How Much Can D2D Communication Reduce Content Delivery Latency in Fog Networks with Edge Caching?

A Fog-Radio Access Network (F-RAN) is studied in which cache-enabled Edge Nodes (ENs) with dedicated fronthaul connections to the cloud aim at delivering contents to mobile users. Using an information-theoretic approach, this work tackles the problem of quantifying the potential latency reduction that can be obtained by enabling Device-to-Device (D2D) communication over out-of-band broadcast links. Following prior work, the Normalized Delivery Time (NDT) --- a metric that captures the high signal-to-noise ratio worst-case latency --- is adopted as the performance criterion of interest. Joint edge caching, downlink transmission, and D2D communication policies based on compress-and-forward are proposed that are shown to be information-theoretically optimal to within a constant multiplicative factor of two for all values of the problem parameters, and to achieve the minimum NDT for a number of special cases. The analysis provides insights on the role of D2D cooperation in improving the delivery latency.Comment: Submitted to the IEEE Transactions on Communication

Técnicas de equalização para MIMO massivo com amplificação não linear

The dawn of the new generation of mobile communications and the trafic explosion that derives from its implementation pose great challenge. The milimeter wave band and the use of massive number of antennas are technologies which, when combined, allow the transmission of high data rate, functioning in zones of the electromagnetic spectrum that are less explored and with capability of allocation of dozens of GHz of bandwidth. In this dissertation we consider a massive MIMO millimeter wave system employing a hybrid architecture, i.e., the number of transmit and receive antennas are lower than the number of radio frequency chains. As consequence, the precoder and equalizers should be designed in both digital and analog domains. In the literature, most of the proposed hybrid beamforming schemes were evaluated without considering the effects of nonlinear amplifications. However, these systems face non-avoidable nonlinear effects due to power amplifiers functioning in nonlinear regions. The strong nonlinear effects throughout the transmission chain will have a negative impact on the overall system performance and thus its study and the design of equalizers that take into account these effects are of paramount importance. This dissertation proposes a hybrid iterative equalizer for massive MIMO millimeter wave SC-FDMA systems. The user terminals have low complexity, just equipped with analog precoders based on average angle of departure, each with a single radio frequency chain. At the base station it is designed an hybrid analog-digital iterative equalizer with fully connected architecture in order to eliminate both the multi-user interference and the nonlinear distortion caused by signal amplification during the transmission. The equalizer is optimized by minimizing the bit error rate, which is equivalent to minimize the mean square error rate. The impact of the saturation threshold of the amplifiers in the system performance is analysed, and it is demonstrated that the iterative process can efficiently remove the multi-user interference and the distortion, improving the overall system performance.O surgimento de uma nova geração de comunicações móveis e a explosão de tráfego que advém da sua implementação apresenta grandes desafios. A banda de ondas milimétricas e o uso massivo de antenas são tecnologias que, combinadas, permitem atingir elevadas taxas de transmissão, funcionando em zonas do espectro electromagnético menos exploradas e com capacidade de alocação de dezenas de GHz para largura de banda. Nesta dissertação foi considerado um sistema de MIMO massivo de ondas milimétricas usando uma arquitectura híbrida, i.e., o número de antenas para transmissão e recepção é menor que o número de cadeias de radiofrequência. Consequentemente, o pré-codificador e equalizadores devem ser projectados nos domínios digital e analógico. Na literatura, a maioria dos esquemas híbridos de beamforming são avaliados sem ter em conta os efeitos de não linearidade da amplificação do sinal. No entanto, estes sistemas sofrem inevitavelmente de efeitos não lineares devido aos amplificadores de potência operarem em regiões não lineares. Os fortes efeitos das não-linearidades ao longo da cadeia de transmissão têm um efeito nefasto no desempenho do sistema e portanto o seu estudo e projecto de equalizadores que tenham em conta estes efeitos são de extrema importância. Esta dissertação propõe um equalizador híbrido para sistemas baseados em ondas milimétricas para MIMO massivo com modulação SC-FDMA. Os terminais de utilizador possuem baixa complexidade, equipados apenas com pré-codificadores analógicos baseados no ângulo médio de partida, cada um com uma única cadeia de radiofrequência. Na estação base é projectado um equalizador iterativo híbrido analógico-digital com arquitectura completamente conectada de modo a eliminar a interferencia multi-utilizador e a distorção causada pela amplificação do sinal aquando da transmissão. O equalizador é optimizado minimizando a taxa de erro de bit, o que é equivalente a minimizar a taxa de erro quadrático médio. O impacto do limiar de saturação dos amplificadores no desempenho do sistema é analisado, e é demonstrado que o processo iterativo consegue eliminar de modo eficiente a interferência multi-utilizador e a distorção, melhorando o desempenho do sistema.Mestrado em Engenharia Eletrónica e Telecomunicaçõe

