Mobile networks originally were designed to provide outdoor service. While many users receive adequate coverage inside of buildings when using 2G or 3G services, the picture changes with 4G (LTE) service. Distributed antenna systems (DAS) can deliver the in-building coverage and capacity needed to serve LTE users.
Coverage and capacity are the two fundamental challenges for enterprises seeking to deliver high-performance mobile services within their offices and plants. LTE promises multiple megabytes of downlink data per user, and it is impossible for a macro network alone to provide this level of capacity. Effective LTE service will require basestation antennas to be moved much closer to end users to serve fewer users per antenna. DAS enables this type of architecture.
It will also be necessary to move basestation antennas inside buildings to provide the required coverage for LTE users. Many frequencies being used for 4G services attenuate much more quickly than 3G or 2G frequencies, making it more difficult for them to penetrate building walls.
In seeking a solution for in-building coverage and capacity, there are three key considerations:
- Ease/cost of deployment: The network infrastructure must be simple and cost-effective to deploy, or else service providers won’t install it or the speed of network rollout will be unacceptable to users. Existing buildings are particularly problematic since there is a lot of infrastructure to work around, and such installations can disrupt normal business operations.
- Scalability: The infrastructure should easily scale to cover new areas, support higher capacity, and adapt to future implementations of wireless standards such as LTE-Advanced and to shifts from single-input single-output (SISO) to multiple-input multiple-output (MIMO) operation. The in-building wireless solution should, like a fiber optic network, handle whatever applications will be required today and in the future.
- Flexibility: The solution should support multiple mobile operator services. While enterprises may have a corporate purchase agreement with a particular mobile operator, the in-building system must provide service for contractors, visitors, and other people in the building who use other services. The solution should also accommodate network changes as spectrum is acquired or divested.
Potential Solutions For Indoor Coverage
To meet the basic coverage and capacity requirements for in-building coverage, the enterprise or its mobile operators must deploy systems that bring the cellular signal inside the building. There are two basic approaches:
- Deploy new, small mobile basestations (femto, pico, and microcells) to radiate signals from their locations
- Deploy multi-band, multi-protocol distributed antenna systems to propagate signals from a single radio source
Femto, pico, and micro cells are becoming important tools for improving coverage and capacity, and there is a lot of buzz about using them to solve enterprise mobile challenges. Service providers are using pico or micro cells on a limited basis to provide coverage and capacity inside public structures or enterprises, and they are now beginning to offer femto cells for coverage in residences, small offices, and other buildings as small as 1500 square meters in size (Fig. 1).
These devices provide both coverage and capacity, and many of the smaller products are easy to install and require a minimum of space. With each cell you add, you get more coverage and more capacity. But there are some disadvantages as well:
- Each small basestation must be managed, so using these cells as the only solution in a building that could require dozens or hundreds of units, which will create vast new demands on service provider and user resources. With a proliferation of pico and femto cells, service providers may not want to shoulder the burden of managing the equipment and the ongoing system engineering requirements.
- Each small cell provides one or two frequency bands, so if a building needs to support service from several mobile operators, it will need more than one set of equipment in each small cell.
- Each small cell has a fixed amount of capacity tied to the coverage area of the cell. Thus, traffic engineering to meet peak usage demands is critical.
- Service providers have a limited amount of frequency spectrum at their disposal. Service providers manage this problem with large cells by simply alternating frequencies so no two cells using the same frequency will overlap. But since each small cell in an adjacent area must use a different frequency, small cells multiply the chances for interference and make it difficult to manage available spectrum efficiently.
Distributed antenna systems address the potential issues that may arise when using a small basestation as the sole solution to coverage and capacity problems. A DAS works with an available signal source (either an antenna/repeater that captures the signal from the service provider’s macro network or directly from a small cell or basestation) and then uniformly distributes that signal and channel capacity throughout a given area via a series of amplifiers and antennas. A DAS can deliver signals from one or multiple service providers, depending on the number of signal sources to which it is connected.
There are several advantages of using a DAS in conjunction with basestations to create a small-cell architecture:
- A DAS extends the signal from one or more basestations, so service providers or enterprises can use them to provide service to a large area while reducing the number of basestations required, maximizing the use of existing radio resources.
