Bo Xu

A multi-mode wideband patch antenna is proposed for indoor base stations, with the antenna achieving a wide impedance bandwidth and the equipment realizing specific absorption rate (SAR) touch compliance with 0.375 W time-averaged accepted power from 3.4 to 3.8 GHz. For comparison, a narrow-band canonical fundamental-mode patch antenna allows only 0.11 W under the same conditions for touch compliance. The wide bandwidth of the proposed antenna is achieved by combining three even modes excited at close frequencies. The use of even modes results in split SAR maxima and thus lower SAR levels compared to odd modes to achieve touch compliance. To further prove the applicability of the antenna, its performance is evaluated using power-dependent multiplexing efficiency based on indoor channel models. The proposed design demonstrates superior performance, e.g., 4.9–6.0 times higher median levels of the power-dependent multiplexing efficiency in various scenarios, over the canonical patch antenna.

Multiple-input multiple-output (MIMO) is a technique that has been extensively studied and successfully implemented in various communication systems over the past few decades. It has been a key technology in both 4G and 5G mobile communication systems and will be a key enabler in 6G to support the extreme requirements of data rate, energy efficiency (EE), etc. However, meeting these requirements necessitates evolution in the architecture of massive MIMO systems, and distributed MIMO (D-MIMO) has garnered significant attention as a potential solution. This article articulates one of the key findings on adaptive large MIMO systems from the Hexa-X-II European 6G Flagship project and provides a vision of what large MIMO systems may entail in 6G. To achieve this, we examine an adaptive deployment strategy in a joint co-located and distributed MIMO setup, including the utilization of different carrier frequencies and coordinated transmission under an ideal channel state information assumption. We evaluate EE and data rate across various user densities. Additionally, we provide a framework to select the MIMO architecture based on user density and spatial distribution. The key challenges and future research directions on robust large MIMO systems towards realistic 6G networks are also discussed.

Sub-THz communications are envisioned to unlock ultra-high data rates, ultra-low latencies, and massive connectivity in future wireless systems by utilizing bandwidths of up to tens of GHz. This paper presents the holistic perspective on sub-THz communications developed within the European sixth-generation (6G) flagship project Hexa-X-II. According to this perspective, successfully deploying sub-THz systems will require addressing several physical-layer (PHY) challenges associated with the harsh signal propagation and hardware limitations. In addition, sub-THz systems will drive advancements in essential wireless applications beyond communications, such as joint communications and sensing (JCAS). In this context, we present a collection of technical contributions and key findings from the Hexa-X-II project that have shaped this perspective. In contrast to existing works, these efforts collectively tackle the fundamental PHY challenges of sub-THz communications, i.e., understanding and modeling of radio propagation, radio-frequency (RF) power consumption and complexity, and hardware impairments. To achieve this, we present several results across three core areas, i.e., signal propagation and channel modeling, RF transceiver design, and PHY enablers, offering useful insights into the development of sub-THz systems. Lastly, we provide an overview of sub-THz JCAS, emphasizing it as one of the most promising applications in the sub-THz range.

Antenna design for indoor base stations (BSs) needs to consider various requirements to maximize the performance of the entire system in complicated environments. In this article, antenna design and performance evaluation are conducted from a holistic perspective using a modified multiplexing efficiency. This metric considers power constraints due to electromagnetic field exposure compliance and indoor channel environments for multiple-input multiple-output (MIMO) antenna systems. In the case study, using the reference monopole antenna, the accepted power needs to be reduced from 0.5 to 0.28 W to achieve specific absorption rate (SAR) touch compliance, resulting in a reduction in the modified multiplexing efficiency. The improved monopole antenna design can meet SAR touch compliance with an accepted power of 0.5 W by altering the electric current distribution on the monopole and ground plane. The system performance evaluated using the modified multiplexing efficiency is therefore improved by approximately 2.4 dB in the indoor environment for an indoor BS with four antennas. This holistic antenna design and evaluation process provides a valuable basis for comparing and optimizing MIMO antenna systems while considering SAR touch compliance and deployment scenarios for indoor BSs.

In this study, the actual time-averaged equivalent isotropic radiated power (EIRP) levels of nine 5G massive multiple-input multiple-output base stations (BSs) located inside the Melbourne Cricket Ground were assessed. Measurements were conducted during the 2023 Australian Football League Grand Final with full stadium capacity of over 100,000 attendees, under extreme mobile traffic and user density conditions. Precoding matrix indicator and time-averaged output power data of the BSs were collected using a network monitoring tool to obtain actual time-averaged EIRP statistics corresponding to the spatial distribution of power transmission in different beam directions. The results show that, even under exceptional traffic conditions, the actual time-averaged EIRP levels were significantly below the maximum, with the mean and 95 $^\rm{th}$ percentile values being 7.6% and 9.5% of it, respectively. This corresponds to a reduction of the EMF compliance distance by about 60% compared to the theoretical maximum exposure scenario.

