Approaching universal frequency reuse through base station cooperation

Base Station (BS) architectures are a promising cellular wireless solution to mitigate the interference issues and to avoid the high frequency reuse factors implemented in conventional systems. Combined with block transmission techniques, such as Orthogonal Frequency-Division Multiplexing (OFDM) for the downlink and Single-Carrier with Frequency-Domain Equalization (SC-FDE) for the uplink, these systems provide a significant performance improvement to the overall system. Block transmission techniques are suitable for broadband wireless communication systems, which have to deal with strongly frequency-selective fading channels and are able to provide high bit rates despite the channel adversities. In BS cooperation schemes users in adjacent cells share the same physical channel and the signals received by each BS are sent to a Central Processing Unit (CPU) that combines the different signals and performs the user detections and/or separation, which can be regarded as a Multi-User Detection (MUD) technique. The work presented in this thesis is focused on the study of uplink transmissions in BS cooperations systems, considering single carrier block transmission schemes and iterative receivers based on the Iterative-Block Decision Feedback Equalization (IB-DFE) concept, which combined with the employment of Cyclic Prefix (CP)-assisted block transmission techniques are appropriate to scenarios with strongly time-dispersive channels. Furthermore, the impact of the sampling and quantization applied to the received signals from each Mobile Terminal (MT) to the corresponding BS is studied, with the achievement of the spectral characterization of the quantization noise. This thesis also provides a conventional analytical model for the BER (Bit Error Rate) performance complemented with an approach to improve its results. Finally, this thesis addresses the contextualization of BS cooperation schemes in clustered C-RAN (Centralized-Radio Access Network)-type solutions.As arquitecturas BS cooperation são uma solução promissora de redes celulares sem fios para atenuar o problema da interferência e evitar os factores de reuso elevados, que se encontram implementados nos sistemas convencionais. Combinadas com técnicas de transmissão por blocos, como o OFDM para o downlink e o SC-FDE no uplink, estes sistemas fornecem uma melhoria significativa no desempenho geral do sistema. Técnicas de transmissão por blocos são adequadas para sistemas de comunicações de banda larga sem fios, que têm que lidar com canais que possuem um forte desvanescimento selectivo na frequência e são capazes de fornecer ligações com taxas de transmissão altas apesar das adversidades do canal. Em esquemas BS cooperation os terminais móveis situados em células adjacentes partilham o mesmo canal físico e os sinais recebidos em cada estação de base são enviados para uma Unidade Central de Processamento (CPU) que combina os diferentes sinais recebidos associados a um dado utilizador e realiza a detecção e/ou separação do mesmo, sendo esta considerada uma técnica de Detecção Multi-Utilizador (MUD). O trabalho apresentado nesta tese concentra o seu estudo no uplink de transmissões em sistemas BS cooperation, considerando transmissões em bloco de esquemas monoportadoras e receptores iterativos baseados no conceito B-DFE, em que quando combinados com a implementação de técnicas de transmissao por blocos assistidas por prefixos cíclicos (CP) são apropriados a cenários com canais fortemente dispersivos no tempo. Além disso, é estudado o impacto do processo de amostragem e quantização aplicados aos sinais recebidos de cada terminal móvel para a estação de base, com a obtenção da caracterização espectral do ruído de quantização. Esta tese também fornece um modelo analítico convencional para a computação do desempenho da taxa de erros de bit (BER), com um método melhorado para o mesmo. Por último, esta tese visa a contextualização dos sistemas BS cooperation em soluções do tipo C-RAN

