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Optimising distributed antenna system installation

Posted: 03 May 2016     Print Version  Bookmark and Share

Keywords:distributed antenna system  DAS  cable  fibre optic  coax systems 

As installation costs can reach 50 or 60 per cent of the total cost of a distributed antenna system (DAS) deployment, it's important to select systems and cabling that minimise total cost of ownership (TCO). This article will take a look at three key factors in DAS installations that can impact TCO, which includes cable selection, future proofing, and facilities use.

Cable selection
The first factor impacting TCO is the cable infrastructure the system uses. The most common choices are coaxial cable, Cat 5/6, or fibre. The medium you choose has a lot of impact not only on TCO, but also on the system's performance.

Coaxial cabling was the first transport medium used for DAS installations. The older systems, as well as some of the newer ones, use a mixed infrastructure of fibre optic cable (generally used from the head end of the system to a closet-mounted remote unit) and a half inch diameter coax (the "last mile" from the remote unit to the passive antenna). When looking at material cost, a half inch coax is the most expensive of the various cable media typically used in a DAS solution. From an installation standpoint, a half inch coax is problematic because it is heavy, has an extremely limited bend radius – which if exceeded will stop the "flow" of RF similar to a kink in a garden hose – and often requires special cable trays to support its weight. In addition, connectorising a half inch coax can be both time-consuming and costly.

When considering all of these factors, the use of a half inch coax can add around 25 per cent to 35 per cent to the cost of installation, and it doesn't necessarily deliver the broadband capacity one would expect from such large cabling. In fact, RF signal attenuates as it travels over coax cabling, so there will be different output performance at different antennas, depending on the length of the cable run in each case. This makes system design and planning the antenna placement difficult and time-consuming. Note that a few DAS systems use thinner CATV (RG-6/U) cabling now, so the cable itself is easier to pull, but many of the same performance limitations still exist.

Figure 1: A half inch coaxial cabling.

Another point to be considered regarding coax cabling is that if the DAS deployment demands use of multiple input/multiple output (MIMO) technology. This will require two antennas at each remote location and, therefore, two separate coax cable runs. MIMO is increasingly popular in DAS deployments because it delivers greater capacity than single input/single output (SISO) implementations (which have been the norm until recently), particularly for those applications where there is either a high user density, such as a stadium or airport, or a significant amount of wireless data use. But using MIMO means that installers must pull twice as much coax to support such an architecture, thereby doubling the material cost and greatly increasing the installation cost.

Cat 5/6 Ethernet cabling has been used by a number of DAS solutions, but many of these systems were introduced a number of years ago, prior to the proliferation of available mobile frequencies. This cable type is inherently a narrowband medium, so it's constrained in the amount of frequency it can practically support. In one system, for example, the vendor was only able to support a maximum of 37MHz of bandwidth on a single Cat 5/6 cable. This is problematic in today's mobile-centric world, as some spectrum bands come in much broader swaths than that. So supporting a relatively meager 37MHz of bandwidth only allows you to support a single mobile band or frequency (and in some instances only a portion of that band). Meeting today's frequency support requirements (which can require support of as many as 6-8 bands) would require independent layers of equipment and cabling for each band supported. Clearly, this kind of solution would radically impact the total cost.

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