Sinh Le Hong Nguyen

Hybrid Beamforming for Massive MIMO: A Survey

Hybrid multiple-antenna transceivers, which combine large-dimensional analog pre/postprocessing with lower-dimensional digital processing, are the most promising approach for reducing the hardware cost and training overhead in massive MIMO systems. This article provides a comprehensive survey of the various incarnations of such structures that have been proposed in the literature. We provide a taxonomy in terms of the required channel state information, that is, whether the processing adapts to the instantaneous or average (second-order) channel state information; while the former provides somewhat better signal- to-noise and interference ratio, the latter has much lower overhead for CSI acquisition. We furthermore distinguish hardware structures of different complexities. Finally, we point out the special design aspects for operation at millimeter-wave frequencies.

On the Mutual Orthogonality of Millimeter-Wave Massive MIMO Channels

Mutual orthogonal user channels in multiuser (MU)-multiple-input multiple-output (MIMO) systems are desirable and can be approximately obtained under independent and identically distributed (i.i.d.) Rayleigh fading assumption with a very large number of base station antennas. However, it has been shown that at millimeter-wave (mmW) frequencies, this assumption is not valid due to the limited number of multipath components and spatial channel correlation. In this paper, we examine the mutual orthogonality of a realistic 60-GHz outdoor propagation channel with practical large antenna arrays, and determine the factors deciding it based on the channel data generated by means of deterministic field prediction. The results obtained reveal relationships between mutual orthogonality, inter-user distance, number of active users, transmit array dimensions, and downlink system capacity at 60-GHz band, which are useful for designing future mmW massive MU-MIMO systems.

Millimeter-Wave Channel Characterization at Helsinki Airport in the 15, 28, and 60 GHz Bands

Airport terminal is one of the indoor scenarios envisioned for mm-wave 5G deployment. In this paper, we characterize the propagation channel in the Helsinki airport at 15, 28, and 60 GHz bands by means of directional wideband channel sounding. The radio environment was characterized by studying the specular propagation paths, specular and diffuse power contributions, polarization, and the delay and angular spreads. All the studied metrics in different bands were compared and the frequency dependency was analyzed.

Spatio-temporal channel sounding in a street canyon at 15, 28 and 60 GHz

Spatio-temporal channel sounding was performed at frequency bands of 15,28 and 60 GHz in the backhaul-link street canyon scenario. Analysis of the measured channels from the sounding show 1) path loss exponents close to the free-space in the line-of-sight condition, with extra losses of 5 to 30 dB in the non-line-of-sight conditions; 2) the azimuth angular and delay spreads mostly less than 25° and 30 ns at the three frequency bands; 3) decreasing angular and delay spreads as the link distance is longer, 4) up to fourth-order specular reflection observed in the 100-m long street canyon when looking at the strongest 30-dB dynamic range of the channels, and finally, 5) no decisive frequency dependency of the angular and delay spreads and the reflection order, though the presence or absence of cars in the street canyon during the measurements seemed to make some impacts on the spread values given the antenna height of our channel sounding.

Dual-Band Multipath Cluster Analysis of Small-Cell Backhaul Channels in an Urban Street Environment

Thanks to the large spectrum chunks available above 6-GHz bands, several centimeter-wave (cm-wave) and millimeter- wave (mm-wave) bands are being exploited for mobile communications to meet 5G high data speed and low latency requirements. The 5G standardization has been started with the channel modelling for the bands from 6 and up to 100 GHz, and the current models adapting the 3GPP stochastic cluster-based approach. This paper presents a recent measurement campaign and multipath cluster characterization of 15 and 28 GHz channels in an urban street canyon. The measurement environment and setup are targeted for small-cell backhaul scenarios, whose model parameters are currently lacking in current standardized channel models. Inter-cluster and intra-cluster characteristics are presented and compared between two measured frequency ranges. The results show that the number of MPCs per cluster is noticeable higher in 28 GHz, and that is mainly attributed by the higher measurement bandwidth in this band. The higher number of MPCs in 28 GHz leads to little wider cluster delay and angle spreads, while other model parameters are similar or just little varied with carrier frequency. The paper also provides other relevant cluster statistics required to establish 3GPP channel model for the scenario of interest in the 15 and 28 GHz bands.

On-Site Permittivity Estimation at 60 GHz Through Reflecting Surface Identification in the Point Cloud

Accurate site-specific radio propagation simulations provide an important basis for cellular coverage analysis. The quality of these simulations relies on the accuracy of environmental description and electrical properties of constituent materials. This paper presents a novel method of on-site permittivity estimation. The method utilizes an accurate geometrical database of the environment for identifying flat and smooth surfaces producing reflections. The method exploits a limited number of on-site channel sounding to extract reflected multipaths and compare them with ray tracing based on the environmental database. The permittivity of the identified reflecting surfaces is estimated by solving an inverse reflection problem. The method was experimentally tested with limited radio channel measurements at 60 GHz in a large empty office room. The identified reflecting surfaces are classified according to their mean permittivity estimates, showing their consistency with physical material evidence and the permittivity database in the International Telecommunication Union Radiocommunication Recommendation (ITU-R P.2040-1). The estimated permittivity values are visualized as a 3-D map, giving an intuitive understanding of materials constituting the environment. This paper demonstrates on-site permittivity estimation and material classification without the need for isolated measurements of composite materials in an anechoic chamber or in situ measurements of built environments.

Attainable Capacity of Spatial Radio Channels: A Multiple-Frequency Analysis

This paper reports an attainable channel capacity and spatial degrees-of-freedom of multiple-antenna radio channels at 15, 28 and 61 GHz. The analysis is based on channel sounding in a same street canyon. The attainable capacity is evaluated so that it depends only on the antenna aperture size but is independent of implementation of antenna elements on the aperture. The analysis shows that multipath richness decreases as the frequency increases, as indicated by the smaller spatial degrees-of-freedom at higher frequencies for the same electrical aperture size of the receive antenna. The analysis furthermore reveals that channels at the three radio frequencies can attain almost the same level of capacity for a given transmit power and physical size of the receive antenna. The result is explained by a greater electrical size of the antenna aperture at the higher frequencies that can leverage higher gains and finer angular resolution and hence normalize the availability of fewer multipaths.