Signal Attenuation and Transmission Media in Communication Systems
Attenuation of Course (I)
Fundamental physical limitation. Depends on:
- Distance and frequency of the transceiver
The signal loss due to distance is exponential, not linear. To establish a linear relationship between losses and gains, we change the unit to decibels (dB), allowing us to add them.
The parameters that influence attenuation are:
- d (distance between transmitter and receiver)
- λ (wavelength)
The wavelength depends on:
- c (speed of transmission channel – air: 300,000 km/s)
- f (frequency of transmission)
Typically, we only need to ensure the minimum power required by the receiver for a quality signal. Signal gains and losses are multiplied; in decibels (dB), this becomes linear, allowing addition and subtraction.
Guided Transmission Media (I)
The bandwidth or transmission speed depends on distance and whether the link is point-to-point or multipoint.
Classification of Guided Transmission Media (II)
Types of Lines
- Bare Wires (Airlines): Formed by a group of conductors (usually copper), separated and supported by insulators, arranged for specific electrical conditions and mechanical stiffness.
- Cables: A group of insulated conductors bundled together with protective covers, mounted on supports or placed in underground pipes.
Cables
In low frequency, just two wires are used. At high frequencies (microwave), power is contained in a physical structure known as a transmission line. They may be buried (directly or in ducts) or suspended in the air. Whether buried or hanging, a certain length is needed for auxiliary equipment: load coils (for low frequencies, in pairs) or repeaters/amplifiers (for carrier systems).
Primary Parameters
- Resistance
- Inductance (L) per kilometer
- Capacitance (C) per kilometer
- Admittance (G) per kilometer. Depends on insulation type; varies little with frequency.
Skin Effect: Significant because a smaller penetration depth increases resistance per unit length. As frequency increases, the penetration depth of boundary currents decreases for electromagnetic waves.
Physical Constitution
- Conductors: Usually copper, with gauges ranging from 0.3 to 1.3 mm (0.91 mm is widely used).
- Insulation: Separates conductors; can be paper or plastic.
- Covers: Protect conductor pairs; must be flexible, mechanically resistant, corrosion-resistant, and impervious to moisture. Materials include lead (now disused), steel, aluminum, and increasingly plastics like polyethylene and polyvinyl chloride (sometimes with aluminum tape for electromagnetic shielding).
Pair Cable
Also called symmetric pair cables, these have balanced two-wire-to-ground pairs. They consist of twisted pairs arranged in layers. Used for low-frequency applications (telephony) and digital transmission (computer networks).
Quad Cables
Cables where insulated conductors are grouped in fours, forming small squares. The total number of quads can vary. Types of quad core cables:
- DM: Transmission at low frequencies.
- Star: High-frequency transmission.
- Peer-Star: Exclusively for low-frequency transmission.
Rating:
- Twisted Pair Cable
- Unshielded Twisted Pair Cable (UTP)
- Shielded Twisted Pair Cable (STP)
- Overall Screened Twisted Pair Cable (FTP)
- Narrowband Coax Cable
- Broadband Coaxial Cable (High Performance or Measurement)
Fiber Optics, Constitution (I)
Very flexible and fine, used for guiding optical energy (visible, infrared, ultraviolet). Optical fibers are dielectric structures, usually silica, with cylindrical symmetry. They have three basic areas:
Fiber Optics, Constitution (II)
- Core: Transmits light; made of optically transparent material (glass or plastic fibers).
- Cladding: Surrounds the core; made of the same material but without doping (pure silica). Confines light in the core.
- Coating: Protects and strengthens the structure, preventing coupling with other fibers. Can be one or more layers of acrylic or similar material, solidified by UV radiation. Its refractive index must be greater than the cladding.
A cover (plastic or similar) isolates the contents from crushing, abrasion, moisture, twisting, vibrations, etc., providing mechanical properties for handling and harsh environments or rodent resistance.
Fiber Optics, Characteristics
Attenuation has evolved from 1000 dB/km to 20 dB/km in the early 1970s. Today, it’s down to 0.16 dB/km, allowing link lengths of 200 km without signal regeneration.
Advantages of fiber optics:
- High bandwidth.
- Low transmission losses (less attenuation).
- Total insulation against electromagnetic interference.
- High mechanical stability.
- Abundant raw material (25% of the earth’s crust).
- Small size, lightweight, flexibility.
- Low distortion.
Fiber Optic Transmission Method (I)
Light propagation can be approached from several theoretical perspectives, resulting in four models (each includes the previous):
- Ray optics model (simplest).
- Wave optics model (Maxwell’s equations, complex).
- Optical model (electromagnetic field analysis).
- Quantum optics model (electron-photon analysis).
Propagation by ray theory or geometric theory: Studies reflections and refractions in the fiber, yielding results close to experimental values for multi-mode fibers.
Modal theory or electromagnetic field theory: Provides a closer relationship between calculated and measured values for single-mode fiber analysis. However, it involves more complex mathematics and is harder to interpret physically.