There is a message of X2-AP protocol. one eNodeB sends a set of information to other eNodeB. "Relative Narrow band Tx Power" information is related to eICIC of LTE-advance. Basically to minimize the interference among cell edge users both the eNodeB which are operating in intra-frequency environment should co-ordinate is such a way so that cell edge throughput can be maximized.
For more details please refer 3GPP 36.423 release 9.