Optimising DAS installations
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 percent to 35 percent 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.
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 37 MHz 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 37 MHz 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.
The other limitation with Cat 5/6 which must be considered is run length. The distance is limited to only about 100 metres, so some installations, particularly those in larger venues, won’t be possible with Cat 5/6 cable.
In contrast, fibre is a broadband transport medium that is thin, light, flexible, and can support many frequencies. While a few solutions can use either multi-mode or single mode fibre, single mode fibre is most commonly used in DAS systems. Given the broadband nature of fibre transport, some DAS systems which use fibre transport end-to-end – effectively from the headend to the antenna – can support mobile, public safety, and Gigabit Ethernet services on the same cable.
Fibre is also commonplace in any Class A building. Contractors put in bundles (generally speaking, using a minimum 12-strand fibre in a single bundle) in order to accommodate current and future services, so there may be existing fibre that can be leveraged for a DAS deployment in a building, thereby reducing the turnkey cost of the solution.
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.
Future proofing
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.
To add that additional amplifier to a conventional DAS you may need to add another card or blade into the head end, the intermediate hub (if one is being used), and in the remote unit, assuming the remote is a chassis which is closet-mounted and can accept cards or blades. With a self-contained ceiling-mounted remote – which is typically found in an all-fibre DAS – you would have to insert an add-on module, or you would add a second remote. That second remote would have to be daisy-chained off the primary remote, and if that’s not possible, then you would end up deploying a whole new second layer of equipment simply to support one additional frequency. And this may be the case in any event if there is no additional space (card slots) in the head-end, intermediate hub, or remote chassis.
A truly wideband DAS is different; it uses a single amplifier that covers every frequency. Because of the high-power nature of this amplifier, it allows you to spread the amp’s power across multiple frequencies – as well as operators using those frequencies – while being able to balance the power output at the end of the remote so that coverage is consistent for each and every frequency.
Facilities use
When deploying a DAS, space for the needed equipment is always an issue. Equipment uses space, power, and cooling resources, so the size of the overall system impacts TCO.
There’s a difference in size between types of DAS systems. Hybrid fibre coax systems, in particular, can be fairly large. Most hybrid fibre coax systems have a head-end hub which is rack mounted and can easily be accommodated in a standard telecom or IT room, but the remotes, which are typically 4-6U high rack- or wall-mounted units, go in the tight confines of a wiring closet. Finding the necessary space and power for these units can often be a challenge. You should also consider the amount of space the cabling takes – it spreads from the remote unit to 4-8 antenna points, and that’s a pretty heavy footprint caused by the cable infrastructure when you are using heavy coax.
The head-end is also a space consideration. Space is precious in equipment rooms and data centres, and, while the head-end hub of a DAS solution may not be a space issue, most DAS systems require attenuation panels between the carrier base station and the DAS head-end to condition the carrier base station’s signal. This is necessary to ensure that the signal is of an appropriate strength so it can be fed to the DAS head-end without damaging the equipment. Most DAS manufacturers now use active integration panels to reduce the amount of such “plumbing” required, and some even use direct digital interfaces so the base station can talk directly to the head-end without a lot of intermediate “plumbing.” But even with these improvements, the space requirements – as well as power and HVAC – can be significant, particularly for larger buildings supporting multiple wireless operators.
Recommendations
While Cat 5/6 Ethernet may be a tempting DAS system cabling option because of its ubiquity, its distance and capacity constraints make it suitable only for smaller buildings and applications requiring support of only one or two wireless frequencies. As we’ve seen, while the use of coax cabling provides more available bandwidth, even hybrid fibre coax systems have their drawbacks in terms of cost, reach and performance. A pure fibre cabling infrastructure offers the most advantages, particularly in terms of key attributes such as cost, performance, and flexibility.
Ultimately, given the relative characteristics of cable types and DAS architectures, it seems clear that a DAS which uses end-to-end fibre connectivity and a truly wideband amplification architecture will deliver the lowest TCO. This type of system will deliver a genuinely future proof solution, which is critical in today’s rapidly changing wireless landscape.
The author
John Spindler joined Zinwave as VP marketing and product management in November 2015 and has over 30 years of product management and marketing experience in the wireless and telecommunications industries. His most recent role as director of product management was with TE/CommScope, and prior to this as VP marketing at ADC Telecommunications. John holds a BA degree in Psychology from UCLA and has an MBA from the Marshall School of Business at USC.
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