As carriers and enterprises consider how to best extend their mobile cellular networks to improve in-building coverage, one decision centers on system architectures. Some vendors offer passive systems which consist primarily of large coaxial cables installed throughout the venue, some offer active systems with electronics extended out into the network, and still others offer hybrid systems that combine active and passive components. In this article, we’ll consider the pros and cons of active, passive, and hybrid architectures, and then look at some guidelines that should help buyers make informed decisions.

Basic In-Building Architecture

The fundamental purpose of in-building wireless solutions is to extend strong, clear cellular signals inside buildings or public venues such as airports or stadiums. Cellular carriers use networks of outdoor base stations to distribute their signals to outdoor users, but building materials such as concrete and steel tend to attenuate these signals. As a result, mobile phones often work indoors when the user is near an outside window (provided that there is a cell tower nearby), but they don’t work well or at all as users move into interior offices, stairwells, or underground parking garages.

To overcome this issue, in-building wireless systems work by distributing a wireless signal throughout a building. In an in-building system, the wireless carrier installs a base station (or in some cases, a rooftop antenna and a repeater that picks up the signal from a nearby external cell tower), and the system picks up that signal and transmits RF throughout the building via a distributed antenna system (DAS).

The system may distribute the signal from just one cellular carrier (for example, when the building is owned and solely occupied by an organization that has a single-carrier cellular service plan for its employees), or it may support multiple cellular carriers. In either case, the system requires input from a base station or repeater installed by each carrier whose signal is being distributed.

Alternative Solutions

In-building wireless systems fall into three main categories: passive, active, and hybrid.

Passive Systems

Passive systems use rigid coaxial cable (1/2” to 7/8” in diameter) to distribute the wireless signal from a repeater or base station to a set of distributed antennas. These systems may use coaxial couplers or splitters to achieve the proper geographic distribution of cabling, but they are passive because the DAS itself uses no electronic components (Figure 1).

The coaxial cable used to distribute radio signals is inherently capable of supporting multiple carrier frequencies. These systems are often touted as “broadband” systems because the DAS itself supports any wireless frequency delivered to the coax system.

Active Systems

Active in-building systems take an approach that more closely mirrors standard LAN architecture. Rather than relying on fat but “dumb” transport cabling from the RF source to the antennas, these systems use managed hubs and standard building cabling such as fiber and Cat-5. To distribute the signal from the RF source through the DAS, these systems use active electronics (managed hubs and Remote Access Units, or RAUs) that amplify the signal (Figure 2). In many cases, an active system uses fiber running up a building riser to link a main hub with expansion hubs, and then uses CAT-5 cabling to connect each expansion hub to its RAUs and antennas (an RAU can support several antennas if needed).

Hybrid Systems

Hybrid systems combine attributes of both active and passive systems. They feature active electronics in the form of a head-end unit connected via fiber home runs to remote units, but use passive coaxial cabling to link these remote units to passive antennas. As such, they deliver some of the advantages of an active system (particularly in terms of greater system reach), but still suffer from some of the same performance and installation issues as passive systems because they use coaxial cables.

Solution Criteria

With the general differences between in-building systems in mind, let’s look at some basic business and operating issues that will affect the decision about which technology approach to take.

Single- or multi-carrier: The different types of systems have unique capabilities for supporting multiple carriers, but that may not matter if the building needs coverage for only one carrier. Remote manufacturing or distribution plants with company-wide cell phone plans provided by one carrier will have different requirements than urban offices or public facilities where multi-carrier coverage is essential. And even in a situation where multiple wireless services are required, each carrier may require its own system due to capacity or maintenance concerns.

Applications: A system designed to support voice-only cellular traffic may have different coverage and performance requirements than a system that will also be carrying 3G voice and high-speed data frequencies along with public safety traffic.

Performance: The performance of a wireless system (typically signal strength, measured in dBm and noise figure) impacts the system’s capacity, its ability to effectively support wideband data, the call quality, and the battery life of user devices. Systems that offer lower performance may not easily meet tomorrow’s requirements in terms of capacity and applications support.

Expandability: Like many IT projects, an in-building wireless deployment can be expensive, disruptive, and time-consuming. Once the system is in place, it should be able to accommodate new carriers or frequencies without requiring a reconfiguration of the system.

Deployment disruption: Most building owners want to minimize the disruption to their ongoing operations when an in-building system is deployed.

Manageability As with critical computer networking systems, an in-building system should be fully manageable, enabling company administrators or carrier personnel to know instantly when an antenna has gone down, for example. And in buildings with systems that host several different cellular carriers, individual carriers may want to manage their own services. Extensive management capabilities also reduce the life cycle cost of the system, since any problems can be easily diagnosed and pinpointed without unnecessarily dispatching a technician or spending excessive amounts of time troubleshooting.

Investment protection: In-building systems represent a significant investment, and like other IT-related projects, companies want to preserve their investment as much as possible, even if they relocate to a new facility. Costs: Finally, the overall cost of an in-building system is always a consideration, regardless of whether the cellular carrier(s) or building owner(s) are paying for it.

