 idea underlying a mobile communication system. We have a person in his car with a mobile phone, which is made up of three parts. When he talks into it: 1. the phone turns the voice signal into a digital signal using a device called a codec (coder/decoder); EM wave EM wave 825 MHz 875 MHz station High frequency High frequency signal signal Digital version Digital version of voice signal of voice signal Electrical signal Electrical version representing voice of voice signal signal Figure 2.21 Mobile communication system fundamentals Chapter Two 2. the phone takes the digital signal it just created and turns it into a high-fre- quency signal (for example, centered at about 825 MHz) using a device called a modem (modulator/demodulator); the high-frequency signal feeds an antenna, and that antenna turns the high- frequency electrical signal into an electromagnetic wave (of frequency 825 MHz). The electromagnetic wave then travels through the air and is picked up by an antenna on a tower called a base station. The antenna on the base station turns the signal into an electric signal. The base station, with help from a switching center called the MTSO (Mobile Telephone Switching Office), figures out where the call is going, and then resends the signal down to the receiving mobile phone at a different fre- quency (for example, at about 875 MHz). The receiving mobile phone has an antenna on it, and this antenna picks up the electromagnetic signal intended for the phone, and turns it into an electrical signal (with frequency 875 MHz). This high-frequency electrical signal is turned into a low- frequency digital signal by the modem. Finally, the digital signal is returned to the original speech signal by the codec. This signal travels from phone to person's ear- and there you have it, a mobile communication system. When engineers first built such a system, they had a problem. The government only gave them a small band of frequencies to Cell A use for their wireless communication systems (for example, 824 MHz-849 MHz and 869