Mobile broadband is enabling a host of new productivity applications for enterprises, healthcare organizations, universities, and other organizations, but the mobile infrastructure must support these applications inside buildings. In this article, we will look at the challenges and requirements for 4G coverage inside buildings, and see how to evaluate potential solutions such as distributed antenna systems and femtocells.
Mobile Enterprise Challenges
Mobile phones are now a standard business tool both inside and outside of corporate offices. Unfortunately, the current mobile networks were designed primarily to provide outdoor coverage. While many users receive decent coverage inside buildings when using 2G services such as GSM or CDMA, the picture changes as we move to 3G (HSPA, EV-DO, etc.) and 4G (LTE, WiMAX) services. These newer services often use higher frequencies that attenuate much more quickly, so they can't easily penetrate building walls. 3G users may get coverage in the exterior-facing offices of a building, but those farther inside the building often have poor coverage. 4G services will attenuate even more quickly, making matters worse.
In addition to coverage (getting the signal to the user), another issue is capacity. 3G services promise a few hundred kilobytes of downlink data service at best, but with 4G services promising multiple megabytes of downlink data service, the network must support sufficient capacity to provide every one of potentially hundreds of simultaneous users in a building with a high amount of bandwidth.
Coverage and capacity are the two fundamental challenges for enterprises seeking to deliver high-performance mobile services within their offices and plants. From an implementation standpoint, there are other challenges as well:
--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 roll-out will be unacceptable to users. Existing buildings are particularly problematic since there is a lot of existing 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. 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 others in the building who use other services.
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 to this:
--Deploy new, small mobile base stations (femto, pico, and microcells) to radiate signals from their locations
--Deploy multi-band, multi-protocol distributed antenna systems (DAS) to propagate signals from a single radio source
Femto, pico, and microcells are becoming important tools for improving coverage and capacity, and there is a lot of buzz in the media about using these to solve enterprise mobile challenges. Service providers often use pico or microcells to provide coverage and capacity inside public structures or enterprises, and they are now beginning to offer femtocells for coverage in residences, small offices, and other buildings as small as 1,500 square meters in size (Figure 1).
Figure 1: Single service femtocell deployment in an enterprise office, fixing capacity to location
The advantage of using these devices is that they 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 base station must be managed, so using these as the only solution in a building that could require dozens or hundreds of units will create vast new demands of service provider and user resources. With a proliferation of pico and femtocells, service providers may not want to shoulder the burden of managing the equipment and the hand-off between these cells.
--Each small cell provides one frequency band, so if a building needs to support service from multiple mobile operators, it will need a set of cells for each operator.
--Each small cell has a fixed amount of capacity tied to the coverage area of the cell.
--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 problems caused by using small base stations 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 base station) and then uniformly distributes that signal throughout a given area via a series of distributed amplifiers and antennas. A(Figure 2) DAS can deliver signals from one or multiple service providers, depending on how many different base stations or signal sources to which it is connected.
Figure 2: Multi-operator DAS deployment in an enterprise office simulcasting aggregate capacity throughout the network
There are several advantages of using a DAS in conjunction with base stations to create a small-cell architecture:
--A DAS extends the signal from one base station, so service providers or enterprises can use them to provide service to a large area while reducing the number of base stations required, maximizing the use of existing radio resources.
--Compared with base stations, DAS are quite reliable, easy to manage, and inherently scalable.
--DAS are relatively inexpensive and easy to deploy
There are three basic types of DAS 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 (1/2" to 1" in diameter) to distribute the wireless signal. The main distribution unit is connected to the repeater or base station and placed adjacent to it, 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, however, 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 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 further you are 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 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 DAS use managed hubs and standard building cabling (i.e., single- or multi-mode fiber 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, remote access units (RAUs), and antennas, as shown in Figure 2. (An RAU can support two antennas if needed.) This system aggregates all capacity and simulcasts the signal to each antenna location.
Because the signal is amplified at the RAUs, active DAS deliver strong and consistent signals at every antenna, no matter how far away it is from the signal source and main hub. In the largest airports or multi-facility deployments such as major hotels on the Las Vegas Strip, some active DAS 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.
With their double-star architecture, active DAS can be expanded indefinitely through deployment of additional hubs and antennas. The distributed hub architecture of an active system mirrors the design of Ethernet LANs--it scales easily through 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 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 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.
Hybrid DAS combine attributes of both passive and active systems. They use fiber optic cabling to carry the signal from the base station up a building riser, and then use thick coaxial cabling to carry the signal horizontally across each floor of the building.
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 wide 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 wave forms.
In the same way, hybrid systems only partially solve the management and cost challenges of DAS deployment: they offer management between the base station 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.
How to determine the best coverage solution
Here are some steps that can be taken to improve service delivery in the enterprise:
--Ask your service provider--Public and private organizations should consult the major service providers in their area to learn which companies provide in-building solutions, and which are recommended. Service provider cooperation will also be required for placement of base stations or the use of rooftop antennas as the signal source. In addition, the service provider will ultimately have to approve the design.
--Survey the wireless environment--Site surveys determine where coverage is weak, what part of the area to cover, if you have any priority areas to support, and how many service providers to support. This will determine the system design. For example, in large, uncomplicated buildings such as warehouses, microcells or high power DAS may be the best approach, while enterprise offices with many rooms and a lot of furniture may be better served by a DAS.
--Determine what your capacity requirements are--How many users do you need to support, now and in the future, and what kinds of applications are you using? What mobile applications are being used and relied upon by staff and customers?
--Consider implementation needs--Some systems are more disruptive to deploy than others. Plan a timeframe and installation plan that minimizes disruption to employees and visitors to the facility.
As we enter the 4G mobile service era, in-building mobile network solutions will be crucial to providing strong, clear, high-bandwidth connectivity for all occupants. With the right in-building wireless solution, enterprises will reap all of the benefits that 4G networking promises.