Bringing cell coverage inside

As enterprise users rely increasingly on cell phones for voice and e-mail connections, ensuring in-building cellular coverage has become more critical.

Large office buildings and corporate campuses can use distributed antenna systems to propagate cellular signals from an on-site cellular base station or repeater, but the cost is prohibitive for small to midsize properties. Pico and femto cell products are now enabling in-building cellular deployment in smaller sites, and an integrated pico cell/distributed antenna system deployment is optimum in midsize facilities.

Pico and femto cells are small versions of the cellular base stations that provide outdoor coverage for cellular subscribers. The cell devices look like Wi-Fi access points and connect to an IP-based service such as DSL for backhaul to the cellular network.

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As enterprise users rely increasingly on cell phones for voice and e-mail connections, ensuring in-building cellular coverage has become more critical.

Large office buildings and corporate campuses can use distributed antenna systems to propagate cellular signals from an on-site cellular base station or repeater, but the cost is prohibitive for small to midsize properties. Pico and femto cell products are now enabling in-building cellular deployment in smaller sites, and an integrated pico cell/distributed antenna system deployment is optimum in midsize facilities.

Pico and femto cells are small versions of the cellular base stations that provide outdoor coverage for cellular subscribers. The cell devices look like Wi-Fi access points and connect to an IP-based service such as DSL for backhaul to the cellular network.

Because pico/femto cell devices can be portable they may interfere with the regular cell network. To compensate, vendors often recommend operating pico or femto cells with very low output power, usually in the 1mW-to-10mW range.

Click to see: How it works: Marrying pico cells with DAS

Femto cells have very low output power and limited capacity, and are designed for very small office spaces or residential units. Pico cells are higher-capacity, higher-power, and can typically cover buildings of as much as 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 Carrier to Interference-plus-Noise Ratio (CINR) 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 interference and the CINR value will be too low to allow high data rates.

Another approach is to use a central radio and a distributed antenna system to extend the signal. The traffic is back-hauled to the carrier's network via a T-1 line. Distributed antenna systems can cover facilities ranging from 10,000-square feet to as much as millions of square feet.

Determining how and where to place pico cells or use distributed antennas is a matter of compensating for the three factors that impact the reach, quality and capacity of cellular coverage: signal strength (which impacts cell radius and caller capacity); link budget (equal to transmit power minus minimum field strength); and CINR.

To deliver consistent voice coverage inside a building and prevent devices hunting from one radio source to another, any indoor coverage must deliver a signal that is 8 to 10 decibels stronger than signals coming into the building. 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, less than 5 decibels.