There are several reasons why signals are usually upconverted and transmitted as bandpass signals. Here are few

  1. The size of the antenna that is required to transmit signals efficiently is inversely proportional to the center frequency.
  2. Multiplexing - Often we need to distinguish transmissions from several sources, all of which overlap in frequency. For example, several radio stations transmit audio signals and we need a easy mechanism to separate the transmissions from these stations. This is easily accomplished by using different carrier frequencies for different radio stations.
  3. The propagation characteristics of electro-magnetic signals varies with the frequency and hence it is important to choose a carrier frequency depending on this. For example, a cellular system is designed such that each cell has a coverage of roughly a few miles radius. Signals around the 900 MHz carrier frequency attenuate quickly and over a few miles radius it attenuates such that users can be grouped in to cells and assigned to a base station such that the interference caused from one cell to the other can be kept small.
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