Flat Fading vs Frequency Selective Fading | difference between Flat Fading and Frequency Selective Fading
This page compares Flat Fading vs Frequency Selective Fading in wireless communication and mentions difference between Flat Fading and Frequency Selective Fading.
Wireless system consists of transmitter, receiver and channel. The channel between transmitter and receiver can be time varying or fixed. Fading refers to variation in received signal strength with respect to time due to channel seen by the transmitted signal over the path of travel. Channel fading occurs due to various factors which include atmospheric (rain, lightening) and physical (movement of Transmitter and receiver, path etc.) parameters.
Refer article on types of fading and fading basics>>. Let us understand flat fading and frequency selective fading.
Flat Fading
• Channel response in flat fading impaired signal has flat gain/linear phase over bandwidth (BW)
which is greater than signal BW.
• Spectral characteristics of flat fading impaired signal are preserved over time.
• The figure-1 depicts time domain and frequency domain flat fading channel characteristics.
• As gain of the signal varies over time, flat fading channels are known as
amplitude varying channels. They are also called as narroband channels as signal BW is narrow compare to channel BW.
• Signal undergoes flat fading if following conditions are met
➨ Bs <<Bc
➨ Ts >>στ
Frequency Selective Fading
• In this faded signal, channel response of signal has constant gain/linear phase over bandwidth (BW)
which is less than that of signal BW.
• It is caused by ISI (Inter Symbol Interference) where in received signal consists of multiple delayed
and attenuated versions of the transmitted signal.
• The figure-2 depicts time domain and frequency domain frequency selective fading channel characteristics.
• Signal undergoes frequency selective fading if following conditions are met
➨ Bs >Bc
➨ Ts <στ
• Rule of thumb for channel to be frequency selective is Ts <=10*στ
Difference between flat fading and frequency selective fading
Following points summarize difference between flat fading and frequency selective fading.
➨In flat fading, BW of signal is less than the BW of channel where as in frequency selective fading, BW of signal is greater than BW of channel.
➨In flat fading, delay spread is less than symbol period where as in frequency selective fading, delay spread is greater than symbol period.
➨In flat fading, range of frequencies in a frequency spectrum are equally faded unlike in frequency selective fading where in
one part of frequency spectrum is faded more than the other part of frequency spectrum.
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