What’s the right infrastructure for public-safety DAS?

John Spindler, director of product management at TE Connectivity, argues that public safety distributed antenna systems have to meet unique regulatory, technical and jurisdictional demands, so separate deployment and management is essential

What’s the right infrastructure for public-safety DAS?

Venues large and small are interested in – and in some cases required to provide – safety and security, and that means ensuring that first responders can communicate clearly over their radios. Often, this requires deploying distributed antenna systems (DAS) that extend public-safety mobile coverage and capacity to the interiors of buildings.

Some vendors recommend a separate overlay for the public-safety network, while others recommend integrating public-safety coverage and capacity into the mobile wireless network – but which is the right approach? To make a decision on whether or not to integrate public-safety and mobile DAS, three categories of DAS integration issues need to be considered; regulatory, technical and economic, and jurisdictional.

Regulatory issues
The UK government provides funding and support for local emergency services – including the police, fire and ambulance services – and sets national standards to make sure they are effective and consistent. These standards typically govern DAS regulations for public-safety communications.

For example, the UK’s Department for Communities and Local Government (DCLG) brought together a series of documents relating to fire safety law and guidance for businesses that apply to public-safety DAS. These guides tell businesses what they need to do to comply with fire safety law, as well as help businesses to carry out a fire risk assessment and identify the general fire precautions they need to have in place.

In 2006, the DCLG introduced the Regulatory Reform (Fire Safety) Order 2005, which covers general fire safety in England and Wales. In Scotland, requirements on general fire safety are covered in Part 3 of the Fire (Scotland) Act 2005. In the majority of premises, local fire and rescue authorities are responsible for enforcing this fire safety legislation.

The Health and Safety Executive (HSE) has enforcement responsibility on construction sites, for example, nuclear premises and ships under construction or undergoing repair. All of these requirements are, therefore, driving new installations of public-safety DAS and include requirements such as:

• Fully waterproof installation using NEMA 4X enclosures
• Battery or generator backup
• A specific set of monitoring points and alarm notifications to assure system availability
• More extensive coverage for facilities than typically implemented with commercial networks – for instance, the need to provide coverage in stairwells or elevator shafts.

Mobile DAS systems do not need to meet these extensive code requirements, but a converged DAS would have to meet them, and doing so could significantly raise the overall cost of the DAS system.

Technical issues and associated economics
There are differences between the deployment goals of a public-safety DAS and a mobile DAS. A public-safety DAS is primarily concerned with comprehensive (everywhere in the structure) coverage that provides clear and consistent voice communications for first responders. With a mobile DAS, the goal is to provide coverage in public-use areas and also enough data capacity for smartphone users.

A public-safety DAS requires redundant equipment and fibre paths, along with overlapping coverage to achieve an appropriate level of system reliability. Commercial DAS networks do not require redundancy. Trying to combine a redundant and non-redundant system in one configuration is not consistent with good engineering practices and raises the cost of the deployment.

Interference within a radio network can bring down an otherwise properly working system. As more frequencies are added to a network, the likelihood of interference grows dramatically. An integrated public-safety/mobile DAS carries many frequencies and warrants much deeper intermodulation distortion analysis to determine and understand where direct hits – and, therefore, interference – may occur.

It is time consuming and expensive to understand and manage these non-linearities. Within a combined system, an appropriate safe operating mode may not be achievable. Trying to combine public-safety and mobile frequencies within a single system presents an unnecessary risk of interference.

In addition, public-safety and mobile DAS systems have different handset power control and coverage standards. A public-safety radio handset transmits full power at all times, while a mobile handset adjusts its power level according to the demands of a specific connection.

Therefore, an integrated DAS is more difficult to optimise because of the differences in antenna layout, potential uplink overload issues and overall dynamic range issues demanded by these operating differences. Suboptimal antenna layouts used to address these differences can lead to higher system costs.

Antenna placement is another mismatch between public-safety DAS and mobile DAS. Public-safety systems generally use lower frequency bands – for example, spectrum from 150 to 450 MHz – and it takes fewer antennas to cover a building for these services than it does for a mobile DAS operating at frequencies as high as 2100 MHz.

Putting the two systems together makes it overkill from a public-safety perspective in terms of the number of antennas (and associated “upstream” equipment) needed. Antennas for public-safety DAS also will be required in areas where commercial use is typically not required, such as stairwells, back rooms, and spaces for utility and HVAC equipment. These areas are typically not covered in mobile DAS deployments.

If the commercial DAS is sectorised – as is often the case in larger venues, such as airports – the combined mobile/public-safety system will be burdened with the need to add more antennas and cabling not normally required of a public-safety system alone. More infrastructure means more cost, and additional materials means a more complicated network and higher long-term maintenance expenses.

Finally, combining a mobile and public-safety DAS is economically inefficient, because it requires the deployment of equipment throughout the DAS that wouldn’t be necessary for either of the DAS systems alone. For example, one regulatory requirement for public-safety DAS is that there be battery backup for the system for eight to 12 hours.

If the building has converged mobile/public-safety DAS, it will have to provide backup for the entire infrastructure, which can cost three to five times what it would cost to back up the public-safety DAS alone.

Jurisdictional issues
Public-safety DAS equipment is life-safety and mission-critical equipment. It must not be accessible to others, such as commercial DAS technicians and maintenance personnel. Keeping the two DAS separated allows proper access and security controls to be exercised by the public-safety agency.

Commercial-service maintenance personnel with access to the entire system, including switches and interconnection cables, could inadvertently interrupt public-safety service while making changes to their co-located mobile gear. Public-safety equipment should be under separate lock and key, with access by only authorised personnel.

Changes to a neutral-host DAS that includes public-safety services would require concurrence from all mobile services, as well as Authorities Having Jurisdiction (AHJ). This means the combined network has a commercial committee making decisions on system changes, maintenance schedules and technology upgrades.

A public-safety system must not be controlled by commercial, for-profit entities; it must be controlled and maintained directly by the building owner and the responsible public-safety authority.

Ultimately, looking at these considerations shows that public-safety DAS should be kept separate from mobile DAS. While a casual look at the problem suggests that it’s more efficient to combine the two systems, the reality is that public-safety DAS is unique and should be deployed and managed separately.

About the author: John Spindler is the director of product management for TE Connectivity’s wireless business unit. During his more than 20 years of industry experience, Spindler has held a variety of product management positions with companies such as Nortel Networks, GTE and InteCom. In these positions, he had responsibility for the areas of networking, network management, computer telephony integration and wireless technologies. He can be reached via email: john.spindler@te.com. For more information, please visit www.te.com/das.



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