Basically, it’s nothing more than a telephone connected to a radio similar to the old walkie-talkies – only much more sophisticated. The difference is that, if they worked exactly the same as walkie-talkies, they would need a frequency band for each call that was taking place and there is not so much space available in the spectrum. There would be yet another problem: the greater the distance between the cell phone and the transmission base, the greater the required power, making small devices with battery unfeasible. “The great thing that made cellular systems possible was the division of the areas to be covered into cells, hence the name”, says engineer Michel Yacoub, from Unicamp’s School of Electrical Engineering. Small cells do not require high power and allow the same transmission channels to be reused in another cell, as long as it is not adjacent.
In this way, a channel used for a conversation in the city center can, at the same time, conduct a call in another neighborhood without the two mixing. The city of São Paulo, for example, is divided into hundreds of cells. Each of them has a relay antenna called a radio base station, which, in turn, is connected to a telephone exchange, known as the Commutation and Control Center (CCC). This exchange connects to other CCCs and fixed telephony exchanges – with the rest of the world, therefore.
Mobile telephony divides the city into cells
1 – When Mr. X dials her friend Mrs. Y, the cell phone transmits the number of the other device by radio waves, in a specially reserved frequency range
2 – The number is captured by the radio base station (ERB) of the yellow cell, where Mr. X. The station detects that the destination device is not in the same cell; therefore contact (via cable or microwave) the Communication and Control Center (CCC)
3 – The CCC has a database updated every second, informing which cell each device in the city is in. Upon receiving the communication from the ERB, it sends the call information to the destination cell station
4 – Mrs.’s cell phone rings. Y. That’s because the station of the cell where she is, which received the communication from the Central, advises that her number is being called by Mr. X
5 – When Mrs. Y picks up, each of the two cells looks for a free frequency channel. They inform both cell phones which channels each device will use during the conversation.
MOVING CALL
The same call can go from one cell to another as the user moves around the city. When a cell phone senses that its cell’s signal is getting weak, it looks for another cell closer, which then looks for a free channel and tells the cell phone to change frequency. Everything is done automatically, without the user noticing. Sometimes, however, there are no vacant channels in the new cell – in which case the connection drops.
VARIABLE SIZE
On the outskirts of a city and in rural areas, cells are large, kilometers in diameter. In the city, they are much smaller – reaching a few tens of meters in the busiest centers. That’s because obstacles like tall buildings block radio waves, limiting their range. In addition, the more people circulating in a place, the more cells it needs to have, otherwise there is a risk of not having frequencies for everyone.
HERE YES, THERE NO
Non-adjacent cells (such as those shown in yellow) can use the same frequencies, because there is no risk of interference