Previous generations of cellular networks have relied on distributed antenna systems (DAS) to provide in-building coverage and to fill coverage gaps in complex structures. Though the same is true for emerging 5G systems and rollouts, there is a twist. Where previous generations of DAS antenna usually meant just covering new bands, 5G DAS also needs to support multi-antenna operation for advanced/active antenna system (AAS) beamforming and MIMO techniques that enable 5G. To support 5G speeds and coverage goals, new 5G DAS must also offer superior electrical performance. This is a challenge for network and antenna solutions.
5G REQUIREMENTS & CHALLENGES FOR NEXT-GENERATION DAS SOLUTIONS
Much of the cellular deployment and application discussion focuses on outdoor usage. We are familiar with mobile handset and automotive outdoor usage, but recent studies1 show that over 80 percent of voice and data usage for handsets occurs indoors. This means that much of the demand for 5G services is likely to come from users located in buildings. While cellular network deployments will be able to provide outdoor coverage to users, it is unlikely that these networks will provide adequate coverage in mass transit systems, large venues, office spaces, warehouses and other indoor spaces. 5G will rely heavily on high frequency spectrum and the propagation characteristics of these signals mean they do not effectively penetrate buildings with concrete and low emissivity glass.
Traditional DAS Challenges
These new challenges are similar to traditional DAS challenges: how do we get cellular coverage into large, complex structures that are made up of a diverse mix of reflective and absorptive materials, along with odd geometries and layouts? With traditional cellular deployments, the link to the cellular services is often through an external antenna that connects to an existing cellular base station and tower. Alternatively, if the venue is large enough, it may have its own base station or use a nearby base station to route signals into the DAS.
Much of the design challenge is determining an appropriate layout for the DAS antenna, remote radio heads or nodes depending on the DAS architecture. Network designers must consider the use of RF cable runs, repeater antennas, fiber optic and Ethernet interconnections between the cellular network and the DAS. The DAS architecture must ensure adequate coverage with the desired frequency bands. As Wi-Fi has grown in popularity, DAS designers are facing the new challenge of offering multi-operator cellular services along with Wi-Fi services. The solution to these challenges typically involved upgrading the DAS hubs, antennas and interconnects to accommodate these new frequency bands while ensuring that DAS placement met customer expectations for service coverage. A typical indoor DAS network is shown in Figure 1.
Emerging 5G DAS Challenges
5G currently uses three frequency ranges, with a variety of frequency bands within those ranges. GSMA defines these three 5G frequency ranges as low-band, with frequencies below 1 GHz, mid-band with frequencies from 1 to less than 7 GHz and high-band as frequencies above 7 GHz, but generally mmWave frequencies. The low-band frequencies generally exhibit the greatest range and coverage capability, but at limited data rates and capacity. Low-band networks are being deployed mostly in rural areas lacking good 4G coverage. These areas likely will not be significant for new DAS deployments. The mid-band range covers the frequency capability of most cellular handsets and this band is seeing the majority of current 5G activity. The mid-band frequency range is adjacent to other common wireless services, such as the 2.4 and 5 GHz ISM bands with services like Wi-Fi, Bluetooth, Zigbee and Matter. The high-band frequency ranges are in the mmWave spectrum where there is currently less activity and deployment effort. However, high-band services are likely to continue to be developed and deployed for the next several decades and future DAS deployments will increasingly need to account for services in these frequency ranges.
Beyond extending the frequency range to 7 GHz, or 7.125 GHz to accommodate Wi-Fi 6e, other capabilities differentiate 5G mid-band systems from legacy cellular technologies. Though 4G/LTE standards did introduce multi-antenna MIMO technologies, these are relatively simple arrangements of a handful of antennas, at most. In these cases, MIMO is generally used to achieve peak data rates for a single-user device. 5G standards introduced multi-antenna technologies to enhance coverage for several users in a given area, as well as beamforming technologies to enhance the gain of a given 5G antenna toward target user devices. For DAS to provide the same capabilities as 5G network antennas, they must become more complex and capable with multi-antenna technology.
New 5G AAS technology enables much higher data rates and coverage in complex deployment scenarios, but this capability must be supported by the DAS distribution system and infrastructure. To do this, DAS interconnects must be able to support higher capacities, regardless of the 5G signal source. In addition, 5G DAS must support the legacy 4G/LTE cellular systems along with new 5G frequency bands and technologies.
Fundamentally, DAS is entering a new era with 5G. Legacy 4G/LTE networks and services have evolved with mobile handsets as the focal point. The 5G vision addresses ultra-reliable low latency communications (URLLC), massive machine-type communications (mMTC) and enhanced mobile broadband (eMBB). These additional use cases and implicit capabilities broaden the focus to IoT devices and critical systems requiring reliability and low latency services. This repositioning envisions applications from real-time monitoring systems to autonomous mobile robots operating alongside human workers, customers and/or pedestrians.
