When I hear this term I am somewhat confused as RF is not like water. First a DAS has to be designed around what they call the link budget.
“A link budget is the accounting of all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in a telecommunication system. It accounts for the attenuation of the transmitted signal due to propagation, as well as the antenna gains, feedline and miscellaneous losses. Randomly varying channel gains such as fading are taken into account by adding some margin depending on the anticipated severity of its effects. The amount of margin required can be reduced by the use of mitigating techniques such as antenna diversity or frequency hopping.”
A simple link budget equation looks like this:
Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) − Losses (dB)
Note that decibels are logarithmic measurements, so adding decibels is equivalent to multiplying the actual numeric ratios.
An argument was made years ago that the passive DAS better than an active DAS. The reality this “wirelesss utility” or simply a DAS, has be architected in 5,000 square ft. segments to meet any kind of voice over IP requirements at -65dbm. So in reality this is not a passive design, but an active design that each segment needs power (active amplification)to drive through the coaxial cable infrastructure to meet the link budget.
Then if you want to support 802.11a…you need additional amplification at the antenna element. When this happens you add “active powered infrastructure” that adds risk to a well designed mission critical or this case healthcare, a “life critical” WLAN. The question then becomes why do this?
While in the past you could have made the financial and technical argument that it makes sense to combine 802.11b/g or even a, on a DAS, the cost considerations go out the window if 802.11n or even 802.11ac is considered.
My feeling is coaxial cable being used as a transport is legacy in nature. While it works…it is very, very expensive if trying to combine WLAN services.
That is why supporting true MIMO on a DAS makes no economical sense. Designing a DAS for the future has to do with supporting the backhaul for LTE, 802.11n, and 802.11ac. Fiber is the future.
