Rate Splitting for MIMO Wireless Networks: A Promising PHY-Layer Strategy for LTE Evolution

MIMO processing plays a central part towards the recent increase in spectral and energy efficiencies of wireless networks. MIMO has grown beyond the original point-to-point channel and nowadays refers to a diverse range of centralized and distributed deployments. The fundamental bottleneck towards enormous spectral and energy efficiency benefits in multiuser MIMO networks lies in a huge demand for accurate channel state information at the transmitter (CSIT). This has become increasingly difficult to satisfy due to the increasing number of antennas and access points in next generation wireless networks relying on dense heterogeneous networks and transmitters equipped with a large number of antennas. CSIT inaccuracy results in a multi-user interference problem that is the primary bottleneck of MIMO wireless networks. Looking backward, the problem has been to strive to apply techniques designed for perfect CSIT to scenarios with imperfect CSIT. In this paper, we depart from this conventional approach and introduce the readers to a promising strategy based on rate-splitting. Rate-splitting relies on the transmission of common and private messages and is shown to provide significant benefits in terms of spectral and energy efficiencies, reliability and CSI feedback overhead reduction over conventional strategies used in LTE-A and exclusively relying on private message transmissions. Open problems, impact on standard specifications and operational challenges are also discussed.Comment: accepted to IEEE Communication Magazine, special issue on LTE Evolutio

Multiuser Millimeter Wave Beamforming Strategies with Quantized and Statistical CSIT

To alleviate the high cost of hardware in mmWave systems, hybrid analog/digital precoding is typically employed. In the conventional two-stage feedback scheme, the analog beamformer is determined by beam search and feedback to maximize the desired signal power of each user. The digital precoder is designed based on quantization and feedback of effective channel to mitigate multiuser interference. Alternatively, we propose a one-stage feedback scheme which effectively reduces the complexity of the signalling and feedback procedure. Specifically, the second-order channel statistics are leveraged to design digital precoder for interference mitigation while all feedback overhead is reserved for precise analog beamforming. Under a fixed total feedback constraint, we investigate the conditions under which the one-stage feedback scheme outperforms the conventional two-stage counterpart. Moreover, a rate splitting (RS) transmission strategy is introduced to further tackle the multiuser interference and enhance the rate performance. Consider (1) RS precoded by the one-stage feedback scheme and (2) conventional transmission strategy precoded by the two-stage scheme with the same first-stage feedback as (1) and also certain amount of extra second-stage feedback. We show that (1) can achieve a sum rate comparable to that of (2). Hence, RS enables remarkable saving in the second-stage training and feedback overhead.Comment: submitted to TW

Blind CSI acquisition for multi-antenna interference mitigation in 5G networks

Future wireless communication networks are required to satisfy the increasing demands of traffic and capacity. The upcoming fifth generation (5G) of the cellular technology is expected to meet 1000 times the capacity that of the current fourth generation (4G). These tight specifications introduce a new set of research challenges. However, interference has always been the bottleneck in cellular communications. Thus, towards the vision of the 5G, massive multi-input multi-output (mMIMO) and interference alignment (IA) are key transmission technologies to fulfil the future requirements, by controlling the residual interference. By equipping the base-station (BS) with a large number of transmit antennas, e.g, tens of hundreds of antennas, a mMIMO system can theoretically achieve significant capacity with limited interference, where many user equipment (UEs) can be served simultaneously at the same time and frequency resources. A mMIMO offers great spatial degrees of freedom (DoFs), which boost the total network capacity without increasing transmission power or bandwidth. However, the majority of the recent mMIMO investigations are based on theoretical channels with independent and identically distributed (i.i.d) Gaussian distribution, which facilitates the computation of closed-form rate expressions. Nonetheless, practical channels are not spatially uncorrelated, where the BS receives different power ratios across different spatial directions between the same transmitting and receiving antennas. Thus, it is important to understand the behavior of such new technology with practical channel modeling. Alternatively, IA is known to break the bottleneck between the capacity of the network and the overall spectral efficiency (SE), where a performance degradation is observed at a certain level of connected user capacity, due to the overwhelming inter-user interference. Theoretically, IA guarantees a linear relationship between half of the overall network SE and the online capacity by aligning interference from all transmitters inside one spatial signal subspace, leaving the other subspace for desired transmission. However, IA has tight feasibility conditions in practice including high precision channel state information at transmitter (CSIT), which leads to severe feedback overhead. In this thesis, high-precision blind CSIT algorithms are developed under different transmission technologies. We first consider the CSIT acquisition problem in MIMO IA systems. Proposed spatial channel estimation for MIMO-IA systems (SCEIA) shows great offered spatial degrees of freedom which contributes to approaching the performance of the perfect-CSIT case, without the requirements of channel quantization or user feedback overhead. In massive MIMO setups, proposed CSIT strategy offered scalable performance with the number of the transmit antennas. The effect of the non-stationary channel characteristics, which appears with very large antenna arrays, is minimized due to the effective scanning precision of the proposed strategy. Finally, we extend the system model to the full dimensional space, where users are distributed across the two dimensions of the cell space (azimuthal/elevation). Proposed directional spatial channel estimation (D-SCE) scans the 3D cell space and effectively attains additional CSIT and beamforming gains. In all cases, a list of comparisons with state-of-the-art schemes from academia and industry is performed to show the performance improvement of the proposed CSIT strategies

