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Pico/Femto Cells Lower Costs, DAS Extends Cellular Coverage
Stefan Scheinert, LGC Wireless
Introduction:
Signals from outdoor cell towers don’t provide complete indoor coverage. As enterprise users rely increasingly on cellular telephones for voice and e-mail connections, in- building cellular coverage has become a far more critical problem. Large office buildings and corporate campuses can use distributed antenna systems (DAS) to propagate cellular signals from an on-site cellular carrier base station or repeater, but the cost of these deployments has made them prohibitively expensive for small to mid-size properties. Pico and femto cell products are now enabling in-building cellular deployment in smaller sites, and an integrated Pico cell/DAS deployment is optimum in mid-sized facilities.
Pico and femto cells are essentially small versions of the cellular base stations that provide outdoor coverage for cellular subscribers. Pico and femto cells look like Wi-Fi access points and connect to an IP (i.e., DSL) network for backhaul transport. Pico/femto cells can be portable, so their antennas can easily be placed near a window or an open area where they may cause interference into the macro cell network. To compensate for this potential problem, vendors often recommend operating pico or femto cells with very low output power (usually in the 1mW-10mW range).
Femto cells have very low output power, limited capacity, and are designed for very small office spaces or residential units. Pico cells can typically cover buildings of up to 30,000 square feet.
To deliver adequate coverage and signal strength in larger buildings, it may be necessary to deploy multiple pico/femto cells. However, this must be planned carefully because each cell may use the same frequency, and the deployment may not meet the CINR (Carrier to Interference-plus-Noise Ratio) requirements for high speed data. With multiple pico/femto cells installed, parts of the building may receive multiple signals with similar field strengths, so there will be a lot of interference and the CINR value will be too low to allow high data rates.
In contrast, DAS use a single central radio and extend the signal through a network of remote antennas. DAS can cover facilities ranging from 10,000 square feet up to millions of square feet.
The determination of how and where to place pico cells or use DAS is a matter of compensating for the two factors which impact the reach, quality and capacity of cellular coverage:
Signal Strength – To deliver consistent voice coverage inside a building and prevent device “hunting” from one radio source to another, any indoor coverage solution must deliver a signal that is 8-10 dB stronger than signals coming into the building from exterior macro cells. (Otherwise, user devices continually hunt from one source to another.) In addition, the signal must be pervasive to eliminate dead spots.
Signal strength affects the coverage area as well as its caller capacity. When multiple pico cells are used to cover a space, user devices “see” multiple radio sources. The devices will hunt between radio sources, and (because each pico cell uses a different radio), CINR is very poor (<5dB).
In areas with low CINR (typically where pico cell coverage areas overlap, high-speed data-services (> 3.6 Mbps – HSDPA) are not possible. In addition, hand-offs between radio sources will reduce available network resources and device battery-life.
To compensate for this problem, users can deploy one pico cell as a base station (or BTS) and then extend its signal with a DAS. In this scenario, the entire floor will be covered with one radio (one dominant radio source, so there will be no hand-offs and very high CINR. In addition, high-speed data service will be available throughout the floor.
If the building is small enough and the macro cell signal is not too strong, a single pico cell base station may be sufficient. In larger facilities, it makes more sense to deploy one pico cell and integrate it with a DAS than it does to deploy multiple pico cells.
Since HSDPA and EV-DO cells can carry a lot of traffic, integration of a pico cell and DAS system enables a higher load on the pico cell by expanding coverage. 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. Finally, the lower-cost, DSL-based backhaul for a pico/femto cell base station makes the deployment faster and less costly from that perspective.
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