Subject : Wireless Communication
Doppler Shift
Introduction:
Doppler shift: Due to the relative motion between the mobile and the base station, each multipath wave experiences an apparent shift in frequency. The shift in received signal frequency due to motion is called the Doppler shift, and is directly proportional to the velocity and direction of motion of the mobile with respect to the direction of arrival of the received multipath wave.
Illustration of Doppler Effect:
- Consider a mobile moving at a constant velocity v, along a path segment having length d between points X and Y, while it receives signals from a remote source S
- The difference in path lengths traveled by the wave from source S to the mobile at points X and Y is given by:
Where ∆t =time required for the mobile to travel from X to Y,
- The phase change in the received signal due to the difference in path lengths is therefore
- Hence the apparent change in frequency, or Doppler shift, is given by f_{d},
- Above Equation relates the Doppler shift to the mobile velocity and the spatial angle between the direction of motion of the mobile and the direction of arrival of the wave
Time Dispersion Parameters:
- The time dispersive properties of wide band multipath channels are most commonly quantified by their mean excess delay (τ) and r m s delay spread (σ_{τ})
- The mean excess delay is
The rms delay spread is the square root of the second central moment of the power delay profile and is defined to be