Communication Systems

The One Thing

A communication system transfers information from a source to a destination through a suitable channel. Reliable communication depends on appropriate signal processing, modulation, propagation, reception, and control of noise, bandwidth, and distortion.

Reactions, Processes and Experiments

Key Terms

• Communication system: An arrangement of components that conveys information from a transmitter to a receiver.
• Information signal: The electrical signal containing the message, such as speech, music, images, or data.
• Transducer: A device that converts one form of energy into another; a microphone converts sound into an electrical signal, while a loudspeaker performs the reverse process.
• Transmitter: The part of a system that processes, amplifies, modulates, and sends the signal through a channel.
• Channel: The physical medium through which a signal travels, such as free space, optical fibre, or a conducting wire.
• Receiver: The part of a system that selects, amplifies, demodulates, and reconstructs the original message.
• Baseband signal: The original information signal before modulation; it generally has relatively low frequencies.
• Carrier wave: A high-frequency sinusoidal wave used to transport information over a communication channel.
• Modulation: The process of varying a property of a carrier wave according to the information signal.
• Amplitude modulation: A method in which the amplitude of the carrier changes according to the instantaneous value of the message signal, while carrier frequency and phase remain constant.
• Frequency modulation: A method in which the frequency of the carrier changes according to the message signal, while carrier amplitude remains constant.
• Phase modulation: A method in which the phase of the carrier changes according to the message signal.
• Demodulation: The process of recovering the original information signal from the modulated carrier at the receiver.
• Noise: Unwanted random disturbance added to a signal during generation, transmission, or reception.
• Bandwidth: The range of frequencies occupied by a signal or supported by a communication channel.
• Ground wave propagation: Propagation in which radio waves travel close to and follow the Earth's surface; it is useful mainly at lower frequencies and over shorter distances.
• Sky wave propagation: Propagation in which radio waves are returned toward Earth by the ionosphere, allowing long-distance communication at suitable frequencies.
• Space wave propagation: Line-of-sight propagation used mainly at higher frequencies, including television, satellite, and microwave communication.
• Antenna: A device that converts electrical signals into electromagnetic waves for transmission or converts received electromagnetic waves into electrical signals.
• Amplitude modulation index: A measure of the extent of amplitude variation in AM, given by m = A_m/A_c, where A_m is message amplitude and A_c is carrier amplitude.
• Ionosphere: Ionised atmospheric layers that can reflect or refract suitable high-frequency radio waves and support long-distance sky-wave communication.
• Repeater: A station that receives, amplifies or regenerates, and retransmits signals to extend communication distance.
• Signal-to-noise ratio: The ratio of signal power to noise power, given by SNR = signal power/noise power.
• Digital communication: Communication in which information is represented using discrete symbols, usually binary digits.
• Analog communication: Communication using continuously varying signals; compared with digital communication, it is generally more affected by noise and less easy to store and process.

Easily Confused

• Baseband signal and carrier wave: A baseband signal is the original low-frequency information signal, whereas a carrier wave is a high-frequency sinusoidal wave used to transport information.
• Modulation and demodulation: Modulation places information onto a carrier at the transmitter, whereas demodulation recovers the information from the carrier at the receiver.
• Amplitude modulation and frequency modulation: AM varies carrier amplitude while carrier frequency and phase remain constant; FM varies carrier frequency while carrier amplitude remains constant.
• Frequency modulation and phase modulation: FM changes the carrier frequency according to the message, whereas phase modulation changes the carrier phase.
• Ground wave, sky wave, and space wave propagation: Ground waves follow the Earth's surface at mainly lower frequencies, sky waves are returned by the ionosphere at suitable frequencies, and space waves travel by line of sight at mainly higher frequencies.
• Bandwidth and signal-to-noise ratio: Bandwidth is the range of frequencies occupied or supported by a channel, whereas SNR is the ratio of signal power to noise power.
• Noise and interference: Noise is an unwanted random disturbance added to a signal, whereas interference can result when different signals occupy overlapping frequency ranges.
• Transmitter and receiver: The transmitter processes, amplifies, modulates, and sends a signal; the receiver selects, amplifies, demodulates, and reconstructs it.
• Antenna height and radio wavelength: Antenna height affects line-of-sight distance, whereas wavelength determines practical antenna dimensions; antenna length is commonly about λ/4 or λ/2.

What Gets Asked

• Describe or complete the communication chain: Questions may require the sequence information source -> input transducer -> transmitter -> communication channel -> receiver -> output transducer -> destination. Marks are lost by omitting a transducer or reversing the transmitter and receiver functions.
• Explain why modulation is necessary: A complete answer must include impractically large antennas for low-frequency signals, inefficient long-distance radiation, and severe interference if many signals were transmitted directly.
• Calculate AM quantities: Questions may use $m=A_m/A_c$, $B=2f_m$, $B=2f_m(max)$, $P_t=P_c(1+m^2/2)$, or sideband power $P_c{m}^2/4$. Marks are lost by using the message frequency instead of twice the message frequency for single-tone AM bandwidth.
• Identify modulation types from the changing carrier property: AM changes amplitude, FM changes frequency, and phase modulation changes phase. For AM, $m\le 1$ is distortion-free; $m>1$ indicates overmodulation and distortion.
• Select the appropriate propagation mode: Ground waves are mainly associated with lower frequencies and shorter distances, sky waves with approximately 3 MHz to 30 MHz and ionospheric return, and space waves with higher frequencies and line-of-sight communication. Marks are lost by assigning ionospheric reflection to space-wave propagation.
• Apply antenna and radio-horizon equations: Questions may use $d=\surd (2R{h}_t)+\surd (2R{h}_r)$ or $d=\surd (2Rh)$. Heights must be measured in the same units, and increasing antenna height increases the line-of-sight distance.