The Indoor 5G Challenge

As 5G mobile networks build out, Mobile Service Providers (MSPs) must ensure consistent quality of service for their customers in both outdoor and indoor situations. This is particularly challenging for high-band, or millimeter wave (mmWave), spectrum as subscribers follow the data indoors to where it originates or is consumed 80% of the time.

Unlike low-band spectrum, mmWave signals do not penetrate buildings, so relying on outdoor to indoor signal penetration is a non-starter. The mmWave signal must be brought indoors and, even then, the physics of Radio Frequency (RF) propagation at these frequencies dictate Line-of-Sight (LOS) conditions for good signal reception/transmission. The indoor environment offers many obstacles, such as partitions, walls, doors, etc., which act as reflectors for the mmWave signal, increasing multipath fading and interference in the channel and potentially reducing the data throughput.

The massive throughput and capacity advantages of 5G mmWave when compared to mid- or low-band 5G are compelling MSPs to move 5G signal sources indoors; if the handset or terminal drops the mmWave signal as it moves indoors and falls back to mid-band, low-band, or even Long-Term Evolution (LTE), the subscriber will experience a rapid reduction in throughput and a failure to meet quality of service expectations. However, while there are no standards or methods for indoor 5G mmWave yet, exploratory testing from Qualcomm does show that mmWave is potentially viable indoors.

Notwithstanding the challenges of indoor 5G mmWave, MSPs must evaluate indoor 5G alongside multiple technologies and architectures that are converging in the indoor Radio Access Network (RAN). Multiple advances in technology and architecture alternatives, along with mmWave, unlicensed, and shared spectrum availability, all contribute to the drive toward very high throughput, ultra-dense, multi-technology heterogeneous networks both indoors and outdoors. The complexity of this convergence era is discussed in the ABI Insight Moving 5G Indoors (IN-553).

Deploying 5G indoors is viable for low- or mid-band spectrum but deploying 5G using mmWave spectrum is much more difficult. In addition, given that Wi-Fi 4 and 5 are very widely deployed and “5G-like” Wi-Fi 6 is imminent, deploying mmWave indoors may not offer a viable business case. 5G mmWave indoors needs to be at least as foolproof as its alternatives to succeed.

Perhaps signposting the way forward, Verizon announced that it is partnering with indoor connectivity, Distributed Antenna System (DAS), and Wi-Fi specialist Boingo to “architect a hyper-dense network designed for large and small indoor spaces such as airports, stadiums and arenas, office buildings, hotels, and other public spaces, as part of Verizon’s ongoing 5G network expansions.” No details on timeframe or RAN architecture or equipment were announced.

Another signpost comes from South Korea—which is one of the world’s leading 5G markets, with more than 1 million 5G subscribers so far, and on track for between 4 and 5 million by the end of the year—according to reports in the press. Perhaps in response to subscriber complaints, South Korean MSP KT (formerly Korea Telecom) has announced that it will begin the rollout of 5G networks at large, densely populated buildings such as airports and train stations. KT has been developing 5G in-building repeater equipment since 2018 and is now ready to deploy in commercial networks. KT plans deployments in major train stations for KTX, South Korea's express train service, in addition to large department stores in the metropolitan areas around Seoul and Gimpo International Airport. KT also plans to collaborate with competitors SK Telecom (SKT) and LG Uplus to expand coverage to 1,000 buildings by the end of the year.

While KT and SKTs 5G indoor plans use the mid-band spectrum, ABI Research believes that it is an indicator of the challenges involved that neither MSP is considering mmWave for 5G indoors.

While Verizon’s 5G deployments to date have been in the mmWave band for the highest throughput, given the impossibility of mmWave penetrating buildings from outside, it is not surprising to see a collaboration with Boingo. Leaning on lessons from South Korea and leveraging Boingo’s technology agnostic in-building skill sets in wireless including 4G, Wi-Fi 6, private LTE, CBRS/OnGo, or other wireless technologies makes sense.

MSPs need to take this seriously, as the promises of enhanced mobile broadband in mmWave will enable them to serve larger enterprise customers and get their networks into more spaces. This is even more important for U.S. MSPs like AT&T and Verizon, as early U.S. 5G networks rely more heavily on mmWave signals than KT's networks do.

We have no insight into the 5G mmWave RAN architecture that MSPs are planning and whether it is repeater-based, like KT, a DAS, or a Distributed Radio System (DRS), but we do know that many radio vendors in the ecosystem will be only too happy to help.

For a high-level discussion of the issues surrounding indoor 5G please see ABI Research’s 5G Indoor Deployments (AN-5099)Application Analysis Report.