Wireless is a short-cut terminology for electromagnetic (EM) waves transmitted through free space (without wires). EM waves are fundamental in nature. As waves, they are sinusoidal fields defined by their frequency, which can have any value from very slow vibrations to ultra-rapid vibrations. The EM waves engineers create are radiated by vibrating electrons, electrically driven to emit at the characteristic frequencies needed for wireless technology. Table D.1 gives the range of frequencies used for wireless and provides names for the ranges of frequencies that engineers use. The wavelengths of EM waves are related to the reciprocal of their frequencies. Those EM waves with wavelengths between 1 m and 1 mm are called microwaves. Millimeter waves are the name given to the highest frequencies used for wireless, with wavelengths from 0.10 to 1 cm. The terms airwaves and radio waves are sometimes used for EM waves, and the entire range of EM frequencies available for applications is called spectrum. Governments all over the world control their own airwaves by allocating separate bands of spectrum for separate applications.
EM waves can be used to transmit information if the waves are modulated (their characteristics changed with time). Radio waves, particularly microwaves and millimeter waves, form the basis of today’s wireless technology. For certain applications, light waves can also be used to establish short-range wireless links, although most of their high-speed applications require transmission in optical fibers/waveguides.
Communication requires modulating the EM waves, resulting in a spread of frequencies (bandwidth) that carry the information surrounding the carrier frequency that defines the unmodulated wave. Because the bandwidth is a well-defined fraction of the carrier frequency, it can be seen that higher carrier frequencies can have larger bandwidth and can transmit more information. Modulated EM waves form the basis of today’s wireless telecommunication systems. Today, much of the modulation has a digital format.
Because radio waves do not travel well through good electrical conductors like salt water, submerged submarines are cut off from radio communication with their command authorities at ordinary radio frequencies. ... During the Cold War, however, nuclear-powered submarines were developed that could stay submerged for months
