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What Are Distributed Antenna Systems? Comprehensive Overview

In this article, we explain what DAS is, types of DAS, how DAS works, and how it relates to private LTE / 5G networks.

Interested in Distributed Antenna Systems (DAS) for your facilities? We’ll explain what DAS is, types of DAS, how DAS works, and how it relates to private 5G / LTE networks for enterprise.

What is a Distributed Antenna System (DAS)?

Distributed antenna systems are networks of antennas connected to a common source and distributed throughout an enterprise building / facility to extend public cellular wireless network coverage.

DAS are commonly used for large areas like sport stadiums and event venues that require support for high density of cellular wireless clients - in areas where mobile operator cell towers may not have adequate reach. Here’s how it works.

How does DAS work?

First, a distributed antenna system’s signal source receives cellular signals from a mobile carrier. Then, DAS amplifies the signal and distributes it throughout the area. Consequently, the signal is extended to cover a large indoor area. Antennas for DAS are set up across multiple floors separately in order to ensure appropriate coverage.

In order to understand how DAS works more thoroughly, we’ll need to dive deeper into the details of its architecture and primary components.

DAS architecture & components

DAS are divided into two main components: the signal source and the signal distribution system. The signal source receives signals from and sends signals to mobile carriers. The signal distribution system distributes the signal throughout the building.

Let’s start with the different types of DAS signal sources.

Types of DAS signal sources

DAS have three types of signal sources: off-air antennas, on-site base transceiver stations (BTS), and small cells. All three serve to receive signals from nearby mobile carriers, but each has its pros and cons.

Off-Air Antennas (Repeaters)

Off-air DAS use a donor antenna placed on the roof of the building to send and receive signals from a cell carrier.

Off-Air Antenna Pros

Off-air antennas tend to be the cheapest and fastest option to extend coverage inside a building. Off-air antennas can also work with multiple carriers at a time.

Off-Air Antenna Cons

Off-air antennas’ performance depends on donor signal strength and quality, not to mention macro network congestion. Changes in donor signal can drastically affect network performance.

Off-air antennas do not add extra capacity, often require retransmit agreements with each carrier prior to installation, and can be difficult to optimize for multiple carriers.

Base Transceiver Stations (BTS)

On-site Base Transceiver Stations (BTS), NodeB, and eNodeB signal sources work the same way as cell towers. The BTS typically connects via fiber optic cables to the mobile operator’s core network, separate from the enterprise IT infrastructure that might already be in place. In some cases, multiple BTSes are installed, one for each carrier.

Base Transceiver Stations (BTS) Pros

BTS signal sources tend to provide the most performance, including adding extra capacity for high-occupancy areas like stadiums and airports.

Base Transceiver Stations (BTS) Cons

BTS signal sources tend to be the most expensive and slowest to deploy, partially because each carrier has to provide their own fiber optic cable and BTS. BTS signal sources also require additional space, cooling, and power that can run up high operating expense costs.

Small Cells

Small cells are a smaller version of macro cells. Small cells can be used as an alternative to distributed antenna systems, but small cells can also be used as signal sources in DAS.

As signal sources, small cells create secure tunnels back to carrier networks via internet connection to generate high-quality wireless signal - also designed and built separately from the enterprise IT network and backbone infrastructure. These are used primarily to carry the public mobile network signal throughout intricate indoor areas.

Small Cells Pros

Small cells are cheaper than BTSes, relatively fast to deploy, and can create high-quality wireless signals for large buildings containing hundreds of users.

Small Cells Cons

Firstly, not every mobile carrier offers small cells. Secondly, small cells require reliable backhaul internet connections, even though they rely on venue-provided connections. Lastly, small cells can be difficult to scale up for thousands of users, such as for stadiums.

Off-air antenna relays, on-site base transceiver stations, and small cells are only the types of signal sources. But there are also multiple types of signal distribution systems.

Types of DAS signal distribution systems

The three main types of DAS signal distribution systems are passive DAS, active DAS, and hybrid DAS.

There are also digital distributed antenna systems too, though those are less common.

Passive Distributed Antenna System

Passive DAS use passive radio frequency components like coaxial cables, splitters, taps, and couplers.

Passive Distributed Antenna System Pros

Passive distributed antenna systems are cheaper relative to the other types of DAS, partially because it doesn’t require extra equipment to support multiple carriers. The absence of extra equipment can also make passive DAS simpler to maintain.

Passive Distributed Antenna System Cons

Passive DAS run on coaxial cables, and longer runs of cable suffer greater attenuation. The attenuation problem requires more precise link budget calculation.

Active Distributed Antenna System

Active distributed antenna systems receive analog radio frequencies from the signal source. Then a master unit converts the analog transmissions to digital signals. The digital signals are distributed through fiber optic or ethernet cables to remote radio units (RRUs). The RRUs convert digital signals back to analog.

Master units can convert signals from as single or multiple carriers. Active DAS also do not use coaxial cables.

Active Distributed Antenna System Pros

Active distributed antenna systems don’t use coaxial cables, meaning there’s no limit to the lengths of coaxial cable runs. This makes Active DAS much more expandable for larger buildings.

Active Distributed Antenna System Cons

Active DAS can be much more expensive than passive or hybrid systems, partially because the RRUs are more expensive and require more dedicated power.

Hybrid Distributed Antenna System

As the name implies, a hybrid distributed antenna system is a mix between active and passive DAS. The distributed antenna system’s RRUs are separated from the antennas, enabling the use of both fiber optic and coaxial cables. RRUs on each floor convert digital signals to analog that connect to antennas on that floor via coaxial cables.

