I can receive FM signal about 50km away from the transmitting station but I lose signal from cell towers in about few km (1-3km)
closed as too broad by Dmitry Grigoryev, Nick Alexeev♦ Sep 14 '18 at 19:43
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There are likely more reasons, but consider your tiny hand held device only develops a fraction of a watt of transmitted power. Clearly, then, it is your device that has limited range. And if your device developed more power, then you would be "stepping" on other's signals in other cells normally outside of your range. If you are clear minded, you can easily see how this could reduce the number of simultaneous cell phone calls by orders of magnitude in a populated city.
There are several reasons:
- A cellular network is bidirectional. The base station needs to receive the signal of your mobile phone as well for the communication to work. It doesn't matter how much power you transmit with the base station if it can't hear the tiny transmitter in your phone. Mobile phone transmit power is in the order of 1W, while FM radio stations may transmit with multiple kilowatts of power.
- Higher frequencies are more directional. FM radio frequencies are around 100MHz while cellular frequencies are about one magnitude above that. This means you have more absorption in obstacles and the waves do not diffract (bend) around them as much as with lower frequencies.
- Larger receiver bandwidth means lower sensitivity, because the total noise floor is higher for a larger bandwidth. While an FM radio channel has a bandwidth of less than 100kHz, the bandwidth of GSM is 200kHz and for newer cellular standards even multiple megahertz.