Distributed Antennas vs. Distributed Radios: What's the Best Way to Deliver Cellular Coverage?

By  John Spindler — May 24, 2006

As in-building cellular coverage becomes increasingly crucial for enterprise operations, IT departments are exploring several models for delivering adequate coverage. Like wireless LANs, some in-building cellular systems are based on "pico cells," which are small, individual radio units deployed throughout a facility. In contrast, distributed antenna systems (DAS) use a single, centrally-located radio source and then propagate its signals through multiple remote antennas located throughout the facility. Each of these approaches has its pros and cons, so buyers should understand the differences between them in order to make an informed choice.

Meeting Business Needs
The type of in-building system deployed depends on the specific site to be covered and the business requirements of the user. Here are some criteria to consider when evaluating different approaches:

  • Carrier support -- Does the facility need to support more than one carrier? If the company has a blanket plan with one carrier and doesn't need to provide coverage for customers or clients who use others, it doesn't need a system that offers multi-carrier support.

  • Coverage -- Is coverage required in a few isolated spots or throughout the facility? Some in-building systems are more difficult to deploy, and must be laboriously engineered to provide pervasive coverage if conditions require it.

  • Interference -- If the facility contains sensitive electronic instruments (labs and hospitals, for example), the amount of radio energy delivered to provide coverage may be a concern. Some in-building systems offer very little control over the amount of radio energy at each antenna, while others offer very good control.

  • Disruption and deployment time -- How sensitive will occupants of the space be to disruption during installation? Deluxe hotels and hospitals may want as little disruption as possible. Deployments can be done at nights and on weekends, too, but at a higher cost.

  • Deployment costs -- The total cost of deployment is always a concern. Systems vary, with some requiring higher electronics costs and others requiring higher costs for installation of cabling and antennas.

  • Capacity expansion -- Will the system need expansion at some point? Some systems are much more scalable than others.

  • Service upgrades -- How important will it be to upgrade services offered through the system as they become available? Some systems can't handle service upgrades without wholesale changes to antennas, while others can be upgraded through changes to centralized electronics. However, this may not matter for specific applications such as inventory tracking.

Technology Alternatives
Cellular networks operate through a series of radios (base stations) and towers (cell sites) that provide coverage in specific areas and hand off connections among one another as users move around. Steel and concrete attenuate the cellular signal, so coverage from outdoor cell sites drops off inside buildings. There are two fundamentally different approaches to providing in-building cellular coverage:

  • Pico cells, which are mini base stations that are deployed wherever coverage is desired; or
  • Distributed antenna systems (DAS), which use one base station and a series of hubs to distribute the radio signal through multiple remote antennas.

Pico cells
Pico cell systems are very small versions of the cellular carrier base stations that power outdoor cell sites. They look a lot like wireless LAN access points (APs) in enterprise wireless LAN systems, even to the point of connecting Cat-5 cable and an IP network for backhaul transport. Like the management switch that regulates traffic from the APs in a wireless LAN, pico cell systems use a base station controller (BSC) to manage the flow of traffic out to each pico cell, and back from the in-building network to the carrier's broader network.

Today, several makers of wide-area cellular infrastructure offer pico cell systems, so the carrier or enterprise can buy the whole solution from a single vendor. But pico cell systems also have their drawbacks:

  • Cost -- Pico cells are sophisticated products that are far more expensive than the simple remote antennas in a DAS. And since pico cells are individual radios, a system with multiple units must be painstakingly engineered (like a wireless LAN) to ensure maximum channel reuse and to minimize radio interference among the cells. This is an expensive process, and it must be repeated whenever a pico cell is added or moved to accommodate changes in the user community.

  • Limited user density -- Pico cells typically have a limited channel capacity (again, similar to WLAN APs), so it's necessary to deploy a lot of them in high- traffic areas like conference rooms or cafeterias.

