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

3G Wideband CDMA : packet-based optimisation for high data-rate downlink transmission

A third generation (3G) of mobile communication systems, based on Wideband CDMA, are intended to offer high-speed packet-based services. Network operators wish to maximise the throughput in the downlink of3G systems, which requires efficient allocation ofresources. This thesis considers the problem ofmaximising throughput in an interference dominated channel. Cooperative broadcasting is a theoretical technique to mitigate this problem. Its implementation in practical systems requires efficient resource allocati.on to maximise the thr(oughput whilst meeting system and user-imposed constramts. A resource allocation approach is presented for implementing cooperative broadcasting. Users are paired and a teclmique for allocating resources between the pair is developed. Then, a method for pairing the users is considered. Simulation results are presented, which show a throughput improvement over existing resource allocation approaches. The problem ofcontrolling the distribution ofrandomly arriving data to meet the resource allocation specifications is examined. A single-threshold buffer is proposed, which requires fewer calculations than an existing double-threshold buffer. Simulation results are presented which show a throughput improvement may be realised, greater than that which would achievable using other rate control schemes. Cooperative broadcasting may lead to transmissions to some users being allocated low power. When full channel infonnation is available at the transmitter, a water filling solution may be used to maximise capacity. However, when combined with buffer management, erasure may result. This erasure may be overcome using an erasure protection code. Such a code is examined. When combined with Turbo coding, ajoint detector may be used for providing error and erasure protection. Analysis ofthis detector shows a lower limit on the error rate, dependent on the probability of erasure. Simulation results show that using this approach the error rate is significantly improved. This code can then be used to increase capacity, whilst achieving low error rates.Imperial Users onl

Optimising energy efficiency and spectral efficiency in multi-tier heterogeneous networks:performance and tradeoffs

The exponential growth in the number of cellular users along with their increasing demand of higher transmission rate and lower power consumption is a dilemma for the design of future generation networks. The spectral efficiency (SE) can be improved by better utilisation of the network resources at the cost of reduction in the energy efficiency (EE) due to the enormous increase in the network power expenditure arising from the densification of the network. One of the possible solutions is to deploy Heterogeneous Networks (HetNets) consisting of several tiers of small cell BSs overlaid within the coverage area of the macrocells. The HetNets can provide better coverage and data rate to the cell edge users in comparison to the macrocells only deployment. One of the key requirements for the next generation networks is to maintain acceptable levels of both EE and SE. In order to tackle these challenges, this thesis focuses on the analysis of the EE, SE and their tradeoff for different scenarios of HetNets. First, a joint network and user adaptive selection mechanism in two-tier HetNets is proposed to improve the SE using game theory to dynamically re-configure the network while satisfying the user's quality-of-service (QoS) requirements. In this work, the proposed scheme tries to offload the traffic from the heavily loaded small cells to the macrocell. The user can only be admitted to a network which satisfies the call admission control procedures for both the uplink and downlink transmission scheme. Second, an energy efficient resource allocation scheme is designed for a two-tier HetNets. The proposed scheme uses a low-complexity user association and power allocation algorithm to improve the uplink system EE performance in comparison to the traditional cellular systems. In addition, an opportunistic joint user association and power allocation algorithm is proposed in an uplink transmission scheme of device to device (D2D) enabled HetNets. In this scheme, each user tries to maximise its own Area Spectral Efficiency (ASE) subject to the required Area Energy Efficiency (AEE) requirements. Further, a near-optimal joint user association and power allocation approach is proposed to investigate the tradeoff between the two conflicting objectives such as achievable throughput and minimising the power consumption in two-tier HetNets for the downlink transmission scheme. Finally, a multi-objective optimization problem is formulated that jointly maximizes the EE and SE in two-tier HetNets. In this context, a joint user association and power allocation algorithm is proposed to analyse the tradeoff between the achievable EE and SE in two-tier HetNets. The formulated problem is solved using convex optimisation methods to obtain the Pareto-optimal solution for the various network parameters

STAR-RIS Assisted Cell-Free Massive MIMO System Under Spatially-Correlated Channels

peer reviewedThis paper investigates the performance of downlink simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-assisted cell-free (CF) massive multiple-input multiple-output (mMIMO) systems, where user equipments (UEs) are located on both sides of the RIS. We account for correlated Rayleigh fading and multiple antennas per access point (AP), while the maximum ratio (MR) beamforming is applied for the design of the active beamforming in terms of instantaneous channel state information (CSI). Firstly, we rely on an aggregated channel estimation approach that reduces the overhead required for channel estimation while providing sufficient information for data processing. We obtain the normalized mean square error (NMSE) of the channel estimate per AP, and design the passive beamforming (PB) of the surface based on the long-time statistical CSI. Next, we derive the received signal in the asymptotic regime of numbers of APs and surface elements. Then, we obtain a closedform expression of the downlink achievable rate for arbitrary numbers of APs and STAR-RIS elements under statistical CSI. Finally, based on the derived expressions, the numerical results show the feasibility and the advantages of deploying a STARRIS into conventional CF mMIMO systems. In particular, we theoretically analyze the properties of STAR-RIS-assisted CF mMIMO systems and reveal explicit insights in terms of the impact of channel correlation, the number of surface elements, and the pilot contamination on the achievable rate

