1.2 Principles of Wireless Communications

In this section, we describe fundamental principles of wireless communications and related wireless technologies, including wireless radio and satellite communications. We introduce basic modulation techniques used in radio communications and two fundamental wireless system topologies: point-to-point and networked topologies. We discuss performance elements of wireless communications.

1.2.1 Wireless Technologies

Today, there are many wireless technologies that are used for a variety of applications. Wireless radio communications are based on transmission of radio waves through the air. Radio waves between 30 MHz and 20 GHz are used for data communications. The range lower than 30 MHz could support data communication; however, it is typically used for FM and AM radio broadcasting, because these waves reflect on the Earth's ionosphere to extend the communication. Radio waves over 20 GHz may be absorbed by water vapor, and therefore, they are not suitable for long distance communication. Table 1.1 shows radio frequencies used for wireless radio applications in AM and FM radio, TV, GPS, and cell phones. ^[1]

Table 1.1: Radio Spectrum and Applications
Applications	Frequency Spectrum
AM	535 to 1700 kHz
FM	88 to 108 MHz
TV	54 to 88, 174 to 220 MHz
GPS	1200 to 1600 MHz
Cell phones	800 to 1000 MHz 1800 to 2000 MHz

Microwave transmission is based on the same principles as radio transmission. The microwave networks require a direct transmission path, high transmission towers, and antennas. Microwave equipment in the United States operates at 18 to 23 GHz. There are 23,000 microwave networks in the United States alone.

Satellite communications are used for a variety of broadcasting applications. The two most-popular frequency bands for satellite communications are C-band (frequency range 5.9 to 6.4 GHz for uplink and 3.7 to 4.2 GHz for downlink) and Ku-band (frequency range 14 to 14.5 GHz for uplink and 11.7 to 12.2 GHz for downlink). Recently, the Ku-band spectrum has been opened up to U.S. satellite communication, which receives at 30 GHz and sends at 20 GHz.

The radio transmission system consists of a transmitter and a receiver. The main components of a radio transmitter are a transducer, an oscillator, a modulator, and an antenna. A transducer, e.g., a microphone or a camera, converts the information to be transmitted to an electrical signal. An oscillator generates a reliable frequency that is used to carry the signal. A modulator embeds the voice or data signal into the carrier frequency. An antenna is used to radiate an electrical signal into space in the form of electromagnetic waves.

A radio receiver consists of an antenna, an oscillator, a demodulator, and an amplifier. An antenna captures radio waves and converts them into electrical signals. An oscillator generates electrical waves at the carrier frequency that is used as a reference wave to extract the signal. A demodulator detects and restores modulated signals. An amplifier amplifies the received signal that is typically very weak.

^[1]Rhoton, J., The Wireless Internet Explained, Digital Press, 2002.