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By Stefan Scheinert, CTO, LGC Wireless
Spurred by new products from vendors such as ipaccess, Ubiquisys, 3Way, and Motorola, the marketplace has shown growing interest in pico and femto cells as a solution for in-building wireless connectivity. Some vendors suggest that pico/femto cells will replace the distributed antenna systems (DAS) that have been so widely deployed around the world, but the technical case isn’t quite so clear. In this article, we’ll consider the relative strengths and weaknesses of pico/femto cells and DAS, and see how they can be integrated to provide an optimum solution in many environments.
Pico and femto cell systems are individual radios, essentially small versions of the cellular base stations that provide outdoor (and sometimes indoor) coverage for mobile handsets. The difference is that femto cells have very low output power, limited capacity, and are designed for small spaces like apartments or houses, while pico cells can typically cover enterprises with buildings up to 30,000 square feet.)
Pico/femto cell products look like Wi-Fi access points and connect via Cat-5 cable to an IP network for backhaul transport. Like the management switch that regulates traffic from the multiple access points in a wireless LAN, pico and femto cell systems use a base station controller (BSC) to manage the flow of traffic out to each pico cell and back, from the in-building network to the carrier’s broader network.
Distributed antenna systems (DAS) use one central radio source and extend its signal to multiple antennas, and are designed for buildings of 10,000 square feet or more. Newer “active” DAS technology uses a system of managed hubs (much like an Ethernet network) and guarantees the same high signal strength at every antenna point, regardless of the distance from the central radio. In contrast, older “passive” DAS technology distributes the signal via heavy coaxial cabling, and the signal degrades with distance form the radio source.
So which type of system is the best for indoor coverage? Let’s review the requirements.
In many organizations, mobile phones, smart phones, and other cellular devices are becoming primary communication devices – employees move around throughout the day attending meetings, and large organizations often host visiting customers or employees. This trend has revealed the need for consistently excellent in-building cellular coverage, and has also shown how the lack of such coverage impedes cellular phone use.
Typically, most buildings rely on coverage from a macro (exterior) cell tower. Concrete, steel, and other building materials block cellular signals, so even when coverage from the macro cell penetrates the building, the signal may be weak or nonexistent in some areas. In many locations, the signal does not have sufficient quality because of conflicting signals from multiple macro cells, which cause many devices to continually hunt between one source and another. (This problem is especially noticeable in high-rise buildings.) Moreover, macro cells sometimes have capacity limitations due to demand from other users in the area, which can lead to blockages and longer call setup times.
Pico/femto cells and DAS are designed to handle this problem by providing excellent coverage within the entire building, which allows subscribers to use their mobile phone as a primary communications device.
To deliver consistent coverage, the indoor signal must be both stronger (measured in dB) and more pervasive than signals coming in from exterior macro cells. Otherwise, indoor callers’ devices may continually hunt from one source to another, or the device may camp on a cell site that is not the closest, resulting in reduced service and lower battery life.
Voice signal requirements
The signal strength affects the coverage area as well as its caller capacity. It is relatively easy to prevent signal source hunting and dropped calls with a signal that is 8-10 dB stronger than the signal from the macro cell. However, the signal strength also determines the size of the coverage area. Table 1 shows how dB loss impacts the coverage area of an indoor pico/femto cell or DAS antenna.
 Table 1: Cell radius and area reduction based on dB reduction |
As shown in Table 1, a 6 dB reduction results in a cell coverage area that is about 48 percent as large as the original cell area, and a 15 dB reduction imposes a cell radius that is only 40 percent of the original cell radius. (These are typical numbers for indoor parameters, although naturally the may vary based on the specific environment.)
Data signal requirements
As companies increasingly use mobile data services such as HSDPA, however, there are additional challenges for coverage:
- The link budget (equal to transmit power minus minimum field strength) is reduced – the higher the data rate, the lower the link budget
- Carrier to Interference-plus-Noise Ratio (CINR) requirements are increased – the higher the data rate, the higher the minimum CINR value
Reliable data service requires a sufficient link budget and CINR value. Table 2 shows the actual reduction in link budget and the required minimum CINR for HSDPA services.
 Table 2: HSDPA data service link budget reduction and CINR requirements |
As we can see, the 10.7Mb/s service reduces the link budget by 28.4 dB compared to voice and requires a CINR of + 10.5 dB.
