The Value of Digital Transport in Distributed Antenna SystemsDistributed antenna systems (DAS) come in two basic flavors: digital and analog. DAS vendors are positioning digital DAS as the wave of the future, but many deployments continue to use analog DAS for its familiarity and perceived cost savings. When does it make sense to use digital DAS? In this article, we’ll look at the technical and fiscal aspects of digital and analog DAS and examine key applications where digital DAS is preferable.

Analog vs. Digital DAS Basics

The goal of a DAS is to enable mobile operator signals to be strongly and uniformly distributed through a structure or urban area. DAS attributes that designers look for include reach (the distance a signal can be carried), performance (the signal strength at each antenna), and ease of deployment.

Analog DAS has been around for more than 20 years. Essentially, an analog DAS uses radio frequency (RF) input – the carrier base station (BTS) signal goes through power attenuators and a radio head to convert the BTS’ digital output to RF, and the DAS distributes the RF signal to antennas over either coaxial cable or fiber. Since the RF signal attenuates over distance, analog DAS antennas may have variable output power at the antenna locations if they are different distances from the head-end. Some analog DAS systems may use repeaters to regenerate the RF signal on the way to antennas, but repeaters impact the system EVM performance.

A digital DAS can interact with carrier BTS in two ways: It can take the RF signal from a radio head and convert it to digital signaling for transport over fiber to antennas, or it can use a direct digital base band interface with a BTS to eliminate the need for radio heads and power attenuators. Using a direct digital interface saves up to 50 percent on the cost of the DAS head-end because of equipment, real estate, cooling, and power savings. Moreover, because the signal is digital, power is uniform at every antenna point, regardless of how far that antenna is from the DAS head-end and BTS. A digital DAS can distribute its signal as far as the maximum distance allowed by the protocol. Each protocol is distance limited by the round trip delay of the signal.

Because the digital signal is a 1 or a 0, the signal is much more robust, allowing for operators to use existing fiber network backbones which include flat polished/UPC connectors.  Due to limitation of the analog fiber architecture, standard connectors must with replaced with angled/APC connectors or have the fiber fusion spliced to bypass patch panels.  This could potentially void fiber plant warranties.  

Now, let’s look at some specific characteristics of digital DAS versus analog DAS.

Optical Budget

The optical budget of a DAS is important because signals are often transported through patch panels and splices in existing fiber networks. An analog DAS typically has a 3-10 dBm optical budget, while a digital DAS has a 26 dBm optical budget. The higher optical budget of a digital DAS means it can more easily traverse problematic networks – older networks that can be a rat’s nest of patch panels and splices, for example.

Because a digital DAS has a more robust optical budget, the operator’s signals can travel further through problematic (dirty fiber, multiple patch panels, multiple splices, etc…) networks to reach endpoints at greater distances from the DAS head-end. Analog DAS are generally not considered for widely distributed applications like urban corridors. In fact, analog DAS installers typically splice around existing patch panels in the network because going through each patch panel causes a ½-dB loss in the signal.

Fiber Muxing Options

When deploying a DAS over an existing fiber network, the key is to get as much leverage out of each fiber as possible because unused fibers are scarce. Multiplexing technologies such as Wave Division Multiplexing (WDM), Coarse Wave Division Multiplexing (CWDM), and Dense Wave Division Multiplexing (DWDM) allow operators to divide each fiber’s light into as many as 32 or 64 individual wavelengths, each of which can transport a signal.

A digital DAS can use any of the multiplexing options (WDM, CWDM, or DWDM), while analog DAS can only use WDM or CWDM. Digital DAS thus offers better leverage for maximizing the usage of existing fiber. One application where a digital DAS has an advantage is the ability to leverage an existing PON or FTTH network, where there are a few spare dark fibers available to use for transporting DAS signals in a neighborhood. When you talk about buried fiber in a city, it’s hard enough to get bandwidth on a pair of fibers. It’s a tremendous sacrifice to use multiple fiber pairs in a situation like that, so analog DAS wouldn’t even be considered for such deployments.

Using DWDM also gives operators the ability to use base station hoteling in a city environment. In a base station hotel, you have a centralized group of BTSs in one location in a city. Using DWDM, the operator can multiplex 64 wavelengths over several kilometers, and then break the wavelengths out to cover a number of specific areas such as neighborhoods, campus, sporting venues or business parks.

Another use for DWDM is for using existing trunking fiber to transport DAS signals from central office (CO) to central office before they are distributed out into the network. By trunking bulk RF from CO to CO, the operators can leverage existing wireline assets and allow the operator to access existing distribution fiber.

Delay Timing

In a DAS that is simulcasting the signal to many antenna units, it is necessary to normalize the delay in signal transmission so that the farthest antenna and the

nearest antenna receive the signal at the same time. For some analog DAS, inserting delay may require installers to physically place excess fiber in the way. For example, if the farthest antenna node is five miles away and the nearest node is one mile away, installers would have to have four miles of fiber spooled to normalize the delay.  

In a digital DAS, delay can be managed through software, so it’s not necessary to deploy extra fiber to normalize timing between near and far antenna units.

Simulcasting

In many DAS installations, it’s useful to be able to simulcast the signal – to distribute the same signal to many antenna points at exactly the same time. Simulcasting is useful as it strongly and uniformly distributes a common RF signal. In addition, it allows for the capacity of the BTS/BBU to be shared among multiple node locations, which allows for more efficient use of a BTS/BBU’s capacity.) With a digital DAS, you can digitally simulcast a signal up to eight times for point-to-point antenna designs or 16 times for cascaded antenna designs. With an analog system, you’d have to use splitting hardware to achieve simulcasting, and this adds noise and loss to the system.

Deployment Costs

A fiber-based analog network requires angle-polished connectors and fusion splices. These require highly skilled technicians for the deployment, so costs are higher and deployment times are longer. A digital DAS can use push-on connectors that can be deployed more quickly by less skilled – and less expensive – personnel.

Fiber cascading also reduces fiber installation costs because you need only run fiber from one antenna unit to the next. Digital DAS allows for fiber cascading, while analog DAS requires home run cabling from each antenna back to the DAS head-end.

And as mentioned before, using the digital interface to a digital DAS eliminates the need for power attenuators and radio heads in the DAS head-end. This simplifies the deployment, eliminates the need for skilled RF technicians, and reduces equipment, power, and cooling costs.

Applications

Given the different technical characteristics of analog and digital DAS, it’s not surprising that they are suited for different sets of applications. Analog DAS is particularly useful in new buildings, where the network can be designed to minimize patch panels and other gear that attenuate the DAS signal. Digital DAS, thanks to its high link budget and ability to distribute signals over long distances, are suited for large buildings, metro areas, and campuses with existing fiber. Digital DAS is also best for any application where there’s an existing fiber network and the deployment requires going through multiple patch panels.

Conclusion

When all is said and done, digital DAS can support any application, while analog DAS is more limited in its use. Digital DAS offers high performance, long reach, and simpler deployment than analog DAS. Many network planners still opt for analog DAS because it is familiar and is offered by a larger number of vendors, but as the networks become denser, utilize multiple frequencies and more protocols, digital DAS is the better solution in most cases.