In-building wireless boosts cellular performance across any frequency band, even for low-band frequencies that can penetrate buildings.

The world is buzzing about the promise of 5G but bringing that connectivity inside buildings is an equally important part of the equation. Many consumers are not aware that 5G frequencies may not transfer inside buildings without extra investment and technology. A well-known statistic used in the wireless industry is that 80% of cellular connections begin and end indoors. Furthermore, according to the Ericsson 2021 ConsumerLab report, early 5G adopters rated indoor coverage – at home or in public places — twice as important as 5G speeds or device battery life.

There are few different methods for bringing macro cellular network indoors, but none more common than distributed antenna systems (DAS). It is a proven and commonly used technology and will remain critical for the continued growth of 5G. Before diving into what makes a DAS, it’s important to understand the challenges it solves.

Why is it challenging to bring signals indoors?
The frequency bands used for each wireless generation all have unique characteristics. But to at least some degree, all bands have issues penetrating man-made and natural environments. Most buildings are made of concrete and metal, which are two of the most RF-obstructive materials. Consider that an average cellular phone receives signal levels ranging from less than -70 dBm to -120 dBm, which is akin to full bars and no bars. Metal and concrete can attenuate signals anywhere between -32 dB to -50 dB and -10 dB to -20 dB, respectively.

Depending on the frequency bands used for 5G, it can even be more challenging. While all carriers use some mix of low-band (600 MHz, 700 MHz), mid-band (C-band, 2.5 GHz), and high-band (mmWave) spectrum, higher bands travel shorter distances and are more easily disrupted. While mmWave (28 GHz, 39 GHz, etc.) is viewed as ideal for providing high speed and low latency to small dense areas, it cannot provide cost-effective sprawling nationwide coverage.

What comprises a DAS?
A DAS (Figure 1) is made of multiple connected components that together bring unobstructed RF signals indoors. The head-end is largely seen as the hub of the DAS and connects to a base transceiver station (BTS) or bi-directional amplifier (BDA) with a donor antenna (outdoor antenna), which is a transmitter of cellular signal to a BTS. In a modular DAS, the head-end can support many interchangeable frequency bands and is typically placed in a “telecom closet” or the deep recesses of a building where it isn’t visible.

Remote units (RUs) are dispersed across different sectors of a deployment and connected to the head-end via fiber optic cables. Depending upon output transmission power of RU, each RU can be further connected to many serving antennas dispersed across the facility via coaxial cable to create a network of antennas and provide strong signal throughout the facility. It’s important to note that DAS excels at bringing both coverage for multi-bands or multi-carriers, while small cells are typically used to improve the coverage for a single band or single carrier. In some cases, small cells are used in replacement for a base station to provide a signal source for DAS. For any DAS installed for public safety purposes, each installation requires a battery backup unit (BBU) in case of emergencies so first responders can still communicate if electricity is cut.

Most modern deployments use an active DAS, which uses fiber-optic cable to distribute signals between HE and RUs. The active DAS is scalable and optimal for medium to large size buildings. Passive DAS is optimal for small to medium size buildings. Passive DAS has limited scalability, and signal strength depends on donor site input. However, it can be less costly to install. Passive DAS typically uses bi-directional amplifiers (BDA) to redistribute signal and amplifies and distributes wireless coverage from a nearest tower through a donor antenna.

Design consideration and challenges for each DAS is unique depending on the facility shape, location, and size. For example, a stadium might require multiple sectors of coverage, each with multiple RUs, whereas a miles-long tunnel might require a single sector with a higher number of RUs. These parameters are addressed prior to deployment in RF design software programs, such as Ranplan and IBWave. Making the right decisions regarding frequency bands, sectors, and anticipating interference caused by various obstacles can be the difference between a successful deployment and one that requires thousands or even millions in remediation.

DAS along with several other technologies will remain as a crucial solution to bring 5G indoors, but buildings must also support LTE and many different frequency bands used by the three major US carriers. Therefore, DAS modularity and the ability to support all these different frequency bands in each DAS is more important than ever. There is no silver bullet when choosing in-building communication solutions but selecting one that is future proof can avoid a costly rip-and-replace situation.

Andrew Jun, Ph.D., is the Chief Technology Officer at Advanced RF Technologies Inc. (ADRF), responsible for corporate strategic direction, technological advancement, and management of research and product development for the company. Prior to joining ADRF, he served as the Head of Business Operations at CS Corp and as a Senior Vice President (SVP) and Chief Technology Officer (CTO) at RF Window. He also held leadership roles at Korea Telecom and Alcatel-Lucent. He holds a BS degree in Engineering Science and a Ph.D. in Electrical & Computer Science from the University of Toronto.