Hybrid Distributed Antenna System Pros

Because hybrid distributed antenna systems require fewer (expensive) RRUs, they tend to be cheaper than active DAS though more expensive than passive ones. Hybrid DAS also aren’t limited by lengths of cable runs, at least for the digital part of the backbone.

Hybrid Distributed Antenna System Cons

As above, hybrid distributed antenna systems are still more expensive than passive ones. Their mixture of both fiber optic and coaxial cable can complicate installation, and coaxial cables still require precise link budgeting per each floor.

Digital Distributed Antenna System

Digital distributed antenna systems are relatively new under the Common Public Radio Interface (CPRI) specification. They use a Base Band Unit (BBU) to connect directly to a master unit without analog-to-digital conversions.

In theory, digital DAS should be simpler and cheaper to deploy. But as an emerging technology, digital DAS have competing standards and have enjoyed little deployment so far.

So what does all this mean for enterprise organizations?

Distributed antenna systems for the enterprise

Enterprise organizations turn to distributed antenna systems to extend coverage and expand capacity for public mobile network connectivity - for their visitors and staff who take advantage of cellular wireless connectivity as part of the received service from mobile operators. This can help with large buildings with high-occupancy. This can also help with high-altitude buildings that normally suffer atmospheric interference, or buildings that are very far from cell towers.

For example, hospitals benefit from distributed antenna systems that take strong cellular wireless signals and evenly distribute them through tough to cover areas and high density of users. Alternatively large campuses and remote worksites can use a distributed antenna system to connect outlying regions of their operations with reliable public cellular service.

However, there are alternatives to distributed antenna systems, like small cells. So how do DAS compare to small cells?

Advantages of distributed antenna systems over small cells

Small cells usually only support a couple bands from a couple carriers at a time, whereas distributed antenna systems can handle many more bands for more carriers. So DAS tend to offer more capacity with more carriers than small cells.

DAS only needs a single backhaul connection and is built to support more users, whereas small cells work on their own and generally can support only a few dozen devices. In certain cases it can be difficult to provide backhaul connections to each small cell, where with a distributed antenna system there is only one point of connection to maintain.

Disadvantages of distributed antenna systems relative to small cells

Compared to small cells, DAS require fiber optic or coaxial cables to connect everything, which drive up costs to be substantially costly compared to small cells, especially for large venues like stadiums.

Needing cables for every radio head can require lots of cables, which complicates cable routing, management, and optimization. Where small cells can be upgraded over the air, distributed antenna systems require on-site technicians to replace base stations or modify radio heads, making DAS much harder to upgrade.

Overall, small cells traditionally have a reputation for being cost effective for smaller areas while distributed antenna systems offer increased capacity for larger areas and higher occupancies. However, the difficulty in upgrading DAS’ and the advancements in small cells may change this.

Private LTE & 5G networks for enterprise

What does this all mean for the future of private mobile networks?

For many enterprise organizations, paying per bit to send sensitive information over someone else’s public commercial network was unpalatable. Yet, the need for high-speed, wide-coverage LTE or 5G necessitated it.

But since the FCC opened up the CBRS spectrum in the United States for the use of cellular wireless technology, enterprise organizations can now build and operate their own private mobile networks without expensive licensing fees or metered monthly per-device contracts - instead of going through mobile carriers. Now, enterprises can enjoy the speed, security, coverage, and capacity of private LTE and 5G customized to their specific private applications and environments.

Use Cases: DAS vs. Private LTE and 5G

Private mobile network implementations favor organizations that need large coverage areas, and rely on critical applications that need the highest levels of predictable performance on enterprise wireless. While distributed antenna systems focus on public mobile network connectivity for visitors and personally owned devices, private LTE / 5G wireless focuses on use cases that are private within the four walls of the enterprise - on dedicated enterprise owned and staff operated mobile/IoT devices.

Healthcare Facilities

In healthcare, private LTE / 5G  wireless within medical buildings can scale to meet the reliability required by internal applications, which Wi-Fi would simply be too interference-prone given neighboring networks and increasing density of users. Given device level security used thanks to SIM authentication on the devices, a private LTE / 5G network could help securely transmit HIPAA information to doctors, improve the quality of voice communications and computer vision applications - along with keeping IoT medical devices securely connected.

College Campuses

Colleges are like a small city of their own. CBRS based private mobile networks can serve outdoor spaces in higher education in more ways that we can imagine today: video surveillance for physical safety, staff communication across VoIP applications, secure IoT infrastructure connectivity for parking meters, research facilities and more. A segmented portion of private LTE / 5G wireless can be reserved for partner organizations as they require connectivity across large coverage areas, away from the interference of public mobile networks and student Wi-Fi.

Sports Stadiums

In order to support thousands of devices at once, a combination of DAS and small cells could be used to distribute both public cellular access. A distributed antenna system can use a series of higher power antennas to provide blanket outdoor coverage while small cells ensure that indoor areas such as offices and concession stands maintain similar speed and coverage. Next to venue guest Wi-Fi and DAS that support public mobile network connectivity for visitors, IoT systems and mobile devices used by venue staff, sports teams, retailers for their operations can rely on CBRS based private LTE / 5G wireless. Away from congestion and interference, critical applications such as mobile point-of-sale (POS), push-to-talk (PTT) voice, computer / thermal vision sensors, video surveillance cameras and more can take advantage of the express lane of wireless communication offered by CBRS spectrum and private mobile networks.  

Do you have use cases for a private mobile network?

If your organization is struggling to maintain wireless coverage across very large areas and adequate interference free operation for critical use cases for your private use, we can help. Discover if our private LTE / 5G wireless solution can help by reviewing our use cases and by contacting us about a free trial of our solution within your facilities.

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