- Compared with small basestations, distributed antenna systems are quite reliable, easy to manage, and inherently scalable.
- Distributed antenna systems are relatively inexpensive and easy to deploy.
There are three basic types of distributed antenna systems on the market today: passive, active, and hybrid. Each type of system has specific strengths and weaknesses when it comes to providing in-building coverage for 4G networks.
Passive systems use thick coaxial cable (0.5 to 1 in. in diameter) to distribute the wireless signal. The main distribution unit is connected to the signal source, and then the unit drives the signal over the coaxial cable. The coaxial cable used to distribute radio signals is inherently capable of supporting multiple carrier frequencies. While passive systems are thereby viewed as simpler, one-stop solutions for indoor wireless coverage, there is also a great risk of signal interference, and multiple bands may “mix” and produce noise on the network.
In a passive system, the signal degrades with the length of the cable in any particular run. As a result, passive systems are not well suited to large facilities with long or complex cable runs or to 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 to get a good signal. Signal quality degrades the farther the cable is from the RF source.
Passive systems do not offer end-to-end monitoring and management. The signal is simply being pushed out over copper cabling, so service providers and building owners never know if a particular antenna has failed until users start complaining.
Finally, passive systems are more difficult and expensive to install, because their heavy and rigid cabling requires special expertise and often special cable raceways or hangers. Since the cabling is not as flexible, it is also more difficult to deploy in tight spaces.
Active distributed antenna systems use managed hubs and standard building cabling (i.e., single-mode or multi-mode fiber and CATV cabling), much like an Ethernet local-area network (LAN). In an active DAS, the main hub is deployed in the building’s equipment room, and it distributes the wireless signal from a local or remote basestation through a series of expansion hubs, remote amplifier units (RAUs), and antennas (Fig. 2). The DAS aggregates all capacity and simulcasts the signal to each antenna location. Two antennas may be used when the systems support MIMO applications.
Because the signal is amplified at the RAUs, there is no end-to-end signal loss. Active distributed antenna systems deliver strong and consistent signals at every antenna, no matter how far away they are from the signal source and main hub, making them easy to design and the user experience consistent. In the largest airports and multi-facility deployments such as major hotels on the Las Vegas strip, some active distributed antenna systems extend for miles. Since every antenna has predictable signal strength and coverage, it is far easier to plan the antenna placement in an active system.
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 online. This leaves the most expensive part of the system, the cabling and antenna plant, untouched. Active systems usually support simple network management protocol (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 DAS 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 specialized 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 fiber that runs up a multistory 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. Active distributed antenna systems minimize this cost.
Hybrid distributed antenna systems combine attributes of both passive and active systems. They use fiber-optic cabling to carry the signal from the basestation up a building riser to remote units in intermediate distribution frame (IDF) closets and then use thick 0.5-in. coaxial cabling from the remotes to carry the signal horizontally across each floor of the building. In the case of LTE, this coaxial cable antenna network must be doubled if MIMO support is required.
Hybrid systems partially alleviate the problem with signal loss and variable signal strength at each antenna because there is far less signal loss in the vertical portion of the system. However, these systems have the same signal loss issues in the horizontal cable runs to individual antennas as a passive system. Overall, output at any given antenna will be higher, but there will still be variability in signal strength and coverage at an antenna, depending on its distance from the fiber-optic portion of the system. And, these solutions are still vulnerable to the RF interference issues as signals combine in their native RF waveforms.
In the same way, hybrid systems only partially solve the management and cost challenges of DAS deployment. They offer management between the basestation and the remote units at each floor, but not between those remotes and individual antennas. And while building owners save partly on the cost of deployment on the fiber portion of a hybrid DAS, they encounter the same cost and disruption issues when installing heavy coaxial cabling and antennas on each floor.
As we move into the 4G mobile service era, in-building mobile network solutions will be crucial to providing strong, clear, high-bandwidth connectivity for all users. Service providers and building owners are already deploying DAS in large buildings, stadiums, airports, hotels, hospitals, and other facilities. As 4G services hit a growth curve, we will see more DAS deployed to provide the needed coverage and capacity.