Can Public Safety and Cellular DAS Coexist?
Posted 23 September 2014 by Tom Kuklo
What is a DAS Network
Distributed Antenna Systems (DAS) have been defined as any system that receives an RF signal either over the air or from a Base Station, redistributes or amplifies that RF signal into a defined area through a network of antennas and receives a signal from a network of antennas and amplifies that RF signal back to the Base Station.
DAS networks have been deployed for both public safety as well as cellular communications. The mission for DAS networks may have started out as coverage enhancement for both types of wireless communication, cellular and public safety. However, the DAS mission for cellular networks, due to the increased demand for data coverage, has evolved to one that increases capacity within the cellular infrastructure. In a nutshell a public safety DAS network is deployed to protect lives and property while a cellular DAS network is designed for the user’s convenience and to generate revenue for providers.
DAS Networks and Public Safety Communications
DAS has been an integral part of public safety communications since TX RX Systems designed the first signal booster in the US and installed it in an Illinois mine in 1979. Since their introduction DAS systems have been installed to extend mission critical communications coverage into disadvantaged areas like parking garages, high-rise buildings, tunnels, campuses, shopping malls, and airports. By definition mission critical public safety networks are put in place to protect the lives of both the first responders as well as the citizens that fire, police, and emergency medical personnel have sworn to protect.
Over the past ten years there has been a significant increase in local and state legislation that sets minimum coverage standards for public safety communication systems. These minimum standards as well as new public safety building codes implemented by International Code Council (ICC) and the National Fire Protection Association (NFPA) are driving the current demand for DAS systems.
The requirements imposed by these new regulations include:
— Battery back-up
— Monitoring of the antenna network
— NEMA-4 enclosures for survivability
— Alarming that communicates to the central fire alarm panel
Additionally, local fire and police chiefs demand proven coverage above designated thresholds to ensure that their officers are able to communicate to command vehicles and other safety forces. Public safety communication networks are designed to survive as long as possible in the event of either a natural or man-made disaster. These networks include hardened sites that can’t be overwhelmed by a spike in voice or data traffic.
Converged vs. Separate Networks
DAS networks supporting mission critical public safety communications have typically been installed as independent networks. This means that cellular communication is not typically installed on the same network or antenna system as the public safety system. In many cases this separate network concept originates with public safety’s mandate to ensure “always on” communications. Additionally, public safety DAS systems have history on their side. Public safety DAS networks were deployed in major cities and large structures long before there was a need for either cellular voice or data communication. As the demand for cellular coverage and capacity has increased within large structures and venues, the wireless carriers have installed their own DAS networks to ensure that their signals were being transmitted.
Many public safety communications experts today continue to insist on the separation of public safety and cellular DAS systems. Reliability or the lack thereof with cellular networks has been one of the major reasons for insisting on separate DAS networks. However, this reason or argument no longer has the same level of validity that it had five or ten years ago. The cellular equipment manufacturers and providers have invested billions of dollars to upgrade their systems and improve the reliability of their equipment. It can be argued that cellular users are demanding the same level of “always on” communications as the public safety communications officers, albeit for significantly different reasons.
Whether or not independent cellular and public safety DAS systems are installed in a particular venue, the two systems will interact or interfere with each other. A DAS system is a complex network consisting of a head-end unit, RF to light converters, coax and/or fiber optic cable, remote amplifiers, splitters, couplers, attenuators, RF loads, filters and antennas. All DAS networks must be designed with the needs of both cellular users and public safety communications requirements in mind. Additionally, the system(s) must be installed and tested in the same manner.
Recently, Bird Technologies became involved in a DAS systems problem at one of the casinos in Las Vegas. Bird Signal Boosters had been installed and operating in the casino for several years as part of a public safety and internal security DAS network. The casino also had a fully operating cellular DAS network installed on the premises.
After an upgrade to the cellular system the public safety network lost coverage in over 75% of the building. None of the cellular system operators felt that they had caused the reduction in coverage for the public safety network. Independently, both systems had been designed and installed to provide the desired coverage. However, the upgrade to the cellular network introduced an interfering signal into the RF environment resulting in the lost public safety coverage. Additional filtering was installed in the DAS network to eliminate the interfering signal and full coverage was restored.
Advantages of a Converged DAS System
Would a converged DAS system have prevented the lost public safety coverage in the above example? It is impossible to know for sure. However, it is known that the potential for lost coverage is significantly reduced when the system designers and installers understand the potential issues and conflicts between systems and take the necessary precautions to reduce or mitigate the potential for interfering signals.
What are the main advantages of a Converged DAS System?
— The system is designed and planned to accommodate the inherent interaction between cellular and public safety systems such as additional filtering (as used above) and antenna spacing.
— The system is designed to manage the wavelengths for the various bands and frequencies.
— There is design control for the entire system instead of independent control over the independent portions of the total network.
— The converged DAS network is generally less expensive to install than separate systems.
What are the main potential areas of conflict when insufficient attention is given to the coexistence of cellular and public safety DAS networks?
— Interference from one signal, harmonics of a signal or intermodulation products causing a dropped or lost signal.
— The presence of null zones or areas where the competing signals may cancel or distort each other.
— The installation of inadequate filtering (rejection is not sharp enough) creating a multi-path environment where a cellular network installation passes some of the public safety frequencies or the public safety network passes cellular signals.
— The downlink path of a signal interfering with the uplink path of a competing signal. This can most commonly be a problem in the 700 and 800 MHz bands.
Regardless whether a converged DAS network is installed or independent cellular and public safety networks are installed it is extremely important to know and understand the RF environment in and around the installation. This can be accomplished by performing field studies of the RF environment where the RF signals are monitored for an extended period of time, stored and then analyzed. This field study will identify existing RF signal levels, unwanted signals, noise levels and the presence of high level carriers. Additionally, it is critical to design the DAS system with as much information as possible regarding the size of the building, the distance to the nearest base stations, the type of building materials used, restriction on building codes regarding historic structures, the number of floors both above and below ground level, etc.
We are demanding more services and more “always on” coverage from both our cellular networks and our public safety communications networks today. Advances in RF technology over the past ten years have led to a significant increase in the number of wireless devices and signals in the air. DAS networks are complicated and expensive to install when it is desired to provide multi-carrier services as well as police, fire and emergency medical coverage.
When designed correctly converged DAS networks can overcome many of the objections of our safety personnel and provide a robust communications network that meets the demands of both coverage enhancement for mission critical public safety and capacity enhancement for cellular communications.
View detailed specifications and applications for signal boosters that comply with New International Fire Code and National Fire Protection Association (IFC/NFPA) Standards.