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Best solutions to improve coverage in elevators

January 17, 2011

Proving coverage in elevators is always a challenge even to the experienced RF designer. There are various solutions to this problem and they can vary according to the country’s regulation and coverage requirements.

Elevators tend to have a high path loss (15 to 40 dBs), almost behaving like a Faraday cage and are in constant motion. Things can get tricky when it’s a building with a high number of floors. There are some solutions to help to achieve a good coverage in these metal cages.

1-Elevator hall antenna

It’s the most used solution. It’s effective and simple to install since there are no special requirements  An omni antenna placed near the elevator’s entrance can provide coverage when a user passes through that floor. One approach is to place an antenna on each floor, if the elevator path loss is high. A more cost efficient solution is to place an antenna on even floors. The appropriated solution can be validated by running some simulations using a transmitter. It’s also a good idea to keep the elevator antennas of the same sector. This way we can avoid unnecessary handovers when the subscribers are moving.

2-Directional antenna in the elevator shaft

This solution requires special permission of the authorities, but it is very effective when combined with solution 3 – see below. An antenna is placed on the top of the elevator shaft (yagi or panel). The shaft behaves like a waveguide bringing sufficient RF levels inside the elevator. Placing an antenna on the top of the shaft can lead to spending a lot of coaxial cable if the technical room is on the ground floor. It’s a good idea to spend some time doing some math and verify the cost of this solution.

3-Antenna in the elevator

This solution requires to install a passive repeater in the elevator. One example is to install two panel antennas (one outside the elevator and another inside). Remember to use this solution combined with solution 2, so that it can be possible to have enough signal to repeat. Most of the times this installation requires special permission from the authorities.

4-Radiating cable in the elevator shaft

Due to its high cost, installing radiating cable can have a big impact on the project budget. But can be a good solution since obtaining a special permit from the local authorities can be easier. It’s also a requirement to have an adequate link budget for this solution.

As a rule of thumb consider to do some real world simulations, since the path loss can vary, depending on the type of materials the lift car is made. If only the elevator manufacturers could do some tests in most common frequencies, it would be possible to see the pathloss in their product datasheets.

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