Technology Developments for In-Building Wireless Systems beyond 5G

The use of mobile data has shifted toward indoors with users becoming more dependent on mobile networks for business, communication, and entertainment at all times. 5G offers more capabilities, such as multi-gigabit speeds and massive device connections. Traditional solutions are often not capable to deliver the required level of performance, reliability, and capacity. For instance, Distributed Antenna Systems (DASs) do not support frequency above 3 Gigahertz (GHz) and major reconstructive engineering work is required to increase network capacity. A new generation of digital indoor systems or Distributed Radio Systems (DRSs) has been developed to ease these challenges and support advanced features like cell splitting. However, cell splitting is reaching its limits given the skyrocketing mobile data traffic demands. The new generation of use cases beyond 5G requires higher levels of performance and Quality of Service (QoS), so new technologies need to be developed to meet those requirements.

Network operators face challenges when deploying indoor networks. They need to tailor the service quality to the different use case requirements; some require low latency (remote-controlled drones) and some require high throughput (video). All these factors motivate infrastructure vendors to innovate. While 5G is set to experience significant growth in the coming years, many are already looking ahead to pave the way to 6G.

Distributed Massive Multiple Input, Multiple Output (mMIMO): For distributed mMIMO, the antenna elements are distributed over a larger area that is managed by a central unit, rather than being localized to one unit. In 2020, Huawei and China Unicom verified the first 5G indoor distributed mMIMO in the industry, opening a new way to enhance 5G indoor capacity and user experience. This solution integrates mMIMO into indoor networks to enable multi-user MIMO and joint beamforming to deliver gigabit connectivity and sustainable capacity expansion. It can increase capacity by up to 4X of a traditional 4T4R Digital Indoor System (DIS), which is convenient for network operators to keep up with the growing requirement for network capacity. It also eliminates cell interferences and gives network operators more flexibility to deploy radios. Moreover, it offers reliable and fast data rates with 99.99% availability, making it sufficient for supporting mission-critical use cases. This 5G distributed mMIMO solution is ideal for hotspot and enterprises applications, including train stations, subways, airports, shopping malls, stadiums, and hospitals. It has already been deployed at scale commercially in major Chinese cities and provinces, such as Shanghai, Beijing, Chongqing, Zhejiang, Guangdong, and Sichuan. At Guangzhou Metro Line 18, it achieved a peak rate of 1.39 Gigabits per Second (Gbps) and an average rate of 1+ Gbps. At Guangzhou South Railway Station, the average user experience rate has increased by 2.9X, while the number of user connections has increased by 2.1X.

Radio Stripes: A radio stripe is a highly distributed mobile network design that aims to improve network quality and performance, while enabling easy network deployments. In a radio stripe system, the antennas and the associated Antenna Processing Units (APUs) are located in the same cable consecutively to provide data transfer, synchronization, and power supply through an internal connector. This solution is expected to be an ideal solution in both indoor and outdoor environments, including dense urban streets, shopping malls, stadiums, and factories. In 2019, Ericsson showcased this technology at the Mobile World Congress (MWC); it is still a preliminary technology. In 2021, Ericsson joined the REINDEER project to develop 6G multi-antenna-based smart connectivity, and radio stripes are expected to lay a strong foundation.

pCell Technology: pCell exploits interference in wireless networks through large-scale coordination among distributed transceivers and synthesizes a cell for each user. In this way, all users get to use the entire spectrum capacity regardless of location and there is no hand-over with users moving through the coverage area. So pCell multiplies the spectrum capacity with uniform and high data rates in the entire coverage area. This technology has been commercialized by Artemis as the pCell multi-gigabit Long Term Evolution (LTE)/5G Virtualized Radio Access Network (vRAN). Artemis claims that a network powered by pCell can achieve a speed of 1+ Gbps using 20 Megahertz (MHz) of spectrum and 7.5+ Gbps in 150 MHz of spectrum. The pCell vRAN from Artemis now operates in sub-6 GHz, and there has been a live 28-antenna deployment providing service to a 20,000-seat arena in San Jose, California.

Radio Information System (RIS): An RIS is software-controlled to use smart radio surfaces with a massive number of metamaterial elements to dynamically control and shape the radio signal. This technology is nearly passive, so it can achieve low weight and be constructed into any shape to meet the various requirements of the different deployment scenarios, such as walls, ceilings, or lampposts. A RIS also has lower hardware costs and energy consumption compared to technologies based on active antenna arrays.  It is also free of antenna noise amplification and self-interference, and can operate in any frequency level (sub-6 GHz to Terahertz (THz)). Many infrastructure vendors and network operators have started their independent research on RISs. In 2020, a transparent dynamic metasurface was demonstrated by NTT DOCOMO to improve indoor coverage. The metasurface can reflect the incident signal with partial/full power and enable the signal to penetrate with a power loss of almost zero. In 2021, NTT DOCOMO reconducted the trial to show that a transparent metasurface can increase the power level of received signals at indoor focal points. At MWC 2022, ZTE showcased that a transparent RIS, PIN diode RIS, and liquid RIS are suited to attach to glass surfaces of building windows to improve indoor coverage.

The distributed mMIMO solution has already been deployed at scale in major Chinese cities and in South Korea by LG U+. Network operators often do not secure the ideal type or amount of spectrum, so this solution helps lead to significant improvements in terms of capacity, throughput, and QoS in scenarios without it in place, generating significant impact on a network operator’s competitiveness and Return on Investment (ROI). It is a valuable addition to the range of 5G solutions and there will be further evolutions of the concepts. Moving toward 5.5G, ABI Research expects distributed MIMO to be widely implemented in more countries.

Regarding the pCell technology, so far, it is only being commercialized by Artemis, which is a small company. Its current focus is on private networks because network operators would rather wait for one of the major equipment vendors to deliver this technology than engage with a small company. Also, the current solution only operates in sub-6 GHz and there is a clear timeline for when Artemis plans its testing on Millimeter Wave (mmWave). When approaching 5.5G, mmWave may be more accessible and service providers will seek solutions supporting both sub-6 GHz and mmWave.

An RIS can be a real value-added technology with key technical challenges being adequately addressed before it is integrated into future communication standards and the move toward commercialization. So far, standardization of RISs has only kicked off on regional levels. ETSI launched an industry specification group on RIS (ISG RIS) in October 2021 for a duration of 2 years to address some of the challenges and establish global standardization of RISs. Companies including KDDI, ZTE, and China Unicom have proposed a study item on RIS for The 3rd Generation Partnership Project (3GPP) Release 18 to explore the use cases, RIS performance evaluations, and channel models from January 2022 to June 2023. However, many of the companies in 3GPP believe it is too early to kick off for the RIS as it is viewed as a technology for 6G. Radio stripes are also expected to be a technology for 6G, but there has not been much update on development since Ericsson showcased it in 2019 and it is expected to lay the foundation for the REINDEER project, which is a 6G research project.