Flat Fading vs. Frequency Selective Fading

This article explores the distinctions between flat fading and frequency selective fading in wireless communication systems. We’ll delve into how these types of fading affect the signal as it travels from the transmitter to the receiver.

Wireless systems fundamentally consist of a transmitter, a receiver, and the channel that connects them. This channel can be either time-varying or fixed. Fading, in this context, refers to the fluctuation in received signal strength over time, caused by the characteristics of the channel as experienced by the transmitted signal.

Several factors contribute to channel fading, including atmospheric conditions (rain, lightning) and physical parameters (movement of the transmitter and receiver, obstacles in the path, etc.). For more background, refer to articles on the basics of fading and the different types of fading.

Let’s now examine flat fading and frequency selective fading in detail.

Flat Fading

• In flat fading, the channel’s response has a flat gain (or linear phase) across a bandwidth that is greater than the signal’s bandwidth.
• The spectral characteristics of the signal impaired by flat fading are preserved over time.
• Figure 1 (above) illustrates the time-domain and frequency-domain characteristics of a flat fading channel.
• Since the signal’s gain varies over time, flat fading channels are also known as amplitude-varying channels. They are sometimes referred to as narrowband channels because the signal bandwidth is narrow compared to the channel bandwidth.
• A signal experiences flat fading if the following conditions are met:
  • Bs << Bc (Signal Bandwidth is much smaller than Channel Bandwidth)
  • Ts >> στ (Symbol Time is much greater than Delay Spread)

Frequency Selective Fading

• In frequency selective fading, the channel’s response exhibits a constant gain (or linear phase) across a bandwidth that is less than the signal’s bandwidth.
• It is caused by Inter-Symbol Interference (ISI), where the received signal consists of multiple delayed and attenuated versions of the transmitted signal.
• Figure 2 (above) shows the time-domain and frequency-domain characteristics of a frequency selective fading channel.
• A signal undergoes frequency selective fading if the following conditions are met:
  • Bs > Bc (Signal Bandwidth is greater than Channel Bandwidth)
  • Ts < στ (Symbol Time is less than Delay Spread)
• A common rule of thumb is that a channel is frequency selective if Ts <= 10*σ τ.

Difference between Flat Fading and Frequency Selective Fading

The following points summarize the key differences between flat fading and frequency selective fading:

• Bandwidth: In flat fading, the signal’s bandwidth is less than the channel’s bandwidth. In frequency selective fading, the signal’s bandwidth is greater than the channel’s bandwidth.
• Delay Spread: In flat fading, the delay spread is less than the symbol period. In frequency selective fading, the delay spread is greater than the symbol period.
• Frequency Spectrum: In flat fading, frequencies within a spectrum are faded equally. In frequency selective fading, one part of the frequency spectrum is faded more than another.