Updated the QPSK_Rx gain value to multiply by 10 rather than 100. At 100X, the chain works from USRP2->N210, but not from N210->USRP2. With a value of 10x, it works both ways. The reived signal without any amplification works from N210->USRP2, but not from USRP2->N210. After checking the output of both devices, it seems that the range of the N210 is between 1 to 2 orders of magnitude greater than the output from the USRP2; although I'm not sure if this is by design or if the USRP2 has an issue after being used with the LED drivers for VLC.