Radio communication system

A radio communication system send signals by radio. ^[1] Types of radio communication systems deployed depend on technology, standards, regulations, radio spectrum allocation, user requirements, service positioning, and investment.^[2]

The radio equipment involved in communication systems includes a transmitter and a receiver, each having an antenna and appropriate terminal equipment such as a microphone at the transmitter and a loudspeaker at the recieiver in the case of a voice-communication system.^[3]

Description

With the technology of the early twentieth century, radio communication systems required:^[4][5]

1. The production of radio frequency alternating electrical energy at a transmitting station. Alternating currents are set up in the transmitting circuit, of suitable frequency and of great intensity (or high voltage). This insures a high rate of variation of the interlinked electrostatic and electromagnetic fields and a great magnitude of these variations.
2. The radiation of this energy into space. The transmitting circuit is given a shape suitable for producing fields extending to great distances and generally in the direction of the receiving circuits more than in other directions.
3. The absorption of a portion of this electromagnetic energy at the receiving stations and its transformation into some form of energy capable of affecting some one of the human senses. The receiving circuit is given such a shape and position as to link it with as large a proportion of the field of the transmitting circuit as possible.

In modern systems, systems transmission intensity is sometimes very small. The power consumed in a transmitting station varies depending on the distance of communication and the transmission conditions. The power received at the receiving station is usually only a tiny fraction of the transmitter's output, since communication depends on receiving the information, not the energy,^[6] that was transmitted.

Classical radio communications systems use frequency-division multiplexing (FDM) as a strategy to split up and share the available radio-frequency bandwidth for use by different parties communications concurrently. Modern radio communication systems include those that divide up a radio-frequency band by time-division multiplexing (TDM) and code-division multiplexing (CDM) as alternatives to the classical FDM strategy. These systems offer different tradeoffs in supporting multiple users, beyond the FDM strategy that was ideal for broadcast radio but less so for applications such as mobile telephony.

In its consideration of the invention of radio, the United States courts accepted a "definition evolved out of the exhaustive depositions taken from many technical experts..."^[7] as requiring "two tuned circuits each at the transmitter and receiver, all four tuned to the same frequency."^[7][8][9] Such radio communication systems are comprised of a transmitting conductor^[10] in which electrical oscillations^[11][12][13] or currents are produced and which is arranged to cause such currents or oscillations to be propagated through the free space medium from one point to another remote therefrom and a receiving conductor^[10] at such distant point adapted to be excited by the oscillations or currents propagated from the transmitter.^{[14][15][16][17]} Receivers using such a strategy,^[18] known as tuned radio frequency receivers, are less common today, having been superseded by systems with frequency-domain selectivity achieved via heterodyning and filtering at an intermediate frequency.

Transmitter

Main article: Radio transmitter design

A transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications.

Modulation

An audio signal (top) may be carried by an AM or FM radio wave.

Amplitude modulation

Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent. For example, changes in the signal strength can be used to reflect the sounds to be reproduced by a speaker, or to specify the light intensity of television pixels. (Contrast this with frequency modulation, also commonly used for sound transmissions, in which the frequency is varied; and phase modulation, often used in remote controls, in which the phase is varied)

In the mid-1870s, a form of amplitude modulation—initially called "undulatory currents"—was the first method to successfully produce quality audio over telephone lines. Beginning with Reginald Fessenden's audio demonstrations in 1906, it was also the original method used for audio radio transmissions, and remains in use today by many forms of communication—"AM" is often used to refer to the mediumwave broadcast band (see AM radio).

Angle modulation

Angle modulation is a class of analog modulation. These techniques are based on altering the angle (or phase) of a sinusoidal carrier wave to transmit data, as opposed to varying the amplitude, such as in AM transmission.

Frequency modulation

Frequency modulation (FM) conveys information over a carrier wave by varying its frequency (contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant). In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. Digital data can be sent by shifting the carrier's frequency among a set of discrete values, a technique known as frequency-shift keying.

FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech (see FM broadcasting). Normal (analog) TV sound is also broadcast using FM. A narrow band form is used for voice communications in commercial and amateur radio settings. The type of FM used in broadcast is generally called wide-FM, or W-FM. In two-way radio, narrowband narrow-fm (N-FM) is used to conserve bandwidth. In addition, it is used to send signals into space.

Phase modulation

Phase modulation is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave. Unlike its more popular counterpart, frequency modulation (FM), PM is not very widely used. This is because it tends to require more complex receiving hardware and there can be ambiguity problems with determining whether, for example, the signal has 0° phase or 180° phase.

