Wireless communications consists of a set of technologies used to transmit information through a free-space medium that is common to all users. But, as happens in many other fields, this medium, for many years believed costless and inexhaustible, is far from being an unlimited resource. According to what is predicted by information theory, a wireless channel''s capacity (the number of transmitted bits per second) is proportional to the transmitted signal-to-noise-power ratio and to the bandwidth used in the transmission. Thus, for a certain amount of noise power, error-free communication can only be guaranteed at the expense of spectrum consumption, power consumption, or both.
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