Physics Notes on Communication Systems

COMMUNICATION SYSTEMS

  • Definition of Communication:
    • The term ‘communication’ refers to sending, receiving, and processing of information electronically.
    • A communication system refers to the setup to transmit information.

Basic Terminology in Electronic Communication System

  1. Transducer:

    • Any device that converts one form of energy into another can be termed as a transducer.
    • An electrical transducer is a device that converts some physical variable (pressure, displacement, force, temperature, etc.) into corresponding variations in the electrical signal at its output.

  2. Signal:

    • Information converted into electrical form and suitable for transmission is called a signal.
    • Signals can be either analog or digital.

  3. Noise:

    • Noise refers to unwanted signals that disturb the transmission and processing of message signals in a communication system.
    • The source generating the noise may be located either inside or outside the system.

  4. Transmitter:

    • A transmitter processes the incoming message signal to make it suitable for transmission through a channel and subsequent reception.

  5. Receiver:

    • A receiver extracts the desired message signals from the received signals at the channel output.

  6. Attenuation:

    • The loss of strength of a signal while propagating through a medium is known as attenuation.

  7. Amplification:

    • The process of increasing the amplitude (and consequently the strength) of a signal using an electronic circuit called the amplifier.
    • Amplification compensates for the attenuation of signals in communication systems.
    • It is carried out at points between the source and destination wherever the signal strength becomes weaker than required.

  8. Range:

    • The largest distance between a source and destination up to which the signal is received with sufficient strength.

  9. Bandwidth:

    • Bandwidth refers to the frequency range over which equipment operates or the portion of the spectrum occupied.

  10. Modulation:

    • The original low frequency message/information signal cannot be transmitted over long distances; therefore, information contained in a low frequency signal is superimposed on a high frequency wave acting as a carrier of information.
    • This process is called modulation.

  11. Demodulation:

    • The process of retrieving information from the carrier wave at the receiver is termed demodulation.
    • This is the reverse process of modulation.

  12. Repeater:

    • A repeater is a combination of a receiver and a transmitter and picks up the signal from the transmitter with a change in carrier frequency.
    • Repeaters are used to extend the range of a communication system.
    • A communication satellite is essentially a repeater station in space.

Analog Signal and Digital Signal

  • Analog Signals:

    • Continuous variations of voltage or current; essentially single-valued functions of time.
    • A sine wave is a fundamental analog signal.
    • Sound and picture signals in TV are analog in nature.

  • Digital Signals:

    • Signals that can take only discrete stepwise values.

    • The binary system extensively used in digital electronics employs two levels of signals: '0' corresponds to a low level and '1' to a high level.

    • A rectangular wave can be composed into a superposition of sinusoidal waves with frequencies:
      u0, 2 u0, 3
      u0, 4 u0, ext{…}, where n is an integer extending to infinity and
      u0 = rac{1}{T0}.

    • To reproduce the rectangular wave shape exactly, all harmonics
      u0, 2 u0, 3
      u0, 4 u0… are required, implying an infinite bandwidth; however, contributions from higher harmonics can often be neglected for practicality.

  • Coding Schemes for Digital Communication:

    • Various coding schemes such as binary coded decimal (BCD) and American Standard Code for Information Interchange (ASCII) are used to represent numbers, letters, and certain characters.

Bandwidth of Signals

  • Each type of message signal (voice, music, picture, computer data) has different frequency ranges.
    • Speech Signals:
    • Frequency range: 300 Hz to 3100 Hz; therefore, the required bandwidth: 3100 ext{ Hz} - 300 ext{ Hz} = 2800 ext{ Hz} .
    • Music:
    • Approximate bandwidth: 20 kHz required due to high frequencies from musical instruments.
    • Audible range: 20 Hz to 20 kHz.
    • Video Signals:
    • Requires about 4.2 MHz of bandwidth for picture transmission.
    • A TV signal (containing both voice and picture): typically allocated 6 MHz bandwidth for transmission.

Bandwidth of Transmission Media

  • Various transmission media have different bandwidth capabilities:
    • Wire:
    • Coaxial cable offers bandwidth approximately of 750 MHz, usually operated below 18 GHz.
    • Free Space:
    • Radio waves can propagate over frequencies from hundreds of kHz to a few GHz.
    • Fiber Optic Cable:
    • Optical communication frequency range: 1 THz to 1000 THz; optical fibers can offer transmission bandwidth in excess of 100 GHz.
    • Spectrum Allocation:
    • Arrived by international agreement and administered by the International Telecommunication Union (ITU).

Propagation of Electromagnetic Waves

  • Ground Waves:

    • To radiate signals effectively, antennas should have sizes comparable to the wavelength ext{λ} of the signal (at least ~ ext{λ}/4).
    • Longer wavelengths (lower frequencies) require larger physical antennas, which often operate near the ground.
    • Standard AM broadcast uses ground-based vertical towers as transmitting antennas, where the ground significantly influences signal propagation (surface wave propagation).
    • A wave induces a current in ground and is attenuated due to energy absorption.

  • Space Wave Propagation:

    • Radio waves propagated through the troposphere are known as space waves.
    • The troposphere extends up to 15 km from the Earth's surface.
    • Two components of space wave propagation are:
    • (i) Direct line-of-sight (LOS) component.
    • (ii) Component that reaches the receiver after reflection from the Earth’s surface.
    • Suitable for waves with frequencies above 30 MHz
    • The distance to the horizon dT for a transmitting antenna at height hT is given by: dT = 2RhT, where R is the Earth's radius (approximately 6400 km).

