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In this thesis, multicell coordination for wireless cellular networks is studied, whereby various approaches have been conducted to tackle this issue. Firstly, the coverage probability and e ective capacity in downlink multiple-input multiple-output (MIMO) cellular system are considered. Two scenarios are investigated; in the rst scenario, it is assumed that the system employs distance-based fractional power control with no multicell coordination. For the second scenario, it is assumed that the system implements multicell coordinated beamforming so as to cancel inter-cell interference. The base stations (BS) are modelled as randomly uniformly distributed in the area according to Poisson point process (PPP). Using tools from stochastic geometry, tractable, analytical expressions for coverage probability and e ective capacity are derived for both scenarios. Secondly, an adaptive strategy for inter-cell interference cancellation and coordination is proposed for downlink multicarrier cellular random networks. The adaptive strategy coordinates and cancels the interference on the both frequency and spatial domains. Based on this adaptive strategy, two interference management schemes have been proposed. The adaptation process is implemented based on measured instantaneous signal-to-interference and noise ratio (SINR) of the considered user. Furthermore, the locations of base stations BSs are modelled as an independent spatial PPP. Using tools from stochastic geometry, the proposed schemes have been analytically evaluated. Analytical expressions for coverage probability are derived for both schemes. In addition, an expression for average rate has been derived using the coverage probability analysis. Thirdly, low complexity algorithms for user scheduling have been proposed for coordinated MIMO multicell network. The algorithms consist of two stages: multicell scheduling stage and precoding stage. The algorithm works on sequential distributive manner. Two variants of multicell scheduling are proposed. The rst algorithm has less complexity but leads to more di erence in sum rate among cells. While the second algorithm results in better fairness in terms of system performance but causes frequent signalling among the cells. Moreover, the algorithm is extended to multimode selection in addition to the user selection. Finally, an adaptive coordination scheme for energy-effeicient resource allocation has been developed for orthogonal frequency division multiple access (OFDMA) cellular networks. The proposed scheme consists of centralised and distributed stages for allocating resources to cell-edge and cell-centre users, respectively. The optimisation problems are formulated as integer linear fractional and integer linear problems for the first stage and second stages, respectively. The spectral-energy trade-o is analysed under the constraint of fairness among users. In summary, the research work presented in this thesis reveals statistical approach to analyse the multicell coordination in random cellular networks. It also offers insight into the resource allocation and scheduling problems within multicell coordination framework, and how to solve them with a certain objective.

Cellular systems are interference limited in nature. This problem is further accentuated in upcoming commercial wireless standards, which intend to use all the available spectrum in every cell in the network to improve peak data rates. This, however, could lead to considerable interference among neighboring cells, decreasing data rates and causing outages at the cell-edge. Multicell cooperation offers a solution for reducing the high levels of interference. The basic idea is that base stations coordinate transmissions by sharing user information among themselves via backhaul links. With the backhaul being bandwidth limited, cooperative strategies that involve the exchange of only user channel state information (CSI) among base stations offer the best tradeoff between complexity, backhaul load and performance gains. This dissertation focuses on these partial cooperative techniques, known as coordinated beamforming in 3GPP LTE Advanced. In existing frequency division duplex systems, users estimate and feedback the CSI of a single channel over a finite-bandwidth feedback link, using limited feedback techniques. In a multicell cooperative scenario, each user needs to transmit the CSI of multiple channels using the same feedback link. This implies that the available feedback bandwidth must be efficiently shared among different channels to maximize performance gains in the cellular network. This dissertation develops three different approaches to limited feedback in multicell cooperative systems. The first technique, separate quantization, involves each channel being fed back individually using a different codebook. Closed-form expressions are derived to partition adaptively the available feedback bits, as a function of the signal strengths and delays associated with each of the multiple channels. The second strategy is known as joint quantization, where the CSI of all the channels are quantized together as a composite vector. It is shown that though this approach yields higher data rates with smaller feedback requirements than separate quantization, it requires the design and storage of special codebooks. Finally, predictive joint quantization is proposed to exploit the temporal correlation of the wireless channel to reduce feedback requirements significantly as compared to the other two strategies, at the cost of high complexity at the user terminals. / text

