Wireless phones, unlike conventional phones that use wires and cables, use radio signals that are carried through the air.
Electromagnetic radiation with a high frequency carrier wave (e.g., radio wave) and an extremely low frequency (ELF) modulation is considered amplitude modulated (e.g. Time Division Multiple Access (TDMA) mobile phone systems). The modulation can also be pulsed where the carrier wave is switched on and off very rapidly in the rate of about 100 pulses per second (e.g., GSM mobile phone systems), while continuous wave (cw) radiation is generated constantly when the transmitter is on (e.g., analog mobile phone systems). Digital systems transmit information in bursts, thereby introducing an amplitude modulation component onto the carrier. Analog phone systems generally use narrow band frequency modulation, which causes phase variations in the carrier with very little amplitude change. Digital systems, because of their improved accuracy of transmission, are gradually replacing analog systems.
The first generation wireless phones were introduced in Europe in 1981 and in the USA in 1983. They were analog in type and operated at 800 and 900 megahertz (MHz), which are slightly higher frequencies than those used for UHF-TV broadcasting (channels 14 to 67; 470 - 806 MHz). From the early 1990s analog phones have been gradually replaced by the second generation digital systems. Some digital systems, such as the Global System for Mobile Communications (GSM), operate at 800-1000 MHz. GSM also operates at 1800 MHz, as does the Personal Communication Service (PCS).
A third generation (3G) of mobile telecommunications technology is being introduced using frequency bands 1885-2010 MHz and 2100-2200 MHz. In Europe it is called UMTS (Universal Mobile Telecommunications System) and worldwide it is known as IMT-2000 (International Mobile Telecommunications - 2000). Others are planned that will use even higher frequencies. The peak output powers of UTMS handsets (0.125W) are lower than those of GSM handsets. However, the transmissions of GSM handsets are pulsed whereas UTMS handsets transmit continuously. Consequently, the maximum time-averaged power is the same for UTMS handsets as for GSM handsets operating in the 1800 MHz frequency band (0.125 W). The peak and spatial distribution of specific energy absorption rate (SAR) in the head under standard test conditions might be expected to be similar for UTMS and GSM 1800 handsets. In practice, the SAR will depend on the characteristics of individual handsets, in particular the design and location of the antennas (NRPB 2004). The services associated with 3G enable the transfer not only of voice data but also nonvoice data, for instance email or instant messaging.
Another expanding area of wireless communication is WiFi (Wireless Fidelity), which is any type of wireless technology that supports local, over-the-air computer communication via a wireless lcaol area network (WLAN). Usually the transmission frequency is about 2.4 GHz. "WiMAX" is a long-range version of WiFi. "Bluetooth is a term used for digital wireless communication among personal-computer-associated devices - i.e. "digital enhanced cordless telecommunication" between laptops, personal computers, personal digital assistants, cell phones, printers, digital cameras, etc. (Valberg et al., 2007).
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