Noise in Communication Systems

In the field of electronic communication, noise refers to any unwanted electrical energy that interferes with the desired signal. Noise can significantly degrade the quality and reliability of a communication system. Broadly, noise can be categorized into two main types: external noise and internal noise. Understanding the origin of different types of noise is crucial for designing effective communication systems that can minimize their impact.

External Noise Sources Explained

External noise originates from sources outside the communication system or receiver. These are naturally occurring or human-made disturbances that propagate through the medium (like air) and are picked up by the receiving antenna along with the desired signal. The question specifically asks about primary sources of external noise. The main primary sources of external noise include:

• Atmospheric Noise: Also known as static noise, this type of external noise is caused by natural electrical disturbances in the Earth's atmosphere, primarily lightning discharges during thunderstorms. It is more prevalent at lower frequencies (typically below 30 MHz) and can be a significant problem for radio communication at these bands.
• Extra-terrestrial Noise: This external noise originates from sources outside the Earth's atmosphere. It encompasses two main categories:
  • Solar Noise: Produced by the sun, particularly during periods of sunspot activity and solar flares. This type of external noise is significant at frequencies above 20 MHz and can cause severe interference during intense solar events.
  • Cosmic Noise: Originates from distant stars and other celestial bodies in our galaxy and beyond, such as the galactic center and supernovas. This external noise is observed over a wide range of frequencies, particularly between 8 MHz and 1.5 GHz, and is a constant background noise source.
• Manmade Noise: Also known as industrial noise, this type of external noise is generated by human activities and machinery. It is highly variable depending on the environment. Common sources of manmade external noise include:
  • Ignition systems of automobiles (e.g., spark plugs, causing impulse noise).
  • Industrial machinery (e.g., arc welders, high-voltage power lines, electrical motors).
  • Switching circuits, fluorescent lights, and various other electrical equipment.

  Manmade noise is particularly problematic in urban and industrial areas and tends to decrease with increasing frequency, becoming less significant at very high frequencies.

Thermal Noise: An Internal Noise Source

Unlike atmospheric, extra-terrestrial, and manmade noise, thermal noise is classified as an internal noise source. Internal noise is generated within the components of the communication receiver itself, such as resistors, transistors, and diodes. Thermal noise, also known as Johnson-Nyquist noise or resistor noise, arises from the random thermal motion of charge carriers (electrons) within an electrical conductor, even when no voltage is applied. This incessant random motion causes small, fluctuating voltages and currents that contribute to the overall noise floor of the system.

The power spectral density of thermal noise is uniformly distributed across all frequencies, making it a form of white noise. The magnitude of thermal noise is directly proportional to the absolute temperature of the conductor and its resistance. The root mean square (RMS) noise voltage across a resistor can be expressed using the Nyquist formula:

$$\text{V}_{\text{n}} = \sqrt{4kTR\Delta f}$$

Where:

• \(V_n\) is the RMS noise voltage
• \(k\) is Boltzmann's constant (\(1.38 \times 10^{-23}\) J/K)
• \(T\) is the absolute temperature of the resistor in Kelvin
• \(R\) is the resistance in ohms
• \(\Delta f\) is the bandwidth over which the noise is measured in Hertz

Since thermal noise is generated within the electronic components of the receiver due to their inherent temperature and resistance, it is considered an internal noise source and not an external one that enters the system from the environment.

Distinguishing External vs. Internal Noise

The key distinction between external noise and internal noise lies in their origin. External noise comes from outside the receiver, while internal noise is generated within the receiver's components. Here's a brief comparison:

Therefore, based on the definitions and origins of these noise types, extra-terrestrial noise, atmospheric noise, and manmade noise are all primary sources of external noise. Thermal noise, however, originates from the inherent properties of electronic components within the receiver and is thus an internal noise source, not an external one.