Doppler Fading

Moving persons, cars or other objects cause the environmental channel properties to change. Furthermore signals being reflected by them experience a doppler shift which results in a dispersion in the frequency domain. The shifted rays are superimposed at the receiver which leads to doppler fading introducing time selectivity to the channel. The doppler frequency $f_\mathrm{d}$ is described by: $$ f_\mathrm{d} = \frac{v\cos{\phi}}{\lambda} $$ where

- $v$ denotes the relative velocity
- $\phi$ denotes the phase of a reflected signal
- $\lambda$ is the wavelength of the signal

The 802.11n doppler model [3] includes 2 different doppler-power-spectra. The first one is the so called "bell" shaped spectrum simulating slowly moving persons inside a building and is described by $$ S(f)=\frac{1}{1+9\left(\frac{f}{f_{\mathrm{d}}}\right)^2} $$ where $f_{\mathrm{d}}$ denotes the doppler frequency which is defined by $$ f_{\mathrm{d}} = \frac{v_0}{\lambda}= \frac{v_0 f_{\mathrm{c}}}{c} $$ where

- $\lambda$ denotes the signal wavelength
- $f_{\mathrm{c}}$ denotes the carrier frequency
- $c$ the speed of light
- $v_0$ is 1.2 km/h and was chosen after a series of measurements