Guide to 5G: Distributed Antenna Systems (5G DAS)

Feb 6, 2021

The following article explores the impact of 5G on DAS systems. To begin, we will first review what DAS is, along with the benefits, the challenges and what considerations are involved when managing a 5G deployment.

What is DAS (Distributed Antenna Systems)? How does DAS work and why is it a great In-Building Coverage Solution?

What are Distributed Antenna Systems (DAS)?

A DAS is a system that distributes the RF signal across antennas that are installed in the building primarily used to modify, improve or extend coverage of a site. A DAS consists of:

  1. A Signal Source

  2. A Distribution System

The signal source - usually a base station, small cell, or repeater - has its signal distributed by fiber optic cables to remote devices throughout the building. DAS is different from other systems like microcells or repeaters because it uses many interconnected devices within large facilities to boost performance (versus one or several united per facility).

Read on to learn why DAS may be the best solution for you over other in-building solutions.

Why is an in-building solution needed?

An in-building solution is needed because macro coverage will not penetrate all buildings. The use of reflective and dense materials prohibits RF energy from propagating within many structures. Basements and parking garages especially are surrounded in concrete rebar. For newer buildings, LEEDs and Green Building initiatives have requirements to add RF signal attenuation through the increased use of Low-E glass (metals in glass) and reflective (foil backed) insulation. Finally, shadowing can be caused by nearby structures such as buildings, hills/mountains, etc.

First, let's compare some of the in-building solutions.

  • Repeaters or bi-directional amplifiers (BDA) are used in small venues to expand coverage.  DAS antennas and splitters provide coverage to various locations inside buildings from the input of the amplifier.

  • Base Transceiver Stations (BTS) are used in the macro network suitable for wide area coverage supporting a large number of users, but it is expensive and requires a dedicated backhaul.

  • Small cells are low powered (<1 Watt) radio access points to increase capacity and offload traffic in outdoor or indoor coverage.

  • Femtocells provide BTS service to 4 -- 20 users over a limited area (5K sq ft).

What are the different types of DAS?

1. Passive DAS: Radio frequency is distributed over coax cables to each antenna from an RF source (repeater or base station). However, coax cable transmission losses can limit effectiveness in large buildings for higher frequency transmissions.

2. Active DAS: Fiber-optic cabling is used for distribution to overcome the transmission losses that occur with a coax cable system. Fiber optic cables connect directly to the active antennas for radiation.

3. Hybrid DAS: Combines the best of a passive and active DAS system. Like active DAS, fiber-optic cabling is used for distribution. The digital signal is then converted to an analog RF signal via a Remote Radio unit, which is then connected to a floor antenna for radiation.

4. Hybrid DAS Small Cell solution: Similar to Hybrid DAS, but instead of using fiber-optic cabling for distribution, a small cell deployment is used for radiation on each floor.

What are the benefits of DAS?

There are a number of benefits with DAS systems:

  • Very scalable with excellent maintenance. DAS is a scalable network that can meet future capacity requirements and additional carriers by adding additional nodes. This can be a significant benefit for neutral property owners who value the ease of adding additional frequency bands or operators to an existing system. Responding to market dynamics, equipment architecture changes and new technologies is far easier compared to a small cell system, which would require the capital cost of a completely new deployment if a new signal or frequency band is added.

  • Good coverage, especially for highly populated buildings. DAS architecture provides coverage in areas that cannot be effectively addressed with traditional sites. Because of its flexibility and scalability, it is especially recommended for larger facilities (100,000 sq ft to over 500,000 sq ft)

  • Capacity alignment to actual need. The capacity requirements more closely match to actual market requirements. It uses available frequency spectrum efficiently through multiple low-power transmission points.

  • Interference reduction. It reduces interference through low radiation centers and lower output power. DAS provides better data throughput given signal strength and proximity of transmission points to user equipment.

  • Reducing small cell deployments. A hybrid DAS and small cell solution can reduce the number of small cell deployments at a single location. Reducing small cell deployments is also beneficial to reducing potential interference issues.

What are the challenges of DAS?

With higher RF power, a growing number of frequency bands, and next generation 5G systems carrying a wider range of carrier frequencies, Passive intermodulation distortion (PIM) from cross band complications can be a significant challenge for in-building DAS systems. In contrast, small cell systems operate like a WIFI AP, and complicated PIM issues are not a consideration.

The PIM challenge can be complicated further with indoor DAS systems because network components are hidden from view for aesthetic reasons. PIM issues that surface after installation can be a significant OPEX cost burden to resolve and fix.

Is there a 5G Distributed Antenna System?

5G deployments operate in the sub 6GHz bands. Operators and carriers are re-farming their existing spectrum over to 5G, allowing for increased compatibility for newer and older DAS. The 3600MHz 5G band for example, is a popular choice worldwide, and can be supported by DAS components.

To meet the design challenges of PIM and 5G frequency coverage, selecting a component supplier who can offer passive network components and cables with low PIM rejection specifications and wideband frequency coverage is a consideration. Passive components engineered below the absolute lowest noise floor could meet the variability of multiple different RF systems:

  • For UMTS systems the receiver sensitivity is -119dBm, which requires at least -162dBc PIM

  • For LTE systems the receiver sensitivity is -126dBm, which requires at least -169dBc PIM

  • For higher power wireless systems greater than 46dBm such as outdoor DAS and cellular systems, <-165dBc PIM is required to avoid PIM impacts.

One such example is Radiocomm's latest set of Low PIM Hybrid Couplers that achieves -165dBc PIM across a wideband frequency of 520MHz-6000Mhz.

In Summary

Distributed Antenna System (DAS) is an in-building solution that consists of a signal source, a distribution system, and a network of antennas installed throughout the building. DAS is a great option for in-building coverage, especially for larger facilities. It is reliable, extremely scalable and easily maintainable. It introduces PIM issues however, and selecting a low PIM and ultra-wideband frequency component supplier is critical to reduce OPEX; in contrast, small cells do not have PIM issues, but do require completely new deployments if a new set of frequencies or signals are added, which can increase maintenance costs.

Have questions?

Acentury has been manufacturing low PIM wideband solutions for several years and offers superior low PIM RF products such as cables and components required for indoor and outdoor DAS applications. Contact us today for expert consultation and customized solutions.

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