Subband filtered multi-carrier systems for multi-service wireless communications

Flexibly supporting multiple services, each with different communication requirements and frame structure, has been identified as one of the most significant and promising characteristics of next generation and beyond wireless communication systems. However, integrating multiple frame structures with different subcarrier spacing in one radio carrier may result in significant inter-service-band-interference (ISBI). In this paper, a framework for multi-service (MS) systems is established based on a subband filtered multi-carrier system. The subband filtering implementations and both asynchronous and generalized synchronous (GS) MS subband filtered multi-carrier (SFMC) systems have been proposed. Based on the GS-MS-SFMC system, the system model with ISBI is derived and a number of properties on ISBI are given. In addition, low-complexity ISBI cancelation algorithms are proposed by precoding the information symbols at the transmitter. For asynchronous MS-SFMC system in the presence of transceiver imperfections, including carrier frequency offset, timing offset, and phase noise, a complete analytical system model is established in terms of desired signal, inter-symbol-interference, inter-carrier-interference, ISBI, and noise. Thereafter, new channel equalization algorithms are proposed by considering the errors and imperfections. Numerical analysis shows that the analytical results match the simulation results, and the proposed ISBI cancelation and equalization algorithms can significantly improve the system performance in comparison with the existing algorithms

An Adaptive Self-Interference Cancelation/Utilization and ICA-Assisted Semi-Blind Full-Duplex Relay System for LLHR IoT

In this article, we propose a semi-blind full-duplex (FD) amplify-and-forward (AF) relay system with adaptive self-interference (SI) processing assisted by independent component analysis (ICA) for low-latency and high-reliability (LLHR) Internet of Things (IoT). The SI at FD relay is not necessarily canceled as much as possible like the conventional approaches, but is canceled or utilized based on a signal-to-residual-SI ratio (SRSIR) threshold at relay. According to the selected SI processing mode at relay, an ICA-based adaptive semi-blind scheme is proposed for signal separation and detection at destination. The proposed FD relay system not only features reduced signal processing cost of SI cancelation but also achieves a much higher degree of freedom in signal detection. The resulting bit error rate (BER) performance is robust against a wide range of SRSIR, much better than that of conventional FD systems, and close to the ideal case with perfect channel state information (CSI) and perfect SI cancelation. The proposed system also requires negligible spectral overhead as only a nonredundant precoding is needed for ambiguity elimination in ICA. In addition, the proposed system enables full resource utilization with consecutive data transmission at all time and same frequency, leading to much higher throughput and energy efficiency than the time-splitting and power-splitting-based self-energy recycling approaches that utilize only partial resources. Furthermore, an intensive analysis is provided, where the SRSIR thresholds for the adaptive SI processing mode selection and the BER expressions with ICA incurred ambiguities are derived

Unified Framework for Multicarrier and Multiple Access based on Generalized Frequency Division Multiplexing

