what is the mean doppler shift for fixed wireless transmitter and receiver?
is it 0 or not?
Dear Geetanjali Choudari ,
welcome,
The Doppler shift is the consequence of the movement of the transmitter and receiver relative to each other. It is depends on the difference of the speed between the transmitter and the receiver. Specifically, fd= +-v fc/c, where v is the relative velocity, fc the carrier frequency and c is the speed of light in free space. So , if both the transmitter and receiver are fixed , which means v=0 and fd=zero. The presence of the doppler shift results in the socalled coherence time of the channel:
For more information about the coherence time please follow the link:http://www.ni.com/white-paper/14911/en/
Best wishes
Doppler shift can cause significant problems if the transmission technique is
sensitive to carrier frequency offsets or if the relative speed is too high. When
an electromagnetic wave source and the receiver are moving relatively one to
another, the received signal frequency will not be the same as the source signal
frequency. When they are moving toward each other the frequency of the
received signal is higher than the source frequency, but when they are moving
from each other the frequency of the received signal is lower than the source frequency.
This occurrence is called the Doppler shift. The amount of the Doppler
shift depends on relative motion between source and receiver and on the speed of
wave propagation. Maximal Doppler shift for frequency is calculated according
to the formula:
fd =(vr * fc/c)*cos α (1)
where fc is source frequency, vr is the speed difference between objects, c is the
speed of light (3 · 108 m/s), and α ∈ [0, π] is the angle of the velocity vector.
Our aim is to get maximal fd which happens when α → 0. (1) can be changed
to
fd =vr * fc /3.6* 3 · 10^8 (2)
Values of fd for 2.4 GHz carrier and various speeds are listed in Table 1. On
the speed range from 10 km/h to 120 km/h the Doppler shift is from 20 Hz to
300 Hz.
V(km/h) 10 20 30 40 50 60 70 80 90 100 110 120
fd(Hz) 22 44 66.7 88.9 111 133 155.6 177.8 200 222 244 266.7
for more detail please, ref: Estimation of Doppler Shift for IEEE 802.11g Standard
There is also Doppler shift if the signal bounces off a moving object before reaching the receiver. This is how moving target indication (MTI) radar works.
If your transmitter and receiver are not moving, and the signal goes from one to the other directly, there will be no Doppler shift. The mean Doppler shift will be zero.
You can get Doppler shift from antenna vibration. The mean of this will be zero.
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