Radio receiver

Main article: Radio receiver design

A radio receiver is an electronic circuit that receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna, amplifies it to a level suitable for further processing, and finally converts through demodulation and decoding the signal into a form usable for the consumer, such as sound, pictures, digital data, measurement values, navigational positions, etc.^[19]

Crystal Receiver
Antenna - rheostat coil - crystal rectifier
capacitor - headphones - ground

The crystal radio receiver is a very simple kind of radio receiver. It needs no battery or power source except the power received from radio waves by a long outdoor wire antenna.

Portable radios include simple transistor radios that are typically monoaural and receive the AM, FM, and/or short wave broadcast bands. FM, and often AM, radios are sometimes included as a feature of portable CD, MP3 CD, and USB key players, as well as cassette player/recorders.

Self-powered portable radios, such as clockwork radios are used in developing nations or as part of an emergency preparedness kit.^[20]

A communications receiver is a type of radio receiver used as a component of radio communication link. Commercial communications receivers are characterised by high stability and reliability of performance, and are generally adapted for remote control and monitoring.

See also: Radio receiver, Crystal radio, and Communications receiver

Resonance

Main article: Electrical resonance

Resonance occurs widely in nature, and is exploited in many man-made devices. It is the mechanism by which virtually all sinusoidal waves and vibrations are generated. AM radios use resonant coil pickups on ferrite rods as compact aerials (much smaller than the wavelength). Electrical resonance of tuned circuits in radios that allow individual stations to be picked up.

An LC circuit can store electrical energy vibrating at its resonant frequency. A capacitor stores energy in the electric field between its plates, depending on the voltage across it, and an inductor stores energy in its magnetic field, depending on the current through it. If a charged capacitor is connected across an inductor, current will start to flow through the inductor, building up a magnetic field around it, and reducing the voltage on the capacitor. Eventually all the charge on the capacitor will be gone. However, the current will continue to flow, because inductors resist changes in current, and energy will be extracted from the magnetic field to keep it flowing. The current will begin to charge the capacitor with a voltage of opposite polarity to its original charge. When the magnetic field is completely dissipated the current will stop and the charge will again be stored in the capacitor (with the opposite polarity) and the cycle will begin again, with the current flowing in the opposite direction.

The charge flows back and forth between the plates of the capacitor, through the inductor. The energy oscillates back and forth between the capacitor and the inductor until (if not replenished by power from an external circuit) internal resistance makes the oscillations die out. Its action, known mathematically as a harmonic oscillator, is similar to a pendulum swinging back and forth, or water sloshing back and forth in a tank. For this reason the circuit is also called a tank circuit. The oscillations are very fast, hundreds to millions of times per second.

See also: LC circuit

Duplex communication

Main article: Duplex (telecommunications)

A duplex radio communication system is a system composed of two connected parties or devices which can communicate with one another in both directions. The term duplex is not used when describing communication between more than two parties or devices.

See also

Radio

Invention of radio, Timeline of radio, and History of radio

Wireless

Wireless telegraphy, Antenna theory, Cavity resonator

Broadcasting

Radio network

Devices

Two-way radio, Radio electronics

Other

Resonance, RLC circuit

References

Citations and notes

Further reading

Listed oldest to newest

• Sewall, C. H. (1904). Wireless telegraphy: its origins, development, inventions, and apparatus. New York: D. Van Nostrand.
• Mills, J. (1917). Radio communication, theory and methods, with an appendix on transmission over wires. New York: McGraw-Hill book company [etc., etc.].
• Lauer, H., & Brown, H. L. (1920). Radio engineering principles. New York: McGraw-Hill book comapny; [etc., etc.].
• Cockaday, L. M. (1922). Radio-telephony for everyone; the wireless: how to construct and maintain modern transmitting and receiving apparatus. New York: Frederick A. Stokes.
• Hausmann, E., Goldsmith, A. N., Hazeltine, L. A., Hogan, J. V. L., Morecroft, J. H., Canavaciol, F. E., et al. (1922). Radio phone receiving; a practical book for everybody. New York: D. Van Nostrand.
• Da Silva, E. (2001). High frequency and microwave engineering. Oxford: Butterworth-Heinemann.
• Clint Smith, Curt Gervelis (2003). Wireless Network Performance Handbook, McGraw-Hill Professional. ISBN 0071406557, http://books.google.com/books?id=nZAVGBoPevUC&pg=PA25&lr=&as_brr=3&ei=F4-sSKuzO6XmtgO4393BBA&sig=ACfU3U0g0vtYCOzP0LCCzdRfr7bgKwvTNg#PPA24,M1.