Much to think about
As specialized mobile radio systems, or SMRs, gained popularity in the 1980s and high-performance portable radio transceiver units for these systems became available, an upsurge of usage took place. However, it was quickly found that buildings, tunnels and areas that were deeply RF shaded from the base station site suffered from intermittent to a complete lack of coverage. The use of unidirectional or bidirectional filter-amplifiers to enhance coverage has provided an effective, yet relatively simple, answer to the problem.
In-building RF coverage solutions are needed primarily because many cities and counties mandate specific coverage levels to ensure that first responders can communicate inside a structure. Other factors that necessitate the use of in-building systems include the antenna inefficiencies of portable radios and the compromise of portable propagation at VHF due to the body’s absorption of RF energy. Additionally, mines and tunnels require RF coverage for safety and productivity reasons.
Building owners and operators realize many benefits from the installation of in-building systems. These include operational efficiency; the ability to respond quickly to tenant calls; improved tenant safety and security, such as wireless coverage in parking garages and stairwells; and improved mechanisms to track and record service calls in real time. Also, when wireless sensors for building equipment are added to an in-building system, costs are reduced and energy usage is monitored and controlled more effectively.
The basic system consists of placing an antenna array in the clear, aimed at the donor radio site to provide access to the communications system (also known as the air interface). Using lossy radiating cable and/or coaxial transmission line with properly located antennas throughout the areas to be covered represents the basic distribution aspect of the system. For longer runs, fiber optic backhaul and distributed antenna systems provide distribution of the desired signal.
Systems can be designed for more than one band. Customized engineering is required on virtually every system that is configured and furnished.
The use of RF/fiber optic converters to extend system range in a campus application or for use over great distances is increasing, typically where a line-of-sight path does not exist, the RF signal is low or interference is an issue, such as in an urban area. Note that interference also may present obstacles to using a common air interface for accessing the remote repeater(s) or system.
There are several operational and technical aspects to consider when working with management to implement an in-building system. For instance, what is the time frame for the project and who is the purchaser of the system? What entity will act as the project developer — a homeowners’ association, a private corporation, a government entity, a cellular/PCS/iDEN operator, or a neutral-host (NH) service provider? (Knowing who will be paying for the NH systems can be particularly challenging.)
There are many examples of locations where in-building systems have been implemented for public safety. These include college campuses and schools, where implementation has become more prevalent due to student safety and discipline issues; industrial complexes; manufacturing plants, which offer a large market opportunity for radio dealers; military installations, airport hangars and related facilities; stadiums and sports arenas; mines; transportation hubs and terminals; and jails and prisons.
NH systems are being used in casinos, airports, convention centers, shopping malls and hospitals. In fact, the National Public Safety Telecommunications Council’s in-building working group technology committee noted in a November 2007 report that “hospital wireless systems providers are also interested in the possibility of adding patient monitoring, hospital administration and security to an in-building RF distribution system.” Office buildings, tunnels and public transportation are both public-safety and NH applications.
There are other important questions. For instance, who ultimately will own and maintain the system after implementation? Also, is the in-building system part of a new construction? It often is assumed that running cables or fiber is easier when the building doesn’t have the walls or ceiling in place. While this is true for the most part, there may be project management, contractor (union) and sub-contractor issues to consider.
If the system is going into an existing building, logistics will be a major concern. Installers might have to work after normal business hours or be especially mindful of not interrupting those conducting business, and working in restricted-access areas may offer other issues.
On the technical side, running coaxial cables can be quite challenging, particularly when they have to go through firewalls. In contrast, fiber optic transport allows the transmission of the signal over great distances with minimal loss. Note, however, that such systems require optical-to-RF converters that require power and a corresponding antenna system. Fiber optic systems also need a secure location for the converter rack.
In addition, all in-building systems must be designed for public-safety and carrier operation and must have emergency backup power, including the optical-to-RF converter rack, to ensure uninterrupted interoperation.
Generally, it’s safe to anticipate that additional users and operators eventually will want to piggyback onto most in-building systems, particularly if it will be an NH system designed to accommodate multiple carriers, such as cellular, PCS and SMR. In this circumstance, fiber optic distribution may be an easier way to add different services.
When a need for in-building coverage is uncovered, invariably the issue of cost will be raised and become a big factor in system design. A plethora of variables will affect system cost, starting with the complexity of the design. The following represent just a few of the many variables that must be considered.
- Vertical risers for penetrating floors add labor cost.
- Horizontal offset cable runs should be avoided when going floor to floor.
- Plenum-rated cables will greatly increase material costs.
- Multiple bands and services can greatly increase installation cost.
- Cost breakdown typically will be one-third for equipment and two-thirds for installation and systems optimization.
Costs can range from $0.30 to $5 per square foot, or more. For example, a 100,000-square-foot building that is essentially open manufacturing or warehousing with few walls and partitions, and which achieves amplification using a single UHF frequency pair, could cost $30,000. In contrast, a 400,000-square-foot office complex with multiple bands and services may cost $1 million or more. Larger systems for airports, shopping malls, hospitals and entertainment venues are among the most costly installations. Multibuilding campuses such as hospitals, colleges, universities and military installations require much planning and design before price can be established.
Just as there are many variables to consider when deciding where and how to implement an in-building system, there are myriad steps that must be taken to bring the project to completion. (See sidebar.) For example, indicating on building prints the current signal levels, as well as those that are needed at specific locations in the structure that are considered crucial, will greatly aid system design and engineering.
In addition, system engineers need to know whether city, county or federal regulations require minimum RF coverage for first responders. Moreover, some fire codes also may specify fire-retardant cable. You will need to provide a copy of the documents as applicable. This is critical information for the system engineer.
Alan Leffler is with RF filtering and amplification vendor EMR Corp. A 35-year radio industry veteran, Leffler is a Radio Club of America Fellow.
Ira Wiesenfeld, P.E., is a consulting engineer who has worked on commercial radio systems for more than 40 years. Both hold amateur radio licenses.
KEY STEPS IN ENGINEERING AN IN-BUILDING SYSTEM
Identify signal levels (in dBm) at the outside pickup antenna location.
Identify the transmitter output power and the distance to the donor site. (Hint: Look at the FCC license for this information.)
Determine whether any barriers exist between the transmitter antenna and the site, such as buildings, structures, hills or mountains.
Provide building diagrams and layouts that are marked with current and desired signal levels, in dBm, at crucial in-building locations (-85 dBm is typical).
Define the footprint of the building, particularly the number of floors to be covered.
Identify construction materials used in the building, such as concrete or reinforced concrete, wood and plywood, and the type of window glass (reflected, mirrored, dual pane).