The use of multiple antennas at base stations and user devices is a key component in the design of cellular communication systems that can meet the capacity demands of tomorrow. The downlink transmission from base stations to users is particularly limiting, both from a theoretical and a practical perspective, since user devices should be simple and power-efficient, and because many applications primarily create downlink traffic (e.g., video streaming). The potential gain of employing multiple antennas for downlink transmission is well recognized: the total data throughput increases linearly with the number of transmit antennas if the spatial dimension is exploited for simultaneous transmission to multiple users. In the design of practical cellular systems, the actual benefit of multiuser multiantenna transmission is limited by a variety of factors, including acquisition and accuracy of channel information, transmit power, channel conditions, cell density, user mobility, computational complexity, and the level of cooperation between base stations in the transmission design. The thesis considers three main components of downlink communications: 1) estimation of current channel conditions using training signaling; 2) efficient feedback of channel estimates; and 3) allocation of transmit resources (e.g., power, time and spatial dimensions) to users. In each area, the thesis seeks to provide a greater understanding of the interplay between different system properties. This is achieved by generalizing the underlying assumptions in prior work and providing both extensions of previous outcomes and entirely new mathematical results, along with supporting numerical examples. Some of the main thesis contributions can be summarized as follows. A framework is proposed for estimation of different channel quantities using a common optimized training sequence. Furthermore, it is proved that each user should only be allocated one data stream and utilize its antennas for receive combining and interference rejection, instead of using the antennas for reception of multiple data streams. This fundamental result is proved under both exact channel acquisition and under imperfections from channel estimation and limited feedback. This also has positive implications on the hardware and system design. Next, a general mathematical model is proposed for joint analysis of cellular systems with different levels of base station cooperation. The optimal multicell resource allocation can in general only be found with exponential computational complexity, but a systematic algorithm is proposed to find the optimal solution for the purpose of offline benchmarking. A parametrization of the optimal solution is also derived, creating a foundation for heuristic low-complexity algorithms that can provide close-to-optimal performance. This is exemplified by proposing centralized and distributed multicell transmission strategies and by evaluating these using multicell channel measurements. / In reference to IEEE copyrighted material which is used with permission in this thesis, the IEEE does not endorse any of KTH Royal Institute of Technology's products or services. Internal or personal use of this material is permitted. If interested in reprinting/republishing IEEE copyrighted material for advertising or promotional purposes or for creating new collective works for resale or redistribution, please go to http://www.ieee.org/publications_standards/publications/rights/rights_link.html to learn how to obtain a License from RightsLink.QC 20111026

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24211号 / 工博第5039号 / 新制||工||1787(附属図書館) / 京都大学大学院工学研究科社会基盤工学専攻 / (主査)教授 中北 英一, 准教授 山口 弘誠, 教授 田中 賢治 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Conventional multicell wireless systems operate with out-of-cell interference treated as background noise; consequently, their performance faces two major limitations: 1)Signal processing is performed on a per-cell basis; and 2)Intercell interference detection is infeasible as intercell interference, although significantly above the noise level, is typically quite weak. In this thesis, we consider a multicell downlink scenario, where base-stations are equipped with multiple transmit antennas, the remote users are equipped with a single antenna, and multiple remote users are active simultaneously via spatial division multiplexing. We propose solutions for the above limitations by considering techniques for mitigating interference. The first part of the thesis proposes solutions for the first limitation. It considers the benefit of coordinating base-stations across multiple cells, where multiple base-stations may jointly optimize their respective beamformers to improve the overall system performance. It focuses on the design criteria of minimizing either the total weighted transmitted power or the maximum per-antenna power across the base-stations subject to signal-to-interference-and-noise-ratio (SINR) constraints at the remote users. The main contribution of this part is an efficient algorithm for finding the joint globally optimal beamformers across all base-stations. The proposed algorithm is based on a generalization of uplink-downlink duality to the multicell setting using the Lagrangian duality theory. An important feature is that it naturally leads to a distributed implementation in time-division duplex (TDD) systems. Simulation results suggest that coordinating the beamforming vectors alone already provides appreciable performance improvements as compared to the conventional per-cell optimized network. The second part of the thesis considers the transmission of both private and common messages for the sole purpose of intercell interference mitigation. It solves the issues of the second limitation mentioned above. It considers the benefit of designing decodable interference signals by allowing common-private message splitting at the transmitter and common message decoding by users in adjacent cells. It solves a network optimization problem of jointly determining the appropriate users in adjacent cells for rate splitting, the optimal beamforming vectors for both common and private messages, and the optimal common-private rates to minimize the total transmit power across the base-stations subject to service rate requirements for remote users. Observe that for fixed user selection and fixed common-private rate splitting, the optimization of beamforming vectors can be performed using a semidefinite programming approach. Further, this part of the thesis proposes a heuristic user-selection and rate splitting strategy to maximize the benefit of common message decoding. This part proposes a heuristic algorithm to characterize the improvement in the feasible rates with common-message decoding. Simulation results show that common message decoding can significantly improve both the total transmit power and the feasibility region for cell-edge users when base-stations are closely spaced from each other.

