Ada,
>>once the signal has been up converted. Now I'm using 0.2 amplitude and
35 gain. But I don't understand, how come that the smaller the power is,
the better transmission performance there will be. And also, why does
power should be in the range of 0 to 1.0? How to calculate transmission
power using the -tx-amplitude and tx-gain?
Better performance is not due to smaller power but it is due to the non-linear effects of RF-amplifiers. As per my observations, Tx-Dsp amplitude beyond 0.15 to 0.2 causes the USRP daughter-boards Tx-amplification-stage to go into non-linear region thus error-rate of PSK/QAM increases. Non-linear modulation e.g. FSK/GMSK are more robust to Amps non-linear effects. so u can experiment increasing TX-amplitude both for FSK-variants & PSK/QAM and observe performance.