Aside from massive MIMO (mMIMO) and beamforming, additional 5G technologies will impact how DAS are designed and deployed. While things like network slicing and network function virtualization are more of a consideration for 5G network operators, these features will influence the DAS requirements. New and emerging business cases will also influence DAS requirements as many large enterprises, government organizations and large campuses are considering 5G private networks alongside public 5G networks.
DAS EVOLVES TO ADDRESS 5G COVERAGE CHALLENGES
To address these challenges, the underlying infrastructure, interconnects and DAS antennas must evolve. Despite a large diversity of DAS solutions, more product solutions and availability are needed to maintain, upgrade and deploy new DAS installations.
The cellular network is an extension of the internet backbone. A mobile core in the cellular access network is the bridge between the radio access network (RAN) and the internet infrastructure. In 5G networks, that mobile core may be deployed at the edge and the mobile core and RAN may be co-located. A backhaul network connects the RAN to the base stations and then a remote radio unit (RRU) distributes wireless service from the base station to the user equipment. In these systems, the DAS may connect to the cellular network through an antenna, a base station or a small cell.
Using small cells to drive DAS is becoming a more common solution to support the throughput and capacity expected from 5G services. This solution requires multiplexing small cells from each service provider to the backhaul internet connection. Aggregating rapidly growing 5G traffic may strain transport capabilities and this may necessitate the installation of higher capacity transport networks. Worst case, this increases the DAS costs and complexity, but much of this 5G traffic may be replacing already existing traffic, resulting in little to no need for increased capacity. Small cell connections are likely to be the biggest growth sector as DAS seeks to enable 5G performance expectations and features. In response to a perceived need for in-building 5G services, small cell manufacturers are enhancing their DAS-like distribution systems. The result appears to be a convergence of small cell and DAS technology for in-building applications.
There are three main strategies for DAS signal distribution: passive, active and hybrid. Passive DAS uses a DAS head end connected to a signal source, which feeds a passive DAS network of antennas and other passive components. Common passive components in this arrangement include splitters, coaxial cable, coaxial adapters, tappers, combiners, dividers and couplers. Passive DAS systems rely on RF signal generation, so they are only as capable as the signal source. These systems have limitations in 5G installations and may be useful only for applications like the last stage of distribution in a hybrid DAS system. An active DAS system uses digital data transmitted over a fiber or Ethernet network to a master unit. The master unit converts the source data to digital signals for distribution to the active DAS antenna. Electronics in these units convert the data stream to RF or digital as needed. A hybrid DAS is similar to an active DAS, but the RF/digital conversion takes place in a separate unit that feeds several RRUs.
It is difficult to determine the mix of signal distribution techniques for 5G services. All methods will likely be necessary to accommodate various installation dynamics and cost considerations. To integrate DAS with 5G services, the antenna units or RRUs must support 5G multi-antenna or AAS technology. DAS antenna units must evolve from omnidirectional and directional antennas to arrays supporting MIMO and beamforming technologies. These technologies require much higher interconnect densities and signal distribution complexity than previous cellular technologies. To reduce costs and maintain small footprints, DAS systems will become more integrated and incorporate denser board-level technologies.
DAS antennas and distribution systems must support higher throughput and higher frequency signals and provide better performance than prior generations of cellular networks. Higher frequency 5G signals will experience higher transmission and RF component losses. Offsetting these losses may require higher transmit power, but this will create concerns about passive intermodulation distortion and other nonlinear characteristics. Degrading performance in these areas could reduce the signal-to-noise ratio, bit-error rates or packet-error rates. The active components in a DAS will need higher levels of linearity over a much wider bandwidth than with previous cellular generations. The RF-to-digital and digital-to-RF conversion electronics will require much higher conversion rates in response to the new 5G frequency bands. Conversion electronics in these bands are expensive and this may drastically increase the cost of active and hybrid DAS compared to passive DAS.
Future mmWave DAS
The previous discussion focuses primarily on sub-6 GHz 5G implementations. Incorporating mmWave 5G technologies into DAS systems presents a different set of challenges. mmWave 5G enables high throughput and much more targeted signal distribution. mmWave 5G is seeing some success in the fixed wireless broadband market where 5G wireless services are used to deliver home internet and other media services. Enterprise and in-building applications have been slow to emerge at these frequencies and it seems unlikely that millimeter DAS will be deployed in the short term. Enabling faster millimeter DAS adoption will require an ongoing evolution of RF and digital technologies, along with new building construction material and layout considerations.
The outcomes and exact steps along the way are never clear with any emerging technology. This is the same with DAS evolving to include 5G technologies. What is clear is that there is currently competition from small cell manufacturers, service providers and DAS installers. The lines are blurring between DAS and small cells when it comes to providing 5G services in venues, stadiums, arenas, mass transit systems, campuses and enterprise locations. Much more flexibility will be needed for DAS installers and service providers to develop methods of delivering 5G services in buildings, where most 5G services will be used.
- Ericsson, “Planning In-building Coverage for 5G: From Rules of Thumb to Statistics and AI,” Ericsson Mobility Report, June 2021, Web: www.ericsson.com/en/reports-and-papers/mobility-report/articles/indoor-outdoor.