Despite the general similarities in power output, coverage area and size, DAS and the various small cell solutions differ greatly in terms of how they operate. DAS is a point-to-multipoint solution in which the DAS headend shares and receives signals with all remote nodes simultaneously. By simulcasting radio channels throughout the building, it creates a single large cell, as opposed to the network of individual cells typical of the various small cell solutions.
Centralized power management enables the DAS operator to change the coverage and capacity characteristics of each node in order to respond to changes in the RF environment.
As a result, DAS and small cells offer significant differences in terms of functionality, interference issues, capacity, complexity, and cost. It must be stressed that these network architectures and technologies are not interchangeable, and each is suitable only for the particular purposes and environments it is designed to address. So it is important that network owners and operators have a clear understanding of the strengths and weaknesses of each.
One of the biggest differences between DAS and femtocells, picocells and microcells is the ability to support multiple carriers. DAS systems can be shared by multiple operators, each connecting their own base stations to the shared RF distribution system.
As a result, DAS allows carriers and venue owners to take advantage of neutral host opportunities in which the CapEx of the network can be shared by all participants, making it more affordable. As a class of solutions, small cells typically do not provide multicarrier support. Although multiband/ multi-technology small cells are in development—and may eventually support more than one operator—today’s systems are highly limited in this regard.
DAS was designed to scale in order to meet the growing needs of the network. By adjusting the power to the antennas, a single BTS can serve up to about 1,800 users and provide a coverage radius of several miles.
Picocells and femtocells were designed to deliver coverage and capacity over a relatively small area, similar to a Wi-Fi access point (WAP). Adding more coverage requires installing more nodes.
DAS solutions are also multi-frequency, able to handle 2G, 3G and 4G commercial frequencies that operate in a range from 700 to 2500 megahertz (MHz), as well as public safety UHF and VHF frequency bands (e.g., 150 and 450 MHz band channels). Femtocells
remain exclusively a single-frequency, single-carrier solution.
As stated earlier, a DAS network functions by creating a single unified cell with blanket coverage within its prescribed area.
This eliminates multicell interference along with the need to hand off from one cell to the next as the user moves about. While potential exists for interference from nearby macro networks, this is easily managed by adjusting the power at the DAS headend or the power amplifiers, if they are in use.
The quality of service within the DAS network, therefore, is excellent. The large capacity of a DAS enables it to be used in tightly packed venues such as sports stadiums, where 50,000 users or more may be downloading data, posting photos, etc.
The system also provides the ability to dynamically adjust to changes in capacity demands per area and per carrier. Femtocells, picocells, and microcells operate on a different principle. These small cell solutions create a network of discrete cells, each with a fixed and fairly limited capacity and coverage.
For defined and small “rifle shot” applications, small cells provide excellent quality of service.
Their short range and ability to detect and adjust to other femtocells in the area help to negate multicell interference.
This does not mean small cell solutions are immune to service issues. When used for larger applications involving dozens of nodes, the potential for interference increases significantly. The sheer number of cells in use and the carrier’s inability to control their position and use—as well as issues with the handoff between these ad hoc cells and the overall network—create significant challenges in the spectrum and interference management.
Small cells can also experience interference problems when using low-band spectrum, and diminished range when using high-band spectrum.
In comparing DAS versus small cell solutions, the cost is perhaps the most curious characteristic of all. It also makes a convincing case that making the best decision for a specific application requires a solid understanding of the strengths and weaknesses of each solution.
One argument against DAS has been high costs to deploy with RF engineers. In fact, when deployed for smaller, low-density applications, DAS may not be the best choice. The reason is simple: DAS is not designed to excel in these types of scenarios.
Femtocells, picocells, and microcells, however, are much better suited for these applications. In order to support a few dozen users and a single carrier, small cells maybe the better choice for these applications.
As capacity and coverage requirements increase, the cost analysis begins to tip in favor of DAS. For large deployments, it is much less expensive to deploy a DAS for in-building coverage than to deploy dozens or hundreds of picocells or femtocells. Operating expenses are lower as well in a multi-carrier environment.
The challenge for the network operator or owner is determining precisely where that tipping point is. In addition to cost, one must also consider the importance of functional needs, interference control, scalability and the other characteristics discussed here.