Data rate USRP2

hi all

i am programming the usrp2 with WBX daughterboard using GNU radio. the
maximum data rate that i had is 638 Kbit/s at the sample rate 700kS/s. i
am transmitting a video file using QPSK modulation i tryed to increase
the sample rate but i had error on the video file. how can i increase
the data rate?

thank you in advence