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Designing and installing fber-optic cabling to support distributed antenna systems continued
end” equipment room. The amplifier amplifies the signal to
provide strength and reliability sufficient for the facility. A repeater repeats the signal without
The signal is distributed from
the equipment room to the antenna nodes. These nodes occupy
locations that provide the desired
coverage. Determining these locations by survey is the most important factor in providing acceptable service. Such distribution
is over coaxial cable or fiber cable.
Small systems use coaxial cable to create a “passive” DAS. The
coaxial is “leaky” so that the cable functions as the antenna.
However, modern building materials can block such relatively
Large systems use fiber cable
to create “active” systems. The
optoelectronics convert the incoming radio frequency (RF) signal to a digital optical signal,
with return to RF at the distributed antennas. Such transmission can be analog or digital.
However, the preference is for
digital transmission. This preference is driven by cost; digital transmission is less expensive to implement and
operate than analog. In addition, digital transmission can be repeated without limit. In comparison, analog signals
have limited repetitions, since the signals gain noise and distortion with each
repetition or amplification.
The current configuration includes
both coaxial and fiber cables. This configuration is fiber cables feeding digital
signals to remote antenna units (RAU),
which, in turn, feed multiple antennas
over short coaxial cables. RAUs are also
known as RRUs (remote radio units), as
in fiber-to-the-antenna systems.
The use of fiber, which has essentially
no power loss, to the RAUs, results in all
antennas having the same sensitivity
and power, regardless of their distance
from the head end equipment room.
This is not the case with systems connected with coaxial cable.
Most DAS transmit digital signals with the Common Public Radio
Interface (CPRI) protocol. Most companies producing DAS and FTTA equipment use this protocol.
DAS have the same architecture as
do local area networks (LAN). In ad-
dition, DAS use the same fiber and ca-
ble types. Because of these common
characteristics, a DAS and a
LAN can share the same ca-
bles and structured cabling
system. Such cables can
have both multimode and
singlemode fiber. In most
current-day systems, the
advantage of potential in-
crease in bandwidth without
limit favors the use of sin-
Because of the low optical
power loss, DAS can be implemented as a passive optical network (PON). With this
approach, each RAU requires
a single fiber with wave-length-division multiplexing (WDM) to enable bidirectional communication.
As distributed antenna sys-
tems us the public radio
spectrum, all distributed an-
tenna systems involve a cell
phone provider, also known
as the carrier. Most DAS pro-
vide service to only the own-
ing carrier. Others provide
“carrier neutral” service; that
is, the DAS allows commu-
nication to multiple carriers. For such
a DAS, the entrance facilities and head
end equipment room space require-
ments are higher than those for a carri-
Most designers of communications
systems prefer to work from appropriate standards. While such an approach
simplifies the design process, the number of variables in design and in the conditions within which DAS must function conflict with the two goals: 1)
economy of implementation and 2) efficiency in use. While no DAS standard exists, the Telecommunications
Industry Association (TIA) developed
An active DAS configuration includes fiber and coaxial
cables; fiber cables feed digital signals to remote
antenna units (RAUs), which in turn feed multiple
antennas over short coaxial cables.