From passive to active

An abundance of passive and analog elements in today’s infrastructure introduces loss and noise that stymie the ability to keep pace with user demands, and these elements are inflexible when it comes to redistributing wireless signals among various network endpoints. Next generation networks have fibre directly from the RF source all the way to the DAS radio head, with any switching or combining being done digitally, eliminating the need for coax cabling and providing a much more robust signal. This will enable a network to reach farther and perform more efficiently, particularly through migration from passive to active DAS networks.

DAS basics

A DAS is a system of hubs and antennas that distributes wireless signals from mobile operator base stations to the antennas located in the area the operator wishes to serve. DASs are important because signals from macro cell tower networks don’t penetrate to all areas, and even when they do penetrate, they don’t always deliver the required network quality and capacity for the area being served. A DAS takes the signal from the base station, routes it through hubs and network cabling, and outputs it at the remote antennas or radio heads.

Passive systems use thick coaxial cable (1/2” to 1” in diameter) to distribute the wireless signal between the head-end (located near the mobile operator base stations) and the remote antennas or radio heads. The head-end is connected to the signal source, and then it drives the signal over the coaxial cable, as shown in Figure 1. The coaxial cable used to distribute radio signals is broadband and inherently capable of supporting multiple carrier frequencies. While passive systems are thereby viewed as simpler, one-stop solutions to address wireless coverage and capacity concerns, there is a great risk of signal interference and multiple bands may ‘mix’ and produce noise in the network.

Figure 1: Passive DAS networks use coaxial cable to drive the signal to broadband antennas

In a passive system, the signal degrades as a function of the length of the cable in any particular run. Typically, a wireless signal can be transported only up to 100 metres over any coax cabling; beyond that point the signal degradation will be too great. As a result, passive systems are not well suited to large facilities with long or complex cable runs, or facilities that require high call capacity or high signal strength. Even in a relatively small deployment with as few as 16 antennas, users may need to stand very close to the antenna in order to get a good signal. Signal quality degrades the farther the cable is from the RF source.

In addition, passive systems do not offer end-to-end monitoring and management. The signal is simply being pushed out over copper cabling without any active elements, so service providers and building owners never know if a particular antenna has failed until users start complaining. Failures can commonly occur due to such things as disconnected antennas or damaged cables.

Finally, passive systems are more difficult and expensive to install, because their heavy rigid cabling requires special expertise and, typically, special cable raceways or hangers. Since the cabling is not as flexible, it is also more difficult to deploy in tight spaces.

Active DAS

Active DASs use managed hubs and standard building cabling (i.e., single- or multi-mode fibre and CATV cabling), much like an Ethernet LAN. In an active DAS, the main hub is deployed next to the base station or repeater in the building’s equipment room, and it distributes the wireless signal through a series of managed expansion hubs and antennas, as shown in Figure 2. This system aggregates all capacity at the head-end and simulcasts the signal to each antenna location, so it can be connected to more than one mobile operator base station at the head-end.

Figure 2: An active DAS distributes the wireless signal through a series of managed expansion hubs and antennas

Because the signal is amplified end-to-end, there is no signal loss and active DASs deliver strong and consistent signals at every antenna, no matter how far away they are from the signal source and main hub. In the largest airports or multi-facility deployments, some active DASs extend for many kilometres. Since every antenna has predictable signal strength and coverage, it is far easier to plan the antenna placement in an active system.

With their double-star architecture, active DASs can be expanded indefinitely through deployment of additional hubs and antennas. And re-sectorisation for added capacity is relatively easy. The distributed hub architecture of an active system mirrors the design of Ethernet LANs - it scales easily through the addition of new antennas and hubs, and the hub electronics can be upgraded to support new services as they come on line. This leaves the most expensive part of the system - the cabling and antenna plant - untouched. Active systems usually support SNMP alarms as well, so a company’s IT staff can monitor the status of all remote antennas in the network using the same network management tools used for the LAN.

Active DASs can be less expensive and are less disruptive to deploy because their standard cabling is inexpensive and the job can be handled by IT cabling contractors or electricians rather than specialised technicians. Standard cabling can be run across suspended ceilings and in tight spaces like conduit just as easily as LAN cabling. In many cases, an active system can use existing, unused fibre that runs up a multi-story building’s utility riser to link a main hub with expansion hubs and then use new CATV cabling to connect each expansion hub to its RAUs and antennas. While multiple sets of electronics may be required to support all service providers (depending on the service providers’ requirements), the cost of cable runs is a larger factor in the overall price of a system in all but the smallest facilities and with active DAS this cost is minimised.

Key migration issues

As mobile wireless networks evolve, they are supporting more and more data traffic as a percentage of overall capacity. As a result, mobile operators need networks in which they can distribute capacity as needed to any given endpoint. Wireless spectrum is a finite resource, and there isn’t enough of it to over-provision every location a DAS serves. Therefore, the mobile operator must be able to shift capacity to various pain points in the network at will.

For example, suppose a DAS covers a sports stadium, commuter rail station, and office building. The mobile operator needs to be able to shift network capacity to the stadium during events, to the rail station during peak commute times and to the office building during business hours. In a fibre-fed DAS with active components, the mobile operator can redeploy capacity by simply redirecting the traffic electronically, whereas a passive DAS would have to be physically reconfigured to accomplish the same thing.

Another reason for moving to active DAS is that mobile operators are increasingly sharing facilities known as ‘base station hotels’, where several operator base stations are located in the same place and feed the same DAS head-end. This so-called neutral host arrangement helps operators share costs and get to market more quickly than they could if they had to build their own, discrete facilities for each DAS installation. An active DAS can easily integrate signals from multiple base stations and direct each signal to the antennas needed, whereas passive DAS would require a separate head-end and cabling for each base station.

Finally, mobile operators deploying LTE are migrating to active DAS because many LTE systems use multi-input, multi-output transmission schemes that use two or more antennas in each location. It is much more cost-effective to deploy multiple antennas with a fibre-fed active DAS, because the same set of electronics and cabling can feed multiple antennas at each location. In a passive scenario, the operator would have to deploy a second passive coaxial cable to support a second antenna. This would effectively double the cost of the cable infrastructure, a cost that already represents a significant percentage of the total cost of the DAS.

In summary, active technology brings high performance, capacity distribution flexibility, and lower costs to a DAS deployment. As mobile operators increasingly support 4G services, active DAS will be a key component in these deployments, and we will see a migration away from passive DAS.