# Multipath Fading ## Physical Cause Interference between two or more versions of the transmitted signal, which arrives at different times. ## Size Changes in attenuation in the order of wavelengths ## Effects 1. Signal attenuation [^1] ## Independent Dimensions 1. Multipath (time dispersive) dimension 1. The channel spreads your signal over time - receive multiple copies 2. Time varying dimension 1. Channel may have gain, then attenuate signals etc. 2. Channel characteristic may change over time. ## The Radio Channel Radio channel is a natural multipath channel because you radiate away [isotropically](isotropic%20radiator.md), with reflections etc. Received signal is the [superposition](superposition.md) of the signal arriving by many paths, with different times. [^2] ### Multipath Dimension [Delay spread](Delay%20spread.md) and [coherence bandwidth](Coherence%20bandwidth.md)  The channel has spread the signal across time. [Flat fading](Flat%20fading.md) is actually not a good thing! This helps with: 1. Equaliser design 2. Achieving frequency [diversity](diversity%20scheme.md) 3. Achieving multipath [diversity](diversity%20scheme.md) ## Multipath formula Discrete [Power Delay Profile](power%20delay%20profile.md) ## Uses 1. [Physical layer](Physical%20layer.md) modem design 1. Codec 2. Modulator 3. Interleaver ## References 1. [UNSW Lecture 1 on Wireless Fading Channel Modeling and Estimation](../../03%20-%20University/Thesis/src/UNSW%20Lecture%201%20on%20Wireless%20Fading%20Channel%20Modeling%20and%20Estimation.mp4) ## Footnotes [^1]: Received power can vary up to 30dB within a few wavelengths [^2]: Due to reflections, diffraction, scattering