecture is more attractive due to the low complexity when compared to the fully connected one. Also, it is expected that millimeter waves be wideband, however, most of the works developed in last years for hybrid architectures are mainly focused in narrowband channels. Therefore, in this dissertation it is designed a low complex analog precoder at the user terminals and a hybrid analog-digital multi-user linear equalizer for broadband sub-connected millimeter wave massive MIMO at the base station. The analog precoder at the transmitter considers a quantized version of the average angle of departure of each cluster for its computation. In order to remove the multi-user interference, it is considered a hybrid sub-connected approach that minimizes the bit error rate (BER). The performance results show that the proposed hybrid sub-connected scheme is close to the hybrid full-connected design. However, due to the large number of connections, the full-connected scheme is slightly better than the proposed sub-connected scheme but with higher complexity. Therefore, the proposed analog precoder and hybrid sub-connected equalizer are more feasible to practical applications due to the good trade-off between performance and complexity.Nos últimos anos, a necessidade por elevadas taxas de transmissão de dados tem vindo a aumentar substancialmente uma vez que as comunicações móveis assumem cada vez mais um papel fundamental na sociedade atual. Por isso, o número de utilizadores que acedem a serviços e aplicações interativas tem vindo a aumentar. A próxima geração de comunicações móveis (5G) é esperada que seja lançada em 2020 e é projetada para fornecer elevadas taxas de transmissão de dados aos seus utilizadores. A comunicação na banda das ondas milimétricas e o MIMO massivo são duas tecnologias promissoras para alcançar os multi Gb/s para as comunicações móveis futuras, em particular o 5G. Conjugando essas duas tecnologias, permite-nos colocar um maior número de antenas no mesmo volume comparativamente às frequências atuais, aumentando assim a eficiência espectral. No entanto, quanto se tem um grande número de antenas, não é viável ter uma arquitetura totalmente digital devido às restrições de hardware. Por outro lado, não é viável ter um sistema que trabalhe apenas no domínio analógico. Assim sendo, é necessária uma arquitetura híbrida analógica-digital de modo a remover a complexidade geral do sistema. É esperado que os sistemas de comunicação baseados em ondas milimétricas sejam de banda larga, no entanto, a maioria dos trabalhos feitos para arquiteturas híbridas são focados em canais de banda estreita. Dois exemplos de soluções híbridas são as arquiteturas completamente conectada e sub-conectada. Na primeira, todas as cadeias RF estão ligadas a todas as antenas enquanto na arquitetura sub-conectada cada cadeia RF é ligada apenas a um grupo de antenas. Consequentemente, a arquitetura sub-conectada é mais interessante do ponto de vista prático devido à sua menor complexidade quando comparada à arquitetura completamente conectada. Nesta dissertação é projetado um pré-codificador analógico de baixa complexidade no terminal móvel, combinado com um equalizador multiutilizador desenhado para uma arquitetura híbrida sub-conectada, implementado na estação base. O pré-codificador no transmissor assume um conhecimento parcial da informação do canal e, de modo a remover eficientemente a interferência multiutilizador, é proposta também uma arquitetura híbrida sub-conectada que minimiza a taxa média de erro. Os resultados de desempenho mostram que o esquema híbrido sub-conectado proposto está próximo da arquitetura híbrida completamente conectada. No entanto, devido ao grande número de conexões, a arquitetura híbrida completamente conectada é ligeiramente melhor que a arquitetura sub-conectada proposta à custa de uma maior complexidade. Assim sendo, o pré-codificador analógico e o equalizador sub-conectado híbrido proposto são mais viáveis para aplicações práticas devido ao compromisso entre o desempenho e a complexidade.Mestrado em Engenharia Eletrónica e Telecomunicaçõe

Precoding Schemes for Millimeter Wave Massive MIMO Systems

In an effort to cut high cost and power consumption of radio frequency (RF) chains, millimeter wave (mmWave) multiple input multiple output (MIMO) deploys hybrid architecture in which precoding is implemented as a combination of digital precoding and analog precoding, accomplished by using a smaller number of RF chains and a network of phase shifters respectively. The mmWave MIMO, which usually suffers from blockages, needs to be supported by Reconfigurable Intelligent Surface (RIS) to make communication possible. Along with the hybrid precoding in mmWave MIMO, the passive precoding of Reconfigurable Intelligent Surface (RIS) is investigated in a downlink RIS-assisted mmWave MIMO. The hybrid precoding and passive precoding are challenged by the unit modulus constraints on the elements of analog precoding matrix and passive precoding vector. The coupling of analog and digital precoders further complicates the hybrid precoding. One of the approaches taken in proposed hybrid precoding algorithms is the use of alternating optimization in which analog precoder and digital precoder are optimized alternately keeping the other fixed. Analog precoder is determined by solving a semidefinite programming problem, and from the unconstrained least squares solution during each iteration. In another approach taken in the proposed methods, the hybrid precoding is split into separate analog and digital precoding subproblems. The analog precoding subproblems are simplified using some approximations, and solved by using iterative power method and employing a truncated singular value decomposition method in two different hybrid precoding algorithms. In the prooposed codebook-based precoder, analog precoder is constructed by choosing precoding vectors from a codebook to maximize signal-to-leakage-and-noise ratio (SLNR). The passive precoding at the RIS in a single user MIMO is designed to minimize mean square error between the transmit signal and the estimate of received signal by using an iterative algorithm that solves the joint optimization problem of precoding, passive precoding and combiner. The problem of designing energy efficient RIS is solved by maximizing energy efficiency which is a joint optimization problem involving precoder, passive precoding matrix and power allocation matrix. The proposed hybrid precoding and passive precoding algorithms deliver very good performances and prove to be computationally efficient