Multipath Fading
Understanding Multipath Fading
Multipath fading occurs because the signal reaches the receiver via multiple paths of different lengths. As these copies add at the receiver, they can combine constructively (increasing signal) or destructively (decreasing signal). The fading changes with frequency (frequency-selective) and location (spatial fading).
Fading Types
- Flat fading: All frequencies in the signal bandwidth are affected equally. Occurs when delay spread is small compared to symbol duration.
- Frequency-selective fading: Different frequencies fade independently. Occurs when delay spread is comparable to symbol duration. OFDM handles this by dividing into narrowband subcarriers.
- Fast fading: Channel changes within one symbol period. Occurs at high speed.
- Slow fading (shadowing): Large-scale signal variation due to obstacles. Log-normal distribution.
Frequently Asked Questions
What is multipath fading?
Multipath fading is signal fluctuation caused by multiple copies of a signal arriving via different paths and combining constructively or destructively. It can cause deep signal nulls (30-40 dB) and is the dominant challenge in mobile wireless.
How is multipath fading combated?
Diversity (space, frequency, polarization), OFDM (converts frequency-selective to flat fading per subcarrier), MIMO (exploits multipath as parallel channels), equalization (compensates for channel distortion), and interleaving with coding.
What is Rayleigh fading?
Rayleigh fading models the amplitude distribution when many reflected signals combine with no dominant line-of-sight path. The amplitude follows a Rayleigh distribution. When a dominant LOS path exists, the fading follows a Rician distribution (less severe).