What is mean by channel inversion in wireless communication
"The transmitter adapts its power to maintain constant SIR at the receiver."


When only one transmitter is involved, that might seem like a pointless thing to do.

When multiple different transmitters, perhaps at very different distances, are involved, it means that each transmitter arrives at the single receiver at the same power level, which optimises their signal to mutual interference ratio. Without this control, the nearest transmitter may swamp the weaker signals from the other more distant transmitters.