As previously I write about fading effect in GSM here I write about Fade Margin Calculation in GSM.
- Cell area probability (CAP ) is the percentage of the cell area that has signal strength greater than the receiver sensitivity.
- CAP is dependent on the radio environment, primarily the standard deviation of the log normal faded signal (s) and the propagation loss constant (n)
- The CAP is calculated using the following equaion
PCA=½ ( 1+ erf (a) + exp (2ab+1/b2)(1 – erf(ab+1/b)))
Where:
PCA = Cell area probability
A = Mfade/s
B = 10nLog10(e) / sÖ2
MFADE = Fade margin applied
s = Standard deviation of received signal
N = Propagation constant
Outdoor Fade Margin
- The outdoor fade margin depends on the standard deviation of the lognormal shadowing and the propagation constant
- The propagation constant depends on the environment and the frequency.
- For urban areas propagation constant varies from 2.7 to 5 , with a typical value of 5 for both 850 Mhz and 1900 Mhz.
- Standard deviation also varies on environment and frequency , and may vary slightly with frequency.
- The urban areas have higher standard deviation than rural areas.Typical value ranges from 5-12dB with a typical value of 8dB
- Outdoor fade margin can be calculated using a plot of the CAP equation.
- The next figure shows the CAP plot for a propagation constant of 3.5 and standard deviation of 5, 8 and 12.
From the figure fade margin to be applied to the Link Budget may be selected depending on the standard of the received signal.