Small Scale Fading vs. Large Scale Fading: Key Differences

Fading refers to the time variation of received signal power caused by changes in the transmission medium, propagation paths, or the presence of obstacles. In essence, it’s how the signal’s strength fluctuates as it travels from the transmitter to the receiver.

Wireless systems consist of three key components: a transmitter, a receiver, and the channel (the medium through which the signal travels).

In fixed scenarios, fading is largely influenced by variations in channel parameters due to atmospheric conditions like rainfall or lightning. However, in mobile scenarios, fading is primarily affected by obstacles along the path that change over time.

Furthermore, path loss contributes to the attenuation (weakening) of the transmitted signal as it propagates.

Based on the channel model parameters and the position or movement of the transmitter/receiver, we encounter various types of fading. Fading arises from reflections off the ground and surrounding buildings, as well as scattered signals from trees, people, and towers within a larger area.

Let’s dive into the specifics of small scale fading and large scale fading.

Small Scale Fading

• Small scale fading is characterized by rapid fluctuations in the received signal strength over very short distances and time periods. Think of it as the signal wiggling rapidly as you move just a few steps.
• These multipath fading types are heavily influenced by the propagation environment. If you’re in a dense city with lots of buildings, you’ll see more multipath fading than in an open field.
• Small scale fading is broadly divided into two main categories:
  • Multipath Delay Spread: How much the signal gets smeared out in time due to arriving via different paths.
  • Doppler Spread: How much the signal’s frequency changes due to the relative motion between the transmitter and receiver.
• Based on multipath delay spread, we have:
  • Flat Fading: The channel has a constant gain and linear phase response over a bandwidth which is greater than the bandwidth of the transmitted signal.
  • Frequency Selective Fading: The channel has a non-constant gain and/or non-linear phase response over a bandwidth which is less than the bandwidth of the transmitted signal. Different frequencies experience different amounts of fading.
• Based on Doppler spread, we have:
  • Fast Fading: The channel impulse response changes rapidly within the symbol duration.
  • Slow Fading: The channel impulse response changes slowly within the symbol duration.

Large Scale Fading

• Large scale fading occurs when an obstacle obstructs the direct path between the transmitter and receiver. This obstruction causes a substantial reduction in signal strength because the electromagnetic wave is shadowed or blocked.

• It’s associated with large fluctuations of the signal over significant distances. This represents the more gradual changes in signal strength as you move around a larger area.

• It primarily includes:

  • Path Loss: The average signal power decrease as the signal propagates over distance.
  • Shadowing Effects: Deviations in the received signal power from the average value due to obstructions.

• The free space path loss can be expressed as:

  $P_t/P_r = \frac{(4 \pi d)^2}{\lambda^2} = \frac{(4\pi f d)^2}{c^2}$

  Where:

  • $P_t$ is the transmitted power
  • $P_r$ is the received power
  • $d$ is the distance between transmitter and receiver
  • $\lambda$ is the wavelength of the signal
  • $f$ is the frequency of the signal
  • $c$ is the speed of light

• Shadowing represents the deviation of the received power of the EM signal from its average value. It results from obstacles along the path between the transmitter and receiver. It depends on the geographical position and the radio frequency of the electromagnetic waves.