Conventional multicell wireless systems operate with out-of-cell interference treated as background noise; consequently, their performance faces two major limitations: 1)Signal processing is performed on a per-cell basis; and 2)Intercell interference detection is infeasible as intercell interference, although significantly above the noise level, is typically quite weak. In this thesis, we consider a multicell downlink scenario, where base-stations are equipped with multiple transmit antennas, the remote users are equipped with a single antenna, and multiple remote users are active simultaneously via spatial division multiplexing. We propose solutions for the above limitations by considering techniques for mitigating interference. The first part of the thesis proposes solutions for the first limitation. It considers the benefit of coordinating base-stations across multiple cells, where multiple base-stations may jointly optimize their respective beamformers to improve the overall system performance. It focuses on the design criteria of minimizing either the total weighted transmitted power or the maximum per-antenna power across the base-stations subject to signal-to-interference-and-noise-ratio (SINR) constraints at the remote users. The main contribution of this part is an efficient algorithm for finding the joint globally optimal beamformers across all base-stations. The proposed algorithm is based on a generalization of uplink-downlink duality to the multicell setting using the Lagrangian duality theory. An important feature is that it naturally leads to a distributed implementation in time-division duplex (TDD) systems. Simulation results suggest that coordinating the beamforming vectors alone already provides appreciable performance improvements as compared to the conventional per-cell optimized network. The second part of the thesis considers the transmission of both private and common messages for the sole purpose of intercell interference mitigation. It solves the issues of the second limitation mentioned above. It considers the benefit of designing decodable interference signals by allowing common-private message splitting at the transmitter and common message decoding by users in adjacent cells. It solves a network optimization problem of jointly determining the appropriate users in adjacent cells for rate splitting, the optimal beamforming vectors for both common and private messages, and the optimal common-private rates to minimize the total transmit power across the base-stations subject to service rate requirements for remote users. Observe that for fixed user selection and fixed common-private rate splitting, the optimization of beamforming vectors can be performed using a semidefinite programming approach. Further, this part of the thesis proposes a heuristic user-selection and rate splitting strategy to maximize the benefit of common message decoding. This part proposes a heuristic algorithm to characterize the improvement in the feasible rates with common-message decoding. Simulation results show that common message decoding can significantly improve both the total transmit power and the feasibility region for cell-edge users when base-stations are closely spaced from each other.

Thesis (PhD (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The multicell inverter, being a widely used multilevel converter, has received much attention in recent years due to problems associated with cell capacitor voltage. In this dissertation we study the balancing problem with a focus on steady-state unbalance. This is achieved by systematic and mathematically rigorous study of the natural balancing mechanisms of the three-phase 2-cell and 3-cell multicell converter, undertaken by using dynamic modelling of the multicell converter, Bennet’s geometric model, steady-state and time constant analysis. Space vector analysis is also performed for the three-phase 2-cell multicell converter. The theory is verified by comparing theoretical results with simulation results / AFRIKAANSE OPSOMMING: Die multisel omkeerder as algemeen-gebruikte meervlakkige omsetter het die afgelope jare groot belangstelling gewek op grond van die probleme wat met selkapasitor stroomspanning geassosieer word. In hierdie proefskrif word die balanseringsprobleem met die klem op die ewewigswanbalans bestudeer. Dit is verrig deur ’n sistematiese en streng wiskundige studie van die natuurlike balanseringsmeganismes van die drie-fase 2-sel en 3- sel multisel omsetter te maak. Dit is gedoen deur die gebruik van dinamiese modellering van die multisel omsetter, Bennet se geometriese model, ewewigtoestand tydkonstante analises, en ruimtevektoranalise is vir die drie-fase 2-sel multisel omsetter gedoen. Die teorie word bevestig deur die teoretiese resultate met die simuleringsresultate te vergelyk

This thesis looks at new methods of achieving reliable data communication in wireless communication systems using different antenna transmission optimization methods. In particular, the problems of exploitation of MIMO communication channel diversity, secure downlink beamforming techniques, adaptive beamforming techniques, resource allocation methods, simultaneous power and information transfer and energy harvesting within the context of multi-antenna wireless systems are addressed.

In multicell coordinated networks where multiple base stations cooperate to jointly combat interference from adjacent cells and fading to receivers, one of the outstanding questions is what is the role of receive antenna and receiver processing. Multiple receive antennas not only enable additional degrees of freedom at each receiver to combat the other-cell interference but also can change the transmitter design because transmitter and receiver beamforming design is often closely coordinated. In this dissertation, we investigate the role of the multiple receive antennas in multicell cooperative systems under different interference conditions. We then present novel non-iterative and iterative coordinated beamforming and precoding algorithms with different receiver processing. We present comprehensive performance comparison of various multicell cooperative systems and explore the feasibility of achieving much higher throughput via hyper-densification of heterogeneous and small cell networks with mandatory multicell cooperation. / text