Multiple-input multiple-output (MIMO) systems will be applied in wireless communications in order to increase the performance, spectral efficiency, and reliability. Theoretically, the channel capacity of those systems grows linearly with the number of transmit and receive antennas. An important performance metric beneath capacity is the normalised mean square error (MSE) under the assumption of optimal linear reception. Clearly, both performance measures depend on the properties of the MIMO channel as well as on the considered system approach, e.g. on the type of channel state information which is available at the transmitter. It has been shown that even partial CSI at the transmitter can increase the performance. In this thesis, we analyse the performance and design optimal transmit strategies of singleand multiuser MIMO systems with respect to the statistical properties of the fading channel and under different types of CSI at ...

Feedback in wireless communications is tied to a long-standing and successful history, facilitating robust and spectrally efficient transmission over the uncertain wireless medium. Since the application of multiple antennas at both ends of the communication link, enabling multiple-input multiple-output (MIMO) transmission, the importance of feedback information to achieve the highest performance is even more pronounced. Especially when multiple antennas are employed by the transmitter to handle the interference between multiple users, channel state information (CSI) is a fundamental prerequisite. The corresponding multi-user MIMO, interference alignment and coordination techniques are considered as a central part of future cellular networks to cope with the growing inter-cell-interference, caused by the unavoidable densification of base stations to support the exponentially increasing demand on network capacities. However, this vision can only be implemented with efficient feedback algorithms that provide accurate CSI at the transmitter without ...

During the last decade, there has been a steady increase in the demand of high data rates that are to be supported by wireless communication applications. Among the different solutions that have been proposed by the research community to cope with this new demand, the utilization of multiple antennas arises as one of the best candidates due to the fact that it provides both an increase in reliability and also in information transmission rate. Although the use of multiple antennas at the receiver side dates back from the sixties, the full potential of multiple antennas at both communication ends has been both theoretically and practically recognized in the last few years. The design of proper multi-antenna communication systems to satisfy the high data rates demand depends not only on the chosen figure of merit or performance metric, but also on ...

In order to meet ever-growing needs for capacity in wireless networks, transmission techniques and the system models used to study their performances have rapidly evolved. From single-user single-antenna point-to-point communications to modern multi-cell multi-antenna cellular networks there have been large advances in technology. Along the way, several assumptions are made in order to have either more realistic models, but also to allow simpler analysis. We analyze three aspects of actual networks and try to benefit from them when possible or conversely, to mitigate their negative impact. This sometimes corrects overly optimistic results, for instance when delay in the channel state information (CSI) acquisition is no longer neglected. However, this sometimes also corrects overly pessimistic results, for instance when in a broadcast channel (BC) the number of users is no longer limited to be equal to the number of transmit antennas ...

The aim of this thesis is to investigate key issues encountered in the design of transmission schemes and receiving techniques for Ultra Wideband (UWB) communication systems. Based on different data rate applications, this work is divided into two parts, where energy efficient and robust physical layer solutions are proposed, respectively. Due to a huge bandwidth of UWB signals, a considerable amount of multipath arrivals with various path gains is resolvable at the receiver. For low data rate impulse radio UWB systems, suboptimal non-coherent detection is a simple way to effectively capture the multipath energy. Feasible techniques that increase the power efficiency and the interference robustness of non-coherent detection need to be investigated. For high data rate direct sequence UWB systems, a large number of multipath arrivals results in severe inter-/intra-symbol interference. Additionally, the system performance may also be deteriorated by ...

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 ...

The last ten years have seen a massive growth in the number of connected wireless devices. Billions of devices are connected and managed by wireless networks. At the same time, each device needs a high throughput to support applications such as voice, real-time video, movies, and games. Demands for wireless throughput and the number of wireless devices will always increase. In addition, there is a growing concern about energy consumption of wireless communication systems. Thus, future wireless systems have to satisfy three main requirements: i) having a high throughput; ii) simultaneously serving many users; and iii) having less energy consumption. Massive multiple-input-multiple-output (MIMO) technology, where a base station (BS) equipped with very large number of antennas (collocated or distributed) serves many users in the same time-frequency resource, can meet the above requirements, and hence, it is a promising candidate technology ...

Code-division multiple-access (CDMA) systems with random spreading and channel uncertainty at the receiver are studied. Frequency selective single antenna, as well as, narrowband multiple antenna channels are considered. Rayleigh fading is assumed in all cases. General Bayesian approach is used to derive both iterative and non-iterative estimators whose performance is obtained in the large system limit via the replica method from statistical physics. The effect of spatial correlation on the performance of a multiple antenna CDMA system operating in a flat-fading channel is studied. Per-antenna spreading (PAS) with random signature sequences and spatial multiplexing is used at the transmitter. Non-iterative multiuser detectors (MUDs) using imperfect channel state information (CSI) are derived. Training symbol based channel estimators having mismatched a priori knowledge about the antenna correlation are considered. Both the channel estimator and the MUD are shown to admit a simple ...