Performance analysis of massive MIMO receivers

We face now an exponential increase in wireless devices and to allow good user experience, it is imperative that the next generation of mobile (5G) communications ensures reliable connections, high data transfer rates and low latency. One way to increase the data transfer rate is to use massive Multiple-Input, Multiple-Output (MIMO) systems, that is, systems with multiple antennas to emit and multiple antennas to receive thus allowing spatial diversity. In these systems, to increase the battery life of the devices it is preferable to use the Single-Carrier with Frequency-Domain Equalization modulation in the uplink as this modulation reduces the complexity in the emitter, transferring it to the receiver, in this case the Base Staion, where it is quite acceptable. This dissertation studies the performance of massive MIMO receiver systems, comparing it to the performance achieved with the Matched Filter Bound (MFB). The Iterative Block Decision-Feedback Equalizer (IB-DFE) receiver presents a very similar performance to the MFB, however, the algorithm requires matrix inversions, which in the systems under study, where the size of the matrix is high, implies an increase of the associated operations increases. Thus it is very important that low complexity receivers, such as the Maximal-Ratio Combining (MRC) or Equal Gain Combining are used. In this dissertation, a simple receiver is proposed combining the IB-DFE receiver with the MRC receiver, thus creating a low complexity receiver with excellent performanceAtualmente, sente-se um aumento exponencial nos dispositivos wireless. De modo a permitir uma boa experiência por parte dos utilizadores é fundamental que a próxima geração de comunicações móveis (5G) assegure fiabilidade nas ligações, uma elevada taxa de transferência de dados e baixa latência. Uma maneira de elevar a taxa de transferência de dados é utilizar sistemas massive Multiple-Input, Multiple-Output (MIMO), ou seja, sistemas com múltiplas antenas a emitir e múltiplas antenas a receber permitindo assim diversidade espacial. Nestes sistemas, para aumentar a bateria dos dispositivos é preferível usar no uplink a modulação Single-Carrier with Frequency-Domain Equalization pois esta modulação reduz a complexidade no emissor transferindo-a para o recetor, neste caso na Base Station, onde isso é bastante aceitável. Esta dissertação estuda o desempenho dos recetores dos sistemas massive MIMO, comparando o desempenho alcançado com o desempenho do Matched Filter Bound (MFB). O recetor Iterative Block Decision-Feedback Equalizer (IBDFE) apresenta um desempenho muito semelhante ao do MFB no entanto, o algoritmo do receptor inverte matrizes, o que nos sistemas em estudo, onde o tamanho das matrizes é elevado, se reflecte no aumento da complexidade das operações associadas. Deste modo, é importante que sejam utilizados recetores de baixa complexidade tal como o Maximal-Ratio Combining (MRC) ou o Equal Gain Combining. Esta dissertação propõe um recetor simples que combina um recetor IB-DFE com um recetor MRC, criando desde modo um recetor de baixa complexidade e com excelente desempenho

Reduced complexity detection in MIMO systems with SC-FDE modulations and iterative DFE receivers

This paper considers a Multiple-Input Multiple-Output (MIMO) system with P transmitting and R receiving antennas and different overall noise characteristics on the different receiver antennas (e.g., due to nonlinear effects at the receiver side). Each communication link employs a Single-Carrier with Frequency-Domain Equalization (SC-FDE) modulation scheme, and the receiver is based on robust iterative frequency-domain multi-user detectors based on the Iterative Block Decision Feedback Equalization (IB-DFE) concept. We present low complexity efficient receivers that can employ low resolution Analog-to-Digital Converters (ADCs) and require the inversion of matrices with reduced dimension when the number of receive antennas is larger than the number of independent data streams. The advantages of the proposed techniques are particularly high for highly unbalanced MIMO systems, such as in the uplink of Base Station (BS) cooperation systems that aim for Single-Frequency Network (SFN) operation or massive MIMO systems with much more antennas at the receiver side.publishe

Nonlinear effects of radio over fiber transmission in base station cooperation systems

In this paper we consider the uplink of Base Station (BS cooperation) systems, where each Mobile Terminal (MT) employs a Single-Carrier with Frequency-Domain Equalization (SC-FDE) modulation scheme. The combined signals at each BS are detected and/or separated by a Central Processing Unit (CPU) with Iterative Block Decision Feedback Equalization (IB-DFE) receivers. We consider a Radio-over-Fiber (RoF) link between the BS and the CPU, the electrical and optical conversions are performed by a Mach-Zehnder (MZ) modulator, which introduces nonlinear distortion. We design robust receivers that take advantage of the statistical characteristics of the nonlinear distortion.info:eu-repo/semantics/acceptedVersio

Performance Evaluation of Low Complexity Massive MIMO Techniques for SC-FDE Schemes