The advancements in wireless communications are the key-enablers of new applications with stringent requirements in low-latency, ultra-reliability, high data rate, high mobility, and massive connectivity. Diverse types of devices, ranging from tiny sensors to vehicles, with different capabilities need to be connected under various channel conditions. Thus, modern connectivity and network techniques at all layers are essential to overcome these challenges. In particular, the physical layer (PHY) transmission is required to achieve certain link reliability, data rate, and latency. In modern digital communications systems, the transmission is performed by means of a digital signal processing module that derives analog hardware. The performance of the analog part is influenced by the quality of the hardware and the baseband signal denoted as waveform. In most of the modern systems such as fifth generation (5G) and WiFi, orthogonal frequency division multiplexing (OFDM) is adopted as a favorite waveform due to its low-complexity advantages in terms of signal processing. However, OFDM requires strict requirements on hardware quality. Many devices are equipped with simplified analog hardware to reduce the cost. In this case, OFDM does not work properly as a result of its high peak-to-average power ratio (PAPR) and sensitivity to synchronization errors. To tackle these problems, many waveforms design have been recently proposed in the literature. Some of these designs are modified versions of OFDM or based on conventional single subcarrier. Moreover, multicarrier frameworks, such as generalized frequency division multiplexing (GFDM), have been proposed to realize varieties of conventional waveforms. Furthermore, recent studies show the potential of using non-conventional waveforms for increasing the link reliability with affordable complexity. Based on that, flexible waveforms and transmission techniques are necessary to adapt the system for different hardware and channel constraints in order to fulfill the applications requirements while optimizing the resources. The objective of this thesis is to provide a holistic view of waveforms and the related multiple access (MA) techniques to enable efficient study and evaluation of different approaches. First, the wireless communications system is reviewed with specific focus on the impact of hardware impairments and the wireless channel on the waveform design. Then, generalized model of waveforms and MA are presented highlighting various special cases. Finally, this work introduces low-complexity architectures for hardware implementation of flexible waveforms. Integrating such designs with software-defined radio (SDR) contributes to the development of practical real-time flexible PHY.:1 Introduction 1.1 Baseband transmission model 1.2 History of multicarrier systems 1.3 The state-of-the-art waveforms 1.4 Prior works related to GFDM 1.5 Objective and contributions 2 Fundamentals of Wireless Communications 2.1 Wireless communications system 2.2 RF transceiver 2.2.1 Digital-analogue conversion 2.2.2 QAM modulation 2.2.3 Effective channel 2.2.4 Hardware impairments 2.3 Waveform aspects 2.3.1 Single-carrier waveform 2.3.2 Multicarrier waveform 2.3.3 MIMO-Waveforms 2.3.4 Waveform performance metrics 2.4 Wireless Channel 2.4.1 Line-of-sight propagation 2.4.2 Multi path and fading process 2.4.3 General baseband statistical channel model 2.4.4 MIMO channel 2.5 Summary 3 Generic Block-based Waveforms 3.1 Block-based waveform formulation 3.1.1 Variable-rate multicarrier 3.1.2 General block-based multicarrier model 3.2 Waveform processing techniques 3.2.1 Linear and circular filtering 3.2.2 Windowing 3.3 Structured representation 3.3.1 Modulator 3.3.2 Demodulator 3.3.3 MIMO Waveform processing 3.4 Detection 3.4.1 Maximum-likelihood detection 3.4.2 Linear detection 3.4.3 Iterative Detection 3.4.4 Numerical example and insights 3.5 Summary 4 Generic Multiple Access Schemes 57 4.1 Basic multiple access and multiplexing schemes 4.1.1 Infrastructure network system model 4.1.2 Duplex schemes 4.1.3 Common multiplexing and multiple access schemes 4.2 General multicarrier-based multiple access 4.2.1 Design with fixed set of pulses 4.2.2 Computational model 4.2.3 Asynchronous multiple access 4.3 Summary 5 Time-Frequency Analyses of Multicarrier 5.1 General time-frequency representation 5.1.1 Block representation 5.1.2 Relation to Zak transform 5.2 Time-frequency spreading 5.3 Time-frequency block in LTV channel 5.3.1 Subcarrier and subsymbol numerology 5.3.2 Processing based on the time-domain signal 5.3.3 Processing based on the frequency-domain signal 5.3.4 Unified signal model 5.4 summary 6 Generalized waveforms based on time-frequency shifts 6.1 General time-frequency shift 6.1.1 Time-frequency shift design 6.1.2 Relation between the shifted pulses 6.2 Time-frequency shift in Gabor frame 6.2.1 Conventional GFDM 6.3 GFDM modulation 6.3.1 Filter bank representation 6.3.2 Block representation 6.3.3 GFDM matrix structure 6.3.4 GFDM demodulator 6.3.5 Alternative interpretation of GFDM 6.3.6 Orthogonal modulation and GFDM spreading 6.4 Summary 7 Modulation Framework: Architectures and Applications 7.1 Modem architectures 7.1.1 General modulation matrix structure 7.1.2 Run-time flexibility 7.1.3 Generic GFDM-based architecture 7.1.4 Flexible parallel multiplications architecture 7.1.5 MIMO waveform architecture 7.2 Extended GFDM framework 7.2.1 Architectures complexity and flexibility analysis 7.2.2 Number of multiplications 7.2.3 Hardware analysis 7.3 Applications of the extended GFDM framework 7.3.1 Generalized FDMA 7.3.2 Enchantment of OFDM system 7.4 Summary 7 Conclusions and Future work