Distributed multi-input multioutput (D-MIMO) is one of the promising technology components for the 6G mobile communication systems. In this article, electromagnetic field (EMF) exposure from D-MIMO deployment scenarios in an industrial indoor environment is evaluated using a hybrid simulation approach, based on ray-tracing and full-wave simulations, for downlink transmission at 3.5 GHz. For comparison, EMF exposure from a massive MIMO (mMIMO) deployment scenario is also assessed in the same environment. Both single-user equal gain transmission precoding and multiuser centralized zero forcing precoding schemes are considered. EMF exposure is assessed with the metrics of incident power density, local specific absorption rate (SAR), and whole-body average SAR. The dependence of the exposure on the number of simultaneously served users and number of distributed radio units is investigated. The simulated exposure at the ground level is well below the limits specified in the international EMF exposure guidelines considering realistic output power levels for both D-MIMO and mMIMO scenarios. It is observed that the 95th and 99th percentiles of the assessed EMF exposure levels from D-MIMO are lower than those from the mMIMO deployment under the same condition. However, a trend is not observed for the median exposure levels.

When the electromagnetic field (EMF) compliance boundary of a radio base station (RBS) is determined based on the actual maximum EMF exposure condition according to the international standard IEC 62232:2022, software features to monitor and control the time-averaged transmitted power or the equivalent isotropically radiated power (EIRP) are recommended or required in many countries. In this paper, a new software feature that maps the EIRP contributions from massive multiple-input multiple-output (MIMO) RBS to eight azimuthal segments within a cell is presented. The power control is much less often triggered using the new feature than a previous one based on the calculation of the transmitted power only. Furthermore, operators can set the threshold per segment by using the presented feature. The counters associated with the feature provide a conservative estimate of the EIRP in all segments. Part I describes the design principles of the new feature and feature validation tests based on in-situ measurements. The results confirm that the new feature ensures that the time-averaged EIRP is maintained below the set threshold in all test cases. Part II describes time-synchronized measurements for a live network deployment and compares the performance of the described directional feature with the cell-wide feature. The results confirm that the new feature has less impact on the network performance.

When the electromagnetic field (EMF) compliance boundary of a radio base station (RBS) is determined based on the actual maximum EMF exposure condition according to the international standard IEC 62232:2022, software features to monitor and control the time-averaged transmitted power or the equivalent isotropically radiated power (EIRP) are recommended or required in many countries. In this paper, a new software feature that maps the EIRP contributions from massive multiple-input multiple-output (MIMO) RBS to eight azimuthal segments within a cell is presented. The power control is much less often triggered using the new feature than a previous one based on the calculation of the transmitted power only. Furthermore, operators can set the threshold per segment by using the presented feature. The counters associated with the feature provide a conservative estimate of the EIRP in all segments. This publication is divided into two parts. Part I describes the design principle of the new feature and feature validation tests based on in-situ measurements. The results confirm that the new feature ensures that the time-averaged EIRP is maintained below the set threshold in all test cases. Part II describes the time-synchronized measurements in a live 5G site and a network-level deployment by comparing the new directional feature with the cell-wide feature. The results confirm that the new feature has less impact on the network performance.

User equipment (UE) needs to comply with regulations limiting the exposure of the human body to electromagnetic fields (EMFs). In this article, three exposure metrics, including specific absorption rate (SAR), incident power density (IPD), and absorbed power density (APD), are quantified for different UE antenna designs. The exposure ratios of the metrics are quantified according to the relevant EMF limit values. The investigations are performed at or close to the transition frequency between different exposure metrics: the range 6–10 GHz being of research interest and currently attracting attention for emerging mobile technologies. The study demonstrates that the IPD does not correlate well with the APD, within the investigated frequency region, when the reactive near-field of the antenna is of interest. This is in accordance with the International Commission on Nonionizing Radiation Protection 2020 guidelines meaning that the APD should be used for compliance purposes. For validation purposes, prototypes are fabricated, and SAR and IPD are measured. The average difference between the numerical and experimental results is 0.4 dB for SAR and 0.9 dB for the IPD, meaning that good agreement between simulations and measurements is obtained.