Massive-MIMO technology has emerged as a means to achieve 5G's ambitious goals; mainly to obtain higher capacities and excellent performances without requiring the use of more spectrum. In this thesis, focused on the uplink direction, we make a study of performance of low complexity equalization techniques as well as we also approach the impact of the non-linear elements located on the receivers of a system of this type. For that purpose, we consider a multi-user uplink scenario through the Single Carrier with Frequency Domain Equalization (SC-FDE) scheme. This seems to be the most appropriate due to the low energy consumption that it implies, as well as being less favorable to the detrimental effects of high envelope fluctuations, that is, by have a low Peak to Average Power Ratio (PAPR) comparing to other similar modulations, such as the Orthogonal Frequency Division Multiplexing (OFDM). Due to the greater number of antennas and consequent implementation complexity, the equalization processes for Massive- MIMO schemes are aspects that should be simplified, that is, they should avoid the inversion of matrices, contrary to common 4G, with the Zero Forcing (ZF) and Minimum Mean Square Error (MMSE) techniques. To this end, we use low-complexity techniques, such as the Equal Gain Combining (EGC) and the Maximum Ratio Combining (MRC). Since these algorithms are not sufficiently capable of removing the entire Inter-Symbol Interference (ISI) and Inter-User Interference (IUI), we combine them with iterative techniques, namely with the Iterative Block with Decision Feedback Equalizer (IB-DFE) to completely remove the residual ISI and IUI. We also take into account the hardware used in the receivers, since the effects of non-linear distortion can impact negatively the performance of the system. It is expected a strong performance degradation associated to the high quantization noise levels when implementing low-resolution Analog to Digital Converters (ADCs). However, despite these elements with these configurations become harmful to the performance of the majority of the systems, they are considered a desirable solution for Massive-MIMO scenarios, because they make their implementation cheaper and more energy efficient. In this way, we made a study of the impact in the performance by the low-resolution ADCs. In this thesis we suggest that it is possible to bypass these negative effects by implementing a number of receiving antennas far superior to the number of transmitting antennas

Receiver design for the uplink of base station cooperation systems employing SC-FDE modulations

The presented paper considers the uplink transmission in base station (BS) cooperation schemes where mobile terminals (MTs) in adjacent cells share the same physical channel. We consider single-carrier with frequency-domain equalization (SC-FDE) combined with iterative frequency-domain receivers based on the iterative block decision feedback equalization (IB-DFE). We study the quantization requirements when sending the received signals, from different MTs, at different BSs to a central unit that performs the separation of different MTs using iterative frequency-domain receivers. Our performance results show that a relatively coarse quantization, with only 4 bits in the in-phase and quadrature components of the complex envelope already allows close-to-optimum macro-diversity gains, as well as an efficient separation of the transmitted signals associated with each MT

Performance evaluation of IB-DFE-based strategies for SC-FDMA systems

The aim of this paper is to propose and evaluate multi-user iterative block decision feedback equalization (IB-DFE) schemes for the uplink of single-carrier frequency-division multiple access (SC-FDMA)-based systems. It is assumed that a set of single antenna users share the same physical channel to transmit its own information to the base station, which is equipped with an antenna array. Two space-frequency multi-user IB-DFE-based processing are considered: iterative successive interference cancellation and parallel interference cancellation. In the first approach, the equalizer vectors are computed by minimizing the mean square error (MSE) of each individual user, at each subcarrier. In the second one, the equalizer matrices are obtained by minimizing the overall MSE of all users at each subcarrier. For both cases, we propose a simple yet accurate analytical approach for obtaining the performance of the discussed receivers. The proposed schemes allow an efficient user separation, with a performance close to the one given by the matched filter bound for severely time-dispersive channels, with only a few iterations

Towards a new cloud-based planning and optimization methodology for mobile communication networks

