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Friday, November 5, 2010

Cellphones in Elevators

Metropolitan carry and talk on their mobile phones everywhere. Unfortunately, most elevators are shielded metal rooms that radio waves cannot get though. Since these invisible electromagnetic waves cannot travel though metallic objects, cellphone reception (or the signal strength) would drop significantly, causing call drops or breaks in the conversations.

"I am in a lift, can I call you back later?" We all kind of knew why cellphones would not work in lifts. Let's talk about why cellphones would sometimes work in elevators!

Elevator with good cellphone reception
Depends on the construction of the lift and the elevator shaft, they are not always completely sealed metallic boxes and tubes. Depends on the position of the elevator shaft relative to the nearest cellphone base station, cellphone signals may be just strong enough to have a decent conversation. The "biggest" challenge for cellphone signals is when the elevator shaft is located right at the centre of a concrete building, and the elevator and shaft are so well constructed with solid metal walls. Otherwise, there is still a chance that there is decent cellphone reception.

Have you ever wonder how come you have decent cellphone reception in the basement lift lobby? Many commercial buildings in major cities have installed micro-cell sites within their building, some even have more than one micro-cells on each floor. Although the transmit/receive power of these micro-cells are usually lower, they made cellphone conversations within an elevator possible. These are certainly great investments made by your mobile network operators. Unfortunately, some elevators within these buildings travels at very high speed and causes problems in hand-overs, ie. the two cell sites did not have enough time to respond to each other, causing call drops.
Indoor micro-cells (Note: Technically speaking these are antennas covered by radomes. The actual  transmit & receive equipment are hidden else where.)
The final reason that has made cellphone conversations possible in elevator is related to your mobile phone. Some cellphones can transit in a "high-power-mode" when the reception is poor. It can be a blessing for some as they can continue their conversation, without realizing that the transmitter next to their brain is transmitting at maximum allowed (usually still safe) power levels. Perhaps it might not be a blessing for those who are sharing the lift with that person. 

The fact that cellphones do not always work in elevators can be used to our advantage. It can be one of the many excuses that we can use to buy 2 to 5 more minutes. I once over-heard a businessman in the gentleman's room said, "I am in ah... this... ah... lift, I'll call you back." It's certainly a handy piece!