Mobile devices such as cell phones, tablet computers, laptops and the like have an enormous impact on our daily lives. Cell phones, for example, now provide numerous software applications (apps) in addition to classic telephony that enrich our everyday lives in terms of Internet access, navigation, communication, control of devices, security and social media. Life in Western society is hardly imaginable without mobile devices.
Many apps require Internet access. This is achieved by receiving radio signals through any number of cellular base stations equipped with microwave antennas. These sites are typically mounted on a tower, pole, or building located in populated areas and then connected to a wired communications network and switching system. Cellular phones, as an example, have a low-power transceiver that transmits voice and data to the nearest cellular sites, which are typically no more than 8 to 13 km (about 5 to 8 miles) away. In areas of low coverage, a cellular repeater can be used, which uses a high-sensitivity dish antenna or Yagi antenna to communicate over long distances with a cell tower far beyond normal range and uses a repeater to retransmit to a small local antenna with a short range that allows any cell phone within a few meters to function properly.
When the cellular phone or mobile device is turned on, it registers with the cellular exchange using its unique identifier and can then be notified by the exchange when a phone call is received or a network connection is to be established. The mobile device constantly searches for the strongest signal received from surrounding base stations and can seamlessly switch between locations. As the user moves around the network, “handoffs” are performed to allow the device to change locations without breaking the connection.
Cell sites have relatively low-power radio transmitters (often as little as one or two watts) that broadcast their presence and relay communications between mobile terminals and the switching center. The exchange, in turn, connects, for example, a call to another subscriber of the same mobile operator or to the public telephone network, which includes the networks of other mobile operators. Many of these sites are camouflaged to blend into the existing environment, especially in scenic areas.
The dialog between the cellular phone and the cell site is a digital data stream that also includes digitized audio data (except for first-generation analog networks). The technology used to achieve this depends on the system that the mobile operator has implemented. Technologies are classified by generation. The first generation systems, introduced in Japan in 1979, are all analog and include AMPS and NMT. Second-generation systems, introduced in Finland in 1991, are all digital and include GSM, CDMA and TDMA. Modern wireless standards include third (3G), fourth (4G or LTE), and fifth (5G) generation.
Some older wireless technologies make phones susceptible to “cloning.” Whenever a cell phone goes out of range (e.g., in a road tunnel), once the signal is restored, it sends a “reconnect” signal to the nearest cell tower to identify itself and signal that it is ready to transmit again. With the right equipment, it is possible to intercept the reconnect signal and encode the data it contains into a “blank” phone – for all intents and purposes: the “blank” phone is then an exact duplicate of the real phone, and all calls made with the “clone” are billed to the original account. This problem was widespread with the first generation of analog technology, but later digital standards from GSM onwards significantly improve security and make cloning more difficult.
In an effort to limit the potential harm from having a transmitter close to the user’s body, the first fixed/mobile cellular phones that had a separate transmitter, vehicle-mounted antenna, and handset (known as car phones and bag phones) were limited to a maximum 3 watts Effective Radiated Power. Modern handheld cellphones which must have the transmission antenna held inches from the user’s skull are limited to a maximum transmission power of 0.6 watts ERP.