Competition for limited bandwidth, power, and time resources is an intrinsic aspect of multi-user wireless networks. There has been a recent move towards optimizing coexistence and confidentiality at the physical layer of multi-user wireless networks, mainly by exploiting the advanced capabilities of multiple-input multiple-out (MIMO) signal processing methods. Coexistence of disparate networks is made possible via interference mitigation and suppression, and is exemplified by the current interest in cognitive radio (CR) systems. On the other hand, MIMO communications that are secure at the physical layer without depending upon network-layer encryption are achieved by redirecting jamming or multi-user interference to unauthorized receivers, while minimizing that to legitimate receivers. In all cases, the accuracy of the channel state information (CSI) available at the transmitters plays a crucial role in determining the degree of interference mitigation and confidentiality that is achieved. This dissertation ...

MULTIPLE-INPUT MULTIPLE-OUTPUT (MIMO) CHANNELS constitute a unified way of modeling a wide range of different physical communication channels, which can then be handled with a compact and elegant vector-matrix notation. The two paradigmatic examples are wireless multi-antenna channels and wireline Digital Subscriber Line (DSL) channels. Research in antenna arrays (also known as smart antennas) dates back to the 1960s. However, the use of multiples antennas at both the transmitter and the receiver, which can be naturally modeled as a MIMO channel, has been recently shown to offer a significant potential increase in capacity. DSL has gained popularity as a broadband access technology capable of reliably delivering high data rates over telephone subscriber lines. A DSL system can be modeled as a communication through a MIMO channel by considering all the copper twisted pairs within a binder as a whole rather ...

In this thesis, we consider four different scenarios of interest in modern satellite communications. For each scenario, we will propose the use of advanced solutions aimed at increasing the spectral efficiency of the communication links. First, we will investigate the optimization of the current standard for digital video broadcasting. We will increase the symbol rate of the signal and determine the optimal signal bandwidth. We will apply the time packing technique and propose a specifically design constellation. We will then compare some receiver architectures with different performance and complexity. The second scenario still addresses broadcast transmissions, but in a network composed of two satellites. We will compare three alternative transceiver strategies, namely, signals completely overlapped in frequency, frequency division multiplexing, and the Alamouti space-time block code, and, for each technique, we will derive theoretical results on the achievable rates. We ...

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 ...

In the last few years, wireless devices have evolved to unimaginable heights. Current forecasts suggest that, in the near future, every device that may take advantage of a wireless connection will have one. In addition, there is a gradual migration to smart devices and high-speed connections, and, as a consequence, the overall mobile traffic is expected to experience a tremendous growth in the next years. The multiuser interference will hence become the main limiting factor and the most critical point to address. As instrumental to efficiently manage interference between different systems, this thesis provides a thorough study on cooperative techniques. That is, users share information and exploit it to improve the overall performance. Since multiuser cooperation represents a very broad term, we will focus on algorithm design and transceiver optimization for three cooperative scenarios that capture some of the main ...

Mobile communication networks of the third and future generations are designed to offer high-data rate services like video-telephony and data-transfer. The current Rake receiver architecture will create a shortage in available bandwidth offered to the users. This is not due to a shortage in spectrum but results from inefficient receiver architectures. Spectral efficiency can be increased considerably through multi-user detection techniques in the receiver algorithms. The present thesis investigates iterative re- ceivers for encoded CDMA transmission in the uplink. The iterative receiver is a suboptimal receiver algorithm with manageable complexity. It consists of an inter- ference mitigating multi-user detector, a bank of single-user decoders, and a channel estimator. Instead of deciding on the transmitted symbols right after the first decod- ing, the receiver feeds back tentative decision symbols to mitigate multiple-access interference in the next iteration. Similarly, soft decision symbols ...

When data is transmitted over the wireless communication channel, the transmit signal experiences distortion depending on the channel¢s fading characteristics. On the one hand, this calls for efficient processing at the receiver to mitigate the detrimental effects of the channel and maximize data throughput. On the other hand, the diversity inherently present in these channels can be leveraged with appropriate transmit processing in order to increase the reliability of the transmission link. Recently, in [1] it was shown that the channel characteristics can be exploited to maximize the total data throughput in the interference channel where multiple user pairs rely on the same resource to communicate among themselves. In this PhD dissertation, we first propose novel equalizer designs for frequency selective channels. We then present new results on the diversity gain of equalizers in fading channels when appropriate precoding is ...