Digital subscriber line (DSL) technology is a very popular broadband access technology. It uses the existing telephone infrastructure to provide broadband access. In order to cope with the increased bandwidth demand to support broadband services, such as, Video on Demand (VoD), real time multimedia streaming, it is important to further improve the DSL. The main performance degradation of the DSL system is caused by channel impairments, such as, crosstalk and inter-symbol interference (ISI). Furthermore, the discrete Fourier transform (DFT) based discrete multitone (DMT) system has very poor spectral properties, which prohibit the use of tones at the band edges in order to meet the power spectral density (PSD) constraints of the system, thus reducing the achievable bit rate. In order to mitigate the channel impairments as well as to combat the poor spectral properties of the DFT based DMT, sophisticated ...

Digital subscriber line (DSL) technology is currently the most widely deployed broadband internet access technology and will continue to play an important role during the next decade. However, one of the major sources that limits the performance of current DSL systems is crosstalk, which is a channel distortion that is caused by the electromagnetic coupling among the different copper wires (DSL connections). Multi-user resource management is a very promising approach to prevent or even remove the impact of crosstalk, and that can significantly increase the performance of DSL systems. In this thesis, multiple efficient algorithms are proposed for multi-user resource management that only require a very low computational complexity and that can be applied to large-scale DSL systems. The application of these algorithms allows to significantly increase the data rates of DSL systems. It is furthermore shown that the proposed ...

In xDSL technology, high-speed data are transferred between the central office and the customers, or between two or more central offices using unshielded telephone lines. A major impairment that hinders the increase in data-rate through the twisted-pair line is near-end crosstalk (NEXT) between the adjacent twisted pairs. DSL systems with overlapping transmit and receive spectra are susceptible to NEXT which significantly increases the interference noise in the received signal and also reduces the reliability and availability of the system. One way to cancel the NEXT in the received signal is to deploy adaptive filters. However, if adaptive filters are deployed to cancel every possible NEXT signal from the other twisted pairs, the computational complexity increases in proportion to N^2 where N is the number of twisted pairs in the bundle and, therefore, it becomes prohibitive even for small values of ...

The ability to easily exchange and access data has transformed the way we work, study, inform and entertain ourselves. In particular, the Internet has had an effect on people’s lives in the past two decades that is profound. Profound as this effect may be, people seem not to grow tired of it. On the contrary: as of today, the Internet revolution is far from over. The thirst for bigger amounts of data at higher speeds and biquitous connectivity seem not to abate. This thirst for more, faster and better quality data is both a huge challenge and a huge opportunity for the broadband access industry. The opportunity lies on the fact that, as of the end of 2012, there were 600 million subscribers to broadband services around the world. Plus, even though the market is already enormous, it still has ...

Multi-antenna processing is widely adopted as one of the key enabling technologies for current and future cellular networks. Particularly, multiuser downlink beamforming (also known as space-division multiple access), in which multiple users are simultaneously served with spatial transmit beams in the same time and frequency resource, achieves high spectral efficiency with reduced energy consumption. To harvest the potential of multiuser downlink beamforming in practical systems, optimal beamformer design shall be carried out jointly with network resource allocation. Due to the specifications of cellular standards and/or implementation constraints, resource allocation in practice naturally necessitates discrete decision makings, e.g., base station (BS) association, user scheduling and admission control, adaptive modulation and coding, and codebook-based beamforming (precoding). This dissertation focuses on the joint optimization of multiuser downlink beamforming and discrete resource allocation in modern cellular networks. The problems studied in this thesis involve ...

Digital subscriber line (DSL) is a technology to provide broadband communications over the existing twisted pair telephone network. The signals received by a DSL modem are typically corrupted by channel induced noise, background noise, radio frequeny interference (RFI) and undesired echo. In this thesis we focus on the design of digital signal processing algorithms to improve the bit rate and/or the loop reach of current and future DSL systems. Furthermore, in the proposed algorithms we aim at keeping the hardware cost as low as possible. The transmission format of many DSL systems is based on discrete multitone modulation (DMT). To combat channel induced noise, DMT-based receivers perform an equalization step by means of a time domain equalizer (TEQ) and a one-tap frequency domain equalizer (FEQ) per used tone. Despite the variety of TEQ design methods presented in the literature, we ...

Broadband communications over the local telephone loop has become feasible nowadays by means of Digital Subscriber Line (DSL) technologies. Asymmetric DSL and one proposal for Very high bit rate DSL adopt Discrete Multitone (DMT) as modulation scheme. In this thesis we develop new equalization and echo cancellation structures for DMT-based receivers. Our main motivation is the application to DMT-based DSL-modems. In literature, a DMT-based receiver containing a multitap time domain equalizer (TEQ) and a 1-taps frequency domain equalizer per tone, has been presented. The TEQ is usually initialized by means of the so-called channel shortening algorithm. This does however not correspond to bit rate optimization, which is a major disadvantage. In part I, we aim at improving upon the channel shortening algorithm. In a first attempt, we maintain the receiver structure and only change the TEQ-initialization algorithm. In a second ...

The demand for high-speed communications required by cutting-edge applications has put a strain on the already saturated wireless spectrum. The incorporation of antenna arrays at both ends of the communication link has provided improved spectral efficiency and link reliability to the inherently complex wireless environment, thus allowing for the thriving of high data-rate applications without the cost of extra bandwidth consumption. As a consequence to this, multiple-input multiple-output (MIMO) systems have become the key technology for wideband communication standards both in single-user and multi-user setups. The main difficulty in single-user MIMO systems stems from the signal detection stage at the receiver, whereas multi-user downlink systems struggle with the challenge of enabling non-cooperative signal acquisition at the user terminals. In this respect, precoding techniques perform a pre-equalization stage at the base station so that the signal at each receiver can be ...

