Revision: Communication Systems >> Communication Systems Physics (Theory) ISC (Science) ISC Class 12 CISCE

Physical medium which connects the transmitted and the receiver is called the channel.

Bandwidth of signals is a range of frequencies required for the satisfactory transmission of message signals.

The medium in between a transmitter and tli.e receiver is called transmission medium.

The physical medium used for establishing a communication between a transmitt~ng station and receiving station is called communication channel. If the channel is a conducting, physical path or line it is called line communication channel.

The process of changing some characteristics like amplitude, frequency or phase of a carrier wave in accordance with the intensity of the modulating signal is called modulation. 

m_a = \[\frac {\text{Maximum change in amplitude of carrier wave}}{\text{Amplitude of unmodulated carrier wave}}\] = \[\frac {E_s}{E_c}\]

• A communication system consists of four main parts: an information source, a transmitter, a channel, and a receiver.
• The transmitter converts the message signal into a suitable form for transmission through the channel, and the receiver reconstructs the original message.
• Communication systems are classified based on information source, nature of signal (analog/digital), channel type, and type of modulation (AM, FM, PM).

• Electromagnetic waves propagate in three ways: ground wave, space wave, and sky wave.
• In ground-wave propagation, radio waves travel along Earth's surface.
• In space wave propagation, waves travel directly in a straight line between transmitting and receiving antennas, while in sky wave propagation, waves are reflected by the ionosphere back to the Earth.

• Speech needs about 2800 Hz, music about 20 kHz, and video about 4–6 MHz bandwidth.
• Digital signals consist of square pulses composed of multiple harmonics.
• Ideal digital signals require infinite bandwidth, but practical limits on bandwidth cause slight distortion.
• Different transmission media have different bandwidth capacities.
• Optical fibre has a very high bandwidth, higher than coaxial cable and free space.

• A communication satellite is an artificial satellite used for long-distance microwave signal transmission.
• In satellite communication, signals sent from Earth to the satellite are called uplink, and signals retransmitted back to Earth are called downlink.
• A satellite that receives, amplifies and retransmits signals is called a transponder or relay satellite.
• Geostationary satellites orbit at about 36,000 km above the Earth and appear stationary relative to the Earth.
• Microwaves (GHz range) are used because they can penetrate the ionosphere and provide high bandwidth for communication.

• AM has three frequencies: carrier, upper sideband, and lower sideband.
• AM bandwidth = 2 × highest signal frequency.
• AM has noise, low efficiency, and poor sound quality.
• In FM, the frequency changes while the amplitude remains constant.
• FM gives clear sound, less noise, and wider coverage.

• Communication is the process of sending, receiving, and processing information from one point to another in an intelligible form.
• A signal is a physical quantity that varies with time or space and carries information in electrical form.
• Attenuation is the weakening of a signal as it travels through a medium; noise is an unwanted disturbance that distorts the signal.
• A transducer converts one form of energy (like sound or pressure) into electrical signals and vice versa.
• Amplification is the process of increasing the strength of a weak signal without changing its shape.
• Modulation is the process of mixing a low-frequency message signal with a high-frequency carrier wave for long-distance transmission.
• Demodulation (or detection) is the process of recovering the original message signal from the modulated wave.
• A transmitter processes and transmits the signal, while a receiver receives it and extracts the original message.
• Bandwidth is the frequency range occupied by a signal, and a repeater is a device that amplifies and retransmits signals to increase the communication range.

• Ground wave travels along the Earth’s surface and is used for short distances (500 kHz–1500 kHz). Its strength decreases due to absorption by the ground.
• Space wave (LOS) travels in straight lines and is used for TV, FM, and satellite communication.
  Maximum range:
  d = \[\sqrt {2Rh}\]
• Frequencies above 40 MHz are not reflected by the ionosphere, so they require line-of-sight or satellite transmission.
• Sky waves are reflected by the ionosphere and are used for long-distance communication (a few MHz to 30–40 MHz).
• Critical frequency: f_c = 9\[\sqrt {N_{max}}\]​​.
  The first receiving point is called the skip distance, and the region with no signal is the skip zone.