Novel transmission and beamforming strategies for multiuser MIMO with various CSIT types

In multiuser multi-antenna wireless systems, the transmission and beamforming strategies that achieve the sum rate capacity depend critically on the acquisition of perfect Channel State Information at the Transmitter (CSIT). Accordingly, a high-rate low-latency feedback link between the receiver and the transmitter is required to keep the latter accurately and instantaneously informed about the CSI. In realistic wireless systems, however, only imperfect CSIT is achievable due to pilot contamination, estimation error, limited feedback and delay, etc. As an intermediate solution, this thesis investigates novel transmission strategies suitable for various imperfect CSIT scenarios and the associated beamforming techniques to optimise the rate performance. First, we consider a two-user Multiple-Input-Single-Output (MISO) Broadcast Channel (BC) under statistical and delayed CSIT. We mainly focus on linear beamforming and power allocation designs for ergodic sum rate maximisation. The proposed designs enable higher sum rate than the conventional designs. Interestingly, we propose a novel transmission framework which makes better use of statistical and delayed CSIT and smoothly bridges between statistical CSIT-based strategies and delayed CSIT-based strategies. Second, we consider a multiuser massive MIMO system under partial and statistical CSIT. In order to tackle multiuser interference incurred by partial CSIT, a Rate-Splitting (RS) transmission strategy has been proposed recently. We generalise the idea of RS into the large-scale array. By further exploiting statistical CSIT, we propose a novel framework Hierarchical-Rate-Splitting that is particularly suited to massive MIMO systems. Third, we consider a multiuser Millimetre Wave (mmWave) system with hybrid analog/digital precoding under statistical and quantised CSIT. We leverage statistical CSIT to design digital precoder for interference mitigation while all feedback overhead is reserved for precise analog beamforming. For very limited feedback and/or very sparse channels, the proposed precoding scheme yields higher sum rate than the conventional precoding schemes under a fixed total feedback constraint. Moreover, a RS transmission strategy is introduced to further tackle the multiuser interference, enabling remarkable saving in feedback overhead compared with conventional transmission strategies. Finally, we investigate the downlink hybrid precoding for physical layer multicasting with a limited number of RF chains. We propose a low complexity algorithm to compute the analog precoder that achieves near-optimal max-min performance. Moreover, we derive a simple condition under which the hybrid precoding driven by a limited number of RF chains incurs no loss of optimality with respect to the fully digital precoding case.Open Acces

Content delivery over multi-antenna wireless networks

The past few decades have witnessed unprecedented advances in information technology, which have significantly shaped the way we acquire and process information in our daily lives. Wireless communications has become the main means of access to data through mobile devices, resulting in a continuous exponential growth in wireless data traffic, mainly driven by the demand for high quality content. Various technologies have been proposed by researchers to tackle this growth in 5G and beyond, including the use of increasing number of antenna elements, integrated point-to-multipoint delivery and caching, which constitute the core of this thesis. In particular, we study non-orthogonal content delivery in multiuser multiple-input-single-output (MISO) systems. First, a joint beamforming strategy for simultaneous delivery of broadcast and unicast services is investigated, based on layered division multiplexing (LDM) as a means of superposition coding. The system performance in terms of minimum required power under prescribed quality-of-service (QoS) requirements is examined in comparison with time division multiplexing (TDM). It is demonstrated through simulations that the non-orthogonal delivery strategy based on LDM significantly outperforms the orthogonal strategy based on TDM in terms of system throughput and reliability. To facilitate efficient implementation of the LDM-based beamforming design, we further propose a dual decomposition-based distributed approach. Next, we study an efficient multicast beamforming design in cache-aided multiuser MISO systems, exploiting proactive content placement and coded delivery. It is observed that the complexity of this problem grows exponentially with the number of subfiles delivered to each user in each time slot, which itself grows exponentially with the number of users in the system. Therefore, we propose a low-complexity alternative through time-sharing that limits the number of subfiles that can be received by a user in each time slot. Moreover, a joint design of content delivery and multicast beamforming is proposed to further enhance the system performance, under the constraint on maximum number of subfiles each user can decode in each time slot. Finally, conclusions are drawn in Chapter 5, followed by an outlook for future works.Open Acces