Currently, the telecommunications market has brought changes to the design of the old model of the telecommunications network. The emergence of new technologies for higher speed access was inevitable in order to meet the requirements of the appearance of the multimedia services (VoD, online gaming etc.). The latest technologies for broadband access over telephone pairs are Digital Subscriber Lines or DSL. This set of xDSL technologies allow the transfer of binary high speed over telephone twisted pair by using a suitable type of line codes. They allow a flow of high-speed information both asymmetrical and symmetrical over the telephone loop. This thesis presents the state of the art of Dynamic Spectrum Management (DSM) technologies suggested to improve the performance of DSL systems and proposes a new approach to this issue. The main contributions of this thesis includes extended bandwidth channel ...

In the last years, research has focused on multiple-input multiple-output (MIMO) wireless technology due to the capacity and performance improvement it provides, offering a higher spectral efficiency. In addition, when multiple users take part in the network, the scenario becomes much more complex, since resources like bandwidth, time or transmission power must be shared. Furthermore, the performance of the system is degraded as a consequence of the noise and multiuser interference (MUI). When the transmission is conducted from a base station (BS) to multiple users, a pre-equalization stage called precoding is applied. By means of this, each user will be able to interpret the signal independently, without the knowledge of the channel. Precoding techniques are classified into linear and non-linear. In fact, the non-linear Tomlinson-Harashima precoding (THP) and vector precoding (VP) techniques have been shown to achieve very good results ...

The widespread use of mobile internet and smart applications has led to an explosive growth in mobile data traffic. With the rise of smart homes, smart buildings, and smart cities, this demand is ever growing since future communication systems will require the integration of multiple networks serving diverse sectors, domains and applications, such as multimedia, virtual or augmented reality, machine-to-machine (M2M) communication / the Internet of things (IoT), automotive applications, and many more. Therefore, in the future, the communication systems will not only be required to provide Gbps wireless connectivity but also fulfill other requirements such as low latency and massive machine type connectivity while ensuring the quality of service. Without significant technological advances to increase the system capacity, the existing telecommunications infrastructure will be unable to support these multi-dimensional requirements. This poses an important demand for suitable waveforms with ...

In this thesis, we consider a multiuser system with a transmitter equipped with multiple antennas and only one antenna at each receiver user. This system, which is termed MUMISO (Multi User Multiple Input/Single Output), is of use to model the downlink of a wireless communication system, where multiple antennas at the base station transmit to several users with usually only one antenna at each receiving unit. This downlink channel is also called Broadcast Channel (BC). When considering this broadcast channel, the centralized transmitter clearly has more degrees of freedom than each of the receivers. Therefore, it is appropriate to separate the signals by applying precoding at the transmitter. To be able to design precoding, the transmitter needs knowledge about the channel states of the different receivers. In the case of Frequency Division Duplex (FDD) systems, this knowledge can be obtained ...

Signal processing techniques for multiple-antenna transmission can exploit the spatial dimension of the wireless channel to serve multiple users simultaneously, achieving high spectral efficiencies. Realizing such gains; however, is strongly dependent on the availability of highly accurate and up-to-date Channel State Information at the Transmitter (CSIT). This stems from the necessity to deal with multiuser interference through preprocessing; as receivers cannot coordinate in general. In wireless systems, CSIT is subject to uncertainty due to estimation and quantization errors, delays and mismatches. This thesis proposes optimized preprocessing techniques for broadcasting scenarios where a multi-antenna transmitter communicates with single-antenna receivers under CSIT uncertainties. First, we consider a scenario where the transmitter communicates an independent message to each receiver. The most popular preprocessing techniques in this setup are based on linear precoding (or beamforming). Despite their near-optimum rate performances when highly accurate CSIT ...

Wireless communications are important in our highly connected world. The amount of data being transferred in cellular networks is steadily growing, and consequently more capacity is needed. This thesis considers the problem of downlink capacity improvement from the perspective of multicell coordination. By employing multiple antennas at the transmitters and receivers of a multicell network, the inherent spatial selectivity of the users can be exploited in order to increase the capacity through linear precoding and receive filtering. For the coordination between cells, distributed algorithms are often sought due to their low implementation complexity and robustness. In this context, the thesis considers two problem domains: base station clustering and coordinated precoding. Base station clustering corresponds to grouping the cell base stations into disjoint clusters in order to reduce the coordination overhead. This is needed in intermediate-sized to large networks, where the ...

The thriving development of wireless communications calls for innovative and advanced signal processing techniques targeting at an enhanced performance in terms of reliability, throughput, robustness, efficiency, flexibility, etc.. This thesis addresses such a compelling demand and presents new and intriguing progress towards fulfilling it. We mainly concentrate on two advanced multi-dimensional signal processing challenges for wireless systems that have attracted tremendous research attention in recent years, multi-carrier Multiple-Input Multiple-Output (MIMO) systems and multi-dimensional harmonic retrieval. As the key technologies of wireless communications, the numerous benefits of MIMO and multi-carrier modulation, e.g., boosting the data rate and improving the link reliability, have long been identified and have ignited great research interest. In particular, the Orthogonal Frequency Division Multiplexing (OFDM)-based multi-user MIMO downlink with Space-Division Multiple Access (SDMA) combines the twofold advantages of MIMO and multi-carrier modulation. It is the essential element ...