How They Stack Up

When measured against the foregoing criteria, we find some significant differences between the three types of in-building systems.

The coaxial cable in passive systems is “lossy” – the longer it runs, the weaker the wireless signal. As a result, the performance of passive systems is generally poor for applications requiring long cable runs (large facilities), high call capacity, or high signal strength. Passive systems must be painstakingly engineered to deliver sufficient wireless coverage everywhere.

Active systems typically offer better performance than passive systems. Because they use active components end-to-end, such systems can deliver radio signals with virtually no loss to any antenna. In addition, these systems generally offer higher output power at the antenna, which translates into larger coverage area per antenna and fewer antennas needed to cover a given area. (Higher power also means that additional capacity can be added without having to reengineer the system.)

With higher output power at the antenna and lower system noise throughout, an active system provides better overall performance and allows individual users’ mobile devices to use less power, thus increasing their battery life.

On the cost and disruption fronts, passive and hybrid systems have other problems. Rigid coaxial cable requires specialized expertise to install, and costs about $4.50 per foot to deploy compared with about $1 to $2 per foot for the Cat-5 Ethernet used in active systems. The cabling used in passive and hybrid systems can’t be run through existing conduits used for networking – bend radiuses are poor, so cable routing can be a significant issue. Moreover, deploying rigid coaxial cabling above false ceilings typically requires special hangers and removal of the ceilings for extended periods.

In contrast, active systems’ standard cabling can be run through existing conduits above ceilings. Active systems’ higher output power means fewer antennas, less equipment, and less cabling, so the overall system is easier and less expensive to install.

The advantage of a hybrid system is that its electronics help counter the signal loss found in all-passive solutions, particularly in large deployments. Also, because the transport mechanism from the remote unit to the antenna is passive rather than active, these systems may also require fewer electronics than full-active systems, and thus may have lower equipment costs than active systems in some instances. However, these systems’ reliance on coaxial cabling means they come with high installation costs, so the turnkey cost of a hybrid system is frequently more than that of an active system.

When it comes to manageability, there are other clear differences. A passive system does not support end-to-end management. Since all the connections to antennas are passive, there can be no remote diagnostics or alarming. If a distributed antenna goes down, there’s no way for the system administrator to know about it until users start complaining. Hybrid systems can offer management capabilities on those pieces of the system which are active. But, due to the coax element of the systems, they cannot provide end-to-end management and supervision.

In contrast, active systems are fully manageable. The distributed hub architecture of an active system mirrors the design of Ethernet LANs – it is infinitely scalable and fully manageable. Active systems usually support SNMP alarms as well, so IT staff can monitor the status of all remote antennas using standard network management tools.

In terms of investment protection, the phrase “you can’t take it with you” can be applied to passive systems because there is no practical way to move the coax cabling in the system. With hybrid solutions, some of the investment could be recovered, since the active elements could be removed and installed in the new facility. However, the investment in the coax cabling infrastructure—which can represent 60 percent or more of the total cost of the system—will be lost. With an active system, most of the system can be reused in the new facility.

There are also big differences in investment protection and expandability. If the building owner or carrier wants to deploy data service upgrades like EV-DO or add more capacity to a passive or hybrid system, the coverage area of each antenna may change enough to require re-engineering of the system, changing antenna placements, and moving or installing new cable. This can be an extremely difficult and costly exercise. In an active system, the upgrade in many instance won’t require any system changes, or may just involve the addition of some Cat-5 cabling and a few antennas.

In terms of cost, the difference between the three types of solutions has more to do with the distribution of these costs than the overall total. With passive systems, there are effectively no electronics costs, so almost all of the cost is in the cable and the installation. These costs can be considerable, as described previously.

Hybrid systems may have lower costs for electronics because there are fewer boxes in the system. However, due to the higher costs of installing the coaxial cabling (plus all of the fiber home runs) about 60 percent of the cost lies in deployment, while 40 percent of the cost is in equipment. In fact, hybrid systems are often the most expensive, because they have higher electronics costs than passive systems and higher deployment costs than active systems.

In active systems, the cost ratio is generally reversed, with 60 percent of costs going to equipment and 40 percent going to deployment. As the size of the deployment increases, however, the cost of an active system per square foot will decrease significantly. With passive and hybrid systems, the opposite is true – the larger the venue, the higher the cost per square foot.

In-building wireless systems are becoming increasingly critical to ensure adequate cellular coverage in buildings and public venues. With carriers rapidly enabling 3G services via EV-DO and HSDPA this year and delivering new services such as video and music, we can expect customer demand for in-building coverage to escalate. By carefully weighing the alternatives and investing in systems that minimize costs while maximizing scalability, performance, and manageability, wireless carriers and building owners can make their facilities 3G-compatible to ensure a consistently reliable wireless connection.

John Spindler is Vice President of Marketing for LGC Wireless.

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