Subject : Wireless Communication
Practical Link Budget Design Using Path Loss Model
Introduction:
By using path loss models to estimate the received signal level as a function of distance, it becomes possible to predict the SNR for a mobile communication system.
Practical Path Loss Estimation Techniques:
- Log-distance Path Loss Model
- Log-normal Shadowing Model
Log-distance Path Loss Model:
- Both theoretical and measurement-based propagation models indicate that average received signal power decreases logarithmically with distance, whether in outdoor or indoor radio channels.
- The average large-scale path loss for an arbitrary T-R separation is expressed as a function of distance by using a path loss exponent, n.
Equation 1
In equation 1 n is the path loss exponent .d0 is the close in reference distance, d is the T-R separation distance
Equation 2
- The bars in equations 1 denote the ensemble average of all possible path loss values for a given value of d.When plotted on a log-log scale, the modeled path loss is a straight line with a slope equal to 10n dB per decade
- The reference path loss is calculated using the free space path loss formula given by equation 2 or through field measurements at distance do.
Log-normal Shadowing Model:
- The log-normal distribution describes the random shadowing effects which occur over a large number of measurement locations which have the same T-R separation, but have different levels of clutter on the propagation path.
- This phenomenon is referred to as log-normal shadowing. The path loss PL(d) at a particular location is random and distributed log-normally is given by
Or
The probability that the received signal level will exceed a certain value Υcan be calculated from the cumulative density function as
- Q is the error function & defined as
- Similarly, the probability that the received signal level will be below Y is given by