Quality margins also apply. For example, if a service must have a 90 percent confidence level (i.e. 90 percent of the time, the signal has met the minimum quality requirement), 1.3 sigma (the standard deviation) must be added. If sigma is 8dB, an additional 10.4dB is required. For the 10.7Mb/s service, the resulting CINR is 10.5 dB (minimum quality) + 10.4 dB (the quality margin for 90 percent) = 20.9 dB, which is a real challenge for outdoor cells. If the cell area for voice is compared with, for example, 7.2 Mb/s data service, there is a 23.9 dB reduction in link budget, which according to Table 1 (24 dB reduction) shrinks the cell radius by 77 percent and the cell area by a whopping 94.5 percent.
Pico cells and DAS must both work with the existing macro cell network, providing high enough link budgets and high enough CINR to act as the dominant radio source for indoor voice and data users. Therefore, it is essential that both types of in-building solutions be integrated into the macro cell network. There are two major aspects to consider: frequency planning and handover.
Frequency Planning
For successful deployments of in-building solutions, interference must be minimized. In many networks (UMTS/HSDPA, for example) there are very few frequency channels available. As we have seen, 8 to 10 dB are needed for the mobile to reliably camp on the indoor cell within the building for voice services. For cellular data, the requirement may increase up to 20 dB, depending on the type of data service (HSDPA, EV-DO, etc.). So, for example, if the building receives a signal of -65 dB from a macro cell, the minimum field strength from the indoor signal source must be at least -45 dB to guarantee high-rate data services.
To provide adequate coverage in large buildings, it might be necessary to use multiple pico/femto cells. But because each cell is using the same frequency, this approach does not meet the CINR requirements for high speed data—a large area within the building will receive multiple pico/femto base station signals with similar field strengths, so the CINR value will be low and will prohibit high data rates.
Backhaul connectivity
Furthermore, each pico/femto cell requires backhaul connectivity. The optimal location of a pico/femto cell may be determined by the need for backhaul connectivity for each cell, and may not be the optimal location to provide the best coverage. In contrast, DAS installations place the radio source in a convenient location for backhaul (typically the communications room or data center), and the antennas may be placed as needed for optimum coverage.
Output power and range
Another aspect of pico/femto cells is that they can be portable, so their antennas can easily be moved and can be placed close to a window or an open area where they may cause interference into macro cells. To compensate for this potential problem, vendors often recommend operating pico or femto cells with very low output power. It is usually suggested that pico/femto cells be operated in the 1mW-10mW range to minimize the potential for interference into the macro cells.
So, even though a single pico/femto cell deployment has some advantages, it can generate interference into the macro cell and—because of its limited range—it doesn’t necessarily establish a dominant signal source inside the building. Deployment of multiple pico/femto cells guarantees good coverage, but it does not allow high data rates in a lot of locations within the building because the need to minimize interference among pico cells limits their placement. Table 3 summarizes the pros and cons of different deployment scenarios.
Due to these contrasting technical considerations, the best results for in-building wireless deployment can often be gained by combining a pico or femto cell base station and a DAS system.
 Table 3: Pros and cons for different in-building deployment scenarios |
As we have seen, the best performance for enterprises with voice and high data rate services is achieved by deploying one pico base station in combination with a DAS system. If the building is small enough and the macro cell signal is not too strong, a single pico base station will be sufficient. However, buildings of several thousand square feet may result in a high output power of the pico base station, resulting in interference with macro cells. A dominant server cannot be guaranteed since some obstructions within the building may reduce the signal strength by 20 or even 30 dB.
Multiple pico base station solutions may solve the minimum signal strength issue, but won’t provide sufficient CINR within the building. Therefore, high data rates will only be available in less than 50 percent of the building. Furthermore, the network integration can be challenging for this scenario since each pico cell base station must be integrated with the network.
Pico cell base stations with a DAS system provide the highest performance and lowest interference while minimizing network integration requirements. Since HSDPA and EV-DO cells can carry a lot of traffic, the DAS system enables a higher load of the pico cell by expanding coverage. Even so, the current generation of pico cells can handle a relatively small number of calls and data connections (16 – 32), while the next generations of pico base stations may actually exceed the capacity of a macro cell because of the better radio environment.
The DAS system also separates the location of the pico base station (which should preferably be in the telecom room) and the antennas (which are distributed throughout the building). When existing cable (like RJ-45) can be used, the installation of a DAS system can be easy, straightforward, and extremely economical.
Stefan Scheinert is CTO of LGC Wireless.
