Multiuser Random Beamforming in Millimetre-Waves Channels

This thesis aims to show that in mmWaves channels, schemes based on randomly-directional beamforming allow to harness both the spatial multiplexing and multi-user diversity characterizing the broadcast channel by using only limited feedback and a simple transmitter architecture. The number of necessary users with respect to the number of transmitting antennas for optimal performances is investigated as well as the fairness issue, for which the use of NOMA is shown to be advantageous w.r.t. OMA

An Overview of Physical Layer Security with Finite-Alphabet Signaling

Providing secure communications over the physical layer with the objective of achieving perfect secrecy without requiring a secret key has been receiving growing attention within the past decade. The vast majority of the existing studies in the area of physical layer security focus exclusively on the scenarios where the channel inputs are Gaussian distributed. However, in practice, the signals employed for transmission are drawn from discrete signal constellations such as phase shift keying and quadrature amplitude modulation. Hence, understanding the impact of the finite-alphabet input constraints and designing secure transmission schemes under this assumption is a mandatory step towards a practical implementation of physical layer security. With this motivation, this article reviews recent developments on physical layer security with finite-alphabet inputs. We explore transmit signal design algorithms for single-antenna as well as multi-antenna wiretap channels under different assumptions on the channel state information at the transmitter. Moreover, we present a review of the recent results on secure transmission with discrete signaling for various scenarios including multi-carrier transmission systems, broadcast channels with confidential messages, cognitive multiple access and relay networks. Throughout the article, we stress the important behavioral differences of discrete versus Gaussian inputs in the context of the physical layer security. We also present an overview of practical code construction over Gaussian and fading wiretap channels, and we discuss some open problems and directions for future research.Comment: Submitted to IEEE Communications Surveys & Tutorials (1st Revision

Multiuser random beamforming in mmWave channels

Cellular communications exploiting the mmWaves frequency range are com- ing within our technological reach. However the specificities of propagation at these frequencies calls for new transmission schemes. Concerning the downlink there are signs that opportunistic beamforming may be an effec- tive solution. This thesis aims to show that in mmWaves channels, schemes based on randomly-directional beamforming allow to harness both the spatial multiplexing and multiuser diversity characterizing the broadcast channel by using only limited feedback and a simple transmitter architecture. It is well- known that performances of random beamforming schemes become optimal when the number of users tends to infinity. Hence, the number of necessary users with respect to the number of transmitting antennas is investigated and the necessity of a linear relation between the two is confirmed. Opportunis- tic beamforming is furthermore analysed under the aspect of fairness. The possibility to combine it with proportional-fair scheduling with only a small sum-rate loss is shown. Finally, the allocation of multiple users per beam is considered and the advantage of NOMA over OMA under the point of view of fairness is displayed

A rate-splitting approach to multiple-antenna broadcasting

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 is available, we show that such techniques exhibit severe losses under CSIT uncertainties, even when optimally designed. We depart from this conventional approach and adopt an unorthodox transmission strategy based on Rate-Splitting (RS), which relies on broadcasting a common data stream on top of the private data streams precoded using partial CSIT. We propose an average Weighted Minimum Mean Square Error (WMMSE) algorithm to maximize the ergodic sum-rate performance. While the ergodic sum-rate measure captures the long-term overall performance, it is not well suited for delay-limited or fairness based transmissions. Hence, we generalize the RS strategy to formulate the problem of achieving robust max-min fairness over one random fading state under a bounded CSIT uncertainty model. We derive new performance limits in terms of the symmetric-DoF under heterogeneous CSIT qualities across users to identify the RS gains. Then, a robust WMMSE algorithm based on the cutting-set method is proposed to solve the semi-infinite optimization problem. This framework is extended to address the problem of power minimization under Quality of Service (QoS) constraints. Finally, we consider the problem of achieving max-min fairness in a multigroup multicasting scenario, where each message is intended to a group of users. We assume perfect CSIT in this setup, where the presence of multiple users in each group is thought of as a source of (finite) uncertainty. The DoF performance of conventional beamforming techniques are derived from which their limitations are identified. The RS strategy is then extended to this scenario, where we show that significant DoF gains can be achieved. The RS precoder optimization problem in this setup is then solved using the WMMSE approach.Open Acces

D11.2 Consolidated results on the performance limits of wireless communications

Deliverable D11.2 del projecte europeu NEWCOM#The report presents the Intermediate Results of N# JRAs on Performance Limits of Wireless Communications and highlights the fundamental issues that have been investigated by the WP1.1. The report illustrates the Joint Research Activities (JRAs) already identified during the first year of the project which are currently ongoing. For each activity there is a description, an illustration of the adherence and relevance with the identified fundamental open issues, a short presentation of the preliminary results, and a roadmap for the joint research work in the next year. Appendices for each JRA give technical details on the scientific activity in each JRA.Peer ReviewedPreprin