Modelling the Limiting Path

Inevitably the paths from base station to mobile and from mobile to base station are imbalanced.  That is to say that there will always be more signal arriving at one receiver than the other even though the paths themselves are reciprocal.  Generally, because the mobile or handheld is of relatively low RF power output compared to the base station, the uplink path from mobile/portable to base station is the weakest link.

There is no point in painting an optimistic view suggesting that viable communications exist when in fact this is true on one direction only.

This can be checked by drawing up a path budget for both uplink and downlink paths.  Then compare the maximum permissible loss for both.  The lowest value of maximum permissible loss is the weakest link.  The difference between the two maximums gives the path imbalance.

The following shows a path budget for a TETRA radio equipment.

The figures given here are indicative and for illustration of the point. The user must make his own calculation. 

The Land Mobile Coverage Plot computes the link in one direction only - from base station transmitter to mobile or portable receiver. The user must therefore make the necessary adjustments to the path budget to simulate the weakest link.  The above example shows that by subtracting 4dB from the downlink transmitter power, the uplink will be simulated showing locations on the ground where bi-directional communications will exist.