Downlink Performance of Superimposed Pilots in Massive MIMO systems

In this paper, we investigate the downlink throughput performance of a massive multiple-input multiple-output (MIMO) system that employs superimposed pilots for channel estimation. The component of downlink (DL) interference that results from transmitting data alongside pilots in the uplink (UL) is shown to decrease at a rate proportional to the square root of the number of antennas at the BS. The normalized mean-squared error (NMSE) of the channel estimate is compared with the Bayesian Cram\'{e}r-Rao lower bound that is derived for the system, and the former is also shown to diminish with increasing number of antennas at the base station (BS). Furthermore, we show that staggered pilots are a particular case of superimposed pilots and offer the downlink throughput of superimposed pilots while retaining the UL spectral and energy efficiency of regular pilots. We also extend the framework for designing a hybrid system, consisting of users that transmit either regular or superimposed pilots, to minimize both the UL and DL interference. The improved NMSE and DL rates of the channel estimator based on superimposed pilots are demonstrated by means of simulations.Comment: 28 single-column pages, 6 figures, 1 table, Submitted to IEEE Trans. Wireless Commun. in Aug 2017. Revised Submission in Feb. 201

Heterogeneous Rank Beamforming for Industrial Communications

This paper proposes a novel hardware beamforming architecture, which is capable of utilizing a different number of Radio Frequency (RF) chains in different parts of the bandwidth. It also shows that a proportional fairness scheduler will effectively utilize the high rank part of the bandwidth in a multi-user setting, thus operating more efficiently and effectively than classical beamforming schemes.Comment: 10 pages, 14 figures. Submitted to IEEE Transactions on Wireless Communication

28 GHz NLOS Channel Measurements Revealing Low Path Loss and High Angular Spread in Container Ports

This paper presents results from a comprehensive measurement campaign conducted at 28 GHz inside a container canyon within a commercial port environment. The measurements are performed at various points inside the container canyon, considering two types of container stacking and two different Transmitter (TX) locations, using a narrowband channel sounder equipped with a rotating horn antenna. The measurements are used to evaluate the azimuthal spectrum and spatial correlation, as well as the impact of a vehicle inside a canyon on these parameters. Further, the measurement data is utilized to validate a simulation setup from which the path loss and the elevation spectrum inside the canyon is obtained. Lastly, a propagation model inside the canyon is hypothesized and shown to be consistent with the measurements. The analysis show a low path loss compared to free space, as well as a high angular spread and short spatial correlation.Comment: 10 pages, 19 figures. Submitted to Transactions on Antennas and Propagatio

Extreme Communication in 6G: Vision and Challenges for ‘in-X’ Subnetworks

The 6th Generation (6G) radio access technology is expected to support extreme communication requirements in terms of throughput, latency and reliability, which can only be achieved by providing capillary wireless coverage. In this paper, we present our vision for short-range low power 6G ‘in-X’ subnetworks, with the ‘X’ standing for the entity in which the cell in which the subnetwork is deployed, e.g., a production module, a robot, a vehicle, a house or even a human body. Such cells can support services that can be life-critical and that traditionally relied on wired systems. We discuss potential deployment options, as well as candidate air interface components and spectrum bands. Interference management is identified as a major challenge in dense deployments, which needs to handle also non-cellular types of interference like jamming attacks and impulsive noise. A qualitative example of interference-robust system design is also presented

Digital Twins for Industry 4.0 in the 6G Era

Having the Fifth Generation (5G) mobile communication system recently rolled out in many countries, the wireless community is now setting its eyes on the next era of Sixth Generation (6G). Inheriting from 5G its focus on industrial use cases, 6G is envisaged to become the infrastructural backbone of future intelligent industry. Especially, a combination of 6G and the emerging technologies of Digital Twins (DT) will give impetus to the next evolution of Industry 4.0 (I4.0) systems. This article provides a survey in the research area of 6G-empowered industrial DT system. With a novel vision of 6G industrial DT ecosystem, this survey discusses the ambitions and potential applications of industrial DT in the 6G era, identifying the emerging challenges as well as the key enabling technologies. The introduced ecosystem is supposed to bridge the gaps between humans, machines, and the data infrastructure, and therewith enable numerous novel application scenarios.Comment: Accepted for publication in IEEE Open Journal of Vehicular Technolog

Channel Estimation in Large-Scale Multi-Antenna Systems for 5G and Beyond - Novel Pilot Structures and Algorithms

Efficient use of the limited quantity of available spectrum to cater to the exponentially increasing demand for throughput has been the focus of communication and signal processing engineers for the past few decades. With the advent of technologies such as the Internet of things (IoT) or machine-type communications (MTC), devices and appliances around us which have predominantly been offline are being equipped with sensors that generate data and are now driving the demand for throughput. The forthcoming fifth generation (5G) standard is being developed to cater to these use cases and to also increase throughput for conventional mobile users. One of the enabling technologies of 5G is the use of antenna arrays with orders of magnitude more elements than in conventional fourth generation (4G) transceivers. Large-scale multi-antenna systems impose constraints on channel training and transceiver architecture. In this thesis, we consider the problem of channel estimation in large-scale multi-antenna systems at conventional sub-6 GHz and millimeter-wave (mmWave) frequencies. In coherent receivers, channel state information (CSI) is obtained using training, which involves sending known pilots from the transmitter. In multi-cell networks, these pilots will have to be reused in different cells in order to limit the channel estimation overhead, resulting in a detrimental phenomenon known as pilot contamination. Pilot contamination, which causes interference and decreases throughput, is a fundamental challenge in large-scale multi-antenna systems. In the first part of this thesis, we address the issue of pilot contamination and propose using superimposed pilots for avoiding/mitigating interference. We also consider variants of superimposed pilots such as the hybrid system and staggered pilots to improve throughput. Next, we address the problem of estimating spatial covariance matrices (SCMs) in massive MIMO systems in the presence of pilot contamination. SCMs are useful for mitigating the effects of pilot contamination, but have to be estimated from contaminated observations of the user channels, and consequently, are also contaminated. In the second part of this thesis, we propose a novel pilot structure for estimating contamination-free SCMs. The shift to mmWave frequencies opens up large swathes of spectrum for communication, enabling the large throughputs that 5G demands. However, the channel propagation characteristics at these frequencies are markedly different from sub-6 GHz channels and communicating at mmWave frequencies imposes significant constraints on the transceiver architecture. Both factors in turn influence the design of signal processing algorithms. In the third part of the thesis, we address the problem of channel tracking in mmWave transceivers and develop novel semi-blind algorithms to track the channel with a low overhead