A DAS (Distributed Antenna System) provides the means to take the signal from a radio source and distribute it over a larger area, often at some distance from the original location. In large venues like sports stadiums, DAS can provide service in areas that would otherwise be have a very poor signal (or no signal at all), or that would be uneconomical to cover.

We usually discuss DAS solutions in the context of mobile wireless networks, where the source is a cell site of some sort (a CDMA 1X base station, a UMTS Node B, or an LTE eNodeB, for example). But DAS is, by and large, technology agnostic; as long as the DAS can handle the specific radio frequencies involved, it doesn’t really matter what the signal itself looks like.

So can DAS be used to distribute other types of radio signals? Suppose we wanted to provide wireless Internet access throughout a large facility. Could we deploy a Wi-Fi-based DAS to extend a Wi-Fi access point across multiple floors of a building?

Wi-Fi vs. Cellular

Just about every smartphone, tablet and laptop in existence supports Wi-Fi. Wi-Fi operates in unlicensed spectrum, so expensive licenses are not required, and users on Wi-Fi aren’t charged for their data usage. The current versions of the Wi-Fi standards (such as 802.11ac) support very high data rates (500 Mbps or more); in fact, 802.11ac shares many of the same characteristics and capabilities as LTE. At face value, this plan has a lot of appeal.

So let’s take that Wi-Fi access point and hook it up to a DAS with, say, 12 nodes to provide the necessary coverage. What does the result look like and will it get the job done?

Much Ado About Nothing

Suppose we decided to use a passive DAS for this project, where the radio signal from the Wi-Fi access point is carried over RF coax cable, and distributed to the DAS antennas via splitters to divide the signal across the various DAS segments. By the time the Wi-Fi signal reaches the most distant DAS antenna, its signal has been attenuated by losses over several hundred feet of cable, with the splitters taking out a chunk each time as well. The radio signal generated by the Wi-Fi access point (typically around 200 milliwatts, or 23 dBm) may well end up below 10 milliwatts (10 dBm) at the far end of the DAS, too low to be effective.

We can fix that by using an active DAS instead, converting the radio signal into an optical format carried over fiber optic cable, and then converting back to RF at the DAS antennas. The output power can be set to whatever level we choose, so the resulting Wi-Fi signal can be good and strong. Problem solved, right?

Show me the Money

Well, not so fast. What did our high-tech Wi-Fi DAS system just cost us? In addition to the cost of the equipment involved (the master unit to convert the Wi-Fi signal to optical, the cables to carry the signal throughout the unit, the expansion units to distribute the signal to the different segments, the remote units and antennas to recreate the original RF signal), we also have to factor in the cost of planning, deploying and maintaining the network. Bear in mind that most DAS are placed in the walls and ceilings of the facilities they serve, where space is at a premium and reaching any components that need to be repaired or replaced can be difficult and time-consuming. A DAS is expensive, both from the capital expense (CapEx) and operational expense (OpEx) perspective; our hypothetical system may easily cost us hundreds of thousands of dollars.

In addition, the DAS doesn’t increase the capacity of the source node. Now that Wi-Fi access point doesn’t just serve the ten or so users in that corner of the building; now it has to handle a hundred users scattered throughout the facility. That’s a recipe for a lot of unhappy users.

So what’s the alternative? If we want to provide Wi-Fi access from twelve points within a building, the simplest approach, of course, is to deploy twelve access points in those locations, for a total cost of perhaps $500, a tiny fraction of what a DAS would require. Each access point would require power and a data connection at its location, but then so would the DAS remote units, and if an access point fails, we can simply swap it out for a new one and carry on.

So, could you build a working Wi-Fi-based DAS? Absolutely. Should you? Only if you have a lot of money to throw away, and I know a bridge in Brooklyn that may be a better deal…