When using existing cabling, however, there are some issues to consider. While some DAS installations use existing fibre cabling to save the cost of pulling new cabling, installers need to be careful about what kinds of fibre and connectors have been installed, because some systems don’t support multi-mode fibre. In addition, reflections in the fibre can cause issues; this is particularly true for multi-mode fibre. Typically, if multi-mode fibre is being used it will require the use of angle polished connectors (APC), not only at the start and end point of the cable run, but also at any intermediate point where a patch panel is in use. If APC connectors weren’t used, it may require changing out not only all the fibre connectors but also the patch panels as well. And if the system does not support multi-mode fibre, and single mode fibre is not already available in the venue, then additional fibre will have to pulled to support the system. Another issue with fibre use is making sure that the fibre is not dirty – you will find that even with fibre pulled new, if it sits before you plug it in and commission the system, it can get dusty and you’ll end up with system performance issues.
DAS systems typically consist of a main hub or head-end – fed by an RF source such as a repeater or base station – connected via fibre, usually single mode, to either one or more secondary/intermediate hubs,. This, in turn, distributes the signal via fibre to one or more remote units. Alternatively, the head end may be directly connected to the remote via fibre without an intermediate stage. The remote unit is the last point of amplification in the system; this gear is usually a rack-mounted chassis with card slots accommodating various radio frequency cards, each supporting one frequency.
The difference between hybrid fibre/coax DAS systems and all-fibre DAS systems is that with hybrid fibre/coax solutions, the remote is placed in a wiring closet or IDF, and the “last mile” of cabling is a half inch coaxial cabling used to feed passive antennas. With an all-fibre DAS, the remote and antenna are collocated (usually in the ceiling) and there is no use of a half inch coax cabling infrastructure.
Regardless of the system architecture, the challenge with most DAS systems is that they frequently must be upgraded with new equipment every time a new frequency is added. Operators add new frequencies every couple of years – and even public safety systems will soon be moving to 700 MHz frequencies – and this means adding new hardware and amplifiers to a conventional DAS. This is because, as mentioned previously, conventional DAS solutions use a separate amplifier to support each frequency, so they have to “stack” amplifiers in order to support more than one frequency. Today’s demands typically require support of as many as eight frequencies. When it becomes necessary to add a frequency to the system, yet another amplifier must be added to the stack.