Performance of Adaptive Satellite Antenna Array Processing and Comparison with Optimal Multi-User Communications

Eine Methode eine hohe spektrale Effizienz in Satellitenkommunikationssystemen zu erzielen, ist die koordinierte Wiederverwendung von Frequenzen in ausreichend voneinander entfernten geographischen Regionen. Die Frequenzwiederverwendung und damit die spektrale Effizienz können durch den Einsatz von Maßnahmen maximiert werden, die die Interferenz von nahe benachbarten Gleichkanal-Nutzern effizienter unterdrücken. In der Literatur kann man zwei fundamental unterschiedliche Ansätze finden, die eine bessere Unterdrückung der Interferenz von Gleichkanal-Nutzern ermöglichen. Der erste Ansatz fußt auf Dekodierungsmethoden, die durch die informationstheoretische Beschreibung des Mehrfachzugriffskanals (multiple access channel (MAC)) nahegelegt werden. Als eine der Möglichkeiten stellt Sukzessive Dekodierung (successive decoding) dabei eine attraktive Alternative zu komplexeren Verfahren dar, weil die Komplexität einerseits nur linear mit der Zahl der Quellen ansteigt und weil mit gewissen Einschränkungen dieselben Informationsraten erreicht werden können wie mit der optimalen Verbunddekodierung. Der zweite Ansatz versucht die räumliche Verteilung der interferierenden Gleichkanal-Nutzer effizient zu nutzen, indem eine Gruppenantenne (antenna array) zusammen mit adaptiver Strahlformung (adaptive beamforming) am Satelliten eingesetzt wird. Folgende Fragestellungen ergeben sich innerhalb des oben beschriebenen Rahmens: Ist es möglich, die oben beschriebenen Empfängervarianten, die entweder feste oder adaptive Strahlformung, bzw. entweder unabhängige oder sukzessive Dekodierung anwenden, in einer einheitlichen Weise zu beschreiben? Gibt es eine einfache Lösung für das Problem der Ressourcenzuteilung, die Sendeleistungen den Quellen zuzuordnen, welche notwendig sind, um eine bestimmte Informationsrate für alle Quellen zu erzielen? Wie ist die Leistungsfähigkeit eines Empfängers, der adaptive Strahlformung und/oder sukzessive Dekodierung anwendet, im Vergleich zu einem Empfänger, der feste Strahlformung und unabhängige Dekodierung einsetzt, wie es heutzutage der Standard ist? Wie nahe liegt die Leistung des Empfängers, der sowohl adaptive Strahlformung, als auch sukzessive Dekodierung verwendet, an der des Empfängers, der nur entweder adaptive Strahlformung oder sukzessive Dekodierung verwendet? Ist der Empfänger, der feste Strahlformung mit sukzessiver Dekodierung implementiert, besser als der, der adaptive Strahlformung mit unabhängiger Dekodierung implementiert, oder umgekehrt? Wie hängen die Antworten auf obige Fragen ab von der jeweiligen Quellenverteilung und dem daraus resultierenden Interferenzszenario

Decentralised Distributed Massive MIMO

In this thesis, decentralised distributed massive multiple-input multiple-output (DD-MaMIMO) is considered for providing high spectral efficiency (SE) per user. In the DD-MaMIMO system, a large number of access points (APs) within a coordination region are connected to an edge processing unit (EPU) via fronthaul links, serving the users within a service region. Initially, we investigate a DD-MaMIMO system with perfect fronthaul links and assume that the processing takes place in the EPU. To demonstrate the improved SE, we compare our proposed architecture to cell-free MaMIMO. Furthermore, we discuss the scalability of DD-MaMIMO and give its definition. Secondly, we extend our research to the limited-capacity fronthaul links which is essential in practice. To model the limited-capacity fronthaul links, we adopt the Bussgang decomposition to express the quantisation. We propose two strategies for obtaining channel state information (CSI): estimate-and-quantise (EQ) and quantise-and-estimate (QE). Particularly, in the QE scheme, we derive the closed-form expressions of Bussgang decomposition coefficients for the non-Gaussian distribution input of the quantiser, as the elements of pilots follow complex Gaussian distribution. Both CSI acquisition strategies are analysed with respect to the mean square error (MSE) of channel estimation. Finally, we explore the processing which happens at the AP which is the local estimation in DD-MaMIMO. Here, two approaches are exploited for data decoding at the EPU: simply averaging decoding and large scale fading decoding. We further compare the local estimation scheme with the decentralised processing scheme. The scalability is also discussed as the channel estimation and data detection happens at the AP