Communication is the transmission of information from one point to another. Modern communication systems - radio, television, telephony, satellite links, and the internet - rely on the principles of electronics and electromagnetic wave propagation. This chapter introduces the basic elements of a communication system and the terminology used to describe its performance.
A communication system has three essential components: a transmitter that converts the message into a signal, a channel through which the signal travels, and a receiver that recovers the original message. Signals can be analog, varying continuously like a human voice, or digital, consisting of discrete 0s and 1s.
To transmit information efficiently over long distances, the low-frequency message signal is superimposed on a high-frequency carrier wave in a process called modulation. This chapter studies amplitude modulation in detail, the propagation of electromagnetic waves through the atmosphere, and the practical systems that use these principles, from broadcasting to satellite and optical communication.
The basic communication system consists of: 1. Transmitter: converts the information (message signal) into a form suitable for transmission. It includes the transducer that converts sound or other information into an electrical signal, and the modulator that combines the message with a high-frequency carrier. 2. Channel: the medium through which the signal travels, such as free space, coaxial cable, or optical fibre. The channel may attenuate the signal and add noise. 3. Receiver: extracts the message from the received signal and converts it back to its original form, using a demodulator (detector) and appropriate amplification.
Information sources include speech, music, pictures, and computer data. A transducer converts the message into an electrical signal, and the signal is then processed, modulated, and transmitted. The signal may be analog or digital; digital signals are more robust to noise.
The range of frequencies contained in a signal is called its bandwidth. Different signals have different bandwidths: - Speech signals: about 300 Hz to 3100 Hz, a bandwidth of about 2.8 kHz. - Music signals: about 20 Hz to 20 kHz, a bandwidth of about 20 kHz. - Video signals: bandwidth of about 4.2 MHz. - TV signals: bandwidth of about 6 MHz.
The bandwidth of a communication channel is the range of frequencies it can transmit without significant attenuation. For a channel to transmit a signal without distortion, the channel bandwidth must be at least as large as the signal bandwidth. The information carrying capacity of a channel is proportional to its bandwidth. Baseband signals are signals in their original frequency range, transmitted without modulation, such as voice over a local telephone line.
The message signal is a low-frequency signal that cannot be transmitted directly over long distances for several reasons:
Size of the antenna: for efficient radiation, the antenna size should be comparable to the wavelength. For a 1 kHz audio signal, the antenna would need to be about 75 km long, which is impractical. By modulating to a high carrier frequency, the wavelength shrinks and a small antenna suffices.
Effective power radiated: power radiated is proportional to the fourth power of frequency, so high-frequency signals radiate much more power for the same current.
Mixing of signals: if all stations transmitted baseband signals directly, the signals would mix and interfere, and receivers could not separate them. Modulation assigns each station a distinct carrier frequency band, enabling multiplexing.
Longer transmission range: high-frequency modulated waves travel farther through the atmosphere.
Thus modulation - superimposing the message on a high-frequency carrier - is essential for practical radio, television, and telephony.
In amplitude modulation (AM), the amplitude of a high-frequency carrier wave is varied in accordance with the instantaneous amplitude of the message signal, while the frequency and phase of the carrier remain constant. If the message signal is m(t) = Am cos(omega_m t) and the carrier is c(t) = Ac cos(omega_c t), the modulated wave is:
c_m(t) = (Ac + Am cos(omega_m t)) cos(omega_c t)
The modulation index (or depth of modulation) is:
mu = Am / Ac
For undistorted transmission, mu must be less than or equal to 1. If mu > 1, overmodulation occurs and the message is distorted. The AM wave contains three frequencies: - The carrier frequency fc. - The upper sideband frequency fc + fm. - The lower sideband frequency fc - fm.
The bandwidth of an AM wave is 2fm, twice the message frequency. The sidebands carry the information, and the power in the sidebands is given by P_sb = (mu^2/4) P_c for each sideband, where P_c is the carrier power.