The great concern of telecommunication operators to offer high-quality services to their customers requires a constant care with the state of the networks. These networks can present some problems that imply that the experience offered to customers is unsatisfactory. In order to monitor these situations, operators collect, on a fairly regular basis, data, like drive tests, that allow them to monitor and correct minor issues. This thesis takes advantage of the data collected and uses it in network planning in order to precisely obtain the coverage estimation of a network. In order to automate failure correction mechanisms, a totally automatic propagation model is presented, which precisely describes the state of the network, allowing it to be used for network planning and optimisation. After its implementation, the model was compared to a second model, generated through Artificial Intelligence, which is completely agnostic to all telecommunications knowledge. These models, for the considered scenarios, reached average absolute errors between estimated and actual values of 6.1 dB with a standard deviation of 4 dB. The existence of several real telecommunication network measures and their evolution to Multiple Input, Multiple Output (MIMO) systems motivated not only the investigation on the coverage impact with the change from a Single Input, Single Output (SISO) to a MIMO system, but also the investigation on the reduction of complexity of the receivers used in MIMO systems. The closer the Bit Error Rate performance of the receiver is to the Matched Filter Bound, the smaller will be the reduction in the coverage area with the transition from a SISO system to a MIMO system.A grande preocupação dos operadores de telecomunicações em oferecerem serviços de alta qualidade aos seus clientes leva a um constante cuidado com o estado das redes. Estas redes podem apresentar alguns problemas que implicam que a experiência oferecida aos clientes seja desagradável. De forma a monitorizar estas situações, os operadores recolhem, com bastante regularidade, dados, como "drive tests", que lhes permitem avaliar e corrigir pequenos problemas. Esta tese aproveita os dados recolhidos e utiliza-os no planeamento da rede de forma a obter fielmente a estimativa de cobertura de uma rede. De forma a automatizar mecanismos de correção de falhas, é apresentado um modelo de propagação completamente automático, que descreve de forma precisa o estado da rede permitindo que seja aplicado em algoritmos de planeamento e otimização da rede. Após a sua implementação, este modelo foi comparado com um segundo modelo, gerado através de inteligência artificial, que é completamente agnóstico a todo o conhecimento de telecomunicações. Estes modelos, para os cenários estudados, atingiram erros absolutos médios entre os valores estimados e os valores reais de 6.1 dB com um desvio padrão de 4 dB. A existência de diversos dados reais das redes de telecomunicações e a evolução para os sistemas "Multiple Input", "Multiple Output" (MIMO) motivou não só a investigação no impacto da cobertura com a mudança de um sistema "Single Input", "Single Output" (SISO) para um sistema MIMO, mas também a investigação na redução de complexidade dos recetores utilizados em sistemas MIMO. Quanto mais próxima a "Bit Error Rate performance" do recetor estiver do "Matched Filter Bound", menor será a redução na área de cobertura com a transição de um sistema SISO para um sistema MIMO

Multipacket reception in LTE femtocell networks

Dissertação apresentada para obtenção do Grau de Mestre em Engenharia Electrotécnica e de ComputadoresDriven by the growing demand for high-speed broadband wireless services, LTE technology has emerged and evolve, promising high data rates to the demanding mobile users. Based on the 3rd Generation Partnership Project (3GPP) speci cations,Long Term Evo- lution Advanced (LTE-A) telecommunication services predict the existence of macro base stations, Enhanced Node B (eNB) and micro stations HeNB with low power that complements the network's coverage. This dissertation studies the complementary use of HeNBs (femtocells 3GPP terminology) to provide broadband services. It is essential to maintain the networks performance with the network densi cation phenomenon, which brings signi cant interference problems and consequently more collisions and lost packets. The use of SC-FDE in the downlink of a LTE-A femtocell network - speci cally multipacket reception (MPR), with an IB-DFE receiver employing Multipacket Detection (MPD) and SIC techniques is proposed. A new telecommunications concept named GC emerged with the increasing environmental concerns. This dissertation shows the performance results of an iterative MPR and proposes a green association algorithm to change the network layout according to the mobile users demands reducing the Base Station (BS)'s negative contribution to the network total energy consumption. The overall results show that the technologies employed are a solution to achieve a favorable trade-o between performance and Energy E ciency (EE), responding to the global demands (high data rates) and concerns (low energy consumption and carbon footprint reduction). Keywords: Long Term Evolution(LTE), Single Carrier with Frequency Domain Equalization (SC-FDE), Iterative Block-Decision Feedback Equalizer (IB-DFE), Home enhanced Node B (HeNB), Successive Interference Cancellation(SIC),Multipacket Reception(MPR), Green Communications (GC)FCT/MEC Femtocells(PTDC/EEATEL/120666/2010), OPPORTUNISTIC CR(PTDC/EEA-TEL/115981/2009) and ADIN(PTDC/EEI-TEL/2990/2012) project

