Radio Frequency Wireless Communications

RF technologies employ transmitters and receivers tuned to produce and consume, respectively, radio waves of a given frequency range. The transmitter's power and the receiver's sensitivity help to determine the distance over which they can communicate. High transmission power output is used for long-range communications such as broadcast television while short-range communications generally require much less power; thus technologies that are designed to communicate across only a few meters could be employed in small, mobile battery powered devices. Another characteristic that is relevant for communication applications is the ability of radio waves to penetrate many objects. Obstacles reflect light waves used in technologies such as infrared, but radio waves used in RF technologies in general can (with certain limitations) penetrate many obstacles (although in some cases radio waves can diffract or go around objects too). Thus RF technologies can permeate many obstacles such as clothing, bodies, walls, doors and the like. This means that there is no requirement for a "line of sight" between the transmitter and the receiver.

RF technologies use frequency modulation to generate radio waves within a certain frequency spectrum, which encode information and can be intercepted by receivers tuned to the corresponding frequency. FM radio broadcasts, for example, operate in the 88 megahertz (MHz) to 108 MHz frequency spectrum; some cordless telephones operate in the 900 MHz frequency spectrum; Bluetooth wireless communications and other technologies operate in the 2.4 gigahertz (or GHz; one gigahertz equals one billion cycles per second) spectrum. Because the usable radio frequency space is finite, most governments regulate its use, partitioning frequency ranges and granting licenses to transmit at those frequencies at specified power levels. In the United States, for example, a federal license is required to transmit in the FM radio frequency spectrum except at extremely low power levels that limit the range to no more than about 30 meters. Some frequencies are reserved for use without a license under certain conditions. For example, in the United States unlicensed operation is permitted, with some restrictions, in the 900 MHz and 2.4 GHz frequency ranges (the latter being where Bluetooth wireless communication operates). In fact, through multinational agreement, the 2.4 GHz spectrum requires no license for its use anywhere in the world.^[1] The SIG together with other organizations such as the IEEE 802.11 standards body is working with regulatory authorities in some countries to pursue harmonization of the frequency assignments for unlicensed use within the 2.4 GHz spectrum and of the approval process for wireless communications. In general the chosen frequency spectrum can be used globally without license so long as the rules for operating within this spectrum are followed.

^[1] In some countries there are restrictions and only part of this spectrum is available for unlicensed use; these restrictions are discussed elsewhere in the book, notably in Chapter 6.