How to get a range of receiving SNRs

Dear all,

I’m transmitting some data between two usrps (N210) and using OFDM
blocks in GRC in the two computers. My experiment is getting BER in a
range of SNRs, like 0~20dB. So I need to set the usrps to get a range of
SNRs. However, fixing one usrp and moving the other along a line from 0m
to 3m doesn’t change SNR much. I also tried tuning antenna gain of UHD
source in GRC and found that higher the receiving gain, lower the SNR,
why? Also, the highest SNR I ever got is around 9dB using 16qam.

However, could anyone tell me how to set or locate usrps to get a series
of gradually increasing SNRs from 0~20dB? Thank you very munch! Any help
is appreciated.