Pré-codificação e equalização para sistemas SC-FDMA heterogéneos

Mestrado em Engenharia Electrónica e TelecomunicaçõesMobile traffic in cellular networks is increasing exponentially. Small-cells are considered as a key solution to meet these requirements. Under the same spectrum the small-cells and the associated macro-cell (forming the so called heterogeneous systems) must cooperate so that one system can adapt to the other. If no cooperation is considered then the small-cells will generate harmful interference at the macro-cell. Interference alignment (IA) is a precoding technique that is able to achieve the maximum degrees of freedom of the interference channel, and can efficiently deal with inter-systems interference. Single carrier frequency division multiple access (SC-FDMA) is a promising solution technique for high data rate uplink communications in future cellular systems. Conventional linear equalizers are not efficient to remove the residual inter-carrier interference of the SC-FDMA systems. For this reason, there has been significant interest in the design of nonlinear frequency domain equalizers in general and decision feedback equalizers in particular, with the iterative block decision feedback equalizer (IB-DFE) being the most promising nonlinear equalizer. In this dissertation we propose and evaluate joint interference alignment precoding at the small cell user terminals with iterative non-linear frequency domain equalizer at the receivers (macro base station and central unit) for SC-FDMA based heterogeneous networks. The small-cell precoders are designed by enforcing that all generated interference at the macro-cell is aligned in an orthogonal subspace to the macro-cell received signal subspace. This enforces that no performance degradation is observed at the macro cell. Then, we design an iterative nonlinear frequency domain equalizer at the macro-cell receiver that is able to recover the macro-cell spatial streams, in the presence of both small-cell and inter-carrier interferences. The results show that the proposed transmitter and receiver structures are robust to the inter-system interferences and at the same time are able to efficient separate the macro and small cells spatial streams.O trafego móvel nas redes celulares tem aumentado exponencialmente. As pico- células são consideradas como a solução chave para cumprir estes requisitos. Dentro do mesmo espectro, as pico-células e as macro-células (formando os chamados sistemas heterogéneos) precisam de colaborar de modo a que um sistema possa adaptar-se ao outro. Se não for considerada a cooperação, então as pico-células irão gerar interferência prejudicial na macro-célula. Interference alignment (IA) é uma técnica de précodificação que é capaz de atingir o grau máximo de liberdade do canal de interferência, e consegue lidar eficazmente com interferência entre sistemas. Single carrier frequency division multiple access (SC-FDMA) é uma solução técnica promissora para transmissão de dados em uplink, para sistemas celulares futuros. Equalizadores lineares convencionais não são eficientes a remover a interferência residual entre portadoras dos sistemas SC-FDMA. Por este motivo, tem havido interesse significativo no desenho de equalizadores não lineares no domínio da frequência em geral e em equalizadores baseados em decisão por feedback em particular, tendo o iterative block decision feedback equalizer (IB-DFE) como o equalizador não linear mais promissor. Nesta dissertação propomos e avaliamos précodificação de alinhamento de interferência nos terminais das pico-células em conjunto com equalizadores não lineares no domínio da frequência nos recetores (estação base da macro-célula e unidade central de processamento) para redes heterogéneas baseadas em SC-FDMA. Os précodificadores das pico-células são desenhados de maneira a obrigar a que toda a interferência gerada na macro-célula esteja alinhada num subespaço ortogonal em relação ao subespaço do sinal recebido na macro- célula. Isto obriga a que não seja observada degradação de desempenho na macro-célula. Em seguida, desenhamos um equalizador não linear no domínio da frequência no recetor da macro-célula capaz de recuperar os fluxos de dados da macro-célula, na presença de interferência tanto entre portadoras como das pico-células. Os resultados mostram que as estruturas de transmissão e receção propostas são robustas contra a interferência entre sistemas e ao mesmo tempo capaz de separar eficientemente os dados da macro e das pico células