Maximum Line-of-Sight Distance

  • The maximum distance dM between two antennas of heights hT (transmitting) and hR (receiving) is provided by the formula: dM = ext{√}(2RhT) + ext{√}(2RhR).
    • Area covered by transmission A can be expressed as: A = ext{π}(d_T)^{2}.

Examples of Communication Systems using Space Wave Propagation

  • Television broadcasts, microwave links, and satellite communications.

Solved Numerical Examples

Q) What must be the height of a transmission antenna of an FM radio station so that people in a circular region of 3140 km² can receive the program? Given R = 6400 ext{ km}.

  • Solution: A = ext{π}(2h_TR)
    • 3140 = 3.14 imes 2 imes h_T imes 6400
    • h_T ext{= 0.078125 km = 78.125 m}.

Q) An antenna at the top of a tower has a height of 50m and the height of the receiving antenna is 32m. What is the maximum distance for satisfactory communication in LOS mode? Given R = 6400 km.

  • Solution:
    • dm = ext{√}(2R imes hT) + ext{√}(2R imes h_R)
    • d_m = ext{√}(2 imes 6400 imes 10^3 imes 50) + ext{√}(2 imes 6400 imes 10^3 imes 32)
    • d_m = 25.29 imes 10^3 + 20.23 imes 10^3 = 45.5 ext{ km}.

Sky Wave Propagation

  • Occurs between frequencies 2 MHz to 30 MHz.
  • Radio waves reflect from the ionosphere, which acts as a mirror for these waves, allowing reception over long distances.
  • The ionosphere contains electrons and anions due to solar radiation, with varying densities leading to different reflection characteristics.
  • Only frequencies below 30 MHz are reflected; above 30 MHz they penetrate.

Modulation and Its Necessity

  • Low-frequency signals cannot travel long distances due to various factors:

    1. Length of Antenna: For effective transmission, the minimum length of the antenna must be rac{λ}{4}. If transmitted wavelength is 300 km, frequency about 1 kHz, antenna length would be about 75 km, which is impractical.
    2. Power Radiated from Antenna: The power transmitted by an antenna is inversely proportional to the square of the wavelength: P ext{∝} rac{1}{λ^{2}}.
    3. Mixing Signals: Multiple transmitters using audio signals can mix information, necessitating frequency differentiation for proper communication.
    • Definition of Modulation: The process of superimposing low-frequency audio signals on high-frequency waves is called modulation; the low-frequency signal is referred to as the modulating signal, while the high-frequency wave carries the information and is called the carrier wave.

Mathematical Representation of Carrier Wave

  • A sinusoidal carrier wave can be represented as: c(t) = Ac ext{sin}( ext{ω}ct + φ)
    • where c(t) is the signal strength (voltage or current), Ac is the amplitude, ext{ω}c = 2 ext{π}
      u_c is the angular frequency, and φ is the initial phase of the carrier wave.

Types of Modulation

  • Any of three parameters of the carrier wave (Amplitude, Frequency, Phase) can be adjusted by the message signal, resulting in:
    1. Amplitude Modulation (AM):
    • The amplitude of the carrier wave is varied according to the instantaneous value of the modulating wave while frequency and phase remain constant.
    • It can be represented mathematically as:
      cm(t) = (Ac + Am ext{sin}( ext{ω}mt)) ext{sin}( ext{ω}_ct).

Mathematical Form of AM Wave

  • Derived from:

    • The modulated signal given by:
      cm(t) = Ac ext{sin}( ext{ω}ct) + Am ext{sin}( ext{ω}mt) ext{sin}( ext{ω}ct)
    • Which can be simplified and leads to an amplitude modulated wave with maximum and minimum amplitudes:
      A{ ext{max}} = Ac + Am ext{, } A{ ext{min}} = Ac - Am.

  • Modulation Index is defined as:
    ext{μ} = rac{Am}{Ac} = rac{A{ ext{max}} - A{ ext{min}}}{A{ ext{max}} + A{ ext{min}}}.

    • Percentage modulation can be given by:
      ext{μ}( ext{percent}) = rac{A{ ext{max}} - A{ ext{min}}}{A{ ext{max}} + A{ ext{min}}} imes 100.

  • Example: A 10 MHz sinusoidal carrier wave of amplitude 10 mV is modulated by a 5 kHz sinusoidal audio signal wave of amplitude 6 mV.

    • Frequency components of the modulated wave will include:
    • Original carrier frequency = 10 MHz
    • Upper Side Band (USB) = 10.005 MHz
    • Lower Side Band (LSB) = 9.995 MHz
    • Modulation factor:
      ext{μ} = rac{Am}{Ac} = rac{6}{10} = 0.6.

Production of AM Waves

  • AM can be produced using various methods, depicting the modulating signal added to the carrier signal, producing an output that passes through a nonlinear device (square law device).
  • The output supports various frequencies, and through band filtering, unwanted frequencies are stripped away, leaving only the modulated wave.

Detection of AM Waves

  • Before detection, the modulated signal might be weak post-transmission; thus, a receiver typically includes an amplifier and a detection stage.
    • Detection Process: Recovering the modulating signal from the modulated carrier wave involves rectifying the modulated signal and passing it through an envelope detector.