Antenna Array Enabled Space/Air/Ground Communications and Networking for 6G

Antenna arrays have a long history of more than 100 years and have evolved closely with the development of electronic and information technologies, playing an indispensable role in wireless communications and radar. With the rapid development of electronic and information technologies, the demand for all-time, all-domain, and full-space network services has exploded, and new communication requirements have been put forward on various space/air/ground platforms. To meet the ever increasing requirements of the future sixth generation (6G) wireless communications, such as high capacity, wide coverage, low latency, and strong robustness, it is promising to employ different types of antenna arrays with various beamforming technologies in space/air/ground communication networks, bringing in advantages such as considerable antenna gains, multiplexing gains, and diversity gains. However, enabling antenna array for space/air/ground communication networks poses specific, distinctive and tricky challenges, which has aroused extensive research attention. This paper aims to overview the field of antenna array enabled space/air/ground communications and networking. The technical potentials and challenges of antenna array enabled space/air/ground communications and networking are presented first. Subsequently, the antenna array structures and designs are discussed. We then discuss various emerging technologies facilitated by antenna arrays to meet the new communication requirements of space/air/ground communication systems. Enabled by these emerging technologies, the distinct characteristics, challenges, and solutions for space communications, airborne communications, and ground communications are reviewed. Finally, we present promising directions for future research in antenna array enabled space/air/ground communications and networking

Técnicas de quantização para sistemas de comunicação híbridos na banda de ondas milimétricas com um número elevado de antenas

Since the appearance of mobile communications, the users of this technology have been growing exponentially every day. The escalating mobile traffic growth it has been imposed by the proliferation of smartphones and tablets. The increasing and more intensive use of wireless communications may lead to a future breaking point, where the traditional systems will fail to support the required capability, spectral and energy efficiency. On the other hand, to cover all this current need to have more and more data it is necessary to provide a new range of data rates around the gigabits per second. Today, almost all mobile communications systems use spectrum in the range of 300MHz – 3GHz. It is needed to start looking to the range of 3GHz – 300GHz spectrum for mobile broadband applications. Millimeter waves are one way to alleviate the spectrum gridlock at lower frequencies. MIMO based systems has been researched for the last 20 years and are now part of the current standards. However, to achieve more gains, a grander view of the MIMO concept envisions the use of a large scale of antennas at each base stations, a concept referred as massive MIMO. The symbiotic combination of these technologies and other ones will lead to the development of a new generation system known as the 5G. The knowledge of the channel state information at the transmitter is very important in real massive MIMO millimeter wave systems. In this dissertation a limited feedback strategy for a hybrid massive MIMO OFDM system is proposed, where only a part of the parameters associated to the link channel are quantized and fed back. The limited feedback strategy employs a uniform-based quantization for channel amplitudes, angle of departure and angle of arrival in time domain. After being fed back, this information is used to reconstruct the overall channel in frequency domain and the transmit antenna array, which are then used to compute the hybrid analog-digital precoders. Numerical results show that the proposed quantization strategy achieve a performance close to the one obtained with perfect full channel, with a low overhead and complexityDesde o aparecimento das comunicações móveis, os utilizadores desta tecnologia têm vindo a crescer exponencialmente todos os dias. A escalada do crescimento do tráfego móvel foi imposta, principalmente, pela proliferação de smartphones e tablets. O uso crescente e intensivo das comunicações sem fios pode levar no futuro a um ponto de rutura, onde os sistemas tradicionais não suportam a capacidade requerida, a eficiência espectral e eficiência enérgica. Por outro lado, para cobrir toda esta necessidade atual de ter mais e mais dados, é necessário fornecer taxas de transmissão mais elevadas, em torno dos gigabits por segundo. Hoje, quase todos os sistemas de comunicações móveis usam espectro na faixa de 300 MHz - 3GHz. É necessário começar a procurar a gama de espectro 3GHz - 300 GHz para aplicações de banda larga móvel. Aqui vamos apresentar as ondas milimétricas, sendo esta uma maneira de aliviar espectro em frequências mais baixas. Os sistemas baseados em MIMO foram alvo de pesquisa nos últimos 20 anos e agora fazem parte dos padrões atuais. No entanto, para obter mais ganhos, uma visão mais ampla do conceito MIMO prevê o uso de uma grande quantidade de antenas em cada estação base, um conceito referido como massive MIMO. A combinação simbiótica destas tecnologias levará ao desenvolvimento de um novo sistema de geração denominado 5G. O desenvolvimento de técnicas de conhecimento da informação do canal no transmissor é muito importante em sistemas massive MIMO millimeter wave reais. Nesta dissertação é proposta e avaliada uma estratégia de envio de informação de canal para o transmissor para sistemas massive MIMO OFDM híbrido, onde apenas uma parte dos parâmetros associados ao canal são quantificados e transmitidos para o transmissor. A estratégia de feedback proposta é baseada numa quantização uniforme das amplitudes de canal, ângulos de partida e de chegada, no domínio do tempo. Depois de serem enviadas, essas informações são usadas para reconstruir o canal geral no domínio da frequência e a matriz da antena de transmissão, que são então usadas para obter os precoders híbridos analógico-digitais. Os resultados numéricos mostram que a estratégia de quantificação proposta atinge um desempenho próximo ao obtido caso se conhecesse o canal perfeito no transmissor, com um baixo overhead e complexidadeMestrado em Engenharia Eletrónica e Telecomunicaçõe