Cellular Telephones

Cellular telephones transmit and receive radio frequencies in the range 824.040 - 848.970 MHz. This is described as being 45 MHz below the base frequencies assigned by the FCC, implying that a 45 MHz beat frequency is used in the detection scheme. Communication on a cell telephone involves two frequencies, one for transmission and another slightly different for receiving, an arrangement referred to as "full duplex" transmission. This permits simultaneous input and output signals. In a given city it is typical to have 832 frequencies, representing 395 full duplex voice channels and 42 control channels. Each "cell" of base towers has 56 voice channels, or 1/7 of the total, with the distribution arranged on a hexagonal grid. The channels are 30 kHz apart. Early cell phones transmitted the signal by frequency modulation, but the technology is rapidly moving toward digital sound imaging.