Wireless (Radio) Transmission : Applications of Wireless TransmissionFitness Gear & Equipment
Radio encompasses the electromagnetic spectrum in the range of 3 KHz to 300 GHz. In radio communications the signal is transmitted into the air or space, using an antenna that radiates energy at some carrier frequency. For example, in QAM modulation the information sequence determines a point in the signal constellation that specifies the amplitude and phase of the cosine wave that is transmitted. Depending on the frequency and the antenna, this energy can propagate in either a unidirectional or Omni-directional fashion. In the unidirectional case a properly aligned antenna receives the modulated signal, and an associated receiver in the direction of the transmission recovers the original information. In the Omni directional case any receiver with an antenna in the area of coverage can pick up the signal.
Radio communication systems are subject to a variety of transmission impairments. The attenuation in radio links varies logarithmically with the distance. Attenuation for radio systems also increases with rainfall. Radio systems are subject to multi-path fading and interference. Multi-path fading refers to the interference those results at a receiver when two or more versions of the same signal arrive at slightly different times because of reflections from different objects. If the arriving signals differ in polarity, then they will cancel each other. Multi-path fading can result in wide fluctuations in the amplitude and phase of the received signal. Interference refers to energy that appears at the receiver from sources other than the transmitter. Interference can be generated by other users of the same frequency band or by equipment that inadvertently transmits energy outside its band and into the bands of adjacent channels. Interference can seriously affect the performance of radio systems, and for this reason regulatory bodies apply strict requirements on the emission properties of electronic equipment.
In general, radio frequencies below 1 GHz are more suitable for Omni directional applications. For example, paging systems (beepers) are an Omni directional application that provides one-way communications. Cordless telephones are example of an Omni directional application that provides two-way communications. Here a simple base station connects to a telephone outlet and relays signaling and voice information to a cordless phone. This technology allows the user to move around in an area of a few tens of meters while talking on the phone. Other applications of wireless transmission are given below.
Cellular Communications: Cellular telephone networks extend the basic telephone service to mobile users with portable telephones. Unlike conventional telephone service where a cable carries signal between the telephone exchange and the telephone equipment at the user end, here the radio signal is used for transmission. Both portable telephone equipment and the interface at the telephone exchange are capable of broadcasting and receiving the voice signal which modulates the radio carrier signal. There are two standards in cellular telephone systems. GSM - Global System for Mobile (GSM), Code Division Multiple Access (CDMA).
Wireless LANs: Wireless LANs are another application of Omni directional wireless communications. A Wireless LAN (WLAN) is a stretchy data communication scheme implemented as a conservatory to or as an substitute for, a wired LAN within a building or campus. Using electromagnetic waves, WLANs transmit and receive data over the air, minimizing the need for wired connections. Thus, WLANs combine data connectivity with user mobility, and, through simplified configuration, enable movable LANs. In recent past, WLANs have gained strong popularity.
WLAN technology - Spread Spectrum: Most wireless LAN systems use spread-spectrum technology, a wideband radio frequency technique developed by the military for use in reliable, secure, mission-critical communications systems. Spread-spectrum is designed to trade off bandwidth efficiency for reliability, integrity, and security.
Satellite Communications: Early satellite communications systems can be viewed as microwave systems with a single repeater in the sky. A (Geostationary) satellite is placed at an altitude of about 36,000 km above the equator where its orbit is stationary relative to the rotation of the earth. A modulated microwave radio signal is beamed to the satellite on an uplink carrier frequency.
A transponder in the satellite receives the uplink signal, regenerates it, and beams it down back to earth on a downlink carrier frequency.