An AM wave is produced by a modulator circuit. In a simple square-law modulator, the message and carrier are applied to a device with a nonlinear characteristic, producing sum and difference frequencies. The AM wave is obtained by filtering the unwanted components.
Detection (demodulation) recovers the message signal from the AM wave. A simple AM detector uses: 1. A diode that rectifies the modulated wave, allowing only one half cycle to pass. 2. An envelope detector with an RC filter that smooths the rectified wave, recovering the envelope (the message signal).
The product of the time constant RC should satisfy 1/fc << RC << 1/fm for faithful detection. The detector output is then amplified to give the original message. AM broadcasting uses this scheme, which is simple and cheap, though AM is more susceptible to noise than FM.
The transmission of electromagnetic waves depends on the frequency and the atmospheric layers. There are three main modes of propagation:
Ground (surface) waves travel along the surface of the Earth, following its curvature. They are used for low and medium frequencies (AM radio) and are attenuated with distance.
Sky (ionospheric) waves are reflected back to Earth by the ionosphere, the ionized region of the upper atmosphere. The ionosphere reflects waves in the high-frequency (HF) band, from about 3 to 30 MHz, allowing long-distance communication by multiple reflections. The electron density of the ionosphere and its variation with time determine the reflection.
Space waves travel in a straight line from the transmitter to the receiver, used for very high frequencies (VHF) and above, including television, radar, and satellite communication. Space wave transmission is limited by the line of sight and by the curvature of the Earth, so antenna heights are important. The height h of the transmitting antenna extends the range to about d = sqrt(2 R h).
Television uses frequencies in the VHF and UHF bands, transmitted by space waves, with FM for audio and AM for video. Satellite communication uses geostationary satellites at about 36,000 km altitude, which receive uplink signals, amplify them, and transmit downlink signals to Earth. A single satellite can cover a large area, making satellite links ideal for international communication and broadcasting.
Optical fibre communication uses light (infrared) guided along thin glass fibres by total internal reflection. It offers enormous bandwidth, low loss, immunity to electromagnetic interference, and high security. Optical fibres form the backbone of the modern internet and telephone networks.
Mobile communication uses cellular networks: the service area is divided into small cells, each served by a base station, and frequencies are reused across cells to accommodate many users. The mobile phone is a radio transmitter and receiver operating in microwave bands.
| Quantity | Formula | Remark |
|---|---|---|
| Modulation index | mu = Am/Ac | Must be <= 1 |
| Bandwidth of AM | BW = 2 fm | Sidebands fc +/- fm |
| Sideband power | P_sb = mu^2 P_c/4 | Each sideband |
| Total AM power | P_t = P_c(1 + mu^2/2) | Carrier + sidebands |
| Antenna length | l = lambda/4 | For efficient radiation |
| Range of space wave | d = sqrt(2 R h) | Line of sight |
| Signal | Bandwidth |
|---|---|
| Speech | 2.8 kHz |
| Music | 20 kHz |
| Video | 4.2 MHz |
| TV | 6 MHz |
| Propagation mode | Frequency band | Application |
|---|---|---|
| Ground wave | Low, medium | AM radio |
| Sky wave (ionosphere) | 3-30 MHz | HF long-distance |
| Space wave | VHF and above | TV, radar, satellite |
This chapter introduced the principles of communication systems. Every system consists of a transmitter, a channel, and a receiver, with the bandwidth of the signal determining the requirements of the channel. Modulation is necessary because low-frequency baseband signals need impractical antennas, radiate little power, and would interfere with one another. In amplitude modulation, the modulation index mu = Am/Ac, the sidebands fc +/- fm, and the bandwidth 2fm describe the modulated wave, which is produced by a square-law modulator and detected by an envelope detector. Electromagnetic waves propagate as ground waves, sky waves reflected by the ionosphere, and line-of-sight space waves, enabling radio, television, satellite, optical fibre, and mobile communication systems that connect the modern world.