  • Capacity and investment leverage -- Because pico cell systems must be engineered to handle peak traffic demands, companies must deploy a lot of pico cells to cover large meeting rooms or cafeterias that may only be used part of the time. As such, companies must invest a lot of money for equipment that will be idle 80 percent of the day.

  • Single-carrier limitation -- A pico cell system can support only one carrier, so facilities that must provide coverage for several carriers will require an entirely separate set of pico cells and BSCs for each of them.

  • Scalability -- Each pico cell must maintain a "neighbor" list that identifies neighboring cells for traffic hand-offs as users move from one area to another. However, these lists are limited to 32 names, so large sites will ultimately be limited in the number of cells they can deploy.

Distributed antenna systems, passive and active
Distributed antenna systems (DAS) operate by using one centralized radio source and extending its signal to multiple antennas. Some DAS use large coaxial cable to distribute the signal through a venue: typically, 7/8" coax is used in the building risers, with 1/2" coax tapped off this "backbone" running horizontally and feeding antennas at the run's endpoint. These systems are considered to be passive because they don't use managed electronics in the distribution network.

Passive DAS is expensive to install (large coaxial cable requires special installation labor and installation time can be lengthy). In addition, the signal in a passive system attenuates over longer cable runs, leading to inconsistent signal strength. This makes system design/engineering difficult as each antenna's coverage area will vary, depending on the length of coax to which it is attached. This leads to another issue: with a passive DAS, the whole system may require re-engineering if additional capacity is added or if additional coverage areas are required. Finally, passive systems are difficult or impossible to manage: if an antenna malfunctions, only user complaints will alert the operator. Typically, passive DAS are deployed in smaller facilities where the length of a coaxial cable run is less of an issue.

Active DAS use active electronic hubs, fiber optics, active radio access units (RAUs), and antennas to distribute the signal. These systems guarantee the same high signal strength at every antenna point, regardless of the distance from the central radio. Some active DAS extend over several miles, and many systems support optimum signal strength throughout the interiors of buildings and other facilities measuring millions of square feet. In addition, active DAS offer end-to-end monitoring and management down to the individual antenna, ensuring that outages will quickly be reported and repaired.

Active DAS deployments address all of the challenges that users face when deploying pico cell systems:

  • Costs -- DAS antennas are small and inexpensive. They are much smaller and less obvious than pico cells in environments where aesthetics are a concern. Also, active DAS uses standard cabling to connect hubs to RAUs and remote antennas, so this speeds installation and reduces deployment costs when compared with passive DAS.

  • User density -- With an active DAS, there's effectively no limit on the number of antennas or where they are placed. Since every antenna is simply an extension of the same centralized radio source (all antennas in the system broadcast all channels from the radio source), there is no need to traffic-engineer the system at the individual antenna level to meet the needs of high user density areas such as lobbies.

  • Interference -- Because the system operates off of a single radio source, there is no concern with interference among antennas.

  • Investment leverage -- Because DAS systems use inexpensive RAUs and antennas to support high-capacity areas, users have less tied up in expensive endpoints that may be infrequently used.

  • Single-carrier limitation -- Unlike pico cells, both active and passive DAS can support multiple carriers in a single antenna system, making it a relatively painless matter of upgrading centralized or hub electronics to support new carriers or services.

  • Capacity -- In an active DAS, capacity issues are addressed at the central base station. If additional capacity is required, it is a simple matter of adding additional radios at the central source. Contrast this with pico cells, where it may be necessary to add more radio resources at every pico cell location in the building to support more users -- a costly and time-consuming undertaking.

  • Carrier interfaces -- Every leading carrier in the world has deployed dozens or hundreds of DAS, and the interfaces to base stations and the wider network are well understood. In fact, active DAS are the in-building systems chosen to provide coverage inside most major cellular company headquarters.

As we can see, there are lots of issues to consider when choosing an in-building system, and many different approaches to the problem. The advent of high-speed data services and the trend toward building cellular transmission functions into laptops will make in-building cellular coverage even more important in the coming years. But with the right in-building system, enterprises can ensure "five bars" coverage throughout their facilities.

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